Rich running chevy

[Steve]

Hi. I'm looking for a little help on a bowtie problem I have - I hope you guys can help.

I have a 383 chevy, crane 284, Edelbrock RPM Setup, running 10:1
I'm using a 650 holley vac sec. mallory dual point vac adv. timing 10deg. I had the same carb/inlet/heads/distributor running on my 350 @ 9.8:1 until it broke.

my problem is that I can't get the engine to run well at part throttle openings, it runs rich, sputtering until I get to 3000rpm, then eventually clears with acceleration, I can see a black deposit in the exhaust pipes, and around the inside of the carb, and whilst stationary, if I hold the throttle open at 2000 rpm I can feel lots of hesitation & stumbling which can also be felt whilst on the move

If pressing on hard, it runs well enough, until I drop revs.
I get a lot of popping in the sidepipes with any decelaration,

It seems to do this at most RPM, with gentle throttle, but runs better with more throttle, it sems not to want to run cleanly at anything other than full throttle

I've changed the power valve,(it was fine when checking the old one) jets (dropped from 64 to 62 made no difference), plugs, timing but to no avail.

It has been sugggested that I look at the exhaust, which has a 1 3/4 primaries going into a 4 inch collector, then into a 2 inch side pipe. the suggestion being that I might be getting back pressure at low rpm which is causing the block to run rich . This is a standard Dax setup sidepipe for the SBC but I doubt thats at fault - unless any of you know differently. there are so many chevy/dax's out there I thought if this were where the problem llie then I would have seen something here on the site before now

On occasion I also get high RPM at tickover perhaps 2000 if I get off the gas. this sometimes takes 10 - 20 seconds to get down again. It behaves a bit like it has a stuck throttle but I checked, and the throttle is not part open.

I am gonna try swapping out the carb for another unit 750DP to attempt to see if that makes a big difference in symptoms.

I would be grateful for any help - I feel a little stuck

Steve

[wilf]

Steve -I claim to be no expert on either Chevvies or Holleys, but it sounds like either you have an induction side air leak, or your power valve rating is off. try measuring the plenum vacuum level at tickover/light throttle, and then asking someone who might know better than I what the power valve rating should be. I think it should be somewhere around half your tickover vacuum level, but please check this with an expert. (you could give Paul Hinckley a ring at PD Sportscars, 01283 51505.
Also might be worth giving that carb a real good blow out with compressed air, and rechecking the float settings.

If your vacuum reading is way low (below about 10"wg I think) then you may have other problems,although I don't think that cam is so radical as to cause that. Have you set the valve lash right?? Could be it's too tight and holding off the valves - a compression check would maybe tell - each cylinder should be within 10lbs of the rest.

HTH

Wilf

PS try disconnecting the distributor vacuum advance as well.

[RigMan]

Strange one Steve. If you are back firing then it would appear that you are having incomplete combustion (raw fuel being exhausted and igniting on the hot manifold) and if you are having exhaust deposits on the carburetor that combustion is taking place whilst the intake valves are still open.(Arghhhhhhh!). This must mean that your timing is out. When you test your timing this is usually done with the Vac hose disconnected if I remember. What would cause your miss timing when running could therefore be poor vacume. Check that you have the right hose to the right port on the intake manifold/carb, and that all ports/gaskets/hoses are sealed and that there are no leaks. Get a vacume guage if you are not sure.

Let us know how you get on!


RigMan.

[wilf]

Steve - just seen a thing about power valve ratings at www.mortec.com

courtesy of Dave.

Wilf

PS - any cure yet??

[Steve]

Gentlemen

thanks for your replies

In short I have not yet solved my problem

I have to date:-
re checked timing, float levels
stripped & cleaned carb
replaced power valve, jets, plugs & leads, rotor & cap.

I had to remove my headers to replace a blown gasket (blew subsequent to problem with running), and whilst there noticed that my no 8 plug cap was burnt where it is near the header, & possibly arcing, although by that time I had the headers off & couldn't check. The plug top is within 9 mm of the header pipe.

I think I'll have to re shape the dax stainless a bit to give more clearence, or find boots which don't mind touching the header

I've also arranged to borrow a different carb - which may help me to decide if this is a fuel issue or a ignition issue

Wilf - thanks for te mortec information. I have printed this off & will study tonight.

I'll let you know when I finally resolve this, in the meantime, any further comments welcome!!

Steve

[wilf]

Hmmm, bummer. have you tried removing the vac advance yet? Also I assume this problem is off idle - i.e. confined to light throttle on primaries only? If so, is your primary jetting OK? Don't unfortunatley know enough about your engine or your carb to be able to say for sure.
If the problem is at tickover as well, then its got to be float level??? Tried dropping it? Assuming nothing horrible is happening to the valve clearance adjustment?

Persevere - you will get there in the end.


Wilf

[Steve]

Idle seems OK, stumble is on gentle throttle on primaries. jets at 63 currently - seems correct for block.

removed vac advance, no effect on stumbling.

I have just ordered a "better" vac gauge - the one I have is a little damped and slow to respond, perhaps I'm missing something there so I'll try the new one.

I hope today to pick up a carb from a engine builder to try - see what happens there.

[wilf]

Hmm, I think you might have to do more basic checks to pin down the cause of this - things like compression tests and leakdown tests (to ensure bottom end and valves are sealing properly), and also get a real handle on your vacuum readings with a good gauge. If the top end set up was Ok on your previous engine, then something simple is probably amiss. are you 100% sure that the spark is OK? Nothing funny going on with any of the HT leads? If you can get it to a garage suitably equipped, their "Sun" type tester mught be able to track the fault down.
Does it run clean at higher revs/wider throttle openings? If so I would probably look to the carb some more. Let us know how you get on. As a last, but stupid, question, you sure the fuel pressure is Ok and no filters blocked?

Wilf
PS - just had another thought - are you 100% sure the cam and ignition timing is OK?

[mikey]

I agree with all of the above. Also, one other possibility is warped metering block mating face or metering block itself (can happen if screws are overtightened). If this occurs a complete seal is not achieved and additional fuel runs past the gasket leading to rich running. Also on over run, fuel can be drawn due to the increased vacuum signal leading to detonation in manifold. On high or full throttle problem virtually or altogether goes away as the fuel is in demand. The way to check is with a straight edge over the mating face and block, although it is more likely to be the face rather than the latter as the block is more rhobust. Not saying that this is it but its worth a check for free. I would have also said check gasket but if you changed your jets you will have automatically replaced these.
Best of luck, its a real bummer when you get a problem like this.Let us know how you get on.
Mike.