Carb rebuild kits?

[Ivan]

Conner, if you want to see if what I said makes sense, just put the early needles in with your size 100 mains and go for a ride.... it should be too lean and bog at WOT.

Until you do this test... you can't argue

I have been designing my own carb parts for over 25 years... I know what effects what...

I don't care what Mikuni publishes for the public..... it's just a way over-simplified guide for people to use.

Ivan

[tz375]

Suzuki say in broken ingrish on P25 of their carb manual that the homo-pressure carb needs larger main jets because the pressure signal at speed is lower than on regular vm carb. At least that's what it appears to say but hard to decipher. they def say it impacts main jet size.

[Ivan]

Not significantly... maybe 1 or 2 sizes (if at all)

They were trying to reduce the fuel flow at lower rpm & large throttle openings... (it didn't work very well) as well as altitude compensating.

What does work well is using a needle with a larger tip diameter... You just have to be careful about what other dimensions you change.... a few thou in the wrong place, and it'll be way off.

I understand the principle... but it's not going to be 30% like a way thinner needle...

Ivan

[Ivan]

Here's one with an external bowl vent:

http://www.ebay.com/itm/1971-75-SUZUKI- ... 73&vxp=mtr" onclick="window.open(this.href);return false;


And here's one with what appears to be an internal bowl vent:

http://www.ebay.com/itm/NOS-1968-73-Suz ... b2&vxp=mtr" onclick="window.open(this.href);return false;


Too bad you can't read the ID # on the used carb.


Ivan

[Vintageman]

Tz, Ivan, ConnerVT
Tz
The later 32 for T500 is "Homo” If I remember there is air vent passage from cylinder side of carb slide back into bowl. So when there is a pressure delta as is the case at low speed when slide is down it reduces pressure in bowl allowing less fuel (that's what I see using reason).
I see less affect at WOT for smaller pressure delta. Somewhere Suz says in an add helps reduce fuel consumption low throttle. I can’t see how it affects WOT, but may be wrong.

The T305/350 has the same Homo. In fact they added the second pilot jet (B6 or something like that) to prevent stall when you close throttle quick… Is this phenomena made worse by the Homo design?

I found with chambers added to my later 75 T500 I had to up the pilots a lot and the mains too. Somewhere there is a tuners guide (posted here somewhere?) from the period who also found this to be true for pilot at least on the T500. I think the pipes were stock… why should that matter at low speed anyway.

Adding chambers IMO exposes original tuning imperfection or trade offs: you have to address upper RPM ranges never reached before, and richen things up when on pipe particularly when OEM was tuned on lean side. Domed head limit what you can get for boost and may need to be too rich to compensate for detonation by drowning things a bit.

Conner and Ivan.

The needle end is thinner on early T500 and why larger main. But, still a big change. I almost thought we were talking round versus hex mains, but I hear it was always round mains on T500.

What is interesting about Conner’s bike is that he basically he has all later jets with early intake parts and later cylinders. Yes the Y boot is much more restrictive, That is obvious by looking at it.

I also have early and late air box and assumed the Late was more restrictive, but now that I look at is again if it is not by much. Would have to be proven (maybe Conner did).

I think Conner would have an issue if he uses (could that is) the upper RMP range more. I think the Early T500 had another 500-1000 rpm usable top end and that was tuned for as well. Won’t get that with later cylinders. The higher RMPs is where these added restriction of the late model would show up more and possibly prove too lean main as I found

I like his idea of keeping the early bike with visible early parts for original look even though later cylinders. I am thinking all one need s to do is remove the intake divider on the later cylinder and makes it an early type (that would be too easy)!
I took port measurements from early cylinders and posted them asking if someone could do the same for later cylinders, but no response to that posting yet. It would be nice to know… I will get these someday

IMO: unless one can hold a fixed throttle position (10% 25%, 50% 75% full for example) and cover the full usable RPM range at that fixed position it is not “fully” tuned. Using this criteria and from my tuning experience of late T500 with chambers there needs to be a better needle profile that OEM

Ivan'
I am anxious to see what you find. I think your work on the early T500 will prove useful for all T500 years.

I have acquired Early T500 cylinders for my late T500. I have all early T500 air box intake etc if needed. I really want the extra few HP of the early bike along with the added 1000 RPM top. I hope to use Wossner’s new pistons too (if I can get).. any ne try these yet?
This is my winter project.

Ivan,
Will you have the early T500 tuning complete this winter? If I understand, you also Dyno to prove your work. I suppose you are not using chambers (e,g Jemco), but still interested.

[tz375]

Vintageman,

Homo-Pressure is an interesting arrangement. The float bowl does not have an overflow or a conventional vent. The vent arrangement connects to what looks like a powerjet arrangement on the airbox side of the carb. Normally a the bowl is vented to atmosphere, so the same pressures is applied to "push" fuel through the jets as pressure drops in the venturi.

By connecting the bowl vent to the intake side exposes it to a changing pressure. The reason it's called homo-pressure is that now the pressure in the bowl rises and falls with pressure in the venturi, so in theory at least the same pressure differential applies across the rev range.

