Misc: Everything Else Standard Comet Special

[greg brillus]

Ron the single ignition was one of the newer BTH mags, and the twin plug was the Grosset ignition which is one specifically for a twin plug engine, that is it has about 12 degrees of advance in the ignition unit itself. I am not sure of their advance curves but I'm sure it must be published somewhere. The 600 Comet with the Grosset twin plug set up (Terry Prince top end kit) was inclined to still ping a little under load, so i set the ignition to 26 full advance and we run it on 95 Octane fuel, this engine runs a 36 mm pumper type dellorto carb in case you are wondering, it runs very well and loads of power, enough to make the clutch slip.......

 

[vibrac]

Back when the world was young and Harold Wilson was in charge Ron Kemp fitted a 10mm plug firing through a 1/4" hole in the gap between the valve seats on his Grey Flash, perhaps those who may be right about two plugs may see that as a path to increasing the squish area opposite the 14mm hole

 

[Oldhaven]

Equal Lift

I'm putting my timing chest together now and took this picture of the TP Cam without the wheel on yet. I think it shows the equal lift at 4 degrees poition for a Comet, but it got me to thinking. What if the cam is ground or worn so the two lobes do not give equal lift at this textbook position of the slot in the cam base? Does this excellent shortcut to valve timing depend on new and well designed cams or is it just a function of the valve openings and cylinder filling regardless of the shape of the cams or the differences in the lobe grinds ?

(I just noticed that one of the followers is a bit off the cam lobe, but you get the idea and the question still applies.)

 

[BigEd]

Equal Lift

I'm putting my timing chest together now and took this picture of the TP Cam without the wheel on yet. I think it shows the equal lift at 4 degrees poition for a Comet, but it got me to thinking. What if the cam is ground or worn so the two lobes do not give equal lift at this textbook position of the slot in the cam base? Does this excellent shortcut to valve timing depend on new and well designed cams or is it just a function of the valve openings and cylinder filling regardless of the shape of the cams or the differences in the lobe grinds ?
(I just noticed that one of the followers is a bit off the cam lobe, but you get the idea and the question still applies.)

I'm sure that I've read that the 4º BTDC equal lift method works for lots of engines, not just Vincent assuming a relatively well designed cam.

 

[davidd]

What model TP cam is it?

It is not impossible to have grind or wear issues, but it is a pretty safe spot to take a reading from.

Even with some wear you are going to still be in the area. Many owners set it at tdc rather than 4 degrees before and it is difficult to note the difference in performance on the street.

David

 

[greg brillus]

Terry's cams are usually set up at 0.120" lift on his Mk 3 cams and 0.170" lift on his Mk 2, 4, and 5 cams all set with the engine at TDC on the overlap stroke, this on the inlets the exhausts are fixed so you can't alter that anyway. Be aware that most of Terry's cams have plain bronze bushes which will readily seize to the spindle if not enough clearance is given, when you press the pinion on it will collapse the bush on the drive end and will need honing out, oilite type bushes are better and less prone to seizure.

 

[Bill Thomas]

Ron the single ignition was one of the newer BTH mags, and the twin plug was the Grosset ignition which is one specifically for a twin plug engine, that is it has about 12 degrees of advance in the ignition unit itself. I am not sure of their advance curves but I'm sure it must be published somewhere. The 600 Comet with the Grosset twin plug set up (Terry Prince top end kit) was inclined to still ping a little under load, so i set the ignition to 26 full advance and we run it on 95 Octane fuel, this engine runs a 36 mm pumper type dellorto carb in case you are wondering, it runs very well and loads of power, enough to make the clutch slip.......

Greg, Can you tell us what jets you use, It might help Ben on another thread, Cheers Bill.

 

[greg brillus]

Hi there Bill, I'm sure i wrote the specs down somewhere but hidden in the depths of everything on one of my benches. I don't know much about the different jets and slides on the Dellorto's, although I do find them to be a very good carby. That 600 Norvin single I built had a 36 mm Mikuni on it, straight out of the box...........

 

[Bill Thomas]

Thanks Greg , I know what you mean, Just trying to save Ben buying too big jets, If he starts getting 200 etc , Like Amal , I think he will be miles out, I am thinking 160 or less, Just looking for someone to confirm what they have used. Cheers Bill.

 

[Oldhaven]

The TP camshaft is stamped with a 2. Somewhere I heard that his cam profiles have changed over the years , and as you can see below, it is very different from a similarly stamped Vincent 2. this is why I was wondering about the equal lift theory, since it seems like differently shaped and non symmetrical cams on the front and backside of their curves could be in different angular positions at that overlap. I find all this very confusing. I will try to check lift at TDC when I get there. I made a jig from the Vincent cam to press on the pinion, assuming that Terry's cam would use a similar angular geometry relative to the slot. The idea is to use the pin to locate the camshaft in the new pinion so I can use the pre-etched marks on the pinion for later assembly. The pin locates in the slot to get things started with the screw thread on the 1/2" spindle and is then backed out to allow completion of the press. (All those other holes were there for another purpose)
Thanks particularly for the advice about the cam bushes, and I will check for clearances (is .0015" to .002" OK for plain bronze?) after pressing the camshaft in, though now I wonder if I should swap them for a set of new porous bronze ET64's that I have. I don't think oilite should be reamed and I don't have a hone for that. More to worry about.