Attn; Johnson gurus, 70HP WOT stumble

[ScottSabre72]

My motor runs ok but when I push the throttle past 4200 RPMs the motor stumbles down to 4000. It will only get back up to 4200 if back the throttle down to the point before the stumble. This problem I have been living with but am tired of it. I actually paid a shop to find the problem and they said it was carbs. They said the compression, fuel quality, spark, and timing were fine. I cleaned the already clean carbs including high speed orfices and passages and the problem persists. HELP!

 

[Joe Reeves]

Re: Attn; Johnson gurus, 70HP WOT stumble

Scott.... to lose only 200 rpm, I would suspect that the throttle linkage somewhere along the line is adjusted in such a manner that throttle butterflys are going beyond the wide open position, and at least one is starting to close down in the reverse direction. On some engines, this is very possible. Let us know what you find.

 

[GT15070]

Re: Attn; Johnson gurus, 70HP WOT stumble

Scott,<br /> I agree with Joe Reeves, check to make sure the butterflys don't go past horizontal. If they don't and everything else checks out then it has to be that your high speed jets have worn a bigger hole in them and are flooding out the engine at WOT. They are easy to take out of the front of the carbs and check. Take them to the parts store with a tooth pick and note how far the toothpick goes into your jets and how far into a new jet. I think yours are .029 or .031.<br />Or just buy them ($6 bucks a piece) and try them, I think you'll be amazed and your fuel consumption will improve also. Good luck and let us know.

 

[ScottSabre72]

Re: Attn; Johnson gurus, 70HP WOT stumble

I just went out and verified that the throttle plates on all three carb open wide and not beyond a WOT. What would cause the orfices to be worn to a larger size, unless someone installed a wrong size. Maybe that is why.

 

[ScottSabre72]

Re: Attn; Johnson gurus, 70HP WOT stumble

All 3 high speed jets are stamped with # 54 on them. Is that the size? What should they be? I am going to replace them be cause I and surely others have damaged the slot on them.

 

[GT15070]

Re: Attn; Johnson gurus, 70HP WOT stumble

Scott,<br /> I'm not sure of the jet numbers but the outboard shop should be able to look them up.<br />As for why they wear out, years of gas, oil and debris running through the soft brass jets will wear the hole a little larger and it doesnt take much. I have a '74 70 Johnson and new jets cured the same problems that are nagging you and I can run twice as far on a tank of gas now. Hope this works for you. Let us know how it turns out.

 

[Joe Reeves]

Re: Attn; Johnson gurus, 70HP WOT stumble

Scott.... #54D is the correct size high speed jet.

 

[suzukidave]

Re: Attn; Johnson gurus, 70HP WOT stumble

What's with these old johnnyrudes and their quirks? Last week it was a 1960's era 90hp with a ten inch pitch prop that I still haven't figured out (1-1 gear ratio I guess??), this week a 70's era 70hp running only 4200 rpm w-o-t. Shouldn't it be closer to 5000 w-o-t?<br /><br />Inquiring minds who know zip about old johnsons would like to know?

 

[ScottSabre72]

Re: Attn; Johnson gurus, 70HP WOT stumble

So, while I wait for new jets, My Seloc book says to set the timing at 3 degrees ATC. My flyweel only goes to zero. What the???? The appendix in that same book says set time to zero degrees. Which is correct?

 

[ScottSabre72]

Re: Attn; Johnson gurus, 70HP WOT stumble

New jets finally arrived. I put them in tonight, put the carbs back on and ran it with the muffs on in the side yard. I ran the motor out of gear of course up 5000 RPM! I think it is fixed, only went to 4200 in or out of the water before. Big difference in hole size between the old and new jets. Mechanic at shop said it isn't true but you could see a difference and verified with a toothpick, I showed him this. I will run it this weekend and check the timing. You guys are awesome. Thank you very much for your info.

 

[Scooby]

Re: Attn; Johnson gurus, 70HP WOT stumble

Scott,<br /> Glad to hear things are looking up. I don't know that I would be reving that old motor up to 5000 rpm in the back yard though, you might end up with more than just carb problems. Save that for the weekend river trip. Unless you have adjusted the nuts on the rubber stop I would not mess with the timing if the shop said it was good.<br />Mechanics never like it when you show them something new. <br />BTW GT15070 and I are one in the same, was out of town and forgot password. Good luck.