Author Topic: CB750 idle problems....  (Read 9722 times)

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Offline J.Webster Designs

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CB750 idle problems....
« on: November 13, 2008, 05:52:41 pm »
Hey guys, I'm new to the scene. I just bought a CB750 for 450 bucks yesterday. Well, today I got to work and have it starting on first try every time and idling continuously. however, I am experiencing an issue with it the I'm not 100% certain about and would like to see what you guys think about it.

The bike starts fine now, it will idle around 1300RPM when you blip the throttle off idle above about 2500 it immediately jumps to 4000 RPM and stays there and wont go back down. At first I thought that I might have some sticky carb valves. But when you open them WOT and let go you can hear them shut.(click closed). I'm thinking that its tuning but I have never experienced something like this just from tuning. I was thinking that it might also be ignition. But I don't think its likely.... What do you guys think???

Here is a quick video to show what is happening. ( bike started, idle, throttle blipped, high idle, shut down)

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Offline KRONUS0100

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Re: CB750 idle problems....
« Reply #1 on: November 13, 2008, 06:37:38 pm »
use ether and check for air leaks at the intake boots at idel............then get the carbs synchronized....this is a usual indication on everything I've worked on.    Could be a real faint possible that ignition advance springs are weak.
MATT
current bikes:  1976 CB750F, 1981 GS1100E
bikes owned:1981 GL1100I, 1990 GS500E, 1981 GS850, 1977 and 1979 GS750, 1974 CB750, 1975 CB750, and a 1982 GS750E

Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #2 on: November 13, 2008, 06:44:01 pm »
that video was taken with the air box off the carbs.
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Offline Shenanigans

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Re: CB750 idle problems....
« Reply #3 on: November 13, 2008, 06:56:30 pm »
Spray some wd40 on the intake boots to see if you have a leak.
   This pretty much sums it up.   76' CB592 cafe. 69 750 project, 03 CBR954, 75 750 super sport.

Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #4 on: November 13, 2008, 07:13:13 pm »
will WD-40 do the trick? should i be looking for an increase of decrease with WD-40? I have some ether that i can use....
« Last Edit: November 13, 2008, 07:15:38 pm by midnight08 »
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Offline mrbreeze

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Re: CB750 idle problems....
« Reply #5 on: November 13, 2008, 07:17:09 pm »
If the airbox is off,you should be able to look into carbs and see the slides sitting down. If they are....I agree with everything that has been posted......synch is very important....vac leaks,timing. Just keep at it and you will be rewarded. If your slides aren't sitting down,you will have to do some investigating....carbs hanging up,throttle cables, etc. Have fun with it!!!!
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Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #6 on: November 13, 2008, 07:45:06 pm »
The throttle cables are brand new.. that was one thing that I was thinking off. All the carbs come back down. I think I might just rebuild the carbs before I have them synched. I'll check on the vac. leaks tomorrow and see if that shows me anything. I appreciate the ideas guys.
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Offline mrbreeze

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Re: CB750 idle problems....
« Reply #7 on: November 13, 2008, 08:14:44 pm »
I didn't catch what year your bike is but if its older than 76' it will be easier to deal with.My suggestion would be to steer away from inferior kits(Keyster comes to mind). You will be better off to rebuild them and clean your fuel tank,lines,etc.than to fight it.
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Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #8 on: November 14, 2008, 07:19:24 am »
the bike is a 76... the motor however was swapped in by the PO and is a 78 K motor with 76 carbs. i think that for right now i am going to tackle some of the electrical problems with the blinkers and horn and other things and try to clean the bike up then get to rebuilding the carbs... i appreciate the responses guys.
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Offline taxiscooter

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Re: CB750 idle problems....
« Reply #9 on: November 14, 2008, 07:40:48 am »
May be the fast idle cam for the choke. Take the tank off and the adjustment is near the center of the four carbs at the top. You can back the adjustment off and if this helps, this is your problem. Make sure your choke is pushed all the way down (off) before you back it off. I can't remember what the specs are for adjustment, but you can probably download what you need to look at for the location and adjustment specs.  Hope this helps.

