Throttle Jerk

[rickmcdougald]

Yes, I said, "Throttle Jerk!" (But don’t take it personally!)
Observation #1. That's what I call the condition that, at a very low speed, say when doin' a U-ee in the street, my XJ1100's engine RpMs drop and rise SUDDENLY! Chunk, chunk! Chunk, chunk! Around the tight, little corner. Woah! It takes a lot of practice and right-hand control (read "white knuckles") to keep it smooth when the throttle handle is in that range.
It also happens when I'm in the “slow zones” – you know, the 5 – 10MpH, “Slow Children” signs, and speed bumps. Sometimes I just have to either feather or pull in the clutch and coast a little ways.
It also happens at higher speeds when I release the throttle grip quickly, but with fewer consequences - it's scarier when you're in a low-speed turn!
Observation #2. I also have noticed that when I'm using compression to slow the bike at any speed, there's a mild form of back-firing in the XJ's stock exhaust.
I haven't yet pulled the plugs, but I'd be willing to guess that they'd be black.
She's really got lots of power starting around 3200RpM. If I try to open the throttle too quickly any lower than that, she really bogs down.
For documentation, I have the Yamaha Service Manual, but no good owner’s manual (I’ve got a poor PDF).

Does this sound familiar? Common? What do you guys think?

Thanks for all of your help!
RLMcD

[TopCatGr58]

Quit jerkin' us around!!

Hey there Rick,

I've seen several folks look at this thread, but no replies, so I'll start things off!?

First, check your throttle cable, may be getting kinked or binded, especially in those tight turn situations. When the bars are straight ahead, there is supposed to be a little slack in the cable, if the throttle responds with just the slightest beginning of turn of the throttle grip, then it's slack is set to tight!

Backfiring or popping on decel is usually related more to a leaner condition, so your plugs may not be as black as you suspect!?

Poor throttle response in the lower rpms could be several things! Do you have a new air filter, or old one clogged, snorkel pinched or clogged with "stuff"? When was the last time you cleaned your carbs? Could have varnish in the pilot circuit, so that it doesn't flow well=poor variable throttle response until you get to 3+krpms, starting to get into the vacuum slides and mains!?

Checked your vacuum advance hose lately, it'll be going to that little black box under the tank, the vacuum sensor, if it's cracked, might not be getting the proper signal to advance the throttle under low load conditions making it harder to respond to throttle input??

Recommend putting in some SEAFOAM in the tank, and running it thru, might provide an easier cure than tearing the carbs apart, but nothing beats a good "triple" clean !!!
Report back what you've done, what you've found and further info as needed!
T.C.

[mro]

"Throttle that Jerk!"

I'd check for a vac leak(s) first. Sync carbs. Check/adjust piolt screw (Xj got em??? ) Sync agin......

No help, then clean carbs.


mro

[wingnut]

Could have varnish in the pilot circuit

Has it always run like this or is this new? Has any one else been in the carbs? Sounds like my SH with K&L 4 hole pilot jets. Went back to the stock 3 hole jets, sync'ed the carbs and every thing was fine.

[randy]

Yep,

Pilots.

That is all....

[81xsproject]

I don't know, but with my bike it just seems like there is so much power that even the slightest bit of throttle and it really starts to move, especially at really low speeds in first. Like when taking off at a light behind cars. I always have to immediatly let off or I'll drive right up some cage's @ss. And with all the compression of the XS I start slowing right down. Just kinda herky jerky if I am not really tring to feather the trottle. The fatter grips made a deffinate difference there.

[81xsproject]

oh... what about a sloppy U-joint? Could that do it????

[remccool]

As far as the turning, I had a similiar situation with Sancha when I first got her ... oh yeah, back to the bike... I traced it down to my throttle cable routing. It was routed through the triple tree then back under the tank, I was actually pulling the entire cable on tight turns and didn't see it at first, I moved it to run on the frame side of the triple tree and the problem went away. Just something else to check.

[skids]

Come to think about it, also check to ensure that the tab by the right grip is still there. I mean that piece where the end of the cable exits the sleeve at the "goose neck." If it breaks, the throttle get very erratic.

Originally posted by remccool
I traced it down to my throttle cable routing.

