308gt4 - unvelievable backfire, assistance?

I am not familiar with this system, but is it possible you put the new distributor in 180 off or one tooth off etc?

 

Plug wires mixed up?

How did you set up static timiing on distrib? My trick for TDC on the power stroke is look for cam timing mark thru the oil filler opening.

 

Coils are usually good or bad but I would check the resistance of the secondary and primary windings with ohm meter. Also check the resistance on all the distributor cap terminals. I had an engine that would start backfiring and mising when the engine was warmed. I checked the distributor cap terminal resistance and saw an increased resistance when the cap was heated. The new cap was bad.

 

Well, if you have an issue with only one bank and the thing is running a single distributor, it's probably not the timing of the ignition. It could be the cam timing, but that's hard to verify (without pulling the cam covers). Spitting through the carbs is often associated with running lean. Do you have access to a Fuel/Air meter?

 

No, I don't think so. I think it's more likely that there is an ignition timing issue that is only manifesting itself in one bank for some reason, maybe because of tolerances in cam timing. (?)

Do you have a completely separate exhaust for each bank? On a normal exhaust, you can't tell which bank is producing the issues because they all join together before the muffler. Are you going on just symptoms on the carbs?

Logic is that if the guys who rebuilt the engine said it was fine there and it's not fine with your ignition, it must be the ignition. (That being said, you definitely need to dial the carbs to the new pistons...the engine is going to breathe differently). I expect you will need to go up a size on jets at a minimum.

Birdman

 

Are you sure that you've got the "sensors" hooked up to the correct coil/electronics? The cyl #5 firing event must TRAIL the cyl #1 firing event by 90 deg of crankshaft rotation (45 deg of distributor rotation). If you're using the 5/8 sensor position to fire the 1/4 coil, it would be possible to install the distributor so that the 1/4 side would be perfectly happy, but the 5/8 side would fire much too early (at the end of the intake stroke rather than the end of the compression stroke) -- just a thought... Do you have a timing light so you can confirm/deny when cyl #5 is firing? Or is there some sort of visual way that you can confirm/deny that the triggering feature firing the #5 cyl is trailing the firing of cyl #1 by 45 deg of distributor rotation?

 

NEVER ASSUME. We had a Flight Surgeon up here, who was also a licensed pilot, aircraft mechanic, "AND" FAA authorised inspector. He didnt need to have ANYTHING signed off by ANYBODY, because he was GOD. Well, he put a winshield in his twin engine airplane, and had the SOB come in on him while on final approach. Crash and burn. The more someone knows, the more experience someone has, can often times allow them to overlook the obvious, or skip an important step they themselves would yell at you for.

If you dont pull the cam covers and verify cam timing, you may be wasting time chasing ghosts. If the carbs were balanced, and the ignition is routed properly ((remember, #1 (#5) front bank is opposite end than #1 rear bank) IOW, firing order should be, 1,5,3,7,4,8,2,6, could you possibly have wired it 1,8,3,6,4,5,2,7?)) AND the engine ran okay on a dyno, it "should" run okay in your car.

I would pull the cam covers, verify valve timing, and move through in logical order.

 

Not any kind of Ferrari wrench am I, but on general principles from aviation mechanic school--

If you have a loud bang and bright flame coming out of the exhaust, that's not backfire, that's AFTERfire. Backfire is much worse and means fire is coming back up toward the carb, possibly starting an engine fire. One cause is lean mixture--too hot, burns too soon.

Afterfire probably means the mixture is too rich. Unburnt fuel is leaving the cylinder and occasionally detonating in the exhaust when it again has enough heat and oxygen. It scares people and may start forest fires, but it's too far away from the engine to cause much damage.

Along with whatever else is bugging you, see if your mixture is rich.

 

Listen to Steve. He is on to something!

Nope, they join inside the muffler. The outlets on the muffler mean nothing. one is always giving more oomph than the other because of the internal baffle design. Has nothing to do with banks. Glad we got this sorted early. Now...more troubleshooting...

Before I explain....THIS IS NOT THE PROBLEM. What you are experiencing HAS to be timing related. It's just too violent to be jetting. (Who was it on here, maybe Art, who said long ago that 95% of carb problems turn out to be ignition? That is so true). But anyway, if you do anything that makes the engine breathe better (more flowing exhaust, more flowing intake, higher compression ratio, etc.) then the carbs will go lean because the jets are not large enough to deliver fuel at the right ratio in higher airflow. You swapped out for higher compression pistons, you will ABSOLUTELY need to rejet the carbs and re-synch them.

BUT this isn't why it's backfiring.

Amen brother. Although I would check the timing for certain first. I'm betting something funky is going on and one bank is out somehow.

I agree on afterfire, but I doubt it's richness. I'm betting the timing is off.

Birdman

 

Have to agree with Paul that "They forgot to change the old fibreglass cam shaft pulleys for new aluminium ones during the rebuild, so the guy had to fly over to Ireland to install them here in my garage with the engine out and on the ground" is a heck of a lot more suspicious (and intrusive) than "...so the installation of my single distributor and associated electronic timing sensors on the engine is the only apparent change since run in". Still, I'd confirm/deny the ignition stuff first as B-Man stated (since this can be done without other disassembly), but, if the ignition stuff turns out to be OK as it is (but you still aren't running right), you'd have to confirm/deny the camshaft work IMO -- Good Hunting!

 

Had somewhat of the same backfire "afterfire" problems in my GT4. After all sorts of searching and false starts and replacing things that did not need replacing, the problem turned out to be that the low tension wire from the rear distributor to the coil had an internal intermittent break and so the back cylinder bank was not firing. But all of that raw gas was going into the muffler and meeting the hot gas from the front bank causing big "afterfires". I am still running points, but the principle could be the same.
Martin

 

How did the compression test work out from one bank to the other. If a cam were of a cog, a compression test would surely help to confirm.

 

The ignition systems for 1-4 and 5-8 are still seperate ignition systems that share a cap and rotor, right? Sounds like you only checked 1-4. Might be worth checking 5-8 for proper firing especially since you think this is a 5-8 bank issue. Did you check cyl 1 while it was running?

Is this afterfire regular with every two crank rotations?

 

Plugs 6,7 and 8 are fuel fouled (and may or may not fire, even with the ignition working "properly")...perhaps due to your past problems with ignition system. I'd replace those with new ones and see how she runs then. The other plugs are simply rich...