Offline Geeto67

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Re: CB750 idle problems....
« Reply #10 on: November 14, 2008, 07:47:43 am »
midnight, you forgot to mention that this bike has been siting around for at least a year before you got it and outside to boot.

Stop being a lazy #$%* and rebuild the carbs. I told you that on caferacer.net and I'm telling it to you here. There is no "baseline" when it comes to this - no mater what you do the carbs need to be rebuilt, otherwise the carb problems you are having will mask other problems and could also create a few phantom ones.

When you do rebuild it, foregoe the K&L and keyster kits and use real honda parts. Its the only way to do it proper.
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Offline taxiscooter

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Re: CB750 idle problems....
« Reply #11 on: November 14, 2008, 07:58:33 am »
Found a picture  Note that fast idle adjustment can go up to 4000rpm   

Offline Geeto67

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Re: CB750 idle problems....
« Reply #12 on: November 14, 2008, 08:37:28 am »
Any tuning you do to the carbs now will just have to be undone once you have rebuilt them, stop dicking around trying to figure out why your carbs are acting "funny" while they are full of year old gas crust and who knows what else and get in and clean those suckers.

your baseline should be a clean set of carbs set at factory service settings and then you adjust them to suit your bike.

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Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #13 on: November 14, 2008, 08:43:50 am »
I realize that any tuning will be undone as soon as i rebuild the carbs... thats a given... and thats why i havent done any tuning to them. I am working on electrical problems right now and getting the bike running properly is on hold right now.

now as far as the carbs go... i wouldnt mind spending the money on honda parts... where is the best place to get the parts at?
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Offline cafeconleche

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Re: CB750 idle problems....
« Reply #14 on: November 14, 2008, 10:06:33 am »
I've had this same problem with my 77.  I was told that the throttle bar that connects the 4 carbs is twisted and that is what creates the high idle.

On my bike as its idling high i can push back on the throttle to somewhat correct it.  not a solution but evidence to a problem at the least.

I am presently in the works to rebuild a set of carbs.

does anyone know of this problem i'm talking about?

Offline andy750

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Re: CB750 idle problems....
« Reply #15 on: November 14, 2008, 10:17:30 am »
Geeto is correct on this one. Rebuild them with Honda parts - yes its expensive now but it will give you a great running bike - carbs are everything on these bikes. Check out servicehonda.com - cheap Honda parts. Also TAS motorsports sells carb gaskets very cheap for a set of 4. Carb balancer -borrow one or get a Morgan CarbTube - foolproof!

Go through your connections and make sure they are all clean and making good contacts - blinkers not blinking likely due to flasher relay (nder left hand side battery cover) - get a 12v replacement at local autostore - electrical flasher unit. Open up the headlight shell and look for unconnected wires in there - remember its colour to colour i.e. blue to blue, green to green etc etc...pretty easy really.

And get a manual - learn how to adjust the cam chain - yours sounds ok but may be a tad loose - anyway required before balancing carbs. And check valve clearances - again very easy and something to do routinely - get a free manual online - see the FAQs.

good luck with it
cheers
Andy
Current bikes
1. CB750K4: Long distance bike, 17 countries and counting...2001 - Trans-USA-Mexico, 2003 - European Tour, 2004 - SOHC Easy Rider Trip , 2008 - Adirondack Tour 2-up , 2013 - Tail of the Dragon Tour , 2017: 836 kit install and bottom end rebuild. And rebirth: http://forums.sohc4.net/index.php/topic,173213.msg2029836.html#msg2029836
2. CB750/810cc K2  - road racer with JMR worked head 71 hp
3. Yamaha Tenere T700 2022

Where did you go on your bike today? - http://forums.sohc4.net/index.php?topic=45183.2350

Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #16 on: November 14, 2008, 01:04:38 pm »
thanks for the advice. i did notice quite a chatter coming from it.
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Offline goon 1492

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Re: CB750 idle problems....
« Reply #17 on: November 14, 2008, 01:15:11 pm »
Hondapartsdirect has honda stuff too, I jsut got in my carb boots and they are SWEEEET! Especially compared to the 32yr old rock hard ones I am giong to pull off tonight.  Keep listening to all these good people in here they will walk you through this, beer and all.
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Offline J.Webster Designs

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Re: CB750 idle problems....
« Reply #18 on: November 14, 2008, 02:55:05 pm »
thanks guys.
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Re: CB750 idle problems....
« Reply #19 on: November 14, 2008, 04:57:36 pm »
Just for kicks, take off the air cleaner and look in at the throttle valves while slowly twisting the grip (use an assistant, engine off) and notice if they all open in unison. If they do, slowly back off the grip and see if they all close and seat in unison. If there is ANY variations between any of them, check for any "slop" in the suspected throttle valve(s), indicating a bent needle plate in that carb. just my .02...

Offline mrbreeze

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Re: CB750 idle problems....
« Reply #20 on: November 14, 2008, 05:44:34 pm »
If you have 76 carbs,you have the bales that hold the float bowls on.You also have no fast idle cam or choke cable(its a lever).You have no accelerator pumps and you have adjustable needles. All of this is good!!!! Thats why I said its better for it to be a 76' or earlier.
I don't know what Geeto has against K&L kits but I have been runnin mine for 2-3 years now.However.........I didn't use the supplied needles or needle jets........I just reused the old ones.This winter,I plan to tear back in and replace those parts with OEM.I forgot the price(I think like $12 a pop....$48).Also will do new intake rubbers.As far as all the gaskets and float needle & seat....no problems.The biggest complaint I have heard here about Keyster kits is that the float bowl gaskets always swell up and cause major headaches.I have had my bowls on & off quite a few times and have had no problems.
So...thats my recomendation when you go to rebuild......K&L kits(mine were like $20 per kit at Dennis Kirk a few years ago)....Stock Honda needles & needle jets.....Also......carb dip and compressed air are the way to go.....If you need different size main jets...they are usually available at any Honda(or probably other makes) dealership (mine were $7 each). Keep us informed and if you have questions....ask away!!!!
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Offline mrbreeze

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Re: CB750 idle problems....
« Reply #21 on: November 14, 2008, 05:55:28 pm »
thanks for the advice. i did notice quite a chatter coming from it.
That chatter will still be there after you adjust your cam chain but will be gone after you do the carbs AND SYNCH them.Still....you need to do the chain adjustment ,valve adjustment,and ignition first.The synch is the final and most crucial adjustment.If everything else is right on the $ but its not in synch.....the pistons are fighting each other and it will run like ass.....chain slap and all!!!!!!! Like Andy says.....get a Morgan Carbtune if you plan on keeping this bike for long(or working on others later down the line). I would imagine it would pay for itself after 2 uses as far as you would spend to have a dealership synch your carbs.
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Offline TomC

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Re: CB750 idle problems....
« Reply #22 on: November 14, 2008, 06:42:38 pm »
Hi mrbreeze
     "You also have no fast idle cam" This is good????????????? How??????????????????
          TomC in Ohio
TomC in Ohio
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Offline mrbreeze

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Re: CB750 idle problems....
« Reply #23 on: November 14, 2008, 06:57:00 pm »
Sure its good!!!!!.....Hell...I'm old school....I don't need a cable and a bunch of extra mechanical crap....I can control my choke lever quite well thank you very much!!!! :D :D
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Offline mrbreeze

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Re: CB750 idle problems....
« Reply #24 on: November 14, 2008, 07:00:39 pm »
You guys keep talkin to me!!!!....You're makin me old!!!!!!(about 25 more posts).
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