The consequence is that at high engine speeds, less fuel is drawn up by a homo-pressure carb, so they need a much larger main jet to achieve the same fuel flow at high revs, but it doesn't flow as much at low speed. So they use it as a technique to lean out the bottom end. According to Suzuki, that style of carb was used on T500, T500-2, T500-3 and T500R.

From what I have seen so far, it looks like
T500 VM34 #410 Main jet Large Hex Homo-Pressure
T500-2 VM32 #150 Main jet Large round (= to 275 hex) Homo Pressure
GT500 VM32 #97.5 M/J Large Round, NOT Homo pressure

I have not stopped to look at what year the changes took place or to look at parts books to see what else changed at the same time. Clearly the H-P versus non H-P made a difference as did the change from Hex to round and from 34 with low signal to 32 with stronger signal.

Surely someone has tracked through all those changes somewhere in the world.

What is clear, as Ivan has pointed out, is that carbs are more than a casting with holes and what appear to be tiny changes can have huge effects on pressure differentials and fuel flow. For example a pipe that really creates strong wave activity will probably need to be jetted down and not up as normally expect. It's all about the pressure differentials and that's what is so hard to "see" when we look at modifications.

[Ivan]

I still don't buy the part that internal vent need a much larger main jet.... not yet.
The graph that Suzuki provides in their service manuals shows the biggest difference in fuel delivery at low rpm.

The T305 was hailed as having altitude compensating carbs.... I have 2 of these bikes and a box of extra carbs (8) and none of them have the internal vent..... the passage is there, but there is no hole in the throat.... there is a brass tube pressed into the passage that vents to the atmosphere.... the T350 also does not have an internal vent. They also have secondary pilot jets to help prevent puking excess fuel up on down shifts and decel, and braking.... This is the only bike that I have ever seen this setup on.... a quick band-aid fix covering up some other type of design problem.

What am curious about is whether or not there is a main air jet in the 73 and later T500 carbs? All the early ones don't have one.... it's just open.

I am not 100% satisfied with the performance of my 34mm T500 kit yet. (this is why I havent released it yet)

The bike is way too sensitive to any change to the needle... if you move it 1 clip position, it affects the top end too much. This means to me that something else is wrong.... The main jet should not be as large as it is.... way too much fuel available and this is why it's so sensitive to needle position, length and tip diameter changes.

The standard needle length on this bike (34mm carbs) is longer than the needles that came on the Yamaha R1 which uses 40mm carbs.

I may start over again with a smaller main, in the normal size range for an engine this size and state of tune and see where that leads me.

I also may drill through the vents at the bellmouth on my extra set of 34's and see how it affects main jet size.... I can always tap the hole and install a set screw to close it up again.

As far as power hanging on longer on the early models, I am convinced that the long intake tract on the later ones is the biggist contributing factor... running an H2 with no snorkel on the airbox will end it's power much sooner than using pod filters.... even though the main jet needs to be the same size.

Later T500's have much better power below 4000 rpm than the early ones....

Ivan

[tz375]

Ivan,
That's a deceptive little chart for sure. They explain at length, but not clearly, that the main jet effect is similar to changing the height of the spray bar or fitting a larger air jet. They then explain that the H-P carb requires a larger Main and the graph is with a larger main to equal the same WOT main jet flow as a normal carb. So if the main has been changed to match at the top end, it's way off at the bottom end.

I am not saying it is so. I'm just saying that they are saying that.

Needle sensitivity implies a strong signal over the needle jet and a steep fuel slope. I wonder if a smaller (or larger) air jet might attenuate that signal enough to reduce sensitivity. A shorter needle jet discharge (spray bar) might also help there.

Or go to a needle with slower taper, or try that H-P type air bleed arrangement to see if that tips the fuel slope enough. I'm glad it's you doing the work and not me. By now I would have fitted an Air:fuel gauge, TPS and data logging to see what was happening across the whole 3D map and that doesn't address transient conditions.

Has anyone compared porting specs on early to late engines by any chance. We know that 75 on 750s have a much lower intake floor and that hurts low speed performance far more than the rise in exhaust roof or the removal of couplers. What did Suzuki do to the 500?

[Ivan]

Ivan,

Needle sensitivity implies a strong signal over the needle jet and a steep fuel slope. I wonder if a smaller (or larger) air jet might attenuate that signal enough to reduce sensitivity. A shorter needle jet discharge (spray bar) might also help there.

Or go to a needle with slower taper, or try that H-P type air bleed arrangement to see if that tips the fuel slope enough. I'm glad it's you doing the work and not me. By now I would have fitted an Air:fuel gauge, TPS and data logging to see what was happening across the whole 3D map and that doesn't address transient conditions.

I have over 60 different taper combinations for the 34's so far..... You might get away with a shorter hood on the needle jet if you installed an air jet.... They are all 5 angle needles...

