1989 40hp 3 cyl Yamaha idle issues...

I just recently bought a used 16' aluminum semi-v that has a 1989 Yamaha 40ELF 40hp 3 cylinder 2 stroke on it. It looks like brand new inside and has never been even cracked open from the way the gaskets all look. It sat for about a year without being run, so I rebuilt the carburetors, replaced the fuel filter, and replaced the plugs. Initially it was having a problem to where it would idle really rough, then make a squeak noise and cut off.

I had a mechanic friend come over and tune the carburetors and now we have it idling smooth as silk, but still every 20 seconds it will just die down and cut off. It doesn't seem to be an issue with the timing or spark, and the carburetors are not being starved since their bowls are all full, and they're not overflowing to where they would blow gas out the back of them. It doesn't seem to be a seal issue or bearing issue either as we thought originally because I ran it almost wide open for about 10 minutes in gear and it would have seized up for sure if that was the problem, but it runs smooth as can be at higher RPMs without hesitation or anything.

Does anyone else have any ideas? Thanks...

[Skeeter50]

Remote Control

Had similar issues this year with my 89 50HP. Cranked good, ran at fast idle for 15 seconds and died. If I placed into gear immediately, it ran normal up to WOT, but as soon as I brought it back to idle, it died. Tried everything at the engine with no luck.

Mechanic found a short in the remote control (Type 703) (by accident I might add). I guess a tightly packed box and old wiring and maybe some friction in there caused this. I had changed the neutral switch earlier and I may have actually caused it (and probably did).

This may not be your issue, but something to keep in mind if you've tried everything else.

Good Luck

I may check that out tomorrow when I get a chance to work on it some more. That would make sense, but isn't the neutral throttle control all mechanical? Or was it an electrical wire that got pinched/shorted? And is there a "neutral switch" as you mention, or is that just the throttle lever?

[jb123]

IF you think it is the switch, simply unhook the harness after starting.

[Skeeter50]

Remote Control

The 703 remote control has a mechanical side in which movement of the throttle lever mechanically moves the control linkages. But also inside are a fair number of wires con*****ing the ignition switch, neutral switch, kill switch and choke (if equipped). One of my wires must have been pinched/chaffed or otherwise shorting out. The neutral switch is a small plastic switch that is depressed when the throttle lever is in the 12 o'clock position. On mine, you hear a "click" when the throttle lever depresses the neutral switch. If the neutral switch is not depressed my engine will not even turn over.

Be careful if you remove and open your remote control. Wire management in getting it closed again is quite challenging (which probably led to me pinching a wire). I would not attempt it without a manual. Most manuals have pretty good photo's of the internal remote control workings and how it all fits together.

[Operaman]

IF you think it is the switch, simply unhook the harness after starting.

This is how to roll. Start the engine up and disconnect the wire harness, if you can get at it.

Taking apart a control box is not fun.

[protagy311]

fix for 89 40

I've fixed two motors that make a squeak noise and cut off. What I've found is that the bottom two crack seals go bad. Pull the powerhead and replace seals and squeak noise will go away.
Let us know how it goes....

I've fixed two motors that make a squeak noise and cut off. What I've found is that the bottom two crack seals go bad. Pull the powerhead and replace seals and squeak noise will go away.
Let us know how it goes....

I'm betting on that. We worked on it for a few more days troubleshooting the timing and coils and everything, and after we sprayed some CDC down on the bottom of the powerhead where it goes into the drive unit, the motor dies down which means there is a leak down there. We checked the compression again and it is spot-on, but the bottom cylinder just doesn't seem to be firing at all, even though it has fuel and spark. It must be sucking in air from a leaky lower crack seal and killing that cylinder. Time to pull the powerhead...

On a side note, how hard is it to replace those seals? Do you have to pull the crankshaft, or is it just on the bottom of the powerhead where you can get to it pretty easily after pulling it? Thanks for the heads up!

Also, on a side note, what does that plastic black box do that is attached to the timing/throttle linkage? It says Mitsubishi Electric on it; does this have something to do with timing/ignition?

OK, I pulled the powerhead today and there is a little funnel-shaped aluminum part on the bottom of the crankshaft with an O-ring and 2 seals inside it. Are these the crack seals? They look fine, but I think this is where the bottom cylinder is sucking in air from.

[protagy311]

take small funnel shaped part off. I used a small screwdriver to knock the seals out with. I heated the end of screwdriver and bent it a little bit to get the angle I needed.

Well, after pulling the powerhead and replacing the crack seals and gaskets, it is still doing the same thing. It will run for about 15-20 seconds just fine, and then just die out. I'm at a loss as the only thing I can imagine still causing this is a crack in the block, but when we had pulled it we examined it pretty closely and didn't find anything the naked eye could see.

Does anyone have any ideas? I can't come up with anything else and neither can my mechanic, unless it is just cracked somewhere towards the bottom of the block where we can't see it. I'm about to give up and sell this thing off for parts.

On a side note, do Yamaha and Johnson/Evinrude outboards have the same steering bracket? The reason I ask is that there is a year 2000 50hp Johnson for sale near me and I'm considering re-powering with that but don't want to have to replace the steering.

[jb123]

i was very curious to see what the outcome of the crankcase pressure would be.
still sounds like you have a carb issue to me.

idle up to about 2k on hose and pull each plug wire at a time ensuring power loss, then pull airbox and at same rpm choke each throat with your finger listenin for a HELP in performance.

ive an 89 pro-v 150 and have had to go thru the carbs up to 3 times to get them right.

Well, I did some more troubleshooting today. There is a black box that says Mitsubishi electric on it that I'm assuming controls the timing. I have it adjusted to where it will run at idle now, but for the first 15-20 seconds, it will idle higher, then die down to idle. It seems all 3 cylinders fire until 15-20 seconds, then the bottom cylinder will quit and it will die down, hence the conditions described above.

With it running for a while at near-idle, maybe about 2000RPM as suggested, I pulled each plug wire. When you pull the top and middle, the engine dies down, but when you pull the plug wire on the bottom cylinder, it doesn't make any difference. All cylinders are getting spark (you can see the spark when you hold the plug close enough). The bottom spark plug obviously seems to be a little more damp than the 2 above, but still has a good-looking burn to it. Also, each cylinder is able to crank the motor and run all by themselves (only 1 spark plug wire plugged in); the bottom cylinder will run for 15-20 seconds and cut. The other 2 have trouble, but seem to be able to run longer by themselves. We have already troubleshot the coils and plug wires, and that's not the problem. The carburetors have all 3 been rebuilt twice now, with new gaskets, carb kit, etc.

My only thought now is that somewhere there is a crack in the block towards the bottom where the bottom cylinder isn't getting correct vacuum and just isn't running right. It seems to be getting gas (the plug is damp) just fine, and I don't think it's dumping too much gas (it isn't blowing back out of the carbs).

Any other ideas?

[jb123]

again, i would say it is the # 3 carb.

[protagy311]

I think its suppost to do that

idle for about 15-20 seconds then idle down... Where does the fuel pump get its pulse? If you have a fuel pump that has a bad seal in it, more fuel will get in that cyc.