Smoke and idle struggles runs fine once warm

[Fingerscrossed]

Hi everyone,

I really appreciate this forum. I've been through a lot with this XS, but then life got in the way and it was left to sit for a few months... It was running quite well before that except for this problem, which is apparently still a problem (sometimes I wish letting things sit would magically fix issues rather than compound them). The bike starts up first kick (with choke) but will die after running for 30 seconds or so — it seems to be the right cylinder that cuts out. There is quite a bit of grey smoke after it gets warm, but that goes away once completely warmed up and riding. I have noticed that the fuel disappears from the fuel line overnight (and I don't think anyone is breaking in and sucking it out but I would need cameras to be completely sure). I'm wondering if the smoke is from the fuel somehow getting into the cylinder and making the mixture very rich, but then I don't understand why it only starts with choke.

The last few times I rode it (after struggling to keep it idling) it would accelerate and then cut out (like it lost spark) for a second and then regain power, this would happen twice every time and after that the bike would run and idle fine. I thought perhaps that it was running too rich and the spark plug would start to foul but then under load it would be hot enough to keep it running... Anyway, after starting it today, I pulled the spark plugs—which were wet with fuel—and then noticed quite a bit of light smoke wafting out from the right cylinder and not the left. There was no visible oil on either intake or exhaust valve before or after running. I have good compression on each cylinder (155 and 160)—I rebuilt the engine about 500kms ago. I'm running VM34's, and to my surprise the needle clip was already on the 2nd slot, but I can't remember if I did this for any reason. I'll check jets tomorrow but my hypothesis at the moment is that the gas cap isn't venting properly and that's making it run rich while also letting gas into the cylinder (and subsequently the oil) although I've pulled the cap apart a few times to check already.

I should say, the idle is consistent but it will just die—throttle seems to make it even worse. You can hear it struggling and then open up too much or something. I'll run some tests tomorrow, so please let me know any that you think may be useful. I would love to do a leak-down test but don't own an air compressor. Sorry for the ramble...

 

[gggGary]

Do you have a fuel filter in addition to the petcock filters?
have you drained float bowls into a catch container, any debris in the fuel?
Open and clean float needles?

 

[Fingerscrossed]

Do you have a fuel filter in addition to the petcock filters?

Yes I have an inline filter that goes to a T then into each carb.

have you drained float bowls into a catch container, any debris in the fuel?
Open and clean float needles?

I've just cleaned the carbs, I didn't find any debris. Cleaned the needle and seat (pin is very easy to remove with a automatic punch). The float level did seem on the high side, maybe slightly above 24mm so I took both sides down to the low end—22mm.

I've going to do a complete check over—camchain, valves, points and timing, resync the carbs and see how it goes.

Jetting is:
159 P-5 needle jets
185 main jets
25 pilot jets
6F9 needle
2.5 slides

I also hadn't removed the air jets, so I went ahead and did that.

 

[Fingerscrossed]

Timing was a little off but everything else was fine. It wasn't starting so I checked the spark plugs — who knows why but the right spark plug had no gap at all... Anyway, after I fixed that started right up, with no smoke and sounding perfect. The plugs were a bit wet when I pulled them, and it's hard for me to tell if it's oil or gas but I'll see how it goes. I'm going to say it's gas because I kicked it so many times.

I'm curious whether there will be smoke tomorrow and I'll mark the fuel line to see how much it drops overnight, if at all. Anyway, feel a bit stupid but at least everything seems to be right.

 

[Raymond]

You should never feel stupid, these things are not obvious and anybody can end up running round in circles.

 

[gggGary]

You should never feel stupid, these things are not obvious and anybody can end up running round in circles.

You talk'n to me?

 

[Fingerscrossed]

You should never feel stupid, these things are not obvious and anybody can end up running round in circles.

Haha thank you, circles is right. It wasn't so easy to start this morning, took a few kicks, but ran great once it warmed a little although it was still smoking on startup until warm. I'm still convinced it's fuel, but I guess I can try leaving it for a while with the carbs off or fuel line disconnected and see if it smokes. Anyway, was running well and then blew the main fuse... That hasn't happened before but I could feel it cut out while riding twice, like there was an inconsistent spark. I'll sort out the electricals but it's the smoke that's getting to me... I'm also pretty sure there is a small air leak as the idle once warm wavers up and down slightly. I'll do the WD-40 test for that.

EDIT: won't be able to work on the bike for a few weeks. I need a new reg/rec, so will sort that out—I think that may explain the inconsistent idle and why it was harder to start when I didn't charge the battery first. I've left it without any fuel in the carbs and the lines disconnected so if it smokes again I'll try replacing the intake valve seals.

 

[cra-z1]

Well I feel stupid, my buddy had a screw in the rear but the tire was not flat?. So I remove the wheel break the bead pull one side of the tire off, feel inside but no screw coming through and tube is not punctured? Remove the screw and it was like 1/4 long and never came through.

 

[Fingerscrossed]

Ok, so it's definitely a fuel problem.

I left the bike for 2 a bit over weeks with no fuel and last night it started up easy well with no smoke. After leaving it overnight (fuel off) with carbs attached, it smokes and the right cylinder is struggling to hold idle without choke. I'm talking to Mikuni because I can't get the two carbs to land at the same time—the one on the right (problem cylinder) is landing before the left, even if they lift at the same time. They haven't been so useful yet... any ideas would be appreciated. I've cleaned needle and seat, checked float height and set it to 22mm (the lower end of spectrum).

