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backfiring, then stalls...

  • Thread starter Thread starter zeebob
  • Start date Start date
Z

zeebob

Guest
hoping someone can shed some light on this problem...
patient is a 1979 GS425 (14,824 mi)
I've rebuilt the carbs, changed the oil, new plugs, new pts, new exhaust gasket, oiled the air "sponge", adjusted valves, new carb boot O-rings... Also the cover for the air box is absent... otherwise it's all seems tight to me (all new clamps)

anyway...
starts fine now... it runs much better than it did, at least sounds better, esp after the valve adj, but has a consistent backfire which will eventually stall it. Seems to run better cold than as it warms up.

When I put a timing light on it and increase the RPM's the mark slides back (20 degrees)... is that normal? I thought the mark was 'spose to stay put (spark advance). Back in the day backfiring, on cars, meant a timing issue. Can I assume the same here?
Also the right side seems to run rich (black, dry soot) while the left side plug look OK.

TIA for help... I feel like it's close...
Zeebob
 
Hi Mr. zeebob,

It's pretty important that you have a properly sealed airbox for configuring and adjusting your carbs. But the symptoms you describe make me think that the bike is running rich. I hate to say it, but sometimes it takes two or three carb "cleanings" before they are really, really clean. Is your jetting stock?


Thank you for your indulgence,

BassCliff
 
you hate to say it...

you hate to say it...

and I hate to hear it..lol. I soaked both carbs (BS34's) in Berrymans overnight, they were both dissasembled, except for the butterfly section. I blew everything out. Also adjusted the float heights best I could. I will say that the reading was different from the outside of the float to the inside. They are both angled a few thousents. Both higher on the inside.

Whats your feeling about the observed timing mark "retreating" at higher RPM's? Is that normal? Is there a way to evaluate the advance mechanism?
My gut feeling is this is a timing issue. The TDC mark is pretty jumpy as compared to a V8. But that was 30 years ago and I'm new to motorcycles.
I put the timing light on it last night and coun't get any light on the right side. It's a new induction type. maybe coils? maybe condenser? plug wires?...

thanks so much for your time and attention..
z
 
Hi Mr. zeebob,

Does your bike have the stock points or has the ignition been replaced with an electronic unit?


Thank you for your indulgence,

BassCliff
 
Last edited:
the whole thing

the whole thing

is stock as far as I know (points, jets) etc. Altho the P/O wasn't adverse to "borrowing" certain parts or putting stuff on backwards or just adding stuff for no apparent reason. Makes it just that much for interesting for the newbe! lol

I would love to add a eletronic ignition to this bike but not sure if one ever existed. The Dynatek DS3-3C is not an option, I just spoke with them. :( If you hear of a replacement ignition for these please let me know.

I guess it's back to square one...my plan now is to take another shot at the carb cleaning and then replace the plug wires, condensers and coils if that dosen't do the job.

thanx for gettin' back to me...
Z

FYI: I did find out there is an advance mark on the points plate... so the moving forward (approx 20*) at higher RPMs is normal.
 
Going from memory here, but the timing pointer should have two marks...one for base timing and one for advanced. The letters T and F are coming to mind...but I haven't looked at one in a long time.

Do you have a service manual? I would follow it's instructions for the dynamic test, using a timing light.

~Adam
 
you have a good memory

you have a good memory

there is an advanced timimg mark and an idle mark (10* BTDC)

Got another question for y'all... the pts for the left cylinder are still showing some current, judging by the brightness of the continuity light, where the right is not... does that make sense?
thanx
Z
 
the current ...

the current ...

is when the pts are open ...
Z
 
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