1990 225 missing

[Out of Bounds]

ok we have 1990 225 carb /with a computer contoled motor in my shop that we can not figure out.under excel only it drop cynlnders not the same one jumps around. it wont do it on the dyno or running it under load on the trailer while in the water, the only time is while under load going on plane. we have replaced trigger, plugs, plug wiers, plug wire end , fuel lines,any body have any thing else that we can test i feel like wee looked @ everything. there is 3 mech that have 20 years plus it has us stumped????

 

[rodbolt]

Re: 1990 225 missing

what test wheel are you running it on?
what model peak reading multimeter are you using?
do you have the correct test harness's for pulser coil and stator inputs as well as CDI outputs?

 

[backyard mechanic]

Re: 1990 225 missing

So allow me to recap here... this only happens under exceleration and only under a load when on a plane. Sensors tell the computer to richen/lean out the engine as well as when to fire the plug (timing aspects). You've replaced the trigger and wires. Hmmm.

That the intermitent miss jumps around mystifies me... how do you know that the miss jumps around? Any chance you could run this at night and watch for ignition leaks? Is there a specific RPM range that it does this? Any wires that are frayed, resistance high etc???? I suppose you've looked that over already.

 

[iammarcuse]

Re: 1990 225 missing

Have you tried it with the knock sensor disconnected? I'm no expert but have heard these motors have been known to detonate going on to plane. The computer retarts timing. Manual says 89 octane minimum. I have the exact same motor and it sometimes detonates in quick deceleration.