HELP! Aftermarket piston and cylinder with different timing?

[Yellowking86]

I have a 3120 I replaced the crankshaft, bearings, seals and put on a hyway P/C and when I put it back together it backfires and dies. I checked the flywheel key it was fine. I tried a new carb same thing. I removed the kill switch didn’t help. I tried 2 other known good ignitions. It passed vac and pressure test. I tried another flywheel that I know works. I’m stumped??? I’ve gotten it to idle a couple times and then it backfires and dies. If I wait a day and try to fire it without choking it it just backfires. Anyone have any ideas? I’m pretty new to building saws and I’m ready to throw this thing out the window.

 

[Shane¹]

I have a 3120 I replaced the crankshaft, bearings, seals and put on a hyway P/C and when I put it back together it backfires and dies. I checked the flywheel key it was fine. I tried a new carb same thing. I removed the kill switch didn’t help. I tried 2 other known good ignitions. It passed vac and pressure test. I tried another flywheel that I know works. I’m stumped??? I’ve gotten it to idle a couple times and then it backfires and dies. If I wait a day and try to fire it without choking it it just backfires. Anyone have any ideas? I’m pretty new to building saws and I’m ready to throw this thing out the window.

Did you use sealer on the base? Have you checked to make sure the impulse port isn't blocked.
Aftermarket crank? Is the keyway slot cut in the same spot as the original?

 

[Yellowking86]

No sealer on the base, I will check to see if anything got into the impulse port thanks for the suggestion.

 

[jacob j.]

Is the ignition original to that saw? The 3120 used several different flywheel/ignition combos and most are rev-limited.

You can't mix and match some of the ignitions and flywheels. It sounds like an ignition timing issue to me.

 

[Yellowking86]

Is the ignition original to that saw? The 3120 used several different flywheel/ignition combos and most are rev-limited.

You can't mix and match some of the ignitions and flywheels. It sounds like an ignition timing issue to me

 

[Yellowking86]

Not sure if it’s original, the saw was running with that coil when the crankshaft rod bearing seized.

 

[Mad Professor]

Is the crank OEM?

 

[Yellowking86]

Is the crank OEM?

I believe so, I got it from a member here. Idk if there were more than one and maybe the timing is different. I do have a timing light but I’ve never used it I guess the next step is checking that.

 

[Yellowking86]

Did you use sealer on the base? Have you checked to make sure the impulse port isn't blocked.
Aftermarket crank? Is the keyway slot cut in the same spot as the original?

Just pulled the cylinder and checked the impulse it’s clear.

 

[Shane¹]

Just check where the keyway is cut in the crank at top dead center if it is the same as the original your ignition timing should be the same as it was

 

[Yellowking86]

Just check where the keyway is cut in the crank at top dead center if it is the same as the original your ignition timing should be the same as it was

I’ll check that tomorrow thank you

 

[huskihl]

I think the crank timing is the same on all 3120s. The flywheels are clocked differently. Fw and coil need to match

 

[Yellowking86]

It does the same thing with the coil that was on it when it was running before the crank broke.

 

[backhoelover]

Need to check your spark at no less than 6mm gap, if you can jump a 6-8mm gap the coils is putting out enough volts to firing the plug under compression. have you tried changing the plug? autozone used to sell a adjustable spark tester you can adjust the gap on for cheap, just have to be careful as its open to atmosphere.

 

[Yellowking86]

Need to check your spark at no less than 6mm gap, if you can jump a 6-8mm gap the coils is putting out enough volts to firing the plug under compression. have you tried changing the plug? autozone used to sell a adjustable spark tester you can adjust the gap on for cheap, just have to be careful as its open to atmosphere.

Yes I’ve tried a couple new spark plugs. I’ll try what you suggested.

 

[Ketchup]

Outlier suggestion, but sometimes they’ll backfire if the muffler has the right amount of unburnt fuel in it. It might be flooded. I’ve had that when I put too much fuel down the throat to get a new build started.

 

[Yellowking86]

I’ll look into that thank you

 

[Yellowking86]

I think the crank timing is the same on all 3120s. The flywheels are clocked differently. Fw and coil need to match

This looks like an old one

 

[Yellowking86]

This looks like an old one

 

[huskihl]

This looks like an old one

Old ones are black, new ones are green.