bluegrassz

iTrader: (3)

Ok guys, I am still having this problem.

I did a TPI to carb swap on my 92 L98. I didnt touch the bottom end or heads. Just the wieand Team=G #7525 intake and a holley 750vs with a GM HEI dist.

Every time I Hit the gas hard it will pop and backfire through the carb. It dosent matter were I put the timing at.

Here is what is done to the carb. New rear metering plate, Jet extensions with notch float, Power valve block, 74 jets primary and 76 jets secondary, 25 shooter,and a Quick fuel adjustable secondary diaphram.

The intake has choke and egr provisions, but are blocked. I also had to slot out the 4 center bolt holes to fit the heads.

The dist has new cap & rotor, pick-up coil, weights & springs, New gear, Accel super coil, and accel 8.8 mm wires. New plugs and the module has been checked.

The motor still has the stock cam. I brough it up on TDC and Installed the dist with the rotor pointing to #1. The firing order is correct (18436572) and the wires are not crossed.

When I try to time it with a timing light, It will miss a flash ever so often. The plugs were getting black soot on them, but they seem to be fine now. Maybe just a hint of brown, which would be borderline lean.

The car seems to have a slight miss to it and stays the same even if I pull a plug wire off.

I even did a compression test and Either 170 or 165 on every cylinder.

Did I install the dist wrong or something? I had two mechanics tell me that I would not be able to set the timing with the HEI and the stock L98 cam. I called Because there have been alot of you guys that has done it.

Help please. I am desperate.

todd200

I would try going up in size on your shooter. I had a 25 on mine when it was on the 305 and I had a horrible tip-in stumble and occasionally a lean backfire. I switched to a 31 shooter and it solved the problem.

bluegrassz

iTrader: (3)

yeah, I called holley teck line and they suggested the same thing. They said the 750 was too big for the stock 350. I have a 31 and a 40 that I think I will try just to see what happens.

Im going to try and set the base timing at 8*, clean and regap the plugs from 35 to 40. I will try the 31 shooter first and then maybe go up a size in jets if that dont work.

anyone else got any more suggestions?

1QWIKZ

maybe when you installed the distributer perhaps it wound up a tooth or 2 off?

bluegrassz

iTrader: (3)

Yeah, I thought about that, but would I be able to advance or retard the timing enough to compensate?

After I brought it up on TDC, I turned the oil pump shaft so that the rotor would point directly at the #1 cylinder. I even had the valve cover off to watch the #1 vavles. I even marked the spot on the cap and dist base were the #1 plug wire would go.

If you or anyone has a better way to install the dist, I will try it?
At this point, I would stand on my head if it would help! lol.

todd200

It sounds like the distributer is in right. Thats the same way I do it and and everyone else I know does it. I highly suggest trying a bigger shooter. It sounds like it is starving for fuel at tip-in and isn't able to get over the "hump" before switching to the main system.

bluegrassz

iTrader: (3)

Im going to try and reset the timing to about 8* and try a 31 shooter. I also have some 77-78 jets to try. I may not have large enough jets in it with the power valve block.

Im not sure what pump cam is in it or what would be best to use either? I just want to get it to smooth up and not back fire.

scottland

you can't install the distributer a tooth or two off. because there is no exact correct tooth.

stab it anywhere you feel like, then just place the plug wires accordingly. or stab it with the rotor pointing to the #1 cylinder @TDC and wire the plugs the way it says in your manual.

your problem sounds like a carb problem, if it has tip-in stumble, and then takes off, your ignition system is working fine, its an air/fuel problem, im no master at tuning carbs, but i can tell thats what the problem is.

F-BIRD'88

"Here is what is done to the carb. New rear metering plate, Jet extensions with notch float, Power valve block, 74 jets primary and 76 jets secondary, 25 shooter,and a Quick fuel adjustable secondary diaphram. "

Why did you remove and block the primary power valve?
If you did , put it back in.
The carb will be severly lean at WOT without it.

Use #70/72 primary jetting. use a 10.5" primary power valve.
use #80sec jets with not power valve in the back.

The accelerator pump should squirt fuel right as soon as you move the throttle lever. If there is any delay then readjust he accelerator pump linkage.

