whitedevilTA

iTrader: (8)

I need some info on this one guys. Me and my brother are trying to learn about carb tuning and his car has never idled right. It would always either idle at 1300 or so rpm's when warmed up, or very low around 450-500 rpm when warm. The carb is a holley 4150 mech sec 750 cfm. We have tried to adjust the idle mixture screws and got them to where it makes max vacuum at idle, but we can not get it set at an optimum rpm. Anything that could cause this?

Vac at around 500 rpm is 6 HG's, and vacuum at 1300 rpms is 15 HG's. At the optimum rpm, we should have around 9-10 HG's vac which would be great with the cam in the car. Does the carb possibly just need a rebuild? It's been on a running engine for about 8 years and I'm not sure how often they need to be cleaned and rebuilt.

I'm guessing something is bad or gummed up inside the carb to be causing the problem. The carb has a 6.5 power valve in it so when the idle drops to 500rpm and 6 HG's, I'm guessing it gets really rich which we don't want. Anyways, any help would be great.

Set up is a 350, procomp 210CC aluminum heads, .488/.510 & 234/244 duration cam, performer RPM air gap intake, and the 750 carb.

BTW, turning the fast idle adjustment screw has no effect on idle rpm. It will either be at or go above 1300 rpm when screwing it in, or when turning screw out, it stays at 1300 rpm and then just falls off to 500 rpm at some point. There is no in between rpm.

five7kid

iTrader: (14)

What are you doing with the vacuum advance?

whitedevilTA

iTrader: (8)

What do you mean what am I doing with it??

naf

iTrader: (7)

See what your timing is doing at the two different idle speeds. A big jump in timing advance somewhere in that rpm range can make finding your preferred idle speed difficult.

whitedevilTA

iTrader: (8)

Well the problem persists whether the vacuum advance is connected or not so I am thinking it's most likely a carb problem.

naf

iTrader: (7)

I'd still put a light on it to see what it's doing.

whitedevilTA

iTrader: (8)

Please don't mind my ignorance since I am just curious....but if the rpm is jumping like that in the first place, the timing is going to suddenly change with the rpms whether it's the distributors fault or the carbs fault. Anything specific I should be looking for to point the problem to the distributor?? I guess what I'm wondering is why do you think the problem could be the distributor in the first place? It's my brothers car so I will have to wait for him to get home to try it out anyways but we will def look into the timing and see what it's doing.

Street Lethal

iTrader: (16)

Sounds like a faulty choke....

whitedevilTA

iTrader: (8)

No choke on the carb. It's a holley mech secondary 4 barrel and the choke was removed.

Street Lethal

iTrader: (16)

If your not running a choke anymore, then the following....

... is what it honestly sounds like. That is a very healthy cam you are running, so determine your vacuum reading in gear at your desired RPM, say around 600-RPM, then divide that vacuum reading by two, and that is where your power valve needs to be. Remove that 6.5 power valve and replace it with a 4.5, for starters. If you still need less, then go 3.5.

five7kid

iTrader: (14)

The power valve only affects the main metering circuit. If idle vacuum affects the PV, then the throttle blades are way too far open at idle.

I'd suggest starting with following the recommendations in the Holley carb sticky, and seeing if your problem is fixed by following those steps.

naf

iTrader: (7)

A good general rule when trying to fix a carb issue is to ALWAYS double check your ignition first. I've seen similar idle issues that were caused by a timing advance curve that jumped the timing up significantly at a fairly low rpm. Like 12 degrees at 500 rpm then suddenly 20+ at around 800-900 rpm. No way in that case to get a solid idle anywhere in between. Very likely your problem? Probably not. But putting a light on it takes all of, what, two minutes?

whitedevilTA

iTrader: (8)

^^ Good to know. I have heard to always check the ignition on a carbed car before the carb itself. I will get a light on it soon just to be sure it's not taking a huge jump in timing in between those rpm's. If it isn't, I'm assuming it's something in the carb so we'll have to dig into it.

