1995 Speedster Ignition/Fuel Troubleshoot

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JeremyD615

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Hey guys, I'm a long time lurker, but first time poster here, so I'll try not to ask any of the questions that have been answered a thousand times.

I've had a 95 Speedster with twin 657X's that I've been slowly restoring for the past two years. The port side motor has one season on it, and seems to be running pretty smoothly with a top end RPM of about 6800. The starboard side motor is a fresh SBT rebuild that has been properly broken in and now has about 10 hours on it. It tops out at about 6600, for a top speed on the GPS of 50.5mph. Strangely, this new motor is the one I don't trust yet.

The motor starts up fairly easily with a little choke, and then revs up to 3500 and bounces off of the limiter for a few seconds when on the trailer. If I leave it alone, it will eventually settle down to 3k and idle well, or I can give it a bit of choke while it's bouncing off the limiter and it will settle right in as soon as I let off of the choke handle. That part doesn't really worry me too much, as the port motor behaves the same way. What has me questioning the new motor is its tendency to randomly drop the idle down to 1100 or so after about an hour on the water. When the rpm drops, it obviously idles rough, and seems to smoke more than usual. If I crack the throttle and bring it back to 1500, it feels normal, but drops again as soon as I pull the throttle back. At anything more than idle, it seems to run fine, although I sometimes feel a very slight miss if I hold the revs right around 4000. I did a plug chop after running at that throttle setting, and the plugs were a nice even brown, so I don't think it's a lean spot. Any ideas what could be causing this? I keep thinking plugs, but they don't look fouled, and I've gone through a few sets.

So far this is what I've done:
- Fresh SBT rebuild.
- Rebuilt carbs with OEM kits and new N/S.
- Double and triple checked pop-off pressure of 24lbs plus or minus 1psi.
- Set LS screw to 1-1/4 turns out and HS screws to barely cracked open.
- Bypassed fuel selector valve.
- Pressure tested the carbs, fuel pump, and fuel tank.
- Blown out all fuel hoses that are of the black automotive type.
- Clipped plug wires back.
- Confirmed oil pump arm is adjusted to spec.

When the problem first surfaced, I pulled the air box and found that the oil pump cable had flipped over 180 degrees on me, but I fixed that and it ran fine for another hour or so before the idle dropped again. This time all the linkage was fine. Any help from the experts would be much appreciated. Thanks!
 
So I believe I found my idle issue! The port side motor had rattled the bolts loose at the exhaust pipe to manifold flange creating quite an exhaust leak. When I had the boat out last time, the idle dropped down again and it started running rough, and when given throttle it would rev to about 4K RPM and would bog out from there. After opening the tray to see what was going on, it ran fine for a few minutes before doing the same thing. I put it on the trailer and called it a day.

When I got home, I pulled the plugs and found them all to be black and oily. After putting fresh plugs back in, I fired up both motors with the tray out to find smoke coming from the pipe to manifold flange. On closer inspection I found that the stud was broken off in the manifold, one bolt was missing, and the other two were loose. I’ve gone back through and replaced the bolts, added a healthy dose of blue loctite, and made sure all of the bushings and mounts are tight. Hopefully that will sort out my issues. Fingers crossed and I’ll report back if that does it!
 
An exhust leak will do what you are talking about. BUT......... when working on the exhaust, replace the bolts/studs, lockwashers, and gaskets. The exhaust parts expand and contract a lot. If you over tighten the bolts, or reuse the old bolts... or reuse the old gaskets... you don't have any "Spring" in the system.

I see it all the time here... guys have problems like this, and then just tighten the piss out of it. Next thing they know... the have broken bolts, and leaks again.

Basically, the stainless bolts have a good amount of spring in them. Old bolts will eventualy "Work Harden" and loose that spring. Along with that... new bolts over torqued will have the spring stretched out of them, and won't recover when they cool. Resulting in leaks on the next run.

So... new parts, and only torque to 18 Lb/ft.

Let us know how it works out.


Oh... these are 2-strokes, so the plugs will be oily around the rim, threads, and top of the electrode. They should be clean(ish) on the inside of the outer electrode, and tip electrode. AND... the tip of the ceramic should be very dark brown.
 
Thanks Doc! I replaced the bolts with new stainless fasteners, gasket, and lock washers with blue locktite. My plugs looked very similar to yours until this last trip out. Hopefully I’ve got it all sorted out now. I’ll let everyone know as soon as the rain lets up and I can get back out on the water...
 
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