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I am having a similar mysterious problem with my 86.

It would not idle at the end of the last ride. Even if I ran the idle screws up, it would run at 3500 RPM for a while and then 30 seconds later it would run on one cylinder then idle down and die.

Tonight, I checked the compression and measured 115 and 120 PSI. No clue there.

I started it and the mag side head pipe was 400 F and the PTO side was 250 F. A clue!

I took off the muffler can and felt the exhaust. The mag side was much hotter and popper harder on my hand. Both sides were firing but out of balance.

After about two minutes of idling, the right cylinder started firing back into the intake boot and pushing the balance indicator around. There was also a pop out the exhaust can. I started thinking it must be lean.

I checked the carb sync and found the left throttle cable was about a turn too loose on the adjuster. After that was adjusted, the throttle response was better but the idle was still unstable. Idling at 3500 RPM, there was too much vacuum on the PTO side. I turned the idle speed adjuster up and the RPM came DOWN. The vacuum balanced again at 3500 RPM idle. I turned both idle speed adjusters down a half turn and the engine idled at 2500 RPM and then started slowing down. I flipped the choke briefly and the engine came back up and then started down again quickly.

By blipping the throttle, it will stay at 2500 RPM and then slowly drift down and die.
I can let it idle at 3500 RPM and it will remain steady.
By setting it at 1500 it dies almost immediately.

I noticed that the air screws were almost two turns out. I turned them in but if I had them less than a full turn out, the engine would stumble and die quickly. If I recall correctly, turning them in makes the mixture richer.

A few theories:
E10 is lower in BTU per gallon, therefore I should need to richen the mixture to get the same heat to run at idle. The sticker on the hood says the air screw should be 1/2 turn out. I am over a turn out, therfore I suspect I am too lean, but richening it up makes it die. This does not make sense. Is anyone else having problems with E10?

The fact that the engine runs at 3500 RPM but not at 1500 suggests that I may have a fuel delivery problem with the fuel pumps not putting out enough fuel at the lower RPM but are working fine at the higher RPM. The engine runs fine when moving, The problem only shows up at idle. Maybe I need to replace the fuel pump(s). I have a fuel tee that I can install tomorrow to check the fuel pump pressure at idle. Does anyone know or have a reference for the idle fuel pressure specification? Tweaking the choke brings the engine back when it is dying, therefore the fuel level in the carbs may be getting low and the choke brings the fuel flow back up where it will fire again. Can I put a clear line on the power valve port on the fuel bowl to observe the fuel level at idle and see if it drops at the lower idle speed? Would the fuel pressure test be more efficient?

The balance test indicates the balance is not stable at all RPM and loads. One carb seems to respond faster than the other one both when opening and closing the throttle. I don't understand why since they are both tied to the same throttle lever through cables. This might suggest that I have a vacuum leak. I could check this theory tomorrow as well. Is there another test besides spraying around the boots?

Bob;

You are not alone facing mysterious symptoms.

Well I'm sorry you have the problem too, but at least now I have someone to talk too
Mine idles very high, 4k or better, doesn't want to come down. Right now I'm running it with the belt off, something I don't like to do. .It has a 108 on it which I don'r know how its calibrated, comes in low. I tried until I bent my puller to get it off, no luck.. gave up on that for now.
I think I'm past a carb problem. I trust my SRV carbs, the SRV is actully 550cc and makes a bt more power and they are flawless on it. I switched carbs side to side, no diff. Same thing when I went back to the PJ carbs, problem is always PTO. I do have a Unisync gauge but didn't use it, again regardless of carb its always PTO
I switched to a single inlet, dual outlet pump, same problem. When I went back to the VM dual pumps I tried to use the 1/4 lines on the pumps meant for the smaller tubibing, both sprayed fuel everywhere. Way more then fuel needed at idle. Why would Yami use 2 pumps, I assume for volume, but then use small dia hoses? Whatever, I doubt its a problem here
I just bought this from a guy that said he had new cyl's put on and new standard pistons, but could never get it to start. Their were no stains or color on the piston tops and the plugs looked like they never fired so I believe that. Now after maybe 10 min of running, the mag side plug is getting alittle color but PTO is dead white and always abit wet.
Right now we have no snow, and I live in the middle of a big city, no chance to try it out anywhere. We build 4 cy race engines for karts and 120 sleds for a living and really I'm alittle nervouse when someone I don't know builds an engine, bore not straight or piston to wall clearance could be an issue. Judging by the amount of crud in the float bowls, it has sat for a long time, could be something plugging a pipe.
Joe, have you done any work to your engine lately or did this just suddenly appear

Well Joe, I got good news, and i got bad news

The good news is I found my problem.

The bad new is,








What s on factory pistons form markings? this one has a number, 09-816 or 918-60. odd how you could read that number either way

The only thing I have opened on my engine are: the thermostat, the carbs, the oil pump, and the timing stator. I have not had the cylinders off (yet).

