Only idle at 2000rpm (550)

[Vintageman]

Did you find all the pilot air circuits and fuel circuits were open in carb bodies... nothing plugged? Pilot jets themselves... did they pick up a dirt spec... via attempting to look thru it with a good light (and eye) ?. Dang, I get that every now and again... doesn't take much to partly plug the pilot jet supply hole


Sometimes I have found if you have to turn Idle up (lifting slides up) to 2K to idle you are starting to run on Needle Jet circuit... pilot circuit plugged... more likely than factory pilot jet off by 10% maybe

Let us know when you know

[Allan k]

Has taken all 3 carbs 100% apart. Nothing looks clogged.. With a flashlight i have looked in all holes and no dirt or plugged holes.. But this is the first time i have taken the jet needle ( big brass tube)out of the carbs, and here there was dirt.

Hopefully a great cleaning, new rubber boots, corrected float height. New 105 main jets and new 32,5 pilot jet and rebuild kit and a careful syncronize cure the idle problem.

Will get back

P.s i asked the guys at the mikuni spares center about that idle problem.. They said it is not normal to use a bigger pilot jet to get a lower idle.. But will try this anyway.

W

[tz375]

When teh three carbs are stripped down, grab a can of carb cleaner or WD40 and spray it through the pilot jet air holes and fuel feed holes on all 3 carbs and compare the results. All 3 should spray out of the tiny discharge hole(s) at more or less the same rate.

The hard thing to clean is the tiny hole(s) above the pilot jet where it discharges at the front edge of the slide. Those holes are often clogged.

[GT750Battleship]

+ 1 on TZ's post,everything has to be CLEAN !! my GT750A has never run better after rebuilding & cleaning the carbs 2 years ago....no more surging problems idles happily at a steady 1,000rpm...run her on Premium 98 fuel...
Cheers.
Roger.

[Allan k]

Can you show a picture of where exacly that hole is? Just to make shure..

[yeadon_m]

On the later model 550 linked carbs there are two holes in the engine end of each carb. One is diagonally facing and I believe is called pilot bypass. I measured it as having a bore of just 0.5mm. You can't miss seeing this one.
There is another, hidden under the throttle slides so lift them up to see it. It's vertical so faces upwards into the engine facing side of the carb. That is bigger with a diameter of approx 1.3mm.

To check them, hold the throttles open a bit with a wedge in the bell crank. Removed float bowls and unscrew the small brass pilot jets.

You can gently probe up from the pilot jet side to ensure the inner outlet is clear. The diagonal one can be gently probed from the open end of the carb. Visualise a probe with a torch up the pilot jet hole.

Once cleaned, do as suggested, spray carb cleaner into the small air intake where the pilot air screw sits on the carb air intake side. The spray should spray out of both outlets you've just cleaned.

Finally, synchronise the carb slides so that when closed there is exactly 0.8mm between the bottom and the carb mouth.

I did this today and my 550B will now idle stably as slow as 1000rpm.

Good luck!
Mike

[Vintageman]

yeadon_m

Great job with pics. Perfect. Nice effort

I would only add - remove pilot air screw (can see it is in your pic I think for example ...shinny too). Once out check this too for wear at meeting end/tip.

Allan K

I think most of us feel it is most likely a blocked pilot circuit somewhere (next is air leak)
I disagree if I dare with the advice you got that if it won't idle slow you need bigger pilot jet,
I don't think it would hurt too much if you go next size up on pilot as you said, but if 30 was OK you may be a little rich.

Also changing too many things at once can just cause new issues .. if more guessing than can tell for sure it is a potential problem.

Be careful of the "carb kit" too. Pilot jet and Main jet for price use real Mikuni parts and throw kit ones away The float needle set is OK usually if yours is bad (if original part just may be!). But, I think they use a metal washer which is thicker than oem non metal washer. I still use OEM washer ... either way need to check float height again.

Also if your jet needle is not worn I would not use one from the kit, If you do use it, compare it to the OEM and make sure same shape. If Keyster it should have Yxx (e.g. (Y21) number for example on it and some web sites will say what Mikuni pat it is suppose to be equivalent too. The two version of GT550 early and late used different needle I think…. Which do you get in kit

One last thing I so strongly believe... Once you have the Needle Jet out look at Needle end of it, it should be perfect shape not worn, Also (here is what I believe strongly and rarely here others say). Look a ways inside the needle jet barrel with light at the other end. Use magnifying glass : it'sthat critical, It should be smooth not rough or pitted at all. Shinny is best! This has nothing to do with you idle problem, but your bike would run so much better on street if that is a problem . But here's the kicker I don’t know for that bike, if can you get a new one

What series is that needle jet...anyone know? The early GT550 is not available

[yeadon_m]

My 76 A and 77 B UK 550s (Nikasil cylinders) both had 172 P-4 needle jets.

Selfishlessly, but reasonably (since I paid for both bikes), I combined the best set of three to put in the B resto bike!

Poor pic, but left NJ shows pitting and the right was very crisp, in the centre hole, where the jet needle slides.

Cheers,
Mike

[Allan k]

Both my engines are nikasil type and needle jets are also the same 172 p-4. One needle jet had a bit of " rust" on it.

[Vintageman]

yeoadon_m and allan_k.


Perfect and thank you

Also, did you guys ever look into the barrel as well and see if smooth inside? I find the top can be OK but, inside very pitted. The metering is done by the inside barrel ( just first part of it) with the needle jet so critical that surface is nice
I know you can't take picture of the inside easily. But, if you hold to light your eye can see. One day I'll send one that looks ok on top but, bad inside to a machinist to do a cut away so a picture can be taken of half of the inside .

Allan_k will yours clean up ok?

The 172 series is obsolete. It is supposed to be replaced the 169 series needle jet but, in fact (discussed here before and elsewhere like Kawasaki triple etc.) it is not the same and the shoulder is about 1mm shorter making it set lower in carb (problem(s)?).

I tried shimming them (can be done at top or bottom same series in GT250, but was having other problems and was not sure if this or something elseI got lucky and found new 172s so dropped that concern from my mind

Importantly, one thing I recall was when you lifted the 169 up the 1 mm the air inlet hole in it also lifted, That hole is supposed to align with air passage from the bell mouth.
I thought when I looked into it air passage it made the hole in the 169 needle jet partially block but, not 100% sure...was hard to see. Also if you look at 169 and 172 the shape of the air passage was a tad different. I suppose as long as bigger than air jet passage no issue there. But if blocked if you lift 1mm not good I think.
Just be aware that if it gets lifted it lifts that hole too and verify it still lines up and not blocked with air passage.

If so maybe lift only half way and adjust clip in needle the other…etc.

I'll try again later this winter looking at a 169 vs. 172 for I am putting a T350 engine in that GT250A