Topic: Cylinders not combusting, solved

[tennesseebreeze]

So some of you may remember the carburation issues I was having with this 73 cb500 I've been working on for a while. For the past week I've been riding her to work and all the while getting it tuned closer to what it's supposed to be. Yesterday, while I was at work, I moved the needles down a notch to lean out the mixture. I re-adjusted the air screws and the bike felt really good. Where it was bogging down and burbling when pulling out or under heavy throttle, it was now smooth and strong acceleration. Even the idle was improved.

Long story short, the bike began missing severely and popping through the exhaust. I limped home and discovered the 1 and 4 cylinders had not been firing, their plugs black. Numbers 2 and three, a nice toasty tan color. I took this as a sign that when all were firing, the mix coming in was good. So I thought a simple ignition problem. Coils for 1 and 4 being bad, or a couple bad plug wire connections or something. I checked the connections, took a voltage reading on the b&w wires on the coils (11.6v) and checked the spark coming from the plug wires. After cleaning up the connections betwen the caps and the wires, I was getting spark from all of them.

So I decided to re-check the point gaps and surfaces. I re-gapped them and took some fine sand paper to the surfaces, which looked good. I was getting some slight sparking off of both the points. Then I thought to check the spark advancer, which I cleaned but have not re-installed. Because at this point I got the idea to do a compression check.

I had a problem before with my '79 DOHC 750 where I lost compression because of a faulty tensioner that allowed the timing chain to jump time. So that's what made me think to do a compression check here. Just like my 750, I was getting reading on the cylinders in the 35-60 range, well below spec. Prior to this bike starting to miss yesterday I had heard no unusual cam chain noise coming from it. Just recently I had the valve cover off and had set the timing and checked the tensioner.

I don't think my compression guage is faulty, but if this is the same problem, it is quite a coincidence. What do you all think?

[BomberMann650]

I can't remember who mentioned it here.  But it has been said; cold motors report lower compression.

[tennesseebreeze]

I can't remember who mentioned it here.  But it has been said; cold motors report lower compression.

That's true. Something I didn't think about. But to be less than half of what it should read? I don't know enough about it. I need some of these old timerzs to come to my rescue. I was about to take her to the Meltdown vintage bike show in Hendersonville, N.C. tomorrow, but I guess I'll be going without her.

[harisuluv]

Compression check is best performed on a warm engine.  Also, it is common for people to forget to also open the throttle up all the way during the test.

[tennesseebreeze]

Well I went back out, reassembled th spark advancer and points plate, set the point gaps at .38 mm, put back the plugs and ran it. What I noticed since it's now dark out is that the 1and 4 point is sparking brightly and consistently, and the 2 and 3 point has a much less robust visible spark, that is intermittent. Correct me if I'm wrong but the 1 and 4 point shouldn't be sparking that much as I understand it. What's the cause of that and what's the remedy?

I rode the bike up the rode and it was still missing, but not as much as before btw.

[scottly]

Long story short, the bike began missing severely and popping through the exhaust. I limped home and discovered the 1 and 4 cylinders had not been firing, their plugs black.

Black plugs are caused by a too rich mixture, not a lack of spark. Replace the #1 and 4 plugs with new ones. If/when it happens again, be prepared to do some carb work.

[tennesseebreeze]

I just don't think that's it though. For one I've done a lot of carb work to them. They are very clean, synchronized, floats checked, etc. And I had just dropped the needles down a notch to lean out the mix, precisely to fix a rich condition. All the symptoms of running rich I'd been having disappeared. And they were working very well, running strong for like 30 miles before this problem arose. Then running on two cylinders all of a sudden. Doesn't that sound like an ignition problem though- what I described above?  I'm not saying it isn't possible that it's the carbs, but I'm skeptical.

[scottly]

Read my lips: Carbon Fouled Spark Plugs are caused by too much fuel/lack of air. Swap in two new plugs and report back.

[scottly]

All the symptoms of running rich I'd been having disappeared.

Evidently not. Be aware that fouled plugs will continue to cause problems, even after the original carb problem has been corrected. This was a hard learned lesson for me once or twice.

[Gene]

Condensers? If it's 1 and 4 I believe it has to do with points. Timing, gap, something. It should be that simple. You're getting fire but not at the right time. Check points again, check timing again. Cut back the plug wire a tiny bit and reinstall your plug caps. Don't do this all at once tho- one thing at a time.

P.s. This is based on your points sparking and I am not an expert, just sayin'.

[scottly]

Sparking at the #1-4 points may indeed indicate a bad condenser, but black plugs are ONLY caused by too much fuel. If the ignition system is at fault, new plugs will be clean, but wet.

[grcamna2]

Condensers? If it's 1 and 4 I believe it has to do with points. Timing, gap, something. It should be that simple. You're getting fire but not at the right time. Check points again, check timing again. Cut back the plug wire a tiny bit and reinstall your plug caps. Don't do this all at once tho- one thing at a time.

