Mech

Man that's annoying.. It must be something causing it.. You are onto things so I'm sure you'll suss it eventually.. Could the ignition timing be too far advanced, or, if it's meant to have a start signal going to the cdi to help easy starting, could that start signal be missing/not getting through ? Surely it can't be a mechanical fault, or you would have fixed that by now..

Amen to that Brother..

I think that comment is for the "need help" thread Gw.. You're like me and get lost in here.. haha.. I'm forever forgetting which bike is which and which problem is which.. i waste an awful lot of time going back to page one to check I'm thinking of the right bike and problem, and then sometimes things have moved on and it's a new problem we are discussing, and I tilli get it wrong ! And yeah, a rich mixture can do it too.. And more often in the exhaust. Lean mixtures always seem to whoomph out the inlet though..

haha.. i reckon !

Carbs puff a little fuel out the carb sometimes anyway during normal revving up from low revs, but if the exhaust's really blocked so you can notice it (the blockage) stationary, then I'd expect the carb to start puffing more fuel and air than it should at high revs.. But it'll be a comparative sort of thing.. You'll notice it mainly because of it's abrupt occurrence. Most blocked exhausts don't show up so readily though stationary, because you need to rev them high and for a few seconds to get the back-pressure to build up, and most people are hesitant to keep the revs that high with no load on the engine. People are much happier to rev things right out when the engine's under some load.. I ride bikes to try and detect blocked exhausts. taking the exhaust off and revving it in the workshop doesn't really reveal/prove anything. If you can adapt a vacuum gauge into a steady vacuum point on the carb or inlet it will show it up better stationary.

The "whoomph" noise is the carb backfiring and trying to set fire to your air-filter. Check there's a flame proof mesh between the carb and air-filter. The reason the carb backfires like that is because the mixture is too lean. I'd suggest trying adjusting the idle mixture and speed to cure that, but the high speed symptom sounds like the whole carb is running lean. You could check the carb is getting plenty of fuel right to it's fuel bowl by undoing the drain screw on the bottom of the carb and making sure fuel keeps running out even after the bowl has drained. Then check the carb breathers are clean and not crimped off or anything. Then, if it's getting plenty of fuel to the bowl but not revving right out, probably best to take the carb off and strip it entirely, clean and adjust it, and that should cure the problems.

View File 1987-1990 Yamaha YFM350ER Moto 4 Service Manual YFM350ER Moto 4 Service Manual. !987,1988,1989,1990. Submitter Mech Submitted 02/10/2025 Category Yamaha ATV  

A close-up photo of the fouling might be helpful too.

The spark-plug probably fouls because either, there's too much oil getting into the mixture, the carb is running too rich, or it needs a hotter spark-plug. Too much oil makes a cloud of blue smoke. The idle mixture can be adjusted. A hotter plug is cheap and easy to try..

I see.. Ok well there's another check for blocked exhaust, well there are two symptoms we can look for actually.. apart from going for a drive with the pipe off.. If the exhaust's blocked and you have the airbox off you'll notice that at the revs start to flatten out because of the back pressure, the carb will start to puff fuel and air back out. I'll start sort of abruptly when the power starts dropping. Some carbs spit backwards a little anyway, so you need to use a bit of discretion, but the fact it starts spitting back more as the revs build, rather than the more common , spitting a little at low revs then doing it less as the revs build, that's the clue. The other test is to fit a vacuum gauge, and it will start at idle with moderate vacuum, then the vacuum builds as the revs build a little, but then the vacuum starts to drop abruptly when the blocked exhaust starts to do it's thing.. And I figured why the cam timing came out different after a turn.. It's because the first setting should have been a quarter tooth off one way, then after one turn the mark should have been a quarter tooth out in the other direction.. So the first check might have had the mark just below the head surface, and then the next check should have shown it just above the head surface.. Instead you had it spot on at first and then all the discrepancy showing after a rotation.. It'll be close enough though not to be the trouble with the no revs problem.

haha.. Good onya. So did you suss the stiff shift ?

It was the "shift a little hard" comment that had me worried. When you use the shift lever it operates the plate clutch, and I thought you may have adjusted it so that it was trying to push the clutch release bearing too far.. which would make the shift lever not want to move through it's full travel.. All good though. It sounds like you've got it sussed.

