81 140 evinrude jet motor

[Chris Roberts]

Hallo,
i just rebuilt this motor and so far i'm disappointed. It will only turn 4700 with a 7 3/8"D Impeller. Any ideas??

[mike1]

Is the motor reaching full ignition timing I think its 28 degrees btdc if I'm not mistaken, were the carburators cleaned and synched and do they open up to full throttle with the control and how about spark, do all coils have good spark, also check compression as well, just some things you can initially check

[mike1]

I have johnson 90 with jet and it runs great with a 7 3/8 d, I don't have a tach but it truely sounds like more than 5000 rpm at full throttle, your engine shouldn't have a problem spining that impeller

[Chris Roberts]

Yes it spot on 28. Cleaned and in sync, and the butterflies lay flat. I have a 79 100 Also it seems to be as strong as the 140????

[Chris Roberts]

I have johnson 90 with jet and it runs great with a 7 3/8 d, I don't have a tach but it truely sounds like more than 5000 rpm at full throttle, your engine shouldn't have a problem spining that impeller

No idea what the issue is.

[mike1]

What are the compression numbers and how about the spark? Does it accelerate fast out of the hole or slowly gather speed? And are you sure the tach is correct?

[Chris Roberts]

Mike1,
I'll have to check the compression tomorrow. As for the spark it's good on all 4 holes. It accelates pretty good. Pretty sure the tach is accutate, but i'll verify it with my hand Held this week.

[EMDSAPMGR]

What's the throat size on the carbs?

[Chris Roberts]

Compression is 130 PSI all holes

[Chris Roberts]

1 3/8" carbs. Boyseen reeds, cleaned up the cases,and a 77 140 tuner, 78 model heads.

[mike1]

Sounds like a nice set up and good compression number, what do the spark plug insulators, look like...tan, or kinda white, or very dark after you run it? Also how is your fuel system...fuel pump working well? And maybe the fuel tank venting system is closed or restricted. The main fuel line unrestricted and adequate size...just more stuff the check

[Chris Roberts]

A little lighter than a original hershey bar. So about as good as they could be.

[Chris Roberts]

All new fuel lines and pump. No kinks that I have found anyway. Doesn't make sense it just won't turn anymore with this impeller. Maybe I'm expecting to much!!!

[mike1]

Hmmmm...what kind of boat is the engine mounted on and another thing is the impeller 4 blade stainless or 3 blade aluminum?

[Chris Roberts]

Bass tracker with a 3 blade aluminum(110 to 140) I also have a 4 blade SS v6 it will turn 4600. They are with in 1 mph of each other. I also have a 4 blade SS (85 to 100) impeller it will turn 5700 but 3 mph slower.

I have been busy!! This is driving me crazy!!