ET: Engine (Twin) How Much Compression Reading Should A Twin Have & Leak Down

[CarlHungness]

There are about 16 pages on this forum of sincere replies to my dilemma of trying to get my Shadow running and the problem(s) are beyond my capabilities and seemingly defy logic.
As one of my last resorts I did a compression test and the best I can get is 90 lbs,(with throttle wide open) which is the same as I recall getting a decade ago. Since, I've had new pistons, rings, valve job (by journeyman Glenn Bewley in the US) and it HAD been running well ever since. John Healy from Coventry says 150 lbs are needed to fire the machine. I don't think I've ever had over 90 during my ownership. I'd never taken a leak down test on any engine, so bought a tester that has no instructions but seems straightforward. I put the piston near TDC, hooked up the air hose to the tester and watched the gauges. I got an immediate 100%, and watched the second guage ever-so-slowly depress, and one can surmise there just isn't any leakage coming from the rings or valves.
Compression seems to be the same as it has been in the past as I can stand on the kickstart lever and she won't go over TDC without the decomp. So am wondering what the 'normal' lbs would be on a twin.
The 3-4 times I have made the bike fire it has run for 15 to 60 seconds and seems like it runs out of fuel. The mag is newly rebuilt last week by Doug Wood, I've timed and re-timed and re-timed and made sure the ATD has stayed open, an on and on to the point of madness. So very many members have given me advice and I'm about knackered in trying to get to the bottom of the problem, so had to do the compression/leak-down test.

 

[davidd]

A leak-down tester reads in percentage as opposed to lbs. A new bike should be at .5% to 1%. I would put in new rings at 5-7%. When I broke a ring it read 60%. Thus, 60% of the air entering the cylinder was blowing by the rings (or valve seats).

I would say 120 lbs is the lowest I would go on compression. A Mk2 cam with 8 or 9:1 should be making 145 to 155 lbs. You can refer to a calculator if you want to know specifics:

Compression Ratio Calculator - Wallace Racing

Wallace Racing has a lot of online calculators that can give you some data.

David

 

[robert shennan]

I had 155psi with Mk2 cams and new rings

 

[Chris Launders]

I doubt mine is anywhere near that, it can be kicked over very easily without the valve lifter, but it still starts easily.
You can barely feel the compression on my SV Brough Superior (about 4:1 on a good day) but it still starts and runs well.

Can somebody within the US whip a pair of carbs off a running bike and post them to Carl to try.

 

[fogrider]

Carl, a leakdown test checks everything is sealing with engine stationary. Compression tests rely on the engine's ability to 'breathe'. ie, if the valve timing is adrift, the cylinders won't fill enough, especially by kickstart. However, reading your 16 pages of very trying times, my gut feeling is insufficient fuel in the float bowls. The clear tube check is essential, during my recent carb troubles there was a stage where one float was only lifting 1/8". The tap could not fill the bowl fast enough and the fuel level too low. Are you trying with both taps open, out of interest ?
Fuel height is also crucial.
I did'nt see a response to the blocked exhaust comment. It was a classic when I was a kid, stuffing a spud up someone's tailpipe . Symptoms way too similar !
I admire your persistence,
Regards , Terry.

 

[CarlHungness]

Carl, a leakdown test checks everything is sealing with engine stationary. Compression tests rely on the engine's ability to 'breathe'. ie, if the valve timing is adrift, the cylinders won't fill enough, especially by kickstart. However, reading your 16 pages of very trying times, my gut feeling is insufficient fuel in the float bowls. The clear tube check is essential, during my recent carb troubles there was a stage where one float was only lifting 1/8". The tap could not fill the bowl fast enough and the fuel level too low. Are you trying with both taps open, out of interest ?
Fuel height is also crucial.
I did'nt see a response to the blocked exhaust comment. It was a classic when I was a kid, stuffing a spud up someone's tailpipe . Symptoms way too similar !
I admire your persistence,
Regards , Terry.

I agree with your assessment on insufficient fuel in the float bowls. I do have MK II cams (wish I had left the MK 1's in) and just don't have enough knowledge to know how to fix. I don't think I have ever come anywhere close to 150 lbs
on compression in 30 years. I always open both taps. Since the floats have no adjustment, only a needle with a clip I need some additional knowledge on performing a fix. Will try to get additional banjos for a clear tube test which I've never done.

 

[fogrider]

Best way to check the fuel level is an adaptor into the end of the float arm. 1 BA I believe, but, hold the flexible tube against the banjo rather than the float bowl. There are differing views on where the level should be - from half way up the top band of the banjo to 1/8" below the air screw hole. It needs to be in that area , regardless of any other issues.

Whilst checking fuel levels, it's inevitable there will be fuel trying to get everywhere, be careful where you do it !

