Gt550B Carburetor

[Dav57]

Hi guys

When i stripped the carbs on my GT550B to clean and fit a rebuild kit i noticed that one of the needle jet holder tubes was broken as you can see in the photo.
Does anybody know where i can purchase a replacement please and they are stamped with the number 172 on them.
Thanks
Dave

[willowden]

Motocarb , Liverpool.
Google it , guy called Steve , very helpful & has everything in stock for Mikuni.

[Harrymail]

Dave

I had to find 172 Needle Jets last year for my UK GT550A. Unfortunately the 172 Series of needle Jets are discontinued my Mikuni. They have been replaced by the 169 Series, which are the same internal dimensions, but are made to take Hex main jets, which have a slightly different thread.

To make matters more difficult Suzuki used two different sizes of 172 needle jets. As well as having 172 stamped on them, they will have a size, which for a late UK 550 will be P-4. The earlier 550's had P-0 needle jets, as well as all USA bikes which had P-0 right through to the B model. It is the Nikasil engines which have the P-4 needle jets. You must match the correct needle to the needle jet as follows:

P-4 Jet has 5D1 Needle (late Nikasil engines)
P-0 Jet has 5DH21 Needle (early & USA engines)

The needles have very different profiles, as the jets are 4 sizes apart. If you are using an aftermarket kit to refurbish your carbs, the needles are always 5DH21, which are not suitable for UK 550's with Nikasil bores. It is not a good idea to use any of the brass bits in these aftermarket kits as you will be very unlikely to have the correct sizes of needles or jets for your specific year and engine.

So the options are either find a second hand one of the exact same size (either P-0 or P-4), or tap the thread of a new 169 series needle jet, or even use the comparable hex jet. I managed after a long wait to get some second hand ones. P-4 size is particularly difficult to find as most are P-0 from USA carbs.

Apologies for the long reply, and hope it makes sense

Paul

[Alan H]

Or PM Baz, he restores and refurbs carbs.

[Dav57]

After reading the posts i phoned Motocarb and had a good chat where he explained that as the 172 P4 is not available anymore he'd spoken with Mikuni in Japan and he bought a batch from them that took 6 months to arrive and are stamped 182 now but all the dimensions are identical so i purchased 3 off him.Thanks for the posts and as with other items i have sourced it wouldn't have been possible without this forum.
Thanks Guys

[teazer]

169 series are similar to 172 but are 1mm longer and take Hex jets. It also has a thinner top flange/spigot, so it will sit lower in the bore by 1mm. That's not enough for most of us to notice in the real world.

182 series appear to be the same dimensions as a 172, but also use large hex jets where the 172 had round main jets. My notes don't indicate if they both share the same size bleed hole, but I suspect that it's large enough not to matter.

182 came stock in some 28mm aftermarket carbs and should work in a later model GT550 with appropriate sized hex jets.

[Harrymail]

After reading the posts i phoned Motocarb and had a good chat where he explained that as the 172 P4 is not available anymore he'd spoken with Mikuni in Japan and he bought a batch from them that took 6 months to arrive and are stamped 182 now but all the dimensions are identical so i purchased 3 off him.Thanks for the posts and as with other items i have sourced it wouldn't have been possible without this forum.
Thanks Guys

I would be interested to know if your 182 jets take the round main jets or the hex. Using Hex main jets is not straight forward as the sizes are not a direct conversion.

All my info comes from Teazer helping me in the past when I had this issue

Paul

[teazer]

182 series are hex. I just checked a pair that came out of a pair of new aftermarket Mikuni VM26 carbs.

Small round (GT750 L model on BS40) and large round (GT750 J/K VM32) have a different thread and no not screw in to a 182 needle jet.

There's a couple of charts on the web showing equivalent flow rates between round and hex jets. There isn't a 100% correlation, but stock jets have quite a tolerance range, so if in doubt use the next larger sized equivalent large hex jet to be safe.

Hex jets and round jets use a slightly different thread pitch and it's not uncommon to find the wrong jet screwed in extra hard into a needle jet. Because the sizing systems are so different, Mikuni decided to use different threads to avoid fitting the wrong jet but they forgot to make the design idiot proof. Hex is 0.9mm pitch and round are 0.75mm pitch.

Hex jet numbers represent flow rates, so going from say 100 to 200 would be a 100% increase in flow. Round jets numbers represent the orifice diameter, so changing for 100 to 200 doubles the diameter and therefore increase area and flow to 400%.

According to the chart, 100 round equals 115 hex and 120 round equals 170 in a hex jet. and a 170 round equates to a 370 hex.

[Harrymail]

I got my round jets about halfway in before the thread tightened and I realised it wasn't right. Baz eventually got me the correct 172's so all is well now. I'm amazed at your knowledge Teazer. I quite enjoy knowing the fine detail. It helps if things go wrong.

Paul

[Dav57]

My round jets fitted the 182's ok.I've looked at the jets and are marked 2 x 105 and the other one 102.5 looking at older posts on the forum is it the left hand carb as your sitting on the bike the one fitted with the 102.5 jet?