Carburetor Tuning

bbb88

XS650 Member
Messages
9
Reaction score
4
Points
3
Location
Oregon
I have an 82 XS650. This is my first XS, and first carburetored bike. It's got uni filters and aftermarket 2-1 exhaust with a muffler (unknown brand, maybe Mike's). It's got the stock timing/ignition/charging systems.

When I first got it, it was very low on power, having trouble getting up small hills. Found that one of the spark plugs was not making spark. Swapped in new plugs, firing on both cylinders. I also tested the compression, both cylinders were at 110-120 psi cold. I also noticed a small exhaust leak from where the headers fit into the muffler from the left header.

The next problem was idle creep. I adjusted the idle down to around 1100 rpm after the bike warmed up. I'd take it for a ride and shortly the rpm would creep up to 2.5-3000 rpm. Seemed like a carb problem.

I bought the Keyster premium kit from Mikes, cleaned and rebuilt both carbs. Replaced the float needle, float needle seat/filter, needle jet, float gasket, mixture screw/spring/o-ring, and the main and pilot jets. It had the stock size mains and pilots. I put in the 49 pilot and 138 main. I sync the carbs as you can see here
(yes I know it's the stupid vacuum gauges :) )

The engine doesn't sound the greatest in the video... The rpms go up and down at idle.

I tried doing the dead cylinder tune from the carb guide, but it didn't seem to make any rpm difference turning the mix screw up to a whole turn in or out. So I'm wondering, should I order a smaller main and pilot jet, or try the bigger size mains from the kit? Or maybe something else is wrong...

Here's a couple photos from the rebuild: https://imgur.com/a/64KQVhU
 

Superjet

XS650 Guru
Top Contributor
Messages
1,182
Reaction score
2,371
Points
263
Location
Moncton, NB, Canada
The throttle body shafts could be tired. Spray carb cleaner while the bike is started spray on both sides of the carbs see if idle drops. While you are doing that wouldnt be a bad idea spraying the carb boots at engine and carb mouth. Did you check slide diaphragms for leaks?
 
Last edited:

bbb88

XS650 Member
Messages
9
Reaction score
4
Points
3
Location
Oregon
By throttle body shafts are you talking about the rubber gaskets on the inlets to the cylinders? Like these? https://www.mikesxs.net/yamaha-xs650-carb-holders-set-2-80-84-bs34-cv-oem-3g1-13586-3g1-13596.html

The ones on there are a little cracked. I was thinking about replacing them but reading on here about them made me a bit cautious that they may disintegrate quickly. I was looking for aluminum ones but the only ones available appear to only be for aftermarket carburetors.

I checked the slide diaphragms just by touch and shining a light to check for pinholes, I need to do the procedure specified in the carb guide. Or maybe just order a replacement as there's visible wear on the diaphragms already. It looks like mikes only sells the entire assembly, not just the diaphragm...
 

650Skull

Cockytoo
Top Contributor
Messages
9,999
Reaction score
11,612
Points
688
Location
FNQ Australia
replacing the needle jet without replacing the needle may be causing problems..........the old needle jet and needle would wear as a set replacing one without the other..........
 

bbb88

XS650 Member
Messages
9
Reaction score
4
Points
3
Location
Oregon
Should I put in the matching needle from the kit? The Y503-RR

Also, I forgot to mention in the first post, but I did take it for a quick ride around the block and it was back firing like crazy, no power, died after about a minute.
 

Superjet

XS650 Guru
Top Contributor
Messages
1,182
Reaction score
2,371
Points
263
Location
Moncton, NB, Canada
Not the carb boots but the shaft that goes thru each body of the 2 carbs linked together. Do a search and you will find oem Yamaha part numbers for replacements.
 

peanut

XS650 enthusiast & inveterate tinkerer
Top Contributor
Messages
4,054
Reaction score
2,809
Points
263
Location
England
sounds like a classic lean mixture issue due to inlet air leaks.

You have upgraded filters and exhausts so your carburettors will need to be re-jetted if it has not already been done.
Tell us what jets and needles you have fitted and we can start to advise about jetting etc.

My first step would be to re-fit your inlet rubbers with threebond or similar liquid gasket cement both sides of fresh gaskets. From then on remove just the carbs without disturbing the inlets . this will at least eliminate any potential air leaks around the inlet rubbers and allow you to concentrate on the carbs .
 
Last edited:

bbb88

XS650 Member
Messages
9
Reaction score
4
Points
3
Location
Oregon

peanut

XS650 enthusiast & inveterate tinkerer
Top Contributor
Messages
4,054
Reaction score
2,809
Points
263
Location
England
First thing I always do is once the engine is warmed up is to adjust the idle mixture using the idle mixture screws. Your Pilot jets look to me to be rather large but I'm not familiar with jetting the BS34 carbs.
 
Last edited:

5twins

XS650 Guru
Top Contributor
Messages
23,477
Reaction score
19,676
Points
813
You've probably over-jetted it. Also, you have a mis-matched needle and needle jet now. First thing I'd do is install one of the middle range needles, the "S" or "R" one. This kit is a fairly new offering from Mike's and there really isn't much feedback on it yet. What the kit is calling "Standard" in it's documentation appears to be the Canadian and "world" BS34 specs (130 main, adjustable needle). What's not mentioned is those carbs run a smaller 130 air jet. You may have to do that to get the kit parts to work right. Personally, I would probably start with the "S" needle and kit needle jet, the 133 mains and the 42.5 pilots, along with 130 air jets. That would be pretty close to stock but probably richer in the midrange.

I'll also mention that 2-1 exhaust systems can be difficult to tune for. That could be adding to your issues.

This could be another case of a carb kit designed for a foreign market being introduced here. The Heiden kit was like that and most reports on it weren't very good. It just plain doesn't work right in this country. My best guess is that our fuel formulations differ from those in Europe and Japan.
 

bbb88

XS650 Member
Messages
9
Reaction score
4
Points
3
Location
Oregon
The dreaded PO ground off all the mounts for the left side exhaust... :shrug:

When you say 130 air jets, what are you referring to?

I purchased the Keyster kit because I couldn't find new pilot jets for the BS34s
 

5twins

XS650 Guru
Top Contributor
Messages
23,477
Reaction score
19,676
Points
813
These carbs use common Mikuni jets. The pilots are the BS30/96 type and pretty readily available from many places. The air jets are mounted in the bell mouth where the air filter clamps on. A small round main jet is used just that in this case, it flows air. It is the same main jet that is used in the BS38 carbs .....

XuTgyBl.jpg


M98LIeO.png


Uoaj1xt.jpg
 

bbb88

XS650 Member
Messages
9
Reaction score
4
Points
3
Location
Oregon
That picture refers to the "pilot/air jet" but I thought the pilot jet was the smaller jet that goes in the float bowl under the rubber plug?
 

5twins

XS650 Guru
Top Contributor
Messages
23,477
Reaction score
19,676
Points
813
Yes, the actual pilot jet is the smaller one under the rubber plug. This is the air jet for the pilot/idle/low speed circuit. It adds the air that mixes with the fuel from the pilot jet in the pilot circuit. The stock size on U.S. spec BS34s is a 135. The Canadian and "world" spec BS34s got a smaller 130 size. This jet is another means of tuning your idle circuit. Making it smaller lets less air in and makes the idle circuit richer. It doesn't have as big of an effect as changing a pilot jet size so you can "fine tune" the circuit with it, achieve richness levels in between pilot jet sizes.
 
Top