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Misc: Everything Else Standard Comet Special


greg brillus

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VOC Member
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

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VOC Member
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

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VOC Member
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.)

IMG_1127.jpg
 

BigEd

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VOC Member
VOC Forum Moderator
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.
 
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davidd

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VOC Member
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.
Overlap.PNG
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

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VOC Member
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

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VOC Member
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.
 
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greg brillus

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VOC Member
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

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VOC Member
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

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VOC Member
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.

IMG_1128.jpgIMG_1129.jpg
 

timetraveller

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VOC Member
I have said this before but I will repeat it. If you really want to get a handle on what a set of cams is doing then spend some time and produce a set of lift versus engine rotation, every ten degrees of engine rotation which is every five degrees of cam rotation. Once this is plotted out the one can see what is happening. The four degree BTDC equal lift system might be OK for well known cams but if you are playing about with cams with unknown qualities then it will pay to do the job properly. If you can use a spread sheet then you can produce the graphs you need with very little effort and you can also use the same spread sheet to show the velocity and acceleration curves. If you cannot do that then if you send me a list of lift versus engine degrees then I will set up the whole spread sheet for you and, if it is of interest to others, put the results on here.
 

Oldhaven

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VOC Member
Thanks TT, I'll do that, as it will answer most of my questions, but I am probably two weeks to a month away from getting the engine together enough to do it from the valves themselves. I may try it directly from the followers just to get a baseline.
 

Oldhaven

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VOC Member
I'm working on the Comet timing chest today and was hearing the big idler ring like a tuning fork as I spun the gears. On a whim I brought my phone with a chromatic tuner app over and it is ringing at E flat 7 with overtone at A8. I have the same idler on the Rapide and the tone is very familiar. Has anyone ever applied a damping coating to the center web of the idler?

Anyway, the spindles, oil pump, and bearings are all in now, and once I recheck the spindle shimming I should be able to button up the cases. I did change the cam bushings to oilite.
 

Attachments

greg brillus

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VOC Member
Check the grub screws at either end of the crank pin are tight, and pump some clean oil into the big end via the t/side mainshaft with an oil can. I had one that was missing one of these grub screws, resulted in a seized piston. This was on a 600 Comet with a TP crank assembly and top end kit. This was how I received the engine.
 

Cyborg

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VOC Member
Has anyone ever applied a damping coating to the center web of the idler?
No, but I thought about it. With an Alton added to the equation (with the rare earth magnets), the Comet's idler sounded like hells bells. Some kind soul assured me that the noise would disappear once it got above kicking speed.
 

davidd

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VOC Member
Ron,

Does the flywheel use the two taper roller drive bearings?

I used E flat tuning for years. It's easier on the fingers.

David
 

Oldhaven

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VOC Member
Hi David,

No, standard reduced width rollers. The timing side has been repaired and sleeved but the drive side is standard and OK.

P1050085 (Large).JPG

My wife, who is good in music theory, says that if the high note of the idler was an A flat or B flat it would be a more pleasant cacophany.
 

Cyborg

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VOC Member
I'm assuming it will get drowned out when the rest of the percussion section starts up. Sort of like the 3rd graders playing the 1812 overture.
 

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