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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!
 

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Like you said... put the original distributor back in it. You already put the original coil back in and the stumble problem remained. You just about ruled out fuel\carb by mocking in the Holley. Could the idle stumble have been there before?
Evan though you could get the idle stumble to stabilize with a certain choke setting, the RPM's changed which could lead you back to the dist.

As far as the rich idle, dunno....
 

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awsm502 you've done a great job of approaching this problem like a pro mechanic would.... imo. In fact, as I read, I would think of something, and then, next thing in the post, like vacuum probleml you would have already checked it. The problem is that SO many items have been altered, and so you don't know WHAT is the culprit. I can only suggest that you go back to ANYTHING that you know is both rock solid and worked previously, whether it is a carb or a distributor. I hate a problem like this. Basics say, "Spark or Fuel..." , and I know you are all over that. Keep us posted, ESPECIALLY when you see some daylight here, and I personally will keep thinking on this one, as I know others will.
 

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just a thought...

take your gas tank cap off and see if that makes any difference.

don't forget to put it back on.
 

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I don't want to sound dumb, but how about bad plug or plugswire. Sounds like you checked everything else. That starting fluid thing sounds scary. Good Luck Kevin
 

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Originally posted by Everett#2390:
</font><blockquote>quote:</font><hr /> - 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.
Try a smaller cfm carb. </font>[/QUOTE]850 CFM is not absurd for 502 cubic inches. I don't think the 850 flow rating in and of itself is the culprit for the large liquid drops.
 

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Discussion Starter #8
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.

Thank's for all the support and advice.
 

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elprup asks:
850 CFM is not absurd for 502 cubic inches. I don't think the 850 flow rating in and of itself is the culprit for the large liquid drops.
My thoughts are carburetors are sized by CID and max rpm the engine will see. Granted, 850 CFM for a 502 CID would probably be a good size if one were to work out the formula and if one were to operate the engine at this rpm.

I suggest a smaller carb due to the fact the engine will be operated at a lower rpm the majority of the time. This smaller carb will restrict the air flow some, thus provididng a better vacuum signal for the carb to atomize the fuel better.

The engine creates the vacuum, atmospheric pressure is trying to equalize this vacuum. Thus, a smaller carb will limit this amount, allow the engine to make more vacuum letting the carb do its job better by introducing the main fuel system at a lower rpm and allowing the venturis to atomize the fuel further by the venturis design, ie., creating a high pressure area.

I'm not suggesting a 650 or 700 cfm, just step down to say a 750 or 800 cfm. Borrow one and examine the results.

As jethro suggested, the engine ran good at one time, awsm502 should think about history and back track to help the troubleshooting process.

Probably wouldn't be bad idea to change spark plugs.
 

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Being totally lost and fealing brain dead I'll just throw in did you check torques on all gasketed items like heads and intake manifold. Your probably wondering the same thing if after a ceratin preasure is the cooling getting injected past seals into the intake. I've had heated fuel problems before so I wouldn't doubt that. The big aluminum plates some engines wear extending from the base of carbs is at times really needed. Good luck.
 

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Discussion Starter #11
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!
 

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Good to find/read you're back on the road. There is a difference in seasonal blends on all brands.

Just means we have to drive more often to get a good stock of fuel. Convert to propane and problems go away.
 

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Originally posted by awsm502:
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!

You're not unlucky... I've seen it over and over in Holleys and BG carbs. Read this; http://www.camaros.net/techref/articles/ftecref003.html
 

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I bought a Quick Fuel Technology Carb and I am in love the darn thing. Ran 10.90 out of the box with normal adjusments, float level and idle. I gave Marty all my specs for my car and he built it for my application. Quick Fuel bench tests their carbs on a running engine before they ship them too. I have chased my share of demons tho. As much as I like my MSD Digital 6, I just put my third box, in three years, on the car. If you burn a plug wire and get a spark jump you'll fry the msd before you can say boo. Best thing I did was to buy fiberglass boot protectors for my wires. One good thing about Summit is that if you have a problem with the MSD they replace it no questions asked for one full year. Hopefully I brought some useful info to the table...
 
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