GT550 Carb Alternative

[Srxburns]

Hi,

I am looking at a full rebuilding a GT550 that I just pickup for free, got it from a friend that had it for yrs sitting in a shed due to one of the pistons exploding. I may be thinking of doing some head and porting as per some of the threads that I have read + Chambers . For the Carbs I was thinking of using 33mm Mic flat side carbs off a 1999 SRX 600 Yamaha triple sled. they are all connected like the GT550 and I would assume offer better performance. Has anyone tried this or don't think this would be a viable option? you can pick up all 3 for $60-$90 on ebay.

Thanks
James

[tz375]

I think you will find that the stock 28mm carbs are big enough for any sane level of performance. Sled carbs usually take a fair amount of work to get them to work on a bike - starting with the float valves which are designed for a pump.

[Srxburns]

I thought about the pump, but isn't the pump just there because the tank is horizontal to the engine not sitting over top? I was think of modding the engine like Kushners post he used h1 carbs but said it said he could have went bigger. If the sled carbs are to much of a pain then maybe not worth it. Is there any other possible issues?

[Coyote]

I would think if the intake ports matched the late model Yamaha carbs, it would be somewhat of a miracle. Like TZ said, the 28's are plenty enough. Bigger carbs rarely equal better performance.

[tz375]

Correct that the reason for a fuel pump on a sled is to raise the fuel. But because pump pressure is much higher than gravity feed pressure, Mikuni supply sled carbs with 1.5mm float valves. For gravity feed you need 2.5 - 3.5mm float valves. So the float valves have to be changed out and they are not cheap.

The other issue is that the motor doesn't need bigger carbs and as A.G.Bell demonstrated on an RD400, bigger (34mm) carbs did nothing for top end and lost bottom end on that motor. To put it into perspective, an RZ350 makes almost 60 rear wheel HP with a pair of 26mm carbs. If we glued anothewr cylinder oonto that RZ, it would make say 90hp on 26mm carbs.

Why then would we think that a GT550 would need larger carbs?

In Fact when tested in MOTA, larger carbs made only a very small improvement at the very top end and lost much more lower down.

[Srxburns]

Thanks for the info. I was mainly looking at them not for the size increase but ease of finding them and the cost plus they come attached in a rack like the GT550. Are there any other flat side carbs that are a recommended upgrade? or due flat side even make a difference?

Thanks again

[tz375]

I follow the logic - assuming that the distance between the carbs is close enough to make them fit.

Flat slides tend to respond faster that round slides and are arguably easier to jet because the "signal" to the jets is stronger. Tests seem to suggest that they also make a little more power compared to the same size round slide carbs.

If you absolutely had to buy new (replacement) carbs, flatslides would be worth looking at. 34s are very tall and would not be easy to fit, but 28 flats might work nicely or even 30s if they are available and small enough.

[Suzukidave]

Wasnt there a thread on 73 GT750 carbs on a 550 ?

[tz375]

There was, but so far I don't think anyone has tried it or at least they have not shared the results.

[Vintageman]

In the GT750 carbs or GT550...that thread pointed out (by me?) that the intake port area on the GT550 where it enters the cylinder is very small. I forget (would have to go back to that post) but believe it was nearly that of 28mm carb are itself. I don’t see putting a big carb on small intake ports amounting to poop.

Just today I was looking at pictures of H1 cylinder and GT550 cylinder. One main difference I see is the H1 has a HUGE intake port compared to the little twin windowed GT550. H1 is also wide open from carb into liner... very nice me thinks

Jeff Kirschner(sp?) put 30mm carbs from kawa on GT550 (your tube vid ? search porting or tuning gt550). He also said he spend much time opening up intake port.

I am strongly thinking about reworking the intake on a 550 significantly. Take out most of the intake divider Aluminum and open up the liner windows wider. I plan to stay with 28mm at first... maybe try the early GT750 or H1 carbs later for example.

Something is really holding back the GT550... don't thinks its the 28mm carbs

I know GT550 carbs are hard to come by and pricey these days but, think that is your best option

[tz375]

I.............
Something is really holding back the GT550... don't thinks its the 28mm carbs
...

It's not, but the port time area is not very generous