1981 35 Hp WOT engine "bumping"

[insanclown]

First a little about my motor(j35rcig). I've owed it for about 5 years now, the block and tiller is from a '87 30 hp w/electric start(j30tecub). It also uses the original '81 35hp carb with butterfly choke.

The motor starts/idles/runs great except when the throttle is over 85%, when I open it all the way up it "bumps" or maybe misses probably once a second at WOT. I've lived with this for 5 years because running smooth at 85% throttle moves me up river just fine. What can I do to have a smooth running motor and reach those upper rpms?

I'm not much of mechanic but both cylinders have 130 psi, and the spark plugs looked decent with some carbon fouling and gapped correctly at .30 and I went ahead and replaced them..

Could it be the high speed jet clogging or maybe ignition problem? Thanks to anyone who can help

**Edit** added video in post below

 

[o/b tech]

Re: 1981 35 Hp WOT engine "bumping"

Does the bumping sound like it is trying to jump out of gear as when the clutch dog teeth and forward gear clutch teeth start to round off it will start to do what you are describing. From your desription of the plugs and cylinders, sounds like the powerhead is ok. Need a better description of what the "bump" actualy is.

 

[insanclown]

Re: 1981 35 Hp WOT engine "bumping"

Does the bumping sound like it is trying to jump out of gear as when the clutch dog teeth and forward gear clutch teeth start to round off it will start to do what you are describing. From your desription of the plugs and cylinders, sounds like the powerhead is ok. Need a better description of what the "bump" actualy is.

That's what I thought it was too before last year. But I swapped my lower unit last year for an outboard jet... and the problem persists.

Once I turn the throttle to about 90% open it will thump like a cylinder missing and the engine will jerk intermittently about once every 2 sec... open it up more and it jerks the engine about once every second.. It's still able to pick up some rpm but It's not purring like should.

I need to get a tach to see what rpm this actually starts to occur at.. The motor is kind of a frankenstein and I've had to research what parts the previous owner used.. He may have used larger jets in the carb.. is this possible? and could it be messing up the mixture at wot?

 

[tashasdaddy]

Re: 1981 35 Hp WOT engine "bumping"

try adjusting your shift linkage. if it were carb or spark problem it would show up at lower speeds. increase in speed and frequency, says, lower unit.

 

[insanclown]

Re: 1981 35 Hp WOT engine "bumping"

try adjusting your shift linkage. if it were carb or spark problem it would show up at lower speeds. increase in speed and frequency, says, lower unit.

I have a jet drive lower unit.. So no shift linkage or gears to cause this problem. The problem existed when I had a standard gearcase lower unit too.

Anyways I made a few videos demonstrating what I'm talking about. Thanks to anyone who can help

http://www.youtube.com/watch?v=gaeN85nQo7w
http://www.youtube.com/watch?v=4FPm_o3WERU
http://www.youtube.com/watch?v=AtwIpa9mhO8