You may have a vacuum leak.

Set the ignition timing with the vacuum line disconnected.
Set the timing at idle then rev the motor and see what it advances to at high rpm. Should go to 32/36degrees.
When you're done, connect the vacuum advance in to the "ported vacuum source." Its on the passenger side of the primary metering block.

You may have a manifold vacuum leak.

bluegrassz

iTrader: (3)

well, I removed the powervalve because I am setting the car up for the drag strip. Besides I wasnt for sure that the 65 that was in it was good. I tried a 4.5 with 73/74 jets and had black soot on the plugs and still had the same problem.

I went around the inake and carb with starting fluid looking for any vaccum leaks. Cant find any.

and the accelerator pump diaphram is new and it does squirt as soon as I hit the throttle.

I agree that it is probably leaned out with the powervalve, but I hope to take care of that with larger jets. Oh yeah, I have tried both a 4.5 and a 6.5 powervalve with different jets. Never thought about a 10.5 If larger jets, timing, and larger squirter dosent work, I will pick one up and try it.

Like I said, I just want to get it to run and not back fire, untill I build it up later on.

88ROC1LE

iTrader: (4)

Hope you get this figured out soon!! Maybe we can meet up at the track this spring!

That does sound like a lean condition. I was having the same problem on my Caprice with the Edelbrock carb(750cfm). I put on my 650 Holley and the problem went away.

JakeJr

Try putting a vacuum gauge on the engine to see if you get a steady vacuum reading. If it's jumping around or dropping only to recover, you've got a leak somewhere.

The vacuum gauge can tell you which power valve is best too. You want the PV to be just below your in drive vacuum reading so that as soon as you jump on it, and the vacuum in the engine drops, the power valve will open. The number on the PV tells you when it's designed to open.

4.5 and probably 6.5 are way too low and by the time they open you'd been lean for a good little while.

If you remove and block the PV, you'll need to go up on main jets at least 10 sizes to account for the fuel the PV would have contributed. Driveability will be awful, along with lots of plug fouling and oil dilution from the excessive gas. Removing the PV is really for WOT only running.

Hope this helps.

Jake

F-BIRD'88

"but I hope to take care of that with larger jets. Oh yeah, I have tried both a 4.5 and a 6.5 powervalve with different jets. Never thought about a 10.5 If larger jets, timing, and larger squirter dosent work, I will pick one up and try it. "

Put the powervalve back in.

If you're only going to be drag racing then the vacuum rating of the power valve is irrelevant. When you floor the gas the power valve will open instantly, reguardless of what # is stamped on it.
When the throttle is open the manifold vacuum is 0 or near 0.
As long as the power valve is closed at idle and open at Wot its fine for "drag racing"
The different ratings allow the power valve to open at different manifold vacuums. This allows you to adjust the opening point to different engine loads/throttle positions at part throttle.
But when you floor the gas it will open instantly with
out delay.
If you eliminate the power valve the primary jetting will be too lean. Too lean is a major cause of backfireing through the carb.

You've created your own problem by trying to "set it up for drag racing" With a stock TPI camshaft your engines level of tune is Mild.
No need to hillbilly tune it. Put the power valve back in.


The black smoke is not caused by the power valve, unless it is blown ( diaphram punctured). Put it back in. Probabile cause is a blown power valve, a vacuum leak (underside of intake ports) weak ignition or just fouled plugs.

bluegrassz

iTrader: (3)

Yeah, I hope to be at the track more this spring. I have seen a few thirdgens down there, but didnt get to talk to anyone. I hope to do some head work and a new cam by then.

bluegrassz

iTrader: (3)

I really need to pick up a vaccum gauge and check for leaks. I have tried starting fluid, but it may be a small leak.

I tried 76-78 jets and a 31 squirter tonight, and set the timing a 4*. The motor seemed to start fine at that point. I tried both places on the carb for the vaccum advance line also. the car did do some better, at least quit backfiring as bad. It would free rev great, but would have a dead mis. I took it out and it would just cut out when I accererated, but would smooth out with steady pedal.

I will check my local speed shop for a 10.5 powervalve and pick up somemore jets and try that.

If I do have a vaccum leak, I cant find it. But a vaccum gauge test would tell for sure.

