Author Topic: CB500 One Cylinder Running Cool - Solved (I hope) (Read 2341 times)

jaytee-nz · « **on:** October 07, 2018, 12:39:39 PM »

I started my 72 K1 CB500 engine for the first time on Saturday after a rebuild. Totally stock bike with new valves, rings, air filter, plugs, new original exhaust system, coils and leads but with electronic ignition.
Set the valve clearances during the rebuild. Carbs were ultrasonically cleaned, original jets re-used and then bench synced.
The bike started fine but the idle was high so after some minor adjustments it idled beautifully at 1100 rpm (really smooth compared to my 74 CB750).
After about 10 minutes running I noticed that cylinder No. 1 was running cooler than the others - the header was hot (ish) but the muffler was a lot cooler than the others.
Pulled the plug and it was a tan color. Note that I haven't run it on the road yet as it is not fully assembled.
I checked the float level with the clear tube method and it was at 5mm below the seam so a couple of mm out.
Would this be enough to make it run cooler ?
Other thoughts are to check the valve clearance on this cylinder and balance the carbs (it revs cleanly and idles really well so they don't appear to be badly out of balance)
Any other suggestions ?

jaytee-nz · « **Reply #1 on:** October 07, 2018, 12:56:46 PM »

Thanks Cal - I'll check the emulsion tube out.
I only had time to check No 4 float level and that was right on at 3mm. Given that it runs smoothly my guess is the other two are good as well but I will check them.
I'll also try the drizzle / sizzle test !
Cheers

jaytee-nz · « **Reply #2 on:** October 07, 2018, 11:59:41 PM »

So taking it one step at a time, I adjusted the float level to 3mm below the seam but this made no difference. Tried the drizzle and sizzle test - cylinders 1 to 3 sizzled real good, No 4 didn't.
I also noticed a popping noise on deceleration on cylinder 1 as well. I guess that points to either the low speed jet or the emulsion tube ? Can I remove the emulsion tube with the carbs in place ?

Deltarider · « **Reply #3 on:** October 08, 2018, 10:44:55 AM »

Quote from: jaytee-nz on October 07, 2018, 11:59:41 PM

Can I remove the emulsion tube with the carbs in place ?

Yes, you can. They're not screwed in, nor are the main jets. But I advice you first to make sure ignition 1+4 is correct. All aspects like plugcaps impedance etc. You also may read this: http://forums.sohc4.net/index.php/topic,174506.msg2030492.html#msg2030492
Removing the floatbowls of these oldstyle Keihins is all what it takes to do the necessary cleaning. You can even do it with the carbs in situ.
After you have removed the main jets, open so to speak the throttle wide, so the jetneedles are up. Stick a wooden match, toothpick or whatever in the needle jet's hole from below, cant it and you can wiggle the needle jet (aka emulsion tube) out. Sometimes they just come falling out by themselves. Spraying some WD-40 or penetrating oil at forehand may help.
'There's a lot of emphasis in this forum on the cleaniless of the little holes in the emulsion tubes. However when I cleaned mine (decades ago) I never noticed much difference if at all...

jaytee-nz · « **Reply #4 on:** October 08, 2018, 12:23:46 PM »

Thanks Guys.
Just to correct my previous post re the drizzle and sizzle test, cylinders 2,3 and 4 sizzled real good but No. 1 (my problem cylinder) did not. Sorry for the confusion.
The coils, leads and caps are all new but I will check them anyhow.

jaytee-nz · « **Reply #5 on:** October 09, 2018, 12:34:34 PM »

Update - Checked the spark plug caps, leads etc and they are all good.
Dropped the slow jet and emulsion tube out last night and both look very clean, especially the emulsion tube (looks brand new and super clean). I have a spare low speed jet which I will try anyhow but I'm wondering if I accidentally put the emulsion tube in upside down when I cleaned the carbs some time ago. I didn't take much notice of the orientation when I removed it last night (great tip to use a tooth pick thanks Delta) but is it actually possible to put it in upside down ? Would this cause my cool running ? I'm at work right now so can't test my theory !

jaytee-nz · « **Reply #6 on:** October 09, 2018, 05:26:51 PM »

Cal, I can see light when I cover the small holes. Should the hole of light be perfectly round as it kind of looks like it's clover leaf shaped. It could be my bad eyes but I guess it could also indicate that it's not completely clear.

jaytee-nz · « **Reply #7 on:** October 09, 2018, 11:29:29 PM »

So replaced the pilot jet with one that was perfectly clean (nice round hole) and - no change. I've noticed that it's not starting on that cylinder when cold and then it slowly chimes in but does not pass the header "sizzle" test.
Could it be the pilot circuit within the carb ?

Deltarider · « **Reply #8 on:** October 10, 2018, 12:54:36 AM »

Have you checked the little O-ring around the main jet is still OK and seals well? If not, the cylinder will greedily suck in the extra gas that can pass.

jaytee-nz · « **Reply #9 on:** October 10, 2018, 01:13:09 AM »

O ring is new but I did check it again I'm sure it's sealing good as I need to lube it slightly to get it to go in.

Deltarider · « **Reply #10 on:** October 10, 2018, 01:21:07 AM »

Jaytee, forgive my laziness. I just reread your initial post.

