What is causing this?

[Bear700]

I have a 1979 55HP Johnson. I got the boat about 1 month ago. It did run, however, I believe it was only firing on one cylinder at the time. On occasion, the other cylinder would "wake up". I just rebuilt both carbs, and put new coil and wire kits on both cylinders. When I pulled the boat out and started advancing the throttle, everything seemed great. Boat got up on plane, and we were on out way. Well, about after 1 or 2 minutes, the boat started to surge, almost as if someone were revving a motor at a stop light. Just a slow rythmic rev. I backed off the throttle....long story short....if I come way back on the throttle, the boat will eventually even out, when I get back on the gas though, the same rythmic pulsing begins. I thinks that there must be a fuel issue somewhere....did I get the floats wrong? I checked the pickup line...no debris. Checked the fuel pump....screen seems clear....any help here would be welcome!

 

[noelm]

Re: What is causing this?

before you go pulling anything else to bits, try the very easy non invasive stuff first, like a complete fuel tank and hose with fresh fuel, do a compression test, pull the plugs and see of they look the same, the more you pull to bits on suspicion, the more chance there is of introducing another unknown fault.

 

[jjacobs007]

Re: What is causing this?

vent hole open and clear on gas tank?check primer bulb.

 

[jonesg]

Re: What is causing this?

compression test
spark gap test.

First find out if its fuel delivery or ignition.

 

[Bear700]

Re: What is causing this?

Sorry, I should disclosed a bit more before I told the problem. I have drained old fuel, added new fuel, filter is clean on the pickup...checked compression (both are good) new spark plugs both gapped to 30....fuel screen is clean on pump.

 

[jonesg]

Re: What is causing this?

get a $10 spark GAP tester from any auto store, set the gap to 3/8th inch, remove other plugs, ground them and crank it over, the spark should jump the wide gap with an audible crack and blue spark.
If not ...theres a failure in the ignition etc.

if it passes the spark gap test its probably the carbs, get rebuild kits.

 

[Fl_Richard]

Re: What is causing this?

Engine reving while in gear like cars racing at a stop light.... Spun prop hub?