JakeJr

Back in 86 I pulled the TPI engine from a T/A I had just bought and put in a 468 BB with a Holley 850 DP - 4781-2. Drove it for years and loved every minute, so I know what you're doing can be done.

Apparently you are no longer using the original distributor, right? If you are, is it advancing with the RPMs (checked with the timing light)?

Is the ECM still installed and active; did you try pulling error codes? What part is it playing in this whole thing?

How are you cutting down the fuel pressure from the electric pump in the tank to live with the carbed motor.

At rear of carb is there a power brake hose bib? How about on the manifold in the same location (a tapped hole) in the plenum. Are they both sealed.


Keep us posted.

Jake

bluegrassz

iTrader: (3)

Well, I am running a old style GM HEI dist with everything newinside and out except the vaccum advance.

The computer is somewere in california with the TPI system I sold.

I reduced the fuel pressure down with a bypass regulator from mallory, the 3-port one.

As for the power brake hose, yes I have the bib on the back of the carb and also a fitting on the intake for a pcv hose.

I have tried switch the hoses for the brake booster and pcv. Im am not sure which one is right?

Should the brake booster hose go to the carb, or to the manifold?

bluegrassz

iTrader: (3)

[QUOTE]Originally posted by F-BIRD'88
[B]"but I hope to take care of that with larger jets. Oh yeah, I have tried both a 4.5 and a 6.5 powervalve with different jets. Never thought about a 10.5 If larger jets, timing, and larger squirter dosent work, I will pick one up and try it. "

Put the powervalve back in.

If you're only going to be drag racing then the vacuum rating of the power valve is irrelevant. When you floor the gas the power valve will open instantly, reguardless of what # is stamped on it.
When the throttle is open the manifold vacuum is 0 or near 0.
As long as the power valve is closed at idle and open at Wot its fine for "drag racing"
The different ratings allow the power valve to open at different manifold vacuums. This allows you to adjust the opening point to different engine loads/throttle positions at part throttle.
But when you floor the gas it will open instantly with
out delay.
If you eliminate the power valve the primary jetting will be too lean. Too lean is a major cause of backfireing through the carb.

You've created your own problem by trying to "set it up for drag racing" With a stock TPI camshaft your engines level of tune is Mild.
No need to hillbilly tune it. Put the power valve back in.

[QUOTE]


I forgot to mention, that it was backfiring and had black soot on the plugs and out the tailpipe when I first put the carb on. It had 73 jets in the front, 6.5 powervalve and the stock metering plate in the back.

This was one reason I put the powervalve block in to see it and bigger jets would help.

Im still learning on tuning the carb, so thanks for all the help.

I have 78 jets in the back, so I will try the 10.5 powervalve and some 73 jets in the front. Bump the timing up and go from there.

What pump cam should I be running? I have no idea which one is in the carb, or were to look for it! Like I said I am still learning and could use the help

thanks.

JakeJr

I just went back through the your posts and the responses still trying to figure out what could be causing your backfiring.

Black smoke out the tail pipe indicates extremely rich condition. Back firing most often is a cross plug wire.

The squirter being too small will just give you a bog as you first open the throttle under load (car in gear).

A power valve with too low a number, such as 4.5, ect., will give you a lean condition under load until vacuum drops to 4.5 or below.

Jets too small will give surging at steady throttle and a lean condition at WOT.

A general vacuum leak will give you low vacuum readings (lower than you'd expect factoring in the camshaft you're running).

An intermittent vacuum leak - one that only effects one intake port, will result in the vacuum gauge reading dropping sharply when that intake valve begins to open. The reading will recover when the valves on that cylinder are closed.

So here's what I'm left with:

You wrote you brought the piston (I'm assuming it was #1) up to TDC.

That may be the cause right there.

All pistons are at TDC twice during each RPM; Once on the compression stroke and once on the exhaust stroke.

Did you make sure you were on the compression stroke by sticking your finger in the #1 spark plug hole, stuffing a piece of rolled-up paper towel partically in the hole (it'll blow out on the compression stroke) or carefully watch the #1 rocker arms?

Just watching the dampner won't work since the TDC mark on the dampner will align with the timing chain cover tab twice in each RPM; once on compression and once on exhaust.

