BS38 vs VM34

[Alexs650e]

Hello all,

I've recently did a top end rebuilt. Since I bought the bike back in '16, right cylinder was always weaker, at idle at least, possibly through the whole rev spectrum as well.

Everything was put together fine and it fired right up, when I got the static timing on the dot using the light bulb technique.

I went from airbox to K&N style air filters, and I have stock header with a shorty muffler. I knew I would might need to jet the carbs, and replace all gaskets and seals, but first goal was to start and go pass inspection so I can keep test riding.

After the inspection, I rode for about 30'. At the lights it kept struggling with idle speed and shutting off. Coming to a stop takes a bit for the revs to go back to idle. Full throttle was bogging. Mid range was fine with stock needle clip 1 position down/needle 1 position up(however you want to call it).

I ordered the Keyster rebuild and tuning kits. Cleaned the carbs, went #30 PJ, #138 MJ(could go #142), needle as it was, and synced the butterflies using a feeler gauge(I couldn't find any suggestions on that). Now trying to do the dead cylinder adjustment, LH idles fine at anywhere from 1.5 to 2.5 turns out on the idle mix screw. Right hand can't even do the dead cylinder adjustment, and with both firing It sounds its best at 3.5 turns out. Still missing a few bangs compare to the left and sounds weaker. Off the line I do get a single bog. I'm not going to try and find the source of that issue at this time because it might be related to the RH not idling fine. Mid as is and full throttle didn't bog.

New points, new capacitor, ignition coils read good. I haven't tried flipping them yet. I will try today.

If it's not the coils, could it be that there's something really wrong with the RH carb body, as I saw someone had a similar issue before on this forum?

Should I just buy a set of VM34s and not bother trying to tune a pair of 46yo vacuum type carbs that can be very temperamental? And it's not trying to fine tune them, which I'd be fine, as it seems a major deviation between them). If say I go up one more PJ size on the RH carb, that just doesn't sound right.

Any suggestions?

Thanks in advance.

1978 XS650E with BS38 as stated on the subject.

 

[zrx1100]

I have the same bike model and so does @5twins, the resident carburation expert. A couple of things that I learnerd from 5 twins and from experience, don't use the K&N pod filter on the stock carbs, use UNI pod filters UP4200 instead or the stock airbox. Some pods interfere with vent/air jet openings in the carb intakes and other reasons. These UNI pod filters don;t cause issues. Buy only Mikuni carb components. Niche cycles for jets. check your float height and make sure your floats are not leaking. Also check for any leaks at carburator boots. Either a boot leak or carb float is causing your problem on the right. Make sure that carburators are synchronized.

(5twins): "For mains, you will need the N102.221 type .....

https://www.summitracing.com/parts/mku-n102-221140

For pilots, you will need the VM22/210 type .....

http://www.nichecycle.com/ncs/categ...ts/vm22-210-vm22-210-pilot-jet-by-mikuni.html

If your pilots are the originals, you will notice they have no bleed holes down the sides. The replacements will have them but that's OK, they'll work fine."

If you go with the UNI pods here are some starting configurations that worked for me. @5twins has good advice. reach out to him

muffler		stock	closed end baffle mufflr	closed end baffle muffler	open core muffler
Intake	box	stock	stock	N/A	N/A
	filters	K&N stock replacements	K&N stock replacements	UNI pods: UP4200	UNI pods: UP4200
Carburator	type	BS38	BS38	BS38	BS38
	main jet	135	132.5	142.5	150
	needle	5O2	5O2	5O2	5O2
	needle clip position	3	3	3	2
	pilot jet	27.5	30	30	25