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Comet Crankcase Breather

Phil Baker

Well Known and Active Forum Website User
Non-VOC Member
Hello all

Phil (the Basingstoke Comet) here..... still enjoying riding about and getting soaked in the summer! A bit frustrated by the Comet's slow going on uphill stretches. I was advised to check that the crankcase breather is timed correctly so I have just removed the banjo and created a plastic tube/insulating tape thingy to thrust up the hole (not using Ernie's KY) in the crankcase just in front of the dynamo. Took out the sparkplug and turned the engine over a few times - NOTHING coming in or out of the pipe at any time during the cycle. Not knowing the inner mysteries of this particular setup I don't know what to look for now. A screwdriver passes unimpeded from one side of the cases to the other and there are no visible signs of blockage. Any ideas, please?

Regards

Phil
 

Howard

Well Known and Active Forum Website User
VOC Member
Have you taken the rocker caps off (or something simillar) - if you have the air will take the line of least resistance. I'm not sure what you're doing with the screwdriver, but if you're pushing it into the breather hole it should stop when it reaches the end of the hole in the breather spindle - about 3".

You could try blowing down the tube with the crank at various positions to see if the breather's opening and closing.

H
 

Phil Baker

Well Known and Active Forum Website User
Non-VOC Member
No, rocker caps still on - but on trying again I found that there was no blockage (dunno what that was about!) and was able to blow through the tube to a greater or lesser extent dependent upon the position of the piston. So, took off timing cover, put on degree disc and pointer, found 2 degrees before BDC and rotated pinion until resistance increased to maximum. Reassembled it all and it started up as well as usual. I can't test ride it at the moment as I can hardly see in the glow of the magnificent summer sun that we are all enjoying at the moment! Hopefully it will make a difference - if not, it's try a different silencer time!! Thanks, Howard.
 

Howard

Well Known and Active Forum Website User
VOC Member
Silencers really can stifle the performance. If you've got about 18" of exhaust pipe (and understanding neighbours) try to lash it on in place of the silencer (temporarily). It will show you if the silencer's too restrictive, but don't get used to the lovely sound, it atracts blue lights.

H
 

nkt267

Well Known and Active Forum Website User
VOC Member
What sort of hill are you looking at..A Suffolk hill that rises 2ft in every mile,very flat round here.
What sort of performance are you expecting? When I went to the national in 2007 on the Island I could only just make 60ish on the climb out of Waterworks Corner,3rd gear or 4th made no difference, till I got to the relatively 'flat' top of the Island.With the 17 tooth sprocket I had on the gearbox at the time, top speed on the flat 1 or 2 up showed 80-85mph on the speedo and over 90 on good downhill roads.BUT the engine was trying to climb out of the frame.:eek::eek:
I now have an 18 tooth sprocket and the engine is still not quite run in and the bike goes pretty well, with very short burts up to 80 mph. Cruising at 65-70 is comfortable. I have Mk2 cams and 8:1 pistons as that was all that was available, previously I had a standard piston in..John
 

nkt267

Well Known and Active Forum Website User
VOC Member
It will show you if the silencer's too restrictive, but don't get used to the lovely sound, it atracts blue lights
Stainless Armours silencer, you can hear me a'comin boss. It's straight through but has some?? absorption??that takes the edge off the CRACKLE.:D:D.John
 

davidd

Well Known and Active Forum Website User
VOC Member
Phil,

I am not sure that I am following what is going on, so I will just mention what I think should be happening. I put a degree wheel and pointer on, without removing the timing cover, etc. Then I take the plug out to find TDC. Once the pointer is correct I pull the black rubber tube off the breather pipe and I slip on a long plastic tube so I can sit on the timing side while blowing in the tube. On a single, the window in the breather should align with the window in the spindle sometime just after TDC and it should close around BDC. So, as you are bumping the rear wheel forward the breather will be blocked until sometime just after TDC. It will be free flowing until sometime around BDC. This may vary somewhat as the timing position will most likely take into account the momentum of the charge of air that is being expelled. So it may open just after TDC and just after BDC to account for the momentum. Someone on the Forum may have experimented with the numbers, but I think the range would require you to move the breather gear one tooth once it is opening at TDC to see if it breathes better by opening later. I know on the twin the timing is on the rear cylinder and it opens 70 degrees after TDC and closes at 30 degrees after BDC.

Poking the screw driver in would reveal a blockage if one existed, but if there are no mud daubers nesting in there the shaft will hit nothing until it hits the top of the breather spindle as the windows or ports are 90 degrees to the shaft.

Finally, if the breather restriction is slowing the engine severely, it is most likely the top end that needs rebuilding, but it is good to know what your breather timing is before you begin drawing conclusions. A leak down test will tell you if anything is wrong with the top end.

David

David
 

davidd

Well Known and Active Forum Website User
VOC Member
Phil,

I am not sure that I am following what is going on, so I will just mention what I think should be happening. I put a degree wheel and pointer on, without removing the timing cover, etc. Then I take the plug out to find TDC. Once the pointer is correct I pull the black rubber tube off the breather pipe and I slip on a long plastic tube so I can sit on the timing side while blowing in the tube. On a single, the window in the breather should align with the window in the spindle sometime just after TDC and it should close around BDC. So, as you are bumping the rear wheel forward the breather will be blocked until sometime just after TDC. It will be free flowing until sometime around BDC. This may vary somewhat as the timing position will most likely take into account the momentum of the charge of air that is being expelled. So it may open just after TDC and just after BDC to account for the momentum. Someone on the Forum may have experimented with the numbers, but I think the range would require you to move the breather gear one tooth once it is opening at TDC to see if it breathes better by opening later. I know on the twin the timing is on the rear cylinder and it opens 70 degrees after TDC and closes at 30 degrees after BDC.

Poking the screw driver in would reveal a blockage if one existed, but if there are no mud daubers nesting in there the shaft will hit nothing until it hits the top of the breather spindle as the windows or ports are 90 degrees to the shaft.

Finally, if the breather restriction is slowing the engine severely, it is most likely the top end that needs rebuilding, but it is good to know what your breather timing is before you begin drawing conclusions. A leak down test will tell you if anything is wrong with the top end.

David
 

Phil Baker

Well Known and Active Forum Website User
Non-VOC Member
David

Top end rebuilt by Clev Trev so I'll leave you to fight that one out!!!!:cool:

John

25mph up a 1 in 8 is just a bit inconsiderate to other pedestrians, innit? :eek:

Still, I've retimed the breather and will report back when I have bought a diving suit and been out for a blast!!

Phil
 
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