Oh NO! More carby problems

[buffalorider]

I know, I know, carb issues are the hardest for users to explain and hardest for forum "helpers" to diagnose which is usually why no one ever really likes to reply...or on the other hand...one will get too many replies which then can confuse the issue even more.

I have a '77 RD400 engine that was completely gone through by "Supertune" to include bottom end, bore, new matching piston set etc. Bike also has the 34mm Mikuni's with new manifold kit and crossover tube. Large Uni filters to boot. Older Neutronics (working condition) electronic iginition with Spec-II coils and pipes. (Lets keep the "get rid of the Neutronic's and Spec-II comments to a minimum please) Until I can afford or unless they are the culprits in my issue, those parts are staying put for now.

With both choke levers applied, the bike starts right up. OK guys, here we go!! Strap in!!  When I take chokes off, something magical happens to the left cylinder.  It seems to stop firing. What I mean by that is the exhaust coming out of the pipe gets colder than the right side. When I pull the cap off the plug and get a minor shock through my body and then hold it close to the head fins, I can see plenty of spark. When I pull the cap while the bike is running there is no difference to the idle meaning only the right side is firing and burning the fuel. If I push the choke lever down a bit, the cylinder will start to fire back up.

I am "ass"uming that it is a carb issue...but what?? I have had these carbs off and on more times than I want to remember. I don't want to ride it and try to "blow it out" which I would also think could "blow it up" only running on one piston. Maybe a clogged pilot jet that would come to life when choke was applied? Now that would be an easy fix and maybe I should have checked that prior to sitting here typing all this up so you guys could laugh at me. If it warms up above 20 today, I might get after it.

On a side note, both choke levers point to the left on this bike. Should the right carb be pointing out to the right? Both slide cut outs are facing the rear and are timed as close together as I can get them. Do I have two left carb's...would that even matter? 

[Alain2]

These aftermarket Mikuni carburetors are not distinctively right or left so this is definitely not your issue.

Were the carbs working properly at some point before the rebuild or it is a new setup for you? Were the pilots tuned for your setup?

Your faulty carb is maybe dirty or your float level is wrong maybe.

[buffalorider]

Thanks for the reply Alain2. These carbs were on it but when I obtained the bike, the right piston had exploded, thus I sent engine to Chuck. I added the new manifolds from Chuck and cross over tube. The reed spacers were there. When I reinstalled, the carbs were pouring gas so I had to adjust the floats in both and maybe that is wrong? Carbs are spotless inside. Any idea what level should be on these? I do have manual but that is for the OEM carbs. Thanks

[m in sc]

yup. id say float level or clogged pilot on the offending cylinder. 

[buffalorider]

SWEET!! So do I turn carb over and try to set level so floats are even across? I will keep everyone posted on outcome and all jetting info for my elevation just for future reference.

[m in sc]

in my experience... as long as with fuel you are about 8-10mm belw the gasket surface with fuel when its full, you should be good. how to do this? i fill the carb right side up with bowl screws out, but holding the bowl on, then just lower the bowl a bit so you can peek in.

this is also a handy reference.  http://www.allensperformance.co.uk/carb-kit/carburettor-float-height-chart/

[buffalorider]

Damn that sounds messy. There goes all my nice engine paint. I'll remember to not be smoking for this little endeavor.

[m in sc]

i do it on the bench actually.

[paul1478]

I have seen your comments about newtronics and respect that. having said that, I had that on my bike and swore it was jetting/carbs. seemed too rich. Chuck fount it to be an issue with the ignition. Between resister plugs, resistor wire caps and week signal that was the issue.
just adding my real life experience.
Good luck!

[~JM~]

Perhaps swap the carbs. If the problem swaps likewise, most likely carb problem. If it doesn't, look at spark and seals.

[m in sc]

as much as I dont trust newtronics.... he's stating that it runs on the choke and then immediately drops the cyl when he takes it off, so my guess is float level. the newtronics just hasn't broken , yet.  usually, in my experience, they start going in a different way, when warmed up & under load.

[buffalorider]

UPDATE.....
I pulled left carb off and promptly broke off the oil injector nipple. Glad I had a spare!! I need some more of those damn things. Pulled the bowl and cleaned out pilot which seemed to have a little somin somin in it, then adjusted float to what I "guessed" was a reasonable level, reassembled and she RUNS!!!!! Have not had it under a load yet due to lousy weather but hope to soon. She zinged up to 7 grand with no issues. Thanks for all the help. 

[Striker1423]

EDIT:

Insert foot in mouth.

That is all.

[m in sc]

You need the Yamaha float level spec for the 400 in that year.

.. he has VM34's on it. that probably wont help him.

[teazer]

With carbs off teh bike as someone already commented, set fuel level to 3-4mm below the gasket surface on both carbs.  Job done.