Just off idle

[Guest]

Hello All,
I have a '79 gs750l - 8 valve with VM26s. The bike is pretty much stock with a Jama replica 4 into 2 pipes although I have some short meggas in place of the 3 foot long Jama silencers.
I bit the bullet recently and did the valve clearances, I was to be fair putting the job off as I thought I might have some cover bolts break on me, but in the end it was a really nice job, clearances are now in the .06 - .09 range. Sorting this has highlighted other issues.
The bike always starts well on choke but takes an age to warm up. It idles well hot or cold and goes like hell on anything more than 1/4 throttle. It mostly goes great on a fast run down to the pub. Slow / town riding is a pain however as it hesitates / misses on very small throttle openings. It stutters / runs uneven in neutral when throttle opened slightly and coughs when throttle snapped open. I've switched exhausts back and forth but this doesn't make a lot of difference.

Now:
Ignition timing (on points) is fine on strobe at idle and it appears to advance ok, condensers are newish.
Plugs are dark brown / black on the edge but look clean / lean on the centre insulator, all look about the same but I did have an issue with one being dry sooty which I put down to a faulty plug cap.
Carbs have been balanced and I've set the air screws just off yellow to blue flame at idle using a carbtune. Tank is clean and carbs were cleaned thoroughly a while ago.

So, questions are:
Is it possible to have mixtures looking OK via carbtune at idle but in fact have too little fuel for progression (i.e. both fuel and air screws too far in)? or do the screws just set the idle mixture - there's also a pilot feed passage under the slide?
Could it be I have just offset leaky intake o-rings by richening up the idle, so the idle looks OK but still pulling in enough air to upset it just off idle?
Am I right in thinking this is a lean running issue? Do I need to be looking at upping the pilot jets?

I'm gonna bite the bullet again and replace those intake o-rings (not looking forward to the cheese screws) and while I'm at it clean the pilot circuits and check where the fuel screws are. Should I start again at 3/4 - 7/8 fuel screw and adjust air screws from there?

Any advice appreciated.

Regards

Andy

[Steve]

Did not see any mention of intake. Stock airbox or pods?

Valve clearances are on the "nicely-loose" side.

Use of a Carbtune to adjust color of the mixture? Hopefully you mean a Colortune.

Carb clean/rebuild is (almost) always a PLUS.

If you are looking to replace the intake o-rings and the "aged Cheddar cheese" screws, get them from cycleorings.com. You could probably source the screws locally and save some postage.

It does sound like a lean-running issue. Change the o-rings first, then address the mixture screws. If the fuel screws are at the recommended 3/4 to 7/8 turn, you should be good. Adjust the air screws for proper color. Maybe turn the air screws IN about 1/8 turn after getting good color, just to make it a little bit richer. If that doesn't work, turn the fuel screws OUT about 1/8 turn, then re-check the color and adjust the air screws, if needed.

.

[Andyjwatson]

Yes sorry colortune to set mixture, carbtune for balancing!

No pods, full stock airbox.

Andy

[Andyjwatson]

Now I'm confused!

I fired the bike up yesterday and sprayed some wd40 on the inlet rubbers - I'd swear it was getting sucked inside the motor.

Took airbox and carbs off this morning.

Luckily the inlet rubber cheesy screws came out dead easy with a close fitting screwdriver bit, light tap with a hammer and a 1/4 drive ratchet. O'rings weren't split completely but some of the stubs had muck and corrosion across the back of the rings as if its been sucking stuff past for a good while.

Took each float bowl off in turn, carbs all clean inside , pilot jets clean and pilot passages seem clear, going by https://www.kzrider.com/forum/3-carb...ning-procedure.

Pilot screws are all over the place however, one of the bottom fuel screws was about 3/4 turns out, the other 3 looked visually further out and were between 1 1/2 and nearly 2 turns out.

Now I know the 3/4 one was one that was previously plugged and I've had it out and cleaned it before, I don't recall touching the others. There's nothing wierd about the 750L that has odd settings is there? everything else have seen says start at 3/4 - 1 from lightly seated.

I haven't moved the air screws as yet but looking at them some are screwed right out presumably to compensate for the wide open fuel screws.

750L manual supplement states float height at 24mm - I've always taken this as you tilt the carb until float just contacts needle spring and measure from gasket face to bottom of float, rather than let full weight of float on spring - is this how you see it?

Cheers

Andy

[Nessism]

I'd pull out the pilot fuel screws and make sure the points are all intact and that the hole in the carb body is not plugged. Sometimes people tighten the screws too much and then the tip wedges into the body and breaks off. When setting them try 3/4 of a turn and put the air screws at 1.5.

Float height is just an approximation. What matters is fuel level. Try measuring that. You can make a sight gauge by taking a piece of common vinyl tubing and grinding the edges of one end to create a truncated cone shape. Then screw that into the threads on the float bowl. That will get you through the job anyway. Fuel level specs in your service manual.

[UKAndy]

For your information Mikuni VM Carb pilot jetting has influence up to 1/2 throttle so a blockage in one or more pilot circuits could explain poor off idle performance. There is a lot of overlap in various jetting components. Clearly if it runs well above 1/4 throttle that would indicate your needle, needle jet and main jets are all performing as they should.

[Andyjwatson]

Fingers crossed its sorted.

I've cleaned up the inlet stubs (see below) and fitted new viton o-rings and put the carbs back with 1turn fuel screws, 1.5 turns air screws.

Connected up temporary fuel supply and plugged the vacuum line and it fired up first time on choke, a few minutes later it's idling happily at 1000rpm without choke. Colortune says mixtures look about right too.

That was last night. I've put the tank and bodywork back on but only got as far as riding up and down the garage. Before, it wouldn't pull away from standstill until almost fully warmed up, so I reckon already a massive improvement.

What seemed to be the issue with the o-rings was not so much they were cracked (although they were flat and brittle) but the steel inlet stubs were rusted between the gap with the head flanges. The expansion of the rust was pushing the stubs away from the head creating an air gap.

Hopefully I'm gonna get home early tomorrow and give it a proper run to be sure.

Cheers

Andy