GT750 carbs on a GT550

[tz375]

The relationship between carb cross sectional area and port area is an important part of the equation, but not as important as teh size of carb that the motor needs for its size and power.

Do you have a picture of a 164 series jet and the rest of the dimensions:
- Overall Length in MM if your calipers/micrometer is metric or inches if not.
- Height of the spray tube
- diameter at the bottom
- diameter of the main tube
- diameter of the flange
- flange thickness (you mentioned 4.0mm - most are 1.5, 2.0 or 2.5mm)
- Diameter of the air bleed hole

[Vintageman]

Tz375.

By flange is that what I called shoulder? You know the part that is thinner of the 169 series versus 172 even thought it is suppose to be the equivalent and is not .

That shoulder measured 3.5mm not 4.0 mm. That was very different (compared to 159,169 188, 192,172 series I had as was far thickest.

The spray tube height is the same as 159 series for example. just add this to the length I gave for ~40mm

I'll take a pictures and all the dimension you ask. I suppose there is also the notch length too.

It may not be for a day or two for have to work late tonight.

The relationship between carb cross sectional area and port area is an important part of the equation, but not as important as teh size of carb that the motor needs for its size and power.

Yes, but think my hypothesis of why a 26mm may work well with the GT380 is the windows area is not the limiting factor. Could be on a GT550 trying a 32mm.

Why even the Kawasaki S2 ran 24 mm and that bike sure worked well as far as peek power. The S3 went with 26mm Not sure whatthese had for port area

The only thing I don't know is if the GT380 with its spigot mount versus GT250 flange mount has the same initial inlet size to accommodate 26mm carb without limiting?

Wish if someone had one loose could check. Maybe pull out of this post and start another

Thanks
Mike

[yeadon_m]

Don't know the answer on the 380 inlet port but measured the 550 inlet port/ stub - its a tad under 30mm, certainly bigger than 28mm nominal.

[tz375]

It's not the limiting factor on a 550. There's simply enough intake port/carb area to make the power it makes and partly because as a long stroke motor, it creates relatively long, strong intake waves.

But bottom line is that there's only one way to test a theory and that's to try it. Run a bike on the dyno and then add bigger carbs are run another test back to back with no other changes and see what the dyno says. If you don't have access to a dyno, do timed roll on tests. Run it in any gear and accelerate from say 30-50-80 and then make the change and repeat the test in the same gear.

Try it at part throttle or full throttle and if you use a GoPro watching the speedo, you can simply compare one test run with another to see the differences.

Or do a filmed run through the gears from zero to say 70mph as hard as it will run with both sets of carbs and compare thr results.

[Vintageman]

Don't know the answer on the 380 inlet port but measured the 550 inlet port/ stub - its a tad under 30mm, certainly bigger than 28mm nominal.

Hmmm, my 72/73 was less than 28mm vertical and just 28mm horizontal.

I did not count the chamfer. I went in with my calipers about 1/4 inch and found the min. Be honest I only checked one. I still have two untouched and the other I have opened up nicely IMO.

The problem with divider is it added more flow resistance via two more side than without and port window area is a good match for 28. at 20% more. But Yes try gt750 carbs if you have... I don't the CV one are as responsive as VM, but bet they will help manage if too much throttle via vacuum .

If you had cyls off easy to widen inlet and window. I am staying with 28mm for now for I am making too many changes at once. If you try do tell. I have spare GT550 carbs so I may just bore them out a little some day.

I'll try bigger carbs on GT380 based on your findings and my Gt250 comparison thinking.

[tz375]

Interesting that the 164 series has such a thick shoulder/flange/spigot. Others range from 1.5 to 2.5mm and that suggests to me that the carb is a relatively large body with small bore - therefore the wall thickness is high compared to say a VM34.

If those GT380 carbs were bored from 24mm to say 26mm and it were possible to remove metal off the floor as well as the roof, you could slide in a pair of 182s or if it went up to 28mm a pair of 175s might work. usually that's not possible because of the way the slide fits and because of the way the pilot jet outlets are machined, but it may be possible.

If you think of a 550 as similar to an RD400, that motor runs with 28mm carbs and anything less than full house race tune works best with stock carbs or 29.5mm offset bored carbs. See AG Bell for details of his tests.

[Vintageman]

I think the 159 series is the closest fit. The main jet and washer would hold it up and maybe enough shoulder to seal or add shim. The issue is the air hole location. It would be too high and may not align with air passage. This is problem I had shimming the 169 series to replace 172 in Later Gt250.

If you elongated the air hole maybe? Or bore carb (gulp) .

I don't shown notch slot, but that may need to be lengthened as well (dremmel would do the trick precise enough).

Also the barrel the Needle rides in all start in the same spot relative to top of shoulder. So if shoulder too high or low that would change needle position. Maybe shim up or down to compensate.

I think between my GT/T250 carbs and GT380 I have I am OK finding 3 more good 164.

