Comet Top End Speed

[Gene Nehring]

Hi Guys,

Thanks for all the replies on this thread! Awesome feed back.

I have stepped down the main jet from 210 to 200 and I have picked up 5mph. I will try a 48T sprocket in the near future and give feed back.

I will try the straight through exhaust and see what I come up with.

All in all the bike is running well and the plug colour through the range is looking allot better.

Being an inexperienced Vincent owner this is thread is adding to my ability to work on my bike and for that I am thankful. I am a hands on guy and like the challenge of working on things, much like allot of you out there.

So cheers.

 

[Kansas Bad Man]

Before long you will be choosing off the croch rockets, more power to ya , bang up job!


MAX

 

[Martyn Goodwin]

Genedn,

Care to share the carb specs - make, type and jetting plus the ignition type and settings?

Martyn

 

[Gene Nehring]

Genedn,

Care to share the carb specs - make, type and jetting plus the ignition type and settings?

Martyn

Mikuni VM 28 from Sudco and Lucas Mag

200 Main jet
30 Pilot jet
169 P4 needle jet
5fL14 Jet needle
2.5 slide
2.0 air

Not one hundred percent on timing. I am going to have some mag work done in the near future and will let you know what I come up with.

 

[Martyn Goodwin]

Thanks for that. I have a Lucas Mag and run premium grade fuel (98 RON). After much road testing I have settled on 31 degrees BTDC fully advanced and 6 degrees BTDC fully retarded. To get that spread I had some extra metal welded to the ATD fixed ears then carefully "tuned" the ATD range with a fine file.

Today I swapped out my 48 rear sprocket for a 50 - just a quick run around the block so far - longer run scheduled for this Sunday. Feels much more lively at suburban speeds.

 

[Albervin]

Hi Guys,

Thanks for all the replies on this thread! Awesome feed back.

I have stepped down the main jet from 210 to 200 and I have picked up 5mph. I will try a 48T sprocket in the near future and give feed back.

I will try the straight through exhaust and see what I come up with.

All in all the bike is running well and the plug colour through the range is looking allot better.

Being an inexperienced Vincent owner this is thread is adding to my ability to work on my bike and for that I am thankful. I am a hands on guy and like the challenge of working on things, much like allot of you out there.

So cheers.

If you fit a straight through exhaust then you may want to initially consider a larger main jet for safety sake.

 

[Gene Nehring]

Thanks for that. I have a Lucas Mag and run premium grade fuel (98 RON). After much road testing I have settled on 31 degrees BTDC fully advanced and 6 degrees BTDC fully retarded. To get that spread I had some extra metal welded to the ATD fixed ears then carefully "tuned" the ATD range with a fine file.

Today I swapped out my 48 rear sprocket for a 50 - just a quick run around the block so far - longer run scheduled for this Sunday. Feels much more lively at suburban speeds.

I would be interested in your thoughts once you have had a good run on the 50. I have not run a 48 or 50 and would appreciate your comparison before I switch out the 46.

 

[Martyn Goodwin]

After some more road testing, here is my conclusions.

With my standard comet configured as follows: 7.4:1 CP piston, standard head and valves, Mk1 cam, Amal Mk1 premium carb with 105 needle jet and 210 main, original rebuilt Lucas magneto (points at 0.012") with ATD tweaked to give 6 BTDC at starting and 31 BTDC full advance, Wheels front 19" and rear 18" with Michelin Pilot Active tyres 34 psi rear 30 psi front. Spark Plug, gaped at 0.020", is a Nippondenso W22EP-U

Front sprocket standard with standard 48 rear sprocket uses standard 108 pitch chain (with rear axle around mid point of adjustment) gives somewhat sedate acceleration and the need to change down to 3rd on some hills. Comfortable top speed of 120 KPH.

I then changed the rear sprocket to a 50, the standard 108 pitch chain was OK though the rear axle is now closer to the front of the adjustment, but NOT at the limit of travel. Acceleration is significantly improved and the bike now pulls strongly (in 4th) up most hills where with the 48, 3rd was required. Comfortable top speed is around 118 KPH at what "feels" like the same revs. Sorry I do not have a tacho fitted.

The move to a 50 rear sprocket has made the bike much more tractable and pleasant to ride. With a viciously policed Australian speed limit of 100 KPH the variation in top speed is not an issue to me.

The Spares Co (UK) as well as Coventry Spares (USA) have the 50 rear sprocket. I believe the chain of choice is Tsubaki 530QR, a quad staked non o-ring chain - make Google your friend to find your local supplier. In Australia I purchased the chain from UMS Transmissions ( www.umstrans.com.au ).

Hope this info is of some help.

Martyn

 

[Gene Nehring]

Great info Martyn. Thanks allot for posting!

I will be sure to update you once I adjust my bike.

I got a away with some riding this winter, but alas we have just had a 6 to 8" dump in the last 24hrs so no riding on the horizon this week.

 

[Martyn Goodwin]

If you change from a 46 to a 48 rear then the existing chain MAY be OK, however if you go from 46 to 50 I expect you will need a new chain.

M