+1 on the condensers,how old are they? they can cause your points to arc badly if they're too old.I've had a few sets of points that got some junk flung onto them and I needed to really work hard on cleaning/lightly sanding/filing them until they wouldn't arc any more but make sure you have a fresh(check the date,HondaMan says no older than 5 yrs. shelf life;I even went to the auto parts store and bought some automotive condensers w/ a date code and they were nice) set of condensers first.

[tennesseebreeze]

I get it, Scotty. You have a lot of posts on here. You no doubt have a lot more experience with these bikes than I do. But everyone's minds go down paths where you're sure you are right, and you can't see anything else. Can't see the forest for the trees, so to speak. Happens to me all the time. But as much as I tried to explain on here the history and the evidence in front of me and how I came to my conclusions, some people just need to be right.

So it was the condenser after all. It had nothing to do with the carbs. I replaced the condenser for the 1 and 4 point and it worked like a charm. The bike is back to running like a bee-stung dog.

If you think about it, plugs could become dark if you are not getting a spark on every cycle. Fuel comes in and the un-burnt fuel builds up. When you finally get a spark every five or so cycles. It combusts but you have an abundance of fuel there already. Hence the backfiring. Hence the dark plugs. I guess in a sense, that is a rich condition, but having nothing to do with the mixture coming from the carburetors.

As for the carburetor situation , maybe I wasn't clear enough in my post above. I LOWERED the needles, not raised them (raised the clip, in other words). Once in a while, I halfway know what the hell I'm doing.

[harisuluv]

So you changed the condensers but not the fouled plugs?  Those plugs should have been changed.

[tennesseebreeze]

So you changed the condensers but not the fouled plugs?  Those plugs should have been changed.

Please explain. As I said, the bike is running great.

[el pachuco]

Please correct me if I'm wrong, but shouldn't switching the points wires confirm if the 1&4 points/condenser are bad, by swapping the problem to the 2&3 cylinders?

[tennesseebreeze]

Please correct me if I'm wrong, but shouldn't switching the points wires confirm if the 1&4 points/condenser are bad, by swapping the problem to the 2&3 cylinders?

I imagine that by switching the condensers with each other you could diagnose the problem. What I did was merely put back the original condenser that came on the bike, in place of the new one I'd bought. These cheapo condensers aren't worth a dime.

[enwri]

Please correct me if I'm wrong, but shouldn't switching the points wires confirm if the 1&4 points/condenser are bad, by swapping the problem to the 2&3 cylinders?

That makes 2-3 try to fire when 1-4 need to, lots of backfiring and flame throwing if you crank it long enough, no running though..
Unless you mean just swapping the condensor wires at the points, then yes.

[Don R]

thanks for the report, all too often we never learn what fixed it.

[harisuluv]

So you changed the condensers but not the fouled plugs?  Those plugs should have been changed.

Please explain. As I said, the bike is running great.

Well if you're claiming that x was the problem, the only way you would know that is if you only changed x.  That's why I said what I did about the plugs.  It seems you are saying that you also changed the plugs, which means that you changed more than one thing.  If that's the case you wouldn't know if it was the condensers or not and surely it would run better without totally fouled plugs.

If it's the case that you didn't change the plugs, then why would you not change disgusting fouled plugs?

[tennesseebreeze]

So you changed the condensers but not the fouled plugs?  Those plugs should have been changed.

Please explain. As I said, the bike is running great.

Well if you're claiming that x was the problem, the only way you would know that is if you only changed x.  That's why I said what I did about the plugs.  It seems you are saying that you also changed the plugs, which means that you changed more than one thing.  If that's the case you wouldn't know if it was the condensers or not and surely it would run better without totally fouled plugs.

If it's the case that you didn't change the plugs, then why would you not change disgusting fouled plugs?

I never said I changed the plugs. I brushed the soot off them. I may yet change them, idk. I changed the condenser, and only the condenser. I changed only X, as you put it. Now the bike runs. So what is the problem? Should I spend money when I don't have to? Should I have taken Scotty's advise as gospel, and gone down a carburetor rabbit-hole that would have cost me another day, at the very least? Would you have me buy new plugs, when the ones I have work?

I have gotten a lot advise from you guys that was very helpful, and I am not ungrateful. I have learned more about this stuff than I thought I would. But how helpful is it to 1. always insist that you are right, when the evidence says otherwise. and 2. when proven wrong, either disappear from the thread (as Scotty did) or ask why didn't you fix (insert thing that is not broken)?

Furthermore, I make an effort to be thorough and concise when asking questions of the experts on this thread. I also give back with relaying my experience for others to use. But it seems like lately, some of you don't even bother reading the posts before you weigh in.

[harisuluv]

So you changed the condensers but not the fouled plugs?  Those plugs should have been changed.

Please explain. As I said, the bike is running great.

