Backfire

[LarryA_Texas]

Once again I turn to the o' Wise Ones. Again, on an '81 GS850G that we've talked about here before.

I've cleaned the carbs (yes, they were dipped) I've replaced the o-rings. I've milked the best RPM out of the screws (3 turns seems to do the trick) It has pods and a jet kit. I have moved the needles up and down to no avail. I've changed the jets and currently have 137.5s on it which are giving me some awesome acceleration at wide open throttle right up to the red line.

It's running really well now. Starts easy, idles great, smooth cruise, and good acceleration in the 5000RPM range up to the red line...

but

I just can't get rid of that annoying little backfire. Mostly when cold although it still does it a little when hot. An occasional little pop and a puff of air out the carbs and through the pod filters. I think it's all 4 cylinders.

I checked the timing; it's dead on. Should I go a few degrees off?
I checked the valves; they are all in spec. The only one that was a little off was the #4 exhaust and I re-shimmed it. Spark plugs look clean. Replaced all the spark plug caps. Float bowls are all even and set to the specs.

Anybody have a guess where to go next?

[Guest]

the answer to your question depends on at what exact conditions you get the backfire (deceleration, wot, steady 1/2 throttle, etc)

as the stock main jet is 115 i would think that you've gone up too much in size (for just the pods and stock pipes)

also, with pods you may need a smaller pilot air jet and/or a size bigger pilot fuel jet


all the above is asuming you are not drawing air at the carb boots or at the exhaust gaskets

[duaneage]

If it is popping back through the carbs an intake valve may be hanging up. It could be bent, and when it rotates (all valves do) it hangs up.

Compression test probably won't catch it, you could try and remove the shim, valve spring and keeper and rotate it to see if the stem is bent, but that is a lot of work,

A weak valve spring could cause this too, but that is unlikely on a GS.

[LarryA_Texas]

more info.

Clarification:
The backfire is happening during idle as it warms up, and at steady cruise in the lower RPM ranges. Not as often but sometimes during closed-throttle deceleration. Never at full throttle or during hard acceleration through the gears.

Actually, that's one reason I think the 137.5s may be OK. It pulls great at 5000 RPM and up with any throttle, 1/2 up to wide open. The re-jet kit had 160s in it but with stock pipes that was too much. Yes, it started with 115s, and I may need smaller jets, but right now it seems to do well with more throttle.

The kit also included flow restrictors called the "main air corrector" which may change things a bit. It was a stage III kit with 155, 160 and 165 main jets. Since it's only got pods, not aftermarket exhaust, probably didn't need a stage III, so I'm assuming that's why the 137s work.

Still, it's running well, I just want to see if I can correct the backfire.

NOTE: This in not AFTERFIRE in the exhaust pipe...there is a little 'poof' once every two or three minutes when cold, less often when warm, of air blowing back through the carbs. You can feel it in the pods.

Sticky valves seems like a likely cause. I may look in that direction. Tomorrow I'm also going to look into the 'roll off' test described in the Mikuni web site to check the main jets. Any suggestions as to how to isolate the problem would be appreciated.

Larry

Quote from Mikuni:

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.