GS 850 Air box

[81GS]

I am working on a 81 GS 850 and it keeps back firing. I noticed that the air box has four holes drilled in the back of it . Are these suppose to be there? Could this be why it's backfiring? Too much air? How many turns out should the air/fuel mixture screws be set at?

Bob L.

[mr.spot97]

It is back firing because you have a timing issue. The firing order is not properly timed right. With points it is a easy thing to just go and reset the points,etc. But with electronic ignition, it is a more complicated issue. I would suggest you look in the repair manual and see what they tell you do about timing.

[Nessism]

It is back firing because you have a timing issue. The firing order is not properly timed right. With points it is a easy thing to just go and reset the points,etc. But with electronic ignition, it is a more complicated issue. I would suggest you look in the repair manual and see what they tell you do about timing.

Don't mean for this to sound like a put down but your supposition is almost certainly wrong on this. The backfiring issue is typically related to fuel mixture on these bikes, and the 850 is particularly sensitive. Some backfiring though the carbs is normal when the bike is cold but it should not occur when the bike is warmed up.

Regarding those extra holes, I have no idea since we can not see what's been done. Some photos would help. Also, replacing the foam on the airbox end caps is a time honored GS tradition. Get some weatherstrip foam and get to work if your foam is damaged in any way.

[mr.spot97]

I am use to plugs and points, the T500 Suzukis and the GT Series, I know very well, so whenever something backfires it usually means check the points and the plugs as a starting point, then the carbs. I have seen something similiar to this happen with a bad diaghram with CV carbs.

[doctorgonzo]

Well, an 81 is going to have an electronic ignition, so there are no points. The timing COULD be off, but in the pie chart of possibilities, that is a sliver, and the carbs (air leak at the head, dirty, no air filter, badly sealed airbox, etc...) are 95% of the pie.

[81GS]

The foam for the air filter is new. I will try and put some foam around the air box side covers. I have some reservations about the carb horns that go into the air box. It doesn't seem like there sealing at all. Just stuck in there. Maybe I should try and put some foam around those or buy new horns??? Sounds like the holes on the back side of the air box shouldn't be there???. What about the air/fuel mixture settings???