valves lowering compression or piston rings

[jackinacabin]

Hi , My elderly 1989 Gs 500 slingshot has low compression of about 125ish per side . I did the put a bit of oil in the cylinder trick and it added about 15psi on one side and 10 on the other . I am now wondering if the major compression loss is because of valves - the gap is in spec and the gruntfuttock runs well despite being left for who knows long to rot by the previous dodgy dudes that i bought this high mileage paragon or reliability off several years ago ... come armageddon Mavis the motorcycle will survive , she must already be part zombee ... anyway I like my bike nd ive put a lot of effort  but very little coin  into her . She cost me half a packet of smarties and a guiness and i am considering getting a valve job to aid her onwards  beyond 60000 miles ... Opinions sought on if the culprit is more rings or valves ... she starts easilly runs well pulls ok  but uses a fair amount of oil ... I am going to put in thicker oil next oil change to combat that ... It may be worth getting a valve job but probably not a rebore etc ... hope you can help

[user11235813]

I had one valve having more clearance opposite to what you'd expect. I thought maybe there was some build up of carbon keeping it open a bit, I did a compression check and I was getting 120 and 125, there was at the same time a bit of weird behaviour. In the end it turned out to be an unsusual carburettor problem, the press fit part that guides the slide was a bit loose and leaking air from there.

After I fixed the carburettor issue the GS was running really well, plenty of power despite being "only" 125 or a bit less. We had an almost new Suzuki S40 at the time and I checked the PSI on that, and it was also 125 and similarly ran well, in spite of the minimum spec being 142 like the GS.

What I'm getting at is that in my experience 125 PSI is not a problem. But if you are told that it should be 195 with a minimum of 142 you might find that hard to believe. We should have a thread where members publish their PSI just for reference.

[herennow]

Hi did you check compression Hot/Warm?

Checking it cold is a crap shoot.

was carb wide open?

Cheers.

[Sporty]

The addition of oil temporarily seals up the rings!   So it would indicate ring seal.


However, there are many riders reporting the engines run fine with 125 PSI cranking compression

[jackinacabin]

Hi , thanks for the replies , i compression tested the bike cold  so as suggested I will try it warm . I did have the throttle open ... I am encouraged by the reports that 125psi ones are  reportedly running ok  because that is my experience  ... so i will retest when warm ... Still  not sure about  whether its valves or rings ... it confuses me as the oil jump was not huge .. i e 10 to 15psi ... rather than it zooming up to 160 etc etc  which I think ? would be a sure sign of rings ... Im guessing more advice sought ... On a related not there seem to be a plethera of possibly snake oils that  claim to soften and hence improve valve seals  in cars ... i wonder if they are worth a punt in an older gs . I also  wonder if stuff like seafoam may clear some carbon from  valves and help  so thanks all of you and any more wisdom greatfully recieved ....

[herennow]

125 cold will be perfect warm, i'd bet on it.

[jackinacabin]

Ps ... Yes a thread that sows bikes compression and running state in the real world would be a great idea , maybe I will start one !

[jackinacabin]

I hope so Herenow , thanks for the reply , Will post it up when I ve done that ... Cheers

[jackinacabin]

Just started a whats your compression and does bike run well thread ...  hope this helps people , good suggestion !

[Sporty]

Still  not sure about  whether its valves or rings ... it confuses me as the oil jump was not huge .. i e 10 to 15psi ... rather than it zooming up to 160 etc etc  which I think ? would be a sure sign of rings .

10 to 15psi increase is about all you'd ever get by putting a little oil in.

If it's valves leaking, NO INCREASE from adding oil.

Hope this helps...

[jackinacabin]

Thanks for the reply re the oil and rings . Can you clear something up for me . I get it that the rings are not great . Would the 10/15psi jump indicate that the whole problem is rings or do i also need to get a leakdown test done and listen  for the valves.. Cheers

[Sporty]

Thanks for the reply re the oil and rings . Can you clear something up for me . I get it that the rings are not great . Would the 10/15psi jump indicate that the whole problem is rings or do i also need to get a leakdown test done and listen  for the valves.. Cheers

The oil test just indicates rings

You might have some valve leakage, but that's not causing the oil consumption.

  If it runs well and you can deal with the oil usage, adjust the valve tappet clearance ( normal maintenance) and keep riding it

[jackinacabin]

Yeah , got it , of course ... left my brain in the jar by the bed kinda day ! .... currently thinking thicker oil , warm it up retest compression ... look about for cheap thicker oil in quantity ,,, ie car oil without friction modifiers if all is good enough at that point ... Cheers

[struckjm]

You can always try Marvel Mystery Oil.

Basically pour 2 oz in the cylinder, and let it soak for a couple of days. I've heard this can improve the rings/seal, and increase compression if the bike has been sitting for a while.

[jackinacabin]

Thanks for the reply ... the bike has been started  regularly  but ridden little  for some time due to lousy health  ( mine not the bike ) ... so I doubt marvel oil is the cure but its a thought should the retest of compression when hot still show low psi...using lockdown the sequel ( Im in the uk) to tinker with it ... i expect at 60k miles there is general wear throughout ... If I did get the top end sorted out properly I imagine the valves and valves seals could use work as well ....

[Sporty]

You can always try Marvel Mystery Oil.

Basically pour 2 oz in the cylinder, and let it soak for a couple of days. I've heard this can improve the rings/seal, and increase compression if the bike has been sitting for a while.

That can possibly free up stuck gummed up rings on engine that was sitting or sludged up.

Nothing is going to restore worn rings or cylinders except new parts and machining.

[Sporty]

Yeah , got it , of course ... left my brain in the jar by the bed kinda day ! .... currently thinking thicker oil , warm it up retest compression ... look about for cheap thicker oil in quantity ,,, ie car oil without friction modifiers if all is good enough at that point ... Cheers

I'd suggest 20w50 4t motorcycle oil. The GS has a wet clutch. 

[jackinacabin]

Hi sporty ,thanks for the reply .. I am getting 20w 50 ... After a look about people are finding that the cheaper car oi is ok long  as it has no friction modifiers ... so I will try that as an experiment , it will likely still use a fair bit of oil so it would be useful to know if the clutch will cope with  the cheaper oil and I dont plan on wring the bikes neck and stressing anything .... Im disabled and  typically use a mobility scooter , my gs is a toy so i spend very little on  it , its a wonderful feeling zooming along, wI  i am usually in a mobility scooter or electric wheelchair  to get around smaller shops , I hope i will have a better patch and be able to ride a bit  again once covid is vacinated  againt on mass..