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Discussion Starter · #1 · (Edited)
I want to do a compression test on my bike and have got a hold of a Gunson,s tester but I am not sure what to do next.
Isnt there a de-compression system on the cylinder head that would give a false reading if I just screw it in an thumb the starter and if so, how do I get round the problem.
I have never used this tester before but it is worth the couple of beers it cost me if it works.
Any help on doing a compression test would be greatly appreciated.
 

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don't quote me but I think in the manual it gives 2 different ideal compression values depending on the speed the engine is rotating so I guess the lower speed and lower figure relates to when the automatic decompressor is doing it's job. having never had the rocker cover off I've always been intrigued as to how exactly the thing works but I know it disengages at higher speeds presumably as a result of centrifugal force.
 

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According to the manual you set the RHS exhaust valve clearance to 1mm which prevents the auto decompressor having an effect on the results. Then with the compression tester fitted in place of the spark plug turn on the ignition switch and turn off the handlebar kill switch. This will allow the engine to be spun over on the starter button without sending a spark to the plug lead. Turn the engine over with the throttle held fully open (to allow as much air in as possible) untill the gauge on the compression tester stops rising. as WW64 rightly points out the manual is confusing and lists 2 different compression specs. From memory its 90 something and 170 something psi. Talking to friends in he motor trade they say that 170 sounds very high, they would normally expect around 120psi from a healthy petrol car engine. Given that the dominator engine turns over pretty slowly on the starter and that the compression ratio is pretty low at 8.7:1 I'd be inclined to think that the lower figure is more likely to be the correct one. I carried out a compression test recently but as mine only reached 55psi the results won't help you in any way. I'm currently in the middle of a top end rebuild as both compression rings were stuck firmly in the piston ring grooves.

Personally I would only bother with a compression check unless you are having starting/running problems with your bike. If it's running ok then it's just not worth the expense of the gaskets to strip/rebuild the top end for a look see.

Hope this helps.

cheers
Stewart
 

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Discussion Starter · #4 ·
Thanks for that, the bike is starting fine but the grey smoke on start up has me a little concerned, as one side smokes more than the other.
It disappears when the engine warms up. I need to find out if its using oil and how much.
My speedo is broken so I cant trust the mileage shown,can any one recommend a cheap replacement for it?
 

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so what value is worth concerning? this weekend i`m doing my tests and definetly share them here so we can compare results.
 

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Discussion Starter · #7 ·
We have just had more snow last night so my dommies compression test is on hold as I dont fancy working outside in the garden at the moment.
When the snow has gone, I might just take the head off and do it anyway.:(
 

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1mm valve clearance on the exhaust valve where the decomp is and like said above kill switch so it doesent spark. Then turn the petrol off. Compression test must be done with the throttle held wide open so there is no restriction to the air flow. some people will only do it when the engine is cold and others when it is hot so its up to you. Ive always worked with fours and twins and any test revealing 90 psi would indicate a engine strip down But I am not so sure with big singles.

A bit of smoke on startup isnt too bad considering your probably using a bit of choke so it will be rich anyway. Checking the plug will probably give a better indication if its burning oil . If the bikes running fine I would leave it until its a lot warmer outside so you can remove the choke from the equation.
 

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If the bikes running fine I would leave it until its a lot warmer outside so you can remove the choke from the equation.
.
Remember he said it smokes more on one side than the other so that's why i suggested valve guide seals . If it were the choke it would equally effect both pipes .Also a rich mixture gives a different colour smoke to burnt oil.
 

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valve guide seals are always a good thing to change if they have been there a long time and yes like you said its probably the culprit , but it isnt going to hurt the engine . Another way of telling if its the seals is to get someone to follow you and see if it lets out a puff of smoke when you change gear . A compression test will not show if the guide seals are weeping. if anything it will give a very high reading as there will be oil in the bore.
 

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Discussion Starter · #11 ·
The compression tester I have has a knurled brass adapter on the end so I can see it will be a problem tightening it up so it doesnt leak. I am worried that i wont be able to get it out again.
The spark plug I took out was really badly gummed up with oil and carbon and the air filter was really clogged up which probably didnt help so I think I will wait until the weather warms up and choke isnt needed and take it out for a 50 mile test ride and see how much oil it uses.
This bike appears to have been mis-treated by the previous owner and if the head needs doing then there is no getting away from it.
I am just worried what else will be revealed if I take the head off as money is tight at the moment due to being on the sick and I have other bikes that need sorting out....:(
Valves, cam-chain and tensioner, piston rings,gaskets etc, it could turn out to be expensive:rolleyes:
 

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its not a cheap job , I have just done it and it was painfull. treat it to a new plug and filter and see how it goes :thumbup:
 

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Discussion Starter · #13 ·
The Spark plug, oil, filter, battery fork oil, brake pads and air filter have all just been sorted when I bought it a few weeks ago , just waiting for decent weather due to the huge ammounts of salt we have been using on the roads over here in Denmark.:thumb:
 

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Discussion Starter · #15 ·
Air filter WAS gunked up, that why I replaced it, aint any more.
It looked like it had been in there since the bike was new.:blob2:
 

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ahh I see , well as its was that gunked up then it maybe just crud in the exhaust . Like you said give it untill the spring and give it a long ride to get hot and see what its doing
 

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Discussion Starter · #17 ·
Really looking forward to it, I have owned 3 SR500,s over the years but sold my last one as I got fed up with the kick starting.
There is summit about the low down grunt of a big single that makes it feel and sound cool that my old Zephyr dont have.
Wonder what a Dunstall exhaust would sound like on a Dommie?:toothy7:
 

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Discussion Starter · #19 ·
My neighbours hate bikes but me being the sharing type have something special in mind for them when I got the engine sorted.
You show me yours and I will show you mine...lol :toothy5:
 
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