Early BS38 Carb Woes w/ Specifics

[Jerm]

1972 XS2 with 19,000 showing. Bought it in parts and put it together.
1972 BS38 Donor Carbs from an upstanding forum member. 130 Mains 42.5 Pilots Floats set at 25mm. Mix screws are both set 3/4 turn out. Dead cylinder method. Stock airboxes and new filters.
Pamco w/ Hight Output Coil & E Advance.
B8ES plugs gapped to .035"
Cam chain set.
Intake set @ .006" & Exhaust set @ .012"
Compression test Cold: RH 122 LH 118
Compression test Hot: RH 110 LH 110
Starts first kick or bump of the e start, hot or cold.

I cleaned up the carbs when I got them. Both plugs looked very rich (like the plug on the right in the image). I took them to a local very experienced with vintage metrics and let him go through them. Put them back on and the LH plug still looks the same. RH plug looks completely different now. I took a trip back over to the locals shop and showed him the plugs. He says the RH plug looks perfect. I was worried it might be a bit lean. He said it was probably a motor related issue since the bike was an orphan with no known history. He asked me to swap the carbs to the opposite cylinders and see what happens. He was sure it would stay in the same cylinder. I swapped the carbs to the opposite cylinders. Installed new plugs. LH carb on RH cylinder produced the same black plug. RH carb on LH cylinder produced the same whitish plug with a bit of tan down near the base of the insulator. Swapped the carbs back to their appropriate cylinders and the rich plug followed the LH carb. Is a clogged air jet(s) in the LH carb causing this or something else?

Here is a picture of the plugs for a visual. Black is sooty and not wet at all. Let me know what you think. Thanks in advance!

 

[DogBunny]

His swap test failed, so it's not the engine, it's his carb re-build. Take them back to him.
I do not know if the problem is the air jets, but I do know that it is very simple to test them. Spray carb cleaner into them with the float bowls off, and you should see the spray exiting.
Your compression is supposed to go up when hot, not down. Did you remember to hold the throttle open on the hot test?
BTW, I'm kind of curious how you swapped sides. I have never heard of anyone doing that. For starters, the fuel and the choke cross-over tubes won't match up. And attaching the cables would be problematic.

 

[Jerm]

2' fuel and air line to make the connections on the side swap. Throttles were easy to hookup prior to putting the carbs in the manifolds. Air boxes bolt up and all. I know it has to be the rebuild. Just looking for confirmation. I know compression is supposed to go up when hot. What could cause a decrease? Exhaust valves set too tight? Sorry about the crummy cell images.

 

[TwoManyXS1Bs]

Hey Jerm, next test is to swap carb bowls...

 

[grizld1]

Your valve clearance is incorrect for early motors. Spec for yours is .006" intake, .012" exhaust. As the old saying goes, it's better to hear 'em than to smell 'em. Check what your feeler gauges tell you against the readings you get from a dial indicator the first time around; if the valve stem is cupped, the feeler gauge will give the distance from the adjuster to the edge of the cup, rather than to the true contact point.

The carb problem is probably in the float system in the rich carburetor--leaking float, bad float valve, or error in float level adjustment.

The timing error you describe is dangerous, and the single most common cause of holed pistons in these engines. It's due to worn bobweights in the automatic timing unit. There are a number of ways to patch it up, and you'll find them with a bit of searching, but the best fix is replacement of the ATU.

 

[Jerm]

@ grizld1
I had the valves set to .006" & .012" until yesterday morning. They were a bit noisy. I saw the .003 & .006 info on 650central and thought I would give it a shot. I will set them back to where they were. I will have to look into the dial indicator.

The float level is at 25mm for sure. The float is not leaking. I am sure about these two things. Float valve is the only thing I cannot confirm as fine. No obvious flaws, but the parts are 40 years old. Swap needle and seat? Just the needle? Needle seems like it could be the weakest link. Should I swap the bowls as twomanyXS1B suggested prior to changing anything to do with the float valve?

I replaced ATU with Pete's new E advance about a month ago. Everything was good at install. Pete has requested I remove the advance rod and insect the copper cup for signs it has rotated out of position. I have not done this yet because MOTOGP was in town. I will be investigating this tomorrow.

Any thoughts on the cylinder compression decrease when hot?

 

[grizld1]

Re. bowl swap, sure, try it. If it makes a difference, have a look at the main and pilot jets, could be hogged out by aggressive cleaning, and a PJ of the right size and the wrong type could have been installed. Check the stamps on the float bowl bosses and make sure they're the same; the identifying mark should be "A." Also inspect the needle jet (AKA atomizer) and needle (the one in the slide, not the one in the float valve) on the rich carbie. Those parts are subject to mechanical wear, and NJ's are easily damaged by aggressive cleaning. I've seen real messes made when incorrect parts were installed, and if a wrong needle was in place your mechanic may have missed it. Be sure the O-ring on the NJ is in good shape. I don't know what to make of the hot compression drop offhand, but if I were you I wouldn't worry about it just yet. When you get the ignition and carbs straightened out and put some miles on the motor that issue may clear up on its own.

