T500 bucking when throttle closed?

[Suzsmokeyallan]

Ivan what about those annoying bleedless pilot jets, have you any research data on if simply upsizing the main bore helps these jets.

[tz375]

Which carbs are you referring to Allan? Something other than a GT presumably?

[Ivan]

It's a trial and error thing... if the mod is to be done, there are problem identifiers that need to be carefully evaluated before touching anything.

These are some of the questions that I ask myself when trying to figure some of this stuff out.

Is the problem only when the throttle is closed?
Is it there also at very light throttle also?

Have you exhausted all other options ? (pilot jet size and mix screw settings)
Have you tried different needles with a richer point of origin of taper without changing any other part of the needle?
(This is not for the general public - The public doesn't have the choices available that I have as a needle manufacturer).

If the problem still persists no matter what is done from the above choices, then the pilot holes need to be changed.

Usually if the pilot outlet hole is enlarged to stop the decel problem, then the idle may become too rich.... and then the bypass hole needs to be enlarged as well to weaken the signal to the pilot jet (closed throttle)..... until the correct balance is achieved..... (pilot jet sizes are usually juggled around as well during this process).

The problem is that in order to find the best settings, sometimes you need to go too far and at that point the carbs become junk.

This may become an unnecessary avenue.... I always try to avoid it if I can.


Ivan

[Ivan]

I would have to see the whole carb circuit (carb in hand) to see the layout.... bleedless jets doesnt mean poorly atomized.... the necessary atomizing air may be added by another passage.

I am not at all familiar with GT 750 CV carbs (never had any in my hands and apart) .... but I am very familiar with modern CV carbs on everything made from 1985 and newer.


Ivan

[Vintageman]

Ivan,

I think the 72/73 GT550 has this issue. It had very hard tuning just off idle. The issue is only when at higher RPMs, chambers boosting charge and you back off throttle not quite all the way to full off.
I have increased from 27.5 pilots stock to 37.5 or 40 (forget). The pilot alone has fixed 96.4% fixed, but I can find the spot if I want too. As pilots got bigger they seemed to have diminishing affect. I leave air screw out 2 turns best overall riding, but not best for the more spirited ride and back off throttle issue (3/4 turn). The carb body may limit both fuel and air for pilot circuit

Maybe you’ll make a kit for a GT550 and one with chambers

Separate question. If you don’t get this far I may make a separate post.

Carb Tuning philosophy:

I try to tune each throttle position to work though out full RPM range. Even at very small opening I’ll climb up rpm range and make sure it Acceleratessmoothly. I do this for several positions up to full throttle. Once completed, I find this is best for enjoyable tune for miled and sporty street riding.

What is your tuning approach?

[Ivan]

Ivan,

I think the 72/73 GT550 has this issue. It had very hard tuning just off idle. The issue is only when at higher RPMs, chambers boosting charge and you back off throttle not quite all the way to full off.
I have increased from 27.5 pilots stock to 37.5 or 40 (forget). The pilot alone has fixed 96.4% fixed, but I can find the spot if I want too. As pilots got bigger they seemed to have diminishing affect. I leave air screw out 2 turns best overall riding, but not best for the more spirited ride and back off throttle issue (3/4 turn). The carb body may limit both fuel and air for pilot circuit

If you have to change the pilot that many sizes... there is definetly something wrong with the needle... (taper starts too late - very common on all the 2 stroke bikes from the 60's and 70's).
Raising the needle makes everything too rich because the lower taper(s) are way too rich.

For small throttle openings to be correct and supply seamless power without going excessively lean or rich as the rpms climb, the pilot circuit needs to transition correctly to the needle circuit.... to get this correct, you first need to get the pilot circuit to do what it needs to.
(no bucking on decel and air screw somewhere between 1 - 1.5 turns out.)

