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Discussion Starter · #1 · (Edited)
Ok.. so Ive been having some issues with dieseling with my fairly new (7k miles) ZZ4 crate engine. _Its got a Barry Grant zz4 crate carb on it.

So after flogging the distributor back and forth trying to find a spot where it wouldn't ping so bad and not diesel and simply not finding it I figured I better do it the right way.

So with the engine at operating temp I set the timing to 12 degrees.. The engine runs good at that but still diesels. _So I figured maybe there's something wrong with the vacuum advance. _So I disconnected and plugged the vacuum line and set the base timing to 4 degrees. _As soon as I plugged the vacuum line back in it jumped to 23 degrees. _To me that just doesn't sound right. _

The vacuum advance is connected to straight vacuum not ported.

Any suggestions? _


DUH - I figured it out after thinking about it..

I set the base timing with the vacuum disconnected to 12 degrees. Total timing 36 degrees at 3500 rpm.

THEN connected the vacuum advance back in Sometimes I wonder about myself ;)
 

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I`m pretty sure the vaccum advance is suppost to go to a ported vaccum source. It`s been a while since I used a vac. advance distrib. I could be wrong though. If it`s hooked up straight to vaccum, it`s got the full advance at idle. It`s not suppost to have full advance untill around 2500-3000rpms.
 

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Discussion Starter · #4 ·
Ported vs Vacuum

Ok.. so I read about 10000 posts about the difference between ported and manifold vacuum advance. So it seems that everyone has a different opinion. Is there a definitive answer for a zz4? :)
 

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I have always used ported vacuum, it stops the "tip in" ping that occurs under light loads and the engine generally runs cooler. I have experimented with both types of setups and have never found an instance where full vacuum outperformed ported...for street cruising or road racing.

Sorry guys, I have never seen an instance where switching back to ported vacuum and retuning the ignition timing to take advantage of running more initial advance didn't make the engine run better, stop run-on and drop coolant and exhaust temps. Dual diaphram distributors need both hookups with the inner diaphram getting full manifold vacuum. The proof is in the plugs, check them for signs of detonation and you will see the difference between the two methods.

Feel free to try both setups and determine for yourself, don't forget to recurve the distributor to have all your mechanical advance timing in by 3400-3600 rpm.;)

Just my opinion and I'm sticking to it.:D
 

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Discussion Starter · #8 ·
OMG I cant get this right! help

Ok Still cant figure this out.. heres what Ive found:

14btdc Manifold vacuum = pronounced ping at tip in

14btdc Ported vacuum = Pronounced ping at tip in

12btdc Manifold vacuum = pronounced ping at tip in

12btdc ported vacuum = same ping

10btdc ported vacuum = not so much ping but there

Cant do manifold vacuum as rpms get up to high and cant idle below 900 in park

8btdc ported vacuum = ping going away but also power feels less

6btdc ported vacuum = ping there barely but definite drop in power. Also starting to diesel a lot when shutting off the motor.

So my guess is that the ping would go away at around 4 degrees but I also guess that 40% of the power would also be robbed by this setting.

I checked and with the vacuum disconnected from the distrib. advance I get around 32 degrees of total timing in around 3000k rpm.

What am I missing? Timing is not this difficult but for some reason I cant get this figured out. I tried emailing Barry Grant carbs but got no response. Could the pinging be something other than detonation? It starts pinging from a cruising speed to maybe 1/8th throttle trying to accelerate mildly. It doesnt ping at idle or at 1/2-full throttle.

Im going crazy, theres gotta be something Im missing

Thanks!
Joe
 

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Joe,
call the people where you bought the motor.In my opinion you should have initial timing set at 8-10 btdc.Idle at 700-800 rpms and full manifold vacuum.Thats what I have my 67 setup as and it screams!!!!Good luck bm
 

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Discussion Starter · #11 ·
Unfortunatly thats really not an option for me ;(

Theres gotta be a simple solution to this.. Could the distributor be bad?

