Walms wrote:
Hi PK, any updates on these carbs? They look pretty compact which is what I'm looking for...
Are the brass bits readily available? Do you find the power valve helpful for jetting differential between on and off the pipe?
Hi Andrew, I started replying to your PM but decided to post my answer here instead as others may also be interested.
Yes, I'm still using the RGV carbs. It's been approximately 10K miles now and I'm very happy with them.
Let me try to describe some of the peculiarities around these carbs.
Pilot circuitThe pilot circuit uses standard Mikuni pilot jet (VM22/210), however, these carbs does not have a pilot air screw. Instead there is an air jet located in the pilot air intake. This is a standard Mikuni air jet (BS30/97), and instead of adjusting an air screw you have to change the air jet size. A bit more cumbersome but once you find the pilot and air jet sizes that work, you normally wont have to mess with it again.
I only know of two persons who have tried (or wanted to try) these carbs after I published a paper on the subject, and they were both hung up on the fact that there is no pilot air screw. Dave (Higgspeed) did a 90 RWHP dyno run using these carbs but stopped using them because of the missing air screw. TTNut bought of a set of these carbs and immediately started drilling them for air screws. I don't know how this turned out. I don't think he was able to finish it before he unexpectedly passed away.
Main circuitThe main jet in this carb is standard large hex (4/042) just like most off the shelf Mikuni carbs for 2-strokes (VM/TM/TMX/TMS). The real difference is the needle jet. This carb uses a bleed type needle jet. It still has a primary choke screen but the screen is part of the carb body instead of the needle jet.
There seem to be a common belief that primary type needle jets are for 2-strokes and bleed type is for 4-strokes. This may be true in racing and on dirt bikes but if you research 2-stroke
street bikes I think you will find more bleed type than primary type needle jets (Yamaha RD/RZ, Suzuki GT series etc...).
The needle used in this carb is a TMX/TMS style needle and I don't think you can use regular needles in this carb (like 6DH2 etc...). As you may already know these needles are different than regular Mikuni needles in that they have a defined diameter on the cylindrical section before the first taper. This is a very useful tuning parameter (around 1/8 to 1/4 throttle opening). The diameter is denoted by the dash number of the needle. I'm currently using 6EN11-56 which is a TMX35 needle. -56 mean that the cylindrical part is 2.56 mm diameter. I have -55, -56, -57 and -58 versions of this needle for tuning purposes but ended up using -56 most of the time. The most common needle found in RGV carbs is 6GH8-55. This needle is a little rich on my H1 at 1/4 to 1/2 throttle and low to mid RPM, probably due to double carburation caused by piston port blow back. I have a sneaking suspicion that the 6GH8-55 needle may be perfect for a reed converted H1 (after all, the RGV 250 is a reed engine).
Since this is a bleed type needle jet carb there is also an air jet in the main circuit, and these carbs are very sensitive to changes in air jet size. This is a very useful tuning parameter.
Air bypassesMost of the RGV carbs have air bypass fittings in both pilot and main circuits. On my H1 I have simply plugged these using some rubber caps. On the RGV, these are controlled by electronics and solenoids. When a solenoid is opened, more air is supplied to respective circuit. I believe the pilot bypass is mostly used for emissions reduction. The main circuit bypass is more interesting as I believe it is used to "alter" jetting according to whether or not on the pipe. On an RGV this bypass solenoid is mapped to both RPM and throttle position. I've been playing with the idea of doing the same on my H1 using RGV throttle position hardware and my own electronics, but haven't given it any priority.
Power jetsI can't say much about the built in power jet as I haven't experimented with it yet. It's not adjustable but it can be re-jetted using small round jets (N102/221) located in the float bowls. For now I have only used the largest jet found in these carbs on all three of mine (left and right are different on the RGV). I may try to use my action cam directed to a carb and then take it for a spin without air filters. Should be able to see whether or not the power jet delivers at full throttle high RPM.
Dimensions etc...Spigot O.D. is 40mm.
Bell mouth O.D. is 57mm.
Viton tipped float valves.
O-ring sealed float bowl.
The downwards angle of the venturi matches the forward leaning angle of the H1 cylinders. Float bowl is almost perfectly horizontal when mounted on a H1. No need for wedge shaped adapters. This carb clears both the crank case and the cylinder fins. The top of the carb may be close to the stock petcock but not a big problem.
Future plansI plan to get away from pod filters by building an air box with a large K&N or UNI style filter located where the battery box is now. In this box I will fit the stock "velocity stack rubber boots" from the RGV (see pictures below).
Her is a link to the paper I originally wrote for TTNut:
http://dl.dropboxusercontent.com/u/399336/Pictures/Posted/Using%20Suzuki%20RGV250%20carbs%20on%20a%20H1.pdfPK
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