Backfiring VS 1400

[SerTorm]

I just brought on a ’95 VS1400 which sat for quite a while, though it never had running issues. I have good reason to believe the story. The guy is well known to me and trustworthy, and he was the only owner. Also with the exception of the tiny tip-over dent in the tank that every 90s bike seems to have it is in cherry condition. He put almost a thousand miles a year on it until 2010 or so, then it was very sporadic. Before I came into the picture he had somebody cleaning it up to start riding again, and they cleaned the carbs and changed the oil, replaced the air filters and the like, unfortunately an itemized list was not provided and a single coherent narrative was not relayed, so I don’t know all the details.
When I picked up the bike it was stormy and I was mentally and materially unprepared to ride a pretty bike in those conditions so I was content to hear it run and warm up a bit in the garage before loading it in the truck. It sounded fine, awesome really.
Take the bike home, wait for clear weather. Try to start the bike, it turns over but battery is weak, my unfamiliarity with the choke and this bike's cold weather protocol leads quickly to a dead battery. Fine, bike has tender lead, I have tender.
Next day, weather still passable. Bike starts and warms up fine, notice high idle but shrug it off. Notice bike revs when bars full lock right. Noted and logged for further study. Ride around the block, maybe a few blocks, notice motorcycle backfires in first and second gear while accelerating hard. Bike also backfires once hard when engine is turned off.
Throttle cable adjusted to provide a little slack at the input and remove revving condition at steering lock. Bike started and warmed up, now has idle issue. When I blip the throttle it stalls. If I roll on easy and use a lot of clutch it will keep together, but sudden throttle input is no bueno. Backfiring in first and second under load is diminished but still presenting, bike did not backfire when I shut it off.
I apologize for the stream of consciousness format, but I try to report what happens and in what order so I don’t forget something important (I will have done so anyway) and so I don’t get off topic. I would like to correct the backfiring issue as well as perhaps the cold-bloodedness (if that’s a thing) and any suggestions would be welcome. Thanks in advance.

[navigator]

You mentioned the carbs were cleaned.
I suspect they need to be synced.
Also the backfiring may be due to leaky plug boots or a weak battery.

[Herb]

You mentioned the carbs were cleaned.
I suspect they need to be synced.
Also the backfiring may be due to leaky plug boots or a weak battery.

Running too lean can also cause it.

Have the idle mixture screws been uncapped?

[SerTorm]

Happy new year ever'body, I hope it finds you in good health.

The mixture screws do appear to be "uncapped". I'll look into the syncing threads..

[Herb]

The 1400 has a problem with backfiring in stock form because the idle screws are set too lean.

While you are checking the carbs check the vacuum diaphragm on the slide. I was having trouble with mine and the diaphragm on the rear carb had a bunch of holes in it.

[Lechy]

Happy new year ever'body, I hope it finds you in good health.

The mixture screws do appear to be "uncapped". I'll look into the syncing threads..

Happy New Year.

Carb synch and AF setup info here: viewtopic.php?f=32&t=4139

[SerTorm]

Problem may be worse than I thought.

I started it up again the other night. Not sure what for, just to hear it go, I guess. Anyway while it was warming up I had the bright idea to point a thermometer at the engine and see what was going on in that respect. The back jug warms up pretty quick, but the front remained cold. I'm assuming this means the front isn't firing at all and that I'll need to work out why before any balance issues can be focused on. I took off the plug cover and looked at the plug, little wet (gassy), and narrow. I opened the gap on the plug up to .035, not sure if that would be enough of an issue to foul the system. Thought I'd look for opinions on other possible low-hanging fruit.

I have the tank unbolted, but I'm waiting to remove it fully because instead of hose clamps the fuel lines are secured with zips and I'd like to have a proper replacement on-hand before taking it apart. All the wiring looks in place and unmolested, everything is very clean, but the zip-ties worry me..

[navigator]

It is very common to use zip ties in lieu of hose clamps.
A very out of sync condition will cause the front not to fire.

[Herb]

It is very common to use zip ties in lieu of hose clamps.
A very out of sync condition will cause the front not to fire.

