Issue with 89 Yamaha moto 4 250cc

[Need-help-with-quads]

Ok so I found another problem, when I ride it, the speedometer wont go past 40( it goes up to 50) it almost sounds like it’s cuts out for a second and then keeps going and then cuts out, and sometimes it’ll go like 45 but most of the time it won’t go past 40

[Gwbarm]

That doesnt sound bad at all, there could still be a carb issue, but we know that this isnt the carb that was factory installed on the Quad. The top speed of that bike fresh off the showroom floor was 45 MPH. With age and wear and other issues(carb) it sounds just about right. Plus is the speedometer still dead accurate.

[Mech]

I wouldn't expect that bike to cut out and then run then cut then run at the forty speed. That sort of behavior is common with efi but not an old carb model. I'd expect that bike to get near top speed and then for the acceleration to slowly drop till it was just gaspinng for air but unable to go any faster.. no cutting in and out though.

Depending how long the "cut" is for, and I doubt it is a whole second, and how it responded to throttle changes, it could be fuel problems still, and that would be my suspicion, or it might be an electrical fault.

[Gwbarm]

True, it shouldnt be cutting in and out.

[Need-help-with-quads]

 

So this is normal?

Also with the speedometer I can’t really tell if it’s right because I have smaller tires on the back and I don’t have the right size so it’s actually going like 30 I think

 

 

 

 

[Gwbarm]

I couldnt get the videos tpo load but no it should not be cutting out

[Need-help-with-quads]

Ok I’ll try and explain what’s happening, so I’ll hit full throttle, then once the rpm’s get up it sounds like it cuts out for half a second and then the rpm’s go back up. The thing about this is that I’ve ran it really fast before and it’s only happened like 3 time but the speedometer went past 50 so I know that it can go faster I’m just not sure why it wont

[Gwbarm]

Will it stop cutting out if you let off the throttle a little, or continue. 

[Mech]

Yup, what happens when you ease the throttle off very slightly ? Describe sound,bike behavior, time(accurately), what happens if you hold it steady at the slightly backed off throttle, and what happens if you throttle off further. By behavior I mean, when it cuts, does the engine go entirely dead so the engine braking kicks in, or does it just go flat and no power, but not particularly engine braking.

And..  Does this happen hot or cold ? Is there any backfiring from either the carb or exhaust ?

Do you think the exhaust could be blocking momentarily ? Does it go unusually silent at the exhaust, or gaspy at the carb ?

 

 

[Need-help-with-quads]

So this happens hot and cold but when I ease off the throttle a little bit then it goes the same speed but doesn’t “cut out” quite as long

There is a backfire sometimes if I ride it hard on full throttle

The engine brakes don’t kick in it just loses power for about a half a second

if I back off the throttle too much it obviously slows down but stops cutting out

what do you mean when you asked if the exhaust is silent and the carb is gaspy?

[Mech]

A blocked exhaust makes a muffled sort of noise, I think you'd notice it if there was a flap of metal loose in the muffler blocking the exhaust..  A blocked exhaust also makes the carb start to puff air back out and sounds sort of..  err.. gaspy.. haha. It sounds like it can't suck air in.. which it can't.

[Mech]

Form what you say in #40, it sounds like a fuel shortage problem, especially so if that backfire you mention is out of the carb.. If that backfire is out the exhaust, then it's more likely to because of no ignition of the fuel for a second causing a build up of fuel in the exhaust which then gets ignited when the ignition resumes. That sort of backfire is usually a really load explosion sound. Unburnt fuel can build up in the exhaust though due to a bad mixture, in which case the backfire out the exhaust is not so explosive, and they often give a series of small backfires under certain throttle/load conditions..  That bad mixture induced backfire is linked to throttle..

If the ignition was dying entirely the engine braking should be quite abrupt. The way you describe it losing power but recovering if you ease the throttle, and not really loosing much speed, sounds like fuel.

Ride more, take note of everything you do and what effect it has on the bike..

If you have it at full throttle and it starts to run out of fuel at the fuel tap, it will take several seconds of throttle off before the fuel supply fills the carb again, but if there's a drop of water or a spec of dirt blocking a jet, it will probably come right almost instantly when you throttle off and start using a different jet..

[Gwbarm]

I did finally get the video to load, its like you let go of the throttle slightly every second, while at full throttle, i didnt hear a backfire. Do you recall whaat size main jet it had in it, but it could be not getting enough fuel to the carb fast enough as Mech suggested.

[Mech]

Cant see the video but if that's riding the bike then that definitely sounds like a fuel issue.

I'd drain the carb bowl into something and look for water, then I'd check the tap was letting plenty of fuel through and that it was running out the bottom of the carb at a good rate, then I'd check the float level with a bit of clear hose. Those are all simple.

Check the air-filter's clean and oiled as per the book, wet but squeezed out.

[Mech]

Another thing that makes things run surging like that is too much ignition advance. That's pretty rare on quads but if you have a timing light you should check what the timing's doing at full advance.

[Need-help-with-quads]

Ok fellas the first thing I checked is the timing and here it is.

now for that F, is that the correct line because there’s also another line

the first two is with the f line and the last two are the other line

If this is the issue how do I adjust this?

[Gwbarm]

 The other line should be top dead center, some have a T beside it but i didnt see one on yours. and the F is when it fires. It looks like yours is in perfect time, they most always are, timing advance is controlled by the CDI, yours is so minor i dont think that is the problem, i think i would try the next size larger main jet.

