Light throttle stumble, Holley carb.

hell no, it starts to spin the tires around 1900 rpm. it's on the upgrade list, that or a manual swap.
it runs pretty well on the 670 and is probably in the mid 12's if i could launch it hard judging how it's done against some other cars.

 

I've not had any hands-on experience with this exact carb but this is the kind of thing I'd try first before spending any more money. Especially since you say that it runs fairly well under most other conditions. It just sounded like maybe the accelerator pump shot was coming in skosh too late.

Try adjusting it so there's no clearance betwwen the pump arm and the plunger at idle, or maybe so it's just starting to depress the plunger a bit, and see if that changes the off-idle response. If it's better, then back the adjustment off a little at a time till the hesitation comes back and split the difference.

 

I read on the HAMB, that 90% of all carb problems are electrical.

I had a similar symptom of part throttle stumble that I just finely, after screwing around for 2 years with it, got it solved. I had the softest springs in the mech advance, thinking get the timing in fast. Well, retarding the initial timing helped the stumble a lot, but then the idle was rough. So, I think slowing the advance down a little with the next heavier springs might help. And the putting the timing back to where the idle smooths out. This did it for me.

Easy easy thing to try. I won't bill you anything now, only if it fixes the problem.

Frank

 

i'll have to try that. however, it doesn't happen at all taking off from a light, just when i'm rolling with steady, light pressure on the pedal. could this still be the pump? on a side note, it will barely spin the tires if i mash it from a light, but it does pull hard above 3500rpm.

 

i believe i'm running 1 blue and 1 silver, and run a low vacuum advance can. retarding the timing makes the stumble more prevalent. both the spring change and the advance can change had positive seat of the pants changes. the can in particular has made the in-gear idle much smoother (doesn't labor like it used to a bit) and it feels stronger on a low rpm pull in 3rd gear.

 

Then maybe with your combination, more timing in the mid range might be the answer. Try the two lightest tension springs.

sounds like you are real close to everything right, it's just gonna be one simple thing to solve this, I think.

 

i've had a lot of those moments on this car, hahahahaha

 

I must have missed that reading thru your posts. I was thinking more along the lines of an off idle stumble or flat spot. Didn't mean to get you off track, but it would still be easy enough to experiment with some of the accel. pump adjustments.

 

I agree with the guys who think the carb is too small. A 750 dp would be better, and if it was mine, I would probably look at an annular 850, but i think i am more comfortable with major carb tuning than a lot of guys. To fix the stumble with the one you have, it sounds like its lean transitioning onto the mains. The ideal fix would be a slight reduction in the size of the high speed air bleed. But this carb doesnt have changable airbleeds, and drilling and tapping the bleeds is probably alot more work than you are interested in. Changing the power valve down one number would also probably do it, but hurt the gas mileage more than a change in air bleed.

PS; I would also be ditching those 3.55s in short order if it was mine. You should probably keep that in mind bfore you rush out and buy an annular 850.

 

We are on the same track here, but I am looking more at trying to get the mains to initiate earlier, rather than fattening up the rest of the bottom of the fuel curve, which is going to hurt mileage more. But we are both on the same track here.

 

I hear ya George. ClayMart's last post with a quote puts me in the same boat with ya. And to add, more gear or converter for that cam. Or smaller cam if ya want low end. It just ain't gonna have much below 3-3500.

 

Yea, if it was me, I'd be looking at 4.56's, a 10" converter and some M/T DOT's. Gotta let that dog eat!

 

I ended up with the orange one.

 

http://www.holley.com/TechService/Library.asp

A good read, also I have had great luck with the white cams.

 

i had an off idle stumble, but fixed it with some tuning. of course i forgot now what it was i did to get rid of it.......

 

Yep, if it came down to all that, I'd just step the carb up. As much as I'd LOVE to run 4.11's, it's a street car. I had a built 55 4.11 rear with good axles and diff, but like a dumbass I sold it when I should have installed it and then either put an OD trans or a manual in it. Or maybe I'm just being a pussy about it.