Well if you're claiming that x was the problem, the only way you would know that is if you only changed x.  That's why I said what I did about the plugs.  It seems you are saying that you also changed the plugs, which means that you changed more than one thing.  If that's the case you wouldn't know if it was the condensers or not and surely it would run better without totally fouled plugs.

If it's the case that you didn't change the plugs, then why would you not change disgusting fouled plugs?

I never said I changed the plugs. I brushed the soot off them. I may yet change them, idk. I changed the condenser, and only the condenser. I changed only X, as you put it. Now the bike runs. So what is the problem? Should I spend money when I don't have to? Should I have taken Scotty's advise as gospel, and gone down a carburetor rabbit-hole that would have cost me another day, at the very least? Would you have me buy new plugs, when the ones I have work?

I have gotten a lot advise from you guys that was very helpful, and I am not ungrateful. I have learned more about this stuff than I thought I would. But how helpful is it to 1. always insist that you are right, when the evidence says otherwise. and 2. when proven wrong, either disappear from the thread (as Scotty did) or ask why didn't you fix (insert thing that is not broken)?

Furthermore, I make an effort to be thorough and concise when asking questions of the experts on this thread. I also give back with relaying my experience for others to use. But it seems like lately, some of you don't even bother reading the posts before you weigh in.

Scotty probably has a life and doesn't need to play SOHC forum ninja warrior his every waking hour.  There's no need to "call him out" as being a coward and running away from some internet thread with his tail between his legs.  You can stop with that kind of stuff immediately.

If you "brushed the soot off of them", then you are acknowledging fouled plugs would affect performance.  Therefore you can't conclusively say that it was the condensers. 

It's still possible that the underlying problem was a rich condition which will just manifest itself again once the plugs foul up.

[tennesseebreeze]

Okay. This is completely un-called for. If you look up-thread, I have been totally respectful, even though Scotty was rude, saying "read my lips" and so forth, when I merely expressed skepticism about his diagnosis. The collective ego of this forum is so easily bruised.

[tennesseebreeze]

Okay. This is completely un-called for. If you look up-thread, I have been totally respectful, even though Scotty was rude, saying "read my lips" and so forth, when I merely expressed skepticism about his diagnosis. The collective ego of this forum is so easily bruised.

I think you're well off-base, Tennessee. Scotty's (correct) explanation was that:

Sparking at the #1-4 points may indeed indicate a bad condenser, but black plugs are ONLY caused by too much fuel. If the ignition system is at fault, new plugs will be clean, but wet.

His citing the fuel condition is exactly correct. It is NOT to say that you only have this single problem. He corrected your statement that "...You had corrected all conditions of a rich fuel mixture...".

I understand that you've swapped a condenser and now your bike is running well. Great! But it would now be prudent to perform an actual plug chop on the offending cylinder and confirm that you no longer have soot compiling. It is strange that you swapped condensers (did I read that correctly earlier?) from 2/3 to 1/4 and the problem persisted. That certainly should have moved the problem; except that you still had a badly fouled plug 

You can clean plug to an extent after they're fouled, but a few extra $ for spare sets is a wise investment.

Last word on Scottly: Okay, some times he's brusk. But, he's usually spot on with the accuracy of his advice and some times will gladly point out the inconsistencies of peoples statement. Yep, it can be annoying, but hey, its free advice. Accept it, disregard it, or pay for it. Then you can complain about it!  (Meant with a good-natured "beer toast")

Nah, I didn't switch the condensers. I was answering a theoretical question about if that could diagnose the problem or not. I figured you could. I merely replaced the one condenser that I suspected was bad.

My reasoning for not suspecting a rich carb issue was this:
1. The work I had done on the carbs to lean them out already. Also, having taken great pains to set all the carbs the same and synch them. (Also ruling out a clogged carb, which would probably lean the mixture)
2. The fact that this was happening to the 1 and 4 cylinders (as opposed to one cylinder, or all of them, or to any other combo besides the 2 and 3)(knowing that the coils ignite 1 and 4, and 2 and 3, respectively)
3. Also having tested that it was not a coil or plug wire problem.
4. The fact that I could visibly see excessive spark coming from the 1 and 4 points, and not from the 2 and 3 points, which further confirmed an ignition issue in my mind. The excessive sparking from the points matched the cylinders that were not firing.

The reasoning I was given for a carb problem: That black plugs are a rich condition.
Yes, I will concede, a rich condition causes black plugs. But what caused of that rich condition? A bad condenser.

[tennesseebreeze]

More precisely is the condenser caused a weak spark or intermittent spark. Condensers don't effect A/F mixture, only the strength of spark. I think that if you re-read Scottlys posts, you'll see that was his point all along.

"Black plugs are caused by a too rich mixture, not a lack of spark. Replace the #1 and 4 plugs with new ones. If/when it happens again, be prepared to do some carb work."

This is what he said.