Carb Tuning - T500

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ConnerVT
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Carb Tuning - T500

Post by ConnerVT »

I've been making great headway to getting my (mostly) '71 T500 road ready. In fact, she's been on a number of circuits around my neighborhood, and occasionally we have gone out to play in traffic.

The bike is somewhat a bitsa, as the motor that came on it was trashed. So the 1971 T500 has a 1973 motor on it now. So there really isn't a "stock" configuration that's 100% correct. At the moment, it is configured like this on both cylinders:
  • '73 cylinders (smaller/more restrictive porting)
    Early (short) intakes
    Early Air Box and Boot, with NOS filter installed
    145 Main Jet
    P-5 Needle Jet
    5FP8 Jet Needle, in 3rd slot
    2.5 Throttle Slide (cutout towards rear)
    30 Pilot Jet
    New genuine Float Needle/Seat, 27.0 mm Float height
I have been doing a ton of research, and feel confident I'm heading in the right direction. But input from those who have more experience than me (which, for me, is about zero) is greatly appreciated.

The engine started off way too rich. I did have a late model Air Box (it *is* more restrictive, as I found), but with the new early model Air Box, I now have the idle circuit set pretty well. Screws are out ~2.5 turns, but have 27.5 Pilot Jets on the way to try.

The bike starts on 1 or 2 kicks, and now idles well, with minimal smoke. Where it lacks at the moment is at small throttle openings. Twist the throttle 3/4 to WOT, and she pulls and sounds great. But at a civilized throttle (pulling from a stop, or just holding a cruising speed at 2500/3000 RPM) it sounds/feels very heavy.

Checking the spark plugs after riding a bit, it is obvious that it is still running rich. My gut feeling is the next step should be dropping the Valve Needle down a notch to see if that helps. I also have a pair of both P-4 Needle Jets and 5FP17 Jet Needles, but I figure baby steps, right?

All input is welcomed.
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ConnerVT
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Re: Carb Tuning - T500

Post by ConnerVT »

I dropped both Valve Needles down one notch this morning, to the 2nd notch. What a significant difference! Hard to believe that such a small change in length would have that much effect.

I took the bike down the road, where I could open it up some, and also have some places where I could hold speed at a steady RPM. She certainly seemed much happier than before, especially at a steady RPM. Still runs a bit 'heavy', but it is pulling much stronger than it was at the 3rd notch of the needle.

Waiting for the bike too cool down some, before checking the spark plugs. They have been looking both very rich and a bit oil fouled. I know the rich pilot circuit isn't helping with this, so I may want to wait until the smaller Pilot Jets get here before moving any leaner.
parksie
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Suzuki 2-Strokes: Suzuki t500 69,71,73, Gt500 76
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Re: Carb Tuning - T500

Post by parksie »

On post 72 T500s the main jet was 97.5 with size 30 pilot jets.
1969 T500
1973 T500
1971 T500 (being restored)
1976 GT500 (being restored)
2012 Ninja 650
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ConnerVT
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Re: Carb Tuning - T500

Post by ConnerVT »

thanks, I am aware of this. From 1973 on, there were also longer intakes between the cylinders and carbs (48mm longer), as well as a different air box, which I found the other day is a bit more restrictive than the early, paper element one. There is also a design difference between the early and late boot from the air box to the carb (late model has a longer length tube, internal to the boot).

Along with the Main Jet, later models used a P-4 Needle Jet (leaner) as well as the 5FP17 Jet Needle (suggested as having the same tapers, but 2nd taper starting at a different part of the needle). Both the early 1971 150 Main Jet and the 1973 97.5 main jet are large round type. Both use the 30 Pilot jet.

Given all of the differences, I've limited the difference in my 1971 bike to only be the 1973 engine -- all other of the above differences have been eliminated. The unknown is how much less flow the difference in porting between the 1971 and 1973 engines results in.

Needing a starting point, I went with the 1971 specs, which I was sure would be too rich. I didn't think I would have an issue with the Pilot Jet tuning, but it is still definitely too rich (2.5+ turns to even notice a difference).

WOT improved at 145 Main jet, and overall response has improved by moving the Jet needle down to the 2nd notch, but all is still a bit rich and heavy. I should be receiving the 27.5 Pilot Jets tomorrow, and want to put them in (and get the idle circuit right) before addressing the rest of the issues.
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Re: Carb Tuning - T500

Post by NY500 »

I am going threw the same problem with my 70 t500.I put on later jugs and it changed my jetting from 145 mains down to 105.I am also waiting on 27.5 pilots to see if I can lean it out some.
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ConnerVT
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Re: Carb Tuning - T500

Post by ConnerVT »

If the correct size for the Main Jet is down to near 105, that would show that the later cylinders were the major contributor to Suzuki's late model detuning, rather than the increase in intake length. I wouldn't of thought that it would be that severe a change, and makes me nervous moving that lean.

Looking at the carb specs for the 1970 T500, I see that the Needle Jets call for P-4 for both carbs. Is this what you are currently running? I have P-5 in both, per the 1971 specs.

The Main Jet shouldn't have much of an effect under 3/4 throttle. I haven't done many WOT tests as of yet, as the motor just came back to life a short time ago, and I have been trying to sort out the idle and middle throttle settings. But on the couple of 3/4-WOT runs I have done, I can tell it is rich (both from the way the engine runs, and the trail of smoke behind me).

For the mid-throttle, dropping the needle one notch (now 2nd slot) helped a lot. What Needle Jet/Jet Needle/notch are you running right now?