If you did do it using one of these, then I'll have to go back to the drawing board.

Try putting your timing light on each plug wire one at a time as the engine idles and verify that each one flashes consistently.

Could you have cracked the porcelain on one of the plugs when you installed it?

Engine idling in a completely dark area; any arching of the plugs or plug wires?

Inducted cross-fire; are any two plug wires running in parallel and touching or very close to each other? You need at lease a 1/4" seperation. If you don't have swire seperators you can fabricate them using cable ties.

Any plug boots up against the header?

Any sign of cross-fire inside the distributor cap; carbon tracking, etc. Rotor installed correctly and tight?

Just trying to help.

Jake

bluegrassz

iTrader: (3)

well where should I start:

Plugs are fine, I just repulled them and cleaned them up, regaped at 40. No crossed plug wires or cross-fire in the dist. All the wires seem fine (no arching).

And yes I did have my thumb over the #1 spark plug hole while someone cranked it. I felt a little pressure on the exhaust stroke and then on the second it blew my thumb off the hole. I then turned the balancer about 1/2 inch more to align the marks.

I will try the timing light on each wire and see how that goes. The timing is only at 4*btdc, so I will change it to 6* to 8* and see how that works. I might even try it at 12* if it will start.

bluegrassz

iTrader: (3)

Update

I put the timing light on each plug wire and found one that didnt fire all the time, so I replaced it. The car seems to idle better and the mis is very slight if any.

I put the 6.5 powervalve back in and tried 74 jet in front and 78 in back. The car runs better, but it still seems to stumble when I get on the throttle. It does smooth out, but if I stay on the throttle, and start to get the rpms up, it starts stumbling.

Its like it runs out of gas. Oh yeah, I bumped the timing to about 8* to 10* btdc. Car seems to start fine.

I am almost there, just dont know what else to do to get everything right.

JakeJr

If the stumble you wrote about occurs just as you accelerate from a stop or low speed it could just be that you need a slightly larger squirter.

If it occurs later on, then it could be, again, the power valve is opening too late resulting in a momentary lean condition and, thus, the stumble.

You need to put a vacuum gauge on the engine and see what vacuum it pulls FULLY WARMED UP AND IN GEAR. Choose you power valve based on that. A 6.5 power valve means the power valve doesn't begin to open until engine vacuum drops to 6.5" or lower. You have to be into the throttle pretty good for it to drop that low.

You also set your idle mixture screws using the vacuum gauge. Start with 2-1/2 turns from being lightly seated, then adjust each one a LITTLE at a time until you get the highest vacuum reading. Of course, the engine should be fully warmed up, idling in gear for this to be most accurate (I'm assuming you have an auto trans, if not you'll have to use neutral gear).

Hope this helps; keep me posted.

Jake

bluegrassz

iTrader: (3)

I will try and borrow a vaccum gauge this weekend and check it out. I hope that a different powervalve and some tunning is all that is left. I definatly need new plug wires. They might be breaking down at higher rpms.

The motor seems to stumble up around 4000 rpm, as soon as I let off of it some but still have steady pressure, it smooths right up.

thanks for the help. I will let you guys know what I find.

JakeJr

Even if you have to buy one, a vacuum gauge is a nice tool to have in the garage. They're really cheap too, under $10 or so. All the auto parts stores sell 'em.

Jake

bluegrassz

iTrader: (3)

OK

I hooked the vaccum gauge to a fitting on the intake. the car was warmed up and was in drive. The gauge showed 17" at idle. I would hold it at 2000 rpm and let off it. The gauge would jump to about 25" and then back down. From what I have read, this is normal.

I even tried the vaccum port on the base of the carb. It showed 15" to 16" at idle.

what powervalve should I get?

I also noticed that the pump cam was orange, How is this cam rated?

thanks

JakeJr

If the vacuum reading held steady then you've got excellent vacuum. A 10.5 is the right one to use.

Make sure the squirter is adjusted properly. It's the spring loaded screw on the driver's side front of the carb, right at the fuel bowl. As soon as the throttle is moved fuel should begin to squirt from the two primary squirter nozzles. The point that the squirter begins to contribute fuel can be adjusted by turning the little spring loaded screw. It should rest on the accelerator pump arm with no clearance (when you hold the spring loaded screw upward). That way, as soon as you touch the accelerator pedal, the accelerator pump begins to contribute fuel.

