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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 10-09-2011, 08:28 PM   #1 (permalink)
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My twin is running on 1 cylinder

I am looking for some direction on tracking down why my left cylinder is not firing. On the last ride it sputtered at low rpm - mostly one cylinder than kicked both kicked in. It seemed random but would do it as I came to a stop sign or was starting to get up to speed. At speed it was fine. After a couple of hours riding - on the final mile to my house it ran only on one cylinder - right side, no matter what speed or rpm. Since then, I went through the carbs. I dropped in new vitron float needles and replaced gaskets and cleaned up all passages. I have spark at the points, I have checked both spark plug wires and swapped the spark plugs and determined that these are ok. The bike will only run on the right cylinder. As I see it, the coil, condensor and checking for a wiring problem are my next things to check. The bike is a 1976 Bonneville T140V. Anyone had similar problem to this and what was the source of the problem?

Thanks, lloyds-son
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Old 10-09-2011, 09:02 PM   #2 (permalink)
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Did you say you had spark at both plugs? If so...

I had a similar experience and discovered an exhaust valve had seized in the guide, resulting in zero compression on that cylinder. Even though I had spark and fuel, the bike ran like crap.

Check your tappet clearance - perhaps there is too little and one of the valves is not closing properly.

Check the compression. Maybe there's a burnt piston (hope not)

Please keep in touch and let us know how this turns out.
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Old 10-09-2011, 09:04 PM   #3 (permalink)
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Interesting... I just went thru the same set of problems with a Type I Honda 305 Scrambler. Solved the problem today.

The points should not arc excessively. If they do, that's normally an indication that the condensor is bad. However, if they are contaminated, and the conductivity of the contacts is marginal, then you might see arcing.

Here are some tests to perform.

Disconnect condensors and test them for a dead short to ground. They should charge up from your DMM and then show in essence an open circuit to ground.

Put a light on your points as if you were going to static time the engine. One clip on the coil side of the points and the other to a suitable ground. If the light comes on and stays on as you turn the engine, chances are you have a bad set of points.

Take a piece of emery paper, fold it so you have grit on both sides and with the points closed, open them with your fingers insert the emery and clean them. When finished blow off the points with compressed air and then clean them with a piece of cardboard soaked in acetone or lacquer thinner. Slide the cardboard back and forth and then put a dry piece in there and make sure it comes out clean. A dollar bill is also good for removing the grit. The fabric used to make dollars will let the grit embed in it.

Now retest with your light.

Also open, clean and test all your connections between the coils and the points for continuity and good connections.

Also check the hot side of the coils to make sure you have 12V on each coil with the ignition on. Make sure that wiggling wires does no interrupt the 12v to the coils.

My problem started with a left cylinder that would not fire, but once the bike warmed up, it ran perfect. Did a two day event with the bike and each morning it started on one cylinder and after some warming up, ran on both.

First investigation revealed bad arching on the points. Assumed condensors (original 1967 vintage parts) were failing from moisture (event run in two days of rain). But condensers tested fine. Pulled everything apart, cleaned all the terminals, tested all the wires, reassembled and problem changed sides. Now right side would not fire. I knew I had put everything back where it was so not a case of swapping sides with coils, condensors, etc.. But it was late and decided to call it a night. Started on it again this morning and found that one of the points that worked fine when I static timed the bike was now not making good contact. I had to pressed the points with finger pressure to get the timing light to go out. So this time I really went at the points contacts with emery and then cleaned them thoroughly with acetone. Bike runs like a raped ape now.

What I think happened was the PO had put an excessive amount of grease on the points cam and I think that it had coated and baked on the points. Or the points were severely corroded and need more than a quick touch up with emery. The serious polishing and cleaning did the trick. Will still put new points in engine and probably a new consensor pack. But only time will tell if today's fix will last with the present components.

Do do hit and miss troubleshooting. Go piece by piece. you can swap plug leads and coils to see if the problem changes sides as a quick check. Then work your way thru the pieces. You'll find it. It might just be a faulty/high resistance connection.

regards,
Rob
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Old 10-09-2011, 10:00 PM   #4 (permalink)
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looseparts,
Great suggestions, I have not checked spark I will do, then go on to compression test if needed. Thanks!
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Old 10-09-2011, 11:37 PM   #5 (permalink)
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Hmm. Checked the spark. None at the spark plug, noticed a weak spark at the points. So I took off cleaned the points, and sanded the contacts. No improvement. I also had two extra condensers, so I checked both by connecting with proper ground and kicking it over looking for spark. Still no spark. I wonder if I should swap the coils?
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Old 10-10-2011, 04:24 AM   #6 (permalink)
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There could be bad contact at the ignition switch,and especially at the kill switch.
Measure the voltage at the coil -VE terminals,with the ignition on and one set of points closed.Or just run a hotwire from battery -VE to the coils,and see if it all works better.
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Old 10-10-2011, 04:51 AM   #7 (permalink)
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Maybe your bike has one of them fancy variable engine maps like those fancy sports bikes. Is there a switch on the bars somewhere that says Track, Wet, and Road? Maybe on Wet it only runs one cylinder...
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Old 10-10-2011, 06:33 PM   #8 (permalink)
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When you say no spark I assume you are talking at the spark plug with the plug out of the engine and resting against the head for a good ground? If you are looking for spark at the points, don't. That's a waste of time unless they are arching badly which is an indication of a bad condensor.

If the left cylinder has spark and the right does not, start at the root as Mr. Pete suggests. Look for 12V at the left coil -VE with the points for that coil open. If you have 12v then go to the points and connect a light between the coil side of the points and ground and when the points close the light should go out. If not, then your points are not making proper contact or for some reason are not grounded. If the light goes out consistently with each closing of the points, and the points are not severly arcing (making a snapping sound), then your coil, HT plug lead, plug cap or spark plug is faulty or one or more of the connections between those components.

You can swap the plugs and leads on the coils to see if that cures the problem on the left side and if so, then you know it is whichever one solved the problem. If that does not solve the problem, then try swapping the coils.

If you just cleaned the points but did not test them for proper function, you cannot eliminate them from the list of potential causes.

regards,
Rob

Last edited by Snakeoil; 10-10-2011 at 06:37 PM.
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Old 10-11-2011, 04:35 AM   #9 (permalink)
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Lloyds-son,
Check to see the spade connector to the l/h cylinder condenser is securely soldered?
I suffered near identical symptoms to you a while back & it turned out the soldered spade connector on the condenser parted company. Trailered my Tiger to nearby club mates workshop and resoldered both condensers properly and the old girl hasn't missed a beat yet.
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Old 10-11-2011, 11:25 AM   #10 (permalink)
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Thanks to all who responded - save for Demonic. Many good directions that would not have been on my top of mind list of things to check. Problem solved - bad coil. I am now wondering what could have caused it to go bad? Perhaps ignition left on and overheated it???
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