91camaro248

check timming, then try the 4.5 power valve.

whitedevilTA

iTrader: (8)

Will do....thats was what I was planning on doing. This weekend my brother wants to tear into it so we will check the timing and then he's pulling the carb apart to check it out. We've been wanting to richen up the secondaries a bit anyways since the new engine combo has much more compression, so while were in there, we may end up changing out the power valve for the 4.5 since the cam doesn't make the greatest vacuum. I'll keep you all updated on the outcome.

whitedevilTA

iTrader: (8)

Got an update on the carb situation guys. Me and my brother pulled the bowl off the secondary side and were going to clean the passages out and put some new gaskets in it just to freshen it up. I'd like to get a baseline point for jets, PV's, etc to put in there before we put it back together. The secondary jets were 79's and it had a plug in place of the PV. On the primary side, the PV is a 6.5 currently and were still unsure of the jets but we will find out tomorrow.

I'd really like to get a recommendation on what jets to run since the new engine is up around 40 or more HP than the old engine but I don't know how much each jet size will affect how much richer it will make it. Anybody that can give me a good starting point?

Also I have heard that putting a power valve in and lowering the jet size will help in leaning it out more during idle and cruise. Any tips you guys can give me and a starting point would be great! Thanks for all the help.

pancherj

If it were me, I would go on Holley's website and find out what the 750 Double Pumpers come with stock...including the rear PV! There is no reason for your rear PV to be blocked (unless this is more of a race car). As for what PV to chose, check out this link http://speedtalk.com/forum/viewtopic.php?t=14523

Even if the PV opened at idle, it should have no affect on the AFR at idle (a leaking gasket is another story). Forget most of the rules you have heard for chosing a PV (1/2 idle vacuum, 2" less than idle vacuum, etc.). Put a vacuum gage on the carb that you can read inside the car while driving. See what the normal cruising vacuum is (probably around 16-20"). Then, see where it drops to when you moderately accelerate (like going up a hill). It will probably drop to around 8-10" of vacuum. You want your PV to be in that range. If you do this, then you can lean out your main jets so that it cruises much nicer.

Again, return the carb to stock specs and tell us about your timing curve. Don't even try to tune the carb without getting your timing set up correctly (base, vacuum advance, full mechanical, etc).

whitedevilTA

iTrader: (8)

Holleys factory jet sizes in this carb were 70's in front, 80's in rear. They never came with a secondary PV because it's not really needed from what I've read. It would only function when the secondaries are open which is basically wide open throttle only, and the secondary jets can take care of that.

As far as why we pulled the carb apart....this car does not have the power it should. It's comparable to a 13 second car and it should be WAY faster than that. We have played with timing and tried adjusting it all over the place, even going in increments of 2 degrees from 29 to 36 and the car just doesn't go like it should. With more timing, it hesitates in upper RPM's as well. I think it's either not getting enough fuel for WOT or the carb is gummed up because it looks filthy when looking inside with the air cleaner off. The car should be an easy mid to low 12 second car and the performance is absolutely not there. This car was built to be a weekend grudge race car for the most part so we really want WOT to be the first thing we get right. Then from there we'll play with the thing to get it to run nice on the street. So with that said, does anyone think that going from a 79 secondary jet to an 83 or 84 will richen it up too much?

We don't plan on messing with the primary side except for making PV changes since it seemed to cruise pretty well before.

pancherj

You could up the jets. Make a drastic change, like four jet sizes. This way you can tell if you are on the right track. With that being said, what leads you to believe the car should be faster? A 350 with a small hydraulic cam and a dual plane intake may not be a real screamer (low 12's ina full weight street car). Also, I would be suspect of the procomp heads as far as geometry at the rocker arm is concerned. I have no personal experience, but I have heard some nightmare stories. If the geometry is bad, the engine may not want to rev. Just some things to think about if the carb changes don't get you where you want to be.

whitedevilTA

iTrader: (8)

I'll definately keep the rocker arm geometry in mind. The car runs very well but has 4.10 gears and just seems to struggle in upper rpms. Don't get me wrong, it still goes good but after being in many different cars with many different engines and gear set ups, I just know something is up with this car. We should have the carb back together today and hopefully it makes a big difference....if not we have bigger fish to fry LOL.

And low 12's would be GREAT right about now, but we have raced a few freinds with mid 13 cars and just BARELY pulled by them. A 350 with this compression and cam should go way better than that.