[Hawksmoor]

Please help. I’ve been trying for a long time to get my 1974 cb360 g5 to Idle nicely. (It’s actually a 250 but everyone ignores me if I say that, they’re the same bike almost).
I’ve stripped the carbs over 10 times and replaced all the parts I thought needed replacing/cleaned etc. I also put in new diaphragms. Then I discovered I had bad compression on the left side so I overhauled the top end and replaced rings and ground the valve seats, great compression now.. still no idle. I also replaced the carb manifolds on the cylinder head. I also put good hose clips on both sides of the carbs. I also put filters in my fuel lines. Plus a load of other stuff I can’t remember. NO IDLE. It seems fine when cold but always goes haywire when hot, sometimes the revs go up by themselves but most of the time it will idle high at 2000 + then die (unless I throttle). Getting a bit tired of it. Can anyone please suggest anything? Thanks, H

 

[J-T]

Vacuum leaks?

 

[spirograph]

Did you bench sync your carbs when rebuilding them? Bench sync them to get it running, then vacuum sync the moment is runs well enough to do so.

It's really easy to overtighten things on the intake boots and the hose clamps. Overtightening them will cause them to warp and give you a vacuum leak. It sounds like you might have a couple of conflated issues though, so we should work them one at a time:

High idle:
Usually this is a vacuum leak or a hanging throttle cable. Do you have some slack in your throttle? Mine goes about 1/8 turn before engaging the carb - it feels like a lot, but anything less does wonky things with the throttle when turning. Otherwise, I'd get it all put together, and make sure you aren't overtightening stuff - new gaskets between the cylinder head and the boots, using JIS or Philips screws (some people stick hex bolts on there, and it lets you put way too much torque on the bolts and warps the base of your intake boot). Set your idle/air screws to their base value:
(Stolen from Mydlyf):
745A: 1 & 1/8 TURNS OUT
745B/749A/750A/754A: 1 & 5/8 TURNS OUT
747A: 1 & 3/4 TURNS OUT

And then see what it does. If it's still wonky, get some WD-40 and carefully spray around every joint where air might get in (including the ends of the throttle shaft, that felt washer can dry up and screw you). Go slow so you know exactly where it started leaking. And then let us know if that worked!

 

[Hawksmoor]

Great, thanks Pat. I'll try those things and let you know how it goes. Fingers crossed

 

[Scotto75]

I had a similar thing on my 250 after rebuilding the carbs with a keyster kit. Mine’s an earlier model so can’t use a manometer on the carbs, instead I have a Gunson Colortune.

I remove the plug out of one cylinder (say the right for this example) and stick the colortune in the other cylinder (left side). I adjust the throttle screw on the left carb to increase the revs, so that the engine can run on that single cylinder. I then adjust the fuel/air so the flame just turns from orange (rich) to blue. I then reverse the process on the other side.

Once each cylinder/carb has been adjusted separately and both plugs back in, the engine should start but rev high. I then adjust the throttle screws to bring the revs down, feeling the pressure out of the exhaust pipes to keep them the same. (This is on a hot engine and the cam chain, valve clearances and timing are done before.)

It may or may not help, but it made me find which side was causing issues, in this case a leaner mix on the left from a crack in the inlet rubber.

 

[Bill H]

I agree with spirograph I was also thinking throttle shaft seals. LDR has had good luck with orings instead of felt. You can buy felt to make your own seals at a craft store. Worth a try.


Bill

 

[83XLX]

My guess would be either an intake air leak or a carb synch problem. CV carbs are sympathetic, that is the idle on one affects the other, so they need to be close to being perfectly synchronized for the idle to be consistent at the desired rpm.

 

[remcodost]

Did you put in a keyster set? If so, measure the length of the slow jet: on my cb200 these were too short causing all kinds of trouble until I replaced with the cleaned original set

 

[Hawksmoor]

Thanks all. Good news, the bike now seems to idle well. I did a bench sync on the carbs, adjusted the points timing and valve clearance and also put a load of slack in the throttle cables. It now idles at about 1400 or lower and hangs on better when it drops close to dying. Its been a long journey to reach this point and have basically replaced or adjusted everything in the long chain of fuel delivery. Wow, now I just have to keep up my maintenance. Learning a lot, thanks, H
ps.I just have to retrieve the valve clearance lock nut which dropped into the engine now. I've put up a new post about that.