T500K - carbie issues?

[Fritz500]

The saga continues...to improve.

My 500K was still popping and farting under low load and suffering jerkiness on a trailing throttle. I found some original air filters and tried those - didn't notice any difference.

I screwed the pilot air screws both to one full turn out and went for a long ride, stopping every 500 metres or so and turning each of the pilot screws out 1/4 of a turn, then riding another 500m and so on. When they got to about 2 1/4 turns out the bike was running like a dream (almost). Far less popping after each gear change, far less chain snatch cruising down hill and it still idles OK. I screwed out the slide screws so it idles about 1300rpm - this has helped with the trailing throttle jerkiness too.

I'm going to try one or two sizes smaller pilot jets - any ideas?

Thanks

Geoff

[ConnerVT]

Did you replace the Main Jets with new ones, as you suggested earlier in this thread?

[Fritz500]

Did you replace the Main Jets with new ones, as you suggested earlier in this thread?

Yes I put new (same size) main jets in just to be sure. Didn't make any difference. I really seems to be up to about 1/4 throttle opening that it has issues. I was going to try pilot jets that are say 10% smaller, I was hoping a forum member would give me some sort of direction - should I go 20% smaller or only 10%?

I wound it out this afternoon to see if the genuine air cleaners were better than the first set of foam air cleaners I made (before going to thinner ones). I think they are still a little restrictive but I got to 65mph OK. Curiously it wasn't vibrating as much today - the weather was cool and it was raining lightly when I was scooting along. I couldn't stop smiling. It was just so...refreshing and therapeutic.

Tomorrow I'm going to see how far I need to screw the air bleed screw out to eliminate the slight popping I've still got.

All good fun.

Cheers

[ConnerVT]

Did you replace the Main Jets with new ones, as you suggested earlier in this thread?

Yes I put new (same size) main jets in just to be sure. Didn't make any difference. I really seems to be up to about 1/4 throttle opening that it has issues.

Good. That eliminates some possible issues.

I would try going down one size, to 27.5 (get the correct style, the OEM is the wrong one, trust me).

I've needed to do a lot of experimenting, as I have '73 cylinders and early carb setup on my '71. I originally started with 30 Pilot Jets, and *way* too rich Main Jets. Switching to the 27.5 Pilots was a great help, as everything was running too rich, fouling plugs, etc.

As you probably know, the pilot circuit is always supplying fuel mixture. Not just at idle. So at part throttle, it is an influence on your total fuel mixture, becoming a smaller percentage as the throttle opens. before I switched to the 27.5, I had to open the Air Screw up a bunch, *and* drop the needle one notch, just to make things run.

Now, with the 27.5 (and more reasonable Main Jet), I'm back with a P-4 Needle jet with the Jet Needle in the 3 slot, the air screw at 3/4 to 1 turn, and the bike runs much better. The spark plugs look good, no more oil fouling.

The troubles still may be a bit too restrictive air supply, but going down one step on the Pilot Jets is an easy experiment to try. Put in a fresh set of plugs, and keep an eye on their condition, to make sure you haven't made things too lean for the engine's own good.

[tz375]

That air screw certainly setting suggests that you have rich stumbles at small throttle openings. The question is whether it is the air jet or the fuel jet that needs to be changed. All large two strokes tend to stumble at low speed. Yamaha has that issue with their 400s and the solution there was to drill a hole above the exhaust port to bleed off pressure at lower speeds.
On 750s with CV carbs we change the air jets and fit a smaller threaded air jet upstream of the large pressed in jet to richen up the low speed on small openings. Your issue seems to be a rich stumble on trailing throttle.

The question here is what is out of synch. Are the pilot jets oversized (poked with wires perhaps) or the wrong type?

Are the air jet circuits clogged or partially obstructed?

Are the air screws the right ones or are they damaged or too thick?

Do your intake boots partly obstruct the air circuit intake?

I would pull the carbs and pay attention to the air jet circuits. Order a pair of VM22/210 slow jets in stock 30, plus a pair of 27.5 and a pair of 25s from IWT in SA or through your local Mikuni supplier. They are cheap and postage on 3 pairs is the same as on one.

They key here is that if it idles OK and pulls well on acceleration, but stumbles on slowing down or at small openings, it's probably the air jet that needs to be changed (not so simple). If it accelerates cleaner with smaller pilot jets then it's those jets that are the issue.

If your bike has the Homo-Pressure carbs then it's also important to make sure that long pathway from above the bellmouth to the float chamber is open and clear. That's the one that looks like it's cast for a power jet, but it's an air passage. It has a significant effect on the air: fuel ratio.

[Fritz500]

Firstly a big thanks to you two (tz375 & ConnerVT), you have confirmed my suspicions.

I'm curious about the jet (OEM) being the wrong one. Damn you Mr Suzuki!!!

I'll buy the two sizes suggested and see what happens.

To briefly answer the other questions - I have had the carbs out a few times and they are now so clean inside (and all holes etc) I can say I have eliminated that as a variable. I have got everything else aligned, set, adjusted, fiddled with and was mostly convinced it was just the pilot jets and now you have firmed that belief that I'm on the right track. [imagine sound of a deep sigh]

I'll order the jets tomorrow and hopefully be able to report in a few weeks I (we) have solved one more problem.

Cheers and thanks!!!!

Geoff

[ConnerVT]

I'm curious about the jet (OEM) being the wrong one. Damn you Mr Suzuki!!!

The 'OEM" jets (as some sites list them) are for the BS carbs. Look very similar, but are much too rich for the VM carbs.

The correct ones are the VM22 series.

[tz375]

That makes more sense. It's not that OEM jets from Suzuki are wrong but that some sellers incorrectly list BS30/96 jets for all years/models. I follow.

[ConnerVT]

Yeah, got bit on the butt by that one. Been tempted to start "What's the dumbest thing you've done..." thread. I have more than a couple I'd be embarrassed to share.

[Fritz500]

I'm curious about the jet (OEM) being the wrong one. Damn you Mr Suzuki!!!

The 'OEM" jets (as some sites list them) are for the BS carbs. Look very similar, but are much too rich for the VM carbs.

The correct ones are the VM22 series.

Ah now I understand. Thanks


Cheers

[Fritz500]

Got my new pair of VM22/210 slow jets in the mail today, a pair of size 27.5 and a pair of size 25.

Now to find time to fit them and test them. Might have to cancel Xmas...

[tz375]

Do not be too surprised if the new jets are too small after all.

It is entirely possible that a PO cleaned out the jets with wire and gouged them oversize. I don't have the capability of flow testing jets but I did find a pair in some crabs recently where the pilot jets were reamed by wire and one allowed a .032" wire through it which is about twice the size it should be. The other was just over .020" which was also oversize, but not as badly. Stuff happens to old bikes.