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Rebuilt RD needs choke on to pull load

Started by 4schizzle, March 06, 2023, 10:50:01 AM

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Alain2

If your bike is hard to start you need to use a bigger pilot jet, try something like a 32.5 Mikuni and tell me how it starts.

Only use genuine Mikuni jets.

My RD350 has foam pod filters and use a 35 pilot jet. It starts great with choke no problem.
1973 RD350, 1977 RD400, 1979 RD400, 1980 RD400, 1985 MJ50, Goped Zenoah 30cc.

m in sc

verify spark and that the ign rotor didn't slip. nothing else should be an issue barring an air leak. you did join the 2 cabr sback at the choke crossover?
but a slipped rotor will give hard starting and bad performance overall.

4schizzle

I started the bike again.  It started right up and idled with choke on and off.  Idle was constant with choke on and off.  It did have some unburnt black gas oil mix leaking at one pipe/muffler connector, but i believe that this was from the other day when it did not want to start.  I want to blame the starting issue on the inline fuel filters (clear gold stone looking ones).  I do not like how they seem to restrict the flow of gas.  I am only using them as i just cleaned out the tank.  They fill the float bowls but the left carb fuel line does not fill up all the way with gas. I recently rebuilt the petcock and no flow restrictions were present.  Carbs are also rebuilt and and float level set to 15mm I think (whatever is spec in manual).   I am going to delete the fuel filters as soon as possible. 
It still does not want to pull a load without choke.  It does seem to be lean on the pilot circuit as someone else suggested.  With the way i have been running the bike i have not been able to get it up on the main jet circuit.  I am still going to verify timing just to make sure.  I should be getting the correct exhaust setup on soon to really verify the jetting.  Just wanted to make sure the bike ran and did not blowup into a million pieces before i dropped all the money on the exhaust. 

I what to thank everyone for the help/suggestions.





verify spark and that the ign rotor didn't slip. nothing else should be an issue barring an air leak. you did join the 2 cabr sback at the choke crossover?
but a slipped rotor will give hard starting and bad performance overall.
[/quote]

m in sc

ah. yeah ditch the brass cone filters, they will absolutely starve a motor. i run screened ones or very large brass element onee (canister types).

the other thing that may have been the issue w starting is if the petock seeps when it sits into the carbs, it can fill the crankcases a bit and immediately foul the plugs on startup attempts. *not uncommon

raise the needle to one clip richer, and where are the airscrews? run them in to 1/2 turn in. see if it runs better, if it does, then def go up 1 or 2 on the pilot jet. ideally you want to be at 1-1/4 to 1-3/4 out ont he airscrew to keep circuit velocity where it atomises correctly. But for tuning purposes, ive run them in to as far as 1/4 out then adjusted the pilot size up after.

.02


4schizzle

I was going to put bigger pilot jets in the carbs before i took the bike to get the pipes built.  I noticed that both float bowls did not have a hole on the bottom.  I had another float bowl with the hole on the bottom for the choke, and I put it on the left side/choke lever carb.  Is this correct?  I am going to start it today to see if this resolved the choke issue.

Float bowl with hole on bottom belongs on the left/choke lever carb.  The float bowl with no hole goes on the right/no choke lever carb.   

Correct? 

Thanks

m in sc


SoCal250

Also on an RD250/RD350 you can verify after assembly that the bowls are on the correct side. The left (choke) carb should have a brass drain tube nipple that points forward. The right carb drain nipple points down. :twocents:
75 Yamaha RD125B   75 Yamaha RD125B (project)
75 Yamaha RD250B   75 Yamaha RD200B (project)
73 Yamaha RD350     77 Yamaha RD400D   79 Yamaha RD400F  
91 Yamaha TZR250R  89 Yamaha FZR400   05 Yamaha FZ6   
05 Yamaha XT225TC  82 Honda MB5  02 Aprilia RS250 Cup (sold)

4schizzle

Also correct as i noticed this after i installed the carbs and reconnected the vent hoses. 

Quote from: SoCal250 on March 29, 2023, 12:00:02 PM
Also on an RD250/RD350 you can verify after assembly that the bowls are on the correct side. The left (choke) carb should have a brass drain tube nipple that points forward. The right carb drain nipple points down. :twocents:

4schizzle

I am still trying to figure this out.  Got the bike back from Jemco.  The expansion chambers look awesome.  The bike starts right up and will idle.  I moved the clip 1 slot leaner and it appears to have have gotten worse when this was done.  What size reed block spacers are members using on their RDs. I have a set on this bike.  I was wondering what size 9mm to 13mm thickness?  Thanks

m in sc

6-8mm is fine. that won't cure your problem though. the point behind the reed-spacers, primarily, is to unshroud the boot port. it can move the power band around a bit as well.

try the clip the other way then. always err rich.

4schizzle

I am going to put in the 30 pilot jets that came with the Dave F carb mod, and move the clip back to the middle.  I still have the 27.5's in there now.  Moving the clip was a test to see how the expansion chambers might have changed it.  I am going to try and video what it is doing.  Once i get the revs up over where its bogging when trying to start out it seems fine.  I am guessing it is still lean.  Just hard to believe with the large increase in jet size that I have already done. 

m in sc

you should be able to determine where the pilot jet needs to be by experimenting w the air screw. crank the screw in to about 1/2 turn, if it gets better, go up on the pilot.  anything under 1 turn 'good' will tell you this. anything over 2.5 out means you typically need to go down.

IF it idles fine and you crack the throttle and it returns to idle ok w out hanging up, then its probably in the needle. move it 2 clips rich from where you currently are. then try it. theres 5 clip positions, don't be afraid to use any of them if need be.  :twocents:

4schizzle

I will give that a try hopefully today.  Thanks

4schizzle

#43
Quote from: m in sc on June 14, 2023, 11:45:34 AMyou should be able to determine where the pilot jet needs to be by experimenting w the air screw. crank the screw in to about 1/2 turn, if it gets better, go up on the pilot.  anything under 1 turn 'good' will tell you this. anything over 2.5 out means you typically need to go down.

So last night I tried what you said.

The air screw was initially set 1.5 turns out.  I went in a .5 turn and it was a bit better.  Then went 1 turn in and it was a bit better.  Then I went 1.5 turns in and it was better but still bogging when letting the clutch out.  Also at this point the air screw was closed off.  I also tried turning out on the air screw and from the "stock" 1.5 turns out to 2 turns out which was was way worse as it would stall when letting the clutch out.

I  am going to put in the 30 pilot jets that i have and also order 32.5 and 35 pilot jets.   

4schizzle

I did some jetting this weekend.  Started with 200 mains and 30 pilots, air screw 1.5 turns out, needle clip in middle position.  The bike could pull out with minimal bogging. but was still bogging.  Once the bike is moving it does not bog.  I kept turning the air screw in till closed in 1/2 turn increments and it kept improving. Tried 200 mains and 32.5 pilots, air screw 1/5 turns out.  I kept turning air screw in in 1/2 increments till closed and it was still improving, however breaking up in high revs 1st/2nd gear.  Ended up going with 210 mains and 35 pilots with air screw 1/5 turns out (need to order more jets). Same as above kept turning air screw in and same situation. 

Bike specs: 75 rd350, bored .25 over new pistons/rings, all new cranks seals, yamaha bond (case), and gaskets, new HPI ignition no battery, yz85 reeds, and reed block spacers, banshee crossover tube, uni pod filters, stock vm28 carbs with Dave F carb mod, Jemco expansion chambers. 

I am thinking about ordering 37.5 and 40 pilot jets but it seems pretty large.  Thanks again.