happydad

iTrader: (1)

I have a 383 with a Powerdyne S/C..750 Holley B/T..7-8 psi to the carb..A/F is 13.2 @ idle..The problem is when starting from a standstill theres a small miss or hessitation..How do I get rid of this..once it's going it runs great..It's just the miss when you 1st start out

mw66nova

iTrader: (13)

sounds like an accelerator pump shot issue. either the squirter is too small or the cam is the wrong color/position. what size squirters are you using?

happydad

iTrader: (1)

I want to say they are 35's..It's a dual pumper I havent messed wth the adjustment screw yet

xpndbl3

iTrader: (5)

A/F ratio at idle should be upper 14s easily. Is it a slight tip-in hesistation or when you quickly stab the throttle? What pump cams? in what hole? what size squirters? What does the wideband show when it dips for a quick second then recovers? or does it dip then not recover? Is this just driving around or are you hitting boost?


details....it's all about the details.

383backinblack

your idle mixture is WAY TO LEAN......you should be idling around 12.3-12.5

my car did the same thing.......until i learned the hard way that even at idle the car isn't TRULY n/a.......if you don't have that mixture richer, you'll never get rid of the hesitation even with a HUGE accelerator pump shot. Mine has 50cc pumps, with big nozzles on it ( i think they're 40's right now)

but ya, like i said, you're WAY too lean there man.......your idle circuit is important especially on the part throttle transition when you stab the throttle.....if its not rich enough, there just won't be enough fuel available to cover the lean spot even with a big pump shot.

we tested this on a 5-gas analyzer not just a wideband o2......the air fuel was at 13ish, and the 5 gas was reading SUPER lean.......and if you got on the throttle with light to no load, the headers would get CHERRY red, even though the a/f dropped to 11.5:1 under load.......the problem was tip in, and idle/part throttle......

your idle circuit is the problem.....richen it up to 12.5 and put a bigger pump nozzle on there....try a 37........and check the color of your cam and what hole its in.

i hope this helps man, had this exact same issue when i first built my procharger setup.....i made the mistake of assuming the car could be tuned like an N/A car at idle/part throttle.......i was mistaken. Bob Vrbancic from the carb shop helped me out (they built my carb) i told him what i saw on the 5gas, and he told me what i just told you.......as soon as i did that all the problems went away.....even with my HUGE bypass the carb is seeing slightly more than atmospheric pressure even at idle......it needs more gas

does your blower have a bypass? if it doesnt it'll probably be even more pronounced of a problem

383backinblack

14:1 idle mixture on a blow-through carb huh? thats a great idea if you want to melt your headers at part throttle cruise dude.......

you're still using the idle circuit on the carburetor at part throttle (off idle), you don't transistion to the main circuit until you get into the throttle......the idle circuit is still contributing a major portion of the fuel curve at that point......if its leaner than the mid 12's (depends on the engine, some a little richer, some a little leaner) there isnt enough of fuel available and you'll see the lean spike on the wideband at tip-in and on 5gas analyzer at part throttle if you have access to one.....no matter how big you make the squirter

xpndbl3

iTrader: (5)

soo sorry all the turbo blow thru carbs are set up that way from everyone that i've seen online. We tuned my buddies S/C SBC the same way with no issues at all on the dyno. He was in the 14s at part throttle cruising, but there was no boost showing on the boost guage at all until he got into it. The dyno operator agreed with us that's how he set them up usually as well. I'm always up for hearing other ideas though, so I'll add your thoughts into that category if we ever run into cherry headers.

383backinblack

well, thats how blow through carbs are tuned.......i don't know how its possible to get it to run right that lean, it will ALWAYS miss......especially if its a holley carb......theres no intermediate circuit except custom-high end metering blocks......usually in dominators

the basic reason without all the explanation is this, the transition circuit isn't big enough (even with a 50cc pump housing) so the idle has to be kept richer.....

don't get me wrong, its going to smell and run rich at part throttle, but if you don't tune it that way it'll never run right and you can end up hurting some parts.....because your exhaust temps will be ridiculous at part throttle.

