Still no power.....at a loss

[mstritt]

I have a 1983 Johnson 70 HP which I have been working on to no avail. It starts and idles fine but won't reach RPM when under power. Here's what I have done. I've rebuilt the carbs(even rechecked them), new fuel pump and fuel lines, checked the compression, and put in new spark plugs. When the boat is in the water it runs smooth just won't go full power. I also tried pumping the bulb while running and that makes no difference. Any help would be greatly appreciated!!<br />Thanks<br />Martin

 

[Joe Reeves]

Re: Still no power.....at a loss

Mstritt.... I'm assuming that you've checked the compression and spark on all three cylinders, and found the compression to be approx 100 psi and even on all cyls.... and the spark will jump a 7/16" gap on all cyls with a strong blue flame?<br /><br />If not, do so now. If you have done the above, and the throttle butterflys are opening to a full horizontal position at full throttle, I would suspect that you did not remove and clean thoroughly the fixed brass high speed jets from the center of the float chambers.<br /><br />If you have cleaned those fixed high speed jets, check to see that the timer base under the flywheel is sliding all the way against the full spark advance screw at full throttle (the one with the black rubber bumper stop. If the timer base sticks, the engine will be running with retarded spark.

 

[mstritt]

Re: Still no power.....at a loss

Thanks I'll give that a try tomorrow. I made sure that all the jets were clean when I rebuilt the carbs, its just strange that it runs fine just won't power up.<br />Thanks again

 

[mstritt]

Re: Still no power.....at a loss

Got another question, sorry I forgot to ask when I replied. Is it possible that a bad power pack or ignition coil could cause the problem? Would this lead to a spark plug that fires but not as strongly as it should thus lowering the RPM?<br />Thanks Again<br />Martin

 

[Joe Reeves]

Re: Still no power.....at a loss

Mstritt.... Any component in the ignition system could affect the spark. That's the reason for the spark test I mentioned. However a powerpack in itself would not create a weak spark.... it could create a no spark condition though. The weak spark situation is usually caused by a failing (melting or cracked) stator, or a bad coil.

 

[mstritt]

Re: Still no power.....at a loss

Thanks for the responses Joe! One final question, I think that the boat may have a weak spark on one possibly two of the cylinders(they all do fire), would this cause the motor to not develop full power?<br />Thanks in advance<br />Martin

 

[fireman 757]

Re: Still no power.....at a loss

Yes it could. Coils can be fine at low speeds but break down and short to the block under a load. If you can run it in a barrel or on the trailer in the dark you might see a blue spark arcing to the block. Are there any cracks in the outside of the coil?

 

[mstritt]

Re: Still no power.....at a loss

Dear Fireman<br />Thanks for the response, I do see an arc going to the block when I pull the lead off of the spark plug. What do I need to replace to fix this?<br />Thanks<br />Martin

 

[fireman 757]

Re: Still no power.....at a loss

IS the spark jumping from where the plug goes in to the block or from the coil to the block. It should jump from where the plug goes in. This is what you want it to do. If all cylinders jump to the block (check it with all plugs pulled as it cranks easier) then you have good spark and there is nothing there to fix. If it does not jump then swap the coils and see if the problem goes with it. If it does then change that coil.