'Backfire' through #1 carburettor...

[tfb]

New O-rings and pilot needles ordered.

[Big T]

If you have broken tips on your fuel needles, make sure the tip isn't stuck in the carb (that would really make it lean)

Also, the seat is easily expanded, so check the diameters there. if they're expanded, a bit less of a turn on the fuel screw

[OldVet66]

Since your Pilot fuel needle seats have been enlarged by someone tightening the needles too much I would suggest starting with the needles closer to 1/2 turn out. Pilot fuel needle adjustments require air screw adjustments for highest idle. I would play with them until your highest idle is closest to two turns out. That seems to be the happiest place for Pipes and pods, and then fine tweak it from there. With a perfect carburetor, you should just be able to see the tips of the Pilot fuel needles below the holes in the throat of the carburetor when lightly seated. You can judge the damage by how much above that level the new needle tips rise into or above the hole when lightly seated. The worst of them would probably be adjusted closer to closed than the better ones to emulate a perfect needle and seat.

[tfb]

BigT, yes... I've had the joy of broken-off fuel needle tips in carbies before, on a z400 Kawawaki. I wondered why it was so hard to tune until I made that discovery. Ended up having to get a whole new carbie, as the brass tip was well-and-truly chemically 'welded' into the cast aluminium.

But happily that is not the case at the moment; the tips are all present and accounted for.

Oldvet66, thanks for the wisdom there; I'll wait for the new fuel screws to arrive. Then shall check them for the amount they protrude through the hole, etc.

After a short jaunt yesterday morning, I feel I'm fairly close to getting this thing sorted out. The spitting-back from carbie #1 is now much less, and the bike accelerates quite smoothly, with only the slightest hesistation as you move from the pilot circuit through to the mid-range. The bike is sounding nice, too.

[Chuck78]

how did this issue end up working out? I have had the same problem, and wondered if my pilot jets were just too small now after some very large and incredibly free flowing K&N air filters.

I cleaned the carbs very extensively, but wondered if water/simple green in the ultrasonic maybe left some oxidation in the pilot circuit passages. Preceeding that I dipped them in Berrymans 4 nights in a row with a cleaning halfway through with aerosol carb cleaner and compressed air, & a cycleorings.com kit with intake boot orings also. Original boots "looked" good.

The symptoms got much worse after upgrading my ignition system to some "Dynamite" gree dyna coil copies made in China, & a used Dyna-S.

my buddy's GS850 had some horrendous backfiring problems that were caused by his igniter box having the improper advance curve due to deteriorated electronic components. I was wondering maybe if it could be a timing issue but I thought it looked great when I set the timing with a light. I will have to double check that and the coil voltage which I also thought was great previously, and then I will probably pull the carbs again and blow out the pilot passages extensively with cleaner and air, and buy some 17.5 pilots.

[tfb]

G'day Chuck78,

how did this issue end up working out?

Thanks for asking! It's been a long story... but after trawling through the electrics, replacing points, condensers, getting the timing spot-on, changing the plugs (again), swapping out the coils (nup, they weren't the problem), dismantling and cleaning the carburetors (again), AND SO ON...

... it has turned out to be getting the fuel mixtures correct in the carbies. From 3500rpm onwards the engine pulls like a train, but light throttle with the motor at 3000rpm will see it spit back through one of the carbies once in a while.

So after lots of fiddling, I have realised that I can EITHER (i) tune the pilot mixture so that the spitting-back is eliminated but the idling mixture is too rich and fouls the plugs, OR (ii) I can get the idling mixture spot-on but then I get the spitting-back at 3000rpm on a light throttle.

The nub of the problem is, I think, that I have pod filters + high-lift cams + free-breathing 4-into-1 exhaust system, and it's simply asking too much of the original spec. carbies to be able to deliver the correct air-fuel mixture right across all revs/ throttle openings.

I am going to try a little more to coax the carbies into some sort of acceptable compromise, but if I just can't get them to come to the party, then I'll pull them out and put in a set of CVs instead.

That's a bit of a pity, as I really do like the looks and mechanical simplicity of the original VM28 carbies, but at the end of the day I'm not going to ditch my prized Yoshimura camshafts. For the most part, this machine absolutely flies and is a blast to ride. So I'll be looking for a set of CV's off a GS1100 or some such machine, to get things running correctly in all situations.

In short, I began by thinking that this was an electrical problem, but with a lot of elimination and many hours (more than I care to admit) of working at things, I came to realise that it's a fueling problem that I may not be able to solve with the existing hardware.

Of course, if anyone has any further tips or ideas for me in solving these fuel-mixture problems, I'd love to hear them. Otherwise it's going to be a set of GS1100 CV's that are going to be installed!

[portdave]

The CVs are not going to fit without a lot of modifications. The intake boots dont match up. Have you tried getting dyna s ignition instead of the points etc? I look at your build site. Very impressive. With the fuel screws out that far (2.5) have you tried larger pilot jets? I am not an expert by any means but these are just a few thoughts. Good luck. Enjoy.

[Big T]

So, did you ever change the pilot or main jets?

What sizes do you have in there now

You should be 17.5 pilot, about 122.5 mains, fuel screw about 1 and air wherever it tunes to

The VM carbs are endlessly tunable, you can get different needles and slides to obtain the results you want

CV carbs, not so much.