Problem with RD 350 fouling one cylinder spark plug at start up

[Still biking]

I have an RD 350 which is fouling one spark plug sometimes at start up, same cylinder all the time.  If the bike warms up with out fouling, it runs fine.  Bike has good compression and seals.  Any ideas?  checked timing and cleaned points. 

[Striker1423]

Check your float height and ensure the needle and seat is clean. Too much fuel and you will foul plugs at idle easily.

[SUPERTUNE]

Check your float height and ensure the needle and seat is clean. Too much fuel and you will foul plugs at idle easily.

Yes, Needle valves cause issues like this.
What cylinder do you have issues with?
Is this on oil pump or premix?
Have you ohm and voltage tested the coils and feed wiring?
Are the plug wires and resistor caps new? If switching over to non-resistor caps you can run resistor spark plugs like a BR8HS.
Don't run both a resistor cap and resistor spark plug.
How modified is the bike and jetting changes done?
Is the charging system working correctly and battery new and in perfect condition?
Could be any of these things.

More info is needed...
Chuck

[Greaser Greg]

I just replaced aftermarket points with Oem and what a difference. Springs are stronger, no wiggle on the post, winds right up. One cylinder was randomly intermittent at idle and no power after six k.

[SUPERTUNE]

I just replaced aftermarket points with Oem and what a difference. Springs are stronger, no wiggle on the post, winds right up. One cylinder was randomly intermittent at idle and no power after six k.

Very good point! (Get it? POINTS!!!) 
That never crosses my mine as I always use Yamaha oem points or aftermarket ignition...
C

[Still biking]

I just replaced aftermarket points with Oem and what a difference. Springs are stronger, no wiggle on the post, winds right up. One cylinder was randomly intermittent at idle and no power after six k.

Where can you purchase OEM points now?

[SoCal250]

You need to find someone selling NOS inventory. Ebay or a vintage Yamaha parts supplier like Speed & Sport.

RD250 RD350 R5 DS7
278-81621-20-00  (L)
278-81622-20-00  (R)
*See advice from Chuck

RD400 C/D/E
1A0-81621-20-00 (same part for L & R)

RD400F 
2V0-81621-60-00  (L)
2V0-81622-60-00  (R)
(supposedly you can substitute either RD350 or RD400 points depending on type installed by the factory, as there were 2 designs used)

[triple1972]

I was thinking the points were the same on 350 and 400 but backing plate was different. I could be wrong.

[Greaser Greg]

There are other models than rd etc that use the same contacts on a different plate. The Oem plates are thicker so once you have two of those you can swap the contacts. There's a recent thread on this site or the air cooled rd club you can find by searching "points" with some info on #'s. Also check the pics on eBay.
https://www.ebay.com/itm/NOS-YAMAHA-CONTACT-BRKR-ASY-1967-YL2-1968-YL2CM-1968-YLCM-1971-CS3C-164-81221-20/232989615583?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Same breaker, different plate. I bought all four of these, but there are others.  First digits of part # is different but they are the same contact breaker for many models.

[SUPERTUNE]

I was thinking the points were the same on 350 and 400 but backing plate was different. I could be wrong.

Yes, different.
I drill out the RD350 plates to fit the 400 points when needed.
Chuck