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No compression

Ady00

Website User
Non-VOC Member
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

Well Known and Active Website User
Non-VOC Member
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

Well Known and Active Website User
Non-VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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
 
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Tom Gaynor

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
VOC Forum Administrator
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
 
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Comet Rider

Well Known and Active Website User
VOC Member
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.:D

Cheers
Neil
 

Pete Appleton

Well Known and Active Website User
VOC Member
VOC Forum Administrator
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?
 

Comet Rider

Well Known and Active Website User
VOC Member
Off topic

Hi Pete,
Whilst I maybe the new S/O for the Herts & Beds I'm not the rally organiser. That honour remains with Ray for the moment (depending on how Ray's feeling):D

Back to the leakdown tester. If you are able to do the test in a quiet area, it can be possible to detect where the leak is, so identifying rings/liner valvles and or head joint. When racing we used to use a cylinder of CO2 so I could hear where the leak was.:rolleyes:

Neil
 

Tom Gaynor

Well Known and Active Website User
VOC Member
Comets with no compression

A local rider had a Comet with almost no compression. It would cough, but wouldn't really run. There were two things wrong with it.
One was that the valve timing, although correct to the marks, was wrong. I believe that Comets with timing marks that are wrong are far more common than twins similarly afflicted. A quick check is that both valves should have more or less equal lift at TDC.
(The other was that the muff was so slack on the liner that there was evidence of gas transit between the two. (The piston to bore seal was actually pretty good.) How slack? When it was sent to have an o/s liner fitted, the fitter put the barrel on his bench and lit the gas axe to heat the muff. As he did, the muff slid off the liner under its own weight.)
Prior to all of this the bike had been running: not well, but loudly.
 
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macvette

Well Known and Active Website User
Non-VOC Member
This is definitely off topic. In the early 70's there was a significant explosion on one of the nations then leading petrochemical company's facilities. It was thought to have been caused by a crane's diesel engine ingesting hydrocarbon gas, bouncing it's valves as a result of over revving and backfiring though the intake into the gas cloud. I was charged with coming up with a solution to this. Problem was the theory was wrong because it was impossible to over rev to valve bounce speeds without destoying the engine. The actual mechanism was that the engine govenor racked back to compensate for the gas in the atmosphere and the engine ran quite smoothly on the gas and diesel mixture. The gas retarded the normal ignition point with the result that flame fronts travelled quite quietly out of the inlet manifold and ignited the gas cloud during the valve overlap period.
I and another friend came up with a device that detected levels of gas in the atmosphere and shut down the diesel by injecting co2 before the situation developed dangerously. We are both named on the patents covering this( no cash because it was before the law was changed to compensate the inventors).
Back on topic, to stop the diesel we had to inert the engines and decided to use co2 fire extinguishers as a source of gas. Our first attempts resulted in fractured pistons as the freezing gas hit the hot pistons. The cure turned out to be simple. We turned the cylinders upside down ( they feed from the bottom) and this cured the problem. Of course , we were stopping large equipment engines so there was avery significant temperature difference between the co2 and the pistons. Nevertheless I wouldn't do a leak down with co2 on a hot engine and would make sure that on a cold engine, the co2 bottle is upside down.
Regards Mac
 

mercurycrest

Well Known and Active Website User
VOC Member
Ady,
You just described a near "text book" case of piston seizure. If you had "holed the piston, or seized it really bad, there would have been oil coming out from your oil cap due to the crankcase being pressurised. Hopefully, a quick job with a hone and a new piston and you'll be back on the road. Don't always rely on the settings listed in books, especially. when using new carbys.
Cheers, John
 

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