Could the tip in pinging be caused by the idle mixtures not being set correctly?

Im beating my head against the wall
 

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ZZ4

Yall are looking in the wrong place. There is too much air coming from somewhere. Usually the problem is when the idle screw is cranked down and the idle transfer slots are uncovered. Look to see if the carb has bleed holes drilled into the butterflies. Usually if you can uncrank the idle screw and get the idle back up by adjusting the idle mixture screws. Some old cars had a anti-diesel solenoid that dropped the throttle to nearly zero degrees when you shut off the ign. Just for kicks idle the carb way down to like 500 rpm. If you need to keep you foor on the throttle to keep it running than do so. Drive the vehicle as usual and then stop and shut the ign off and lift your foot at the same time. If this stops your dieseling than you have too much throttle angle at idle. If there is no way to adjust this out of the carb then you have a carb that is too lean on the idle circuit. If it's an aftermarket carb then you can install a smaller air bleed jet or pull the metering plate and enlarge the fuel jet NO MORE than .001" that will give you about a 7% enrichment in fuel flow at idle which should make up for the lean condition at idle.
OR...You could just try another carb! :thumbup:
 

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Discussion Starter · #16 ·
The sound is pretty obviously from the vacuum advance kicking in but yea Ive never heard one klunk either.. seems like theres a spring missing or something holding the diaphram back.

I just dont know what else it could be. But would that explain the high idle speed?
 

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I would recommend you private message docvette...this guy is super smart and really knows motors.tell him everything that is going on.I'm sure he can help you with your motor.(he helped me quite a few times)I'm stumped.You must be going crazy.I didn't sleep for days until I got my motor dialed in.(i was in the garage at 4:30 am cussing and throwing wrenches)All is good now.What kind of car are we talking about?Good luck.bm
 

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problem with zz 383

I know this is an old post but thought you could help . I'm running in the same problem that you were having.what did you finally do to resolve it . i would love to get it right just not sure what to do
 

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JoeS said:
So when I plug in the vacuum line to the distributor I can hear a distinctive clunk. Could it be that the vacuum advance canister is kaput ?
Well let's eliminate what it is not, based on your post. It appears your timing marks are set properly relative to TDC and the distributor is phased properly because your findings when advancing and retarding timing all make sense. The Vac advance seems to be working based on some of your reading so the can appears to be operating as designed. So I'll assume those issues are non issues.

Now what it could be. The most basic is simply octane intolerance. You simply may need a higher octane. You did not indicate what you are burning. I suspect 93 octane would be the minimum I would want to burn.

If I recall the ZZ motor has the fast burn heads. So timing should work out to 10 initial and 35 total by 3000. Thats where I would tune to for base timing and for total that should be within 1 degree plus or minus.

If your mixture is dead lean on the bottom end that could also cause problems. Under low speed steady state cruise if you sense the motor is surging then that would be a symptom of a lean condition. You may want to try and richen the idle mixture 1/4 of a turn and see if that helps.

Operating temps also can make a motor sensitive to detonation. A 180 degree thermostat seems to work best for overall operation on the street.

Although BG has said they work hard to avoid defects out of the box we always tear down Demons prior to bolting them on the dyno. We inspect and clean/blow out all pasages. We also take time to set floats correctly. Demons can be a bit more touchy setting floats it seems versus a typical Holley.

Plugs that are too hot can also cause detonation. Make sure you have the correct plug installed. Going one step colder drops tip temps about 100c (212 degrees) and sometimes will cure a minor detonation problems. Extended tip plugs are more prone to detonation than standard tip plugs.

Proper fuel pressure is very important as well and low pressure can give you a lean condition.

The benefit to manifold vacuum is you can tune in a much smoother idle. Combustion is slower at idle and the additional advance makes a drastic difference in EGT temps by keepign the combustion heat in the chamber and not blown out the exhaust. Once the throttle is cracked both manifld and ported operate the same.
 
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