Somewhere I have Yamaha DS7 and R5 maybe PE175 and Yamaha DT250 ones. One day I cleaned up old pistons, bad cranks bad cyls, mufflers, and bad jets etc. and dung them. Boy I regret that now

Note the short guy far right. This is early GT550. Obsolete too. It looks like 169 if you cut bottom, re-tap, add notch and center with main you may be OK. What else are you going to do. Put up with screwed up Needle Jets. Sure just Idle and straight to WOT

Here are the pics compared to some jets I had loose and similar.

If others have jet series I don't show take some pics. Hard to get those guys to stay still. Tape did the trick


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http://i266.photobucket.com/albums/ii25 ... /013-1.jpg" onclick="window.open(this.href);return false;

[tz375]

Thanks for posting those pictures. My 72 550 had 192 plus a short adapter than made them a little longer and changed from hex to large round jets.

[Vintageman]

Tz375

Can you clarify what you did to the early GT550 carbs to have a working Needle jet?

[Suzukidave]

Thanks for posting those pictures. My 72 550 had 192 plus a short adapter than made them a little longer and changed from hex to large round jets.

Alright a answer to your question about the short GT550 needle jets you had in your carbs

[Vintageman]

I think I understand. You're not taking about a replacement. You are just stating the adapter that screwed into the needle jet that held the round main jet. I have these. Odd not shown in current fiche, just Needle jet, small washer, and the main jet

JetsRUS shows 192 series but start at O-2 then jumps to P-0

Early gt550 was either O-5 or O-8.

Buy O-2 and hand ream to what you need.

Also shown is a 182 (for 26mm carb) that shoulder looks like the 164 series? for later gt380. Need to get one and see

What is the series is the later GT550?

[yeadon_m]

What is the series is the later GT550?

174 (I've O-9 on some 550L NOS carbs, and P-4 on 550A and 550B old carbs)

Mike

[tz375]

182 have 2.5mm thick flange/spigot - the same as a 176/270 and are 1mm taller overall than 169 according to my notes

[Vintageman]

Just got a spare set of 1975 GT380 cyls. The intake spigot casting is ~1.12" wide or 28.4mm! This is NOT counting chamfer to help match boot/carb.

The left and right a little less Vertical ~ 1.09" or 27.7mm The center vertical is also ~1.12" as well (smoother more open initial inlet start then L and R ..Need to check GT550 again to see if center different there too)
The holes aren't all well centered and jagged some but, if rubber boot lines up well then no intake widening is need to take a larger carb 26mm – 28mm

The Exhaust port height is 33mm on all three cycl. My 74 GT250 is 32.5mm I think some years were 32mm before the 29.5mm GT250 A/B etc.

The GT380 Ex port shape is more square (straighter sides) than GT250. The gt250 has those little extra two Ex ports but, the main port area may be a little less than gt380 for near the bottom it has larger radius corners to accommodate the little port either side. Odd design and other mfgs (e.g. Rotax) who did this put the extra ports up high to help blow down but avoid transfer short circuiting? I think the extra ports were marketing gimmick for adds said “new 7 port design” I think I have seen. Drop them I think with Gt250A.

The GT250 Al at bottom of main Ex port has a longer horizontal shelf whereas the GT 380 is more evenly sloped towards outlet.

The intake height and windows sizes are the same GT380-74 GT350 So same area. But, the bridged is a little wider on 380 for it is has a short Aluminum divider. The oil injection brass piece is well centered in front of bridge unlike GT550.

The transfer look the same but the GT 380 seems to have a little more Al right in corner toward intake side right at bottom of exit. No way I can get to that. Not sure if it matters much. I did not compare transfer, but look the same size at least. The SRIS inlet pipe looks large relatively and must affect flow on that side

The cut out in GT380 liner for transfer is couple mm wider, but, both use same piston cut out to match I think. Did not see how the two match yet. The GT250 chamfers liner where it enters transfer, gt380 does not.

I order Crusin Image pistons 1.00 OS. I think I am going to make Ex height 32mm clean up width, 1mm off intake skirt, clean up intake port, shave head and or T250 al. head gasket, bore carbs to 25.5mm ( have later style GT550 carbs, but needle jets not so good), blend casting into transfer, and cut back SRIS pipe. Someday soon that is Lord willing/providing

FYI

[Vintageman]

The Cruzin Image GT380 piston are very nice. They weigh the same. Pin weight the same. Same OEM circlip style, Same key dimension that I can discern.

Also, it has the wider ring locator pin spacing so you can use in GT250 A/B with the extra boost ports in the rear. Those were hard to find


Yeadon_m.

Did you notice how large the GT380 intake spigot dia. is ala my previous reply?


Yeadon_m (on anyone).

Can you measure dimensions of the later GT550 174 series needle jet please (length, shoulder...). Or compare( to a 159 series if you have for example.