1988 Yamaha FZ600U Throttle reaction/stalling problem

[Bean_FZ600]

Yamaha FZ600 no throttle response and stalling
Hello everyone! I recently purchased a 1988 Yamaha FZ600U sport bike! Very nice bike and only has 10K km on it! We had it running great! We changed oil, spark plugs, drained old gas, synced carbs, and got it all shined up! One day I was riding and then the bike would just be idling and have no throttle response at all! It would Adventually stall out and will not start back up! MAIN ISSUE: it will just glide and have no throttle response but will still idle at 1000rpm! Although after the bike stalls out it will not start until battery is charged! But when the battery is full it fires up no problem! PLEASE HELP

 

[iSteve]

I'm guessing you still feel resistance on the throttle so the cable is still fine and connected. My guess is something is loose somewhere in the throttle linkage. Just follow the linkage and find where it stops working.

 

[Bean_FZ600]

I'm guessing you still feel resistance on the throttle so the cable is still fine and connected. My guess is something is loose somewhere in the throttle linkage. Just follow the linkage and find where it stops working.

Ill check that out today! I'm also suspecting that not enough fuel is getting to the carbs! It is very confusing because the bike will stop and have no throttle reaction! It will idle until it shots off!

 

[Motogiro]

If your voltages at idle seems okay (at least 12.5 VDC.) I would suspect the bike is leaning out. As soon as you open throttle the air to fuel goes lean and the fire goes out. Maybe go back to your carbs and see if junk got reintroduced.

 

[Bean_FZ600]

Will do! After the bike does after it fails to idle and to react to throttle, it will not start back up even if you choke it. After you try starting it a few times the battery will die. I had the battery tested and it said good, but I seems to die so fast!....anyway, could the air/fuel mixture screws have any effect?

 

[FinalImpact]

A quick test is unscrew the bottom of a float bowl ((USING EXTREME CAUTION SO AS NOT TO BURN THE BIKE DOWN // ANYTHING ELSE FOR THAT MATTER)).

If there is fuel present the metering into the carbs is OK but the carb to the spray bar is plugged.

If no fuel is present in the float bowl, then you have issues from the Pet0ck on down. So that issue could be in the tank, in the petc0ck itself, in an inline filter, or even the needle seat/diaphragm in the carbs.

Have you ever pulled a spark plug wire and checked for SPARK right after it dies? Carry an extra spark plug. When it dies, pull one cap and insert the used plug. Crank it over with the plug body touching the head. YOU SHOULD SEE a nice purple arc between the electrode and the ground strap.

 

[Bean_FZ600]

Thank you all! It's great having so many people with so much knowledge! After 3.5 hours of hard work from 2 former Motorsport mechanics we think we have have diagnosed it. Now we need a compression tester is what we need to find find out further issues..... The problem is the engine (air cooled) runs great at the start! But when it heats up, it just stalls

 

[TownsendsFJR1300]

Just to add to Final's post, if you hook up a clear hose to the drain nipple on each carb(one at a time) and open the carb drain screw, while HOLDING THE TUBE UP, it will SHOW THE FLOAT LEVEL and if your getting fuel. You of course can also drain the float bowl into a small very clean can and check for crap.


If you have a vacuum petkock, they'll be a rubber line to one of the carbs. Check that closly for any cracks. If its cracked and leaking, it'll turn off the fuel valve(the valve thinks the engine is NOT RUNNING and turns off the fuel).

Did you ever have the tank valve out and check the filter inside the tank? Should your bike have a manual valve, it'll have an OFF, ON and RESERVE setting. With the valve out of the tank, they'll be two plastic pipes sticking up with a filter on the end. The shorter one is the reserve. Make sure both are clear. *You can test them by hooking up another fuel line to the valve and then setting the valve and checking different positions...

Here's a pic and link of your tank and the valve as posted above:

Yamaha Motorcycle Parts 1988 FZ600U FUEL TANK (NON-CALIFORNIA MODEL ONLY) Diagram


Looking at the illustration, (double click to enlarge it), you do have a valve lever (on, off, reserve) and another hose that MAY be a vaccum line .

Also, you mentioned syncing the carb's. If you haven't had them apart, it's very likely there's some varnish/junk in there..

**I'd also be checking the throttle slide DIAPHRAMs (part #10)(in each carb). Its not unusual for over time, for the material to break down. Once there's a vacuum leak there, the slides will not respond to engine vacuum (as the vacuum is limited / gone) and thus no throttle response. It idles now (high vacuum, slides are closed).