 

thanks!!

 

good info there, thanks!!

 

Check this out, http://www.aedperformance.com/Tuning Tips.htm. Given your circumstances, tuning it will get it a lot better. But ya gotta understand, you have a mismatched combo. You aren't gonna have as much low end as ya want. In my opinion, the cam is too big for what ya want it to do. With the converter/gears and carb ya got, you will be faster with a different cam.

 

No, no, no! Thats entirely the wrong philosophy! The cam is just right. In fact, its a little small. The gears are too tall, the carb is too small, and the converter is too big. Trust me on this.

 

^^ kidding aside, heres another thought. You say at 16 degrees initial the stumble wasnt there, right? You are running underhood air, correct? How about running a cold air package? That will reduce the intake temp. enough to let you crank the initial back up to where it should be, plus you will pick up a sustantial amount of power. Also you said perf. RPM not air gap. If its a regular perf. rpm, have you blocked the heat risers? Ihat by itself might buy you enough octane tolerance to crank some more initial in. 12 degrees is not much initial for a 240ish hyd. on 110 lda.

sorry, just re-read that, you said it was less, not gone.

 

heh, i'd rather swap the gears/convertor/cam and go faster.

 

^^atta boy!

 

i'm not sure of the intake temps, but the temp gauge doesn't get above 160 after i put an aluminum radiator and a shroud in it. i have been thinking about getting the spectre dual ram (thunderbolt style setup) filter/intake kit for it.

 

Well things are getting clearer on what you expect from it. I suppose you are now not worrying about this statement, "I went with the 670 to keep the throttle response crisp, and I shift it at 6000rpm due to the production bottom end pieces (did use ARP bolts in the bottom end, though)" which lead me towards more of a street combo.
I will agree, not enough initial for that cam. Check out the AED link.

 

Air temp at the carb, even at less than 160 degrees will have a BIG effect on the engines octane tolerance. Been there, done that. If it was pinging a little before,I will bet that if you put the spectre kit on, you will be able to go back to 16 deg. initial with no detonation. I would install the cold air set-up, advance the timing, THEN go in and try to tune out any remaining tip-in stumble. You could even just try raising the float a hair. A little more "head" on the other side of the jets, combined with the increased initial, could be enough to get rid of it.

 

It's a street car? Stay away from double pumpers. Go with a vac sec 750/780, you'll be much happier.

To tune the existing carb, have a friend follow you at cruise speed (30-40mph). Punch it and have the buddy report if it blows any smoke from the exhaust. If it blows black smoke during the stumble then cleans out and goes, the squirter is too big. If it stumbles but doesn't blow any black smoke, the squirter is too small. Go 2 steps at a time.

Make sure the accelerator pump is adjusted correctly. Wide open throttle (please, not running at the time) you should have .015 clearance between the pump arm and the pump lever.

For the power valve, you need your buddy again. Have the vac gauge hooked up, start the car, drop it in gear with the brakes on and see what your gauge reads when the car idles against the converter. Drop 1 point down to the next valve. For example, you read 5.7 vac at idle in gear, use a 4.5 valve.

Start there anyway.

SPark

 

You take the reading of your vacuum gauge and divide by 2. If you have 9 inches you would use a 4.5 or a #45 P/V. If you have 8 inches, there is no #4 so you would go down to next lowest and use the 3.5 or #35. 5.7 divided by 2 equals 2.85, so you would use the 2.5 or #25.

 

I would go with the 750 with vacuum secondaries, and I bet it will bring the engine to life. You should be able to easily spint he tires at launch, even with 3.55 gear ratio.
If you want to try and make the existing carb work, an fool with the carb, then swap out the accelerator pump to a 50cc pump, and the black cam. That should provide enough fuel to trick the engine into thinking it's a bigger carb.

 

Seems as though others should check the AED link too.