Author Topic: 1973 cb500 dead cylinder (Read 1460 times)

scott jones · « **on:** July 29, 2019, 05:20:47 AM »

Good morning. I have taken the carbs off and went through them. Cleaned & rebuilt. Good spark but I have a dead cylinder #2. Pipe is cold when running. I thought about running a compression test. Is this the best place to start? I am super frustrated because the cylinder was firing before I took the carbs off. I know this post is a little vague but I have done everything I can think of short of a compression test. Thanks, Scott.

robvangulik · « **Reply #1 on:** July 29, 2019, 06:12:48 AM »

Quote

I am super frustrated because the cylinder was firing before I took the carbs off.

Why would taking the carbs off make compression bad?
I'd start by making sure carb#2 gets fuel, by turning the drain screw at the bottom of the floatbowl a few turns out.

scott jones · « **Reply #2 on:** July 29, 2019, 07:03:01 AM »

My thoughts exactly. The carb is clear. I loossend the drain plug first & fuel came out so I took the carb back off to make sure I had put it back together correctly. I swear all 4 cylinders were firing before I took the carbs off. I only took the carbs off for the second time to put my new jets in.

Bodi · « **Reply #3 on:** July 29, 2019, 07:19:37 AM »

If it's not firing but getting fuel, the plug will be wet with gas after a bit of running, check that. Dry - fueling problem. Wet - ignition problem. I've had several bad NGK plugs lately, and the plug cable could be sparking to frame or tank so that the plug gets no spark.

Mr. Mike · « **Reply #4 on:** July 29, 2019, 07:21:25 AM »

Scott,

You had a good firing #2 before taking the carbs off and cleaning them.
Do you have spark?
If not, check as to why, starting with that same plug.
Is that same plug fouled?
Might it be possible the float height got goofed up? If your plug is dry, do a clear tube test of the actual fuel level in the bowl.

scott jones · « **Reply #5 on:** July 29, 2019, 07:22:29 AM »

Plug is wet & firing. Took the plug out and spun the engine over. Good spark. The only thing I can think of is maybe a stuck valve? Thats why I want to do a compression test. Can't figure it out. Crazy motorcycle.

scott jones · « **Reply #6 on:** July 29, 2019, 07:35:34 AM »

I checked the float height. 22mm dead on the money. I tell you it's just weird. #2 is not firing. Pipe is cold. Stangest thing I have ever seen.

scott jones · « **Reply #7 on:** July 29, 2019, 08:32:06 AM »

The only thing I can think of is that the wire coming form the coil is rubbing on the frame & causing a loss of spark. But whats weird I pulled the plug out & laid it on the head & cranked the engine & I had spark. $:-\$

TwoTired · « **Reply #8 on:** July 29, 2019, 08:35:20 AM »

Air/fuel mixture, compression, and properly timed spark. Cylinder has to run, provided standard earth physics are still in force.

So, what's missing?

Cheers,

scott jones · « **Reply #9 on:** July 29, 2019, 08:46:23 AM »

I agree. Stuck valve? Hole in piston? Dont know. I have to missing something.

Bankerdanny · « **Reply #10 on:** July 29, 2019, 10:13:59 AM »

Double check your intake valve clearance and make sure the valve appears to be opening and closing. Then I would do the compression test.

I had the #1 kick out on my a couple years ago because the lock nut on the adjuster backed off and the valve wasn't opening. A wet plug would indicate that you are getting fuel, so the valve is opening, but maybe it isn't closing properly.

scott jones · « **Reply #11 on:** July 29, 2019, 10:45:55 AM »

Thanks. I was going to do that tonight. I still have the tank off so it should be easy. I do have one question though. I want to adjust the valves but my feeler gage wont go under the rocker. Very small in there. Do I need a special feeler gage?

Bodi · « **Reply #12 on:** July 29, 2019, 12:53:21 PM »

Try swapping plugs. The bad NGK (or counterfeit...?) plugs I have had sparked just fine out of the engine but would not fire the cylinder.

BenelliSEI · « **Reply #13 on:** July 29, 2019, 01:06:24 PM »

Bodi...... glad to hear that duff plug issue is fooling a few people. Drove me nuts a few weeks ago. Now, 4 new plugs is my first solution to every problem!

