Carburetors @5200'-7500' Altitude For RD350

[mlakritz]

Working on my RD250 to 350 build and I need a carb recommendation for daily riding at about 5200' and sometimes up to 7500' (stock carbs are toast). I'm good working out jetting/premix as I rode a CR250 for decades at 10k' and higher. This RD will be a street performance bike with some minor porting (on ebay cyls), Vape ignition, chambers (more midrange than top end), Vito's crank, Boyesen reeds with 3/4"spacers and stock reed blocks, pod filters. I see all the different Mikuni options on HVC and Economy but am not sure what size or type is optimal (round slide, flat slide?) for this application and altitude. Looking to make this thing feel fast as hell on the street firing from light to light, but I will not be racing it so top speed is not a concern and I do want to make it feel "rideable" on the street. Thanks in advance for hopefully a lively discussion. :-)

[TeeZee]

I'd be interested in how this goes ...

I'm running a RZ350 and did make few changes but long ways to go still.

Here's, what I've been running - https://www.2strokeworld.net/forum/index.php?topic=6963.0

Goodluck!

[mlakritz]

I'd be interested in how this goes ...

I'm running a RZ350 and did make few changes but long ways to go still.

Here's, what I've been running - https://www.2strokeworld.net/forum/index.php?topic=6963.0

Goodluck!

TeeZee, looks like you are in CO? I read your thread above, all good info. I used to calculate jet sizes based on actual O2 concentration. So if O2 concentration is about 15% less at say 5200' compared to sea level I would multiply main jet size by 85% and go with the closest jet to that number. That always worked really well with a single cyl CR250 at 10k', not sure how it will apply to the RD but that is my basic calculation starting point. Here in Boulder the O2 concentration is about 83% less than sea level.

[1976RD400C]

Here's a graph from the Mikuni manual, note it is in meters.

[m in sc]

has more to do with air density vs 02 content.  remember, atmospheric pressure 'pushes' the air in, this affects the speed of air thru the venturi. less pressure difference has a large effect as well. o2 content matters of course as well but not quite as much as density of the air itself which is what ^ document illustrates. 0.02

[mlakritz]

I did just find this thread on here https://www.2strokeworld.net/forum/index.php?topic=775.0 which directly and indirectly answers most of my questions. 28mm VM's seem like a can't go wrong option even with some performance mods and per that thread, anything bigger would require some mods to the cases, which doesn't seem like something I want to get into, got my hands full as is with this thing. But...since I do have the performance mods going on, will 28's be a power bottleneck and negatively affect the gains from the chambers, reeds, Vape etc? If that is the case (bad pun), then I'd rather do it right, even if it requires more work or expense, modifying cases, intakes etc. I'd hate to miss out on significant front wheel liftability with carbs too small for this build. I mean really, is there such a thing as an RD with too much power?   

[m in sc]

you dont nec need to mod the cases to fit bigger carbs. as far as 28s, you can get 70hp thru them believe it or not. but a set of flatslides will typically give better response . IMHO just use the 28s.  also, yes, you can have too much hp to be enjoyable to ride to a degree

[SoCal250]

This should help with tuning for altitude. See the very end of the doc.
Mikuni-Motorcycle-Carburetor-Theory-101.pdf
Ian Williams has some great info on his site. He's a very experienced Australian 2T tuner who works primarily on racing karts.

[mlakritz]

SoCal that chart is great, it isn't far off from my back of napkin calcs I've used forever. As far as the flat slides vs. round, I do like the idea of better response with the flats. Here is an absolute encyclopedia of info on the VM/TM carbs that I found, maybe somebody posted this elsewhere but this is quite the dissertation on Mikuni carbs. Not a quick read but lots of great info. https://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

[TeeZee]

I'd be interested in how this goes ...

I'm running a RZ350 and did make few changes but long ways to go still.

Here's, what I've been running - https://www.2strokeworld.net/forum/index.php?topic=6963.0

Goodluck!

TeeZee, looks like you are in CO? I read your thread above, all good info. I used to calculate jet sizes based on actual O2 concentration. So if O2 concentration is about 15% less at say 5200' compared to sea level I would multiply main jet size by 85% and go with the closest jet to that number. That always worked really well with a single cyl CR250 at 10k', not sure how it will apply to the RD but that is my basic calculation starting point. Here in Boulder the O2 concentration is about 83% less than sea level.

Yes, I'm in Monument, CO with an elevation varying from 7500 to 8500 where I ride. I'm no expert in this and still learning as I go but I've been told that at elevation we're at, the expansion chambers are not so much of change but as soon as you remove the standard air box is when the jetting comes in. One of the local mechanics I talked to (Yak's at Monument) says that he generally starts with standard air box and usually the jets are not too far from factory numbers as you tune in.

Unfortunately I don't have factory air box that I can test with. With life happening I haven't found much time so far this season to meddle further but here's my rescue story if you're interested - https://www.2strokeworld.net/forum/index.php?topic=6232.0

[m in sc]

he's right. for example, stock rd350 w stock airbox w lid, etc and old school spec 2 pipes only, 150-160 main up from the stock 140. same stock carbs, pipes etc with big unis,  or a k&n in y boot.. 240 to 260 main. I'm being serious. thats at 750 ish from sea level.  0.02 (as a data point)