E: Engine Lame duck Comet

[Black Flash]

Hello to all of you,
I am writing on behalf of my friend Dieter again. He has recently bought a C Comet to use for shorter trips rather than his shadow.
The bike had been restored 25 years ago and used since. It used a lot of oil and he bought a new 8:1 piston from the club and since fitting it ran the bike about 1600 miles.
The bike received a Mk II cam during restoration and the cam timing is spot on.
The ignition timing is 33 degrees, the bike has a 30 mm MKi concentric carb, inlet track is very smooth. First the bike had a Campbell spiral baffle silencer, now a straight through absorption type.
Gearing is standard for a comet.
Now to the problem, the bike does not perform!
Starting is great, idle as well. No hickups during acceleration.
He is of a 5 foot 5 size about 70 kg but the bike won't run faster than 60 miles on a straight line., slipstreaming maybe 70 miles with a very low slow pickup of speed.
We rechecked everything and ran out of ideas. The 60 MPH are reached with partial throttle but twisting the last 2 cm on the twist grip only changes the inlet sound of the engine but gives no more speed.
Rolling back the throttle also makes no difference, so the 200 main jet seems to be okay.
Do you have any idea how to make this Comet go?

Cheers Bernd

 

[chankly bore]

First I'd check no problems with valve lift-i.e. shortened lower guides, I'd maybe go to 260 main jet on a Concentric and work back to 240. After that I'd be checking compression. What ignition system?

 

[Bill Thomas]

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

Morning Bernd, This is a good read, There are so many different jets and jet holders, Some for two stroke bikes, Also they say there is a lot of fake jets sold, So even if it says the right size on the jet, It could be wrong.
I would check there is enough petrol coming out the Petrol tank taps, Filters blocked ?.
Are you running an air filter ?.
Might be quicker to take a Carb' off his Shadow ?. Just for a test ride.
Cheers Bill.

 

[Black Flash]

Hello to both of you,
I already suggested to change to his shadow carb,as he knows it works very well on the twin.
The ignition is a by magneto but there are no signs of misfire or so, also starting from hot or cold is not a problem.
Last weekend he attended a brit bike meeting near the Swiss border. He said the bike ran beautifully on the 450 mile trip, just that he can't exceed 60 mph.
His consumption on that trip mainly on B roads was 50 mpg if that is any help.

 

[Bill Thomas]

Have you had a ride of it Bernd, I find when I get off my Twin, On to the Comet, It takes me a while to adjust my Expectations !, As you know , If you want top speed quick, You need to buzz it in 3rd gear before getting 4th, That also for the Twins, Because of high gearing.
Like you, I think it will be more easy to put his Shadow Carb' on.
If you are going into a head wind, It makes so much difference on a Comet, Some days mine seems to Fly,
Other days, Not so much !. He might try the Best Petrol, I always use the most expensive.
50 mpg is not the best for a Comet, But not too far off.
I don't like the Mk1 Carb', No two seem the same, Some were made in Spain, I have been told that about the Mk2, But I am happy with the 30mm ones on my Twin and Comet, My Engines are Mk2 cams and 9 to 1 pistons. Cheers Bill.

 

[Len Matthews]

Hello to both of you,
I already suggested to change to his shadow carb,as he knows it works very well on the twin.
The ignition is a by magneto but there are no signs of misfire or so, also starting from hot or cold is not a problem.
Last weekend he attended a brit bike meeting near the Swiss border. He said the bike ran beautifully on the 450 mile trip, just that he can't exceed 60 mph.
His consumption on that trip mainly on B roads was 50 mpg if that is any help.

Could it be that the ATD doesn't advance?

 

[timetraveller]

This might be a silly suggestion but has he checked that the rear sprocket is for a solo and not a sidecar? If it has a large rear sprocket he could be revving the engine a lot but not moving all that quickly. If it is not the rear sprocket it would be worth while getting the rear wheel off the ground, into top gear, spark plug out, one tappet cover off and rotate the rear wheel to check the overall gear ratio. Has he checked for clutch slip?

 

[chankly bore]

All the above is good advice. Also, after checking jet size and type, A.T.D, sprocket and fuel filters, check breather hole in fuel cap.

 

[nkt267]

Is there a chance the gearbox sprocket may be a 19 tooth instead of an 18 tooth. my comet with Mk2 and 8:1 piston will pull to just about 70 mph in third ( 65 mph is kinder) before engaging top..

 

[Mike 40M]

The description is the same as how my C Comet goes. About half throttle, then nothing more.
Same main jet 200, same riders weight.
Differencies is that I'm 5" taller and have a 928 Concentric. Campbell silencer and 7.3 compression.
No idea what cam in it. No mention of air filter, I have a KN-filter.
If you solve the problem, I'd be happy to make same mods.