Runs rough on start up, right cylinder gets hot

7000 · Nov 15, 2015

This morning I wanted to go for a ride up and down the coast a bit (Carlsbad Ca). The 1973 750 started right up on second kick, ran rough for about five seconds, and shut down, as though it ran out of gas. I did this two or three more times, then held it to about 3000 rpm, just to keep it running, which it did. A few seconds of this produced smoke off the right side header pipe, which I assumed was oil from the kickstart, but the smoke kept getting worse, so I killed it with the key. The right side muffler was extremely hot compared to the left. Any ideas on where to start looking for the cause?

Thanks, Dave

7000 · Nov 15, 2015

I forgot to mention, lots of back firing during those few seconds.

Rohan · Nov 15, 2015

Sounds like its either gummed up carburettor(s)
and/or the oil has wet sumped,
and/or a sticking auto advance unit,
and/or a dud or oiled spark plug.

Could be a combination of any one or all 4, so a bit of investigating and checking is called for.
All 4 are not unknown for Commandos, brit bikes and other carbed bikes in general....
Ask here if any of these need further explanation.
Ain't old bikes fun !

pete.v · Nov 15, 2015

Compression test.
Blown head gasket or broken ring with scored cylinder.

o0norton0o · Nov 15, 2015

the hot header side isn't firing correctly (or maybe not at all) so it's pumping unburned gas into the header pipe where it's getting ignited by the occasional spark that sends a hot charge of exhaust into that header pipe... Or if you have a balance pipe connecting your headers, then the hot charge from the side that is firing is igniting the unburned gas in the side that isn't firing correctly.... That's my guess..

the cause could be as simple as a poor ground, stuck auto advance, fouled spark plug, or the dreaded clogged idle jet passage...

Torontonian · Nov 15, 2015

When was it last fired up ? Was it sitting for months ,this might be a clogged idle jet. How strong is the battery. Many possibilities here to explore.

B+Bogus · Nov 16, 2015

Torontonian said:
When was it last fired up ? Was it sitting for months ,this might be a clogged idle jet. How strong is the battery. Many possibilities here to explore.

+1 on the clogged pilot jet - easy to unscrew the adjuster and poke through with a fine wire (the brass stuff they wrap round wine bottles works well, and comes with free wine :wink: )

texasSlick · Nov 18, 2015

o0norton0o said:
the hot header side isn't firing correctly (or maybe not at all) so it's pumping unburned gas into the header pipe where it's getting ignited by the occasional spark that sends a hot charge of exhaust into that header pipe... Or if you have a balance pipe connecting your headers, then the hot charge from the side that is firing is igniting the unburned gas in the side that isn't firing correctly.... That's my guess..

the cause could be as simple as a poor ground, stuck auto advance, fouled spark plug, or the dreaded clogged idle jet passage...

+1 .... Whatever the cause, the hot header is indicative of fuel burning IN the header pipes. The right cylinder itself is likely relatively cold.

Slick

1973x75 · Dec 5, 2015

Finally got around to cleaning the carburtors as suggested, and the Commando works like a charm. A filter had broken off the petcock inside the tank, so I got an inline to replace it. Thanks again for the help.

By the way, my name is different now...changed it when i was having trouble signing in. It's in honor of my Triumph Hurricane, which I sold quite a while back. Heartbreaking, but it was either that or the Norton.

Craig · Dec 5, 2015

tough choice on the sale .... but you did make the correct decision ... :wink: ... Glad you found the cure for trouble
Craig

gortnipper · Dec 5, 2015

Craig said:
tough choice on the sale .... but you did make the correct decision ... :wink: ... Glad you found the cure for trouble
Craig

+1. Bought an x75 once and prefer my Commando.

Runs rough on start up, right cylinder gets hot

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