As for Mk11 cams, I put a pair in my first twin, an easy kick starter, ticked over fine and was a real flier too.

 

[davidd]

When kicking a bike over the throttle opening often determines how much compression you feel on the KS. On many bikes you will feel zero compression with the throttle closed.

With only 90 lbs of compression, you may also have breathing problems. Robert's experience getting 156 lbs is more the norm. I believe that is why John Healy said 150 lbs. It is not unusual with high-performance engines to get 200 lbs. When you get near or over 200 you have to lower the compression a bit because the friction pumping losses begin to erode the power.

David

 

[CarlHungness]

Best way to check the fuel level is an adaptor into the end of the float arm. 1 BA I believe, but, hold the flexible tube against the banjo rather than the float bowl. There are differing views on where the level should be - from half way up the top band of the banjo to 1/8" below the air screw hole. It needs to be in that area , regardless of any other issues.

Whilst checking fuel levels, it's inevitable there will be fuel trying to get everywhere, be careful where you do it !

As for Mk11 cams, I put a pair in my first twin, an easy kick starter, ticked over fine and was a real flier too.

Your reply makes me feel a little thick. In order to put an adaptor

When kicking a bike over the throttle opening often determines how much compression you feel on the KS. On many bikes you will feel zero compression with the throttle closed.

With only 90 lbs of compression, you may also have breathing problems. Robert's experience getting 156 lbs is more the norm. I believe that is why John Healy said 150 lbs. It is not unusual with high-performance engines to get 200 lbs. When you get near or over 200 you have to lower the compression a bit because the friction pumping losses begin to erode the power.

David

Don't think I've ever had over 90 in 30 years. There is virtually NO leakdown. And my problem MAY have been
my installation of the ATD 3 times...not setting it hard enough..will know tomorrow. Rings, pistons top end are all
virtually new, less than 5,000 miles and it SURE feels like it has great compression. You cannot get it over TDC by
standing on the lever, absolutely have to pull the decomp. I only weigh 160 but it'll hold me just fine and no easy trick to kick it with no decomp and just over TDC...feels like it has for decades, but I can't get over 90 out of it
by kicking.

 

[CarlHungness]

After all of the aggravation I've caused to the membership regarding my starting problems I still think
the hiccup is my own installation of the ATD on my Lucas mag. It's been 20 odd years since I installed the Lucas
and did so 2-3 time years back thebn, and each time it fired. Recently, the first time I put it in, the bike ran momentarily and died, as reported, then did so again after fiddling what who knows what. I failed to check the timing after the bike fired and ran a couple of times, my mistake. Since I have such a devil of a time getting it set and starting the bolt, I resorted to taking the timing cover off each time and was as sure as I could be it was installed properly and in time.
However, the last two times, once this morning, I've checked the timing before removal for certain it has been out of time, telling me the ATD is slipping on the magneto shaft. I lapped the ATD to the shaft, twice, and if it could be over-done then there's another problem. I used valve grinding compound and turned the ATD on the shaft vigorously both directions. I took out the mag, put it in the vise with soft jaws, put the ATD on, and set it with a socket, and could see it moved into position, moving back a full quarter inch or so. My problem when putting it on the bike is when I set it, the ears on the ATD are stuck in the full advance position. So I flattened the horseshoe washer with a brass hammer on one of my railroad rails that's flat. Then I flattened the washer further by rubbing on my diamond stone. I can barely get the horseshoe washer behind the bolt to install the plot, so on a couple of occasions I've started the bolt far enough to j-u-s-t get the unit on the mag, then backed the bolt off, put the washer in and finger tightened. Then I've set the unit with a socket..gently. Evidently too gently if it is slipping. Then I've set it firmly, and fiddle-fiddle with the bolt-the unit itself, trying to get a balance where the bolt is just about tight, and the ears will pop open. I can't locate the problem with the ears sticking and once again at wit's end. I don't recall taking the timing cover off years ago to re-install the mag but now it's a must. I just can't get the bolt started, even with pulling the ATD back far as I can, I'm catching the left-hand threads. Of course I'm using a dowel to keep the ears open, and took a tip of master mechanic Glen Bewley to use needle nose vise grips. So, I can get it set, but can't keep the ears from binding..and when I get them freed up the unit must not be all the way on, or else it wouldn't slip. I should have surmised the problem as I did get the bike fired, and wouldn't have had the aggravation of digging through the carb float bowls.
I've had the exhausts, timing, etc all off about 5-6 times minimum here in the past two weeks and not sure how I am ever going to get the ATD seated once and for all. I sure miss the B-TH and the simple rod used to get it in time.
There just has to be a 'binding' problem from the ATD to the shaft.
Didn't I used to see new or rebuilt ATD's in MPH? I want to fix the problem but have run out of talent.