Again, it ran perfectly last night but after leaving it overnight...

EDIT: Just thinking... maybe 22mm is the higher setting. I put on my old BS38 carbs which have a hanging idle problem but the bike starts and runs fine outside of that. I might just oil the cable and try sort those out

 

[gggGary]

Lets see couple pics of the carbs one looking at the intake bells.

 

[Fingerscrossed]

Lets see couple pics of the carbs one looking at the intake bells.

For sure - let me know if you see anything. It's idling at 1300rpm with the screw all the way out. If I push the stop it holds idling at 900 but will only get that low if I push it. The hanging idle is most noticable when I blip the throttle to change gears, it sticks quite a bit. But its running well outside of that... Starts easy.

If I put my finger over the air intake and let the bells drop, the left one falls slower than the right. The right one doesn't fall straight down but takes 5 seconds or so.

The bike is idling strong though and I bet it won't smoke next time I start it up. I'll change the oil if it doesn't because there's probably fuel in there...

 

[MikeyDeepXS]

Did you get the electrical all sorted yet? When you said it blew the main fuse and had some odd hesitations/cutting out while running I began thinking that perhaps an inconsistent spark might be causing unburnt fuel to get pulled thru the engine and leaving residue in the exhaust causing the smoke.

Then again, it almost also sounded like a stuck open float could have been causing the problem, or even that gapless spark plug.

Are you using stock exhaust or something different?

What kind of ignition coil setup, dual output coil or single coil per plug?

 

[Fingerscrossed]

Did you get the electrical all sorted yet? When you said it blew the main fuse and had some odd hesitations/cutting out while running I began thinking that perhaps an inconsistent spark might be causing unburnt fuel to get pulled thru the engine and leaving residue in the exhaust causing the smoke.

Yes I did - the rectifier had gone bad, seems to be charging fine with a new one and no blown fuses anymore.

Then again, it almost also sounded like a stuck open float could have been causing the problem, or even that gapless spark plug.

Yeah, I've checked and cleaned the floats on the VM34'S but there's no visual evidence of anything in there.

Are you using stock exhaust or something different?

What kind of ignition coil setup, dual output coil or single coil per plug?

The muffler isn't stock, don't know about exhaust, they both came with the bike (attached photo). And points with a single coil for each plug. The coils are the stock replacements.

I think it's sorted electrically. I'm going to try loosening the butterfly plates and snap them shut again to see if that fixes the high idle. I'll also have another look at the needle and seat on the VM34s. Won't be able to check it for a few weeks again...

 

[gggGary]

Thanks for the pics!

so 78-79 bs38's in rather sad shape.

Have you vacuum synched?
this doesn't look good/right.

one inlet blocked off, the crossover connection sketchy as-f..
I see a choke body goobered up, stripped float screws. A poorly seating choke will cause a slide to drop, not open correctly, run rich at idle.
cleaning the emulsion tubes and replacing their o-ring often missed on carb cleaning.

 

[MikeyDeepXS]

Good eye gggGary!

 

[5twins]

Yes, I think your BS38s are out of sync. The fact that the idle speed won't drop any lower than 1300 with the idle speed screw all the way out tells me that. The idle speed should drop all the way down to nothing and the bike should stall out.

 

[5twins]

You need a new throttle cable as well, that one is toast, lol. To avoid kinking the end of the inner cable like that, install the end into the lever first, before placing the sheath in it's holder above. That way, you can come at it from the side, keeping the cable straight and not needing to bend it.

 

[Fingerscrossed]

Yes those BS38s have seen better days, which is why I ended up getting the VM34s. I can run an extra fuel line instead of blocking the inlet off, that's easy. I also have a spare throttle cable for them so I'll do that 5twins.

Unfortunately that was me using yamabond on the choke gasket... I have learnt a lot since putting together the bike...

I only synched them quickly by feeling the exhaust pressure, and feel like I got then pretty close but I'll make the DIY manometer and do it properly.

The spark plugs actually look quite good after running it but could still be rich at idle. I'll have a go at synching them properly and try to sort out the needle and seat in the VM34s as well.

 

[Jim]

I only synched them quickly by feeling the exhaust pressure, and feel like I got then pretty close but I'll make the DIY manometer and do it properly.

I've found that once you properly train your "feel" and "ear", you'll do just as good as a proper manometer.
Cup your ear so you only hear one exhaust at a time. Adjust the sync so they sound identical, then feel the pressure on both... adjust sync accordingly.

 

[FB71]

I've found that once you properly train your "feel" and "ear", you'll do just as good as a proper manometer.
Cup your ear so you only hear one exhaust at a time. Adjust the sync so they sound identical, then feel the pressure on both... adjust sync accordingly.

I do a lot by feel, eye, and ear. A few years ago, I had a younger coworker trying to align his '63 Nova on our very nice, very expensive, and (usually) very accurate Hunter alignment machine. Each attempt made it worse. He asked me to look it over. I pointed out some hidden damage/distortion to the front sub frame that he missed. That Hunter wasn't going to compensate for that. I had him put on the ground, and I did an eyeball alignment with a floor jack and prybar. Took about 10 minutes. Told him to take it for a drive. He said it hadn't steered that straight the whole time he owned it! Of course, he understood this was only a temporary fix until he found a replacement front sub frame. He lucked out a few weeks later, got an entire front frame with brand new suspension and brake components for a few hundred bucks! Seller had upgraded to some fancy front end to accommodate it's change into a drag car. Just wanted the stock frame gone.