If not, first unscrew the squirter (be careful not to drop it into the bores) and make sure the little silver colored pin (it's removable) is present. The little pin is about 3/8" to 1/2" long.

Check the holes in the squirter to make sure neither is blocked.

A rule of thumb is: If you mildly wing the throttle and black smoke comes out the tail pipe, the squirter is too big.

If the engine stumbles, then catches up, it's too small.

Pump cams determine how quickly/slowly the accelerator pump contributes fuel through the squirter. The ramps on the cam are different resulting in faster/slower pump activity. They're sold in kits that come with an instruction sheet showing which color does what. Been a long time for me, so I can't remember what the colors mean. Generally, though, pump cams are about the last thing that ever needs changing; they're pretty much for fine-fine tuning.

Hope this helps. Keep me posted.

Jake

bluegrassz

iTrader: (3)

so the 10,5 is definatly what I need? I thought a 8.5, but wasnt for sure.

The squirter has good flow and is now blocked, but it is a 31 and it does stumble at tip-in and then picks up. So I guess a 35 then?

I will pick up some new plug wires, a 31 squirter and a 10.5 powervalve and see what happens. I might bump the timing to 10* to 12* and see what happens also.

Do you think the 74 front and 78 back jets will be ok with the 10.5 powervalve?

As far as the pump cam, I will leave it as is for now.

JakeJr

Remember, the number of the power valve - 4.5, 6.5, 8.5, 10.5 - doesn't relate to how MUCH fuel it flows, only WHEN it begins to allow fuel flow. An 8.5 won't BEGIN to allow fuel flow until vacuum drops to or below 8.5", by that time the engine would probably have stumbled due to the lean condition.

Conversely, if your engine only pulled 10" of vacuum in gear (as would be the case if you have a long duration/high over-lap camshaft installed) and you installed a 10.5 power valve, the engine would run extremely rich because the power valve would always be open at idle.

As far as jetting: Start with the stock jets. Obsent a dyno that can read air/fuel ratio, you first do a cruise check. Drive down the roadway at a set speed, say 55 MPH. If the engine speed remains steady and throttle response is good, then you are probably ok.

If it surges up then down then up then down, etc., the engine's lean and you need to go up in jet size; go one jet size increase at a time until the condition stops.

For WOT jetting, you need to do a 2nd gear WOT pass listening for any ping or detonation. If you hear any, lift immediately and increase the secondary jet size and/or reduce the ignition timing.

It's up to you to find a safe place for this and to use good judgement and common sense. At the end of the WOT pass, immediately turn off the engine to cut it clean, then brake and come to a stop. DO NOT let the engine idle as you come to a stop; that will mask the plug color.

You'll need to be on a straight road for this because your power steering will shut down with the engine.

You then need to immediately pull all the plugs and inspect them for color and any signs of detonation. Be careful, the engine will be hot. Write down the description of each plug's color. A lighted magnifying glass will help.

The color should be light tan and there should be a dark ring around the porcelain deep in the plug shell; the deeper the better.

No "Fly ****" specs should be present on the procelain either, "Fly ****" specs are signs of detonation.

If the plug porcelain is whiteish, jet up the secondaries, if dark brown, jet down the secondaries.

Jake

F-BIRD'88

Just a word of note.... Once a spark plug is carbon fouled, it cannot be "cleaned"..... The porcelin becomes impregnated with carbon which conducts electricity. The plug will always misfire especially at WOT. (reguardless of how many times you clean them or how good you think you are at cleaning plugs)
It takes quiter a bit of extra unburned fuel to turn a plug black but once they are fouled, they are finished.
Being a few jet sizes off will not cause this.


This "extra unburned fuel" can come from an ignition problem, a blown power valve, flooding, etc.


Its most noticeable when you first stand on it. The motor will stumble and beak up. Don't confuse this with a lean stumble.
Its not the same thing.
All the jetting and accelerator pump cams and shooter adjustments will not cure it. Going richer will mask it thou.

