70HP Johnson WOT stumble

[scottsabre]

Motor is a 1978, I have rebuilt fuel pump, adjust throttle plate to WOT when at full throttle on control, set the roller on the cam correctly, base timing good, cleaned fuel sys. Adj. highspeed timing 1 degree +&- It does well up to 4500rpm, higher than that it stumbles down to 4300 until I back off throttle some. Still have inch of throttle left that can't be used. Plan to shorten 9ft fuel line and replace bulb. What else? 15ft center console, heavy old fiberglass boat. HELP!!<br /><br />------------------<br />1978 70hp johnny

 

[JB]

Re: 70HP Johnson WOT stumble

Howdy, Scott. What happens if you pump the ball when it is bogging at w.o.t.?<br />JB

 

[scottsabre]

Re: 70HP Johnson WOT stumble

I have not tried it, bulb is in the back of the boat. That would tell me if the motor was starved at top end, right? It also wants to die when fully warmed up at idle when in gear. If I bump the choke, it does fine for 10-15 seconds. Real pain when positioning at dock or for the trailer.

 

[kalloway]

Re: 70HP Johnson WOT stumble

I think you have three carbs on the engine and if you haven't cleaned them, you should. Both problems, high speed and idling, could be carburetors. The lowest one will normally be the worst one. Pull it and check it first. I'm assuming you have new spark plugs.

 

[foosballpaul]

Re: 70HP Johnson WOT stumble

I have a 1975 three cylinder 70 hp Johnson and recently went through similian problems. I replaced the needle valves and seats on all three carbs and solved my problems. The lower carb is going to be the most fouled and gummed up. You should be able to get the needle valves and seats from local parts dealer for a bout 4.00 each.

 

[scottsabre]

Re: 70HP Johnson WOT stumble

I will check out the carbs and squeeze ball. Thanks to all of you. Will post with results.

 

[scottsabre]

Re: 70HP Johnson WOT stumble

OK, I pulled all three carbs. They were so clean you drink out of the bowls. Looked at the reeds and they looked perfect. I had run Johnson throttle cleaner a few months ago. What else should check for with the WOT problem?

 

[scottsabre]

Re: 70HP Johnson WOT stumble

If I change from 5/16 to 3/8 hose and a new squeeze bulb, would it help? I saw Tempo make one that claims 14% more flow than OEM. A mechanic said to stay with OEM.

 

[freddie]

Re: 70HP Johnson WOT stumble

Hi!, Scotsabre. A rule of thumb when I'm diagnosing a problem is :#1 Make sure all the adjustments are to factory specs,Compression,Air/fuel adjustment needls,(if applicable),etc. Then try to isolate the problem. Is it one cyl. Cyl drop during failure.Changing parts and fixing things that dont need it can get costly.

 

[scottsabre]

Re: 70HP Johnson WOT stumble

As you can see from this thread, I have check to make everything is in spec and synchronized. I have not been able to take the boat out since rebuilding the carbs. I did shorten the fuel line and moved the tank to the back of the boat. If I still have a problem, I think I should focus on ignition. Compression is healthy on all 3 cylinders and spark is strong while cranking.

 

[scottsabre]

Re: 70HP Johnson WOT stumble

What a pain to find this old string! Took the boat out first time since last posting. With clean carbs and new needle/seat I though it would run better. Same old problem. Stumbles at wide open, back of throttle a little and runs good. Dies when hot at idle, stumbles to get on a plane. Mid range is fine. HELP!!!

 

[clanton]

Re: 70HP Johnson WOT stumble

5/16 fuel line ok. Timing in my book is idle timing at TDC, 17 degrees at WOT. Check stator for melted epoxy, check charge coil and power pack output. If checks ok cold, warm it up recheck.

 

[Boatman30]

Re: 70HP Johnson WOT stumble

This sounds dumb, but I ran acrossed this problem with a 50 hp Evinrude I was repairing ofr a guy. I went through the carbs, basically all of the above that you have done. I foound the flywheel key in the crankshaft partially sheared (bent).<br />Replaced that and it ran fine. I threw the WOT timing off a bit.<br />Good luck.<br />Pat

 

[leafs]

Re: 70HP Johnson WOT stumble

were would the flywheel key be on a 1974 70hp johnson