No compression

[Ady00]

Hi
Late last year a friend I met through the club Roy Germain helped me replace the large idler gear in the timing chest as a precaution, this turned out to be a blessing as we found that at some point work had been done but shims were not replaced correctly. With this sorted I continued to enjoy about 150miles of cracking riding on our local back roads before the winter set in. The end of the silencer had been very sooty, Roy said it was running rich and this showed on the plugs, on checking the carbs the needles were on the notch below the middle notch so I adujusted them to the middle as per book, they are standard new carbs from Burlen. The following day I took the wife out on the back we got about six miles down the road and I lost compression on the front pot, there was'nt any 'death rattle' just a loss of power. We limped back home on one pot with a trail of smoke in our wake, im going to have a look very soon and get it fixed, I know things happen but prior to this the bike had been running very well I had the mag rebuilt by Barry Basset last year and I had checked the timing with as timing disc and put in new engine oil and filter when the timing chest was done, if anybody has any ideas please let me know, sadly Roy passed away before christmas so as well as lossing a good friend I have also lost his invaluable knowledge, thanks.....Ady

 

[Tnecniv Edipar]

The colour of the smoke is a good indicator of the possible cause. Blue is oil , so it could be a holed piston if the smoke is blue. Remove the spark plug and check if it wet with oil.

 

[Bracker1]

Don't know how long it was sooty, but carbon buildup might be preventing exhaust valve from closing. Hopefully it's not a blown piston. The other thing to check is simply the adjustment on the valve lifter to front exhaust valve. This could be why the original timing gear was shimmed, might be binding the rod... I've seen many a bodge on my own bike. Good luck, Dan

 

[Tom Gaynor]

Lost compression

Put your thumb over the plug holes, and kick it over.

My bike wasn't going very well at a Scottish camping weekend. So I retimed, and it went much better. On the way home however, it clearly wasn't right, so I took the timing cover off (no, when I got home, not by leaning over on the move). The front inlet pushrod had jumped out, jammed itself and the cam follower in the only place that the motor wouldn't be wrecked, and such that enough mixture would get in to the front pot (valve off seat...) to keep it firing.

Jesus: does a Vincent Shadow with two fully functional cylinders GO! Having replaced the new, bent, pushrod with one of the old ones, I spent the next hour thrashing it up and down the road thinking WOW! This is what they're SUPPOSED to be like. It really WOULD cruise at the ton, if I didn't need my license.

Be lucky.

(The club shadow developed about 13 hp (exaggeration for dramatic effect only) as standard. To get it to go with modern fuel it was dramatically weakened. This bumped the hp up three-fold. (If my memory is correct, and that's possible, ignition timing was not particularly critical.) So the chances you've holed a piston aren't zero, but they're slim. The bad news is that this can only really be checked on a serious (i.e. not Dynajet) brake with an exhaust sniffer.)

Tom

 

[John Appleton]

Put your thumb over the plug holes, and kick it over.

The bad news is that this can only really be checked on a serious (i.e. not Dynajet) brake with an exhaust sniffer.)

Tom

Could this mean we need Tom Thumb?
A much simpler way(to find your lost compression) is to use a normal piece of garage equipment called a cylinder leak tester. This is a delivery hose which screws into the spark plug hole. A measured amount of compressed air is then passed into the cylinder and the amount leaking past rings , valves etc. is shown as a percentage on the gauge. 20% is an average reading, anything over this is easily pinned down by listening at the exhaust for air leaks, showing a faulty exhaust valve, at the carb for inlet valve, and the breather or open valve cap for rings and piston sealing.
All of this is done with the cylinder in question on T.D.C. of the compression stroke, having first made sure there is valve clearance.
Leaks around the head joint are best checked with soap water.
I would also be checking valve timing as the last time I experienced this type of symptom the front cam shaft had turned in its pinion and I rectified matters by welding it in.
John

 

[Tom Gaynor]

Welding cams

I tell you this only for information, speaking as someone who, like you, would have used a dab of MIG or TIG.

A friend in Shetland (NOT "The Shetlands" or they'll cleave your head with a Viking axe...) had a Chater Lea. There is no Chater Lea spares scheme, which adds poignancy to the problem. He sent a camshaft off to be refurbished, and after they ground the lobes (super job..) they ground OFF the raised interference portion that the cam wheel was shrunk on to. Oh dear, 0.001" interference is now 0.010" clearance.
Call for Captain Bearing-Fit Loctite. Apply.Three years later it was still running.

 

[Comet Rider]

Leak Down Tester

Hi Ady,
I'm not sure whereabouts near P'boro you are, but I do have a leakdown tester as described by John Appleton.

Should you wish to make use of it, please send me a message and let me know.

Best of Luck
Neil

 

[Pete Appleton]

Try the easy things first

All of the above will have you happily dismantling the bike but what about the boring things? As the carbs were the last thing interfered with I would be looking there. If the smoke is black then it is possible that the choke cable has become detached and the choke slide has dropped down to the 'full on' position. This is easily checked by opening the throttle and looking up the air intake.

I know this wouldn't give loss of compression but has this actually been measured?

Pete

 

[Comet Rider]

The easy things

Hi Pete,

The leakdown tester is an easy way of establishing wether there is any mechanical problems within the cylinder head/barrel assembly.

After all, all you need to do is to pull a plug.

Cheers
Neil

 

[Pete Appleton]

leak tester

Yes, Neil the leak tester will certainly detect a hole in a piston or a burnt valve but the actual leakage value is best treated with a degree of caution. One of my frst jobs in the garage, on leaving school, was on our morris marina van. This machine would leave a trail of blue smoke that would put the red arrows to shame but on carrying out a cylinder leak test the reading was the lowest that I have ever seen! I can only assume that the oil passing up past the control ring was sealing the bore and giving a false impression. ( a rebore cured the problem)
My only reason for questioning the 'lack of compression' theories is that I hear of this complaint often, at work, from people who are actually just complaining of a misfire.

Pete

P.S Congrats on becoming Herts & Beds section organiser. Does this mean we can blame you this year when we don't get any breakfast?