Wanted Needle Jets for GT550 172 P-4

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Harrymail
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Re: Wanted Needle Jets for GT550 172 P-4

Post by Harrymail »

UPDATE:

Fitted 169 P-4 needle jets from Motocarb. I have been able to use my existing large round main jets. I suspect the threads are different but the difference is only very very slight. They screwed in fine until the last quarter of a turn when a very slight resistance could be felt. Added a dash of oil and they screwed in easily. Being brass there will be a bit of give.

Managed a road test and the difference is very noticeable. Between 1/4 and 1/2 throttle the pickup and acceleration is greatly improved. There is even a power band now. Can pull away in 3rd gear from 1500rpm with no hesitation. No bogging down with a handful of throttle. Its taken some time to get the carbs right but it has been worth it. The bike feels so much more lively off the throttle. I can now forget about about the carbs and get back to putting some miles on the rebuilt engine, fingers crossed that all will be well.

Thanks all for the input on this. I like things to be spot on.

Cheers

Paul
Currently without a GT, but looking
2003 Yamaha Fazer 600 Blue
2020 Triumph Street Twin Red
1968 BSA Bantam Sports Red
barney01
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Re: Wanted Needle Jets for GT550 172 P-4

Post by barney01 »

Great news Paul :D nice to see your persistence paid off! Very good info as well for other 550 riders too.
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stevewharton
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Re: Wanted Needle Jets for GT550 172 P-4

Post by stevewharton »

Well done Paul, great news at last. What a pity you're "double booked" and can't make the rally, because that would've been a really good "road test" for you and the bike, how is the wrist, any better?
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Harrymail
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Re: Wanted Needle Jets for GT550 172 P-4

Post by Harrymail »

stevewharton wrote:Well done Paul, great news at last. What a pity you're "double booked" and can't make the rally, because that would've been a really good "road test" for you and the bike, how is the wrist, any better?
Hi Steve. Wrist is about 50% better. Its just ok for riding the GT550 but no good for my Fazer. Its mainly my palm now that is painful. It was at last years Rally that I was running in my 550 after the first rebuild. It was after the rally that the crank seals went. Its taken a year to get back to where I was. Shame that the change of date for the Rally has gone against me this year. It would have been a good test for the engine.

Paul
Currently without a GT, but looking
2003 Yamaha Fazer 600 Blue
2020 Triumph Street Twin Red
1968 BSA Bantam Sports Red
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yeadon_m
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Re: Wanted Needle Jets for GT550 172 P-4

Post by yeadon_m »

Paul,
Really heartening news, well done.
Could you please summarise what the dimensional change has been? For example, is the clearance between your jet needle & needle jet now bigger or smaller?
Cheers,
Mike
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Harrymail
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Re: Wanted Needle Jets for GT550 172 P-4

Post by Harrymail »

yeadon_m wrote:Paul,
Really heartening news, well done.
Could you please summarise what the dimensional change has been? For example, is the clearance between your jet needle & needle jet now bigger or smaller?
Cheers,
Mike
Hi Mike. What I have been trying to achieve is have the carb exactly how it would have been when it left the factory in 1976. Since the rebuild I could tell that the engine just didnt have the go that it used to. Having checked and checked everything numerous times I realised I hadn't knocked the needle jets out to check them. And there was the problem. Having checked that the main jets, pilots, needles etc were all correct I had omitted to look at the needle jets. I had recently got a book which is very useful, Motorcycle Tuning by John Robinson (Two Stroke version) which was written in the 80's. It has a very good section on Mikuni carburettors and the relationship between the needle and the needle jet. I realised that the taper on the later needles is very different to the early ones. The later needles are "fatter" from the taper to the point. To compensate for this, the needle jet is of a bigger diameter. Hence the change from P-0 needle jets to P-4 for the Nikasil engines. Having an early jet with the later needle reduces the flow through the jet. It does not help that Mikuni needles and needle jets are not available new in either of the sizes, making choices limited.

So, to summarise, I now have a bigger clearance between jet needle and the needle jet. Both are now correct for an A model with Nikasil bores.

Cheers, Paul
Currently without a GT, but looking
2003 Yamaha Fazer 600 Blue
2020 Triumph Street Twin Red
1968 BSA Bantam Sports Red
barney01
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Re: Wanted Needle Jets for GT550 172 P-4

Post by barney01 »

Some good info and measurements here too
(I know its from Kwak site but same carbs)
http://3cyl.com/mraxl/partid/carb/jetid/jetid.htm
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Harrymail
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Re: Wanted Needle Jets for GT550 172 P-4

Post by Harrymail »

barney01 wrote:Some good info and measurements here too
(I know its from Kwak site but same carbs)
http://3cyl.com/mraxl/partid/carb/jetid/jetid.htm
Thanks Barney, confirms the thread / main jet type. Will save that for future reference :)
Currently without a GT, but looking
2003 Yamaha Fazer 600 Blue
2020 Triumph Street Twin Red
1968 BSA Bantam Sports Red
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