Half a cylinder short

Riggles

Forum User
Non-VOC Member
We have a twin that has started to half idle on the rear cylinder, & occasionally spit back through the Mk.1 carby. There is no noticable difference with a few revs. It will idle on the front only with the rear plug lead off, but not the other way around. The plug also is more damp than the front, but a compresion check shows both cylinders about the same. (It has a Grosset starter, so a compression test is easy to do.) With the kick starter lever, it is noticibly easier to kick over the rear pot than the front. Tappet adjustment has been checked, as has the carby. We don't know the motor's history, other than the last owner fitting the starter. We have since converted it to coil ignition.
Have we missed anything before we pull the head off?
 

Albervin

Well Known and Active Forum User
VOC Member
Rig, if compression is the same front & rear then why is it easier to kick through the rear? Is the valve lifter sticking? Have you had a visitor from Broken Hill recently?
Alyn.
 

timetraveller

Well Known and Active Forum User
VOC Member
The compression reading might be the same but what are they. They should be whatever your compression ratio is times atmospheric pressure, say 7 x 15 = 105 lbs/sq inch. That is checked by using the starter and holding the throttle wide open. Have you tried doing the test wet, that is with a little oil pumped into the cylinder first to check for broken piston rings? Have you changed anything recently like tightening down a valve spring cap which might be holding a valve just off its seat? Have you recently moved one of the throttle cables? Check that both carbs are open the same amount at about half throttle by using either a piece of rod or tube or one of the rear brake trunnions which are about the right diameter. Good luck.
 

Tom Gaynor

Well Known and Active Forum User
VOC Member
I've just solved a similar problem. The cause was a tiny piece of clear plastic film in the carb idle circuit, which was a bugger to find since it settled out of harm's way on slow tick-over and blocked the jet as soon as fuel flow increased even by a fraction. Once the motor went on to the slide / needle phase the problem went away, returning when the throttle was closed.
Another possibility (goes over the rear easier than the front) is a deficiency in the valve lifter, which is a bugger to set, particularly the old type but your compression test would seem to eliminate that.
 

van drenth

Well Known and Active Forum User
VOC Member
I read that you have the twin converted to coil ignition, did you run the engine before on the magneto ??. Most probaly it went wrong with the timing setting of your ignition and have you a system fitted for a parralel twin (180 degree) by mistake. Did you timed the engine on the front cilinder.

Regards, van Drenth JC.
 

notap

Well Known and Active Forum User
Non-VOC Member
I had a similar problem at idle only, with the bike running just fine at higher revs. I also pulled the MK1's apart etc.. Then I tried something very simple - I put the bike on the centre stand and off the side stand. Viola! I can only put it down to fuel starvation/float effected by the angle of the bike at idle.

Rob Paton
Sunny Blue Mountains, Australia
 

Riggles

Forum User
Non-VOC Member
Thanks for all the tips. The bike was converted to coil ignition about 4 years ago & has been runing OK. The carbs have been stripped & cleaned several times. As the bike has a chair fitted, leaning is not the problem. The owner, (a VOC member), has a gammy leg but is able to push it over on the rear cylinder with the kick start lever, but struggles to do it on the front pot.
We are now starting to pull it down & figure it is a crook valve.
And for those wondering about Bob Butler's recovery, I visited him on Tuesday & can report he is just like his old self! (Perhaps his stutter is 2% worse, but his sense of humour is better!) A very lucky lad.
 
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