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Backfiring cylinder

  • Thread starter Thread starter Fa-bam
  • Start date Start date
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Fa-bam

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Hey, New to the forums so thanks in advance for any input. I've got a '77 GS 750. I bought the bike for 200 bucks and have only rebuilt the carbs and replaced various cables and electrical components. Roughly 350 US dollars invested with price of bike.

I was riding one day and cylinder number one (the one by your left foot while riding) started to backfire. The motor overheated and the lower engine oil seal ( the one at the very bottom of the bike below the oil level viewer.) I pulled off the road, let it cool, and then took off down the road. The cylinder continued to backfire above 4000rpms.

Basically, I don't want to replace the seal if the rest of the motor is screwed. Here's what I've gathered:

1. All plugs black.
2. No vacuum leak apparent. (sprayed ether around boot while reving motor)

3. Will not rev above 6000 rpm when cold, but does not backfire until then.
4. Only backfires out of number one cylinder, regardless of engine temp.
5. Will only rev to 4000rpm when warm, backfires out of number one cylinder at 4000rpm

6. Points gapped appropriately.
7. New gas, have run Seafoam through motor (mixed with 3 gallons of fuel)
8. Charging system checks out.
9. Carbs have not been synced and bike has not been timed.


I understand that syncing the carbs and timing the bike are necessary, but being that the problem is isolated to the number one cylinder, I tend to believe it is a problem with that particular cylinder, not with something that effects all the cylinders.

Erik
 
Could be valves way out of spec. Have you adjusted them yet? Add that to non synched carbs and you would see the issues that you are describing. Is the bike stock (airbox and exhaust)?
 
I don't know how to adjust the valves. I do not have the airbox. They are pod filters. I have read this post and do not know what the induction O-rings are. Can someone help me here. It may be the carb leaking air because I only tested the boots for leaks....
 
Valve shims are too tight, redo them before you burn out your valves.
carbs running rich............... turn the top screw (if CV's carbs) IN a turn.
Phil.
 
Find out if the carbs were jetted for the pods.
Is the battery good?
 
I'll have to refer to my Hayes manual for the valve shims. I've done a lot of work on autos but never motorcycles. They are CV carbs, and they have been jetted for the change to pod filters, however, I've never actually used a manometer to sync them. The jets are probably too big, however, I'll worry about that after the bike runs worth a damn.


Why would the valve shims suddenly become too tight? The bike ran fine, then suddenly started having these problems.
 
If that was mine, without knowing any history of the bike, I'd want to start with basics......and the most basic is whether your engine is a good air pump - compression - valve lash - mechanical timing. Once this is verified and/or fixed THEN it becomes predictable to try to fuel it equally and light it at the correct time while keeping the electrical system up to spec.

Looking at an unknown engine in this manner will set you on the path to logical diagnosis and predictable performance.
 
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