Topic: Cb750 starts revving...

[koendd]

So last ride last year 5mins before i got home my k2 750 started acting up...
When idling and between shifting gears  the bike started revving. One blip on the throttle and it would idle fine again.
Few weeks ago first ride of the year same problem...

Yesterday started with ignition, cam chain, valve adjusment and carbs synch.

Lube the cables and slide shafts, checked all manifold rubbers and thightend if necessary, same problem!

Only does it when warm, with one (sometimes 2) pulls on the throttle it idles again after a split second.

My guess is it isn't drawing any false air, checked with brake cleaner, no change in rpm...


Any ideas?


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[flybox1]

check for throttle cable binding, return spring function, slides snap closed...etc.
do you have both push/pull cables?
does the 'push' cable help bring the idle back down?

[drumstyx]

Vacuum leak maybe? I've never had any luck testing for small vacuum leaks with a spray can of stuff. Smoke is probably a better way to see, but do you get this issue when you're idling in your garage?

[koendd]

I have pull push yes. Checked when revving and cables and shaft are free so cables not involved. Pushing the cable has no effect.

And will check again for vaccuum leaks...

Does the same not under load so when i got back home after test ride earlier left it running at idle on the driveway. Twisted the trottle, started revving again...

No idea where it would be takkng false air though. Bought new oem
Boots last year. Ran fine ever since all summer until last ride last year in november.





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[drumstyx]

Could you be describing a hanging idle? This can be caused by a lean condition, or, as I've discovered, very unbalanced carbs that have been compensated for with the idle screw being set too high.

[flybox1]

All your vac sync port screws securely in place?
Boots removed and checked for cracks not visible unless the boot is bent out of round?
when was your last vacuum sync?

[Grabcon]

Have you checked you ignition advance? Is it clean and does it move freely?

[Spanner 1]

I believe this is caused by one float bowl overfilling, maybe a float valve 'cocked' a little sideways. When the fuel level is too high the engine vacuum will pull fuel from the main jet at idle and cause the engine to rev as if you had twisted the throttle . An air leak between the carbs and motor would do the same thing, but you say you have checked that. I would take the carbs off ( ugh ) and polish the float tabs where the float needle sits to be sure there is no' wear spot' the float needle can get stuck in, doesn't take much to upset the float needle !

[koendd]

Could you be describing a hanging idle? This can be caused by a lean condition, or, as I've discovered, very unbalanced carbs that have been compensated for with the idle screw being set too high.

Well it ran fine last year with same setup. Original airbox only open 4/1 exhaust but no problems with it ladt year...


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[koendd]

All your vac sync port screws securely in place?
Boots removed and checked for cracks not visible unless the boot is bent out of round?
when was your last vacuum sync?

Did a synch 2 hours ago as last part of the tune up. Weren't to much out of synch though. No improvement


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[koendd]

Have you checked you ignition advance? Is it clean and does it move freely?

I did and not jammed ...


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[koendd]

I believe this is caused by one float bowl overfilling, maybe a float valve 'cocked' a little sideways. When the fuel level is too high the engine vacuum will pull fuel from the main jet at idle and cause the engine to rev as if you had twisted the throttle . An air leak between the carbs and motor would do the same thing, but you say you have checked that. I would take the carbs off ( ugh ) and polish the float tabs where the float needle sits to be sure there is no' wear spot' the float needle can get stuck in, doesn't take much to upset the float needle !

Ok maybe but only revvs when twisting throttle. Idles perfect otherwise....

Guess i'll just have to pull the carbs and check boots carbs etc as suggested... Grrr




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[koendd]

pulled the carbs earlier.
didn't find anything disturbing. boots still in excellent condition. One float valve had a slight 'ring' so polished it with 800grit.

other then that nothing really. So will refit them tomorrow.

I did notice however there's much more tension on the spring of the return when off the bike. they don't close as fast when the cables are attached. So maybe some tweaking needed on the cables after all...

[flybox1]

Aftermarket cables?  Check cable routing...no sharp bends.  Slides should forcefully snap shut.
A friends cb750 had slides that wouldnt snap shut. 
Aftermarket return cable was tight(short) beyond adjustment, runs without it now with no issues.

[ekpent]

 I had a K4 750 doing that once and all was good when I cleaned the slides/needles in the carbs,one may have been hanging up with some build-up. When was the last time your carbs were off the bike for a service ?

[evinrude7]

koendd, did you ever solve this issue?  i have the same.  although as i ride the bike more and more it improves. 

[koendd]

well maybe didn't have time for a testride yet.
I have another 750 in the shop and when you release the throttle it snaps back, mine didn't.

Have an aftermarkte cable grip so opened it up again, lubed it al again and now it snaps back as well.
Because I noticed when I would loosen the 2 bolts a bit it would snap back to...

So now that is fixed, will try to do testride tonight and get back to you

[evinrude7]

Looking forward to it.

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[koendd]

did a testride, no change unfortunatly...

Snaps back when not running, but when I ride it and stop to check when it's revving again it idles again with a little push on the accelerator arm on the carbs.
So defitinatly something in the carb linkeage or maybe throtlle cable has to much tension.

so I can allready rule out an airleak..

[evinrude7]

keondd, here's the list of things i plan to go over to try to solve my revving issue.  i also have a back fire issue with 1 and 3.  3 is worse. 

replace advance springs, points gap, timing, vacuum leaks, tappet valve clearance -again
jet needle clip settings? fix those for stock air box
657A  floatheight26mm  main110  slow40  jn4 screw1out
service carbs again
clean all air passages in carb including air screw mixture holes.
check plug wires and boots (boots should be 10K ohms)

i did find a vacuum leak on #3 airbox boot even after i tightened the hell out of it when i finally got my stock airbox on.  over the course of 2 weeks the clamp on there was loose.  someone on here pointed out that attaching the boots then pulling the airbox back to it's attachment brackets was the way to go. 

keep us posted on your progress. 

[flybox1]

do the carb slides snap back just as well with the bars turned right or left?

[evinrude7]

mine do.