Backfire/misfire thru carb

[Beno]

I was getting my old engine going and inspecting the reed valves and such, as I was working on it I caught a few backfires with exhaust coming out the carburator.
this is something I’ve seen occasionally over the years and never thought much about it.. but then as I was thinking about the design of the reed valves …..

the reed valves close to stop air/fuel/whatever to pressurize the crankcase.. how could misfire exhaust make it back thru the reed valves? wouldn’t they be closed?

Perplexed???

[Ronaldu]

Wires crossed? Wrong cylinder firing ? Maybe

[oldcollector]

This sounds like nothing more than a too lean a carburetor mixture adjustment,which causes a spiting back thru the carb.

[Beno]

Ok yeah, I’ll check that.. and try opening up the low idle pin a 1/8th turn maybe?…. but the base question still perplexes me:

if the reed valves close in order to stop pressurized gas, or pressurized air, or exhaust, or flame… how could there be a backfire thru the carb if these reed valves are intact and functioning properly??

its like seeing a prisoner get out of his cell, run across the road. even tho the cell door is closed.
?

[need2fish]

To answer the reeds question, yes the reeds for the chamber on the downstroke (compression) will be closed. The reeds for the chamber on the upstroke will be open (inhalation). So, it if there is an ignition source, the fuel could ignite in the crankcase and blow back.

Reeds can weaken and be ever-slightly open.

1) Blowback could originate in the combustion chamber (at close to TDC) and make it past the rings as the reeds are open at the time just before the piston descends.
2) What you are seeing is vaporized fuel, not combusted fuel when an engine sneezes from lean fuel-air mix.
3) Reeds are bent
4) Spark plug leads are reversed. This means the spark occurs while the exhaust ports are open.

• This reply was modified 4 years, 10 months ago by need2fish.

[fleetwin]

What year/brand/horsepower engine do you have?

[outbdnut2]

On an old 4.2 HP Champion model 1J, there has always been a throttle setting a bit above idle where the carb can’t decide between the high and low speed jet where it repeatedly backfires. I just move the throttle a tad up or down and it goes away. All motors have a similar rough-running throttle spot, but that Champion is the only one I’ve seen repeatedly backfire.
Dave

[Beno]

140hp (.030over I believe) V4 OMC 1978

[fleetwin]

OK, well the place to start on this engine is proper syncronization and linkage set up… The engine will never idle properly, and will sneeze and cough if its throttle plates are not fully closed at idle. Sounds like this engine has been rebuilt before, do you know its history? When was the last time it idled properly?

[billw]

Those V4s will sneeze at the drop of a hat, if everything isn’t right, for sure. Fleetwin is right (as usual, of course) in starting with a careful link and synch. Next step in my book would be a thorough carb cleaning. IF those two things don’t cure the problem, I guess I would start looking for a leaking crank case sealing job, from the rebuild….

I think the answer to the original, theoretical question, is that when there is a back fire event in the crank case, it takes a few fractions of a second for the reeds to close and stop the backfire, so you will see a bit of a blow-back out the carb. Be thankful for the reeds. If you had a rotary valve engine, you’d have a good chance of seeing flames come out.

Long live American manufacturing!

[Author]

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