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Strange small problem GS450 one weak cylinder

  • Thread starter Thread starter cletus@live.se
  • Start date Start date
C

cletus@live.se

Guest
Hello!
I have a strange problem on my GS450 81 custom:

One cylinder has weak idle and does appear a bit weaker in general, judging by exhaust. If I disconnect the right hand spark plug the bike runs fairly all right. If I disconnect the left hand spark plug it barely runs at all. If I block the left hand exhaust with my hand it dies. If i block the right hand exhaust with my hand the bike barely reacts.

It does run fairly well but probably lacks some power, and I do believe it consumes quite much fuel (haven't done a really serious calculation).

Since it's got one single SU carburettor off a mini and acted the same with original carbs I do not believe it's fuel connected. I did try switching place of the coils but no change.

Anybody recognize this? The engine has only run 30000 km

Cheers
/Viggo, Ume?
 
How long since your last valve adjustment?
 
Forgot to mention that compression is similar on both cylinders and same goes for voltage to the coils
 
cletus@live.se said:
No idea. At least 1000 km since that's when I purchased it. But no ticking sounds... Could of course check it out.
Click to expand...

Please check the Newbie Mistakes thread linked in my signature block. Basic maintenance, including valve adjustment, is not optional. Your presumption about the valves and ticking sounds is incorrect as it relates to shim and bucket GS bikes.
 
cletus@live.se said:
No idea. At least 1000 km since that's when I purchased it. But no ticking sounds... Could of course check it out.
Click to expand...

No ticking sounds is a bad thing. Adjust the valves and put the proper tick back into it before you burn some valves.

I keep reposting this...

Valves heat up and like anything else they expand, they get longer. The clearances are set cold with this expansion in mind, so the valves can still seal even if the engine gets too hot. Now as the engine runs several thousand miles, the valves contacting the seat eventually wear into the seat. After a while it can go farther in before it contacts the seat, the valve actually recedes into the head. This leaves the stem poking out a little bit farther, hence the clearances tighten up as the miles add up. Not always, there are other things in there wearing too, but they usually get tighter.

The only path for heat to get out of the valve itself is through mechanical contact with the seat, which is connected to the head, and to the cooling fins. Maybe a little bit by contact with the valve guides, but not much. There is no other path for heat to get out of the valve. Now if you neglect the valve clearances, and if they get tighter as they generally do, the valve eventually doesn't contact the seat as well as it should, so the valve doesn't cool itself as well as it should. It gets hotter than it should be. When it gets hotter, the valve stem gets longer, and so the valve contacts the seat even less, and the valve gets even hotter. It is a vicious circle, the valve gets hotter and hotter, longer and longer, and cools less and less...

The end result is a burnt valve. When the valve/seat contact is so poor that actual flames start to leak past the edge of the valve under pressure, the valve gets extremely hot in that one spot. Sometimes the edge of the valve gets messed up, then it can't seal so well. Sometimes a pie wedge shaped piece of the valve burns away, but even if it's not that burnt, it won't seal again as well as it should. The cylinder may still run, at least until it burns more, but it won't run right.

Anything else that makes the engine hotter than it should be makes the problem worse, such as a vacuum leak or other lean running problem, or retarded timing.

Adjust the valves correctly, then check the compression. Hopefully you caught it before any damage was done.
 
Last edited:
tkent02 said:
No ticking sounds is a bad thing. Adjust the valves and put the proper tick back into it before you burn some valves.

I keep reposting this...

Valves heat up and like anything else they expand, they get longer. The clearances are set cold with this expansion in mind, so the valves can still seal even if the engine gets too hot. Now as the engine runs several thousand miles, the valves contacting the seat eventually wear into the seat. After a while it can go farther in before it contacts the seat, the valve actually recedes into the head. This leaves the stem poking out a little bit farther, hence the clearances tighten up as the miles add up. Not always, there are other things in there wearing too, but they usually get tighter.

The only path for heat to get out of the valve itself is through mechanical contact with the seat, which is connected to the head, and to the cooling fins. Maybe a little bit by contact with the valve guides, but not much. There is no other path for heat to get out of the valve. Now if you neglect the valve clearances, and if they get tighter as they generally do, the valve eventually doesn't contact the seat as well as it should, so the valve doesn't cool itself as well as it should. It gets hotter than it should be. When it gets hotter, the valve stem gets longer, and so the valve contacts the seat even less, and the valve gets even hotter. It is a vicious circle, the valve gets hotter and hotter, longer and longer, and cools less and less...

The end result is a burnt valve. When the valve/seat contact is so poor that actual flames start to leak past the edge of the valve under pressure, the valve gets extremely hot in that one spot. Sometimes the edge of the valve gets messed up, then it can't seal so well. Sometimes a pie wedge shaped piece of the valve burns away, but even if it's not that burnt, it won't seal again as well as it should. The cylinder may still run, at least until it burns more, but it won't run right.

Anything else that makes the engine hotter than it should be makes the problem worse, such as a vacuum leak or other lean running problem, or retarded timing.

Adjust the valves correctly, then check the compression. Hopefully you caught it before any damage was done.
Click to expand...

Thank you for the advice. I may have to add it doesn't tick that loud. But still of course I should check the valve clearance. The good old too much to do too little time stands in the way but I'm on it. The newbie thread hit me as a safe from a high building :lol:

cheers!
 
Thank you for the advice. I may have to add it doesn't tick that loud.
Click to expand...

Happy valves with the proper clearance may emit a light tick when they close.
Sad valves lacking the proper clearance generally won't tick because they never fully close.

So light ticking can be a good sign, but it's not to be relied upon as a diagnostic method. The only way you can tell whether the valve clearances are good is to go ahead and check.
 
Day in the workshop:

Compression test right hand cylinder (bad cylinder) 10
left hand cylinder 9.5

Valve clearances all between 0.03-0.08 mm

As Haynes said I measured the resistance between brown and green/white cables closest to ATU unit. It should be between 60-80Ω and if LOWER suzuki dealer should be contacted. Mine was 135! It seems this might have to do with my problem?

Also Haynes say that close to Ignition Control Unit multimeter should be wired to black/white and the green/white and brown cables to see if the unscrewed spark plugs shoot off sparks. To do this ignition shall be on and multimeter set to x1 Ω. Mine only has 200Ω and up. With 200Ω setting no spark. With spark plugs unscrewed and just hitting starter button visible sparks occur on both spark plugs though.

Any ideas?

Cheers!
 
Has anyone had an experience similar to this? And if it seems to be the ATU does anyone have a spare one?
 
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