Then, the needle has to start it's taper in the exact position (within .005")... sometimes the needle jet is not the correct size (68 T500 34mm)

The lower tapers will affect small throttle openings on these older carbs even though their predominant effect is at larger throttle openings... The only way for me to get seamless power at all throttle openings is to make 50 or 60 different combinations of various tapers and point of origin of taper and trial and error it.... It's very time consuming and frustrating at times when finalizing the last needle design.... but it's always satisfying to me when I finish a kit and the bike runs as flawless as I can make it.... always a marked improvement over "as delivered"


Maybe you’ll make a kit for a GT550 and one with chambers

I would if I can get a good bike to do it with...

Separate question. If you don’t get this far I may make a separate post.

Carb Tuning philosophy:

I try to tune each throttle position to work though out full RPM range. Even at very small opening I’ll climb up rpm range and make sure it Acceleratessmoothly. I do this for several positions up to full throttle. Once completed, I find this is best for enjoyable tune for miled and sporty street riding.

What is your tuning approach?

I think I answered your 2nd question already...

Then there's EGA tests (on my dyno) to confirm the seat of the pants...


Ivan

[Vintageman]

Ivan,

I appreciate the reply and everything you say makes sense.

Let me give a little more info, which doesn’t conflict with you (agrees), but explains why I choose pilot

If I raise the needle it helps solve the problem as well, But the issue becomes poor MPG. I do not notice the bike getting too rich as I open throttle, but of course it is. If this fully fixed issue I would put up with poor mpg, but it does not when working with stock needle.

Since needle raise (Clip 4 and clip 5) won’t get the spot fully (pinging- just off idle, higher RPM) I choose the pilot approach versus needle raise. Solves same way but little less MPG hit for average riding.

I wanted to find the next size needle jet, but that style is obsolete. I was lucky and found a hand reamer that was equal to the next needle jet size, that helped. If I had a richer slide cut I bet that works too.

Yes a new needle should fix while not compromising other areas. Maybe thinner on top is one attribute?

What happens when I use the pilot only on the early GT550 it seems like carb has a limit? I am sure it does. Other bikes I have will drowned you out if pilot gets too big, the carb body does not limit

My second question I am not sure you answered. I think your point is more dynamic tuning ensuring as you change throttle position one circuit transition to the next nicely. Maybe that’s the point. Important
My perfect tuning is let’s say you leave an intersection, the road is straight, flat and clear, your open the throttle a bit and the rpms climb. You just don’t feel like shifting right away and the bike climbs a little higher in RPM, but it is so smooth and may even pull harder (boosting or bikes power band) you let it rise and enjoy. I find when I street ride I tend to do this lazy performance riding. If I find I have to change throttle to increase or decrease with RMP, the bike takes too much work and not as enjoyable.

These domed head bikes (not squish head) with chambers are just tough to prevent pining. Maybe I try and drowned problem with fuel. Still I am sure the GT550 would benefit highly with a better needle.

Thanks

[Vintageman]

Ivan,

I appreciate the reply and everything you say makes sense.

Let me give a little more info, which doesn’t conflict with you (agrees), but explains why I choose pilot

If I raise the needle it helps solve the problem as well, But the issue becomes poor MPG. I do not notice the bike getting too rich as I open throttle, but of course it is. If this fully fixed issue I would put up with poor mpg, but it does not when working with stock needle.

Since needle raise (Clip 4 and clip 5) won’t get the spot fully (pinging- just off idle, higher RPM) I choose the pilot approach versus needle raise. Solves same way but little less MPG hit for average riding.

I wanted to find the next size needle jet, but that style is obsolete. I was lucky and found a hand reamer that was equal to the next needle jet size, that helped. If I had a richer slide cut I bet that works too.

Yes a new needle should fix while not compromising other areas. Maybe thinner on top is one attribute?