I put a 108 on mine last year. It has the tell tale black line around the fixed drive sheave, so I know I need to modify the secondary as described elsewhere in the website. The 108 works like the 106 except it needs a .120 inch spacer on the end of the spring to take up the slop from using a 1.25 inch belt instead of the 1.375.

Both the 102 and the 108 need to start moving the sled at 4000 to 4400 RPM. If it is lower, the engine will bog. If it is higher, you get too much trackspin resulting in you getting stuck more often than necessary. Some like the aggression. I don't.

You can learn about my experience getting the 102 off by consulting the archives. Long story short: Tighten the puller to max torque and heat the cylindrical portion of the clutch with a rosebud on an acetylene torch. It takes an amazing amount of heat to pop some of them off the crank. The heat expands the outer part of the taper to let go of the inner part of the taper. Be aggressive with the heat. You want the outer part to heat up faster than the inner part. If you are not aggressive, the inner part will heat up as fast as the outer part and it will not let go.

I use a manometer with antifreeze in it so I can see the color in the vinyl tubing. Since I e water, the column needs to be about 7 feet tall. I connect one end of the manometer to each test port. The carbs are synchronized when the levels are the same in both sides of the manometer both at idle and with the throttle open. Be careful that both tubes from the manometer to the test ports are clear of any antifreeze because it will offset the calibration of the gage This can be tested by reading the manometer with the engine off. If it reads and offset with the engine off, check for liquid in the test port hoses.

I have not had any trouble with my Yamaha's, but my Kawasaki had eaten two pistons in the last two years. Last year it was a failed anti-rotation pin in the upper ring groove This year, the ring land eroded away from the anti-rotation pin and dropped the compression. It takes only an hour to change a piston in the Kawasaki with a $50 kit from Dennis Kirk. It takes longer to drag the dead sled back to the trailer than it does to replace the piston.

Yamaha used two fuel pumps on the higher performance models to increase the fuel flow to keep up with the fuel flow demands. I presume they had trouble with a single fuel pump keeping up with the horsepower increases. They may not have had access to the higher volume pumps that are now available. It is also possible the engine did not have adequate pressure pulses from a single crankcase to pump enough fuel so they used two pumps to exploit both crankcase pressure pulses.

The 'won't idle' problem developed during the last ride. It idled fine when we started the ride, but would not idle by the end of the ride. My diagnosis last night seems to suggest that the idle speed adjuster vibrated out during the ride. Once I synchronized the carbs, the problem seems to be much reduced. It used to idle just fine at 1500 RPM. I want that back, therefore I will continue to troubleshoot.

What happened to the piston that would cause the engine to not run, yet have compression on both cylinders? The way the aluminum is scuffed over the ring grooves, I would expect the compression to be lower than normal.

It might just be the pictures, the rings look ok and the scuffing is very light. Not sure what I'm going to do yet, might just throw in a piston for the rest of this winter, Might just let it sit for awhile. I have other sleds if it snows here again.
I have a friend that raced these for years, he's up at a race in Canada this weekend, he found sleeves and pistons that will make these into a 614, might just go that route. He was happy with his when they were that big.
I've had plenty of experience changing pistons, had several sleds with Nitrous on them, changed them in the trailer many times
Just looks to me like whoever went through this engine, wasn't an engine builder I should go through everything. Will measure the bore with a bore gauge later see if their straight. My guess is not and they have taper which is why they scuffed, right now were off to a night race at the snow X track. Bob

Is there any place I can find spec'sfor these engines? I have 1to 1 1/2 th on the side that scuffed and 2 - 3 th on the other side. Does anyone know what it calls for on piston to wall for these engines?

Clearance between Piston Cylinder 0.065~0.075mm (0.0026~0.0030")
Piston Ring End Gap 0.35~0.55 mm (0.014~0.022")
Piston Ring Coating Tufride

Cylinder bore 73mm (2.87")
Taper Limit <0.05mm (0.0020")>
Out of Round Limit <0.01mm (0.0004")>

From my Parts list
Piston 8U900 8U9-11631-01-94 OEM
Piston 8U900 w/rings & pin 8U9-11631-01-94 OEM
Piston 8U9 8U9-11631-01-94 OEM

Piston w/rings, pin & bearing 009-816 After market
Piston w/pin, clip & bearing 009-816 After market
Piston 009-816 After market
Piston w/ringsR09-816 & pin After market SPI
Piston w/rings,pin & c clips 9816 A.M. LLP
Piston w/rings #R09808 09-808 A.M. Lemans
Piston w/rings, pin & c clip 844-0816 A.M. Kimpex
Piston rings 2ea. std. Chrome bore R09-816 A.M. Kimpex

Once again, thanks for the help. Bob

I would also clean the pilot jets in both carburetors the holes are tiny and when they plug up your sled will run but only after 1/3 throttle like 3000 to 3500 . It will run lean at low rpm but not bad once in the midrange.