Heavy throttle stumble.

jdavis · Post by **jdavis** » Sun Mar 09, 2008 5:30 pm

I have an engine that i built over the winter for my 82 gmc 1 ton.

Engine is a 454 .030" over
Approx 9.0 compression
212/218 110 lsa comp cam
peanut port heads with slight rework
Performer 2-0 intake
3310-6 holley 750 vac sec carb
Initial timing 12* initial 37* total all in by 2800
Vacuum advance is hooked up locked to 12* max
14" air-cleaner 5" tall element spun aluminum air cleaner.

Engine is extemely mild, just really getting it all set up it is built to tow heavy loads and get me around during winter. Makes high vacuum at idle 17 at 750rpm. Idles very cleanly and will rev very well under light/medium throttle. Will rev and act fine under hard enough throttle to hit 5" of vac, but when you hit the throttle hard enough to get zero vac it has a lean stumble and backfires through the carb and tries to die. Will rev perfectly fine if you just roll into the throttle as well to as high of an rpm as you wish. I just want to hear from some of the experts here about how they would go about curing this. I thought perhaps i would step up the squirter a few sizes or try a different pump cam, but i wanted opinions first. Carb is entirely as delivered from factory, floats set to just barely dribble from sight holes if you bump the tire with your leg.

All opinions/ ideas appreciated.

MadBill · Post by **MadBill** » Sun Mar 09, 2008 6:19 pm

First thing I'd do is go over the ignition system very thoroughly: plugs, wires, coil, cap & rotor.
You're not by chance using an HEI distributor are you? I've seen several cars where these have been retrofited and wired in through the stock resistance lead or with a 16 GA or smaller wire, which cuts the available plug voltage way down and produces exactly these symptoms. (The highest voltage demand is upon a sudden WOT, steady WOT is somewhat lower)
A quick test would be to knock the plug gaps down to 0.025"; if the problem diminishes or goes away, you know where to look...

A possible actual carb problem

would be a mis-assembled vacuum diaphragm system, missing check ball or missing/incorrect spring, allowing the secondaries to flop open way prematurely, producing a momentary lean spike.

jdavis · Post by **jdavis** » Sun Mar 09, 2008 6:27 pm

It has an hei, which was apparently original equipment on this truck. The dist is a brand new msd unit, new msd wires, msd coil. The hei 12 volt source is a 12 gauge wire rather than a 16 gauge piece of floss. Plugs are gapped at .035" and look good upon inspection. This lean pop occurs under relatively fast throttle under no load, so i'm fairly certain here that it is a fuel related issue, as it is accompanied by a pop and flames blowing out of the carb. But I will give the ignition a once over in case i was having one of those......... moments when i was installing things.

Being as this is a no load situation and i have verified the secondaries are not coming into play, I highly doubt it is them buggering the signal.

MadBill · Post by **MadBill** » Sun Mar 09, 2008 6:34 pm

[quote="jdavis"]It has an hei, which was apparently original equipment on this truck...quote]

A little brain fade there

; yes, from ~ '75 up to late eighties HEI was standard.

ed nix · Post by **ed nix** » Sun Mar 09, 2008 7:17 pm

I would try more intial timing. The carb probaly needs a cam that has a
longer pump shot you might try two or three sizes larger squirter also.
ED

Ron Golden · Post by **Ron Golden** » Sun Mar 09, 2008 7:45 pm

Just a thought: Disconnect the vacuum line to the vacuum advance and see if the problem goes away. If it goes away the vac. adv. may not be retarding the timing fast enough.

Ron

jdavis · Post by **jdavis** » Sun Mar 09, 2008 7:51 pm

Ron Golden wrote:Just a thought: Disconnect the vacuum line to the vacuum advance and see if the problem goes away. If it goes away the vac. adv. may not be retarding the timing fast enough.

Ron

I shoulda put that in the original post. I've already tried that with no effect, as well as advancing and retarding timing approx 5 degrees from the 12 where its at now. I know sometimes i've been able to advance timing and eliminate stumbles altogether by doing that. Sorry, shoulda already said all that.

lasermike · Post by **lasermike** » Sun Mar 09, 2008 7:59 pm

is it a new carb? or just new on the truck? how did the old carb run? if there was one?

RayJE Carburetors · Post by **RayJE Carburetors** » Sun Mar 09, 2008 8:12 pm

go up in squirter size to 35 ...... and a 8.5 power valve .... and lift the secondary float up a couple of flats

lasermike · Post by **lasermike** » Sun Mar 09, 2008 8:17 pm

>as well as advancing and retarding timing approx 5 degrees from the 12 where its at now. ....... was there any change?

have you run it under a load? what happened?

if you hold the rpm's at 1500, and then go "wide open" for a second, what happens?

jdavis · Post by **jdavis** » Mon Mar 10, 2008 12:24 am

Lasermike: cant try the truck under load has no doors fenders or hood at the moment, bought all new panels and have been slow getting things in shape for paint. But no there was no change with the timing change with no load.

I will try your 1500 rpm test and report back when i get a chance. Neighbors wont like it if i do it right now.

Carb was previously on a 350 in my 3/4 ton 85 chevy, it ran fine the whole time it was on there. I removed it when i went to efi on it just before i started working on this truck.

RayJE: I have equipped the carb with a 7.5 power valve after swapping it over from the other truck, with the idea that this engine has quite high vacuum and in order to get enrichment soon enough it should be a little higher than stock. Is it still worth it for me to go up to a 8.5 power valve from here?

I understand the effect of the 35 squirters, should i also perhaps consider a pump cam change? i think it has a brown one now? I'm a little confused about the secondary float adjustment though, not argueing effect but i always like to know why i do something

so i know when to do it.

Appreciate the help fellas, keep it comin.

Ron Golden · Post by **Ron Golden** » Mon Mar 10, 2008 12:33 am

I think I'd change the pump cam to a white one. The best I remember the brown cam is usually used with the 50cc pump. But I can't always trust my memory.

When you adjust the secondary fuel level, turn the float adjustment nut an additional 1/3 turn richer. That way the fuel level will be higher and the "tip in" (when the venturi's start flowing) will be sooner and richen the mixture.

By the way, is there any slack in the accel. pump linkage? there shouldn't be any play.

Ron

jdavis · Post by **jdavis** » Mon Mar 10, 2008 12:48 am

Ron: I checked just now, aparently my memory is faulty. The pump cam is orange. I was thinking of maybe changing to a pink one.

There is zero slack in the accel pump linkage. Squirters begin to spray immediately even a slight tap of the throttle will send fuel out of them.

I'm still fuzzy on the secondary float level, i mean i understand the effect but to my understanding if you're in nuetral the secondaries should not begin to comin as there is not enough airflow demand to activate them. Perhaps thats where my thinking is faulty.

lasermike · Post by **lasermike** » Mon Mar 10, 2008 3:29 am

jdavis wrote: cant try the truck under load....

with all this help you may get it to run good in neutral.....

but the real tuning starts when it's in gear......