What happens when I use the pilot only on the early GT550 it seems like carb has a limit? I am sure it does. Other bikes I have will drowned you out if pilot gets too big, the carb body does not limit

My second question I am not sure you answered. I think your point is more dynamic tuning ensuring as you change throttle position one circuit transition to the next nicely. Maybe that’s the point. Important
My perfect tuning is let’s say you leave an intersection, the road is straight, flat and clear, your open the throttle a bit and the rpms climb. You just don’t feel like shifting right away and the bike climbs a little higher in RPM, but it is so smooth and may even pull harder (boosting or bikes power band) you let it rise and enjoy. I find when I street ride I tend to do this lazy performance riding. If I find I have to change throttle to increase or decrease with RMP, the bike takes too much work and not as enjoyable.

These domed head bikes (not squish head) with chambers are just tough to prevent pining. Maybe I try and drowned problem with fuel. Still I am sure the GT550 would benefit highly with a better needle.

Thanks

[tz375]

Vintageman,
You appear to be describing the same issue that Ivan has touched on - idle through say 1/4 throttle. Raising the needle doesn't help there because the needle taper starts lower. You used a larger pilot jet to cover the problem area and it does up to a point. A slightly larger Needle jet would help at those small openings but it also changes fueling at higher speeds too. The approach I think Ivan explained was that he comes up with a different needle and one that the taper starts earlier so that's what corrects that low to mid throttle opening area.

At less than 1/3 throttle all of the circuits play some part to a greater or lesser degree and that's the zone you like to ride in - not too much throttle but run it all the way through the rev band. And that's what Ivan is fixing, more power, less bucking at all throttle settings and all revs. His EGA system will tell him whether for example the Air:Fuel takes a dip just off idle or wonders off as revs rise.

For example. The Air:Fuel on my bike on the last dyno run was fine at peak rpm and WOT but was too lean at lower speeds, so the fuel slope has to be tilted. If I did a conventional plug chop at WOT it would look fine on the main jet, but it would be lean at 99% of the throttle positions I ride at. I don't have Ivan's expertise or skills, so I have to try other ideas to get there. Do I change to a larger needle jet and smaller main jet or can I try a larger main air jet or would I be better off with a taller spray bar or different air bleed holes? Who knows. That's why we need guys like Ivan to work it out.

If I had the cash and Ivan had the time I'd stick my bike on his dyno and let him work through the options but that takes time and how many jet kits would he have to sell to cover the development costs? All good clean fun though.

[Vintageman]

Tz375, Ivan and all,

Thanks again

This specific GT550 issue is not at 1/3 opening it is about 1/8. Needle is just starting to help sure, but to me (and VM Mikuni manual) not its proper place. Same can be said about pilot circuit. Looks like the slide cut would cover the spot best. I wish I had a richer cut slide. But update to pilot and needle probably over laps the spot too.

Tune: If I can contribute one thing to my fellow registered two stroke street riders is this state of tune I am endorsing. Yes, I gave one scenario last reply of just opening throttle a little and letting it climb from there, But I mean anywhere on throttle. Hold a steady throttle and engine will work for you all over the band without spit, ping or sputter.

Say you were at 1/3 throttle, it still should run smooth from low RPM pull (climb a slope) and then all the way up to full rpm (road levels off maybe) without any sleight of hand with throttle. I want to be clear as I try to explain this for I am confident when achieved you’ll be elated the whole ride. Next I’ll say I have had a few sleds and bikes and this tune is typically not present or long lost. And if not aware you may not realize it can be perfect.

I never knew things could be even better until I had two of the same exact 2 stroke machines I could compare. I could not figure out why one was just a little smoother than the other. One I just preferred to ride over the other. I checked many things until I found in one case the Needle Jet viewed under a microscope, had a dull finish inside versus the other (still have this needle jet somewhere and it don’t take much at all to feel a difference… sensitive creatures aren’t we).

Try this on your bikes or sled. See if you can hold throttle steady Let engine change rpms up or down and see if it still runs well. If you have to close throttle even a tad or open it a pinch, realize you are always hunting:working. Like managing a steering wheel on a car with a bit of tie rod play.