I spent alot of time understanding this problem....and the guy that owns "the carb shop" (excellent blow throughs btw) was good enough to kick it around with me and school me on the concept as it applied to blown cars while we sorted it out, it also has alot to do with how the signal in the venturi is manipulated with the extra air flow/pressure differential.

turbo's are alot different.....although the tip in should encounter the same issues, the part throttle and idle shouldnt be as affected because of the wastegate, you could have alot less residual pressure in the intake tract with the bypass open in theory........so you might not see the same problems with the static idle mixture, but the same lack of available fuel will occur with tip-in throttle response

xpndbl3

iTrader: (5)

Basically everyone is running the boost referenced PV setups by CSU on the turboforums.com website. This with larger PV restictor holes adds the necessary fuel during boost, and running lengthened vent tubes that run up into the carb hat and are boost referenced up there. Check out that site sometime, guys are running without boost in the 14s then getting the 11.3-11.5 ish needed while in boost so it's the best of both worlds. I've only been researching and messing around with them for a year probably, but it really seems like a few guys on there have them setup amazingly well.


Seen that you edited a little blurb, my buddy ran the blow thru carb by CSU with his supercharger, and the tip in issues were solved with a blue pump cam in the #2 hole and 35 squirters up front and 37 squirters in the rear IIRC the rear was 50cc but the front was still the factory type diaphragm.

383backinblack

how do you think my carb is setup? you can't have a boost referenced carb that even works without a boost referenced PV......the vents have to be like that or fuel would shoot out of them, they have to be at the same pressure as the intake tract/carb hat.

if its a race car it doesnt really matter that much, but for drivability its a must to have it setup that way........my car did the same thing.....you can tune the idle circuit down to 14:1 if you want, and as soon as you get under load and into boost it'll hit 11.5........but that doesnt change the behavior of the tip in......its impossible to do that with a holley unless you have an intermediate circuit thats precisely tuned.......4150 series carbs don't have those......4500 dominators can, but thats a really really big carburetor even for my motor.

another thing to consider, is people usually aren't forth coming about details like that. Id bet a good amount of money that alot of the blow through s/c cars that are tuned that way have miserable throttle response......i had the same attitude until i saw the evidence of what was really going on "look my car runs at 14:1 at cruise, and its blown" then i tuned it the right way, and said "oh my god i was an idiot" it was a whole different car
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my carb also has a 30cc pump on the primary and a 50cc on the secondary, but i switched them both the 50cc for the drag strip.....more fuel available on the gear changes (i dont powershift it, my driveline isn't disposable lol)

xpndbl3

iTrader: (5)

Is yours making boost at slight tip-in? Just trying to understand this better. Or is it due to the restriction from the blower? I'm a little confused.

happydad

iTrader: (1)

Wow Thanks for the info guys...I'm putting in 50 acel pumps to see if it helps..Then I'll recheck the A/f..

xpndbl3

iTrader: (5)

you need to tell us how the A/F ratio reacts when you jump on it and squirters play a role as well.

383backinblack

thats true.......you can't go by what your wideband reads while you're operating the car though......unless its data logged.....a) its wicked dangerous to try and read it like that and b) its not nearly as accurate and controlled as you need it to be......if you can get on a chassis dyno and watch your a/f curve in a graph you'll probably see a BIG lean spike where you hit the throttle......the 50cc pumps will give you MORE of a shot that lasts longer......the nozzle size will control how quickly that fuel is delivered......for the same pump cam.

this is what the lean spike will look like on a dyno when you nail the throttle....keep in mind, this one is REALLY bad haha

happydad

iTrader: (1)

Ok I only had one acel pump kit so I ordered aother one and some squirters...It was mentioned to adjust my idle circuit ..How do you do that..Sorry for being ignorant on some of this stuff..I don't wanna blow anything on my new build..Thanks for your help

383backinblack

thats why its good to ask people that have done it before man

none of us would be doing what we do with any success at all, if it weren't for learning from people that did it before we did......

the idle circuit is adjusted with the idle mixture screws......there should be 4 of them, 1 on each side of each metering block.....they should all be adjusted incrementally the same amount, i usually just watch the wideband and move them an 1/8 of a turn at a time.....

like loosen all 4 screws an 1/8 of a turn (with the motor running) then snap the throttle a couple times, and read the wideband.....repeat until you get what you want.

make sure you're timing and idle speed are set how you want them first......after you're done adjusting the idle mixture, you'll most likely have to re-adjust your idle speed....richening it usually slows it down.

junkcltr

iTrader: (1)

Idle AFR depends on the overlap of the cam that is installed. A cam with a fair amount of overlap will not read AFR correctly at idle. This is a well known widely published fact. Usually high HP engines have large overlap cams.
A wideband reading at idle is useless on these engines. Big cams need more fuel at idle because so much air/fuel is pumped out of the exhaust and EGRing has a large affect. Tune the idle to the highest vacuum. Old school and always works properly.