Quote from: jaytee-nz on October 07, 2018, 12:39:39 PM

The bike started fine but the idle was high so after some minor adjustments it idled beautifully at 1100 rpm (really smooth compared to my 74 CB750).

Congrats, nothing abnormal here.

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After about 10 minutes running I noticed that cylinder No. 1 was running cooler than the others - the header was hot (ish) but the muffler was a lot cooler than the others.

Ignore the muffler. Is the no.1 header much hotter than the rest? BTW, you never will have them show the same temperature.

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Pulled the plug and it was a tan color.

Good, however usually the CB500s run quite rich at idle.

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Note that I haven't run it on the road yet as it is not fully assembled.

Let's wait for that then.

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I checked the float level with the clear tube method and it was at 5mm below the seam so a couple of mm out. Would this be enough to make it run cooler ?

Not likely. You'd expect the contrary. Also I would like to stress that you haven't checked the float level but rather the fuel level. Do verify fuel flow. Usually the float itself meters OK for decades and decades, as long as it is not tampered with, which seems to be a hobby in this forum. I suspect the presentation in the Clymer manual on this has to do with it.

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Other thoughts are to check the valve clearance on this cylinder and balance the carbs (it revs cleanly and idles really well so they don't appear to be badly out of balance)

Maybe, there's not much of a problem and you're overthinking it. I mean, it idles fine and you have not been on the road with it. So, what do we know?

jaytee-nz · « **Reply #11 on:** October 10, 2018, 01:36:59 AM »

Ok, thanks Delta. Some valid points there.
I'll run it on the road (soon) and re-assess then.

jaytee-nz · « **Reply #12 on:** October 16, 2018, 12:45:42 AM »

So after checking the valve clearance (good) and timing (slightly too advanced) I then tried to balance the carbs. Initial reading showed that my problem No 1 cylinder was way out compared to the other three but no matter how much I adjusted things I couldn't get them to all play nice together. Also, when I revved the engine my problem cylinder showed some weird readings on its gauge. Time to remove the carbs. All jets and airways were spotlessly clean (I was confident they were as I had completely stripped and cleaned everything ultrasonically last year and had not run the carbs). Float levels were good so I started to scratch my head. Thought I would check the needle jets as I might have might accidentally put the clip in a different position to the others.
Removing the top cap soon showed the problem - both screws that attach the slide to the actuating arm were sitting on top of the slide ! Cannot explain how or why that happened. Workshop fairies maybe.
Will put in all back together in the next few days and hopefully have 4 cylinders and not 3.5.

RustyF · « **Reply #13 on:** December 17, 2018, 05:56:29 AM »

I had similar issue after rebuilding the carbs on my 78 550. I used a Taiwan made rebuild kit and reproduction pilot & main jets. I chased over flowing carbs with properly set levels - fixed after I put the old float bowl needles in. Then chased your issue with 3 cylinders showing 225-235 and one showing 186 with infra red temp gauge. After much head scratching I tried "moving the problem" by swapping ALL the internal carb parts from #3 (cold) to #4 (normal). The problem moved. Found that the pilot jets had small burrs causing restriction. I cleaned up the burrs and that fixed the issue. My experience with aftermarket carb parts on my 550 has not been good.
Hope this helps.

Deltarider · « **Reply #14 on:** December 17, 2018, 08:06:59 AM »

Quote from: calj737 on December 17, 2018, 06:03:51 AM

You’ll need to re-synch the carbs again.

When he has changed the needle jet sets, yes. If the change was limited to slow and main jets, no need to sync.

Deltarider · « **Reply #15 on:** December 17, 2018, 11:06:11 AM »

Quote from: calj737 on December 17, 2018, 10:54:56 AM

Quote from: Deltarider on December 17, 2018, 08:06:59 AM
Quote from: calj737 on December 17, 2018, 06:03:51 AM
You’ll need to re-synch the carbs again.
When he has changed the needle jet sets, yes. If the change was limited to slow and main jets, no need to sync.
Read what was written. The small screws that hold the slide were found loose. That means his slide adjustment is off- again. Dope.

Calj, in this forum it's general practice to formulate answers thus, that more than just one reader receives info. Or do you really fear my answer above has left RustyF confused?

jaytee-nz · « **Reply #16 on:** December 17, 2018, 11:57:55 AM »

You guys are mixing my original post / problem - no screws attaching the slide to the arm, with a NEW posting from RustyF where he is commenting about the quality of aftermarket carb parts.

jaytee-nz · « **Reply #17 on:** December 17, 2018, 01:03:03 PM »

Cal - my last post was on October 16. You did not reply to this.
7 minutes after RustyF posted, you added "You'll need to re-synch the carbs again".
If you were replying to me, it would have been good to clarify that, given the 2 month gap.
As it turns out, I did re-synch them after putting the screws back in.

RustyF · « **Reply #18 on:** December 18, 2018, 05:52:44 AM »

I am not trying hijack the thread...just sharing on how I fixed a similar problem and what I found using reproduction parts. After I verified similar exhaust temps when I switched carb internals I then did readjust all 4 carbs air/fuel mixtures, set the idle and then re-sync'd the carbs. Hope this helps others finding this condition.

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Author Topic: CB500 One Cylinder Running Cool - Solved (I hope) (Read 2341 times)