[rickmcdougald]

The "Jerk" is dead!

Hi guys,
I know it took a long time to complete this thread, but I've been busy!
And the answer is:
The idle mixture screws needed further tweeking!
Remember that this bike had spent a lot of time, in boxes, as an "exploded diagram" because when I got her, she needed a lot. I decided to completely (did I mention COMPLETELY?) disassemble the bike. Of course, you know, that means all new rubber seals, gaskets, grommets, Yamabond #4, and etc!
When I got to the carbs, (or anything else for that matter) I wrote down any settings. The mixture screws, in the front of the Mikuni carbs, had wildly different settings. Also, they were supposed to have a little, solid brass plug over the top of them so that you couldn't mess with them. A hole had been drilled in all four of them. I pulled them out with a screw and a slide hammer (it didn't take much), then went on with my work.
When I reassembled, after their cleaning, I set them to 1.5 turns OUT (from the seat). Then synchronized the carbs with my four-tube "carb stick". It really ran great, but had the "throttle jerk" I'd described.
Now she's ALL BETTER! I tried adjusting them OUT at .5-turn intervals, then riding. I did that a few times then went back to the original settingand turned them IN at .5-turn intervals. I ended up adjusting them IN so that they were now .75-turns OUT from the seat.
I checked out everything else that was suggested in this thread, too.
Thanks for the help, guys - MRO, Wingnut, and Randy hit the nail on the head this time!

[apolloant]

same issue

I put 142.5 mains, 42.5 pilots, syched everything, had the idles out 1.5 and went for a ride. First nice ride I've had on this bike. Then the weather got cold again, and it was back to the herky-jerky at take off to about 3200 rpm.(hard to look at the tach when you doing the jitterbug and trying not to pop over the car in front of you). So, I started adjusting the idle mix screws out.5 at a time, seems to make the problem worse. The only thing that kept me from getting run over last night, @ 35 degrees, was pulling the enricher out to the first notch, smoothed her right out, but hard to sit at the light at 2000 rpm idle, so in and out as needed. Got real old real fast. Pull enricher out, pull in the clutch, put in gear, go. So, this leads me to believe that it's lean on the bottom. I could go on and on. Anyway, I'm trying this next.

[Yamaniac]

sounds like my issue, also

Have to choke it to start (75 degrees out), and then it idles at 2k when warm. Carbs are just bench synch'd now. have a merc stick coming from JCW this week. Sounds like I need a little tweaking of the pilot screw as well.

[Ken Talbot]

Re: sounds like my issue, also

Originally posted by Yamaniac
Have to choke it to start (75 degrees out), and then it idles at 2k when warm. Carbs are just bench synch'd now. have a merc stick coming from JCW this week. Sounds like I need a little tweaking of the pilot screw as well.

Choking to start is normal. High idle whan you bench sync is also normal. You adjust the idle speed with the idle adjuster located between #2 and #3, not with the pilot screws.

[Yamaniac]

Ken,

think I have the idling figured out. I got it adjusted to just over 1k. Just thought it was odd that, with the air temp almost warm, I would have to choke it. Haven't messed with the pilot screws at all, yet....1.25 turns out.

Went riding tonight.....26.5 mpg. cruising speeds of 55-65mph. Something is out of whack. floats at 25mm. New K&N air filter in stock box (not drilled). All jets are original. New K&L needle valve, seat, and gasket.

don't know whether rich or lean. Prob. do a plug check tomorrow to see. Other than that, not sure where to begin until I get the carbs vacuum synched.

[prometheus578]

"I hope you choke...!"

Just thought it was odd that, with the air temp almost warm, I would have to choke it.

For successful combustion, the fuel needs to be a finely atomized mixture. Regardless of the outside temperature, the engine is still cold and the incoming fuel will tend to condensate in the intake tract and on the cylinder walls. It is no longer vaporized and the engine will be hard to start. Hence the need for the choke to richen the mixture. (Also, more fuel is needed to run at low RPMs, like when starting) After the engine warms, the choke is turned off to prevent flooding.
26.5 mpg... sound like you're going through a lot of gas. I'd say the plugs will show rich. Question then would be why?