Revisions to the BS 40s,,,again.

[Waterbuffalo]

I had the carbs fairly well dialed in when the bike lived at approx 3500 ft above sea level in southern Alberta, but since its now in Florida at sea level with more humid air I've gone back at it again for some obvious reasons.
The biggest problem was a re-occuring surging issue, no surprises there, and a lean section in the lower part of the throttle circuit which only made the surging worse.
It appears the whole lower end has gone to the side of lean, so here's whats fixed it. Remove the 0.8 air corrector jets in the bell mount Suzuki tells you to install to cure the surging, and now I've put in the next smaller size of 0.7 instead.
Take out the stock 45 pilots and put in 47.5 like the 74L models use. So far I'm running the mixture screws at 3/4 turn out and its nice and smooth with a stronger low end and a crisp burble from the pipes.
I also run the float levels a tad bit higher than stock which on the BS 40s carbs I've found to be a good thing.

[yeadon_m]

Interesting. I too found that smaller pilot air restriction jets helped, I went as small as 0.5mm and that erased surging, but I had a very rich just off-idle feeling, burbling as if the chokes were open unless I twisted the throttle to 1/8th or more. I tried one size smaller pilot jets but never dialled it in fully. Why did you pick 0.7 (richer) and larger PJs (richer)?
Cheers,
Mike

[Waterbuffalo]

Hi Mike, I picked the 0.7s to fatten up the mixture by reducing the air passing through them. This was in an attempt to reduce the surging with stock timing and not going over too far on the scale to start with.

The 0.7s seem to be fine, further testing will reveal if 0.6s may be needed, but at this point ,, not yet. Your 0.5s seems a bit extreme, I would not have thought anyone would have to go that far down in size.
So far the testing I've done seems to be bearing fruit as far as the surging goes, but sections of the carbs overlap, so its not like I'm just trying to fix one area.

I also noticed the engine temps went up a bit (about two needle widths) when using very little throttle input with lots of near level road cruising. I could also feel as well as hear the exhaust note sounding a bit weak, right in that pilot circuit area.
Both of these issues are definitely leaning the mixture so it was time to address it. It appeared to me that a move up slightly to the 47.5s from the 45s would be a match to address the pilot circuit leanness working with less air on the pilot air input circuit to deal with the surging.

At the present time I need to go down the interstate for about 80 miles to verify how well its working, on the light cruise mode with flat roads to see what the temps are doing and then to test the surging using those long off ramp runs down from speed in fifth.
I would say its very close now, hopefully I can finalize it by the mixture screws and a dab of ignition timing. I may take a short video of the bike running if I can get around to doing it.

[teazer]

Allan, it does seem a little extreme to have richened both aspects of idle jet usage - no load and "negative load", but all bikes are different and it is quite possible that your bike ridden on your roads with that gas, may need to be richer than a different bike ridden in different circumstances.

If the bike feels/acts lean at just off idle riding conditions, it is very likely that the slides are lifting slightly and that they need either a richer needle jet or needle or perhaps a change in the air feed to the chamber above the diaphragm.

Have you had a chance to try your bike on a dyno with exhaust sniffer to see how lean it goes? All the dyno charts I have seen for GT750s go lean as the throttle is opened up, including my own before I changed to Z-6 needle jets. Your set up may require a different solution. I don't ride long distances on mine, so a richer mixture has no down side for me, but may not be so good for you.

[Waterbuffalo]

Richard you have to trust me on this, it was lean on the pilot circuit, not by much but just enough to be noticeable if you listen to the engine under the required circumstances.
The observed temp rise of a needle width or two under the no load light on the throttle cruising speeds also clued me in to a leanness there, right back into that part of the circuit where the pilots are influencing the mixture.
Also bear in mind my airbox is not stock, the restrictive fine mesh and its mounting frame has been removed so air is flowing more freely to the carbs from the filter. Some of the L model 750s also had no mesh in the filter case and they used 47.5s so its not like I strayed far off the course from what was a stock item.

