E: Engine Damaged quill

[Bill Thomas]

My Comet has always been Manual advance/retarded BTH old type, I have tried playing retarding it with the lever at all sorts of engine speed, Wishing for some extra speed !, But found once it's over 2000 revs ish, There is no point, Also my L/ning rep had Manual, Same thing, Once it's on the roll, Left it alone. Cheers Bill.

 

[Chris Launders]

If you're looking for Specialloids I was talking to John at Sheffield British Motorcycles last week and he said he had some in his stock.

 

[Bill Thomas]

I bought a barrel and piston from the late Pat Cole, For my" new" Comet !. Thought it was a Specialloid, But was not, Don't know if I should take the risk ?. Cheers Bill.

 

[Martyn Goodwin]

In my experience there is nothing wrong with the low expansion pistons from CP Carrillo or others other than the supplied installation instructions are NOT suitable for our air cooled engines (but they are OK for liquid cooled motors). I have fitted CP's with their recommended clearance and encountered seizure.

Based on my hard learnt (and $$) experience I suggest a more liberal piston to bore clearance. The CP specification sheet says to use a clearance of 0.0030"; I have found 0.0050 to 0.0060"" to be a much safer figure to use.

I also suggest that you set the ring gaps a bit wider than CP suggest - an extra 20% should cause no issue but may prevent problems later.

Also check the small end pin; it must be able to move freely in the piston - if you need to mechanically press the pin into the piston then its too tight and you need to very gently ream out the holes on the piston till you are able to push the pin in with your fingers. Do the same with the small end bush on the conrod. Again it should be a push in with your finger job - any tighter and you need to ream the bush in the rods small end

Martyn

 

[Bill Thomas]

Plug is an NGK B7EV, nice tan color, no sign of blistering, or tiny shiny spheres
So far the Comet has returned home on the back of my truck anytime I attempted a ride that was longer than 10 miles. I now realize the first time it stopped it must have seized, although it was free when I stopped, and started up again after it cooled off. The second time there was no doubt about what happened, that was mile 126 of my ownership. I am now at mile 205, new piston (7.3:1 diecast VOC part) and cylinder, and nervous about riding any distance.
I timed it with a degree wheel and the head removed, the tricky bit is making sure it does not move when tightening the atd. Next time I will make a piston stop out of an old spark plug. All my other bikes have points except my 07 Guzzi, many years ago I had an Atlas with a magneto. They were all much easier to set. Interesting factoid, I have a Chrysler 9.9 hp two cylinder water cooled outboard motor, the speed control twist grip moves the timing plate, a lot, not the carburetor. My first outboard, so I don't know if this is normal or unusual. It will empty the gas tank without blowing up as will all my other bikes.

Out of interest, What skirt clearance was supplied with your new VOC piston and barrel ?. Cheers Bill.

 

[Black Flash]

When timing the mag I always give the atd a little tap with a hammer to make sure the tapers are engaged before tightening the nut.
This way the timing rarely moves during tightening

 

[Bill Cannon]

Everiman's problem is high exhaust side temperature, which, to me, suggests insufficient advance rather than too much.

Bill

 

[greg brillus]

And/or too lean a mixture...........I prefer the plugs to be slightly sooty black over clean/tan colour. Still if the engine isn't spitting back, that's usually not too bad.

 

[davidd]

I was told by someone (it may have been Gene Aucott) that was around at the time that Vincent once decided to set their piston clearance at .004" for the entire production. The result was a rash of seizures that required a re-examination and an increase in clearance. It created an enormous amount of work for the dealers and cost for the Factory.

If this were true, it does say a bit about the older high expansion pistons and their clearance requirements. I don't have any problem with the low expansion pistons, only the recommendation that they be set at .001" or .0015".

Comet engines often seize as they are going uphill with the throttle open. I think many Comets see more WOT than twins and my guess is that a Comet running uphill fast will increase in temperature to the same area as racing temperatures. I think this is simply an area where the piston is getting so hot that the piston is reaching expansion rates that were not anticipated at what the engineers felt were normal operating temperatures.

Finally, I do think these engines are petrol cooled. I am not sure that helps much, but if you have gotten out of the wooly tuning area, that is probably good enough.

David

 

[Bill Thomas]

Hello David, Are you saying one to one and a half thou ??.
The other thing is some bores are not straight, T. T. told us a bit some time ago.
I nipped up my ex L/ning on the road, When I went down to 7 thou some years ago, It was done by a car bloke, A tight 7 !, I was not happy about the new barrel I was sold.
If you think the piston is not in line, It will be tight at the top on one side and at the bottom on the other side,
So it's not 7 at all !.
Cheers Bill.