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UPDATE: Backfiring and starter bendix gear

Docksidemarineservices said:
Every time you run the engine up, it starts backfiring because of a lean condition. Lean Condition= excessive heat.
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Update from last weekend:
Verified fuel at 6psi all the way up to the point of backfiring, so pressure to carb is good. Swapped carbs and the problem stayed with the engine. Not a lean condition.

fastjeff said:
Actually, no. Lean conditions at start up do not yet involve excessive heat.

Have you tried opening up the idle mixture screws a turn? If that won't do, it's possible that the idle passages are too small. (Don't laugh: I had to drill them out on one of my carbs.)

Jeff
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Engine purrs all the way up to 2500. We can enjoy a day of fishing with no problems as long as we keep it under 20kts (~2500RPM both). Nothing to do with idle mixture, but thank you both for the replies.
 
major_tom said:
Backfiring could aslo be a timing issue, or worn exhaust cam lobe.
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Measured all valve stems at full up and full down positions using a crude, albeit sufficient, method of using a piece of wire, marking height at valve closed, then marking low when valve fully open. Took that "gauge" around all valves, cranking engine by hand, and they are all moving equally. Not going to rule out cam lobes yet, but putting that idea on the back-burner.

A buddy is a marine mechanic, and he spent the day on the water with us last weekend. We checked vacuum, fuel pressure, swapped carbs, swapped coils... He thinks it is either the module or the pickup coil on the Delco EST. With him on the timing light, my brother at the helm, and me on the distributor, we were able to get the engine up to 3200rpms before it started popping. We ran out of time and did not get to swap the dizzy's, but hopefully do that this saturday. A little more difficult to work on the engines out in open water than in the slip, hehe.

Been busy with Master100 Ton courses, so after class this saturday I will have the answer on the distributor. If that is not the problem, he agrees we are either looking at a valve or cam lobe.

Found this thread last night which gives me hope for just being a distributor problem. Skip to the last post of the first page for his answer to the same problem.
http://forums.iboats.com/showthread.php?t=44210&page=1
 
Your method for checking valve operation would best serve you if a dial indicator had been used, but considering that you did not find a substantial difference, doubt this is your issue.
The Delco EST system will have no advancing occuring at/in the distributor itself, as I'm sure you know.
Not a bad idea re; the pickup coil on the Delco.
However, you may want to swap modules first, not necessarily distributors!
Either way, it is one item in your P of E list to check!

If your balancer is marked off, you can check dynamic ignition advance while increasing RPM............ again, as I'm sure you know.

Just a few suggestions!
 
A worn cam lobe would definitely cause full throttle backfiring and at the same time, partial throttle operation would not produce any backfiring. Certainly easy to check on with valve covers removed.
 
bobbyc said:
A worn cam lobe would definitely cause full throttle backfiring and at the same time, partial throttle operation would not produce any backfiring. Certainly easy to check on with valve covers removed.
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Wow! Resurrecting the dead!

This is more than four years old, but reading back through, I guess I never did post what I found. The problem was a bad tachometer. It was pulling too much current from the coil and caused a weak spark. Warrantied the 3-month-old tach and problem was gone. Went through the heads and MUCH MORE to find out the backfiring was caused by a tach?!?!

Who'd-a-thunk-it?
 
Mulletwagon said:
Interesting !! How does one troubleshoot a bad tach (without changing parts or just disconnecting wires) ?
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That was exactly it. After all that troubleshooting... finally had a thought... what else could possibly be drawing off the coil?? Well, the only thing left was the gray wire for the tach.

Had a spark plug wire off, held it near the dizzy, and watched the dull orange arc as I snipped the gray tach wire. The spark instantly turned blue, the "snap" got intensely louder, and instead of an arc that would not even reach 1/2" before breaking, I could pull away about two inches.

Now THAT is what it should look and sound like! Revved the motor... no more backfiring. Took her out and ran it WOT... both engines finally purring like a kitten. Well... 454cc kittens.
 
Nice success story !! Suspect the tach was grounding internally and precluding the distro from completely collapsing the generated field. Thanks for the final chapter.

FL Panhandle
 
That was one of the first lessons learned at the boat shop eons ago....if intermittent ignition is suspected, pull the grey wire from the coil or disconnect the other end at the tach....

As far as general troubleshooting goes, I'm unaware of techniques that don't involve either an inspection or altering the operating conditions....specific to a tach, we had a tach tester in the shop.....nothing more than a fixed frequency pulse generator (and I suspect with a voltage regulator). it also allowed assessing the "accuracy" of the tach....but that's another story.....

As far as the guts go, tachs are typically nothing more than "frequency to voltage" converters (behind a low pass filter) with a relabeled voltmeter....they are "high impedance input devices" so they don't disturb the function of the ignition circuits....from my experience, one of the more frequent failure modes alters this feature and impacts engine performance....the last couple I took apart were heavily corroded on the inside....
 
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