Bankerdanny · « **Reply #14 on:** July 29, 2019, 01:26:59 PM »

Quote from: scott jones on July 29, 2019, 10:45:55 AM

Thanks. I was going to do that tonight. I still have the tank off so it should be easy. I do have one question though. I want to adjust the valves but my feeler gage wont go under the rocker. Very small in there. Do I need a special feeler gage?

They do make a special feeler set that is intake on one side and exhaust on the other. It is a very hand tool (I have one), but it isn't necessary to check the valves. I have found that with a standard feeler you need to bend the tip at about 45 degrees 1/2 inch or so from the tip.

scott jones · « **Reply #15 on:** July 29, 2019, 01:34:08 PM »

You mean to tell me that NGK plugs are that sensitive? I'll try a new plug & see what that does. Thanks.

scott jones · « **Reply #16 on:** July 29, 2019, 01:35:17 PM »

Ok thanks for the tip on the feeler gage.

scott jones · « **Reply #17 on:** July 30, 2019, 04:09:47 AM »

Tried a new plug last night. Still a dead cylinder on # 2. Took the carbs back off again.Spun the engine over with the carbs off. Good manifold vacuum. Checking the carbs to make sure I didn't do something wrong putting them back together. Going to do a compression test tonight. Frustrated.

Scott S · « **Reply #18 on:** July 30, 2019, 04:37:57 AM »

While the carbs and tank are off, remove the tappet covers on #2 and spin the engine over. Either by hand or with the kicker. Watch the rockets move and make sure everything is kosher.

scott jones · « **Reply #19 on:** July 30, 2019, 06:27:28 AM »

Did that last night. All good. I still think it's a carb issue. This has got to be one for the books. Really got me puzzled. #2 carb just aint getting fuel to the cylinder like I thought it was. Unless I have something in the carb not right. Like I said I am going to do a compression test tonight. But what really has be baffled is all 4 pipes were hot before I took the carbs off which led me to believe that all 4 cylinders were firing.

scott jones · « **Reply #20 on:** July 30, 2019, 08:49:38 AM »

The only other thing I can think of is I replaced the intake manifold seals with non oe rubber orings. Manifold to head. I matched them up out of an oring kit. They seemed to fit the intake manifolds fine. Maybe one of them isnt sealing properly.

Bankerdanny · « **Reply #21 on:** July 30, 2019, 01:05:46 PM »

Have you confirmed that the T fitting that routes the fuel to the #1 and #2 carbs is completely clear? What about the bowl vent? If either is partially blocked it might let some fuel in, but not enough.

If a new plug didn't make a difference, try switching the leads 3 onto 2 and 2 onto 3. The both fire at the same time anyway, so if the #2 lead or plug end is bad then #2 will work and #3 will not. If that actually works, then the next test is to just swap the plug ends.

Even though this started after the carb work it is still possible that the issue is ignition and it is just coincidence that it happened at the same time you had the carbs off.

jaytee-nz · « **Reply #22 on:** July 30, 2019, 02:16:02 PM »

Did it run on all 4 cylinders before the carb work ? If it did, then it must be carb related or a million to one chance that an ignition problem has developed at the same time. I don't believe in coincidence when it comes to mechanical apparatuses !

scott jones · « **Reply #23 on:** July 31, 2019, 04:39:40 AM »

Well guys. I really appreciate all of your comments but I mentioned earlier that I had put some new orings in the intake manifolds that I had matched up out of an oring kit. I took them out last night & put the old Honda orings back in & the bike now runs on all 4. I would have never dreamed that that might be my problem but It was the only thing I could think of that I did different. Thanks again! Now on to the clutch adjustment. Clutch seems to work fine but after sitting 35 years I'm thinking the clutch needs servicing. Should I take it apart & check everything out or just leave alone?

TwoTired · « **Reply #24 on:** July 31, 2019, 06:33:03 AM »

If it works, don't "fix it".

Having said that. Be sure it doesn't drag when the lever is pulled in. If it lurches when placed in gear, it's dragging.
If it is hard to select neutral when running, it's dragging.

A dragging clutch will eventually cause shifting problems that require lower case disassembly and parts replacement.

It's a fine line between a dragging clutch and a slipping clutch with these bikes.

Cheers,

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Author Topic: 1973 cb500 dead cylinder (Read 1460 times)