Bike Idles at 1000, but blip throttle and rockets past 4k and stays or climbs

[Snow]

This is making me pull my hair out. I just swapped motor and put my original back in, albeit with 95 more cc, and have run into one final problem.

If I just let her idle, she is fine and dandy sitting right at 900-1100, wherever I decide to put the idle screw.

But if I just lightly blip the throttle, she shoots up to 4-4500rpm and either sits there or keeps climbing, in which case I kill the ignition.

This is a 79 standard motor but with , what I believe, to be 81 Special carbs. Right now, I have 115 main jets and the rest *should* be stock, but there is no guarantee.

The throttle cable is not getting stuck, and in fact, is a brand new cable bought last week.

There are no leaks in the carb boots, verified by spraying wd-40 all around.

On the old motor, these carbs were fine, no oddities whatsoever. The only difference is that they now have 115's instead of 110 main jets and the motor is different. Even the carb boots are the same. Also, the advancer is the same unit. I have already synced the carbs on this motor also.

ANY, I repeat, ANY thoughts, suggestions, or tips are *greatly* appreciated.

[skids]

Re: Bike Idles at 1000, but blip throttle and rockets past 4k and stays or climbs

You did you set the float height? Was it something like 0.91 inches (unlike earlier carbs)? When you checked/adjusted the synch, at what rpm did you do it (should be near idle)? Did you adjust the pilot screws once you synched the carbs (this is an important step). Are the starter jets all tightened to the slider bar?

Originally posted by Snow
If I just let her idle, she is fine and dandy sitting right at 900-1100, wherever I decide to put the idle screw.
But if I just lightly blip the throttle, she shoots up to 4-4500rpm and either sits there or keeps climbing, in which case I kill the ignition.

[Snow]

No sir, I haven't adjusted float height, I guess that is first on the agenda tommorow. When I synced the carbs, I had her idling at 1000 rpm.

Not exactly sure what you mean by pilot screws (I know too incredibly little about carbs at this point it is scary). Also, I'm not sure what the starter jets are :/

[uk1.1s]

Carbs hanging

Snow,

Now I'm not going to be you saviour here as my carb knowledge is limited, but I have suffered the same thing (on another '70's Jap bike) - ending up with the pulling of hair, kicking of the garage wall and giving it to someone else to fix

That's usually against my nature as I don't often trust others to fix my bikes (usually for good reason in my experience).

I can't quite remember now if it was explained as being an over-rich or over-lean problem, but it is a common symptom and I'm sure someone here will be along in a minute with details (I hope).

From memory the bike I had trouble with came back with new main jets (smaller) - so too rich is my guess - have you tried fitting the 110's back?

If the only change to your carbs are those bigger jets then there's your trouble I suppose - try changing back and working from there. Smaller adjustments in fuel enrichment can be made then by:

1. The fuel/air adjustment screw on the carbs
2. The height the slide needle rests in the slide (some you can't adjust too easily I'll admit)
3. Float height.

All this is coming from someone who messed up so much playing with the carb settings that I got lost in what I was doing and ended up CAUSING this sort of problem!!

Take heart that it should be an easy fix.

[Snow]

Ok, I've been doing much searching and reading on the forums and I had no idea the floats could affect the fuel/air mixture, I thought they were just there to stop the gas from overflowing the floatbowls...

It seems that the 80 carbs require a 23mm measurement, so I am guessing my 81 carbs require the same. Now, I read that the measurement is from the top of the gasket (bottom if right side up). My new question is: what exactly am I measuring here? I do have a manual, but it is out at my folk's garage with my bike and I am trying to make sure I understand everything before I go over and start tinkering.

Also, my mixture screws have the factory plugs on them and I really don't want to mess with them if possible.

[amatco]

Hi,
I just posted a problem and I noticed that your RPM was almost exactly the problem I have. If I can figure it out I will pass it on.
I have a 1981 Venture xs1100H.
I will monitor your replys. There is help on this site and guys know what their doing.
Good Luck, Allen

[Snow]

Well, still have the problem, but it is a tad better at best. I set the float height to 23mm, was at 20mm for some reason. Also threw in my 112.5 main jets.

[Snow]

Ok, I am trying another method and setting these carbs aside. I have a set of 78E carbs that were mostly together and ran, just not well and am setting everything to spec and putting my 140's in them (has reeeally old nasty looking 137's).

I'll post whatever happens...

[Snow]

...now I remember why I wasn't using those carbs...my bro had already broken the mixture needles off and replaced them, but apparently they don't work at all...back to the 81 special carbs :/

[CA.XSer]

Snow

If you remove the throttle cable from the carbs and use your hand to work the carbs, does the rpm still stick and/or climb? Will the throttle shaft return to closed position when released (from any position)? Is the throttle shaft(s) binding up? Do you have a cruise control on your bike that may be binding the throttle control? Just some thoughts and questions that I came up with.

[Snow]

I'll double check, but I've already put in a new throttle cable and it has slack after I "blip". I'm heading back out to battle the best a in little bit.
Also, I do not have a cruise control.

[Dennyz]

here is an idea:

What might be happening is one or more of the carbs may have their idle speed set too high. This allows enough air volume to cause a slide or two to stay up. The engine idle initially stays down due to incorrect idle mixture settings, but once air starts to move (open throttle valves), the added fuel increases the idle rpm.

Typically this happens when the idle mixture is too lean, causing the engine idle to be too low. However, rather than correct the mixture problem, the idle speed adjust is turned up to obtain a proper idle speed.

Try this:

-First off, if you have not synced the carbs, you should before continuing.

-Record the current location of the 4 idle screw positions.

-Lower the engine idle speed down as far as possible without killing the engine with the main idle speed adjustment.

-Adjust no. 1 cylinder's idle mixture for an increase in idle speed. This is a trial and error process. First try clockwise a 1/4 turn and see what happens. If the motor speeds up, you’re turning the correct direction. If the idle speed slows down, go the other way (ccw). If neither direction produces any increase. Try the next carb (return mixture setting to the orginal position).

If you get through all four carbs and not able to increase the idle speed, then it is likely your problem is something else. Don’t forget to return the idle mixture screws to their original location.

If you do find one or more of the carbs causes an idle increase, then the problem is in your idle mixture settings. I found the Colour Tune to be the only way to set the idle mixture correctly.

Good Luck,

[wpc57]

Hey guys ...... My old YZ would act the same way when the AIR FILTER wasn't sealing properly. I might be way off but it might be worth it to check out if his filter is sealing properly first.

[Snow]

Today I cleaned, washed, and re-oiled my K&N filter, made sure there where no leaks in the airbox, and carefully reinstalled teh filter and airbox, still no go.

Is it possible this might have something to do with the timing of my motor? We did have troubles setting it when putting the motor back together, or has it have to be related to the intake system?

[skids]

The synch trick seems as if you did it at the right rpms. Pilot screws are idle enrichening screws. They are on top of each carb and are hidden unless you have a 78E model. The caps covering them on the 80 and 81 carbs are a bugger to remove. It is difficult to get them right without a colortune. I am not too sure that I could even do it "by ear." Someone once said that you turn the screw inward until the engine begins to slow and then you back it off about an eighth turn. Do that to each screw. Then you need to color a new set of plugs to examine them. I am not saying that the idle screws are giving you the problems, but it might help. Sorry I can't be more definate.

Originally posted by Snow
No sir, I haven't adjusted float height, I guess that is first on the agenda tommorow. When I synced the carbs, I had her idling at 1000 rpm.

Not exactly sure what you mean by pilot screws (I know too incredibly little about carbs at this point it is scary). Also, I'm not sure what the starter jets are :/