Xs650 backfires at 2k-3500 rpm on left side

Hey everyone been lurking here for a while and finally I have decided to just make a post about my problem. A little info about my bike
1979 xs650 special
Points ignition
New coils and condenser
New carb boots with no vacuum ports
New petcocks (on, off, reserve)
Xs charge system with capacitor (kick only)
New spark plugs and wires and plug ends

Basically what has happened is on Sunday I went for a ride everything seemed great ran well no back firing no sputtering rode for 2 hours straight.

Next day I went to start the bike and it appeared to only be running on the right cylinder the left only seemed to get hot after I raised the throttle to 2500 -3500 rpm and it would occasionally backfire out the exhaust and sometimes the left carb. I have tried switching the plug wires to see if it was a bad coil and it still didn't fix the issue. I've had the carbs off 3 times cleaned them and used a air compressor to blast out the jets and corresponding ports and it doesn't seem to make a difference. When idling I can pull off the left plug wire and the bike will run fine off the right cylinder but if I pull the right plug to test the left side it just immediately dies. Is anyone able to point me in the right direction? Thanks in advance

Sounds like a lean condition on the left side, check for vacuum leaks around the left carb and intake boot.

I'll get some starting spray and when its running I'll spray it around the carb boot? See if that makes a difference. Thanks man.

Djfknwoo said:

I'll get some starting spray and when its running I'll spray it around the carb boot? See if that makes a difference. Thanks man.

Click to expand...

Could be leaking somewhere on the carb body itself as well, shafts, enrichner, diaphragm covers, diaphragms.

Mr Pyles is of Course well informed and knows his stuff
But speaking for myself
I have never had Carburetor problems ( Almost ) in 40 years
I have had massive electric problems

And here is no mention of " New Points " " Adjusted Points " " Ignition timing "
And 3 times carburetor cleaning

If the switch left to right has been performed in full complete the whole chain .
It changes the assumption --- But otherwise I would replace the Points and clean inside with some solvent.
Normal service activity

Jan_P said:

Mr Pyles is of Course well informed and knows his stuff
But speaking for myself
I have never had Carburetor problems ( Almost ) in 40 years
I have had massive electric problems

And here is no mention of " New Points " " Adjusted Points " " Ignition timing "
And 3 times carburetor cleaning

If the switch left to right has been performed in full complete the whole chain .
It changes the assumption --- But otherwise I would replace the Points and clean inside with some solvent.
Normal service activity

Click to expand...

I was leaning towards having to clean and adjust the points and timing. Only thing is I changed to a pma system and no longer have the tdc or fire marks. I know how to find the tdc but am still figuring how where to set the timing to without the original markings I also must assthe bike has a complete new electrical system that has been working perfectly.

Djfknwoo said:

I was leaning towards having to clean and adjust the points and timing. Only thing is a changed to a pma system and no longer have the tdc or fire marks. I know how to find the tdc but am still figuring how where to set the timing to without the original markings

Click to expand...

Sir
Please give information how the shift of wires between left and right was performed
So we can check if the problem follows with the shift.

I also have a non marked rotor ..
If this was me I would open the points side -- mark the current position
Start the bike and adjust the setting Observing the sparking at the points
And listen to sound and throttle response

There is the possibility to find Top dead center via the spark plug hole a wooden stick or better equipment
It is difficult to get exact .. But close enough

A video helps.

post # 2 there about ignition timing

https://www.xs650.com/threads/xsb-1-timing-retard.61508/

Jan_P said:

Sir
Please give information how the shift of wires between left and right was performed
So we can check if the problem follows with the shift.

I also have a non marked rotor ..
If this was me I would open the points side -- mark the current position
Start the bike and adjust the setting Observing the sparking at the points
And listen to sound and throttle response

There is the possibility to find Top dead center via the spark plug hole a wooden stick or better equipment
It is difficult to get exact .. But close enough

A video helps.

post # 2 there about ignition timing

https://www.xs650.com/threads/xsb-1-timing-retard.61508/

Click to expand...

So I switched the 2 spark plug wires and changed the wires coming from the Points to the opposite coil it was still only running on the right side. When you say mark the points position what would be the best way to do that. I'm sorry for the noob questions this is my first xs650 so I'm learning along the way

Sounds to me like carb problems.

