Topic: Cb750 k4 throttle sticking...only when running

[Court750Cafe]

Hi Guys,

I searched the forums but didn't really find anything for my specific problem I don't think..

I Recently adjusted the carb floats off the bike to spec. Put them back on, put the tank and everything else on, checked the throttle and it was binding a bit. Adjusted the cables and now it snaps shut very very well. Put the tank on, everything seems great. Carb synced the bike so looks pretty good on all 4 cylinders.

Problem: Once I start the bike, if I rev it, the throttle will be hung up and not snap back to its starting place. If I force the throttle a bit the RPM's will die down immediately.

I don't know if it's the throttle cables (haven't replaced them because when the bike isn't running, they seem great), but it feels like something in the carbs are causing the throttle to not go back to the idle position.

I turn the bike off, and check the throttle multiple times, and it seems to snap back well just as it should again. I can't for the life of me figure it out.

If you could give me some insight that would be great.
Carb spec:
stock main 105
Idle: stock 40

K&n stock replacement filter
4-2 exhaust

[flybox1]

does the problem persist with the tank OFF the bike while it is running?

[Harsh]

I am thinking a bit outside the box here.  Check the bolts that secure the arms to the carbs.  I recently ran across a set that had worked themselves loose and were causing the throttle to hang.  With the bike off and I rolled the throttle they would snap back, but all four carbs were at different positions and it wasn't noticeable until the bike was running.

[Court750Cafe]

does the problem persist with the tank OFF the bike while it is running?

Thanks fly box for your post. I believe it still does the same thing with or without the tank on the bike. I can try it though without the tank and see.

[Court750Cafe]

I am thinking a bit outside the box here.  Check the bolts that secure the arms to the carbs.  I recently ran across a set that had worked themselves loose and were causing the throttle to hang.  With the bike off and I rolled the throttle they would snap back, but all four carbs were at different positions and it wasn't noticeable until the bike was running.

This is definitely so,etching that could've happened. I know I loosened those same bolts when the carbs were off the bike. I know I tightened them but sounds like you had the exact same issue as me. Thanks for your response I will check it out

[Court750Cafe]

Also, should I rule out a vacuum leak? Because if I physically roll the throttle back to its starting position, the engine responds as it normally should.

[Harsh]

Also, should I rule out a vacuum leak? Because if I physically roll the throttle back to its starting position, the engine responds as it normally should.

That is what I initially thought was happening and I had the same result on the bike I was working on.  Then out of pure luck I caught the brackets that connect the 1&2 carbs and the the 3&4 carbs weren't moving together.

[Court750Cafe]

Update:

I took the tank off and the carbs were still yielding the same results. I started the bike, and blipped the throttle and it would stick. So then I took the throttle pull cable off of the carb rack, and manually pushed down the rack while the bike was running to see if it would continue to stick. And yes it would. I don't know if this means that the throttle pull cable is forcing it to not go back down or not, but it seems like something inside the carbs are causing them not to slide back down when I let go of the throttle..

Thoughts?

Thanks,

[spazz1984]

How does your return spring look ? And is this a 2 cable system?

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[Court750Cafe]

How does your return spring look ? And is this a 2 cable system?

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I don't know really how to tell if my return spring is shot.. When the bike is off, it feels very "snappy" when I let go of the throttle.

And yes these carbs have throttle push and pull cables.

Thanks

[spazz1984]

Try disconnecting the push n just use the pull
And for the spring it happen to the rubber around the spring wouldn't let seat all the way sometimes
That's all I can think of besides lubing the rod the connects the forks together or even the lift rods coming out of each carb

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[Court750Cafe]

Try disconnecting the push n just use the pull
And for the spring it happen to the rubber around the spring wouldn't let seat all the way sometimes
That's all I can think of besides lubing the rod the connects the forks together or even the lift rods coming out of each carb

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I'll try that tonight thanks for the post!

[Harsh]

Any chance we could get a pic of the top of the carbs?  I am curious how many threads are showing.  Did you check the bolts that secure the arms to the rod to see if they are tight?  However, I think spazz might be on to something.

[CB750K4]

Something that I found. 
My tank was a bit rusty, so had some very,very,very fine rust mixed in with the fuel.  I guessed that this could be causing a bit of friction between the slides and the carb bodies. 
First I tried inline filters.  Caused more problems than it solved. 
Then I tried adding a small amount of 2 cycle oil to the fuel.  Sticking problem solved.  Ran it that way for a year or so. 
Then just cleaned the tank.   

[Court750Cafe]

Hey guys,

I checked the carb spring and it doesn't look like the rubber is slowing it down. It acts as it should when the bike is off. I can take a pic of the threads of the top of the carbs this weekend. FYI barely 1 thread is showing though. Thoughts? Also I notice when I turn the idle screw completely out, the bike still doesn't die. I feel like the idle screw adjustment should stall the bike if turned all the way out correct? I don't know if that has anything to do with the issue.

Seems like when the bike is off... Everything is working great. When on, throttle sticks. Feels like something in the carbs are causing them not to drop down without a bit of force from me. I don't know if vacuum has to do with this or not? Should I just pull the carbs and do a cleaning? Bike is running pretty well I didn't think there is any junk in them.

