Any thoughts on this. 1986 Johnson 90 V4 Acting Crazy

[jimbojonesmc]

1986 Johnson 90 V4 Starts great and idles great. 1/4 throttle good, 1/2 throttle good, Full throttle good for about 2 min then without moving the control
the rpm drops down to about 1/2 throttle. Pull the control to a idle and back up up to full throttle and it takes off like a bat out of hell for about 1 to two min
then drops back to 1/2 rpm. It does this over and over. I prime the bulb and it makes no difference.

 

[emdsapmgr]

You may have a weak fuel pump. If an engine consumes more fuel than the pump can supply, it will run out of fuel. Usually at the higher rpm's where the consumption is the highest. Once the bowl runs out of fuel, the engine will slow, the pump will recover and fill the bowls back up and the engine will take off again. This up/down scenario will repeat again and again. A test is in order. When the engine starts to slow at high rpm's have your buddy constantly squeeze the fuel hose bulb. This will supply extra fuel to the carbs. If the engine runs just fine at high rpm's as long as you constantly prime the fuel hose bulb, you've got a fuel supply problem, which could be a weak fuel pump.

 

[jimbojonesmc]

Have tried that and it didnt help. The fuel pump prob less than 10 hrs on it as i just put it on a few months ago. And even though its new i have checked the out put and all is good with the pump. Whats strange is i can just pull the control off WOT and back up to WOT as fast as i can and it takes off like it should.

 

[jakedaawg]

When it dorps off is it immediate or a somewhat gradual slowing of RPM's? This would indicate wether it is fuel/air or electrical.

 

[schematic]

could also be a partially blocked needle and seat. (common with flaking off fragments of fuel line on older unit)
If applying the enrichment valve (pushing the key) helps, time to clean carbs and change hoses.

 

[jimbojonesmc]

Its like some one flips a switch. Running good at WOT and then bam without a sputter or miss 1/2 throttle and the control is still WOT. And i dont even have to pull the control all the way back to idle i can just pull the control down a little bit and and pusn it right back up to WOT and its takes off at WOT for a min or two

Pushing the key to prime it dosent help
All fuel lines and any rubber hoses are new on the motor. New fuel line/filter/primer bulb new. Fuel tank clean.

 

[racerone]

Run with a timing light hooked up and see how spark behaves.--Possibly a broken wire somewhere.

 

[emdsapmgr]

Also could be another type of fuel delivery issue. Crud in the anti-siphon valve, defective ball valves in the fuel hose, a leak in the plastic fuel pickup tube inside the tank, air leak at a fuel hose connection, even at the pump, etc. Any of these can reduce the flow of fuel to the pump/engine. When you said you installed new fuel lines to the powerhead. Did you purchase the 329614 fuel hose ( pump to manifold) from Bombarier? If you used bulk hose, it tends to kink when bent at sharp angles. When that happens, kinked bulk hose can result in a fuel restriction at the bend and cause the problem you noted.