The clutch plates settle into place(in the basket grooves) and wear a little of the fluff off, and need adjusting a few times, then they stay in adjustment for so many hours that it can amount to years in some bikes.. You can sort of check it by using your toe under or on the shift lever, and seeing where it starts to let the clutch grip, and where it locks up, then checking/comparing that occasionally. The adjustment.. The book says to turn the adjustment screw clockwise until it becomes hard to turn, then turn it counterclockwise until it becomes hard to turn, then lock the nut there.. But they don't really mean turn it till you can't turn it any more.. haha.. Try it lightly, and then firmly wound counterclockwise, and see if you can feel the action difference using your toe on the shift lever.

haha.. All's well that ends well.. the plastics do keep the dirt off you.. And over here most of the roads are 1/2" ground rock stuck down in tar, but out in the country they just put 2" ground rock straight down, and grader it back to the center every so often.. That's the ones I like.

Download the manual. Check the resistance readings as per the book, and also then try to check there is some Ac voltage coming out of the cdi charge windings, and a brief pulse oi power coming out of the trigger coil. The brief pulse will show up best on an analogue gauge.. The needle will jump.

If it's got a carb, the carb's float level needle/jet is probably leaking allowing the fuel level to get too high so it overflows the carb and goes into the cylinder, down past the rings and into the sump. It is fairly common though for needle to leak very slightly and it not to be a problem during a days riding, but if you leave the tap on over night they can try to fill the sump.. So check the tap turns off. Another thing that can cause it is if they have a vacuum operated fuel pump that plays up..

Do you mean it has a slow leak, or you can't get the bead to contact the rim to start inflating it ? If it's a slow leak you can put something called "slime" in the tyre, then rotate the tyre and wheel while it's tilted on it's side so the slime gets around the bead and seals the leak. If you can't get a new tyre to fit on one side, take it to a bike shop..

i may have overstated the ten minute recheck.. I'd use the plate clutch a bit, using my toe on the shift lever to slip it a bit. I'd do that on a ten minute run so I could readjust it before returning it to a customer.. You can probably drive it for an hour or two of regular driving before the plates will have settled in and need readjusting..

Did you drive it with the pipe off ? Are you sure all the carb and tank vent hoses are connected right, and have you tried opening the air inlet to the carb ? Taking the lid off the air-filter box perhaps..

Yes. After fitting it needs adjusting, and then checking a couple of times in the first few hours riding, but I'd make the first recheck after only about five or ten minutes. I hope you soaked the shoes and plates in oil before installing them ?

All good points Bellarmine. As far as that cam mark changing goes.. There's something wrong there, worn sprocket/s, tight link in the chain that doesn't like going around corners, play in the cam bearings perhaps.. If you start it, listen for a rhythmic surge/rattle noise from the tensioner/chain. And, another thing that can cause really dull performance and no revs, is retarded ignition timing.. Usually accompanied with a lot of heat.. It makes exhausts glow red.. Oh heck.. I hadn't realised it was a belt drive.. That might not be operating properly..

And does it start repeatedly ? And can you get it to run any longer by carefully managing your hand ? Have you held the fuel hose and checked it has pressure ? Does the pressure drop while it's running, or does the hose stay stiff right till the engine has stopped running ?

The cam timing should come out the same no matter how many turns of the crank you make. If it lines up on one T, it should be the exact same on the next rotation. Make sure you are rotating the crank the right way. Turn it some more rotations and check the timing doesn't slowly creep around.. Caused by a wrong/worn sprocket. It doesn't matter whether the cam's mark is at the front or back of the head.

The book I looked in said T in the peep hole, and then, the mark on the camshaft lined up with a mark on the sprocket,(the cam mark is on the back of the sprocket), then the sprocket lined up with the head surface. There shouldn't be any second T mark. There will be a F mark. If you turn the crank one whole rotation from one T to the next time it appears in the hole, the cam's marks should have done a half turn and gone from front of the head to the back, or from the back to the front. You need to be sure all the slack is out of the tight side of the chain when you are checking the alignment, and it will be possible to get the cam marks less than a half tooth from perfect. If it looks like it wants to be more than half a cam tooth off, yell out and I'll explain how to fix that. The oxy-acet will be the death of the old muffler.. Just unbolt it and go for one fang.. They you'll know it's the exhaust blocked and not a bad spark-plug..