Hello Everyone,

I have been working on a problem that I can’t seem to isolate. I’ve been at it for a month now and I am at the end of my rope. In an attempt to give as much detail as possible, I have laid out the whole story below. It is very long, but if you have the time, I would appreciate a third party opinion.

Thanks!

Combination:

Stock ZZ502
Crower Stainless Steel Roller Rockers
MSD Ready-to-Run distributor w/MSD Coil and Wires
BG 850VE Speed Demon
Holley “Blue” Electric Pump with Fram canister filter
Boesch Platinum Plugs
Power Source 140 Amp Alternator

Problem 1:

After pulling the car out for the first time this season, the engine ran fine. I have always had a rich idle circuit, but I had resolved to just live with that. I burned about ¾ of a tank of the fuel that was in it during the winter (which had Sta-Bil in it), and filled up with fresh fuel.

Shortly thereafter, I started having problems with the engine running rough and dying at temperatures over 180 degrees. It acted very much like the carb was flooding out (black smoke out of the tail pipes). It was almost like a switch, very consistent as soon as the water temp reached 180. I considered vapor lock, but that was never a problem I had in the past. After speaking with BG, I decided to rebuild the carb and look for any debris that would have stuck in the floats. I also checked the fuel filter, and replaced it even though it looked clean. The fuel tank is only 3 years old.

After disassembling the carb, I found that one of the idle fuel feed holes in the base plate was mis-machined so that it did not match the main body. It baffles me that I was able to see any change at all in that idle mixture screw. Even thought the carb was three years old at this point, BG took it back and sent me a new 850 VE with annular discharge boosters, believing this carb should work better on the ZZ502.

New 850VE with Annular Boosters:

This carb, to my confusion, had a Secondary Accelerator pump squirter installed, complete with “needle” check valve. I questioned BG why a Vaccum Secondary carb would have a secondary squirter. At this point, the BG tech said that the main bodies between a VE and Mech. Secondary carb are the same, so it should not be a problem because it was a dead circuit without an actual accelerator pump. At this point I was skeptical, but gave them the benefit of the doubt.

I installed the carb (with fresh spark plugs), started tuning it for base idle at about 170 Deg., and to my dismay the engine started to stumble and die above 180 deg.

So now I start think I lost something in the ignition. So I try the following, one at a time (I had these new parts on the shelf for another project):

- New Coil
- New Coil Wire
- Different MSD Distributor with 6AL Box

The engine acted exactly the same! As a last ditch effort, I decided to drain the tank and put in 5 gallons of fresh fuel. Problem 1 Solved!! I could idle at 180 degrees without any issues. I even took it up to 210 by disconnecting one of my electric fans, and she idled without mishap. Now, I still seemed to be rich at idle, but I was not about to go drilling out idle air bleeds at this point. I put my original MSD coil back on.

Enter Problem 2:

As I start to re-check base timing and other operations, I notice I had a no-load miss throughout the rpm range. Now what??

Tried new distributor cap and rotor – ran the same.

Now my base settings are:

Idle speed 800 rpm
Base Timing Mech: 16 Deg Adv.
Base Timing Vac. Adv.: 36 Deg Adv.
Total Timing Mech: 36 Deg. Adv.
Total Timing Vac Adv.: 56 Deg. Adv.
Idle Vaccuum: 14 in Solid
Transfer slot exposure: Primary 0.020”, Secondary 0.005”
(New BG procedure has secondary blades closed 100%, but I had to open them slightly to smooth out idle a bit).
Idle mixture screws: All four out between ¾ and 1 full turn.
(Any leaner it would stumble, any richer it would not run well and vacuum fell off).

During the miss:

Vacuum stays steady in the high 20’s depending on RPM.
Timing remains steady. There is no jump or bounce. I put a timing light on every plug wire looking for a timing fluctuation but found none.

BG had sent me a 1” spacer back when we thought Problem 1 could be vapor lock. Grasping at straws, I installed it. This allowed me to back out the idle mixture screws to about 1 and ¼ turn, but the no-load miss was still there.

New Test:

Believing my problems were still carb related, I attempted to find out if it was a lean or rich miss by slowly closing the choke. By doing this, I could make the engine run smooth at a particular rpm, by varying the choke position. It seems like I am lean at some RPM’s, and rich at others, to the point which causes the engine to misfire. At the extreme, I started playing with primary jet size. I could “tune” the car under no-load to run smooth in a 100 rpm band by changing the jet size, but either side of that RPM window I would get a miss, sometimes rather severe.

Now, my observations are:

- Fuel is not pulled through the main boosters under no-load conditions until 3000 rpm.
- When it does come through the main boosters, it comes out as large liquid drops. This is very easy to see with the annular boosters.
- At no time do I get fuel through the secondary boosters or squirter, which I pretty much expected.
- When I pulled the carb off, I find that the base plate gasket is saturated with fuel.
- Fuel pressure is a steady 6 pounds under all conditions
- Voltage to the electrical system is a steady 12.5 to 13 Volts (Using direct ignition source, not factory resistance wire).

After working with BG now for a month, they are starting to doubt that secondary squirter, and are sending me a new carb. I will hopefully get it by next week.

However, last night I decided to bolt on a fairly new 750cfm Holley with a 4 corner idle circuit as a test. Same rich idle, same damn no-load miss.

My next test would be to but the original distributor back in, but now I feel I am just blindly changing parts.

I have also plugged of every external vacuum sorce, including the brake booster, without noting any change in behavior whatsoever. I have sprayed down the intake/head seal with starting fluid and have not noted a vacuum leak in that area either.

