1969 Commando

Hi - is there a maximum size for the main jet?
my trusty 1969 commando does not like to rev above 2000rpm - when i open the throttle above 2000rpm is pops and backfires as if it is starved of fuel
i have recently had the carbs ultrasonic cleaned and new jets etc
running boyer with single coil ignition and have checked the timing 31degrees
any thought please would help

2000 rpm isn't getting anywhere near the main jet.
It is really starving for fuel.

Have you checked the float levels, and the fuel float chamber needles.
Its likely they are blocked or restricted somehow - if your fuel taps aren't blocked or restricted somehow.

Do you have a working choke system on this ?
Can you pull the choke on a little - to try if it will rev over 2000 rpm.
Or even just try tickling the floats, to see if more fuel helps.
This can be done outside the garage, without trying it riding...

P.S.
The chokes aren't stuck ON, are they ?
Or one or both needles disconnected from the slides, so they don't lift when you open the throttle...

Should Throw a HALF INCH Blue & White SPARK . Priority Neumero Uno .

then .

If the plugs melting youve got trouble .



in decadand days of yore , Chockolate outside , straw yellow core , white electrode meant it was biased rich for Max Tourque ,
acceleration , and rip snorting pperformance .

If youre into trundling you back of the ignition & dont need hard plugs . But never run W F O continuos thus - or meltdown .
such as a long trip laden and hot and at altitude . :shock: or . . . meltdown .

thus ' We ' ( myself prefer Rich ( slightly ) to L E A N . 220 to 270 mains .http://www.max-boost.co.uk/max-boost/in ... eading.htm

terencem58 said:

Hi - is there a maximum size for the main jet?
my trusty 1969 commando does not like to rev above 2000rpm - when i open the throttle above 2000rpm is pops and backfires as if it is starved of fuel
i have recently had the carbs ultrasonic cleaned and new jets etc
running boyer with single coil ignition and have checked the timing 31degrees
any thought please would help

Click to expand...

Why did you have the carbs cleaned in the first place........ Where you experiencing the same/similar problems prior to carb cleaning....

yes- i had same experience before cleaning the carbs and thought that was the problem...

Old saying - "99% of carb problems are electrical, and 99% of electrical problems are carbs".

Has this bike ever run 'good' ?

If you tickle the carbs when it gets to 2000 rpm, does it improve ?
Tried a different coil in it ?
Boyers have this funny problem with the pickup wires cracking, that sounds like it could well be.
And can also be fussy about low battery voltage.

One Might Say , ' of Concentrics ' if theyve seen 5000 miles ' , theyve seen their best ' :lol: :lol: :lol: :shock: :cry:

If the slides are scoured , or a finger up the orrifice , slides up - fingertip on lower edge ( slide cut out ) you get rattle -
theyve seen their best . :lol: :lol: :lol: :shock: :cry:

Warped at manifold face from overtightening , warped body from overtightening , leaky float needles ( you MUST have ' Viton ' tipped ones ) . Checked Float Levels ? . Matched Wear ? :lol: :lol: :lol: :shock: :cry:

One can almost understand why some people might fit m er mik Er um , miku Er ya know - those Jap things . :lol: :lol: :lol: :shock: :cry:

You could spend a few weeks studying CONCENTRICS on ' search ' Here .

Basically , Meticulous Familiarity that all systems are to Spec. & Functioning , will see that all systems are to spec & functioning .
This will see it goes as it might , but theres room to see that it goes as it may , by seeing things work better than to spec . :wink:


perhaps you should consider upgradeing the carburation . 8) https://www.flickr.com/photos/9383961@N ... 429153358/