[Court750Cafe]

Something that I found. 
My tank was a bit rusty, so had some very,very,very fine rust mixed in with the fuel.  I guessed that this could be causing a bit of friction between the slides and the carb bodies. 
First I tried inline filters.  Caused more problems than it solved. 
Then I tried adding a small amount of 2 cycle oil to the fuel.  Sticking problem solved.  Ran it that way for a year or so. 
Then just cleaned the tank.   

Tank isn't rusty, PO used that tank sealer in it before I had it. Doesn't look bad in the tank though. But something could be stuck in the slides of the carbs regardless. I can pull them apart and clean them up this weekend.

There's only so much that can be the cause of this correct?

[Court750Cafe]

Any chance we could get a pic of the top of the carbs?  I am curious how many threads are showing.  Did you check the bolts that secure the arms to the rod to see if they are tight?  However, I think spazz might be on to something.

Just grabbed a couple quick pics. Sorry one might be a bit blurry. I can record a video this weekend if you guys think it will help solve this mystery.

Thanks again for the help.

[spazz1984]

Any chance we could get a pic of the top of the carbs?  I am curious how many threads are showing.  Did you check the bolts that secure the arms to the rod to see if they are tight?  However, I think spazz might be on to something.

Just grabbed a couple quick pics. Sorry one might be a bit blurry. I can record a video this weekend if you guys think it will help solve this mystery.

Thanks again for the help.

Q: are you running it with boots like that in the photos
But anyway did you try just running the one pull cable or try any lube on moving parts that's if you have had the time that is ?
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[Court750Cafe]

Any chance we could get a pic of the top of the carbs?  I am curious how many threads are showing.  Did you check the bolts that secure the arms to the rod to see if they are tight?  However, I think spazz might be on to something.

Just grabbed a couple quick pics. Sorry one might be a bit blurry. I can record a video this weekend if you guys think it will help solve this mystery.

Thanks again for the help.

Q: are you running it with boots like that in the photos
But anyway did you try just running the one pull cable or try any lube on moving parts that's if you have had the time that is ?
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Nope I just removed them to show you the threads on the carbs. I did lube the slide rods but no change after that. I will try out just the pull cable method and see if the push cable is causing anything. I did detach both cables at one point and tried running the bike and manually pushing on the carb rack to increase rpm's. It still was sticking so I figured I could rule out throttle cables.

[spazz1984]

Any chance we could get a pic of the top of the carbs?  I am curious how many threads are showing.  Did you check the bolts that secure the arms to the rod to see if they are tight?  However, I think spazz might be on to something.

Just grabbed a couple quick pics. Sorry one might be a bit blurry. I can record a video this weekend if you guys think it will help solve this mystery.

Thanks again for the help.

Q: are you running it with boots like that in the photos
But anyway did you try just running the one pull cable or try any lube on moving parts that's if you have had the time that is ?
Sent from my SPH-L710 using Tapatalk 2

Nope I just removed them to show you the threads on the carbs. I did lube the slide rods but no change after that. I will try out just the pull cable method and see if the push cable is causing anything. I did detach both cables at one point and tried running the bike and manually pushing on the carb rack to increase rpm's. It still was sticking so I figured I could rule out throttle cables.

The only other thing I can think of really is the turn spring or even tho you sync the carbs is that one of lift rods is just a little higher or one is just a little lower
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[MikeKato]

Sounds like your tank liner is failing if it only happens when gas is flowing. I would drain some gas from the tank into a clean glass jar and hold it up to the light to look for residue of your liner. Inline filter(always) and a small amount of 2cycle oil also sounded like good suggestions. Carbs sound like they are dirty.

[spazz1984]

Bump

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[Court750Cafe]

Sounds like your tank liner is failing if it only happens when gas is flowing. I would drain some gas from the tank into a clean glass jar and hold it up to the light to look for residue of your liner. Inline filter(always) and a small amount of 2cycle oil also sounded like good suggestions. Carbs sound like they are dirty.

I'll check the tank and do what you suggested this week. I went out for a couple rides this weekend on it and it seems to not stick when driving in higher gears. I'll check again and give you guys an update if it's only at idle that they stick.

Thanks again.

[harisuluv]

I think you are overthinking this.  Look at your first post and ask yourself what changed.

The floats being adjusted wouldn't change anything.

1) you did a carb sync
2) you are showing almost no threads on top
3) you cite lack of response from idle adjust screw

Your carbs may be synced with each other, but they may not be synced in a position where your idle adjust screw allows adjustment each way.  In other words, your idle screw might not even be touching the stop. You want end idle screw touching and then some, allowing for adjustment each way, THEN sync the carbs.

For example if you synced all your carbs at wot then set your idle screw, we'll guess what, when the slides come back down that screw is touching nothing and is just a paperweight turned in a few threads.

Check contact on your idle adjust.

Also you have had your frame cut right around where your carbs are living and when you hit the throttle they are moving around right next to a place that was never intended to be cut.  Check for weld seams or clearance issues from the frame shenanigans.

Also, a drop of oil for the lifter slide rods is ok but in my experience there is usually more of a binding issue with the linkage forks and the adjusters.  Try some lube there.

[ekpent]

Agree with harisuluv if your cables check out OK that if your idle adjustment screw will not work or kill the engine you do not have enough headroom in the carb slides or in other words they are set too high right now. The fix is changing the position of the synch adjuster screws/nuts. Other cause is usually from gummy or dirty slides. A very small amount of slide opening can drastically change the idle so they need to be able to bottom out well.