poor compression

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scubadanski
Posts: 3
Joined: Tue Oct 06, 2020 8:08 pm
Location: Perthshire UK

poor compression

Post by scubadanski »

Hoping someone can help :). I have 46 G80, bit of a reluctant starter when cold but fine when warm. I tested the compression at 50psi :( so did a leak down test and its losing at least 35%. Nothing coming out the head gasket or valve guides, a just perceptible flow from the crank case breather and a barely perceptible air noise which I cant pin down to either the exhaust or the carb. The valves were ground recently and I leak tested them with fuel which sat in the head for over an hour. I have the tank and rocker cover off, just trying to decide what to do next, is it time to look at the rings? They havent covered more than a few hundred miles, though over a long period of time. .
Dan Richardson
billbeavis
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Location: OXFORDSHIRE UK

Re: poor compression

Post by billbeavis »

I don't have experience of compression testing but there are a couple of points I saw online. Do the compression test with a warm engine. That means all the metal bits have expanded and so better sealing. If the reading is low, squirt some oil in around the rings and re-test. An improved result could indicate poor ring sealing. I did find once in the past some rings took a long time to bed in. (sorry can't remember how long it took). Easier to ride it for a few more miles to see if that helps. Bill
g80csp11
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Re: poor compression

Post by g80csp11 »

even with low compression , if the carburation is correct and timing correct you should be able to get a easy start hot and cold
i assume you dont have a compression plated fitted and therefore compression ratio is 4.24:1
if over advanced does it kick back ? if that happens then retard slightly and should start

do you have the correct carburetor and settings ? , and has it always been a bad starter ?

My 46 G80 starts well , hot and cold , although I do give it a few priming kicks with decompressor pulled in just to get fuel around the system
scubadanski
Posts: 3
Joined: Tue Oct 06, 2020 8:08 pm
Location: Perthshire UK

Re: poor compression

Post by scubadanski »

Thank you both. I tried the small squirt of oil it made about a 7psi improvement, which doesnt seem to justify investigating the piston rings? No compression plate, so 6:1 I think, does that mean I should hope for about 90psi? It does kick back if over advanced, but retarding a bit doesnt seem to mean it will start, just not kick back.. carb is correct original type 89b/1ak and I have checked the jets, needle etc. yes its always been a bit reluctant! I'm thinking I should just put the rocker back on set up a bottle and see how it runs, maybe test again when its warm, if I can get it started!
Dan Richardson
SPRIDDLER
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Re: poor compression

Post by SPRIDDLER »

scubadanski wrote: Sat Jul 02, 2022 3:06 pm .......haven't covered more than a few hundred miles, though over a long period of time. .
Yes, I reckon you should be getting a reading more towards say 80 to 90 psi.

Although you say the rings have only done a few hundred miles over a long period of time did you fit them and/or Is the bike new to you or have you ridden it a fair bit and it has always been a reluctant cold starter?

A couple of idle thoughts since you haven't mentioned them......

I realise the issue you have is with cold not hot starting but nevertheless (eggs...granny, so forth) I'll just throw in a couple of idle thoughts about basic initial checks that you haven't mentioned...

if it has been standing a while as you say it may be that the tiny pilot drillings have become gummed up/partially obstructed. A common problem after only a short lack of use, especially since ethanol was added. (I don't think the Amal 89 has a removeable pilot jet as fitted to the Monobloc type).

Have you tried another plug? My '46 was a devil to start when I fitted a new Champion but no problem with the c40 yr old KLG FE80 plug. Yours may be sparkling fine in fresh air but become weak or fail to spark which may not be an issue when the engine's already warm.

As we haven't 'spoken' previously on the Forums I have to assume that you know that when taking a comp reading the throttle should be held wide open. ;)
'There is a tide in the affairs of men
Which taken at the flood............'
scubadanski
Posts: 3
Joined: Tue Oct 06, 2020 8:08 pm
Location: Perthshire UK

Re: poor compression

Post by scubadanski »

Thank you spriddler, I took the head off to re grind the valves and have also dismantled and cleaned out the carb, poked the pilot with a bit of wire etc..I was hoping this would solve the starting issue - and maybe it has, I havent tried starting it yet as the rocker is still off, it just seemed unlikely to have worked when I did the comp and leak tests. Good point about the throttle being open and one I was thinking on earlier but Im a bit confused about why this is. If you put the tester in the plug hole with the piston at bdc thats as much air in there as you can get at 1atm which would be the same as you would get via the inlet port? I have a few NGK plugs to try out, Ive been looking for an old Lodge to no avail. Maybe I should try another make.
Dan Richardson
SPRIDDLER
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Re: poor compression

Post by SPRIDDLER »

scubadanski wrote: Sat Jul 02, 2022 7:16 pm If you put the tester in the plug hole with the piston at bdc that's as much air in there as you can get at 1atm which would be the same as you would get via the inlet port?
Yebbut, It is customary (well I do) to kick the engine over smartly 3 or 4 times (not just the once) to get a valid reading. Most of the air on the compression stroke is driven out on the following zorst stroke and the cylinder needs to 'refill' with unobstructed airflow through the inlet. I imagine it's quite difficult to give just one really good kick from BDC as the piston will unavoidably rise a bit when you take up the 'slack' to engage the k/start quadrant.

I have a few NGK plugs to try out, Ive been looking for an old Lodge to no avail. Maybe I should try another make.
Don't even bother trying a plug if you've cleaned it with a wire brush. On today's plugs the central electrode insulator isn't glazed and the spark can easily track along the minute cleaning scratches.

I've never seen one but word on the Web says that there are/have been some fake NGK plugs at jumbles and from eBay sellers.
NGK fake plugs (640x398).jpg
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g80csp11
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Re: poor compression

Post by g80csp11 »

I run L82C , short reach plugs on my G80 1946 , and found they are better than the NGK equivalent
Agree with Nev that the idle jet could be blocked
When the bike is running can you alter the idle mixture from lean to rich ( idle speed then speed drops when too rich )
In the hand book there is a check on full retard that they suggest to check if carburation is OK without faltering ( dont run too long in that condition )

As far as compression goes , I can stand on my kick start on compression to resist compression pressure , if you cant do that then there may be an issue
Is the decompressor clearance set correctly ? ( try disconnecting to do the kickstart check)

http://archives.jampot.dk/book/Owners_m ... Manual.pdf
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