kalli

Hi all,

maybe you can help out.
car's idle is choppy, and there doesn't seem to be any rythm whatsoever in it.
I wasn't too much concerned about it, but when measuring vacuum the needle jumps very quickly between 8 and 14 on the gauge. It's nearly like a vacuum leak, but I can't find any

that normal with that cam? 31-242-3 (comp cams xe268h). once i floor it the car is moving and it's not overheating in idle either. but it sounds like it could cut out anytime

I have a demon and that has the 4corner idle setup in middle of carb (under aircleaner stud). and I have mixture screws on all 4 corners.
it seems best with them just under 1 turn out. (instead of 1 and a half each).

i'm loosing the will to live. I don't know anymore what is right and what is wrong ...

kalli

read a few docs. will start from scratch resetting the butterflies. Will probably do that in lunch break again, quickly pull the carb

Stepman

I know you don't want to her this but, a vacuum reading that has sudden drops like that, usually indicates valve issues. Is it a steady, rythmic drop, or is it just an irratic drop?

scootchu

This page is out of an old 1973 or so manual. It may help.
I know what you are saying though, the more you think you are improving, the worse things seem to get. It always works out. Sometimes you just have to step back and take a break.

MBDiagMan

Let not your heart be troubled! You will work through it.

Since I have had my head in this problem several times throughout the day I am going to refrain from commenting at this point, so that you can get some fresh ideas without my influence. I have some thoughts, but I have touched on them in the other thread.

It is often helpful to walk away from a problem for awhile in order to completely get your mind off of it, then come back and start over.

In order to help and clarify, it sounds like the needle is sweeping through the 8 to 14 in.Hg. range and not staying still at any one point, correct?

Hang in there!

kalli

It's gotten a bit better. I re-checked on intake. no vac leak there. not at carb. i pulled the carb, set the butterflies, pump cams, smacked it back on. used 1 full turn from closed for all 4 idle screws. seemed to be best, set the idle with the 4corneridle system. lowered the floats a bit. just under middle in sight glass
idle is very high, but seems to be better than lowering. i guess i'm running at around 900 now (haven't measured).
the idle is now between 13 and 15 still jumping, but it seems more normal ...

Ignore the exhaust leak ;-)


I have measured A/F and it's impossible to tell at idle, as it jumps around a lot. during cruise it's mid 14s and flooring it is around 13. it stumbles a bit at high gear very low speeds (like 30 in 4th), so I'll probably go one jet size back up from here

http://www.youtube.com/watch?v=Vkz-B4eZ57I

MBDiagMan

I'm not familiar with speed demon. Is that a 650 Holley Double Pumper? With that cam 13 inches with the minor fluctuation you're seeing doesn't seem all that bad.

If this IS a Holley, what power valve is in it? I don't ask this in relation to the idle, but rather because of the stumble that you mentioned. If the power valve is too low for the carb, then a higher value power valve might be better to try before fattening the main jets.

Double pumpers are a real challenge. Unless I were setting up an all out road race car, I wouldn't personally use one. A vacuum secondary has always worked well for me on street engines.

kalli

it's not too bad of an osscillation, but as soon as I drop the idle to lets say 750 the needle is going babanas. the PV is stock 6.5, i put an 8.5 in there recently. the jets are all 3 down from stock if I remember correctly (front and back).
i have a 570 street avenger in the trunk that I rebuilt only 3 weeks ago, so I'll plant that on later today just to see if there's a difference

MBDiagMan

Due to the time difference the morning is almost gone for you, so I was anxious to see if you've made headway.

Since you're down four sizes on the main jets, I would expect that to be related to your stumbling. This ASSUMES that there is no problem in the carb such as a leaking metering block gasket, blown power valve, dribbling accerator pump or some other issue.

Your bonkers vac gauge at lower idle speed with that cam puts me in "I'm not sure" area. Really wild camshafts can require some strange steps sometimes, but this camshaft is not that wild. That said, it could be providing a mysterious variable.

I think that setting the other carb in place and seeing what it will do is a good troubleshooting step. Even if the other carb induces different problems, then you should be able to assume that the problem is carb related. When you're looking for alligators you drain the swamp.

I'm an old dog and the names of these carbs like Demon and Street Avenger don't mean anything. I think in terms of List numbers, CFM and features such as vac secondary and center hung float bowls. I assume the Street Avenger to be a vac secondary carb. If so IMHO it could be a better carb for your application. 570 would feed that engine well unless you plan on spinning it to 6,500 regularly.

I'll be out in the shop shortly and then off to town for groceries and errands, so I'll check in tonight, probably after you're gone to bed.

Good luck!

TommyK

http://www.4secondsflat.com/Demon_Tuning_Guide.html

Might want to invest in this tuning guide.