Hesitation from idle to cruise or WOT is usually a pump shot problem. Add more accell. cam or accell. pump. More fuel at idle will help cover the problem but is a waste of fuel if it is already tuned properly.

This problem occurs a lot with carbs. Gotta love EFI.

383backinblack

thats the issue though, is that the accerlator pump circuit isn't big enough to cover the lean spot when you go from part throttle, to WOT with boost instantly.....the idle circuit needs to be richer than normal to compensate.

what you say about cams is true, but blower grind cams have VERY little overlap.....even big ones.....my cam is 262/272 at .050......but its on a 114lsa......very big, but not much overlap.

junkcltr

iTrader: (1)

You have (262+272)/2 - 2*114 = 267 - 228 = 39 degrees of overlap. That is a LARGE overlap and the WBO2 will not read correctly at idle. Yes, a typical boost cam has a small to mid overlap. Yours is on the large side. Once you go to a large duration like you have then the LSA has a smaller affect on overlap.

383backinblack

its alot smaller overlap than you would have on 110 or 112, its very little overlap compared to the same grind for an n/a application, this grind is specifically for high boost applications.

If you use a 5 gas analyzer you get a much more accurate picture....most good shops have one

Batass

I had the same problem on my car with a holley and a vortech. It was getting slight boost at idle and screwing everything up. Real bad hesitation. And choppy at part throttle. It was running rich as hell at idle, so I stuck small peices of wire in the Idle fuel restrictions,and then it would hesitate bad off idle. I finally ended up drilling holes in the butterflies to get more air in, and I got it close. No more hesitation, but it still wasnt right. Would run pretty lean cruising sometimes, and it was still getting some boost at cruise. Kinda dangerous.
If I were you, I'd dish out the money for a shop-built blow-through carb. I picked up a Mighty Demon, we'll see how it works. Havent put the boost to it yet. It was 450 bucks vs 750 from reputable shops.

383backinblack

my carb is a holley 950HP blow through built by the carb shop.......it was about $1,000 but its worth every penny.....it was SPOT on out of the box, perfectly flat fuel curve.....just needed some minor tweaking.......all o-ringed and sealed throttle shafts, etc

happydad

iTrader: (1)

O.K. Update.......Changed to 31 squirter in primary..72 jets in primary..Timing @ 14 btdc..cant get A/F below 20 no matter what I do with the metering screws...Should I go up in jets??? Retard Timing ??? I idles pretty well no bog when throttle is cracked...What do ya think this noob should do.I know I'm close

Batass

Im sure its awesome man. I just didnt think I'd need something that fancy for under 12 lbs. Now I'd like to run more, but cr is 9:1 and stock block. Probably not a good idea.

Batass

20?! Main jets have nothing to do with your idle afr. At idle you use only the idle feed circuits, at slight throttle your using transitions circuits and then main jets. Wide open throttle work power valves.

So your reading over 20 afr but it runs well? What kind of sensor and gauge are you using? Where in the pipe is it?

If you want optimum timing at idle then you should use a vacuum gauge. If you have to lock your total timing then you just deal with what you get at idle.

What cam do you have? A longer duration cam will require more base timing, or you will have a hesitation. If anything advance your timing. Easiest thing to do would be to lock it out at say 28 degrees and start from there.

Im not sure what the holley B/T is....is it built for blow-through? You really need all the adjustments a true blow through carb has to offer to get it to run as right as possible.

Heres an important question......did you use this carb on the engine before you put boost to it? Im wondering how far you have your idle screw in. Sometimes you have to turn it in pretty far to bring up the idle and you get out of the idle feed circuits (although they still slightly used) and into the transition circuits. But I do believe that makes you terribly rich.

