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Discussion Starter · #1 · (Edited)
I will assemble some previous posts to explain my problem.

The bad cylinder is not totally dead, the temperature of the header pipes is close to the same. Sometimes the tach gets some signal but not enough to read the rpm. Coil wires in read .4 ohm resistance, out reads about 5.8 or 5800. One plug wire reads 7.57 the longer wire reads 10.7.The new NGK caps read about 5.0, 5000. Do I have a plug wire with high resistance???? The wire that reads higher is 6 inches longer than the other??? Maybe I need different plug wires??? Bike starts first kick and idles OK but a little rough.

Well, things just got worse. I have been working on the CB400 and others. I start the CB occasionally just to see how it runs, been really good. Today I started it and it is back to 1 cylinder. Before I did the GM coil mod the left side was dead, now the right doesn't fire. I am using a contact tachometer to test the spark. Left cylinder shows 1280 rpm, right side shows nothing. What do I do next??? Can this be a stator problem??? I will remove the GM coil and check the connections but beyond that I have no clue.

And finally the latest post:

I do not know what to do next???? I have installed the GM Coil Mod and the bike ran good for a few minutes then went back to the same old problem, running on 1 cylinder. Today I put in new spark plugs and tried 2 sets of plug wires, switched the wires from side to side and nothing changed, left side is very weak, barely runs. After all this I do not think the problem is ignition. The side that runs so weak has a much higher header pipe temperature that the other side??? Stuck exhaust valve??? I will adjust the valves tomorrow when the engine is cold and see what I find. The engine has no unusual noises in it, runs very quietly. I have cleaned the carbs 3 times and I do not think this is a fuel issue. I made a tiny hole in the boot to the air cleaner and I squirted starting fluid into the intake of that carb and nothing happened. There have been instances where the bike ran normally for a few minutes then reverted to
it's old habits. Suggestions any one???

As far as I can tell the problem is in the left cylinder, right side seems OK. Bike starts and runs mostly on the right cylinder but the left side comes on if you raise the rpm's to 2000 or so. In the past it seemed like the problem changed cylinders with changing plug wires but now I think it only left side. I have worked on motorcycles and small engines for years and this is the most confusing one of them all.
 

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Discussion Starter · #3 ·
Compression 130 right, 150 left, tested and retested, new NGK D8EA plugs, Carbs cleaned 3 times with ultrasonic cleaner, did not remove the emulsion tubes, afraid of breaking something, carbs bench synced, not sure about engine ground but the starter works OK. After all this ignition work, nothing has changed. If I pull the choke out and get the rpms up about 2200 the weak cylinder kicks in??? This could be a dirty idle circuit??? The wonky readings with my hand held tach may be worthless as a diagnostic tool. I have not adjusted the valves, doing that tomorrow morning. Going to do one more test, give the right cylinder some more fuel by using a hypodermic needle and injecting gasoline through the intake boot....and that did not help. Could the carbs be so out of sync to cause this??? I will check the sync and adjust the valves in the morning... Through all this the engine sounds good and revs freely with both cylinders running. Just rough at idle. Pull either plug wire and the engine dies??? The bike has the stock air box with a new filter and a stock collector box with stubby exhaust tips, no mufflers.
 

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Dirty idle circuit is definitely a possibility. You can see in this picture how it's routed, Red line, and why it's such a booger to get cleaned. It can be tested by spraying carb cleaner thru the mixture screw port and blocking off each of the 3 ports in turn. Once all 3 ports are blocked the spray should have a good flow out the Primary main if the rubber plug is in place or if not out that passage.
Auto part

While compression isn't great it's enough that you should have a somewhat even idle. It'll be rougher than usual and down on power.
Carb sync could be a source of some of this, I've taken to using 2-3 strands of 18 gauge wire inserted under the throttle plate a testing for drag. That's gotten me extremely close to correct.
Test the engine ground, may be good enough for higher amperage but not for low. Set your VOM to volts. Positive lead to positive battery terminal, check battery voltage first then negative lead to the engine case. Voltage reading should be identical, no more than .05V lower.
 

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Discussion Starter · #6 · (Edited)
Another day another puzzle. Side with low compression runs hotter than other side??? Adjusted the valves, they were OK, checked engine ground, OK. Syncing the carbs will be challenging, how do you get to the nut and screw??? I have the Motion Pro tool that works for some of the CX/GL bikes but may not reach where I need to reach on the CB. I may be too particular about how this bike runs. There are no silencers so I can hear everything. Sometimes I hear a noise like a spark jumping, the noise has rhythm like the rhythm of the engine??? I am going to run the engine in the dark shop tonight and try to sort that out. It could be valve noise but the valves are set OK, within .0005", is that close enough???. With mufflers/silencers some of my problems may go away. My 1979 CX500 never misses a beat, am I asking too much of this CB???

Let me add that top end oiling is good. DAMHIK.
 

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Valve are loose and tight at the same time at .005". Intake is .004" and Exhaust is .006"
W/o the mufflers you'll here every little thing going on, we used to listen to the pipes of race cars to hear what was happening. Might be part of why I'm deaf :lol:
The sync nut is an 8mm and a short wrench from between the carbs works. Really long thin screwdriver from above the frame works for the screw. Remember that when you tighten the nut it will change the sync slightly since it pulls up on the threads.
 

