Right carb is sucking and producing no power

[Castle551]

Hello! Long time listener, first time poster. I've got a 1975 xs650 with Mikuni carbs.

I apologize if this question has been answered. I've searched, but I couldn't find it.

I was out on a ride and everything was fine until I started losing power. I stopped on the side of the road and manually actuated the throttle on each carb. The left side is fine and producing plenty of power without the choke lever engaged. When the butterfly valve is actuated on the right side, all it does is produce a sucking sound without power. With the filter pods off, I made sure that the slide is facing the correct direction and slides freely. With the bike running, I covered the air intake with my hand and it shot fuel onto my hand and produced power and white smoke for a short amount of time and then went right back to producing no power.


I pulled the right carb, sprayed down all the parts with carb cleaner (I didn't pull the jets). To the best of my knowledge, it is seated correctly and all of the lines connecting to the other carb are in place. Keep in mind that this happened on a ride and at the beginning, it was running just fine. I'm going to pull off the bowl and see if its full of fuel.

I'm not SUPER familiar with carbs, but Im generally mechanically inclined and open to learning. I'm at a loss and would really appreciate some backup.

Thank you!!

 

[Castle551]

Update: Pulled the bowl off the right side carb and it’s full of fuel. Float and valve seem to move freely.

 

[jpdevol]

Welcome!

Have you looked at right spark plug; sparking while out (grounded to engine), color, wet (or dry)?

 

[Castle551]

Thanks for the response, jpdevol! I just bought the bike and it has newer plugs and wires. I just took both plugs out and the left side is a little black and sooty. Below is a picture of the right side. Wet and black. Right side (problem side) seems to be sparking, but it looks fouled. Both pictures are of the problem side from different angles.

 

[jpdevol]

I don't think you're getting proper spark there. Can you test? Spare plug?

https://www.xs650.com/threads/broken-bobber-ignition.60139/

 

[Castle551]

I don't have a spare plug yet, but I'll pick one up tomorrow. I put a multimeter to the spark plugs and it looks like the right side spark plug might have a short. Just out of curiosity, I swapped the spark plug leads and I couldn't get the bike to start.

It looks like the bike has new coils, leads, and spark plugs. Could it just be a faulty plug? Could my wiring have caused the plug to fail? The bike was running great until all of a sudden it wasn't.

 

[jpdevol]

Well, you know sometimes new parts indicate an old problem and sometimes are the cause of a new problem.

Your spark plug may have continuity to ground depending on whether those points were closed or open at the time. Best to work logically and the ignition system is the best first step. Start by checking battery voltage at terminals, then voltage at ignition coils, then check points - see the thread I linked above. Post back after you've run through some of the tests.

 

[jpdevol]

Mind you it could be other things - perhaps there's a carb problem that fouled the plug - still best to work in an ordered way to diagnose.

 

[Kevin Werner]

Well, you know sometimes new parts indicate an old problem and sometimes are the cause of a new problem.

"Hall of Fame" right there. You win the internet today...
My bike came with 2 sets of new plugs, turns out it was jetted richer than Jed Clampett's hunting grounds...

 

[grizld1]

Don't blame the malfunction on the carburetor until proven otherwise! Check compression. Then inspect valve lash. Then inspect ignition. If your B-model has the original breaker point ignition, that could be the root of the problem. Be sure to check timing at both full retard and full advance.

By "Mikunis" do you mean aftermarket VMs or OE BS38s? If aftermarket, a good baseline for VM34s at your altitude is the following:

Slide and pilot jet: 2.5 and 22.5 to 25 or 2.0 and 15 to 17.5. Needle jet: 159 P5 or P6. Needle: 6F9. Main jet: 180 to 190. Main air jet : remove.

 

[Castle551]

Ok, So I'm trying to run through it methodically as I've had time.

Tested the battery and sure enough, the battery is bad. I replaced the battery and tested the coils and leads. primary on the coils is measuring 4.5ish. The secondary is measuring about 13k on each side. I bought new spark plugs, but I messed up and bought some BR8EIX plugs without a removable terminal. I'm going to find new plugs and then test my points.

I haven't had a chance yet to test for compression and valve lash, but I will. My carbs don't look like VM34s. I believe I have BS38s, but there isn't a bridge linking the two. The only thing linking the carbs are the 3 hoses.

I also found this terminal that wasn't attached to anything. I don't think its the cause of my problem because the bike was running fine, but I don't know.

THANK YOU!! I really appreciate your suggestions!

 

[jpdevol]

Don't blame the malfunction on the carburetor until proven otherwise! Check compression. If aftermarket, a good baseline for VM34s at your altitude is the following:

Slide and pilot jet: 2.5 and 22.5 to 25 or 2.0 and 15 to 17.5. Needle jet: 159 P5 or P6. Needle: 6F9. Main jet: 180 to 190. Main air jet : remove.

Good catch there; if OEM, one would not normally just "pull right carb": I'm learn'in to read better! lol

 

[jpdevol]

Ok, So I'm trying to run through it methodically as I've had time.

Tested the battery and sure enough, the battery is bad. I replaced the battery and tested the coils and leads. primary on the coils is measuring 4.5ish. The secondary is measuring about 13k on each side.

We're also looking for voltage reading entering the coils - should be red with white stripe wire. New battery should have fixed that,but....

 

[jpdevol]

I also found this terminal that wasn't attached to anything. I don't think its the cause of my problem because the bike was running fine, but I don't know.

THANK YOU!! I really appreciate your suggestions!

Can't really tell from pic; that appears to maybe be connector to original light checker or dash lights.

 

[Castle551]

Tested the red w/white stripe using the bike as a ground and it’s showing 4.5-5 volts. Just installed the correct spark plug. Bike starts really easily, but the left cylinder is the only one firing. When I actuate the right side butterfly valve, the sucking noise persists.

I still have to check the points, valve clearance, and compression.

 

[grizld1]

The original factory recommendation for B8 plugs was made before NGK offered extended nose plugs, which are better cooled by the fuel stream. Use BPR7EIX if you want iridium plugs. Right, your carbs are OE BS 38s. Before the C-model ('76) the carbs were separate. Aftermarket carbs don't have throttle plates in the throat or diaphragm lifted slides. Refer to the Carb Guide (link in Tech section) for original jetting of your carb set.

 

[Castle551]

Thanks grizld1! I might dive back into the carb.

I think the thing that confuses me the most is that the bike was running great till it wasn't. If it was the points, valve lash, compression, or some of the other electrical problems, it seems like it would happen more gradually. This was a shift from cylinder firing to carb sucking noise within a 10 minute span.

 

[grizld1]

I strongly recommend that you service the points before opening the carbies. Nine times out of ten a sudden failure will turn out to be electrical, and if breaker point ignition is present it's a likely culprit.

 

[jpdevol]

Tested the red w/white stripe using the bike as a ground and it’s showing 4.5-5 volts. Just installed the correct spark plug. Bike starts really easily, but the left cylinder is the only one firing.

That wire (R/W) runs from switch, unhooked from coil it should show within 1V of battery voltage (some drop). Will be interesting to see findings on RS points as this happened suddenly.

 

[grizld1]

The standard max is 0.1V per switch and connector. If you're seeing a full 1V drop, make sure of power out from the key switch. You don't want to see a big drop going to the regulator; the regulator will "read" the drop as a low charge condition and overwork the alternator rotor, with the potential for serious damage. I don't like to see more than 0.25V drop from battery voltage anywhere.