Author Topic: Compression  (Read 19406 times)

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Offline bazza

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Compression
« on: 23 Mar 2008 at 06:46 »
Hi All,Think I've got around the carbrettor problem by using and modifying Amal parts.Now have another problem there seems to be no compression when I kick the bike over also hasn't been able to fire either.So whare do I go from here obviously check timing but why no compression the engine has new pistons and rings has been bored out to 80 thou  oversize,any suggestions???????

Bazza

Offline alwyn

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Re: Compression
« Reply #1 on: 23 Mar 2008 at 07:20 »
.........oversize,any suggestions??????? Bazza

Hi Barry,
How's the valve springs - did you renew valve guides - or are they the old ones tacky with carbon - maybe they are lacking clearance to the valve stems and sticking the valves in their open position?

Alwyn
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Offline bazza

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Re: Compression
« Reply #2 on: 23 Mar 2008 at 08:06 »
Alwyn,The valve springs are new the valve guides are old but clean the valves were cleaned and lapped in.What if I made the gap on the compression rings wider from 6thou to 8 thou ????

Bazza
« Last Edit: 23 Mar 2008 at 08:12 by bazza »

Offline alwyn

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Re: Compression
« Reply #3 on: 23 Mar 2008 at 08:50 »
.........What if I made the gap on the compression rings wider from 6thou to 8 thou ????Bazza

Hi again Barry,
I'm not strong on these engineering factors - best wait for Doug or others who can speak more authoratively about bore/pistons/rings clearances to respond, as I am sure they will.

Alwyn
Quotable Quote - "640 k should be enough for anybody"! - Bill Gates - 1981.

Offline Doug

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Re: Compression
« Reply #4 on: 23 Mar 2008 at 18:28 »
Bazza,

Two common places for loss of compression are pressure getting past the piston rings or past the valves. Check the valves first, since they are easier to inspect. I am assuming it had compression at one point, and it vanished suddenly.

Did a valve adjuster work loose and unscrew, holding the valve slightly off its seat? Check that you still have the appropriate clearances. If the valves and the guides are clean, then sticking in the guides is unlikely, unless the engine is running so foul it has already carboned up the exhaust stems. Cheap, poorly made valves of the wrong material could stretch in use. (Only buy genuine Kingswood spares!  :wink:)

If not the valves, then the piston rings are not sealing. Unless you are racing, 0.010 gap on the top ring, 0.008 on the second, and 0.006 on the scraper (if fitted) is usually appropriate. Too little gap could cause the rings to end bind on expansion. Besides causing a partial or full seizure, it can also fracture the rings into multiple segments which would cause them to leak terribly. Or, if the piston partially seized instead due to lack of clearance or insufficient oil, a little of the aluminum (if aluminum pistons) could have been smeared over the rings, trapping them down in their groves, unable to function. You also need 0.001-0.0015 side clearance for the ring in the groove so the gas can get behind the ring to force it out to seal against the cylinder wall. Again if the engine is heavily oil fouling, the rings could quickly get gummed up and stuck in their grooves.

-Doug
« Last Edit: 24 Mar 2008 at 05:00 by Doug »

Offline bazza

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Re: Compression
« Reply #5 on: 23 Mar 2008 at 19:31 »
Hi Doug and Alwyn,Thanks for your help gonna be busy checking everything out got a feeling it may be rings sticking.At the moment I'm grounded had an operation on my right hand for carpel tunnel (tendon problems)should be ok in a couple of weeks.

Bazza

Offline Doug

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Re: Compression
« Reply #6 on: 24 Mar 2008 at 05:02 »
Ah, you must have pulled a carpal tendon whacking on that throttle lever!   :)

Offline Ian

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Re: Compression
« Reply #7 on: 24 Mar 2008 at 06:46 »
We cam across a TS Douglas last year at the ACT rally that had been sitting for 30 years. Graeme and I played with it to see if we could get it running for the guy. It had no compression at all. When I pulled one of the barrels off the whole thing was completely dry inside. Oiled everything and suddenly we had compression and it started !!

Offline bazza

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Re: Compression
« Reply #8 on: 24 Mar 2008 at 09:02 »
Ian,Thanks mate I think youv'e knocked the nail on the head because my bike hasn't been started since I don't know when and oil even though I used the manual pump hasn't circulated enough to create compression.i didn't realise the importance of oil and compression.This whole excercise has been a terrific learning circle for me even though I have some mechanical knowledge having served 3 years as an apprentice motor mechanic and been around cars and bikes all my life,Dad always said experience is a dear school and fools will learn in no other thanks Dad.

Bazza

Offline Ian

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Re: Compression
« Reply #9 on: 24 Mar 2008 at 21:15 »
I hope you will have it all sorted by the Douglas rally in October !!