The 27.5 Pilot Jets arrived late yesterday, so I'll be putting them in this morning. I'll post what changes they make. It is a bit stormy today, so I'll be lucky if I get to do more than just set the idle before needing to go back to work for the next few days.
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Re: Carb Tuning - T500

Post by NY500 »

P4 on my needle and my clips are in the middle position I have good idle and great response from 2000 rpm up below that way to rich
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Re: Carb Tuning - T500

Post by NY500 »

Im trying to get it running as good as possible without moving the clip then try and fine tune with the needle
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Re: Carb Tuning - T500

Post by NY500 »

I think the problem is in my air intakes. The old airbox,filter,and runners with newer jugs I can hear the restriction
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ConnerVT
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Re: Carb Tuning - T500

Post by ConnerVT »

I'll assume that you have a 2.5 throttle slide.

Not much between idle and 2000 RPM. I had a tough time setting the idle -- needing to really crank up the throttle speed screw. I figure this is because the Pilot Jet is too big, and needs the throttle open enough to get more air to idle correctly. Dropping the Jet Needle one notch seemed to help some with this, but I know the idle is still not tuned correct. Hope the 27.5 Pilot Jet will help. It is so rich at idle that it will foul the plugs while warming up at idle in the driveway.

I have a pair of P-4 Needle Jets, as well as brand new 5PF17 Jet Needles. I try to only change one thing at a time, but I may make a wholesale change and swap those in when I have the carbs off today.
I think the problem is in my air intakes. The old airbox,filter,and runners with newer jugs I can hear the restriction
I just swapped out the late model air box for the early one. I do believe the late model may be a bit more restrictive, but not really by a great amount. The late model box needs to be modified (top mounting bracket cut and drilled) to make it fit with the early intakes, and the early frame doesn't have the mounting points to change everything over to the longer, late model intake setup.
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Re: Carb Tuning - T500

Post by NY500 »

Im hoping to get my new pilots today if they work great if not I might just go with unit filters and start the process all over again.
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Re: Carb Tuning - T500

Post by NY500 »

Im also running chambers with mild porting to jugs
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Re: Carb Tuning - T500

Post by ConnerVT »

I would think with chambers, your engine should be 'breathing' more than mine at (nearly) stock. Can't imagine why such a small (105) Main Jet, unless the cylinders themselves are so different from the 1970/71 cylinders.

There was another thread running recently - 5FP8 needles. It was a discussion about 5FP8 vs. 5FP17 Jet Needles. To quote some of the interesting posts:
ConnerVT wrote:The T500 switched from 5FP8 to 5FP17 in 1973 (with the extended intakes). To make it even more interesting, they changed the Needle Valve on the left carb in '72:
1970 -- Left/Right = P-4 / 5FP8 (Added this for clarity, not part of other thread)
1971 -- Left/Right = P-5 / 5FP8
1972 -- Right = P-5 / 5FP8 Left = P-4 / 5FP8
1973 -- Left/Right = P-4 / 5FP17
tz375 wrote:I don't have all the details for every Mikuni needle, and the old copies I have of needle sizes are really hard to read, but in general a needle of the same letter sequence has the same taper(s) but the tapers start at different points.

In your example of the 5FP8 and 5FP17 the two are identical tapers but the tapers start at different points and that makes one slightly richer than the other. I don't have those charts with me, so I can't check which is richer. With a smaller needle jet it would be possible to move the taper (equivalent to lifting or lowering the needle) and that way it could be made slightly richer at low throttle openings and have it increase fueling at the same rate as the other needle. A larger jet would make it richer all the way through but proportionately more at low openings and a different taper would change how fast it got richer as the throttle is opened.
tz375 wrote:The main jet discharges through the needle jet and with two stroke (primary) type needle jet, air is mixed with the fuel as it discharges. The needle and jet combo restrict the amount of fuel that can be discharged by the main jet until they are about 3/4 open and after that, the annular area inside the jet around the needle is greater than the main jet, so they no longer act as a control mechanism.
tz375 wrote:Are the later FP17 needles still available by any chance? Match them to a pair of new P-4 needle jets
Between the feedback from tz375, and what I am reading here, the P-4/5FP17 combination might work better with the 1973 cylinders. Suzuki switched from the 5FP8 to the 5FP17 at the time of the cylinder change, and given the difficulty we've had with over rich mixtures at lower RPM, the difference (so far unknown) in the taper between these two needles could resolve some of the problem.

I think I will configure my carbs to P-4/5FP17/145/27.5 today, and see what I end up with.
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ConnerVT
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Re: Carb Tuning - T500

Post by ConnerVT »

To follow up, I did configure both carbs to P-4/5FP17/145/27.5

The idle has improved tremendously. Idle air screws are now best at 7/8 turn on the left, and 1 1/8 turn on the right (the right carb always wanted a little leaner). At idle, the motor is not missing as much, and smokes less from the exhaust.

Takeoff is nice and smooth, runs well up to 3500 RPM or so, and feels well behaved and happy riding around the neighborhood. Holds a steady RPM under 3500 without complaining. Very enjoyable.

Opening the throttle more than 1/2 is where things break down. It fights to pick up revs, and just doesn't work there. Didn't have enough time to figure it out today, as I needed to get the bike back home, for thunderstorms were minutes behind me.

@NY500 -- How did you come to choose the 105 Main Jet as being right for your setup?
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Re: Carb Tuning - T500

Post by parksie »

The BIGGEST change in the later cylinders was the intake port which was much MORE RESTRICTIVEt han the previous models, hence the 97.5 main jets.
I would say that Ny500 has gone up from 97.5 to 105 is because of his chambers which usually require a richer mixture.
1969 T500
1973 T500
1971 T500 (being restored)
1976 GT500 (being restored)
2012 Ninja 650
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