Mikuni VM Jetting
- Thread starter mtrace535
- Start date
mtrace535
Been Around the Block
I have no idea what the carbs are from, i got them free from my dad. Im trying them on a CB360 with pods and a 2 into 1 exhaust on a stock motor. When i try my richest settings with my needle all the way up and 175-p6 needle jet(bleed type) its so lean it wont go past 1/3 throttle. Yet with a primary needle jet 169-04 and a leaner needle all the way down its so rich it sprays gas out the exhaust.
hillbillyengineer
New Member
who are you getting your mikuni parts from? call Sudco they will help with custom jetting if you buy the parts from them.
175 series are bleed as you said, so they flow a mix of fuel and air out of the orifice. the #169 are a primary type and only meter raw fuel, so a bleed type needs to be larger than a Primary type to flow the same amount of gas by 3 0r 4 sizes.
They sound like Yamaha OEM carbs at that size and with those needle jets. #169 was stock in the R5 and #175 was in the RD250 or RD400 C/D. R5 uses O-0 and RD350 uses O-8 so let's use that spread as indicative of the differences.
169 O-4 would equate to approximately a 175 P-6 but you say that one is lean and the other is way too rich. Odd, but my guess is that the 169 is not breaking the fuel up enough and it's not burning. We had something similar with #182 series in 26mm carbs on a CB175 racer. Lots of unburned fuel even when it was running lean. Are the air jets/drillings clean?
At full throttle the main jet does most of the metering and I suspect that the main jet is too small and is the issue. What M/J is in there at the moment? And what needle is in there 5DP7, 5I4, 5J6?
They sound like Yamaha OEM carbs at that size and with those needle jets. #169 was stock in the R5 and #175 was in the RD250 or RD400 C/D. R5 uses O-0 and RD350 uses O-8 so let's use that spread as indicative of the differences.
169 O-4 would equate to approximately a 175 P-6 but you say that one is lean and the other is way too rich. Odd, but my guess is that the 169 is not breaking the fuel up enough and it's not burning. We had something similar with #182 series in 26mm carbs on a CB175 racer. Lots of unburned fuel even when it was running lean. Are the air jets/drillings clean?
At full throttle the main jet does most of the metering and I suspect that the main jet is too small and is the issue. What M/J is in there at the moment? And what needle is in there 5DP7, 5I4, 5J6?
mtrace535
Been Around the Block
the main jet is a 200 when ive got the 175 needle jet in and ive tried all the way down to a 125 with the 169 type. Ive got a set of 5DP7 and 5F3's and tried both. The pre jetted VM30's from dime city have 159-P5 needle jets in them so i was thinking of trying them to see if that is better than the 169-04's i have.
It sounds like you are trying all teh right things and know what you're doing.
#159 series are 2mmm longer than 169s and has a taller bottom spigot so it would retain less fuel in the well.
#159 has 2.0mm thick retaining ring where #169 is only 1.5mm so the #159 will sit slightly higher
#159 has 2.5mm air bleed hole where #169 is 3.0mm. One would expect that not to make much difference because the bleed air is metered though the air jet/drilling, but there must be a reason for that difference.
5F3 is a single taper needle. The 5DP7 has two taper angles, 1 degree at the top and then 3 degrees closer to the tip and that makes it much richer at the top than the 5F3 and slightly leaner in the middle. That's ideal for a piston ported 2 stroke motor that needs more fuel at WOT than other engines.
So I'd suggest try that 5F3 as the simple starting point.
#159 series are 2mmm longer than 169s and has a taller bottom spigot so it would retain less fuel in the well.
#159 has 2.0mm thick retaining ring where #169 is only 1.5mm so the #159 will sit slightly higher
#159 has 2.5mm air bleed hole where #169 is 3.0mm. One would expect that not to make much difference because the bleed air is metered though the air jet/drilling, but there must be a reason for that difference.
5F3 is a single taper needle. The 5DP7 has two taper angles, 1 degree at the top and then 3 degrees closer to the tip and that makes it much richer at the top than the 5F3 and slightly leaner in the middle. That's ideal for a piston ported 2 stroke motor that needs more fuel at WOT than other engines.
So I'd suggest try that 5F3 as the simple starting point.
OK. Finally the penny drops.
Those carbs are off an RD Yamaha and they do not work with Primary type nozzles because they have a very small main air jet.
R5 carbs use 169 series and had larger air jets than the RD series carbs which is why those nozzles are not working in the RD bodies.
To make it work with those nozzles, you need to drill out the air jet passage to 2.0mm or 5/74" or drill and tap to fit air jets. That would make them suitable for any series of needle jets and will let you trim the fuel slope.
I'd drill and tap the main air jet passageway and install 1.5mm air jets. They are 4mm by a fine thread and I buy taps from ACE, but they must be the right thread pitch - obviously.
Drop in a huge main jet to be on the safe side and see if that works. If it runs, get the main jet zero'd in better for WOT high revs and then go back to the midrange/mid throttle.
One trick that works is to take the main jets out. If it runs OK up to say 1/2 throttle and then goes way rich, the NJ is close enough.
Those carbs are off an RD Yamaha and they do not work with Primary type nozzles because they have a very small main air jet.
R5 carbs use 169 series and had larger air jets than the RD series carbs which is why those nozzles are not working in the RD bodies.
To make it work with those nozzles, you need to drill out the air jet passage to 2.0mm or 5/74" or drill and tap to fit air jets. That would make them suitable for any series of needle jets and will let you trim the fuel slope.
I'd drill and tap the main air jet passageway and install 1.5mm air jets. They are 4mm by a fine thread and I buy taps from ACE, but they must be the right thread pitch - obviously.
Drop in a huge main jet to be on the safe side and see if that works. If it runs, get the main jet zero'd in better for WOT high revs and then go back to the midrange/mid throttle.
One trick that works is to take the main jets out. If it runs OK up to say 1/2 throttle and then goes way rich, the NJ is close enough.
mtrace535
Been Around the Block
Nothing solid yet. But the spitting fuel out the exhaust was a bonehead move on my part. I forgot to tighten one of the pilot jets and it started to fall out. Its currently running good in the garage but its been pissing rain all day. This weeks forecast is looking better. Well see.
mtrace535
Been Around the Block
Teazer, thanks for the help. ive got it to a rideable although no perfect state. So far 160m, 22.5p, 5F3 needle 1st clip, 159 P4 N.J. The 159 isnt made for the 28mm carb but i havent found a 169 or 175 type that runs as well. If anyone has a sugestion let me know.
