I know this might be a "Lars" issue, but he surely gets so many questions that I dont want to bother him before I have tried anything else.

From start:

Egine is a stock zz502 with 5 speed manual, but with Demon Sixshooter Carb that looks like TriPower.
MSD AL 2.

Some weeks ago I noticed jerking while cruising with low speed in town. It turned out that the zz502 had eaten
the dizzy gear of the old original TI dizzy I was driving. So too much gear play, non constant spark.
Because the distributor needed extended work I replaced it with a MSD unit. Advance curve is from 18-36°.
Engin run perfect right after installation. No issues for ~2000mls.

Then came a corvette weekend meeting, I noticed some fuel moisture at the hose between filter and carb.
Maybe a clamp was not tight enough, I replaced the hose and filter.
Drove the weekend without issues, drove back home with some WOT fun on our german autobahn.

Since there I have a jerking when cruising at low rpm (1000-1800) in town.
The engine runs smooth and every scond or so a jerk comes in.
Checked the distributir gear: looks good. Also the side on the camshaft.

Improtant to know is that is dispappears immediatley when i open the throttle a little more.

I guess it´s a carb issue now.
A small particle could have come into the carb and blocks the transistions circuit, leaning it out.
Could this be a reason?

Next thing im going to is to open it and clean it.

Other things that come into my mind are related to blown valves...

Any suggestions?

Important to know is that the jerking

 

It may be a ignition issue. It could be a bad wire, cracked spark plug or a cracked coil head. Jerry

 

I have forgotten to tell, that I already changed all plugs. The old ones did not look bad, but they are cheap, so all replaced. Also new air cleaner.
I also ran the engine without vacuum advance - no difference. Retareded mechanical timing a lot to see if it makes any difference, exhaust noticeable
louder, because more throttle needed, but still jerks.
As soon as I put the food a bit more on the pedal the jerking goes away, while rpm is still the same.
At all other load situations (where the high speed circuit in the carb starts to work) the engine runs perfect, no issues.
After all i doubt it´s an ignition issue.
I run the good wires from MSD, not very old, but for testing I could try another blaster coil.

I guess I will also measure the valve movement, maybe a lobe is worn?

What I noticed too was, that when I close my secondaries better, the previous adjustment was not perfect, the jerking got a bit less.
Leads me also to a too lean mixture.
I also drove with the idle mixture much richer and leaner, but this had no effect. The question would be how much the idle screw
will influence the transistion mixture.
Could a particle partially clogg the transistion jet, causing lean condition at low rpm cruising, but let it run good as soon
as theres some slightly load?
The power valve is only for the high speed jets, so this cant help here. Btw. it´s not blown, the engine dies when idle screws are full in.

 

Sounds like trailer hitching. Like the timing is going too advanced.

 

It´s similar yes.
While trailer hitching happens when i let it idle at ~700-800rpm in gear. In first gear you get a bump now and then.
But if you rev up to 1000 it´s gone - well this was before the new issue occured.

Years ago i noticed that the engine does not like advanced timing in this condition, thats why i put the vacuum advance
to ported, problem solved. That´s why already tried it without vacuum advance and with much retarded timing, but it made
no difference. Important to know is that the engine ran good with this setup for years. But something has changed now...

I also noticed that it´s worse when engine is cold, that leads me also to a lean AFR. Will take the carb appart later, we will see if I can find
any particles that came in when i replaced the fuel hose.

 

Do You have a light flywheel? I noticed this happening when I went from a 30 lb to a 12 pounder.

 

If it’s worse when cold is this a choke delete carb?

any chance you floated a valve during your WOT fun?

 

I have a stock flywheel from a old 454. And i did not change it

Today I removed the carb, took it appart, it was clean inside.
Checked idle vacuum, the needle is flickering at idle, but at 1000 rpm it smooths out.
So no hint for a defective valve. I get 16Hg rock solid vacuum when cruising when the problem occures.
Checked timing, is also rock solid at 18° until 2000 or 2500 rpm, so the mechanical advance ist far away
from where im talking about.

