Carb Tuning - T500

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Re: Carb Tuning - T500

Post by Vintageman »

Frank T wrote: Mon Jan 04, 2021 11:37 pm
Do you still think the jetting
difference between early vs late was due to porting changes?
I don't think that is the case. Here is why. Opposite of you, I I run the early cylinders on a later 1975 T500 and runs well. From the cylinder all the way back it is all the later T500 intake stuff. It looks stock and no one can tell early cylinders by viewing my bike. At the time I did the swap, I acquired all the early intake parts, thinking I would need them. I wanted my bike to look stock so I tired the later setup first. The bike ran so well I never tried early stuff to see if more power or not. By well I mean my bike gained more power and felt like I did jump from ~44 hp to ~48hp as Suz claimed the late and early models had. The bike would rev higher and not run out of breath. The last thing a few years later was I widened (only) the exhaust port and somewhat straighten the flow path from Piston to where muffler pipe mounts. That made even more of difference :shock: adding even more top end with no loss elsewhere in power band No change in jetting, all stock sizes, all fresh, all OEM mfg jetting parts. Some are Hard to find now, obsolete but always very important these items are pristine.

When I added expansion chambers , I believe I had to make the main jet a bit richer to cool things down as it was pinging otherwise when on the pipes only. Chambers, when they are boosting, as one hopes, raises compression and a little extra fuel can help cool things down in there. I resist retarding timing for love the mid range torque as I ride streets. I have no problem taking into into red-line and frequently do, and no issue with my pistons upon inspections. I Run 87 octane (91 or 93 did not help much ping) it just needed a little more fuel ( or better head design and liquid cooling :wink: ).

I had fooled with a 1976 GT250A. Stock It has later air box design concept similar to later T500. I put the early GT250 Airbox on it thinking flow would be better and get more power ( already was a little rocket, but you know, never enough). I did not see any difference (waste of time) nor had to changing jetting on the richer 1976 stock jetting, I put the original stuff back on eventually. However, I would later learn that the 250-350 early air box varied with things intake tube diameter, how many it had and how long the tubes are , and other openings etc and tubers in Y boot or not. The later T350 airbox has larger tubes that 250 and T305 air box is is even less restrictive with more tubes and opening. I did not try it on the GT250A for it was now had T350 with mix of T305/350 air box bits (balance of best flow vs how bad my ears hurt from intake howl). The T305 is really loud and see why it was changed at T350.

I never studied the early T500 airbox for such contributing factors to see if changed over time and if restrictive not vs smaller twins. I think there are some simple mods you can do to later T500 air box to improve its flow and look stock outside. My later T500 airbox is stock and is not holding back my bike from reving well into redline ( I have ti watch it and back off for still cast pistons not forged as of yet). Fun bike

Why not just put early cylinders back on and get the ~48HP vs 44hp ?
Current Bikes
74 GT250 (T350 upgrade),
76 GT250 (T350 upgrade),
71 T350,
70 T350,
74 GT380,
75 T500,
73 GT550,
75 GT750,
72 Yamaha DS7 (R5 upgrade),
77 Yamaha RD400 (Daytona Cyls),
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