You have to tune a motor with clean plugs if you want good results.

You should end up with 70/72 primary jetting and 78 to 84 secondary jets.
If you end up with something much different from this, you have an ignition problem or a vacuum leak or a leaky powervalve.

Save yourself a lot of tuning grief and start/start over with new plugs.

bluegrassz

iTrader: (3)

I will replace the plug wires, plugs and add a 10.5 powervalve. I will also change the jets to a starting point and go from there.

I hope this helps. The car accutaly ran ok at one time with 73 primary jets, the stock metering plate, 25 squirter and a 6.5 powervalve. The car had a bad mis and would only run 14.97 in a quarter.

I hope to get everything fixed. I will keep everyone posted

thanks.

305sbc

iTrader: (3)

Just FYI I'm pretty darn good at cleaning mine. It's a common practice (twice per year per plug) since I run a very cold non-resistor racing plug. They tend to foul very easily, but my method has always worked.
I sand blast them, dab the tip into muratic acid, then I hit them with a propane torch for about 2 minutes each.
Has worked every time.

bluegrassz

iTrader: (3)

update.

I replace the plugwires, plugs, added a 8.5 powervalve and went from a 31 squirter to a 35.

Everything seems 100% better. I have good idle, and the motor does not hesitate at tip-in now.

Only thing is that a around 4000 rpm it starts sputtering. I can get a wot pass through the 1/4, but at around 4000 rpm it starts.

I put a timing light on it with the vaccum advance plugged. It read 8*. When I rev the motor to 2500-3000 it only goes up to like 12-14*. It has new weights and springs(naturel) in the dist. I think they are sticking or something. Shouldnt I get about 36-38 * total when I rev the motor up?

Momar

I had that problem w/ my timing recently. I figured out that the piece in the middle of the weights was upside down. I flipped it over and was geting full advance after that. You might want to check that out. I went through a lot of trouble to figure this out. I even tried grinding out the slots that determine total advance, but it wasnt even advancing enough that those were a factor.

Ben

bluegrassz

iTrader: (3)

I bought a new weight set, but the were just a little different than stock. It was the spectra brand. I will check it out and maybe clean the old set and put them back on. I can still use the new springs. If I get this fixed the car will be running great. I cant wait to see what it runs like getting the advance working right.

I will keep you guys posted

thanks.

Momar

I am pretty sure that is the same brand as was on my dist when I got it from my brother. I dont think it works if you put the middle piece on upside down. I thought it was on right side up by looking at a few pics on line, but I fliped it over and it started working correctly.

Ben

bluegrassz

iTrader: (3)

Thanks ben! I will check it out as soon as I get home. I also noticed the black bushings for the weights were tight when I installed them. I might have to check them again. Too bad they just shut the track down till spring.

Oh well, it will give me time to get the 3.73's in and my exhaust done. I have had two dynomax bullits sitting in the garage for about 3 months and the gears I have had for almost a year.lol

jms

iTrader: (1)

I learned a long time ago that the old style GM HEI distributors must have at least 13 to 13.5 volts going into the distributor.

A lot of guys would convert from points and just wire up to the new HEI but would forget there was usually a resistor somewhere in the circuit, keeping full battery voltage from the HEI.

It may be worth your while to measure the voltage at the connection going to the HEI.

My experience with not enough voltage is much like you are now describing in that the car doesn't want to "rev".

Just a thought.

jms

bluegrassz

iTrader: (3)

Well, The wire I have hooked to the dist was the one that went to the original dist. A large pink one. I checked it with a volt meter and it checked out fine.

I went ahead and put the old weights and springs back on it. After I cleaned them up of course.

The timing seems to advance right now. That fixed that, but I have narrowed it down to what is going on.

If I take off at wot to make a pass im fine, as soon as I let out of the throttle just a little bit it starts shuttering. It does it in the 2500-5500 rpm range. It dosent matter as long as Im at wot, it runs great. When I apply mor throttle, it smooths right up.

I cant figure it out. It just starts shuttering when I ease up on the throttle. Im not raising my foot off the throttle, just easing up on the pressure just a bit.

I even bumped the timing to 12* base. It seemed to like it, but didnt help the problem.

I dont know what is happening?