Runs Poorly For 2 Miles

Cburg74 said:

I thought I had this problem licked when I found a bad spark plug wire. The bike ran great for a while. Then it seemed the same problem started up again early last week. On Friday I went for a short ride and one big POP! later and the left exhaust lock ring, washer and tab washer blew right off the head. The only thing keeping the pipe attached was the muffler bracket. The treads on the exhaust port look a bit mangled and cross threaded. Yeah! Question... what happened, what do I do and how much might it cost?... to do both sides. I'm in Southeastern WI and not to far from T.C. if it matters. Thanks in advance for your help and advice. - Cburg74

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that was probably your problem. My bike use to pop and bang and backfire until I resealed the header flanges. but that problem was there all the time especially when coasting with just a little throttle. My bike is tuned for economy though, and if I ride it very quietly I can get 73 mpg.

Alrighty Keri 70's is remarkable good easy mileage that I've one other brag on BI list some years ago. Owning a Commando is frustration filled so all the more acknowledgement of what it takes to claim a fully fettered one, which eludes me constantly. I say its a real Man's Cycle as it beats me to infantile states and procrastination out of ignorance and helplessness and money resources - routinely yet.

sounds like the poor ignition filled the exhaust with fuel / air , then it got IGNITION . poof .
Seen a split muffler from two year old turning off car key with throttle on , to get backfire .

On a good day , :lol: it blows the muffler seams apart .

Check your Boyer leads to the plate. These can weaken from vibrations. Check spark plug leads. Replace plugs. Those are good plugs. Pull carb bowl ,pull idle jet, check number, and for blockage. Choke only for startup ,then flip it off, A.S.A.P. Check fuel flow to carb. Check age /health of battery and ground connections for corrosion. Good luck.

Thank you everyone. I'll give each suggestion a try. It certainly can't hurt! Thank you. PS... this forum is great. Everyone is always willing to give their advice.

I'll give each suggestion a try. It certainly can't hurt!

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Ugh, don't kid your self as every time you hunt down an issue or solve it once found you have risked knocking a wire loose or a cable fouling or new leak or who know's till ya run into this like i have to onset infantile reaction to remind me i may not be fit to own such a simple complex machine.

Cburg74 said:

Thank you. PS... this forum is great. Everyone is always willing to give their advice.

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Advice is easy to get. Good advice is a bit tricky. It's like panning for gold on most days so read the comments and then use the search function to dig deeper and make your own decisions about what's what.

As for why your exhaust thread blew apart, it might have been near worn out to begin with. A lot of owners have gotten rid of the keepers because they just keep the nut in place enough for them to rattle around and destroy the remaining threads. It is quite common for them to be damaged. The methods of repair vary. One nice thing about a 40-year old motorcycle is that enough time has now passed that bad ideas have become more apparent. Again, do some reading, or consult with someone who has been involved long enough to know the difference. There are a lot of us, like myself, who got back into Commandos. There are others here who have been involved ever since. Those of us who came back tend to reinvent all the shit that has already been done.

Biscuit said:

my idea is a combo of what others have said. Choke on too long can foul the plugs a bit and take some time to self clean (when it starts running good). Or the idle circuit could be a bit rich and 10 min of just idling can foul the plugs that way even with the choke off. Or it's really ok but a bit lean overall so some real heat has to get into the engine for it to be happy. But I agree with the too long a warm -up period.

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I hate to admit it, but I've had this problem, too. Now I realize it's not helping to extend the warm up period. The next time I start up my Mk3 I will turn the idle screw up first and cut the choke after a minute. If the ride is smooth after that I'll turn it back down after a few minutes and stop worrying about my ignition system or anything else ... for a while, anyway.

Great advice from everyone - THANKS!

About the stripped exhaust threads.

This is very very common and is caused by poor original design and owner neglect.

There are several fixes but in Europe and North America everyone seems to go for the insert repair.

I dont understand this frankly because years ago I had mine fixed with helicoils and it works perfectly fine. Unfortunately the shop that did it is in Lower Hutt New Zealand which I doubt is very easy for you to get too

A friend of mine made up a leading and a bottoming tap for the helicoil.

With the engine still in place and without taking off anything except the exhaust you bolt a simple jig to a couple of the engine bolts. This keeps the tap square and straight. Run the taps in. Insert the heilcoil and done. Its a 15 min job per side.

The big helicoils were hard to find. The guys searched the world but eventually found them. Not cheap but much much cheaper than pulling the head off and messing about with inserts etc.

I cant remember the cost but it was less than $150 NZ a side. US$ 130 ?? Most of the cost was the helicoil and they will have gone up because I did this 12 to 14 years ago.

Plus someone has to make the taps but thats easy for any decent bike machinist. John Saywell in the Hutt did these ones. They will cost a bit to make but he is cheap and very good.

As I say they have lasted a good 12 to 14 years. I use a smear of high temp RTV on the threads. Tighten them up, warm the engine, let it cool stone cold and then tighten again. Mine are locked wired and I check they are OK every month or so.

Another thing. The threaded rose clamps are not all well made. Some have really poorly fitting threads so its worth while going through a box of them to find ones that actually fit !!! Some of the ones sold today are useless!

Just a quick thank you for the advice I indirectly received via this thread. I changed my start up routine as it was suggested here. Instead of attempting to warm up my Mk3 with full choke until the idle became stable, I flipped it off after 45 seconds and turned the idle screw in about a half turn on the Mikuni. She only stuttered a couple of times during the first few seconds upon taking off down the road. After that it was a very smooth ride for 14 straight miles. Wow - the simplest things can be so rewarding!

contours said:

Just a quick thank you for the advice I indirectly received via this thread. I changed my start up routine as it was suggested here. Instead of attempting to warm up my Mk3 with full choke until the idle became stable, I flipped it off after 45 seconds and turned the idle screw in about a half turn on the Mikuni. She only stuttered a couple of times during the first few seconds upon taking off down the road. After that it was a very smooth ride for 14 straight miles. Wow - the simplest things can be so rewarding!

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Its great to get feedback and know the problem is resolved... In the future others will also be thankfull.

A well tuned Commando is supposed to be cold natured. Might diddle timing tiny amounts to find a sweater fuel ignition. Thumb screw on slide lift is handy to set rpm up so the enricher chamber carbonizer can be shut off sooner till can finger set back to decent idle.