Bike acting funny at speed

[restorganize]

When riding my 79F at highway speed, if I am not, what always feels like accelerating, my bike will stop running well. So for example, I am riding 60mph, and I move my throttle hand, very little and the throttle moves down a minuscule amount, the bike will all of the sudden have no power, sound like it is running out of fuel and or a few of the cylinders are not firing. I can crank up on the throttle and there is no response. What this causes me to have to do is pull in the clutch, turn off the key, turn it back on and re-fire the bike. All is well, until a little throttle movement and it happens again. I remember the bike feeling like this onetime when I was riding and I didn't know it, but one of the hoses that connects to one of the rubber boots had come off.

I have no idea what could be causing this. It runs great just sitting. I can crank on the motor, keep the rpms up, rotate the throttle a lot or a little, and it acts just fine. It only happens when I am riding.

Any Ideas?

[motoman]

Pick-up coil wires may be a good starting point.

[ViperRon]

Make sure none of the gas lines are kinked. I have done this to myself before. Sitting still the engine takes much less gas flow but on the road at 60 poor flow it can cut out quick.

[BA80]

When riding my 79F at highway speed, if I am not, what always feels like accelerating, my bike will stop running well. So for example, I am riding 60mph, and I move my throttle hand, very little and the throttle moves down a minuscule amount, the bike will all of the sudden have no power, sound like it is running out of fuel and or a few of the cylinders are not firing. I can crank up on the throttle and there is no response. What this causes me to have to do is pull in the clutch, turn off the key, turn it back on and re-fire the bike. All is well, until a little throttle movement and it happens again. I remember the bike feeling like this onetime when I was riding and I didn't know it, but one of the hoses that connects to one of the rubber boots had come off.

I have no idea what could be causing this. It runs great just sitting. I can crank on the motor, keep the rpms up, rotate the throttle a lot or a little, and it acts just fine. It only happens when I am riding.

Any Ideas?

I had that issue with mine when I first started on it. It turned out to be the difference in aftermarket pilot jets. Put genuine Mikuni pilots in and solved the problem.

Pickup wires can cause the same thing though but when they are bad you will occasionally drop 2 cylinders also.

[TopCatGr58]

Hey Judson,

As others have eluded to, it could be an electrical issue, the pickup coils with cracked/broken wires, and as you rotate the throttle the vacuum level changes....especially while cruising...you have much more vac. adv. action and more flexing of the PU wires as the timing plate rotates.

Another problem could be fuel starvation, a few causes, as mentioned first a kink in the fuel line, or a clogged gas cap vent creating a buildup of vacuum inside the tank preventing the fuel from being able to flow out as easily, and a third can be a faulty vac. valve in your petcocks.....you have a standard with vac. controlled petcocks....the vac. hoses to the intake manifolds can develop cracks or leaks so they don't pull on the valve effectively to keep them open adequately. Rotating the petcocks to PRIME when you experience the problem is a quick diagnostic check....if it solves the problem, then you know it's a fuel delivery problem.

T.C.

[BA80]

The slight throttle application is what points me to the pilots TC. If the pilots aren't working correctly it will cause a stumble at a light application.

Partially clogged or incorrect pilots.

The pickup wires are easily checked without a great amount of disassembly though.

The gas cap issue is easily checked also. Just leave the cap unlatched and see if that solves the problem. Not with a full tank though. That could be uncomfortable.

The clogged gas cap will cause TOTAL starvation though. Once the thing gets into a vacuum situation the bike won't accelerate or idle. Saw that happen with Harry's bike.

[jetmechmarty]

Are you runnng fuel filters? Are.they dirty?

[restorganize]

Whole Lot of Info

Hey Guys, thanks for all the suggestions.

Motoman, I will check the coil wires, as they have never been replaced as far as I know, and I have owned the bike for 20 years. That does seem like a good place to start.

ViperRon, The fuel lines are newish, and I haven't taken the tank off since I replaced the lines in 2012, so I would hope that that is not the issue. I will certainly check though.

Jetmechmarty, I do have filters and clear fuel lines, and I can't see an issue with fuel flow.

BA80, I have never changed out the jets so I don't think that would be the issue.

TC, I will check with the clogged gas cap, as that does seem like that is what it could be, but that doesn't answer, at least in my mind, why it would go away when I turned the bike off and on and started it back up, I don't see how more air would get in to the tank. But I will certainly check.

I have a lot of things to look at, but my first order of business is rebuilding the front brakes and master cylinder as they have grabbed and won't let go.

There is no telling when I will get this done with a 7 week old in the house. I will keep you posted.

Thanks

[BA80]

BA80, I have never changed out the jets so I don't think that would be the issue.

But, did the PO? A lot of after market kits come with jacked up after market jets. That's how I got the inferior jets in mine.

[restorganize]

But, did the PO? A lot of after market kits come with jacked up after market jets. That's how I got the inferior jets in mine.

I bought the bike from a Yamaha dealer in '92, and the bike had nothing aftermarket on it. It only had 17k miles, and was completely stock. This is what the mechanic also told me. I have no other way to confirm that, except to tear apart the carbs and check, and who knows, after the 100k I've put on the bike, perhaps they have gone bad. I will not take the suggestion off of the check list.

[BA80]

Ahhhhh..OK. I wasn't aware that you had owned it that long. I just looked at your 30 posts and figured you were a new member. I should have looked at your join date.

I've had old plugs act that way too. JAT

It does sound like a pilot circuit problem though. I just got an E going the other day that had been sitting for quite a while. I had to clean the pilot jets and the whole pilot circuit twice to get it to idle right and not stumble on acceleration.

[Bonz]

Pick up coil wires maybe, or more specifically take off the timing cover and while checking the pick up coil wires, lube the timing plate as you rotate it by hand with a good dose of silicon spray. If it hangs at high vacuum and won't rotate back, that will cause what you describe. Especially if it idles and runs well under no load. That definitely is not carbs IMO.

I had a similar issue with my 80SG where it would bog and not take more throttle, and while the timing plate seemed to rotate freely by hand, after lubing it rotated just a bit more freely and problem gone.

[restorganize]

Pick up coil wires maybe, or more specifically take off the timing cover and while checking the pick up coil wires, lube the timing plate as you rotate it by hand with a good dose of silicon spray. If it hangs at high vacuum and won't rotate back, that will cause what you describe. Especially if it idles and runs well under no load. That definitely is not carbs IMO.

I had a similar issue with my 80SG where it would bog and not take more throttle, and while the timing plate seemed to rotate freely by hand, after lubing it rotated just a bit more freely and problem gone.

I will add that to the list. Thank you for the suggestion.