T500 inlet length

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A.Holley
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T500 inlet length

Post by A.Holley »

Was just wondering what the difference is in main jets with a 32 mm carb on the T500 when you have the short inlet setup and the later long one. Does the long inlet use larger or smaller?
For reference I have always used the early short inlet. Currently using round 125's myself. Along with mild port job, chambers and foam Uni filters . Timing is set to 20 degrees.
rngdng
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Re: T500 inlet length

Post by rngdng »

The long intake uses smaller jets. Changing a GT500 (with long intakes) to the shorter ones resulted in fried pistons in my case from the mains being too lean. I suggest being very careful with your jetting if you are changing intake lengths. Better yet, don't change them.....


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ConnerVT
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Re: T500 inlet length

Post by ConnerVT »

Sorry rngdng, but no. It depends on a number of things. Your answer is partially correct, but could be dangerous to others who read it.

Suzuki changed more than the intake runner length and the Main Jets between 1972 and 1973. They changed the way the fuel bowl was vented - from inside the carb ("Homopressure") to external of the carb. They changed the Jet Needle - slightly richer. They finally decidied to stay at P-4 for both Needle Jets - after bouncing from P-5 to P-4 every year. They changed the airbox (paper to foam). And, as I understand, the cylinder casting is different, which may change things.

It all depends on what you start with, what parts you are using, and what you are trying to end up with. Each component you use will make the jetting requirement either richer or leaner.

On my bike (a '73 engine with '71 components from the intake boots back) I went through a lot of brass to find what worked best. I ended up with 152.5 MJ (think more due to ethanol gas), 30 PJ, 5FP17-2.25 (thin shimmed at the 2nd clip, really wish I could find a new 5FP8 needle, as it is a bit thicker, thus leaner). Idle adjusted close to 1.0 turn. I also have the timing retarded a couple degrees, due to pinging I would get under load at around 4500 RPM. Going richer on the MJ or needle wouldn't resolve that, only makes the bike run like a pig.

The early carb MJ is 150. The later (externally vented) carb MJ is 97.5. It doesn't surprise me that the shorter intake boots may require a larger jet (which may have caused your meltdown). Eleven steps larger (125)? I wouldn't of thought that much bigger. To me, the starting point (the "Big Knob") is the carb body, internally or externally vented, to determine where the MJ should be. Start a few sizes richer, and work your way down from there.
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Re: T500 inlet length

Post by sbaugz »

Interested to see where this goes. On my build, I have a 1975 T500 with UNI filters and Higgspeed chambers. I ended up buying the short intakes for one reason only- my UNI filters wouldn't fit because of the cross bracing I added to the frame. The short intakes allowed me to use them without problem. As far as jetting, I am going to start big at 120 mains and go down from there.
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Re: T500 inlet length

Post by Vintageman »

My 75 T500 was tuned for jetting all stock parts, but Jemco chambers. This took a lot of brass too but, happy with each Mikuni carb fuel circuit. Mainly was just main jet larger and I forget at the moment how much.

Then one day, waiting at a stop light, a SAAB Turbo was beside me , got the jump on me reaction time and I could not catch it :(

So I purchased everything from early model T500 (air box -> carbs-> cylinders). I compared the early and late cylinders and went with that change only first, everything else still 1975, it ran without a need for jetting changes :shock: Note the T500 jetting is affect if the temp is very different. The Jet needle taper profile is not well centered across the wide range it covers in my opinion

And, with just early jugs on it is much faster :up:

I am very happy with my 75 T500 now. I am not sure if the shorter intake would add anymore significant amount of HP given the RPM range (top end) of the power band. So sorry I can't advice jetting impact.

One thing about my T500 if it isn't jetted well, you listen (ping) and feel how the engine is running at each fuel circuit across wide rpm range (not perrrfect) it lets you know if it is not correct.

Listen and check carefully.

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Re: T500 inlet length

Post by ConnerVT »

Shorter intake probably won't add any additional HP. But it makes for a stronger 'signal" to the carbs - a faster change of conditions at the carb, thus a quicker reaction to those changes. The result is a more responsive feel.

Sorry that I keep posting stuff from Klemm's website, but what I've read from them makes the most sense, and has been what I have been experiencing in my trial and error sessions. http://www.klemmvintage.com/mikuni-tuning.htm
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Re: T500 inlet length

Post by Vintageman »

ConnerVT.

Klemm's website is great :up: . I been here many times.. since I don't retain everything I read

So in theory stronger signal, smaller jet, if carb closer :ssh: But I think that is just mainly during transient (throttle pos changse) once you have a steady volume it is overall flow path steady state resistance of air supply path to fuel supply path that dominates.., A bigger carb for example would result in steady state and transient signal impact.

I may or may not be correct ... but, I did sleep at a Holliday Inn last night

But short path should increase HP if your engine is allowed/could climb up in RPMs. There is a test report I posted sometime ago (here in Tuning and Performance) about tests related to intake track length (piston ported engine too if I recall). A longer track can help low end once you have flow inertia.... But too long path may have too much resistance and that will limit flow for higher RPM need.

The tune the older T500 cylinders give me at ~7000 rpm it is still going strong. I back off f and don't over rev for I put those dang OEM pistons in it and not forged! It has no (turbo :P ) lag to throttle pos changes.. I can discern. But have not tried the shorter intake either
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Re: T500 inlet length

Post by rngdng »

I'm just saying be careful to jet it properly. I screwed myself by not completing the job.



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A.Holley
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Re: T500 inlet length

Post by A.Holley »

Thanks for the replies guys, just got back here from a few days of long night shifts.
I don't actually intend changing inlet length, have always run short ones, right f Om when I bought the bike in Jan 75.
I also run bracing from the swing arm pivot area to the top rail, so could not use long intakes and fit the Uni filters.
I used to run 34's on it when I was racing, and decided to go back to 32's when putting it back on the road, so bought a set of originals off flea bay.
Have heard people mention that the early ones used a "homo pressure " type carbs. How do you tell the difference between them?

Andrew
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ConnerVT
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Re: T500 inlet length

Post by ConnerVT »

One way to tell them apart is to install a leaky petcock and a bad set of float needles. Early homo pressure carbs will fill your crankcase full of gasoline. Later ones will spill gasoline all over your garage floor. Don't ask how I know. :ssh:

The early carbs have a bowl vent in the carb throat. The later ones have a vent outside the carb (typically has a barb on it for an overflow line).
A.Holley
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Re: T500 inlet length

Post by A.Holley »

Ok, that makes sense, mine has an over flow line from each float bowl, so must be the later type.

Andrew
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