2 stroke 70 hp that hesitates at acceleration

[Blueboating]

I have a 70 hp Johnson 2 stroke 1987.
When trying to accelerate a 20 ft boat from idle, the rpm increase to about 2500 rpm and no more. If more throttle is given , engine hesitates , slows down and finally dies. If I choke it, just a few light pushes and no more (solenoid type choke) , it accelerates beyond the 2500 and then gladly continues without choke. I can then increase and decrease throttle up to max and down to about 2500 rpm without choke. But as soon as I get below about 2500, I can't accelerate again without choke. So we assume the engine does not get enough fuel around the 2500 rpm but we can't figure out why. The boat is pretty flat bottomed, and max resistance /half planing is not reached before the engine dies. So it dies before max load condition.

If we run the engine in neutral, it idles nicely and responds fast to throttle without hesitation. Carbs are cleaned, spark is strong timing seems correct, fuel pump is new.

Thanks in advance!

 

[ThomW]

I know you said carbs are cleaned...but that seems like it could be a low speed jet that is gummed up...or a float that isn't set correctly.

What do you mean by timing seems correct?

Have you checked compression?

 

[Blueboating]

Thanks for feedback. We are 99 % sure carbsvare clean, we took them apart 3 times. Compression equal on all, vacuum at carb inlet also. Timing we checked with a lamp, goes from TDC to about 18 degrees advance at max.

Floats we are not as certain, found it hard to say what is correct adjustment or not. oAny advice? How could it cause this issue? If fuel level would be too low how come it runs well once we passed that 2500 rpm? We are frustrated and would love to solve this....any ideas very much welcomed!

 

[flyingscott]

Running in neutral means nothing. Does spark jump a 7/16 air gap? Carbs opening all the way? Have you tried a different tank and hose? Link and sync done? Roller on the cam have it's outer layer? What is your compression? Timer base moving freely?

 

[clemsonfor]

I have a 70 hp Johnson 2 stroke 1987.
When trying to accelerate a 20 ft boat from idle, the rpm increase to about 2500 rpm and no more. If more throttle is given , engine hesitates , slows down and finally dies. If I choke it, just a few light pushes and no more (solenoid type choke) , it accelerates beyond the 2500 and then gladly continues without choke. I can then increase and decrease throttle up to max and down to about 2500 rpm without choke. But as soon as I get below about 2500, I can't accelerate again without choke. So we assume the engine does not get enough fuel around the 2500 rpm but we can't figure out why. The boat is pretty flat bottomed, and max resistance /half planing is not reached before the engine dies. So it dies before max load condition.

If we run the engine in neutral, it idles nicely and responds fast to throttle without hesitation. Carbs are cleaned, spark is strong timing seems correct, fuel pump is new.

Thanks in advance!

Have you cleaned the carbs? Are you sure you have spark on all cylinders?

 

[909]

Your lean condition is likely solved with a carb rebuild.

 

[Blueboating]

Running in neutral means nothing. Does spark jump a 7/16 air gap? Carbs opening all the way? Have you tried a different tank and hose? Link and sync done? Roller on the cam have it's outer layer? What is your compression? Timer base moving freely?

Thanks. Yes to good spark, carbs opening full. Tank and hose is new. All carb throttles open and close in sync . Advance timing linkage moves easy, roller ok. Compression is 9.5 to 10 bars on all 3 cylinders. Fuel pump is new and feeds all carbs evenly. Reed valves are good, gaskets carb/inlet new most hoses too.

 

[Blueboating]

Your lean condition is likely solved with a carb rebuild.

So your suggestion is that I don't get enough fuel to go from idle to high speed, due to limitation in lean fuel supply? Makes sense as a bit of choke helps to pass that stage and then all is running well.

But these carbs are really simple, the idle circuit has no adjustable needle valve and all is really clean ( visual and compressed air passage ok). What could be the issue?

 

[Blueboating]

Have you cleaned the carbs? Are you sure you have spark on all cylinders?

Yes, cleaned them twice, took them down 3 times, since it seems to be the fist thing to do apart from checking spark (spark jump test ok at all rpms)

 

[909]

When you reuse gaskets, you squeeze them thinner with each disassembly / reassembly.

Air gets in (where it's not supposed to), then when you demand fuel it sucks in air and starves while choking corrects the lean condition momentarily.

You take it apart to clean it again and nothing changes, or it actually gets worse with every cleaning.