Jetting question on RD350

[4schizzle]

I am still trying to figure out what I think is the jetting on my RD.  Sometimes it runs fine but this problem keeps re-appearing.  I initially thought it was fuel flow problem as the fuel lines do not fill once needle valve closes. (changed petcock and needles and seats) I also adjusted air screw and pilot.  I believe that the jetting is close. The bike is all rebuilt with new pistons and seals.   

I am trying to figure out which way I need to go on the jetting either leaner or richer on the main jet. (I am thinking richer) The bike breaks up and sounds like its starving for fuel high in the revs. The revs flatten off in 3rd gear.  (my opinion)  The revs pull through in 1st and 2nd.


1975 RD350
Stock VM carbs
Dave F carb mod with correct needles.
240 mains
37.5 pilots  Previously had 40's in it but the still had same problem.

Stock YZ 85 reeds
Banshee reed spacers
Uni pod foam filters oiled with foam air filter oil
expansion chambers
no porting

Thanks for any help or suggestions.

Brian

[38special]

Are they US/UK spec cylinders? I fought all sorts of jetting issues until I realized I had been sent India-HT spec cylinders.

[m in sc]

240 way too big for Dave f carbs. usually 190-210. drop it to 200, test it. the lines not filling when the needle valve closes in the carb is not a concern, ignore that

[SoCal250]

Also when tuning carbs and jetting you should not be focusing on gear or RPM but on throttle opening %.
Mark your throttle grip with a paint dot or something similar, and then put corresponding marks on the bar at 0, 1/4. 1/2. 3/4, WFO (I use a strip of duct tape marked with a Sharpie) to show the slide opening.

This tuning guide has some great info and explains it well.
https://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

[4schizzle]

Thanks I will try all the suggestions and let you know. 

They are us spec stock cylinders. 64.25  I believe, can't remember.  1st over. 

I am going to drop it to 200 and try. 

Fingers crossed! 

Thanks
Brian

 

[quocle603]

Not the same set up but I have a my RD with Dave F, uni yboot, stock pipes.

Main: 170
Needle: Position 4
Airmixture: 1 1/2 turns out
Pilot: 35 (might be better with 32.5 or 30)

Your set up is pretty rich right now with Dave F. The mod allows you to use smaller jets for the main (WOT). Your pilot seems a bit rich, but it could work for you. I would confirm your timing and carbs are synced too.

[4schizzle]

Thanks for info.  I am going to lean it out some and try. I will update outcomes. 

[4schizzle]

Update:
I dropped it to 200 mains. It seems better. Not breaking/loading up. What is everyone opinion with my 37.5 pilot. It still seams pretty rich on the bottom. What are the recommendations for the pilots with the Dave F carb mod for the pilots?

Thanks again!

[m in sc]

I run 30s. thats 2 sizes over stock so...

[4schizzle]

Thanks I may also drop the pilots down.  Are the 30's with pod filters?  Thanks

[m in sc]

yeah. contrary to popular belief, the air filters have basically nothing to do with pilot operation.there isn't enough airflow at that low of a throttle opening to affect the pilot stock vs y boot vs unis.

[4schizzle]

Thanks again for all the help.  I am going to drop mine down also.  Hopefully this bike will finally be squared away and I can start my next RD build: RD 400 frame, r5 motor with an RD 6 speed transmission.  Let the fun begin again.     

[quocle603]

Thanks I may also drop the pilots down.  Are the 30's with pod filters?  Thanks

I believe my stockish build tried 37.5 way too rich. 35 was pushing it for my build. 30 is a nice sweet spot to try. I had really good luck with the clip at position 4. It helped with my bottom end and hit the powerband!

[4schizzle]

Thanks