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Discussion Starter · #1 ·
I am slowly trying to dial in

I am slowly trying to dial in my '76 308 GT4. Car has a single dist. conversion with a Mallory Unilite set-up. I have adjusted the advance to spec. Should this be different than spec because of the conversion set up? The carbs have 135 mains and 45 idle jets.

1. I get popping/crackling when I let off the throttle. Research tells me that this could be too lean and so I have ordered some 50 and 55 idles.

2. The car is slow to drop back to idle. When I accelerate and shift, the rpm doesn't drop when I press in the clutch. It's more annoying than anything.
Any suggestions?
Doug
 
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Discussion Starter · #2 ·
Could be intake leaks..very co

Could be intake leaks..very common on all the older cars that haven't had a careful and thorough carb rebuild/remaching process done. The 45's sound too small also considering most of the engines aren't brand new any more. Additionally, the carbs linkages adjustments are critical to nice smooth running. ALL the carbs must be in sync both on air flow at idle - idle stops, and thru their full travel. It takes serious study of the linkages & operations to insure each linkage is exact and correlates to all other linkages..

The timing should still be at the normal Factory specs..with perhaps a touch of tweeking...7 degrees advanced at idle should be perfect for street apps.

Regards, JRV
 
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Discussion Starter · #3 ·
Put in 55 idles and after a li

Put in 55 idles and after a little fine tuning, all is fine. I cant figure out why the rpm's would stay up originally. This problem was cured with the change in idles, but I cannot see how they are related.
 
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Discussion Starter · #4 ·
Last weekend during a spirited

Last weekend during a spirited drive, the higher rpm popping/stumble re-emerged and has not gone away. From idle to @4300 rpm everything is smooth as can be. As soon as I hit @4300 rpm in any gear, the car bogs down and pops/backfires and stumbles. If I back slightly off the throttle, it runs perfectly again. I have it set to 7 deg at 1000rpm.
I'm no expert so I'm just learning my way through this. I figure that since I had changed the jets and had the car running perfect for a while, the carb jetting is not the problem.
Would a bad coil cause these symptoms?
Maybe the Unilite ignition is going?
The cars starts on the first crank and if you never took it over 4300 rpm, you would think it was a new car.
I probably need to clarify the rpm measurement. In neutral, with an RPM timing light, the point of backfiring occurs at @4300. My tach is not hooked up because of the single dist. conversion.
In 1st, 2nd, 3rd and 4th, it reaches this point at @23mph, 40mph, 60mph and 80mph.
In 1st
 
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Discussion Starter · #5 ·
Sure sounds rpm related which

Sure sounds rpm related which would suggest to me to think stuck advance weights or failing coil. I'm not sure of the unilite failure mode.
What type of distributor are you using?

The other usual suspects might be cap/rotor or plug extender burn through

best
rt
 
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Discussion Starter · #6 ·
We really get some tough ones

We really get some tough ones


Have you checked to insure some portion of the throttle linkage hasn't become glicthed and that the engine is getting full throttle?

That there are no loose connections or frayed wires on the distrib & coils?

Done a quicky plug inspection to insure they look good?
 
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Discussion Starter · #7 ·
My dist. is an origianl Marell

My dist. is an origianl Marelli that has been fabricated to accept the single set up. The cap/coil and rotor are all MSD. The rotor and cap are brand new. I feel like it must be an individual pat because the problem is not intermittent. It always happens at the same rpm in all gears. Until that point, it runs perfect.
 
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Discussion Starter · #8 ·
More info that may help in a d

More info that may help in a diagnosis. At the above speeds where the leanness hits in each gear, the car will not accelerate through the problem. In first, 2nd, 3rd at speeds of @ 24,40 and 60mph, the popping/backfiring starts and it will not accelerate anymore. If I back off the throttle a little, everything is fine. It is almost like I have a rev limiter at @ 4400 rpm.
Double checked all my connections and had the battery and MSD coil bench tested. Everything checked out fine.
When a car is run off of a single dist. with electronic ignition, is there anything that happens to the advance that could cause this?
 
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Discussion Starter · #9 ·
>>When a car is run of

>>When a car is run off of a single dist. with electronic ignition, is there anything that happens to the advance that could cause this?<<

possible...

have you checked the timing for BOTH banks to insure both sides are timed and working the same?
 
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Discussion Starter · #10 ·
How do I do that? I'm not

How do I do that? I'm not an expert, just trying to learn as I go. I assume I hook it up to a different plug wire?
 
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Discussion Starter · #11 ·
First check 1-4 by hooking up

First check 1-4 by hooking up the timing light to plug wire cyl #1....check initial timing against the flywheel marks...7-10 BTDC 1-4..I prefer 7 to keep the idle down....then rev the engine to 3K-4K rpms and check Max Advance...it should be right at 32-34 degrees BTDC..then let return to idle you should see the timing drop right back to the 7-10 mark 1-4....(shut off engine) now take the timing light pickup & switch to the cyl #5 wire....start engine....you should see the flywheel mark 7-10 again ....(but 7-10 BTDC 5-8 instead of the 1-4 mark) rev engine again and verify 5-8 is advancing to 32-34 BTDC...or essentially the same as 1-4...both sides have to have identical timings and advance curves for the engine to run correctly.

HTH's
Regards, JRV
 
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Discussion Starter · #12 ·
Ran the check that you said an

Ran the check that you said and all works out with both banks. 7 deg at 1000 and 34 deg at @ 3500 on both line up. When I take the RPM up to the point of the problem @4400 rpm and keep the light going, I notice that the flywheel marks bounce around at this point. Is this caused by the popping/backfiring or does it create the popping/backfiring?
What should I try next?
 
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Discussion Starter · #13 ·
Check cap and rotor. Check plu

Check cap and rotor. Check plug extenders. I'd trw wiring around the MSD and run just on the coil. If that's bad, replace the coil. They're not too expensive.
Best
rt
 
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Discussion Starter · #14 ·
Cap & rotor are new. Plug

Cap & rotor are new. Plug extenders are also new. My configuration is a MSD coil(which tested OK) direct to the cap. My dist. is an original Marelli that has a Mallory UNilite set-up similar to the Millermon ones.
Can the fuel delivery result in the bouncing advance at 4400 rpm or is that is symptom that is onlw ignition/electrical related?
 
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Discussion Starter · #15 ·
Not likely fuel delivery. In a

Not likely fuel delivery. In addition to the coil, are you running an MSD 6A box as well? If so, would definitely wire around and disconnect the MSD and test - easy to do. Only other obvious thing would be the optilite sensor (cheap and easy to replace). Would definitely look next at the MSD trigger box if you have one.
JRV?
best
rt
 
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