Author Topic: Got the CB750k4 running today!!....but the engine is racing  (Read 1667 times)

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Offline nhodges

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Got the CB750k4 running today!!....but the engine is racing
« on: February 19, 2011, 02:02:35 PM »
Good afternoon all,

Picked my bike up just before xmas and have been working on it every wknd since.  Today I fired the old girl up,  honesty I was really surprise it started up so EZ. (never worked on a bike b4)  The only reason I have got this far is bc of this forum.  Really amazing source of information, and most everyone are unbelievable helpful.  So here are some pics and a new question.  When I start it up the bike idles way too high (3k-4k rpm)  the throttle cable is new and not hung up and I just had the carbs apart so I don't think a slide is stuck,  I also had the airboots off and they don't have any holes.  So what could be causing this???



« Last Edit: February 20, 2011, 12:46:56 PM by nhodges »

Offline Duke McDukiedook

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Re: Got the CB750k4 running today!!
« Reply #1 on: February 19, 2011, 02:13:48 PM »
The idle screw near the throttle cam mechanism won't bring your idle down?
If that is the case check for vacuum leaks at the carb insulators (the rubbers between the carbs and engine intake).

You might have to replace those.
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Offline nhodges

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Re: Got the CB750k4 running today!!
« Reply #2 on: February 19, 2011, 02:20:51 PM »
I'll check the engine boots tomorrow,  they looked good though.  I'll also mess with the idle screw

Thanks for the speedy reply!

Offline OldSchool_IsCool

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Re: Got the CB750k4 running today!!
« Reply #3 on: February 19, 2011, 04:02:13 PM »
Sharp looking bike, N!  And congrats on returning her to service.  How long had she sat before you got 'er?
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Offline mrbreeze

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Re: Got the CB750k4 running today!!
« Reply #4 on: February 19, 2011, 05:30:54 PM »
You need to synch the carbs. Thats why it idles high. You will be amazed at the difference after you do this. You want to make sure everything else is spot on first though. Valves adjusted, cam chain tightened, timing spot on, etc.,etc. The synch comes last.
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Offline nhodges

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Re: Got the CB750k4 running today!!
« Reply #5 on: February 20, 2011, 09:40:17 AM »
thanx OldSchool,  the PO said it had sat for a little over a year before I came along.  But I have no way to confirm it,  the only inspection stick on the old set of tube legs said '95. 

MrB,  I'll get to all that in the next couple days.  After spending last week getting all the electrical to work I feel like the rest should be a lot less frustrating. 

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Re: Got the CB750k4 running today!!
« Reply #6 on: February 20, 2011, 09:56:55 AM »
quick ? new to the forum so bear with me. got a 1977 cb750k8. runs great but have noticed it will idle up slightly in hot weather. +/- 100 rpm. i restored this ol gal so everything should be top notch but just a lil puzzed on the idle

Offline nhodges

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Re: Got the CB750k4 running today!!
« Reply #7 on: February 20, 2011, 10:10:33 AM »
from what I've been told it is most likely an air leak around the boots.  I am new to these bikes too.....Mines idling way too high but when I open the choke it idles down to about 1k rpm,   that has me a bit confused.  would this indicate an air leak???

Offline OldSchool_IsCool

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Re: Got the CB750k4 running today!!
« Reply #8 on: February 20, 2011, 10:26:08 AM »
Some of the later carbs had a fast idle cam of sorts.  If your K4 has later carbs, maybe that's the issue. 

To check for intake air leaks, start your engine and get it to settle at idle.  Now spray the boots with carb cleaner.  If the engine revs when you hit the boots with the spray, you have a leak.
Can I have a motorcycle when I get old enough?
If you take care of it.
What do you have to do?
Lot’s of things. You’ve been watching me.
Will you show me all of them?
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Offline nhodges

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Re: Got the CB750k4 running today!!
« Reply #9 on: February 20, 2011, 11:43:35 AM »
I just spray WD-40 on the boots, thinking that would bog it down, and it caused it to race.  The choke helps level the idle out but at times it seems to work in reverse.   ie with the choke open the engine idles down to 1k rpm,  then at other times with the choke closed it will return to the proper idle.  Any thoughts on this??  I am gonna and reseat the boots on the carb to air box side and make sure I have a really tight seal.   Just had then off and wintergreen oiled them and there were no cracks or holes. 


Keep the suggestions comin

Offline nhodges

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Re: Got the CB750k4 running today!!
« Reply #10 on: February 20, 2011, 12:46:26 PM »
Ok,

I reseated the air boots at the box and carbs (loosened up the claps pulled everything up tighter and clapped back down).  started her up and got it idling evenly with the choke them sprayed WD40 all over both sides of the carb and there was no change in idle.  So I am thinking I dont have an air leak.  so whats up??  I guess they'll need a vaccum sync??  but would that make the choke work in reverse?  with the choke wide open it slows the idle with it closed the engine races??  Weird or am I just too new to all this bike stuff??

Offline scondon

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Re: Got the CB750k4 running today!!
« Reply #11 on: February 20, 2011, 01:32:46 PM »
Ok,

I reseated the air boots at the box and carbs (loosened up the claps pulled everything up tighter and clapped back down).  started her up and got it idling evenly with the choke them sprayed WD40 all over both sides of the carb and there was no change in idle.  So I am thinking I dont have an air leak.  so whats up??  I guess they'll need a vaccum sync??  but would that make the choke work in reverse?  with the choke wide open it slows the idle with it closed the engine races??  Weird or am I just too new to all this bike stuff??

When you put the new throttle cable in it is easily possible that you adjusted it too tight, therefore pulling the throttle open. There are two adjuster nuts where it attaches at the carbs, Back these off all the way so that they are slack. This will get you to base point on your throttle cable adjustment. If you are still racing at idle at this point, back your idle adjusting screw back down until you reach a nice even 1000rpm idle. When good idle is achieved, retighten the two adjusting nuts on the throttle cable.

These are the first two areas to check if the only problem is that your idle is too high. Don't waste time with the other thoughts of air leaks, carb sync, or other "problems" until you have at least confirmed this very basic setting.

Your idle slows down when you engage the choke because you are cutting off the air supply and flooding the cylinders with gas. The choke on these older carbs is good for getting the motor started in cold weather, but not so good to use once the engine is running. The only thing the choke lever does is cut off your air supply thereby richening the mixture.

Last, but not least, unless you install a muffler to create at least some exhaust restriction you are going to be pulling your hair out trying to get your bike to idle well at all. At least that's the way it looks in your pics.

Hope this helps :)
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Offline nhodges

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Re: Got the CB750k4 running today!!....but the engine is racing
« Reply #12 on: February 20, 2011, 01:52:12 PM »
thanx scondon,

I will give all that a  try, unfortunately bc of work I won't be able to touch the bike again until next wknd.  But Sat. I will be back at it. 

Yeah the pipes are going to be a problem,  they are chopped but there is baffle in them so I am getting some back pressure.