Topic: 1970 CB750KO - Idle is killing me...help please...

[Johnie]

OK...the short...got this 1970 CB750KO in July with 17,000 miles.  Had not been run since 1990.  Did the usual carb, tank, points, plugs, timing, oil, cam chain and the bike runs great.  Never a miss high or low speed...but...I just can not get this thing to idle.  I have to do it manually everytime I stop.  This is the carbs with 4 into 1 throttle cables.  I adjusted each carb cable 1 full turn out and it was worse.  I adjusted each cable 1 full turn in and it was worse.  So I put them all back where I started and am at a loss of what to do next.  I tried to sync with my Morgan Carbtune just to see where it was at and that was hopeless when I can not even get it to idle.  The side idle screw on each carb did move the vacuum gauge a little, but not enough to help to get an idle speed.  After I did the carbs I turned the air screw out 1 full turn per the service manual.  They were 3/4 turns out before I took them out.  Not sure if that will make a difference.  Any advise would be greatly appreciated.  I feel like I am at the end of my ability to figure this out.  I do use the green Honda service manual to no avail... 

[kghost]

I have found 99% of all idle problems to be....

1.) Dirty idle jets.

2.) Float hight set wrong.


Generally there is a blockage of the idle jet emusifier holes in the jet itself. Did you pull the idle jets and verify they are clean as a whistle?

Secondly a Low fuel level in the bowls will cause erratic to no idle. If you set the float levels wrong thats a result.

[Johnie]

I pulled all jets and cleaned them perfect.  Had the tank cleaned and sealed before I put any gas in the carbs.  I even ran gas through the petcock with the bowl off to be sure no junk was in there.  Now, I did not mess with the float level at all.  Probably because I did not want to mess it up.  I guess I should make that 26mm float gauge I saw here at least to eliminate the float height as the problem.

[Johnie]

I checked all idle jets and they are clean.  I checked the float level with the carbs still on the bike using the 26mm tool made from this board.  Float height across all 4 around 32mm - 33mm if I gently touch the valve seat.  Is the float level supposed to be 26mm or 32mm for this 1970 CB750?  If the float level is to low I can see having a idle problem, but a level too high would cause no idle too?

[Tower]

The float level adjustment is the same at 26 mm for all CB750, CB750 K1-K6.  Model A is set at 14.5mm

The air idle mix will make a big difference to idle.  Try turning 1/4 turn in, in 1/8 turn increments, letting engine cool in between.  That's what that screw is for.  The 1-turn-out position is merely the starting or baseline position.

[kaysystems]

My K0 idles if I set it to around 1200 on the tacho, but I have coil or wire problems I think.

David

[Johnie]

I just pulled the high speed jets along with the idle jets and the thing is spotless.  So is my CB750K that was made in January 1970 a model A and the float should be set at 14.5 or should it be 26mm or the current 32mm?  My green Honda service manual just says to use the Honda float tool but does not give the mm?  This thing is giving me chest pains...

[kaysystems]

I just pulled the high speed jets along with the idle jets and the thing is spotless.  So is my CB750K that was made in January 1970 a model A and the float should be set at 14.5 or should it be 26mm or the current 32mm?  My green Honda service manual just says to use the Honda float tool but does not give the mm?  This thing is giving me chest pains...

26mm with the brass floats. Are you floats good? Take them off & shake them. Even a pin hole will let them fill with gas & then sink.

David

[Johnie]

Yes, I have the brass floats and have checked for leaks and they are empty.  Now I saw another post in the FAQ area that said the KO's are better at 30mm because the brass floats are a little heavier and will sit lower in the fuel.  Keep your ideas coming.  I am on vacation this week and trying all your ideas.  Thank you all again.

[kaysystems]

Now I saw another post in the FAQ area that said the KO's are better at 30mm because they are a little heavier and will sit lower in the fuel. 

Missed that one. Please let us know the result, although I don't want to be too lean on the highway.

David

[736cc]

 I ALWAYS replace the 40 slow jets instead of cleaning; that GUARANTEES they aren't a problem; those holes are teeny-weeny and believe me, cleaning doesn't make them perfect if they were plugged-up.
 Remove air screws and blow carb cleaner in them, then replace to 1 1/8 turn out from bottomed. Make sure carb cutaways are facing airbox (they can be installed backwards). New spark plugs if they're the least bit blackened. Back-out the big idle screws on each carb all the way w/o removing. Screw top of carb adjusters all the way in and verify each slide is bottomed-out, then adjust until all 4 cables all have same 1/4" of play where they enter carb tops. Set-up your vac gauges. Start her up. Adjust the single throttle adjuster at the twist grip so its roaring around 3000 or better rpm. Adjust the the adjusters on top of the carbs so all 4 carbs give same vac reading. Now give the twist grip adjuster the right amount of play at throttle. Adjust the big carb idle screws until all 4 carbs give same reading w/ an idle around 1100 rpm.
Valve clearances MUST be spot-on, along w/ timing and float height (I just pull the bowls while on centerstand w/ petcock off and eyeball they're all around 80% full.)
Of course, the coils can be arcing; see if there are sparks flying around the plug wires while running in a darkened garage.
Even after all that, a PERFECT idle may not happen but at least your chasing it!

[Johnie]

That is the kicker with this KO...it runs perfect from acceleration no matter how much throttle I give her.  Never a miss or sputter.  Cruises like a new bike.  The only thing is the darn idle problem.  Keep those ideas coming guys.  Thanks from WI.

[MRieck]

 Replace all the boots (intake and airbox) with new OEM. They are available. As someone else said....1,000 to 1,100 RPM is a good idle......it keeps oil pressure up!

