E: Engine Big Bore Comet

[Bill Thomas]

Ah, You mean my ex L/ning,
I raced it with T.T. Carbs for 7 years,
Then put it in road trim, With Mk1 Concentrics,
Then about 10 years ago I tried a Mk2 on my Comet and fell in love ,
Had no clue they had been around so long,
So got 2 second hand ones for the Twin All 30 mm jobs.

Manifolds, From the Spares Company, Nicely made, The hole is a nats too small,
But good enough for the road.

With the Mk2 , The slides can be just a nats out and it's like you have switched it off,
We set some up for a friend and it was a bastard to get to run at all !,
Lucky I knew of the trouble and we ordered a pair of slides = Different Bike !.
It depends on the tune of the engine and petrol etc,
They are not for everybody, Some hate them !. Some say Spanish ones are different ?,
Some run them on strange settings, But work for them .

Mine can be hard to start sometimes , It might be me getting too old,
Hate to think that , Worse for me is starting after a stop of about 1/4 hour at the coast,
Always chilly there and need a bit of choke , But Mk2 s are tricky , Choke should be on or off,
But there is a way,
I am finding a nats of choke on just the rear is best from warmish ?.

But my bike is still L/ning spec.

The starting jet is hard to find in the bottom of the bowl,

And the slow running jet can go on top of the bowl or on bottom of the body !!,

The correct needle is 2A1 64mm from centre of top groove to the bottom.

The two stroke one is shorter and some are marked wrong.

 

[TomB]

Hi Bill,

I moved my slow running jet from the bowl to the bottom of the carb body, which seemed to be an improvement! I find that I need to run with the choke on for about 500 yards, then can't run without turning it off, so the lack of any medium setting is awkward but not a real problem.
So your twin is still Lightning compression, heads and cams? I would guess a standardish Rapide would need around 28mm and start with the book setup.

 

[Bill Thomas]

Hello Tom, The Mk2 is a strange beast, Depending on the slow running jet,
I found the slow running screw affects much higher up the rev range than other carbs,
Sounds like you are running too weak ?,
Dangerous with a Vincent ?,
Maybe richen it up with the slow running screw ?, Make a note of what it is now, Before playing !,
Or the slide is too big a cutaway ?,,
Most of our running is on the slide ?, My Twin does 80 mph on 1/4 throttle, Once up to speed,
I can't remember if I put a 30 slow running jet in there ?.

On one of mine , The hole in the front was too small, Can effect things,
If you undo the intake part, 2 screws, Under there is a hole that is important ?.

Maybe mine is too rich , Because I have to take the choke off almost as soon as I start from cold.

I would think 30 mm is OK with a standard engine,
As I said the chart is for the size of carb' , Not make and model ,

Some people have told me to use a 105 needle jet !!, Can't see how that would work on mine,
Mine goes well.

I can just get the choke to stay open half way, Mine is lever type choke,
But that is sometimes for a warm start, And for mine just on the back carb',
As an old mechanic, To put choke on when warm is wrong, But with the petrol we now have ,
I have to think different,
I am not as fit as I was, So some of it could be me, I still try to start it sitting on the bike,
I am tallish, And still running 9 to 1 pistons Mk2 Carbs, Rita electric ignition, I hate that !!,
If it ever packs up , I have a "D" Distributor for it,
McDougleator Alternator.
Good Luck, Bill.

 

[vibrac]

I reccomend an electric start for the Comet Bill
No valve lifter 3 months standing Lithium battery press the button and away we go...

 

[Bill Thomas]

Not for me, They are a lot of money !,
Must admit been having Hip pain of late,
Thought I was in trouble,
But it was the old back trouble,
The nerves in the back go all over the place !!!!,
Can't tell you where the pain was once .

 

[TomB]

Morning Tom , The chart I found for Mk2 carb's ,
Are settings for the size of carb' , Rather than Bike type,
I bought a second hand Mk2 with Methanol jets !,
So had to start again, For my Standard 500, Flash spec' Road Comet,
And the settings they gave for mine made it work well straight away,
I was so pleased I bought a second hand pair for my twin, Same settings !,
Maybe only the slides need changing From bike to bike ?, A small amount.

Hi Bill,

I finally got round to a ride on the Comet (too many other bikes) then took the carb off to check - I have got main 230, so will try larger, needle jet 106, slide 3.5, with a bit more of a cutaway taken off later, and idle jet 25.
What were the perfect settings you had that worked so well? All the best, Tom

 

[DucATIRadeon]

How does it go at full throttle?
My special is 90mm bore, TP 105 cams, 8:1 and I use a 1034 concentric, the main jet is 280.
I'm not sure if this is perfect as the engine won't go over 4500 rpm (about 85 mph) because the bike is way over geared I guess.
But the plug always seems ok, light brown down the porcelain.