I would drop the fuel bowls to see if you can see any junk at the bottom of the bowl. If you can, some dirt may have been sucked into the pilot jet or low speed circuit. Once the bowls are off you can remove the main and pilot jets and blow them out with compressed air. Same for the fuel bowl orifices. Check to see the float is moving freely as well as the fuel inlet valve.

Does adjusting the mixture screw have any effect on the no run condition? I have removed the screw and diaphragm before and given the orifice a shot of compressed air or carb cleaner. Check the diaphragm for tears or holes. This might dislodge some dirt. Make sure the fuel bowl is off when you blow air through, otherwise you’ll collapse the float. Ask me know I know. Lol.

Are you running fuel filters on the petcocks and in line on each fuel line? If not you should.

If your carb cleaning process has only been disassembly and blowing it out with compressed air, you could still have partially blocked passages that need chemical (carb cleaner) or ultrasonic intervention.

Contact Hugh’s Hand Built to buy a timing sticker for your case.

bosco659 said:

Sounds to me like carb problems.

I would drop the fuel bowls to see if you can see any junk at the bottom of the bowl. If you can, some dirt may have been sucked into the pilot jet or low speed circuit. Once the bowls are off you can remove the main and pilot jets and blow them out with compressed air. Same for the fuel bowl orifices. Check to see the float is moving freely as well as the fuel inlet valve.

Does adjusting the mixture screw have any effect on the no run condition? I have removed the screw and diaphragm before and given the orifice a shot of compressed air or carb cleaner. Check the diaphragm for tears or holes. This might dislodge some dirt. Make sure the fuel bowl is off when you blow air through, otherwise you’ll collapse the float. Ask me know I know. Lol.

Are you running fuel filters on the petcocks and in line on each fuel line? If not you should.

If your carb cleaning process has only been disassembly and blowing it out with compressed air, you could still have partially blocked passages that need chemical (carb cleaner) or ultrasonic intervention.

Click to expand...

When I turned the mixture screw in to where it seats it wouldn't pop at all. About 1.5 -2.5 turns out it pops and anything after that it stops. All that is during idle thought as soon as I give it gas the other problem seems to occur. I'm going to take the carbs off fully desemble them blow out all the ports. Then I'm going to check my points gap and change the spark plugs (current ones are very dark from running to rich for a bit) I don't have fuel filters on right now but they are on order.

OK
So if the switching of ignition parts have been performed .And still the right side only runs irrespective of what point is connected
The ignition timing and points replacement can wait .
I would probably inspect and clean with a little solvent but it is not Ignition related as first assumption any more

Upon taking the jets out writing up the sizes helps those who knows more about jetting than I do

But if the bike ran fine before it can be rust in the tank or so

I don't like work with carburetors so I would install a filter and try some carburetor cleaner chemical please consult a bike shop

that can help --

Jan_P said:

OK
So if the switching of ignition parts have been performed .And still the right side only runs irrespective of what point is connected
The ignition timing and points replacement can wait .
I would probably inspect and clean with a little solvent but it is not Ignition related as first assumption any more

Upon taking the jets out writing up the sizes helps those who knows more about jetting than I do

But if the bike ran fine before it can be rust in the tank or so

I don't like work with carburetors so I would install a filter and try some carburetor cleaner chemical please consult a bike shop

that can help --

Click to expand...

There is rust in the tank forsure lol

bosco659 said:

Contact Hugh’s Hand Built to buy a timing sticker for your case.

View attachment 210789

Click to expand...

He won’t sell em solo anymore. Unless you buy his whole system.

Atleast that’s what he told me a few months ago.

WideAWAKE said:

He won’t sell em solo anymore. Unless you buy his whole system.

Atleast that’s what he told me a few months ago.

Click to expand...

About a year ago I got a couple from him but I had to place an order for something. They wouldn’t sell the decals by themselves, too bad.