+1

[SerTorm]

I'm in the process of cobbling together a sync tool with hoses and water bottles, fun little project, having trouble finding adapters to thread into the vacuum ports on the carbs though. I was hoping to avoid buying online, but it looks like 10$ plus 12$ shipping will get me a small handfull of 5mm x 0.8 threaded vacuum nipples.. Regardless, I was poking around under the tank to acquaint myself with the terrain and found this bracket is very loose on it's mounting. It looks like the angle of attack is difficult when the carb is installed and there may have been a failed attempt to fasten it resulting in a partially stripped head (non-JIS bit in a JIS screw, I'm betting). I figure that plate will need to be fastened securely to the side of the carb for the adjustment to mean anything..

[SerTorm]

I got in there with a proper driver and found it is in as far as it'll go. Is it possible there is supposed to be a washer in there or the last guy swapped screws and ended up with one that's too long by a C.H.? I'm in the process of locating a washer in my spare bins, somebody wave me off if the bracket is actually loose by design.

-On the bright side, I manipulated the cable and got the front side to start firing, so that's progress...

[Herb]

I got in there with a proper driver and found it is in as far as it'll go. Is it possible there is supposed to be a washer in there or the last guy swapped screws and ended up with one that's too long by a C.H.? I'm in the process of locating a washer in my spare bins, somebody wave me off if the bracket is actually loose by design.

-On the bright side, I manipulated the cable and got the front side to start firing, so that's progress...

No, you are not wrong, the bracket is supposed to be tight.

[SerTorm]

I put a washer in there and it allowed for secure fastening. [woot]

I have tightened the cable and thought that I had the front firing regularly, the throttle was responsive anyway, and I decided to take a test ride. I rode around the block(s) and discovered the backfire issue is still a thing, and by the time I got home the throttle response had deteriorated to the point that rapid application of gas resulted in a stall.

I’m going to take the tank off again and tighten the sync a little ferther to get the front participating consistently. What should I be looking at to address the backfire? Is there any hope I can find my way to good enough without a rigorous balance job, since it looks like the tool is going to take some time to put together?

[hillsy]

Is there any hope I can find my way to good enough without a rigorous balance job, since it looks like the tool is going to take some time to put together?

All you need is about 12' of hose and a stick to zip tie it to. You don't need all the bottles and stuff - you have a twin. Just one end to one carb.... the other end to the other carb....

[navigator]

I’m going to take the tank off again and tighten the sync a little ferther to get the front participating consistently. What should I be looking at to address the backfire? Is there any hope I can find my way to good enough without a rigorous balance job, since it looks like the tool is going to take some time to put together?

Adjusting that sync cable is sure to throw the sync out...if it is in now.
I doubt you can tighten it and suddenly be in sync....if it is out now, without a proper balance job.
The sync adjustment is very fine...a 1/4 turn will change it drastically.
JMO.

[Herb]

On top of that, the idle sync can be spot on and the high speed can be way off.

[SerTorm]

All you need is about 12' of hose and a stick to zip tie it to. You don't need all the bottles and stuff - you have a twin. Just one end to one carb.... the other end to the other carb....

Maybe I'm looking at the wrong thing, I was under the impression that the vacuum ports on the carb are threaded holes, not suitable for fitting the end of a hose over.

[navigator]

All you need is about 12' of hose and a stick to zip tie it to. You don't need all the bottles and stuff - you have a twin. Just one end to one carb.... the other end to the other carb....

Maybe I'm looking at the wrong thing, I was under the impression that the vacuum ports on the carb are threaded holes, not suitable for fitting the end of a hose over.

Motionpro sells 5mm adapters for about $10.
Part # 08-0013.
https://www.google.com/search?q=motionp ... e&ie=UTF-8

[Herb]

For the front cylinder, you use the port that the boost sensor is hooked up to. For the rear, I put one of the adapters in and just cap it with a rubber vacuum port cap. Getting that stupid little plug in and out is a real PITA.

[SerTorm]

Alright, I have the parts from MotionPro and everything is assembled. Those were the only pieces I was missing, the bottle part was easy and the relative vacuum principle without the impending risk of injecting water into the combustion chamber appeals to me. I'll just work out a fuel I.V. and get to this.

One last thing, I keep seeing references to target RPM, obviously there isn't a tach on this bike.. Do I work by ear or is there an easy to rig temporary tool I'm not familiar with? I guess when it comes down to it I wouldn't necessarily mind adding an aftermarket unit to the dash, but maybe not for this particular job.