Underpowered Mk3

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brf1957

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Hi All Can anyone tell me what top speed I should expect to get from Mk 3 with single carb conversion 270 main jet ,the reason I am asking is I cant get bike to go over 80 mph it runs great up to that and then doesnt want to do anymore ,I have checked the ignition timing it is spot on and cant think what else could be causing this, any help on this would be much appreciated Brian.
 
when at full throttle and you roll off slightly does the bike feel like it accelerates slighty,single 34 mikuni about 100mph.
 
Hi there is actually more throttle to go at control, it just seems to go to 80 and thats it, in answer to your question i will check rolling off throttle next time i am out
 
Two things to be sure of;
1. Looking into the venturi, does the slide completely disappear into the upper carb body at full throttle.

2. Also is there a manual choke fitted? If there is, is this fully retracted into the carb upper body when not in use?

80 mph is certainly far too slow for a Commando top speed :oops:
 
Will the engine hit 7000 rpm in the first three gears, or does it "bog down" before that?
In my opinion, I would say any Commando that is not wanting mechanically should be hitting around the top 90s to 100 mph minimum.

What silencers do you have on? Standard Mk3 will have blackcaps on, but there were some peashooters sold in the 1980s that had baffles in the end, and these severely restricted performance, but as far as I know these are not manufactured anymore, so if it has peashooters, check that you can see straight through.
 
hi brf,what gearing are you using, will it go as quick in third as it does in top, kommando where can i get them miki runi carbs from lol
 
I had a similar issue a while back on my Mk2 850 with a Mikuni 34. The KN airfilter was too small. It was the narrow pancake style and too restrictive. Another thing, with open mufflers pre-Mk3 bean cans, the Amal jet size around 240 to 260 is generally right. Above that with bean can mufflers it could be too rich coming onto the main jet phase of the throttle.

Mick
 
Firstly you're only running at half throttle ie one 32 :wink:

Are the valves are fully opening and is the motor making a piston slap noise?

Cash
 
What I would do is get to a straight bit of road and run the bike for as long as I could at full bore and hit the kill switch, coast to a halt and take the plugs out. If they are white/grey then go bigger with your main jet. If they are sooty/black go smaller. Then do the same at 1/2 to 3/4 throttle and move the tapered needle up a knotch if white/grey or down a knotch if sooty/black.
I have no experience with single carbs on a Commando but 270 sounds big. I run 260's with my twin carb MK1 32's with a single oval K&N filter.
You could also give it a full bore run without the air filter on...
Remember, make only one change at a time, then you can gauge the improvement..
Stuart.
 
Miki runi was a shortened version of Mickey Rooney which is slang for the common Japanese Carb :roll: :D
 
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