[Mech]

The cam timing is meant to be done using the T mark on the crank. The mark on the cam sprocket should be within a quarter of a tooth of perfect. That timing mark is almost a whole tooth out.. which may be right if you were using the F mark, which is for ignition timing and a bit advanced of top dead center. If that was on the T mark though then it could be improved.

I don't think that's your problem though, not the fact it's a bit out, they run fine like that, but, if there is too much slack in the timing chain it might make it surge. You could try adjusting the chain and then checking how much slack there is in it. Or check the slack, then try adjusting it to see that the adjustment is doing something..

To check the ignition timing you use a timing light and at idle speed the F mark should line up, then as you rev it the flywheel should appear to rotate under the light until you see two parallel lines(I think(, to indicate full ignition advance. That full advance should be at fairly low revs still, perhaps two three thousand, and then the timing should stay between those two lines. If the timing goes past those two lines it's too advanced.

If you don't have a timing light it's probably better to ignore the timing advance and concentrate on the carb.

It may need a new larger main jet, but that's not really likely if the bike had been running fine with that jet previously. You could pull the slide and check it's diaphragm carefully for splits, then try lifting the slide needle a notch to check it's not that in the wrong position, or jumping around unconstrained.

I'd check the real simple things first though, like drain the carb bowl into something and check for water, then turn the tap on and check fuel runs out the bottom of the carb at a good rate, then use a clear plastic hose to check the float level.

 

[Gwbarm]

I dont particularly like changing jet sizes either, i tend to stick with factory specs, but since this wasnt the carb that came on the bike, i thought there might be a little difference in jet size, but i have not been able to find the OEM jet size yet.

[Mech]

Depends on whether the bike used to run ok with that jet.  "N" didn't say whether this is a new unknown bike to him. or whether he'd been riding it and then it developed the cutting out problem.

[Gwbarm]

Good point! So many are resurrections, especially on one this old, sometimes i forget that it might have been running good and then this happens.

[Need-help-with-quads]

Sorry been busy but I had bought this quad off of Facebook marketplace and am going to flip it so it is new to me I’ve never had this kind of quad before

It has just done this from the start

[Gwbarm]

So how is it coming, any new problems.

[Need-help-with-quads]

So far it’s still doing it I adjusted the carb a bit and now it doesn’t backfire as bad but it’s still not going full speed, I did find out though that once i ride it for like 10 minutes on high rpm’s I’ll get like a sudden boost of power and it’ll go faster and the speedometer will go up to 50 but other than that I’m not sure what to do there was not water in the carb so yea I’m at a loss

[Gwbarm]

That is strange getting a sudden boost of power, and how long does this sudden increase in power last, does it do it at high RPMs or just randomly.

[Mech]

Cleans it's plug I bet..  If it runs sometimes, and in these circumstances, it sounds to me like the plug is getting wet with fuel and missing, till the right combination of speed and throttle burn it clean.

You should check the spark plug is the right temp and type, then, try lowering the slide needle in it's slide as a first test, then go out and note very carefully whether it takes less time to get it to burn clean. You could also try only opening the throttle till it starts to struggle/starts to sound gaspy or hollow, and then either hold the throttle steady or let it off by the tiniest amount till the engine sounds healthier again, starts to feel like it wants to pull again, then ease the throttle back on, and keep doing that, slowly coaxing the engine to higher revs.

If we drive around in a vehicle that's not tuned just right in the fuel department they can get deposits on them which in the normal course of driving get burnt off again when we start driving a bit faster, but if we drive around for a while slowly, like in a town, then get to the open road and floor it, the soft deposits burn on hard and to a substance that can short the spark. We can drive carefully as I've been describing above to prevent the build up burning hard, and we can even clean fouled plugs by following the same driving techniques.

Try driving like I suggest and if you can coax the plug clean, which is what it sounds to me like you are doing, then you need a different plug, or adjust the carb, or drive differently..

[Gwbarm]

I rewatched your video did you get the surge worked out at full throttle or is that still there.

[Thingfish]

If it starts after sitting for less than a minute it could be floats not adjusted, especially if it does not die under light driving but does using more throttle. But if it has to set for a while to “cool down”, possibly the coil and an earlier response is right. Put a light on it or hook up a spare grounded plug and check the spark. Newbie here. Liking the site. 

[Need-help-with-quads]

It still does it with the speed back off, the spark plug is brand new but ive decided to sell it as is thanks for all your help sorry we couldnt figure it out but i do have one thought, when i bought it, it had been sitting for a couple years and there was like somesort of white hard calcuim buildup or something in the walls of the carb bowl and inside, do you think that has anything to do with it? keep in mind that i rebuilt it so the jets should be fine, i also just replace the petcock valve so theres nothing clogging that up

also i tried coaxing up to higher rpms but that didnt work and i couldnt get it to go higher, but if i ride it for a while it tends to faster as it gets hotter

Edited August 6, 2024 by Need-help-with-quads

[Mech]

Yes the build up in the carb can effect their running. The drilling that the emulsion tube fits in, the emulsion tube being the long brass tube that holds the main jet and that the slide needle goes into, if that drilling gets crud in it, it effects acceleration and from about 1/4 to 7/8 throttle.