Another Jetting question

[X-Man]

I found the Jetting Poll spread sheet to check for jetting my bike. I didn't see anything that was close to my bikes build.
'77 RD400
4th over boar, stock porting, stock heads and stock VM28 carbs w/ Uni pods. HPI ignition, Chambers.

Could anyone please advise? Its an all new motor and dont want to mess it up,

[automan]

so, like, i mean, like
WHAT IS YOUR JETTING?

[X-Man]

I for got to add some other info.

Banshee intakes with X-over pipe. Suzuki RM100 VForce 3 reeds.

The carbs that were on the bike were jetted with pods but all stock bike.
The main is a china jet with no markings, the pilot is a 30.

[m in sc]

stock carb internals? prob a 240-260 main jet w a 27.5 or 30 pilot. try one lean on the needle if the chambers are good, soecii, etc. if dg or something along those lines.. probably middle clip. is the needle stock oem?

[SoCal250]

And I would recommend acquiring some genuine Mikuni jets from a trusted supplier

[X-Man]

Looking at the needles now. They are 5L1 and holders are 175P-2.

[SoCal250]

Looking at the needles now. They are 5L1 and holders are 175P-2.

That is the original needle and needle jet for a US-spec RD400

[quocle603]

Mark is in the right ball park. Go through the ranges and see what you need to adjust. Dave f mod may help out too with your set up

[m in sc]

also, since its a 400 have you removed the brass ball from the bellmouth @ 6.0clock, cleaned the airjet, etc? if not, that needs to be done 1st. if that airjet isnt clean, all the jetting in the world wont matter.   

[RDnuTZ]

curious what is the design/engineering purpose of the brass ball placed there by Yamaha?

[m in sc]

i know the answer to this actually. the idea was to use air from the airbox to feed the air jet to help equalize the pressure on the jet tube. was a still air point plumbed BACK to the airbox.  also used filtered air, some speculate that's the reason.. but... but then again.. so did the open one as the bellmouth.  Weirdly, suzuki tried this w the internally vented carbs ont he t500s, they are terrible. similar concept w the bridgeport H1 carbs.

the downside is its nearly impossible to service when the ball is in there. int he case of the suzukis, it got so weird they (stock) used a different jet tube series left to right on the 500s.

on a dead stock motor, i can see taking the ball out, cleaning it, then putting back in a threaded plug. but thats it.  ive never heard of a case where the ball was taken out, not returned, and it actually making the bike run worse.

 

[X-Man]

Wow. Great info. Never heard of that. Would it be an upgrade to replace the carbs with bigger? Like stock Banshee carbs? Theres a set locally cheap. Anyone use Polini CP carbs? I have good luck with them on small boar bikes and peds. Fully tunable like Mikunis

[RDnuTZ]

stock Banshee carbs had a TORS system and most people chucked that and replaced with good throttle and cable set up to replace stock. I had 1 in 2001 and if I remember correct, they came stock with a thumb throttle instead of twist that was a part of that whole TORS thing.

[m in sc]

banshee carbs, stock, are actually smaller, 26mm.

the stock vm28s work very well.

[2steve]

Much time spent here trying to make RD400 stock carbs work better. Never was happy until I did the Dave F mod.

It's a world of difference. I highly recommend.