I put some wires into the air bleeds of the transistion circuit because I have no matching jets for that circuit.
This enriches the AFR, but did not solve the problem.
Played with the timing back and forth, with and without vacuum advance, everything has an effect, but nothing that would help me.

It really feels like small missfires, I guess there are rellay miss fires.

I noticed another thing, the throttle shaft of the front carb has a lot of play. The rear a bit less.
I made a video of it:

https://www.youtube.com/watch?v=jVtW...ature=youtu.be

I have no experience how much influence this will have, but surely air gets sucked in there, leaning the front cylinders,
maybe this can´t be compensated with a richer mixture.
Tomorrow I will put a sheet metal under the throttle bodies to block them completely and see what happens.

 

Whoa, that's really bad. I' m surprised it doesn't whistle. I had this same loss of vacuum issue on a Holley 4150. Here are the options for repair as I recall.

There is a kit to replace the Throttle Shaft Bushings. Comes with 4 brass bushings, maybe 8 and a drill bit. You will have to disassemble the base plate and using a drill press hope you are a perfect 90* to the shafts hole. I think the kits was $50 or less. This has to be precise or the shafts will bind, leak, etc.

Option B. Send it out to have this done. Google carb repair. But it was pricey at around $70 plus two way shpg.

And last but never least. Find a throttle base plate for your Demon. I found one on eBay and the quality and tech support was excellent. They come in aluminum, blue, black or red colors. Some have extra Vac port. Some have a secondary throttle adj screw on the top, instead of bottom if you know what I' m referring to.
Those throttle plates run $90-$200 depending on quality of CNC machining.

 

Well, engines with vacuum leaks are known for bad idle. This one has good idle.
It seems they used plastic for the shaft?
I have a lathe and a milling machine, I can make any part by myself.
I wonder why just the front carb is worn most, because the outer carbs are used only for
the last 25% of throttle and 99% of time I drive on the primary carb. That ones seems to be better.

 

On a common four barrel the primary shaft bushings, crap-out way before secondary does. That's from all the revving when tuning, setting IGN timing, showing-off, etc.
In your case, perhaps your return spring is putting a unusual side load on the bushing. Or, just maybe the front carb spent most of its life elsewhere. IDK

 

From all what I have read now I doubt it has a big effect, but who knows. When I look on the blades from the top they close the barrel tight, and when the shaft lies on the bottom of the hole
there´s not much gap under the blade where air can get in.

I also thought I could gap the plugs a bit more. Currently I have 0.043 for this 9.6 low compression engine. I will try 0.050 and see if it helps. MSD states I can go up to 0.060.

Btw. forgott to say, yesterday I also replaced the coil, no difference.

 

Stretched timing chain maybe ?

 

Also one of my thoughts. But how to check this without removing the cover?
Does a stretched chain show bumping timing marking when timing? Because mine is stable.

 

Best way to check is breaker bar on harmonic balancer bolt and see how much you can move with out the rotor moving on the dizzy

 

And how much is acceptable?

 

I am not sure but my 427 had about 5 degrees of play and after change the timing set it was under 1 degree

 

1 degree of crank movement? That´s nearly nothing.
I have approximately 5° alone between cam and distributor.
This would be 10° at the crank.

 

Update:
Testdrive with sheet metal under the front carb. Did not help.
Disconnected the PCV, for less external air flow. Did not help.
Plugs gapped to 0.050 did help a bit - at least I think so.
Found the idle was to rich from previous tests, leaned it, did also help a bit.
Leaned it more, jerking again. So back to a point more or less in the middle.

The jerking is now better I can drive 25mph at ~1200rpm without getting shaked too much.
But still not satisfying.

Half tank of fuel blown for test driving, I m fed up now.

 

If the engine is revved up to say 1600 RPM is it steady there when not under a load? Or does the problem only present itself when under a load and it's obviously an engine issue?

That shaft clearance can't be helping anything as it would present as a sort of variable vacuum leak I would think.

If your timing is ported would this not effect your timing at low or no throttle blade openings?