CB360 High Idle / Mess

Some of you may remember my build from last year. Project has been on hold over the winter, and sent out carbs to PJ for jetting, fixed throttle plates and ended up replacing a diaphragm which had a hole in it. She fired up nice, strong and smooth, but still needs some fine tuning.

Bike idles real high with new carb set up, all other circuits seem to run well and good, but I can't get the idle to drop. Looking at about 1800-2000 RPM. Plugs looking pretty whitish-grey. Even at 3-4 counts out on the A/F, throttle speed screw backed out all the way, no noticeable drop in idle. Got about 3/16"-1/4" play in the throttle pull cable, so theres no tension there.

Double checked for vac leaks around carb boots (which i just replaced) and intake.

Running UNI Pod filters and open 2-into-1 exhaust.

I'm thinking at this point that I may need to go up a couple of sizes on the slow jet to richen up the mix and get that idle to where it needs to be, but I havent been able to find a source for different size slow jets for these carbs. I have a drill press, so drilling them out is an option.

If there's something simple I'm overlooking here, let me know. Trying to get her on the road for the warm weather

Any thoughts or sources for jets would be helpful

Re: CB360 High Idle - Pilot Jet Issue?

Even if you've already checked it, I suspect an air leak between the head and the carbs.

Re: CB360 High Idle - Pilot Jet Issue?

Sonreir said:

Even if you've already checked it, I suspect an air leak between the head and the carbs.

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I was having that problem way earlier, so I replaced the boots/gaskets.

Checked by spraying WD around the intake manifolds on head side and carb side, no surging at all. Any "better" way to test?

Re: CB360 High Idle - Pilot Jet Issue?

Hook up a vacuum gauge to each manifold. You should be right around the 17in/hg mark.

Re: CB360 High Idle - Pilot Jet Issue?

Sonreir said:

Hook up a vacuum gauge to each manifold.

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As in, sync ports on the carbs?

Re: CB360 High Idle - Pilot Jet Issue?

Yup.

Re: CB360 High Idle - Pilot Jet Issue?

Vac gauge is reading 7 in/Hg on the left, 13 in/Hg on the right.

Going to throw the OEM carb boots back on and read them again.

Re: CB360 High Idle

Same reading with the old boots/new gaskets installed, though the carbs feel like they seated much better on the old OEM boots. Going to keep the OEM boots on since the fleaBay knockoffs seem too soft anyways. Carbs are supported, so sag is not an issue.

If the reading is consistently inconsistent with both sets of boots, I think its time to look into the intake valves.

Re: CB360 High Idle - Pilot Jet Issue?

Did you adjust the sync to balance the carbs w the gauges installed? a little smear of grease or oil on the manifold boots will help them slip into place easier.

also, next time you pull them off check compression on both cylinders.

CB360 High Idle - Pilot Jet Issue?

I haven't adjusted the sync, I'd just trusted that PJ had them all synced up for me. Guess I should start there before getting too off track.

Will have to get adapters for the compression gauge and borrow a gauge as well.

Re: CB360 High Idle - Pilot Jet Issue?

trek97 said:

also, next time you pull them off check compression on both cylinders.

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Got my hands on a working gauge this time around, since I borrowed my uncles and it was all sorts of leaky.

Kicking this thing over is a PITA. Might have to reinstall the electric start. Anyways...

Read about ~120 PSI on both sides after multiple test. Squirt some oil in there, whaddaya know, reading at 180. So, Looks like I'll be tearing the top end down and looking at replacing rings, maybe pistons. See what happened in there.

It'll have to wait til next weekend, going to motoGP in Austin this weekend. 8)

Have you gone over valves and timing since it started. Would def check those to things before dismantling the engine. Dont forget to adjust cam chain tensioner first. Then i would sync the carbs last to finish the tune up before tearing apart the engine.

Did you take the carbs off or open the throttle/choke befor doing a compresion test?

Carbs were off during compression test.

Been through the valves/timing over and over again since I rebuilt the motor.

just checked my 360 re-ringed engine and its 160 with a crappy push on compression tester ice cold. took me a few tries though.

Looks like a teardown is in your future. Would be looking for a friend with a leak down tester if I was you.

Re: CB360 High Idle - Pilot Jet Issue?

adventurco said:

Kicking this thing over is a PITA. Might have to reinstall the electric start. Anyways...

Read about ~120 PSI on both sides after multiple test.

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Yep sorry to hear it bud, electric start and center stand are both mans best friend with a 360.

hope you have a good time at the moto weekend. 8)

trek97 said:

Yep sorry to hear it bud, electric start and center stand are both mans best friend with a 360.

hope you have a good time at the moto weekend. 8)

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Thanks. Should be a good time, check out the hand built show as well.

As far as the idling issue is concerned, I'm not making the connection between worn rings/low compression and high idle. Would the discrepancy between rings/bore cause excess air to enter the combustion chamber?

I'm going to recheck the valves as well next time I get a minute.

I think a tear down is necessary but I'm wondering if that's the source of all my problems here. I suppose I should rule out one thing at a time. Maybe it's time to resurrect the build thread LOL

Re: CB360 High Idle - Pilot Jet Issue?

adventurco said:

I haven't adjusted the sync, I'd just trusted that PJ had them all synced up for me.

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Your carbs can go out of sync by just pulling them off the bike, so they won't be sync'd after they've been in the mail.


Doesn't sound like out of sync would be the cause of high idle though - are you sure the butterflies are closing properly?

hillsy said:

Doesn't sound like out of sync would be the cause of high idle though - are you sure the butterflies are closing properly?

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Butterflies and throttle operation are normal. Ensured that throttle pull cable was not opening the throttle plates at all when installed (if cable was too short).