Offline bazza

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Re: Compression
« Reply #10 on: 25 Mar 2008 at 07:44 »
So do I mate this bike owes me a fortune it also owes me a few rides but there you go nothings cheap eh!lets hope for my sake when I die it will be worth something might leave it to some museum don't have any kids so I think a museum would be good.

Bazza

Offline Alan Cun

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Re: Compression
« Reply #11 on: 26 Mar 2008 at 23:29 »
Hello Bazza, Been watching all the comments on compression with interest and wonder if it is not just a case of the valve timing.

It is possible to line up the timing marks and have the pistons at the bottom of the stroke. So I guess the next thing is to check that if the valves are rocking on one cylinder, that is one just closed and one just opening, then the pistons should be at the top of the stroke. regards Alan

Offline bazza

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Re: Compression
« Reply #12 on: 02 Apr 2008 at 07:57 »
G'day All,Taking in to consideration my engine on the EW has never been started since its rebore and new piston and rings what sort of compression could I expect after 4 kicks on the kickstart its showing 40psi on the rear cylinder and 30 psi on the front hope someone can help.Its my understanding that compression can be low intitally what do reckon.

Bazza

Offline davebarkshire

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Re: Compression
« Reply #13 on: 02 Apr 2008 at 11:15 »
I'm sure that the experts will be able to give a better answer and as I understand it the compression should change as the engine is run in and the bore becomes smoother. Also  how much oil is on the cylinder walls will make a big difference and possibly the relative position of the piston ring gaps maybe? Mine is very easy to kick over and from what I hear most soft tune Douggies are easy too. When you consider that this is a 350/2 sv motor I'd not expect too much pressure.

Offline Ian

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Re: Compression
« Reply #14 on: 02 Apr 2008 at 22:44 »
My view - just run it with plenty of oil and see what happens over time.

Offline bazza

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Compression
« Reply #15 on: 25 Apr 2008 at 06:03 »
G'day All.Well sh'es all back together timed and primed and ready to go but als she won't fire up.
1, Checked the valve timing all ok
2,Check ignition timing all ok
3.Filled up with fuel and oil.
I'm using a Mikuni carby until I can get bits for my Brown and Barlow its fairly new and bolts up ok to the manifold,it fills up ok in the float chamber ok,but after a few kicks I checked the plugs and they are dry.
Also I have had the magneto overhauled and the spark is great both on the maggy and at the plug end too.
So Gentlemen where do I go from here to find out why the plugs are dry.I do have an Amal 274 which has been overhauled but the fitting is too big and would need a reducer to make it fit should I go down that road but surely the Mikuni should work.
Regards Bazza  :cry:

Edit 25/04/2008: This messsage was posted today under a new topic titled "NonStarter (CW)". It has been merged with the original discussion and renamed accordingly.
Alwyn
« Last Edit: 25 Apr 2008 at 09:02 by alwyn »

Offline eddie

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Re: Compression
« Reply #16 on: 25 Apr 2008 at 07:24 »
Bazza,
           Is the Mikuni a secondhand unit that has been unused for some time? Over here in the UK, the modern unleaded petrol dries out to leave a sticky sort of lacquer coating on the inside of the carb, and after a winter layup, can cause problems when trying to get the fuel flowing again. I would check that fuel is actually getting through the jets - with the throttle open, put your hand over the intake and give the engine a couple of kicks. You should feel a strong suck on the intake  - if there is no suction or even a build up of pressure, you have probably got the valve timing wrong or a tappet is too tight. If there is a healthy suck (at this stage, this is probably more important than having god compression) - check the inside of the carb to make sure that fuel has been drawn up. When checking for wetted plugs - check both of them - the inlet manifold on prewar Dougs can cause quite a bias on the carburation, so that one cylinder may look to be starved of fuel. Having got fuel through to the plugs, if the bike still wont start, then double check that the ignition timing is correct and that you have a good spark. Dont just rely on the timing marks - go back to basics - get the front cylinder on TDC with the valves on the back cylinder rocking - turn the flywheel backwards about 30 degrees (1/12th of a turn) and check the magneto points - they should just be opening. If the timing is out, remove the mag and turn it so that the points are just about to open with it firing on the front cylinder - then replace the mag. This should give you a good starting point (sorry about the pun) for final adjustment of the timing.
                        Good luck,
                                         Eddie.

Offline eddie

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Re: Compression
« Reply #17 on: 25 Apr 2008 at 07:34 »
Sorry,
          I didn't mean to go all religious on you! It should have read "it is more important to have GOOD compression".
                                                Regards,
                                                            Eddie.

