Backfiring ...

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Hi all,

I had my left coil mouting bracket snap in two and left the coil dangling by the plug wire and the two sets of leads for a few days before I noticed. By the time I was able to fab a new one (these coils are USA aftermarkets and have a bigger OD, so the stock coil mounts I bought don't fit).

It sat for a week or two. Now when I start it, I have a very hard time of it. I have to choke, and kick about 10 or more times. When it finally gets running, it's real "ratty", and I have to work the throttle to keep it running -- especially when I flip the choke off. All this while in neutral. Putting in gear and riding is even worse.

If I keep it running by feathering the throttle, I get back-fires (can't tell which side) --- adn a lot more if I'm riding. Actually, I only went a few yards before I turned around and put it back to bed.

I checked and cleaned my connections to the coil and my self-made plug wire. Both pipes feel about the same degree hotness (is that a word?). I have Dunstall pipes which have a small opening in the center. The right feels like a definite "puff" when I put my hand over it. The left feels different ... almost like a "blow" ... more mushy, but with the same amount of gas coming out.

And as you all know, it ran just fine before .... :? :?

My only thought is a clogged jet, so I plan on taking the single Mikuni apart this weekend.
Dana, Try new spark plugs first ( they can gas fouel when the signal gets interupted as in coil problem). Just because they make a spark outside the head dosen't mean they will make a spark under pressure. Also spark plug wires might look strong but if I had left over stuff I would make a new one. The bad running you detail here sounds like weak spark. Is the new single carb a proven piece? last thing you need is to try to tune a carb with a bad ing. system. norbsa
Hi Dana,

I agree with norbsa, sounds more like ignition. I wonder if those skinny little wires that connect to the primary got broken when the coil was dangling from them. Sometimes they break internally and you can't tell just by looking. When the broken ends of the wire make contact you would get a weak spark and then it would cut out when they separate. Or it could be the spark plug lead but that wire is a lot fatter and presumably stronger.

I had a similar problem on my 850 with the wire that runs -12 to the ignition switch. It must have broken from vibration. I ran a jumper wire across the bad area to fix it.

good luck,
misfiring coil

Hi Dana,

We missed you at the Mt Diablo alternate ride this morning...great weather and we ended up touring Arlen Ness's shop in Danville.

If you drove the bike with the coil dangling for a couple of days you may have damaged the coil, such as shorting internally. That will cause misfiring. To test, swap the right coil to the left and left to right. Does the misfire go with the same coil? If so replace the coil.
Hi Dave,

Yea :( I wanted to go, but 1 mile is all I'm comfortable with now. Drat.

Question: I have aftermarket coils. Is there any diff. from the stock ones? A bigger jolt mayhaps? I lean toward the "when in doubt, rip it out!" philosophy. So I may want to replace both. I think there's a way to check the resistance in the primary and secondary, so I may do that for kicks.

I did find some HORRIBLE connections the PO made on the two wires that go from the coils/ballast to the points, so I'm in the process of checking them out as well. I'll also check and/or replace the points. Can't swing a Boyer yet.

I'm gonna need a lot of miles to build up my confidence and drive out West.

Hi Dana,

"If it isn't broken, don't fix it" is the brit bike addage.

We had four Commandos, a '61 Norton Model 50, a '66 BSA 650 clubman and a Harley Sportster go on the Mt Hamilton ride. Not bad for an
"alternate ride".

You have probably sorted the issue by now, but I had some of the same symptoms myself on my 72 Roadster. I used a stroboscope and found one of the plug caps faulty. The roadside plug test gave a fine fat spark - but the stroboscope revealed the sparks were not consistant. So I cut half an inch off the plug lead and got a new cap - now it is OK again.

Regards - Thore
Hi Thore,

Thanks for the suggestion. And no, I haven't gotten it sorted out. I've spent the last two weeks scrambling to finish my Land Speed race bike so I could drive 500 miles each way in order to drive as fast as I could for 1.3 miles.

I've been pondering what to do. Unfortunately a stroboscope isn't in my shop ... I'm assuming it's not the strobe type inductive timing lights. I have one of those.

I will probably replace the points, coils, and possibly the plug wires. Not sure yet.
I was reading through this post and it sounds like a problem I had. One of my condensers was bad, but it only acted up when riding or starting the bike. After a lot of time checking and rechecking did I find the propblem. These have a stud bolt coming out of the top of the condenser. On my machine the stud bolt was loose in the condenser, and the condser would work spuratically. It made the bike miserable to start and it made the bike missfire when running. I replaced them both because they are not expensive and now I have a spare. By the way what did you find?
Sherm 1972 Norton Commando 750 , 1953/56 Triumph Tigger T-110
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