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Changing mixture with carbs on bike

  • Thread starter Thread starter Gs425FridayHarbor
  • Start date Start date
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Gs425FridayHarbor

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Hi all

i was getting some backfiring on my ride this morning when down shifting and letting the engine slow (especially from 4th to 3rd at about 3500rpm) but when I gave it throttle firing would stop. I couldn't tell whether it was backfiring or after firing or where it was coming from. I pulled plugs and think I'm running lean on right and a little rich on the left. Is there a way to tweak the mixture without pulling the carbs?

thanks!
 
I think the GS425 uses BS carbs. I haven't read much about them but I'm sure there is a mixture screw you can adjust.
 
Gs425FridayHarbor said:
Is there a way to tweak the mixture without pulling the carb
Click to expand...

Probably, but that may not be whats wrong. Prepare to be lambasted by the "not enough info " gang. Sherlock Holmes they aint. Tell us what Bike and what condition its in. Has it been running well up to this point. A little history might be nice.

Never mind the Bike history.... I found it :rolleyes:.

i have a '79 425 that hasn't been that well maintained but had fluids replaced, carbs cleaned, and battery swapped last week. I took her on a maiden run and she rode pretty well.
 
Last edited:
Hi Dave,

thanks for for the reply an sorry for lack of details: I've been posting a lot recently and forgot to include details. 79 gs425 just dipped carbs, adjusted valve shims (now in spec), valve cover and breather gaskets, orings all replaced. Did a compression test (hopefully correctly) and I got 80 psi on left (where plug looked a little dark) and 120 psi on right (plug looked pretty white), with both plugs brand new yesterday. Maybe this helps?
 
hannibal said:
I think the GS425 uses BS carbs. I haven't read much about them but I'm sure there is a mixture screw you can adjust.
Click to expand...

It does have BS mikuni carbs (CVs) and I think I seeadjustment screws on the exhaust side (front of carbs on either side) small flat head screwdriver fits, but I'm not exactly sure what these do or how to set them. I went 1.5 turns from lightly seated and am now having the backfiring issue, which I didn't have before.
 
Gs425FridayHarbor said:
I went 1.5 turns from lightly seated and am now having the backfiring issue, which I didn't have before.
Click to expand...
Try 2.5 turns.

Actually, when I rebuild a set of carbs, I start with the screws out 3 full turns, then slowly turn them in for best engine idle. As you continue to turn the screws in, the speed will start to drop. Stop there, turn the screws back out about 1/4 turn, then do the other carb(s).
 
UPDATE: reran a compression test - the right way - and now have 125 psi on left and 130 on the right. Left plug still looks hot (white) and right plug showing signs of whiting also. Combined with the backfire/after fire symptoms I'm thinking I need to make the mixture more rich. How can I do this without removing the carbs? Or do I have to remove and mess with mixture screw down near float bowl drains?

thanks!
 
You were asking about exhaust gaskets in another thread.
Did you ever replace the ones in your bike?
A air leak at the head can cause backfiring.
 
Gs425FridayHarbor said:
It does have BS mikuni carbs (CVs) and I think I seeadjustment screws on the exhaust side (front of carbs on either side) small flat head screwdriver fits, but I'm not exactly sure what these do or how to set them. I went 1.5 turns from lightly seated and am now having the backfiring issue, which I didn't have before.
Click to expand...


Those are your "pilot jets" they control mix from idle to just off idle". Those are the only ones you can get at without pulling the carbs. Backfiring indicates over rich low speed jetting. Are you getting burbling pop pop pops, or BANG! Bang! bang BOOM!?

If your plug chops are inconsistent across cylinders that indicates a different problem than jetting settings. Either an air leak somewhere or you are not getting the same jetting on both carbs.

One of the basic rules of carb tuning is that if some change you made makes things worse, go back to where they were before. Only changing one thing at a time greatly simplifies this process.
 
I think they are like the 450's CV carbs you can in fact change main jets easily with the carbs in place, pilot jets too. They are in the float bowl which you can remove. Those screws are the mixture screws, they are not jets at all.

What you need is to adjust the mixture screw correctly. Don't just guess at it, adjust it properly while the engine is warmed up and idling, then synchronize the carbs. Get the idle speed where you want it, then tweak the mixture screws again to make sure, as a change in synch can slightly change the mixture adjustment. The number of turns out is just a guess to get it running so you can tune it, just like the bench synch is.

If you don't know how search "highest idle method" on this site, there are a lot of threads about it.


This mixture tuning should be done after you have the main jet and needle correct. I can't remember if you changed anything that would affect carburetion like intake, exhaust, cam timing or displacement, but if you did get the jetting right first. CV carbs are tuned from the top down, main jet first.
 
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