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Discussion Starter · #8 · (Edited)
I ran the bike in the dark last night, I saw no sparks of electricity jumping.
So with that I am going to ride the bike a while, we are having very warm
weather for the time being. The bike starts and runs fine except for some
rough idle and sometimes dropping a cylinder. I think the problem is a
combination of things, spark plug wires, carb sync and maybe a dirty idle
circuit. I will start a thread on a restoration of this bike to a near stock form.
The plan is to take the bike apart, clean, paint or polish every part. Frame
and misc part will be black, the tank and side covers will be yellow with some
black accents, wheels will be stock wheels polished and new K70 tires.
Frame will be painted with industrial coating rather than powder coat, I have a
budget that does not allow for powder coating things that can be painted.
I can do all of the work except painting the tank and side covers. Wish me luck.
To all, thanks for the help with this bike. I will report when the restoration begins.

View attachment 217585

View attachment 217593

This is what we start with.

LDR, what I was trying to say is the valves are each within .0005 of correct.
They are very near .004 and .006. Actually .0045 and .006
 

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Discussion Starter · #10 ·
I will do that, I have Sea Foam.
Can't hurt and may help. From what I read in the FSM this engine can be removed by
disconnecting what is necessary and lowering it with a jack. I have a floor jack and a
motorcycle platform type jack. I have removed and installed several CX500 and GL500
engines so this one should not be a problem.
 

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Discussion Starter · #12 ·
I wish this thread could die but as long as this CB causes trouble the thread will live. The bike has been running pretty good in the shop. Yesterday I rode it for the first time. She ran good, started and stopped OK, I parked her for a while and when I went back to ride her to the shop and park her she was back to running on one cylinder most of the time. While the bike is running and missing or misfiring on 1 cylinder there is a small sharp clicking sound at the top of the engine. The clicking noise seems to be in time with the misfiring. Also I saw one puff of whitish/grayish smoke come off the left side of the engine when I revved it up a little, I thought head gasket or header gasket. I have seen that smoke before today but can not see where it comes from. When it is misfiring I can throttle it up a little and the other cylinder will kick in. I can say this, the bike feels light and easy to handle and will be pleasure to ride at some time in the future, I hope.
 

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I'm sure I've asked before but can't remember. The stator tests all were good? The carb insulators are new or known to be semi-soft and pliable? The carbs were rebuilt using the original Keihin jets except for the primary main which should have been increased one size?
 

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Discussion Starter · #14 ·
The stator tested low normal, no leaks around the carbs, carbs have been cleaned 3 times and no new parts were used. What size should that jet be??? I did discover an exhaust leak today on the left side. Seems like the header gasket is bad. Seems like the bike runs better until it is fully warmed up. It will run at higher rpm but misses at idle or low speed. I am going to proceed with the refurbish of the bike and deal with the running problem later. Since it has low compression on the right cylinder, and that seems to be the cylinder that misses, a top end overhaul may solve the issue. I will have the engine out so the top end overhaul will be easy to do. I have not synced the carbs, I may do that tomorrow just for sillies, that may help. More later.
 

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There's a sticky in the Fuel section on what jets were originally in what carbs.
Even though you don't detect leaks I'd be looking at replacing the insulators. I'd also inspect those air cut valve diaphragms.
Since you're going to go into the engine be aware that the valves on these cannot be resurfaced, replacement only. Seats can be cut w/o problems.
 

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When I upgraded my ignition to the 400T, I thought I read from the guy who makes those CDI's that both cylinders fire at the same time. Someone else chime in here to confirm. But if so, and you have the wire length to do it, swap the wires back and forth to tell u if it is indeed ignition related, carb, or mechanical. Just a thought. To test insulators without removing them, once it's idling -and airbox is on and secured- squirt a little starting fluid/carb clean/brake clean around the insulators themselves. If you notice a change in RPM, you've found your air leak/problem. Squirt SPARINGLY, using the little red straw that comes with those types of chemicals to pinpoint exactly where a/the leak is. Are you sure it's a "click" that you hear, and perhaps not a "snap" that CAN indicate an arc, meaning your spark could be arcing before it even gets to the plug.


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He also says that left cylinder is hot, which could suggest lean. Make sync spot on if absolutely positive no insulator leaks. If had bikes before that some previous owner was in that had totally different jet sizes in each carb for absolutely no identifiable reason.


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Discussion Starter · #18 ·
I will do some more "checking" tomorrow but I think the click/snap/pop noise is an exhaust leak at the header pipe where it joins the head. I ran the bike in the shop in total darkness and did not see anything to indicate an electrical problems. I have Sea Foam in the fuel and that seems to be helping the situation. When the bike is running rough I have injected gasoline directly into the air stream with a syringe into the intake boot and that made no difference. This may be a combination of little things that add up to a problem. The bike starts instantly hot or cold. I use the choke for cold starts and no choke for warm starts. I have, I think, determined that it is the right cylinder that fails. The right side of the barrels/heads is cooler than the left side, the right side has 130# compression, the left side has 160#.
I am going to check the compression again. If the low compression is caused by stuck rings it may get better with running the bike. I did ride the bike when it was running OK and I will like the bike once it runs right. I really appreciate the help I am getting from the forum members and I am sure that this will be solved and we may all learn something from it.
 
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