[Clyde]

Not sure if I have missed something, but if you can idle using the twist grip, why can't you just adjust the side idle screws in to get an idle? They all should be screwed in the same amount at the same time to get an even idle and to get a good idle you will need your carb balancer.
They may need to be screwed in a fair bit, but they just do the same thing as the cable:-that is to raise the slide to increase the engine rpm.
As you screw one in, the vacuum on that carb will fall, and on the other carbs should go up.
Regds Clyde

[Tower]

@Johnie, with all due respect , you might be overthinking this.  Set your floats to 26mm.  Only the model A (A for Automatic) sets up at 14.5mm.  Use the idle air screws for their intended purpose - to fine tune the idle after you have synced the carbs. 

Turn the screws the same amount on each carb.  They should not have to be turned more than 1/4 turn from their baseline position - in 1/8 turn increments then cool engine and restart.  Idle speed of about 1000 RPM is best (even though factory says 850 - 900 RPM).

[jmckinne]

I found on my K4 that at idles above ~1000 RPM the main jet & needle are as much a factor as the pilot jet & airscrew. I even saw that documented somewhere. The airscrew has little effect above 1000. If you ordered rebuild kits for your carbs make sure the jet sizes and needle clip position match those called for by the castings on your carbs, not just the year/k# of your bike. There is a table on here that lists those factors by casting number - use it.

I've been burned by this very thing.

[Tower]

@jmckinne, you are right about the growing effect that the needle jet and main jet have.  At 1000 RPM they begin to contribute a significant portion of the total fuel in the airflow.  However, you should not conclude therefore the idle air and pilot have little effect. 

The idle circuit is still providing considerable fuel mixture into the airflow and therefore can provide a fair amount of adjustment.  Indeed the idle circuit is providing about 1/4 of the fuel at 1000 rpm.  More significantly, the idle circuit has a smoothing effect, helping to prevent engine stumbling at idle. 

As the air speed rises across the venturi with rpm, the main jets continue to grow their fuel portion and smoother delivery, whereas the idle circuit is already contributing its maximum fuel flow by the time the engine has reached 1000 - 1200 RPM.  Contributing fuel at its maximum rate, the proportion (not the volume) that the idle circuit contributes, therefore diminishes.

In other words, the idle circuit is the main adjustment circuit for smooth idle.  If a smooth idle can't be obtained adjusting the idle air, only then does it mean the problem is a mismatch in the larger circuits (either too rich or more likely, too lean)

[HondaMan]

I've often found that a non-idling Four, otherwise smooth, is a low float bowl level. And, 26mm for all the "K" models is right. If someone set a brass-floated model to 30mm and got a better idle, that's a new one on me: the idle jet is especially susceptible to low float bowl levels because it sits higher up than the main jet system. The idle jet needs to be at least 1/4 of the length into the fuel.

For many, many years I ran 24mm float levels. This prevents leanness on long, hard. hi-RPM runs (what, me speed?). It also made the bike idle rich, no matter what I did with the air screws, and 2 hours of city traffic would inevitably foul spark plugs. That 2mm (.08") makes quite a difference! Today, I've built it into a commuter bike, with stock 26mm float levels, idles fine with no assistance, at 1050-1100 RPM.

[kghost]

I dunno if I agree with Mreik about switching carb boots...however his premise is sound.

If theres a vacuum leak the thing will never idle. Period.

I'd make sure that there are no air leaks post carb.

bear in mind also...I have found old honda tachs to be less then accurate.

Mine tend to indicate 1200-1300 when the actual RPM is about 900-1000.

[MRieck]

I dunno if I agree with Mreik about switching carb boots...however his premise is sound.

If theres a vacuum leak the thing will never idle. Period.

I'd make sure that there are no air leaks post carb.

bear in mind also...I have found old honda tachs to be less then accurate.

Mine tend to indicate 1200-1300 when the actual RPM is about 900-1000.

Yep....300RPM off all the time.

[jmckinne]

I dunno if I agree with Mreik about switching carb boots...however his premise is sound.

...

I'd make sure that there are no air leaks post carb.

I second the suggestion to take a good hard look at your boots. Mine looked good, supple, no cracks etc. But the first time I hooked up a vacuum gauge I got really crappy response from one of the carbs when goosing the throttle. On closer inspection all the clamps on the engine (boot) side of my carbs were tightened down to the max and I could still move the boots by hand. They'd been over-torqued and the rubber was so nice and "supple" it wouldn't seal well enough anymore - especially at higher RPMs. I'd run the tried and true "wd-40" around the carb test and they had passed, but I'd only doen it at idle. Have you done the WD-40 test btw?

I replaced all my stock clamps with auto-store radiator types for the time being. It's lovely, but they can be tightened much more than the stock clamps - and my vacuum leak is gone. I've ordered all new boots and original style clamps from Service Honda and will put them on next time I pull the carbs. I think my boots were original which made them 35 years old. If you're in doubt at all - replace them.

[HondaMan]

I dunno if I agree with Mreik about switching carb boots...however his premise is sound.

If theres a vacuum leak the thing will never idle. Period.

I'd make sure that there are no air leaks post carb.

bear in mind also...I have found old honda tachs to be less then accurate.

Mine tend to indicate 1200-1300 when the actual RPM is about 900-1000.

Yep....300RPM off all the time.

Mine, too.

[sandcastcb750]

Honda solved the K0 carb problem.............the K1 !

My best running K0 ran great at high speed but idled crappy.

The bike should run without idle screw increase which probably means the pilot air screws are blocked or incorrect. If this is not working other adjustments are probably a waste of time. Usually 1 1/4 turns should run the bike.