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78 GS 1000 Bent exhaust valve?

  • Thread starter Thread starter Fancool
  • Start date Start date
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Fancool

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I believe I have a bent valve on #3 cylinder exhaust port.
Did shims, carbs, carb sync, comp. test, and spark plug test, but exhaust only reaches about 120 degrees C. on the #3 pipe.
All the other exhaust pipe temp. are between 450-300 degrees C.
Also lot of backfiring when de accelerating and smoke.
Did see a slight mark on top of the piston that look like it was caused by the valve when spark plug was removed but was hoping I wasn't going to have to take top end off. Before I take apart is there any other ideas I should check?
Any suggestions will be appreciated.
 
Do a compression test and double check valve clearance. If the valve is bent the clearance will be huge.
 
I did compression test and all 4 cylinders came up the same. I just received a brand new tester today so I'll recheck. I have oil leak on cylinder head so I guess it will have to come apart anyways, as far as valve clearances go I just did the shims and I had to use thinner ones in most tappets but nothing beyond allowable specs. #3 was the worst one but thinner shim gave good clearance.
 
It seems you didn't bend a valve.

Might be a carb issue.
 
OK that is possible and I did remove and check the jets again. Is there a way to check if its getting fuel through the carb when its on the bike? Synced them today and that went well but that wont tell me if its firing?
 
Hope it turns out good. I had a similar experience after a synch. The issue in my case was that at idle one carb butterfly was completely closed (cold cylinder) while the others were not. I dont know if this would translate to VM carbs, but it may be worthwhile to recheck the synch.
 
Stick a cat food can under the drains and open them to see if all the bowls are getting fuel. That will at least verify the bowls are getting fuel to them. After that its next thing inline and that would be proper float hts and see if the jets and emulsion tubes are all open. And be sure to look down the center of the pilots for daylight as there is a small hole down the center of them as well as the big ones around the outside.
 
Emulsion tube may be the problem. New float needles and jets were already installed.
I will be removing carbs again and using some carb cleaner and air to blow though them.
Sorry just a bit off topic but I have pod air filters now and was wondering if I should be going with the next size up on the jets?
 
Did you reassemble carbs in correct order? Each one is unique
Vacuum port on #3 connected to fuel tap, or capped?

Backfiring is from pods causing lean condition. Cheap or quality pods?
What jets are in your carbs now?
 
Fancool said:
I did compression test and all 4 cylinders came up the same. I just received a brand new tester today so I'll recheck. I have oil leak on cylinder head so I guess it will have to come apart anyways, as far as valve clearances go I just did the shims and I had to use thinner ones in most tappets but nothing beyond allowable specs. #3 was the worst one but thinner shim gave good clearance.
Click to expand...

What were the compression numbers? If all 3 came out the same then you DON'T have a bent valve on #3. It's probably going to be a clogged carb. Did you follow the carb rebuild tutorial to the letter?

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf
 
Get some Eagle One Mag Wheel cleaner at the auto parts store. Squirt some in a small can till it covers the jets and emulsion tubes..let it set for an hour. Then rinse and use a bread tie to physically poke out all the holes. Rinse in carb spray and lots of air going both directions thru the jets.

Wheel cleaner has three kinds of acids in it band will COMPLETELY dissolve any crud in those jets. Think they were OK before when you hold them up to the light and look down the pilot jets?? Wait till you see the difference once the wheel cleaner does its job!!!!
 
Carb needed a little extra cleaning especially the #2 and 3 emulsion tubes. Now all 4 cylinders firing and bike starts no problem.
I will redo the card sync now that I know it's all ok. Just have to get that #3 cylinder cleaned out with a good ride.
Thank you for everyone's help and I always appreciate this site.
 
Fancool said:
Carb needed a little extra cleaning especially the #2 and 3 emulsion tubes. Now all 4 cylinders firing and bike starts no problem.
I will redo the card sync now that I know it's all ok. Just have to get that #3 cylinder cleaned out with a good ride.
Thank you for everyone's help and I always appreciate this site.
Click to expand...
Also check and make sure your petcock is not leaking by and sending fuel down the vaccum line. Vacuum on that is on #3 cyl.
 
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