Has anyone out there had a similar experience? Any diagnostic tests I’m missing? Due to the rock solid and high vacuum I had initially eliminated a mechanical issue (such as sticky valve or flat cam lobe), but I am now not so sure.

Please, any suggestions would be greatly appreciated. I feel as if I am in my own little automotive hell.

Thanks and Regards!

Keith a few thoughts

do you think it possible you have a dead cylinder? a dead or non firing cylinder would create alot of unburned fuel through the exhaust causing you to think your idle is rich, and would also explain your miss.

with the engine running and warm feel the primary tubes on your header (if you have em) if one is colder than the rest you have found your dead cyl.

have you checked your plugs since you drained the fuel tank? take note of which plug was in what cylinder and compare them. a black and wet plug that smells like gas is a dead givaway for a dead cylinder.

also i would think about switching to standard copper type spark plugs. from what ive seen platinum plugs dont stand up too well in engines that run fairly rich. AC Delco, Autolite and NGK all make good plugs.

Is it possible the fuel bowl is boiling, sending the gas in the bowl rushing into the carb? A quick check would be a phenolic spacer on the intake to act as a heat buffer, and see if it still acts up.

If it is the problem, you could consider keeping the spacer but really I'd block off the intake's exhaust passage.

I've never heard of this being a problem, it's just a thought.

Damn, sorry I didn't read you already tried that. No other thoughts at this point, but it just seems like you're getting vapor lock from SOMEWHERE.

The no load miss sounds to me like the primary 'transistion' circuit is flakey in the carb, especially if you can 'tune' out the miss with the choke plate. If so, the 'latest replacement' from BG should fix.

try a new sey of AC spark plugs. Its like reliving my own hell hearing yours. New ACs fixed it.

Weekend Update:

I put the original MSD Ready-To-Run distributor back in, with the 750 Holley sitting atop the 1" spacer. Engine ran the best yet at idle and below 3000rpm at no-load (in Neutral). Above 3000 she would sputter a bit, but I attribute that to a lean condition caused by the power valve not being able to open at high vacuum durring no-load.

I had the MSD Mag. Trigger distributor spun on a Sun machine, and it checked out OK. So, I'm think maybe the 6AL box has a problem.

Excited, I desided to go back to baseline on the 850 Demon without a spacer. Alas, the no load miss between off-idle and 3000rpm was back.

I am now waiting on the new replacement carb, to see how that works. Maybe I got really unlucky and have a bad distributor AND carb.

Regarding the plugs, I think I will head in that direction if the new replacement carb does not work. I really hate the screw on tips of the Boesch plugs anyway. I have do idea what the advantage of that style is.

Thank's for all the support and advice.

I would have said check and see if you have a broken valve spring, but if it goes away when you change the carb it doesn't make much sence to me... When I built my engine I got some Lunati springs... It was good for about 2 months and then springs started breaking... I'd break a spring or 2 almost every week... I just broke one of the last 2 that are in there last week... I thoght that it was odd with such a small cam... Must have been a bad batch of springs...

UPDATE!

Received the new 850 Vac Sec. Annular Dist. Carb, and the engine runs MUCH better. No no-load miss.

Engine still holds the best idle at ¾ turn out of all four mixture screws. This was common to all the carbs I’ve tried.

After a few miles of driving and some full throttle pulls, the plugs look the best they have ever looked. Nice and tan on the primary electrode. I need some more tuning in the mid range and I have some ignition tuning to do, but at least I’m back to a good baseline. I may play with the 1” spacer when time allows, but for now I am concentrating on enjoying the car again.

I still found some things in the new carb I didn’t like when I disassembled it. There was the usual machining flash and chips in some of the bores and orifices, and one of the metering block locating dowel pins was missing. I stole one out of the 2nd carb before sending it back.

At the end, even though they have stood behind the product, I don’t think I would recommend a Demon or go with one again myself. There just seems to be a lot of QC issues with them. Either that, or I am very unlucky. Definitely pull them completely apart and clean them out before bolting them on your engine!

To recap – the issue that started all this was a bad tank of fuel from my local Mobil station. Caused what appeared to be flooding (black smoke billowing from exhaust) at anything over 180 F. This led me to believe the floats stuck in my original 850 Demon. After disassembling it, I found some orifices machined wrong and sent it back. Replacement carb had the same tendency. Swapped ignition systems, problem stayed. Finally drained the tank and filled it with BP/Amaco, running problem at above 180 was solved. However, new carb/distributor gave me a problem with a no-load miss.

Final solution – original ignition system with 3rd Demon carb and running BP/Amaco fuel.

I’m noting blaming Mobile specifically, but now I’m wondering if anyone else has had similar fuel related issues. Just to avoid future hassles, I am avoiding Mobile.

I bought the subject fuel back in the early spring, so maybe a got some “winter” blend that is alittle more susceptible to detonation. Chicago fuel is notoriously bad anyway.

Thanks for all the help and support!

Keith
AWSM502

First off I used to haul gas (tanker) for a living. I do not know where you are in the country but in the Winter gas has different additives then in the summer and depending on where you are the formula for winter and summer gas can be way different. Just a note, all gas is the same, it is just the additives that are injected at the terminal where it is picked up for delivery to the gas stations that make different brands different.

These different formulas are supplied at specific dates controlled by the federal government. I think that is what was your first problem.

I was also thinking you might have a bad diaphragm in the vacuum advance in the distributor or vacuum secondary on your carb.

I’m glad you got the problem solved.

Uh Glenn did you happen to notice the thread was from 2004 ?