Backfiring is associated with lean running, running on one cylinder is generally associated with the slide not opening, if all else is ok. Lift the slide up with your finger, stick another finger over the long slot on top of the intake, then let the slide go. If the slide just slams shut you have a leak somewhere in the slide vacuum system. A number of things to check: Check the top cover is flat, they do bend over time if done up too tight. Also check the thick sealing ring around the periphery of the diaphragm, sometimes, after the rubber hardens with age, the rubber compresses, loses it's elasticity and won't seal properly. Also check the diaphragm for small holes. You won't see them without shining a torch from underneath or holding up to the sun. For the backfiring, check for leaks, multiple places to check there.

Djfknwoo said:

When I turned the mixture screw in to where it seats it wouldn't pop at all. About 1.5 -2.5 turns out it pops and anything after that it stops. All that is during idle thought as soon as I give it gas the other problem seems to occur. I'm going to take the carbs off fully desemble them blow out all the ports. Then I'm going to check my points gap and change the spark plugs (current ones are very dark from running to rich for a bit) I don't have fuel filters on right now but they are on order.

Click to expand...

Have you changed the spark plug? As you say they look very dark & the left may be fouled. A fouled plug will cause backfiring & not running all the time on that cylinder. A common cause of plug fouling is warming up the bike too long with the choke on & if yours is running a bit rich it may be the cause.

I'm thinking what you describe points to a blocked pilot jet. It resides in the float bowl (if you are still running the Stock BS38s, which you haven't actually stated) So they can be removed and checked without even taking the carbs off, it is a bit fiddly getting the screws in and out. Some sort of right angle driver helps a lot. a 1/4" ratchet and the right length of phillips bit can be made to fit.

the central hole in the pilots is tiny, easily blocked by a rust particle. A fine copper wire can often dislodge the offender. you should be able to see light though the middle when looking towards a light source. One strand from a motorcycle or automotive electric wire is about the right size to poke it free. Poke from the threaded end cuz the crap will have come up from below.Don't go ape with a steel wire cuz you can change the jet size pretty easily.
the pilot is just above center in this pic. 79 carbs should look like the LH bowl.

your pilots should look like the one on the left. The orifice at the top.

check that the brass is correct for your year. GENUINE Mikuni brass is strongly recommended.

Many carb cleanings ignore removing the emulsion tubes/ (needle jet) and cleaning the cross holes in them,

they are critical to proper operation, tapped out from above, the o-rings are usually bad, tiny and need replacing. Fully reseating the needle jet with new o-rings in place can be a bit of a chore.

For timing; these should give you enough info to make a set of marks


important to check timing on BOTH sides, they need to match closely, this can be more "fun" to achieve than you'd expect.

Confirm the advance is working smoothly and consistently. Advance rods sticking inside the cam causing erratic running are common.

You can play whack-a-mole with symptoms or you can follow a systematic troubleshooting procedure. Back when I first turned a wrench for a dollar, the boss drilled something into my hard young head with these words, often repeated: "Get your fingers off that carburetor! Ignition first, dammit!" Why? Because ignition, compression, and valve train can be directly observed and inspected and fuel/air mixture, in the absence of an exhaust gas analyzer of some sort, cannot.

Since you already know how to locate TDC, Gary has given you the key to making your timing marks. Check with static timing first. Rotate the timing rod manually and be very sure that your points open at the right time in both retarded and advanced positions. If the advance curve has spread, you'll need to correct for wear on the tips of the bob weights in the advance governor (AKA "automatic timing unit"). There are good threads on this, seek and you will find.
(Tip: I like Gary's fix better than the ones I used to use.) When points are free of pits, clean, and static timed, finish timing with a strobe.

Edit: Gotta correct myself here after rereading your first post. The problem didn't switch sides when you shifted the ignition from left to right, a point that wasn't missed by folks who advised getting into the carbies. That would be where the initial problem lies. Since the problem cropped up suddenly you might consider removing and cleaning the pilot jet and cleaning associated passages with aerosol solvent followed by compressed air rather than getting into a full rebuild.

Thank you guys for the replys. I am going to take the carbs off this weekend for a proper cleaning and check the condition of the slide and diaphragm. I will wait to hook the fuel lines back up after I get some fuel filters. I'll also check how clean the contact points are and make some marks for my tdc fire and advance so I can make sure it's properly timed. After all that is done I'll replace the spark plugs and fingers crossed I'll be good to go. I'll post an update after those are completed