5FP8 needles

[joewillybob]

Anyone have a source for 5FP8 needles? Trying to sort out carb issues on my T500R, thinking my needles are worn. 27 thousand miles on it. Can feel slight wear on the original needles. Motorcyclecarbs.com seems to be out of business. No reply to e-mails, phone disconnected. Sudco sti ll lists the part as 002-292, but, no one has them. Is there a different needle # that would work?

[tz375]

That's an unusual needle profile, according to the old Mikuni charts. It has two angles - first is 1 degree 30 minutes and starts at close to 30mm and the second (P) is 4 degrees starts at around 42mm and then tapers to a very small (rich) tip. That's if I interpreted the old copy of an even older chart correctly.

There are not a lot of needles close to that shape, 5DP7 looks like it might be similar at the tip but its single taper angle, so it may be wrong at mid throttle. 5F12 has a more aggressive single angle and even smaller diameter tip

5L1 and 5J9 have similar tips and steeper tapers and were OEM on RD400 models. Sucdco are the best guys to tell you want's close.

[ConnerVT]

I would be interested on what info you could share regarding 5FP8 vs. 5FP17 Valve needles.

The T500 switched from 5FP8 to 5FP17 in 1973 (with the extended intakes). To make it even more interesting, they changed the Needle Valve on the left carb in '72:

1971 -- Left/Right = P-5 / 5FP8
1972 -- Right = = P-5 / 5FP8 Left = = P-4 / 5FP8
1973 -- Left/Right = P-4 / 5FP17

Understanding the difference between these two needles will help me sort out the project I'm working on now.

[tz375]

I don't have all the details for every Mikuni needle, and the old copies I have of needle sizes are really hard to read, but in general a needle of the same letter sequence has the same taper(s) but the tapers start at different points.

In your example of the 5FP8 and 5FP17 the two are identical tapers but the tapers start at different points and that makes one slightly richer than the other. I don't have those charts with me, so I can't check which is richer. With a smaller needle jet it would be possible to move the taper (equivalent to lifting or lowering the needle) and that way it could be made slightly richer at low throttle openings and have it increase fueling at the same rate as the other needle. A larger jet would make it richer all the way through but proportionately more at low openings and a different taper would change how fast it got richer as the throttle is opened.

We get used to changing main jets and pretty much ignoring needles and needle jets but Del Orto carbs on Aprilia racers, most of teh tuning is done with needles and needle jets and very little is done with main jets.

[r3tro74]

In your example of the 5FP8 and 5FP17 the two are identical tapers but the tapers start at different points and that makes one slightly richer than the other. I don't have those charts with me, so I can't check which is richer. With a smaller needle jet it would be possible to move the taper (equivalent to lifting or lowering the needle) and that way it could be made slightly richer at low throttle openings and have it increase fueling at the same rate as the other needle. A larger jet would make it richer all the way through but proportionately more at low openings and a different taper would change how fast it got richer as the throttle is opened.

We get used to changing main jets and pretty much ignoring needles and needle jets but Del Orto carbs on Aprilia racers, most of teh tuning is done with needles and needle jets and very little is done with main jets.

That is a great explination of how that all works- On the Mikuni, does the main jet size affect the amount of fuel available to/through the needle jet?

[tz375]

Yes. The main jet discharges through the needle jet and with two stroke (primary) type needle jet, air is mixed with the fuel as it discharges. The needle and jet combo restrict the amount of fuel that can be discharged by the main jet until they are about 3/4 open and after that, the annular area inside the jet around the needle is greater than the main jet, so they no longer act as a control mechanism.

[joewillybob]

My 71 T500 has the P4/P5 needle jets. Can't say for sure if the DPO changed them or not. Seems unlikely, but, you never know. I think that Suzuki kind of used whatever they had on hand when they built these bikes. My T20 and my T350 both have some parts that appear to be from a different year.

[ConnerVT]

After 40 years, it is hard to know for certain what previous owners (and various MC mechanics) may have done on a bike. I based my previous post on the Suzuki Service Bulletin Specification-6, published July 1975, found on Ozebook.com:

http://www.ozebook.com/compendium/techb ... %206-9.pdf" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

tz375, when you get the chance, it would be fantastic if you would follow up on the characteristics of these two needles.

[tz375]

I'll have to dig out the hard copy print - the scan is almost illegible.

Are the later FP17 needles still available by any chance? Match them to a pair of new P-4 needle jets

[tz375]

I found the scan of the FP8 but no info on the FB17 to compare it to. If someone has those needles I'd be happy to compare them.