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Comet Top End Speed

Gene Nehring

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VOC Member
Hi All,

I know this has been discussed before and I am looking for a little clarification specific to my machines set up. I have repaired the exhaust lifter as discussed in a previous thread, thanks to all that replied.

I find that the bike will cruise really happily at 55mph and up to 60-65mph. If I roll on a little more throttle at the speeds mentioned it starts to feel like the bike is retarding. I am looking for places to start. The bike is running a little rich pulled plug after a good ride (not wide open cut).

Should I expect more speed or is this about right?

The bike is running the following:

Mikuni VM28
Mk3 cams
7.3 to 1 compression
NGK B6ES plug
46 tooth rear sprocket (if I counted right) :)
Lucas Magneto




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VOC Member
Leave it in 3rd gear and see what it accelerates like at 60 mph. A 48T sprocket was standard and even with that the bike likes to be in 3rd gear up even moderate hills. It also comes
down to the ahem, ballast. What ignition timing did you set?
Last edited:

Gene Nehring

Well Known and Active Forum Website User
VOC Member
The ballast is 195lbs or 88kg.

I am not 100% sure what the ignition timing is. I have had the bike 6months or so. I am looking to have a fellow member in Vancouver look at my mag as it has a very slight oil leak through the seal on the timing case side.


Well Known and Active Forum Website User
VOC Member
Gearing sounds suspect.
Main jet may need tweeking. Checking the plug after a run won't show you what the main jet is like, you need to do a plug chop at full throttle.
Then there's the old favourite, the silencer. Some of them are too restrictive.
Always difficult to know what to expect from a "new" bike. My rattly, smokey old Comet is happy cruising at 60, and will clock 80 with me sitting upright on ordinary country roads. Maybe they go better when there's more clearance and less resistance round pistons and valves. :D


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VOC Member
I have never achieved more than a registered 85mph on a standard Comet. But it would regularly do that in third, two up. It would not go faster in top. Thirty years on, I hope I have a little more mechanical sympathy and the bike is now twice the age.
If you want to go faster, lower the gearing and use earplugs.

My current Comet will achieve greater speeds, having sufficient power to pull top gear, but I rarely find the need. Isn't ageing sad?

Len Matthews

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VOC Member
Certainly needs a larger rear wheel sprocket. Even the original Road Test considered the machine overgeared with a 48 tooth sprocket. I tried a 52t sprocket; that transformed the bike. Hill climbing, acceleration and even fuel consumption were improved as the machine was not labouring and the engine could rev freely.


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VOC Member
The best my Comet went was with a lower than standard gearing, 85mph 2 up and accelerating up a slight incline..It had a 17 tooth gearbox sprocket ..
I changed to an 18 tooth gearbox with 48 tooth rear as standard. T he bike is still being run in after I cooked the last piston, I have about 300 miles on it..I am running in at between 45-55 mph with occasional bursts up to 65.With the ignition set at 35 degrees BTDC and I think it may need to be retarded a bit more.
I agree with Len that a Comet could do with being slightly undergeared than standard. I also think that if you do have a 46 tooth rear sprocket the bike will most likely go better in 3rd than 4th.
When my Comet is run in I will expect it to cruise fairly well at 65-70 mph. It is not even trying at 55. John


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VOC Member

It sounds like a pretty nice set-up. Running rich will slow you down and if it is a 46T rear, that will slow you down. I would check the counter shaft sprocket and if it is stock I would run a 48T rear at a minimum. The more sparks per mile the more power, in general. I am sure that someone local will check the ignition and show you what you need to know. Check your carb slide to make sure it is opening all the way when you hold the throttle wide open. If you can check these few mechanical things it will be much easier to tune.

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