Bike revs to redline. . .

[Narcisist]

I was riding home from work the other night and as I pulled onto my street my bike revs up to redline and I had to kill it to shut it down. Now when I start it up it just shoots straight to redline.
I've looked at the throttle cable and it seems fine, I can watch the throttle linkage open and fully close with no effect. I haven't left it running for more than a few seconds. . . Any ideas?

[psycoreefer]

Did you try turning the idle adjust screw in the middle down?

I'm not sure what could have changed mid-ride, but the first thing I'd attempt is adjusting the idle down with that adjuster.

[dbeardslee]

I wonder if that throttle cable was made by Toyota..... When is the last time your carbs were synced? I'd follow psyco's advice and turn the idle screw down (counterclockwise) and check your carb adjustments - they can and do change. Might be a good idea to lube that throttle cable too if it hasn't been done in a while. My $.02

[Narcisist]

Where exactly are all the adjustments on the carbs on this bike?

[Narcisist]

I've only owned the bike since last february, bought it on Friday the 13th. But until now it's been running great, sits at a rocksteady idle at about 900 rpms. It has developed a backfire out of the left pipe when I throttle down on freeway off ramps.

I work on cars on the side sometimes, but I seem to know very little about bikes.

[Ivan]

On a 79, the idle adjustment is in the middle and points up through the carbs. It has a notched wheel, easiest to adjust it with a long flat blade screw driver. The idle screws are on top of the carb forward of the top cover. Inside on the vacuum slide, there is an adjustment for the needle height. Its adjustable by moving the clip. There are main and pilot fuel jets that can be replaced and an air pilot jet that can be replaced. The sync screws are on the throttle linkage between the carbs. Float height is adjusted by bending the tab the needle rides on.

I think thats it...

[DenyP]

I was riding home from work the other night and as I pulled onto my street my bike revs up to redline and I had to kill it to shut it down. Now when I start it up it just shoots straight to redline.
I've looked at the throttle cable and it seems fine, I can watch the throttle linkage open and fully close with no effect. I haven't left it running for more than a few seconds. . . Any ideas?

Sounds like a cable problem. It's hanging up somewhere and not letting the carbs close all the way. You can verify this by disconnecting the cable from the carb assy and start the bike.

Deny

[dbeardslee]

It has developed a backfire out of the left pipe when I throttle down on freeway off ramps.

Backfiring is one symptom of out-of-sync carbs. It can also point to valve clearance issues, but that shouldn't cause it to rev. I think Denyp's idea on disconnecting the cable was a good one.

The idle screws are on top of the carb forward of the top cover.

I think what Ivan meant was idle mixture screws - not to be confused with the big idle screw. I wouldn't monkey around with the mixture screws unless you're confident that you can set them properly - for this problem anyway.

[tadtl1]

new to bikes

Hi,

The fact that this whole thing started when you were turning a corner, indicates to me that its likely that a cable has caught on something during the turning process or a cable housing is out of position perhaps on the stop that holds the carb end of the cable. Ive worked with cables a lot, and an out of position cable stop or housing, can do stuff like this easily. The good news is, that should be easy to fix once you find that. To a person unfamiliar with bikes, and precisely what to look for, it could still have the appearance of open and closed throttle working ok. I dont really know much about the vaccuum parts of the carb, but those vaccuum diaphragms somehow control the throttle slides, is it possible that a vaccuum line has come off and caused this, I doubt it, but it can be checked easily.

[Narcisist]

Well, to eliminate the easy, I have disconnected the throttle cable from the carb linkage and it's still redlining.

[DGXSER]

Just to be sure I got this correctly....

You were riding along fat dumb and happy, bikes running smooth, turn the corner and BAMMM...it just starts redlining? Now after shutting it down and letting it cool, turn the key and redline??

Something somehwere has your throttle linkage stuck. Nothing about these carbs would go THAT badly out of synch that fast. JMO

[Larrym]

Schwinn, Trek, Huffy???

I'm going along with tadtl1's notion about the throttle cable pulling out of the retainer socket either at the carbs or at the grip "J".

New to bikes? Maybe motorcycles.....but the technology/hardware for the bike's throttle cable is the same used in ten-speed bike brakes and shifters:

A cable run through a sheath with little end caps that are supposed to fit inside the recesses and stay there.

When they don't stay put it's either the cable has popped out due to too much slack in the cable or due to too much friction.

I"d consider re-seating the cable ends into both the throttle grip "J" fitting and the one at the other carb end, lube the cable, and adjusting the cable slack per the manual's instructions.

Lubing the cable is a lot easier with one of these:



It slides over the cable end and lets you spray the aerosol lube of your choice in one side of the cable til it drips out the other end.

Course you'd likely have to walk in to a bike shop to get it.

Note: this post written before reading about yours about "disconnecting the cable entirely". Too late to delete. OOps!

[GAXS1100SH]

I had this issue also.

Hello, when I first bought my bike, I took it for a ride after I fixed some wiring issues , anyway, the PO said that he had rebuilt the Carbs and I assumed all was well. Then during the first ride same thing happened to my bike. The problem was that when the PO put the carbs back on he ran the Clutch cable through #3 and #4 boots instead of the out side of the carbs.
What was happening was that the clutch cable was catching on the sync screws and was jaming the throtle open. when I got a manual I realized that the cable runs on the outside of the carbs and I took the carbs off and put the Clutch cable where it was supposed to be and all is Well.

Hope this helps. Ride safe,
Thanks, Steve

[dbeardslee]

Well, to eliminate the easy, I have disconnected the throttle cable from the carb linkage and it's still redlining.

Looks like you have effectively eliminated the cable as the problem. About the only thing left that would cause that kind of problem is carbs. Unless you've fiddled with them or a vacuum line came off (make sure your vacuum advance is connected to the tap on the carb, not the one on the carb boot), to me it's an indication that the carbs either have an internal issue, a linkage issue, or they've gone out of adjustment. I'd check the dumb stuff first - check your vacuum lines, lubricate your linkage with some WD-40 and check that sync. There's a finite number of things that can cause high revving and sync is one of them.

[dbeardslee]

Cheap dirty way to check sync

If you don't have a 4-gauge carb balancer, you can pick up a combination vacuum/pressure gauge at Harbor Freight for $12.99. Put it on the carb boot taps one-at-a-time (after dialing down the idle with the idle screw, and with the vacuum caps in place on the other three carbs), and record the readings on each one (at the same rpm). To be properly balanced each carb should be within 2cm vacuum of the others. Or you could spend a few bucks more and pick up a power bleeder (like DGXSER uses), and use the vacuum gauge on the bleeder - then you'll have a sync tool and a power bleeder. Diagnosing mechanical problems is as much a process of figuring out what's not wrong as it is figuring out what is wrong. With each potential problem you eliminate you're that much closer to the cause.

Combination gauge - $12.99



Power bleeder - $29.99