If all is well and nothing is wrong, then perhaps you truly need larger idle feed restrictions (ifr's). If your carb isnt set up with adjustable ones that is. Which I havent seen you mention I dont believe. Theres instructions online on how to remove/drill/pull your hair out.

happydad

iTrader: (1)

Thanks... come to think of it the Innovate seemed weird towards the end..It just sat at 20 and didnt move...so instead of smashing the laptop on the floor I shut it down and had a cold one....I wonder if it got stupid or something..I was stopping and starting alot....I'm new to this A/F tuning stuff..Learning the Logworks software as I go....But when I first plugged it the laptop it was around 12.5-13 but bounceing all over then I changed the jets..That's when it pegged @ 20 and did'nt move...Hummm gonna try tomorrow am...Yeah it's a Blow Thru Holley D/P 750...Now the Idle screw is turned in for it to idle @ 900 in park..Should I turn it up higher(the idle speed) then turn the idle feed screws ?? I started with them turned about 1 1/2 turns out..Lord knows where they are now...Thanks for all the help...Will post results tomorrow

BDR

I didnt read the whole thread.. I'm running a LM1 also. If you went up in jets and before the change it was at 12.x-13 now its at 20 and not moving.. have you done a 'free air calibration'? Mine was reading like that until I did the same thing.

I have a CSU 850HP carb.. 50cc pump in the front, 30 in the rear.

happydad

iTrader: (1)

Yes the LM1 is pegging at either 7.8 or 20...When I get i to read it jumps all over...wont hold steady..Is there a secret ??? for now I just set my timing @ 1000 curb idle @15* and turned the idle screws in or out so it would run...Runs hot though you can feel the heat off of the headers...Took it out for a quick test spin ,I know it's not right not alot of power...Funny thing also a vacuum gauge shows 15 on manifold vac...Gotta figure this LM1 out 1st

BDR

make sure you have a good ground. I have my LM1 hooked directly to my battery (hot and ground)

383backinblack

hotter than normal exhaust is either a lean condition, or the timing is too far retarded

Batass

Your idle afr really isnt all that important, I set my idle with a vacuum gauge. Its more accurate I think. I set the idle speed, then adjust each idle feed screw together to achieve the highest vacuum, then readjust the idle speed.

15 degrees should be ok. I've had header heat problems with 10-12 before and ended up taking it to 18.

Is Logworks the accessory for the LM-1 that records your rpm and afr? I dont have that, I plan on doing the final tuning on a dyno, but that should be very helpful.

Another thing is they say those wideband 02 sensor are only good for 50 hrs with power on them. If your idle is lopey that will send the gauge all over the place too.

Also are you running a full exhaust? If you just have headers, you can suck up ambient air will will screw with the sensor.

happydad

iTrader: (1)

Thanks..Yes a full 2.5"exhaust..Hi flow cat/Magna Flows.. I'll adjust the carb with an vacuum gage..The engine likes 15-18* at idle..Full advance out of distributer is 20 so total timing will be 35*..That should be O.K...Pisses me off about the jumpy A/F can't set it or have a real clue whats up...I know the carb isnt right ..I didnt have the power it should although I went real easy...Where is the best place to hook up a Vacuum gauge..I don't have any ports off the intake..I'm useing the port on the base of the carb for my MSD and boost gauge..The upper port is for the distributor..And the brake booster is the back one...You can get the Log Works software 2.01 Free off of Innovate's site..Do you have the cables to hook to a laptop?? Thanks for all the help

383backinblack

your idle circuit has a large impact on your part throttle cruise fuel curve.........because there isnt enough of the transfer slot exposed to pull all the fuel from the main circuit yet, with the butterflies almost completely closed.....on a holley carb that is

also, if you find the part number (i cant remember where i put it) that sensor is a VW part made by bosch, you can get it really cheap from most part's places if you the number (I think there's a standard ignition part no. that crosses to it as well)

Batass

Yea, youre right about the part throttle. But your not guaranteed the best performance with a 13.1 afr for idle. You dont want to be too far from that but anything in the range of 11.5-15 would be fine if the motor liked it there.
Since it does affect part throttle, you may have to sacrifice idle quality for better part throttle performance.
Yea, I remember seeing that it was a bosch sensor.

happydad, you have to hook up the vacuum gauge below the carb. What are you using vacuum for msd for? Boost timing control? You could just pull that off and use it.
No, I dont have the cables, but don't you also need another box to read the rpms? I'll have to check into it.

BDR

If you're at 20 a/r you would be glowing red. Before you go around changing this and that CHECK TO MAKE SURE YOUR LM1 IS PROPERLY CONNECTED. Make sure your ground is good also. It WILL CAUSE ERRATIC READINGS (a bad ground).