I am hesitant to shorten the hood though since I did that on my TC305.... It stuck both pistons at 90 mph chasing a burgman 650.... luckily, I grabbed the clutch as I heard the engines note change. It wasn't really bad because I let it cool and restarted it and rode it back to the shop slowly.... just a quick hone and it was good again with new pistons & rings of the same size.... no damage to the bore at all.

Although, I have shortened the hood on the needle jet with success on other 2 strokes, it was always with smaller carbs where the intake velocity is higher and there is a really good signal.

I'll have a lot of these answers pretty soon.... I want the T500 to have a nice improvement to all aspects of how it runs with the stock carbs.


Ivan

[Suzsmokeyallan]

I'll be interested in your T500 kit once you get it finished so let me know when you are happy with the results.
Would you be willing to, or do you plan to make any needles for the BS40 carbs on the GT750 of 74/77.
Most of the needles I've seen seem to suffer from wear issues around the upper portion of the needle from contact/vibration of the orifice.

[Ivan]

I need a GT750 in order to do it... But I would if I had the bike.

I wouldn't make copies of the stock needles, there is always quite a bit of improvement to be had.

I definetly will announce it here and on my website when I have something ready.


Ivan

[Ivan]

TZ,

Generally in these type of carbs that use a non-bleed type of needle jet, the main air jet has it's largest effect in the 2nd half of the power peak with little to no effect in the upper midrange.... also, it really has it's effect at or very close to full throttle.

With bleed type of needle jets, the main air jet has an effect all the way down to the middle of the midrange.... In an R6 for example.... you can see it's effect all the way down to 6000 rpm... even though it's predominant effect is in the 2nd half of the power peak.
In this particular bike, once I got the power peak fixed, I had to re-profile the needle because the midrange fueling got too rich. The air jet had to be reduced from a #120 all the way down to a #68.... This made the upper midrange stuttering rich at 8000-10,000 rpm.

If I reduced the tip diameter/ shortened the needle/raised the main jet, it still went way lean after 12,500 with the stock #120 MAJ... the only way to make power up till 15,500 rpm was to use the smaller MAJ.... everything else had to be redone around this..... Bleed type needle jets are more difficult to work with in this regard, but needle dimension changes are less sensitive...

Ivan

[Ivan]

I just recieved a pair of 32mm carbs that I bought on ebay... they are the 69-72 carbs.

I also just took measurements on the 5FP8 needle and compared it to the measurements of the 5FP17

The '17 is a moderatly richer needle.. it starts it's taper 2 clip positions sooner than the 8's does and I took measurements at 3 different points just to get a rough idea of how much steeper the tapers are on the '17....

At near full throttle the '17 is .006" smaller than the '8.
(This is a pretty large difference)

At appx 2/3 throttle the 17' is .001" smaller than the '8

At appx 1/8th throttle the '17 is .003" smaller than the '8
(this is huge)

I know that they both cannot run in the same position.... so moving the '17 to match the same point of origin of taper as the '8 in order to get the steady throttle cruise and transition off the pilot correct will make the differences between the 2 even more marked.

This would make sense as there is much less fuel available when a 97.5 main is used rather than a # 150 or so.

So getting closer to the bottom of things here....

The question of why does the 73 and up use the small mains.... it's one of 2 things... either the type of bowl venting really does have a large effect on the main jet... or does the 73 and up use a main air jet?

This would make a huge difference as to the main jet size for sure because the 69-72 models don't have a jet installed.... it's just open... (necessitating the need for a large main jet)

I'm going to look for a set of 73 and up carbs... I like to rely on my own first hand info.

Getting a little closer...

Happy new year everyone,


Ivan

[tz375]

That will be interesting to see if they have air jets. I know that the 72/3 GT750 carbs do not have a pressed in or threaded air jet. They are just a drilling.

With Primary type needle jets I regard the air jet as fine tuning at the top end of WOT. On Bleed types they tip the whole fuel curve in a similar way that spray tube height does on a Primary needle jet. On our small Honda 4 stroke race bike, I have used different needle jet heights as well as different air jets to get the fuel slope closer to acceptable.

Interestingly enough, LEDAR used to supply needle jets for RD twins with a taller spray bar than stock 175 series and the last RD400s came with #284 needle jets which also had a taller spray bar.

[Ivan]

Air jets.........

My 305's have open air jets (#170 hex main external vent),
So does my TC200 (#140 hex main, H/P vent)
the 34's on my T500 are open... (#410 hex H/P vent)

I'm not sure about my GT380...#80 (round) main

All my Kawasaki triples 750 (102.5 main), 500 (92.5 main), 400 (#85) have them (#50 air jet)
Interestingly, the H1 has internally (H/P) vented bowls and the main air jets.... and uses small main jets

My X6 hustler (#95 hex main, external vent) has them (#50)

My 66 Yamaha YDS3 (#120 hex, external vent) has #50's also

My guess (if I had to.... ) would be that the 73-77 T500 has main air jets or they are blocked/closed. This is based on the H1 having both internal vented bowls and main air jets.... and it uses small main jets.


Ivan