Offline Stuart Lister

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Re: Compression
« Reply #18 on: 25 Apr 2008 at 17:19 »
Hi Bazza,

This sounds very similar to the starting problems I had with my EW after the rebuild. I went through the same list as you have and eventually, it turned out to be the induction pipe that wasn't sealing properly. I didn't really believe it because I was quite convinced that I had made a good job using the original parts. But I hadn't.

I have now sealed mine using 1/8" gland packing cord (the stuff designed to stop water leaking in through the stern gland on boats). She starts first kick every time now.

Hope this helps,

Stuart.

Offline bazza

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Re: Compression
« Reply #19 on: 27 Apr 2008 at 09:22 »
G'day All,Well I followed all instructions thanks to Stuart,Eddie and everyone else but still no joy maybe I am timing on the wrong stroke but I have followed the diagram in the EW Maintenance book  to the letter.I am presuming that if the keyway on the half time sprocket is facing the rear cylinder that this is the firing stroke and the cams should be inlet pointy end facing 2 oClock and the exhaust facing 4oClock .The inlet and exhaust valves are rocking the engine is the turned back 1 twelve and the mag is set with points just opening.now it still seems that fuel is not getting through I've changed the carby to the Amal 274 (made a reducer from brass)this carby has been reconditioned what I did notice was after a few kicks it was flooding this was happening with the Mikuni also it appears fuel is not getting to valve chamber is this because compression is low (about 30 psi).Also it seems to blow air out from the bell mouth when I kick it over very little suck if any.Should I try bump starting to give the old girl a bit of a chance or am I wasting my breath,or should I consider putting it on Ebay !!!!!!!!!!.

Regards A Very Frustrated

Bazza

Offline Chris

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Re: Compression
« Reply #20 on: 27 Apr 2008 at 09:48 »
HI Bazza
    Upon the only occasion on which I have experienced blow back from the carburettors when kicking over a machine that would not start, I found that the valves were leaking badly on their seats which tied in with low compression. These were cylinder heads that had been overhauled with new valves, guides and springs by a well known "Engineer". Upon removing the heads and stripping I found that the seats looked as though they had just had course grinding paste applied and then a power drill used to grind them in. The seats had to be re-cut and the valves refaced and then carefully lapped in with fine paste until a petrol tight seal was obtained. After re-assembly and without changing any settings, the engine started immediately. Could this be your problem?   Chris.   

Offline bazza

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Re: Compression
« Reply #21 on: 27 Apr 2008 at 10:28 »
Hi Chris,You may have something there ! it may need checking out thanks.

Bazza

Offline Stuart Lister

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Re: Compression
« Reply #22 on: 27 Apr 2008 at 21:17 »
Hi Bazza,

I'll measure the compression on mine in the next day or two. Then we'll know if yours is somewhere close.

Stuart.

Offline Stuart Lister

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Re: Compression
« Reply #23 on: 28 Apr 2008 at 21:41 »
Hi again Bazza,

I just measured the compression on my EW. It was a bit difficult to read, because my gauge goes up to 300 psi, and the poor little 350 side valve hardly even registered on the scale. I was reading somewhere between 25 and 30 psi on both cylinders. That's an engine that starts first kick every time and runs well, so I guess 30 psi must be about right.

Hope that helps,

Stuart.

Offline bazza

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Re: Compression
« Reply #24 on: 29 Apr 2008 at 03:10 »
Thanks Stuart I find that amazing but still think I may need to go down the road Chris mentions and it certainly won't hurt the engine to do that only my pocket I'm waiting on a qoute as we speak.

Bazza

Offline bazza

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Re: Compression
« Reply #25 on: 06 May 2008 at 06:20 »
Hi All,Well Chris you were right,I sent the cylinders up to Canberra to a reputable (hopefully) engine reconditioners and they tell me the seats are buggered as are the valves and the valve guides.I am awaiting a quote so before I start ordering stuff from you we'll se what they come up with.

Bazza

Offline bazza

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Re: Compression
« Reply #26 on: 16 Jun 2008 at 22:09 »
G'day All,Still trying and not much hope on ebay can someone advise,with the front cylinder at TDC and the rear cylinders valves rocking and the engine has been turned back 30 degrees how can one time the magneto so it is firing on the correct cylinder also with the engine at this point which cylinder is on the firing stroke?

Regards Bazza

Offline Ian

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Re: Compression
« Reply #27 on: 16 Jun 2008 at 23:08 »
The cylinder at the top of the compression stroke is the one that should be firing - so the opposite one to where the valves are rocking. To check the magneto, take off the pickup and have a look inside - the one which is firing is the one with the metal contact patch visible inside. The points should also be just opening. One other easy way to check if you have the magneto firing on the wrong cylinder is to just swap the plug leads to the other cylinders if they are long enough so see if it will fire.