40 hp rpm surge

[Tcarazzai]

I have a 97 johnson 40 hp that starts, idles and runs perfect until i get to about 3/4 of full throttle. After that the rpms surge dramatically. It wont stall but goes from 25mpy to 10 and back again repeatedly. I checked the carbs and filter. No success there. Any suggestions?

 

[oldboat1]

Disassemble and clean carbs, special attention to high speed jets. (Soak, carefully use soft wire or monofilament in all openings, finish with carb spray.)

 

[Tcarazzai]

Thanks, but i already did that. Doesnt seem to be missing, so i dont think its spark either. Im stumped...

 

[oldboat1]

Testing on the boat, sounds like. Is motor mounting height an issue (i.e., too high?)

 

[lindy46]

Dropping a cylinder?

 

[Tcarazzai]

Motor heights fine. If it was dropping a cylinder wouldnt it run rough all the time? I have a clear view fuel filter on the engine. It fills and empties while running. Should it retain fuel in the tube or is it only fed the fuel needed by the motor?

 

[oldboat1]

Can give partial answer -- cyl drop normally happens at speed. Can be caused by overheated ignition parts. Under the flywheel power packs can be a problem, if that applies.

On the filter -- problem might be the filter itself. I would try running without it (thinking you have added an inline filter.) I THINK the fuel supply should be constant, not what you describe. (Thinking of the filter bowls on older models, and believe they ran empty, or seemingly empty, but with full fuel flow -- might need correction on that.)

 

[Tcarazzai]

I think the supply is constant but steps up at times, and not necessarily with the throttle. The engine only has 150 hrs on it. Its gotta be something stupid. Atleast thats what im telling myself...

 

[oldboat1]

Maybe tank venting issue, or connector at motor not fully secured. Did you add the filter, or was that original equipment?

 

[Tcarazzai]

Original. Its just a mesh tube in a glass cylinder. I blow it out when it looks dirty. I did try running the motor with the fuel cap open. No difference. Im guessing you would have to do a compression check for the cylinders?

 

[oldboat1]

Two basic checks -- the compression check you mentioned, and measure spark with an open air adjustable tester (tests ignition strength).

 

[Tcarazzai]

The primer solenoid had a little dirt in it that i cleaned out, but the motor starts right up so thats not likely the issue...

 

[Tcarazzai]

Those test are a bit beyond my experience, but theres always youtube. Ill givem a shot. Really appreciate everyones input.

 

[oldboat1]

Tester is about $10 at an auto parts store, and they might loan out a compression tester. (Might post a model number so everybody is on the same page.)

 

[Tcarazzai]

I have a tester.ill sees if i can get a loaner compression tester. The model is j40tleuc. Better late than never. Ill let ya know how it works out. Thanks again

 

[Crosbyman]

sucking air ?? bad orings in the connectors ?

fuel pump bad ?? can't keep up with demand at HS did u try pumping the bulb when it starts acting up ??

crumbling fuel hose… ? the one's with a sleeve liner were known to deteriorate .... try another hose / tank