Needle Jet for GT550

[Vintageman]

Blky2k,

Thanks for the offer, but I just got a spare set. I'll have an late model set and early model set so I can experiment.

Tz375,

I did think of having needle turned as well. I'll have 6 spares so maybe an option.

I think the NJ would have to be reamed for a better finish versus drilled (?). I have looked briefly but see no one on my edge of Earth that sells hand reamers this size (saw some in UK!).

The other idea is to buy 182s and cut off the end and retap for hex or round MJ. Even if I take off the bottom shoulder a keying slot completelyI think it would work.

Let say no issue here

What do you think is a good NJ size.

I think it is the 0-8. Why I don't think the Suzuki parts list I showed early on is wrong here?
Next I would use 1974 gt550 carb specs

25 Pilot
3 clip (not 4th as stated)
NJ = 0-8.

How much more flow would 0-8 versus 0-5 again please ?

It may be a while before I reply again, but I will.

Thanks

[tz375]

Changing from O-5 (2.625mm) to O-8 (2.640mm) is an increase of 14% with a stock needle with 2.515mm root diameter.

If the needle has a larger root diameter, the percentage change is larger because the starting annular area is smaller. At the tip of the needle, the change is only 4%

Of course fuel flow doesn't necessarily rise by that much, but it's a good indication of how much difference a small change can make.

[H2RICK]

If the needle has a larger root diameter, the percentage change is larger because the starting annular area is smaller. At the tip of the needle, the change is only 4%

My old and feeble mind made me read that TWICE before I figured it out, and even then I had to perform some mental imagery for the final interpretation, TZ. Mathematics was never my strong suit........but it's obviously faaarrrr less of a problem for you.

[tz375]

Thanks Rick

The math is simple - that's what spreadsheets are for It took a little effort to set up but it makes it really simple to compare needles or jets - in theory at least. The numbers seem to correlate reasonably well to the charts Mikuni publish.

[tz375]

Crazy thought but did you try making the idle slightly richer? ie rich enough to help out as you crack the throttle but not so rich that it blubbers and dies at idle

[Vintageman]

I think I can call this one closed, with respect to pinging.


I got those used carbs and took out the 192-O5 Needle Jets. I got access to a 0.104” (2.6416mm) Hand reamer and reamed out the NJs. They should now be about a 192-O9. I went in from the Main Jet end to help align/start the reamer and used little light oil. I did not pull reamer back just went one way only. It went easy. Finish was very nice.

I tore down the carbs on bike and put in these pseudo 192-O9s in place. I also put the Pilot Jet back to 27.5, 1.5 turns on pilot air screw, and needle clip on third position; all stock, but the NJ.

The ping was gone around 1/8 throttle. But, if I closed it even further slight ping. At first this was much better for usually when I back off I don’t fully close. So this was hard to hit normal driving (IMO). I suspected now the issue was the Pilot Jet too lean.

I turned the Pilot air screw in ¼ turn for all three carbs at a time and, after third try (now ¾ turn out no ping. I kept doing accelerations with throttle just barely opened and on flats letting the engine approach high RPMs. I settled on just a smidge above ½ turn out on air screw. It was cool that day too (65F/18C). I suppose a 30 PJ would have been better.

I repeated the acceleration tests at just open, ~ 1/8 and ~ ¼ throttles no pinging! I went for a 30 mile ride or so and bike ran well. I even put the timing back to stock (I had backed off a bit on advance) and no issue.

I have not done miles per gallon test yet. I was getting low 30s before. I was hoping for upper 30s. Not sure what bike is supposed to get? I could try the needle at 2nd clip from top. I never had an issue above ¼ throttle and maybe I have room here too go leaner and thus better gas mileage. We’ll see.

It looks like I had two lean regions caused by PJ and NJ. I never tried bike with stock pipes and went straight to chambers. I am guessing the bike is bit lean in those areas and the chambers, when on the pipe, made things worse.

I am very happy . I can’t make my GT550 Ping! After about 20 miles of that ride I stopped trying , relaxed and enjoyed the torque of that old GT550!


Thanks Everyone

[H2RICK]

OK !!! Way to go, vintageman !!! I'm glad to hear your "bore job" of the needle jets solved your problem.
It'll be interesting to hear how your mileage ends up after some more fettling.
My totally stock 550A is NOT great on gas.....even with 2 sizes smaller main jets to cater for the altitude here (3500' ASL) and stock gearing.
Consumption with moderate highway riding (~60 MPH) is ~90 miles to reserve, whatever that works out to. I can't remember the tank capacity with/without reserve.

[Vintageman]

I got 33.5 miles to an USA gallon. About same as before. I did not drive easy all the time though. Maybe if I went easy all the time I would get a few more?

What do you guys get. Any period articles on this for the gt550?

[yeadon_m]

I'm going to start a new thread on GT550 and pinging, if thats what I have, and replying to this thread may make comparisons easier. Interesting to read!
Mike