engine seems to hunt at 60MPH

[fastgpz900]

I have a 87 that I bought last year, did some basic maintenance on it like spark plugs,  clean the carbs and change the needle assembly, sync them, runs on 91 octane no ethanol, did also the gas cap mod for the flap.

Now here is the problem, the bike seems to hunt at 60MPH, hard to describe, the only way I can reproduce that feeling is to grab a little bit of the brake while riding, it seems like the bike slow down a bit reaccelerate and do this all the time. It seems to run better with a full tank of gas compared to 1/2 full but I am not sure if this just an impression.

Engine does not change sound like cutting off, RPM are pretty constant. Just a back and forth motion.

No problem in acceleration, pull hard all the way.

Carb problem, CDI problem, wheel bearings problem?

Thanks for any help.

Pierre

[Pat Conlon]

Hello Pierre, years ago my '84 had the exact same symptoms.

I found that it was a venting problem with the carb fuel bowls. How I found this was by ruling out a tank vent problem by opening the fuel cap while riding, and the surge problem continued, so I knew it was not a tank vent problem.

What happened was I installed longer vent hoses on my carbs. The vent hoses hung down to far and the hose ends melted shut from the heat off my exhaust collector box.
Snip, snip and the problem was solved. The fuel bowls on our carbs need vents to the atmosphere for correct fuel flow.

So....rule out a tank venting problem first (open the cap) also check your carb vent hoses, make sure they are open and unobstructed.
The final thing to check is your fuel tank petcock. The vacuum diaphragm may be failing causing a partial interruption in the flow of fuel.
Do the suck test on the vacuum port, if you can draw air, the vacuum diaphragm is leaking.

Hope this helps.  Pat

[fastgpz900]

Pat

thanks for the tips, good place to start, I ordered a new petcock rebuild kit from Randy (RPM) just in case it is bad.

I checked the tank venting and it is good, the tube going down the frame is free, I could have made something bad when I took the carbs off.

Back to the shop

[Pat Conlon]

Ok Pierre, I'm still betting on a fuel delivery problem.

One last thing to be aware of: On gravity flow FJ's ('84-87) proper fuel line routing is critical.

The single line leaving the petcock should run *under* the 2 branch lines. This is counter intuitive. The petcock outlet hangs low when the tank is in place.
To compound the problem, some FJ's had an incorrect diagram under the seat on the rear fender showing the single line running under only 1 branch line. I recall Yamaha had issued a factory bulletin addressing this error.

What happens is the oem thin wall fuel line heats up and gets warm and pinches causing an obstruction in the flow of fuel.

Let us know how you make out!  Cheers Pat

Safety wire that petcock elbow!!  Do it now!

[fastgpz900]

OK, here is the update, remove the tank and found that the fuel line was pinched at 2 places, I replace the line with something stronger but could not get not to have some king of kink when I put the tank on, I looked at some post and took the idea from one member to change the Y connection and went further with it. Sync the carbs and went for a ride, all is good now.

Here are the pictures of my fuel connection.