Well... I bought the zz502 because I wanted a stupid solid engine that would run without problems until it´s worn after many years.

I put my Demon Sixshooter on it that I already had. It´s basicly all Holley stuff.
And I keept the original TI-system which should perform like a modern HEI-system. It´s set up to 36° with additional ~10° vacuum.
Here´s a pic for better understanding:



Had around one year after a lot of tuning and it run very good.

Then it started with slightly vibrations when cruising in high gear close to idle load. It feels like little missfires. It gets instantly away when you slightly accelerate.
After checking anything without results I changed timing in small and big steps, with and without vacuum without any result.
What helped a bit was richen the idle mixture and transistion circuit.
But I won´t go richer anymore, now I have black plugs, that´s not so good either and the problem is still there.

Well I am out of ideas. Haven´t checked the valve adjustmend, but don´t expect to find something (hydro lifters).

Choke is working correctly.

Im out of ideas now. Could it be something is wrong with my old TI-system? Yesterday I had also missfires under hard acceleration, but thinking the carbon black plugs could be the reason for this too. (New plugs are ordered already)
Now I think a MSD ignition box could help with it´s stronger sparks.

 

Mine does something similar. Seems to surge when going down the road close to idle. Need to either accelerate or downshift to get the rpm up. I had thought that it had something to do with the vacuum advance, but disconnecting it made no difference.

 

Hi Markus,

could you re-activate the AFR gauge you had installed at some point?
My bet is on too lean AFR at extreme low load.
My thought, without having detailed knowledge about your six-shooter circuits: In your attempt to richen it up you might have ended up being too rich at higher loads, thus the black plugs, without having solved the issue in your trouble-causing drive condition.

With all the variation you've already done in the ignition system I think you can put that out of the equation and any mechanical issue seems even more unlikely to me...

Cheers
Carsten

 

^Yep, you have a A/F ratio issue. Sounds like maybe your carb is malfunctioning. Probably some trash made it's way past the filter and now is causing havoc somewhere in a carb passage. I'd pull the carb off, disassemble it, and completely clean it with carb cleaner. IMO

Beautiful engine compartment!

 

Well in this low load area you have only the idle mixture screws and the transistion circuit to influence the mixture.
The transistion circuit is at factory settings now, I drove it a bit leaner all the time.
Yes I will measure the AFR if I can´t find out whats wrong. The curios thing is that turning out the screws and richen the transistion circuit did not make such a big difference.
From my experience, when it´s running too lean you get missfire when you put more load on the engine. (You can see droping the AFR very good with an AFR-gauge)
But in this case a tiny increase of the pedal load takes the "bad vibrations" away.

But who knows what the engine and carbs are doing under vakuum togehter. Defective powervalve could also be a reason. But it´s just one year old and was tested before I put it in.

 

Hmm, well try new plugs first. I mine eats plugs about every 4000 miles or so.

 

What do you have the plug gap set at?

This is somewhat common with BBC's, the plug gap is best at .045" with anything short of a very high output ignition.

Could be something in your ignition getting a little weak.

Not saying this is it but it's something to check.

Neal

 

You say it ran good for some time before the problem started, so I'd look for a small vacuum leak that only affects one or two cylinders. It could even be a crack in the intake manifold, although unlikely.

A small vacuum leak on one cylinder will make that cylinder misfire at low load. As load increases the leak gets smaller, and the air/fuel ratio goes back to something that ignites and the miss goes away.

 

MY 69 had the TI system on it from the factory- the PO had bypassed the TI and wired in a mopar ignition amp. I dumped that and went to a Summit branded Mallory CD with a Rev limiter. Ran that for a while and then got curious to see if the TI still worked. Wired the TI back in and got a huge surprise. Runs way better. Even starts easier. Win-Win with the GM TI. .035 plug gap too.

 

Disassembled the carb today, did not find any particles or dirt inside.
Powervalve found also good.
Found only 2 minor other things:
I had rebuild the carbs with new gaskets 1 year ago, found the screws between throttle body and ventury only medium tight because the gaskets have seated. Vakuum leak here could be possible but unlikely, because there is no vacuum above the throttle body.

Then I found that the primary throttle blades were not set up perfectly. The should be a square visible from the slots of the transistion circuit. Actually it was rectangular, the ported vacuum hole was already active too. Well I doubt this is the reason for my problem, but will fix it anyway.
I guess I have to open the sencondaries until the primary throttle operates in the right idle range, right? My carb does not have a bypass screw for air like other carbs have. And I don´t want to drill holes into the blades, currently there are no holes in them.

 

yes if you don't want to start drilling adjust the secondaries as long as it's not a large amount.

Neal

 

Have you thought of a vacuum advance kit?

 

I was wrong about that. The Choke was active
So one failure possibility less.

 

That is exactly what happens when to much of the transition slot is exposed. And how I would set the transition with out taking the carb off. I would crack open the rear barrels slightly and close down the front 2 with the idle screw as soon as I could make engine want to stall with the a/f screw I knew I was not drawing from the transition slots for my idle .

 

Well now we know that my idle throttle settings are correct. (And you are right, when the transistion slot is too much exposed turning the idle screws will have no big influence)
But what I did was changing the air bleeds to influence the transistion AFR. The factory settings let the engine cruise with 1:13 and I leaned it to ~ 1:15 when cruising.
Now it should be back at 1:13 but have not measured it yet.

Anyways, richen the idle plus the transistion did only make a small difference.

 

I'm interested in that Demon 6 shooter. I'd like to go with a 3X2 also.
I have a 67 rectangular port 3X2 manifold that I'd use. The 67 rectangular port is a little taller; i.e. flows better than the 69 rectangular 3X2.

 

just FYI: The intake that usually comes with the sixshooter has the big oval ports (like the zz502 crate has), and the intake is taller than the stock Tripower intake, it won´t fit under a normal BB hood. But L88 hood works with 1" space left.

 

If those are repro wires from LectricLimited, try swapping wires and see if the misfires diminish.

I've been thru 2 new sets of those wires on our new 396 and had little odd misfires here and there.... A $15 set of Duralast wires from Autozone cleaned it up a bunch...

Something to check.

 

The wires are from corvetteamerica. Don´t know the real manufacturer, but they were expensive. Well I could stick some known good quality wires into the steel braid. (Want to keep the stock look)

 

If you're running full vacuum at idle, try running ported vacuum to see if it makes any difference.