You would think it could be tuned out by the mixture screws but thats a very narrow range of physical adjustment which mostly affects the idle.
I had a similar issue with my Mk1 Bandit 1200s with the stock jetting, again the solution there was to bump up the pilots from 37.5s to some 40s. This was before I fitted the Dynojet kits which added needles, main jets, slide springs and needle shims. Their kit did not address the pilot issue either which meant the 40s stayed to help that part of the circuit they neglected.
My primary concern is to see what happens after a long cruise on the interstate and then coming down from speed in fifth on the exit ramp.
Whats going to be the zone for surging like now?

[yeadon_m]

Allan,
I installed 0.5mm PAR jets because (i) I couldn't obtain any smaller than 0.8 - which I had already - unless I chose 0.5mm and (ii) as a scientist I was happy to go further than I expected needed in a certain direction because, if that didn't delete surging (it did) then I was way off track!
Cheers,
Mike

[PaulD738]

What really confuses me with this problem is it doesn’t affect my 750J which as you know is fitted with the none CV Mikuni VM’s and yet the 750 Kawa triple which is also fitted with VM’s is even worse for surging than the Kettle? I remember when a friend bought his H2 new, it was in and out of the bike shop every other week while they tried to cure it. Eventually he traded it in for a H1 which he still owns and will never be parted from.

[teazer]

Too many variables for one simple answer.

Air filters make very little difference in most cases because at low engine loads there is low volume of air and therefore little resistance to flow. Some carbs are more affected than others, but in general the filtration resistance has more impact as load rises.

Air jest also have more effect with higher gas velocity, so at idle, little effect but deceleration with much higher gas velocity through teh carb the jet size has a greater effect on mixture strength.

Allan is saying his bike was lean at idle and he could be right, but rolling down the road at 1/8 to 1/4 throttle has two circuits in play - idle and needle/needle jet. That's why seat of the pants is only so good. I'm not doubting what Allan has experienced, I am just suggesting that maybe there's a more complex answer and therefore potentially different solutions. Bottom line is the same for all of us. If you find a fix that workd for you, then ut doesn't much matter if there is some theoretically "better " solution. And where circuits overlap that is certainly true.

As I mentioned before, almost all BS40 dyno charts show the motor going lean as the throttle is opened. That's not uncommon with any carburetted machine.

Stumbles on overlap are interesting and hard to cure on large bore two strokes. Part of the puzzle is state of tune - compression ration and porting play a role. On a 64mm bore RD350 there was little problem with closed throttle stumbles but on the RD400 if became a real issue. Yamaha answered that with a small exhaust bleed port that at low revs allowed a bleed off of compression pressure, so the motor was running with a lower effective state of tune.

I would speculate that a 750J with lower state of tune than an H2, probably avoids the chronic shudders on overrun because it had less compression and less noticeable power pulses. My bike is not quite like most GT750s in that I run very high compression and huge (relatively) ports and it has run cleanly with pods, stacks and shorty bellmouths. They all have different effects of where it makes power and how weak it is at the bottom end, but I don't get much shudder because that is not how it's ridden. It rarely sees low throttle gentle riding, so that isn't an issue for how the bike is ridden, but on appropriately ridden GT's that would be an issue.

When I first set the bike up I tried all sorts of things on the dyno and on the strip and even resorted to data logging Lambda, throttle position and revs and there's a lot of data that is not easy to make sense of, so I resorted to more simple data. How fast did it run. With difficult stumbles or rough running, seat of the pants is good, but a dyno run or two can reveal a lot about exactly what is happening and it's easier to change jetting at a warm safe dyno facility than trying things on the street. That said, in the final analysis, it's how it feels to the owner that counts.

Fortunately I don't have to home machine too many needles any more.

[Waterbuffalo]

All very good reading indeed guys and thanks for the answers listed so far.
As seen before all of the BS carbed GTs seem to have their own little quirkiness, some surge some don't, some are easily fixed while some defy logic. You have to work around whats effecting yours and solve what issues the carbs may be influencing your particular engine. Playing around with it in the garage its definitely a different bike with the carb updates.
The weathers been a bit lousy so as soon as its a decent day I'll take it out for an extended run to evaluate whats up with the choice of 0.7s versus the 0.8s and what the 47.5s are doing when cruising along.