What a day... backfiring :-(

Just when it seemed we were getting all our gremlins exterminated, we took the Cheap Hooker out yesterday for a little inshore fishing. About four miles out, we decided to give her a WOT test. We hit JUST over 3k revs, then starboard started randomly popping out the carb. Back her down below 3k, and everything went back to smooth, quiet, and good power. Tried again a mile-or-so later, same deal. We could cruise at 2600 revs no problem, but if we throttled up past 3k, backfires. To note: we have NEVER had the engines over 3k. WOT just would not break 3k before. We have done quite a few things (cable adjustment being a major one) and are now getting 3k at about 60-70% throttle, so we wanted to see what she would do now, but even with more throttle to play with, she gagged at anything over 3k.

Sooo... I have not yet checked the TA on the timing, but I know the initial is good at 10 degrees. On Delco EST's, if I am not getting the full 22 degrees of advance (total 32) out of the distributor, what would cause this? Replace the entire distributor? Coil? Pickup? What actually handles the advance on these?

If the TA checks good, what would be your next suggestions? I will not make it back up to the boat until I return from Kaui, but I would like to have a bit of a game-plan for when I do get up there.

UNRELATED TANGENT: It got better... we moved around, trying fishing a few different spots during the day, then when we decided to call it around 3pm, my brother fired up port, then went to crank starboard... starter free-spooled. He tried again, and it free-spooled again. This time it kept spinning after he turned the key back to off. Tried again, and popped the 50A breaker... sigh... turned out the starter solenoid was shorting out inside and staying engaged. Pulled the starter back at the slip, and the bendix gear was shattered... sigh again... Gonna be fun digging those pieces out of the bellhousing... Just tore down the starter at work today, and the armarture shaft is bent also... What the hell could have caused this? I know the "worst-case scenario", but temps/pressures were good, and I don't even want to THINK about THAT possibility right now! I was in the head and did not notice, but he said it STARTED to crank (he said the tach needle rose up a bit, then dropped as he heard the starter free-spooling). Correct starter, properly aligned... just an old starter had enough? One brass bushing inside is mangled (sits at base of armature shaft).

Update: Showing a buddy the bendix gear, he pointed out how part of the fracture was clean, but the other half of where it broke was dirty; like it was already cracked and just broke the rest of the way. Hoping the shaft just bent from the starter over-revving with no load when the solenoid stuck. I have never seen a starter do this before.

Now for the backfiring...

your backfire is probably the mixture running too lean. Possible causes are restriction in fuel flow and dirty carbs (would also possibly explain the lack of high rpm along with other factors)

Check the plug wiring to see if you have a couple reversed. Don't laugh, it happened to me. The motor started and ran okay--it even planned off--but backfired at higher rpms.

Jeff

don't forget the air valves not opening...same thought as rich - the lean "pop". another (uglier) possibility is a burnt/mushroomed valve.

Check your filters for water in fuel

You should be somewhere in the 4500-4800 RPM range at WOT. The fact that you can't get the engines over 3K means you have a problem. The nature of that problem requires way more investigation. If both engines are doing the same thing performance wise it could be fouled props, the wrong props or bad initial set up of the fuel or ignition system. Whatever the issues it would be good to sort it out.
As to the blown up starter, I had a brand new one do that to one of my Chevy engines, so age is no guarantee. Keep spares aboard.

Lean... possibly, and not doubting it, but please explain how a lean condition could cause a backfire through the carb. I have been reading from other sources, and that seems to be the common recommendation, but being a "new-age" tech, I have honestly not experienced a backfire since my brothers '67 Mustang about 16 years ago. His ended up being a bad mechanical advance.

Mark, I also read the Rochester's air valves are susceptible to sticking closed. I have not visually verified the valves opening, so this sounds like a good place to look first (after getting a new starter on). For the "uglier" possibility, both have nearly perfect compression (147-150), but I have yet to do a leak-down (procrastinating with the installing the timing tapes I already have).

Plug wires being crossed... I will check just to rule it out, but the engines are smooth as silk all the way up to 3k. A couple weeks ago, I started both engines up, and I could "feel" port was missing on one cylinder. Found a plug wire not fully seated on the dist cap. If I could feel one cylinder missing at idle, I am pretty confident I would feel two or more. Again, I will check just to rule it out, and it wouldn't be the first "DOH!" I have had, but I am confident they are correct.

rmm... Filters were just replaced about a month ago, and there was no water in them at all. If the secondary air valves check out as Mark suggested, I will move to the fuel system. On the fuel system... I have read a lot of people blaming anti-siphon valves for similar issues. Ya think? Maybe work slapping a couple new valves on just to be sure? How are they on the wallet?

4500-4800? I thought it was 4200-4500. No? We honestly do not know if it is both engines anymore. That is why we wanted to do the WOT test. The major factor before was the throttle cables not opening the throttles 100%. The bottom of the boat is scrubbed twice monthly, and I personally spoke with the diver last Thursday. He said everything looks to be in good shape except a very small nick in the port prop's leading edge. He suggested just lightly filing it smooth. No fouling going on, though I have no idea the SIZE of the props and whether they are correct for this setup or not (again, hence wanting to do the WOT test).

We are trying to tackle one thing at a time, and at the moment... well after the starter is replaced... the current issue we are focusing on is the backfiring.

Thanks all for the replies! :thumbsup:

I don't know if this is an epidemic but I'm one of the anti-siphon valve guys. They're cheap, less than $20 so I would be inclined to do it myself.

In my case, the boat progressively lost RPM's all season until (after rough weather) my Stb engine stumbled for a while over 2,500 RPMS and finally quit all together the next day.

The starter problem could be unrelated but a cylinder that fired way before tdc could put enough strain on the gear to damage it.....cracked distributor cap or rotor(crossfiring) or worn distributor shaft???...a worn cam or bent pushrod can also cause a backfire through the carburetor at higher loads and throttle openings.

You are going to half to check Total Advance before you do too much more. And perhaps repeat the "pull one plug at a time" test with the engine loaded as Mako Mark suggests. Load it to about 1000 RPMS while tied securely to the dock (or underway if a helper is available). Its pretty tough to consistently detect a change in RPMS at idle by ear.

Hope this is not the case but the backfire could also be caused by a worn cam lobe. I experienced a backfire above about 2500 RPMs due to a wiped cam lobe; at 1200 RPMS the backfire was gone. You can pull the rocker covers and make sure each value is being fully actuated. Mine was a worn lobe operating an exhaust valve, if memory serves.

Best wishes,

Tom

Eric,

AT the time we had the same type of problem, I had chased a few of the same fixes but it was a carb rebuild that eventually solved the problem and unfortunately I do not know what it was in the carb that was a problem. After the carb was installed, it was back to normal, Good Luck

We have done quite a few things (cable adjustment being a major one) and are now getting 3k at about 60-70% throttle, so we wanted to see what she would do now, but even with more throttle to play with, she gagged at anything over 3k.

Click to expand...

It sounds like the carb is now opening all the way when before it was not.
If it is a q-jet, then the mechanical secondaries open and then the air flow opens the air valve at top which lifts the secondary metering rods. If the mixture is too lean, then consider changing rods to thinner ones. BUT, if these carbs were spec'd to the engines, perhaps the fuel flow is not so good going into the float bowl and then it runs lean or misses.

Q-jets also flow only what the engine pulls airwise, so it is like a variable CFM carb. So you may not see the top flap fully open if the engine cant use all that air. That flap also needs to open at a steady rate, not sudden like or it might bog, which is why q-jets have a link from the vacuum pull off to slow down the opening.

See the continuation of this thread here

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