I use a STD BSA gearbox with the smaller sprocket (17T) and a Norton Commando rear hub. There is no way to fit a smaller rear sprocket but I have found a rare Spitfire (16T) G/B sprocket. I'll fit this sprocket as soon as I find the last gear (mainshaft 2nd 22T) I need to build an RR gearbox.
As it is the bike doesn't like the STD gearbox with its huge step between 2nd and 3rd gear.

perhaps (way) out of context but couldn't help noticing this reply as the RE Bullet is identical bore x stoke as the Vin:
my RE bullet 500 (now with 87mm bore giving 535cc) with standard cams (30-60-75-35 timing, 7.8mm valve lift) does over 5000rpm in 4th doing 100+mph....sitting upright!
std primary gearing, 4th gear 1:1, 19/38 final gearing with 19" rear wheel.
valves are now 1.875" (47.6mm) inlet and 44mm exhaust, head ported by ABSAF (The Netherlands), 32mm dellorto and Sachse electronic ignition (8 degrees BTDC static, 34 degrees max advance). did 25hp/47Nm with the original valves, now doing 28hp/50Nm ish.
crank is balanced to 60% (best value according PEI and ABSAF road going racers).

so, The Comet having "better" cams and more valve lift, I wonder why your 600cc+ Comet isn't outperforming my agricultural Enfield??
mind you, my Bullet is from 2002 yet mechanically/design wise indistinguishable from the older '56 design when it moved from UK to India...
If I were to make another mod to my Bullet, it be a larger stroke (106mm if possible otherwise 103mm) and heavier crank (following Egli).

would you happen to know why your Comet isn't going above 4500rpm?
just curious...

 

[greg brillus]

My single 665 race engine (92 x 100) last run on the dyno put out just over 50 RWHP That was with the old piston that had lowish comp ratio.......after that we installed a hand finished piston somewhere above 15:1 (on methanol ), then I set about fine tuning the fueling to lean it off and the power difference was noticeable.......No racing this year so far, as loads of time spent making up roller followers and special one off cam to suit, plus all the time spent getting the replica Grey Flash ready for Goodwood. I am hopeful to see figures upward of 60 but this could be dreaming a bit, but it all looks promising so far. A good road Comet with a 600 kit, Mk 2 cam and 36 mm Mikuni or Dellorto will see around 35 on a good day. It really transforms the bikes, and I tell folk that if the factory could have made them like this, they probably wouldn't have sold as many twins.

 

[DucATIRadeon]

My single 665 race engine (92 x 100) last run on the dyno put out just over 50 RWHP That was with the old piston that had lowish comp ratio.......after that we installed a hand finished piston somewhere above 15:1 (on methanol ), then I set about fine tuning the fueling to lean it off and the power difference was noticeable.......No racing this year so far, as loads of time spent making up roller followers and special one off cam to suit, plus all the time spent getting the replica Grey Flash ready for Goodwood. I am hopeful to see figures upward of 60 but this could be dreaming a bit, but it all looks promising so far. A good road Comet with a 600 kit, Mk 2 cam and 36 mm Mikuni or Dellorto will see around 35 on a good day. It really transforms the bikes, and I tell folk that if the factory could have made them like this, they probably wouldn't have sold as many twins.

I thought Godet lineup included a Flash that gives 60(rw)hp and does 7000rpm with the 600cc setup?
a similar setup should give similar results, perhaps the torque figures vary somewhat.

what rpm are you running, now you mention 36mm carb size? I based my carb size on 100m/s airspeed in the carb at a max rpm.
I'm pretty sure if the carbs on the twin were 32mm minimum and ported to match they'd give an immediate noticeable gain.

 

[greg brillus]

I've not heard of a Godet 600 single........Ours are all 500 cc, but 2 different engine bore/stroke combinations used........Stock French engine uses a 92 x 77.2 mm.......whereas our modified engine uses a 86 x 86 bore/stroke.......This second engine was re engineered by Peter Molloy here in Australia a few years ago.......he was a specialized engine builder who came up with this engine spec combo........Both these engines use a one piece all steel crank with 2 piece shell bearing con-rod.......These engines will rev to 8000 rpm plus, the 92 bore engine even higher......of course they all use a high pressure oil pump and the engine casing has provision for the WR Yamaha type pump.......My engine, i have set the rev limit to 7200 on the limiter but seldom take it past 6200 on the track......It has a huge amount of torque, so simply reving it higher is not really necessary...... i doubt any of the French engines has gone near 60 HP.......The design just simply is not that high a spec......Its power comes from big revs and massive duration cam timing.......There is more that can be done, its just time and money...... The French engine is great, but the top end and cam design is too old school i feel.