 

[Jerm]

The needle on the right is in the properly functioning carb. The one on the left is in the rich carb. The tips appear different to my eyes. Hopefully this is my issue. Please advise.

 

[wesleyonoel]

Not sure if your carbs are identical to those on my '73 TX so this might not be the source of your trouble. But...........the left carb on the stock set-up has an "enrichening" circuit or jet which serves as a "choke". Carburetors are linked with a cross-over tube which allows additional fuel to pass from the left carburetor to the right one when the jet is lifted (open). When the thumb/finger actuated lever which operates the enrichening circuit is in the down position, additional fuel (beyond that which is drawn through the idle circuit by venturi vacuum signal) passes through the jet thereby creating a "rich" mixture condition. If the cross-over tube isn't attached between the carubretors, the rich mixture condition (jet opened) will only exist on the left side and the right will have a vacuum leak leading to a potential lean mixture condition for the right cylinder. Hope your mystery is solved soon........................Wesley

 

[Jerm]

Crossover tube is attached for sure. Thanks for the input.

 

[Jerm]

Adjusted vales back to .006" & .012". NJ looks very clean with no obvious damage. O ring was replaced so it fits nice and snug. Needles were replaced with NOS needles during rebuild. I put a float needle in from another carb I had for parts. I took it for a short spin around the block at low RPMs and the plug looked good. Retarded the timing just a tad so it stopped at full advance, took it out for 5 miles and it ran good. Came back and checked the plug only to find it very black again.

I have figured out that the throttle stop screw on the rich carb is about 3 more turns in than the one on the properly functioning carb. I did not realize this before. I tried backing it out to where the other is, but it will not idle in that range. Had to crank it back down and then back it off until I could get it idling around 600 on that cylinder. Ended up right back where I started. Starts right up, idles around 1200. Right plug still good, left still BLACK. Any other suggestions pertaining to this?

 

[Jerm]

Does anyone think it could be my choke plunger not functioning properly? These carbs only have one plunger and it is located on the rich carb. Has the little black seal on the bottom of the plunger failed?

 

[wesleyonoel]

By your earlier description of the problem it appears the rich condition indeed follows the left carburetor - regardless of position on the engine. Recognizing this is the only carburetor of the 2 with an enrichening circuit, it makes sense to investigate that circuit a little further. Might even consider temporarily blocking the circuit. My '73 TX seldom requires use of the enrichening circuit anyway so should be able to start/run/test with it temporarily disabled....................... Wesley

 

[Jerm]

What would be the best way to temporarily disable it?

 

[gggGary]

Doesn't sound like whoever dumped those carbs off on you did you any favors.....

 

[Jerm]

I have been waiting on you to show up.

It HAS to be this choke plunger.

 

[wesleyonoel]

Would have to get creative. Looks like the entire housing containing the enrichening circuit can be removed from the carburetor. Probably have a look there. Other option would be to block the passage feeding the circuit from the bowl. Idea is to either block the fuel supply to the jet or from there to the venturi. If the housing is removed remember to also block the corresponding orifice of the right carburetor which "shares" the additional fuel supplied by the enrichening circuit of the left carburetor. If not blocked a vacuum loss will occur on the right side..................Wesley

 

[gggGary]

Well for sure, try replacing it...

That 70 in Ioway came to live by me.

Picked up a 72 engine last night, supposed to have 1200 miles on it. Dropped the sump filter and it wasn't torn, that is the first time I have opened a motor that didn't have a torn sump screen. Billy bob must of had a bad gasket leak up in the head, he kind of butchered the head loose and just quit, about 40 years ago? Not looking too bad for an engine that's been sitting 40 years with the head off, pistons are free, bores look good....

 

[Jerm]

I think I will just look for/buy a replacement. The weird thing is that both mikes and the vendors on ebay with NOS parts show the choke plunger not having the groove that my plunger does. Does it matter? Seems like an important detail....

 

[Stone Hands]

I think I will just look for/buy a replacement. The weird thing is that both mikes and the vendors on ebay with NOS parts show the choke plunger not having the groove that my plunger does. Does it matter? Seems like an important detail....

I know this thread is years old. I have been fighting the same problem on both carbs. Plugs black and oily or wet. Fouled out to the point the bike won't start. I did this:

1. Replaced the choke plunger as you did. The replacement one from Mikes has not groove for what I think was for an O ring on the original.
2. I noticed on the main jet, that when I pulled it out, the rubber O rings on both were chipped, brittle and one was missing. I think this was due to a carb soak I did not knowing the carb cleaning solution I soaked them in eats rubber. I ordered 2 new O rings from Mikes and replaced.
3. Put in 2 new plugs and gapped them and the bike took off like a champ. I have not run it enough to see plug conditions but will do so quickly. BTW my bike is an XS2.

A question,,, i know that the choke button is on when in the down position,, so I am sure, on the plunger itself, is the choke on when it is up or down? thank you .