I was/am not sure if a simple (i.e. Mikuni) carb could every do what I am now requiring of all machines, but I have gotten most all to be like this. Some are getting closer.

I hope Ivan understand for he is working on making things better if we agree on criterea

Thanks for reading!

[r3tro74]

it still should run smooth from low RPM pull (climb a slope) and then all the way up to full rpm (road levels off maybe) without any sleight of hand with throttle.

Someone should invent an ignition centrifugal/vacuum advance plate ?

[tz375]

2 strokes typically need to advance timing from say idle to 3,000 and then retard it steadily to say 9000 and then retard faster from there up to provide a soft rev limiter. Some older ignitions have a built in curve that results from simple circuitry and ignition systems like Ignitech and Zeeltronic are fully programmable.

Analogue sytems for say a Banshee rise in a smooth curve from 30 degrees at zero rpm to peak at 33 degrees at 4,000 and drop back to 21 at 12,000.

Those systems come from Europe where they continued to sell high performance two strokes and the technology developed. We are using technology designed at the dawn of the motoring age.

Just as a comparison, a GT750 makes about 50 hp at the rear wheel and weighs about a ton. A 2000 Aprilia RS250 with a Suzuki designed V twin makes about 60 HP at the rear wheel. The 250 'Prilla has more port are than the Buffalo, exhaust power valves, huge reeds and a sophisticated timing curve built into the ECU. Without the timing curve, it would either melt or make a lot less power. The GT would probably benefit at lower revs from more timing but it doesn't need to be as sophisticated as the 250 because it's in such a low state of tune.

The higher the state of tune, the more potential advantage there is of a timing curve.

[Fritz500]

All,

Well after being lazy for a month I decided to swap out the #30 pilot jets in the T500 for some #25 ones to see if that would cure the jerkiness on a trailing throttle. Previously I had the air bleed screws turned out from the factory recommended 1 1/4 to about 2 1/2 to try and improve things, it did make a difference. I put the screws back to 1 1/4 and went for a ride. WOW! The difference was quite noticeable. A lot less chain snatch and jerkiness. I screwed the air bleed out to 1 1/2 and it seems to be even better.

It's quite hot and humid here today so I'll try it at night to see how it goes in cooler temps.

I really enjoyed the ride now that it has smoothed out things. Oh the joy!

Cheers

[Vintageman]

Fritz500.


So opposite of what I saw with my pilot jet needs on a 75 T500. I had to go up on pilot size 35 too. But I only noticed need when I added chambers. my changes is more along with that Classic motorcycle article for t500 (Suzdave pos). They used stock exhaust

I think ConnerVT went down on size too

Curious,

If you just open throttle a tad on flat, light load, and hold throttle position will your bike rev up? Or do you need to open throttle a bit more to climb RPMs more? Will it ping IF it can climb when you hold.

I found when it bucks/surges you are real close on pilot but a bit too lean. So go leaner makes it go away too for not close anymore, but bike won't run well its if you use low throttle for riding (pop corns and won't rev up beyond a point). Up a bit richer and fixes buck and Pop Corning .

You see this stuff on Yamahas (see USA2Strokers.com or AircooledRd.com). Sometimes they are very passionate it is rubber cushions getting week. My R5 and RD400 did this, was not cushions (maybe week yes thus amplifying that fact jetting off won’t argue that), and was fully solved with richer pilots there too. Can cruise around on Idle circuit perfectly

Again interesting but can you share how it well it runs low throttle?

[Fritz500]

Vintageman,
The whole problem with mine was it never ran well at part throttle at any speed. I hated riding at any speed below 40mph as it would pop and fart.

Screwing the air bleeds out helped a fair bit but it still was annoying at lower speeds.

I have to fix a charging issue (it drains the battery with the headlight on) - I want to go for a long evening ride when it's cool to confirm that it works well at lower temps.

I'll post the results in about a week.


Cheers