Overheating Commando

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Jan 3, 2008
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Ok guy's, thought I'd ask all you experts for your thoughts. I've got a 8000 mile 750 Commando. Converted to S spec (High Level Pipes) and Mikuni 34mm carb although this problem happened with the original Amals. The motor (particularly the cyl head ) heats up to a very high temp in a short space of time. It gets so bad that if you are in traffic it starts to heat up that bad it misfires and buggers the plugs.
The thing is running Boyer and it's been changed completely to eliminate that. I've checked the ignition timing and its spot on. Not used the bike in 07 as I couldn't be arsed with the hassle (i've got 3 other Commando's as well all working fine) I miss the 750 though and want to get it back on the road this year. A final clue, when I restored it I got an idea that for the first 15 miles it was OK then something changed and it started to overheat.
Suggestions on a postcard to this address...... :D :D
 
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Jun 14, 2003
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Perhaps an air leak?

If your timing is correct, and your carburetor jetting is correct, perhaps you have an air leak that's leaning out the fuel mixture? Do you notice any pre-ignition symptoms, like pinging? Too hot a plug may lead to pre-ignition and raise the head temperature - but that's a long shot. What about the fuel you're using - is it aircraft gas?
 
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If you are referencing timing to the degree plate in the primary cover then it can be well out, you need to check the degree plate against a DTI and crank degree wheel. IE set it to a true 31 BTDC using the degree wheel, then put the cover back on to see if it points to 31, if it points to 29 then time the bike fully advanced to 29.
 
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No air leak on intake for sure. Kommando I've ridden my original Commando 500 miles in a day (last year en route to Portsmouth for the ferry to the International in Portugal) with the timing over 20 degrees out (don't ask!!!) and it didn't heat up like this. Take your point on board though. :)
 
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Gino, Checking the gage using a dead stop tool is a thirty minute job they are all off about 3 degrees so why not. Yes you are right Commando's will run way out of time, just don't try it on a BSA or a Triumph. Now being 5 degrees retarded would build some heat (first on the list if your over heating) but that shows up on the pipes even more than the head. It gets hot because unburnt fuel continues to be on fire in the pipes.
I am going to go way out on a limb here and ask that you check your cam timing. Yes you need the dead stop here as well. Not just that you have the ten pins between the sprockets but that the lobs of the cam are opening and closing when they are suppose to.Read it on the degree wheel.
Before you try this though Are you dead sure that the Boyer signal has not gone haywire? It can, it will, and it does. Vibration induced from the signal wires, or weakened magnets at the rotor, or voltage drop induced by riding the machine, AC wires crossing the signal wires, Sparx equipment of any kind, or a bad Boyer box but you switched that.
Since you own and ride other Commando's put a new set of plugs in a good one and get them just broke in about 50-70 miles so they have a bit of color but not too much. Now put them in the 750 and ride carry your plug wrench you know the rest. Seeing how they effect the color of the good plugs on a short run might provide some clues.
 
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Gino,
Are you getting enough oil up to the rockers? Commandos are oil/air cooled.

Cash
 
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