Back fire through carb

Wilas89

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Just got my bike on the road. It seems to be running ok on a few very short shake down runs. However it back fires through the carb sometimes when I kick start or more often when it stalls. I’m a bit stuck for ideas why, because it seems to be ok when it’s running. Any ideas?
 

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If the timing is off just a little bit (advanced), it can do that.
 
If the timing is off just a little bit (advanced), it can do that.
I thought it was set right, but I will check. I was suspicious of the auto advance doing silly things but desired my strob was giving some bad readings. I need to fix a bad cable .
 
Many of us have taken to setting our timing a little retarded, over at the right side of the idle timing "range" (at the right slash mark). This helps ward off any possible pre-ignition and pinging, but could also help with your issue. And I actually think the bike runs a bit better like this, stronger at higher speeds and RPMs .....

Vh8F2GU.jpg


I even recently did it to my TCI equipped '83 .....

brRhheT.jpg


But even with the timing set correctly, occasional backfires through the carbs sometimes happen on these bikes. It seems to happen mostly when the bike is in that "gray" area between cold and fully warmed up. Sometimes you'll even blow a carb out of it's manifold, lol. Kinda an XS650 "right of passage", I'm thinking.
 
The key to this is just a little retarded, like 2° or 3°. Go 5° or more and you probably will have issues. What I'm trying to do is mimic the early timing spec, which was 10° - 15° BTDC. The middle of that spec would be about 12° or 13°. The later idle timing spec is 13° - 17° BTDC so setting it over at the right slash mark should put me about in the middle of the old spec.
 
Bringing this back to life. I had a couple of backfires through the carbs - and blew them off - twice today. After retarding the timing. I don’t know if logic applies here, but I’m going to advance it a little bit and see what the outcome is. T.

The key to this is just a little retarded, like 2° or 3°. Go 5° or more and you probably will have issues. What I'm trying to do is mimic the early timing spec, which was 10° - 15° BTDC. The middle of that spec would be about 12° or 13°. The later idle timing spec is 13° - 17° BTDC so setting it over at the right slash mark should put me about in the middle of the old spec.
 
Yes, maybe you went too far. These bikes are very sensitive about their timing. More than a few degrees off can cause issues.
 
Thanks for reconfirming. I dialled the timing back a bit (maybe too much) because it seemed lean and pinging. Although it’s hard to tell given the drive chain, cam chain and valves all singing their songs.
 
I’ll add that up until they went pop the bike ran top drawer at in-town speeds. Low and steady idle, sat happy at red lights, easy off the line, no stumbling or dead spots. Great fun.

Thanks for reconfirming. I dialled the timing back a bit (maybe too much) because it seemed lean and pinging. Although it’s hard to tell given the drive chain, cam chain and valves all singing their songs.
 
Yes, I quite like the '78-'79 BS38s. They run nice at low speeds and idle, better than the earlier sets. I think this is due to the spring-loaded needle set-up they got. It holds the needle in position more precisely. On the older slides where the needle is held in with that large plastic disc, if you hold that down with a finger then grasp the needle, you'll find it has about 1/2mm of up and down "slop". Since the needle clip positions change it's height by 1mm, this means the needle can bounce up and down through half a clip setting, not a very precise set-up to say the least. At higher speeds and RPMs this probably isn't very noticeable, but at low speeds I think it is.
 
Actually, I’m a bit confused over exactly what I’ve got. It’s a Canadian set. They appear to be 78-79 and came to me jetted accordingly but they don’t have the wee spring as you’ve shown elsewhere on this forum, just the disc. Yes, a bit of play in that set-up. But I’ve cleaned them, re-jetted and set the floats and have a healthy charging/ignition.

Yes, I quite like the '78-'79 BS38s. They run nice at low speeds and idle, better than the earlier sets. I think this is due to the spring-loaded needle set-up they got. It holds the needle in position more precisely. On the older slides where the needle is held in with that large plastic disc, if you hold that down with a finger then grasp the needle, you'll find it has about 1/2mm of up and down "slop". Since the needle clip positions change it's height by 1mm, this means the needle can bounce up and down through half a clip setting, not a very precise set-up to say the least. At higher speeds and RPMs this probably isn't very noticeable, but at low speeds I think it is.
 
PS I was at highway speed the other day and it stumbled some. I might go down one size on the main. Currently 140.

Actually, I’m a bit confused over exactly what I’ve got. It’s a Canadian set. They appear to be 78-79 and came to me jetted accordingly but they don’t have the wee spring as you’ve shown elsewhere on this forum, just the disc. Yes, a bit of play in that set-up. But I’ve cleaned them, re-jetted and set the floats and have a healthy charging/ignition.
 
Well, you really need to positively I.D. the year of your carb set because re-jetting is based on what was stock in it, not the year of the bike. It sounds like you have the older style slide with the large plastic disc holding the needle in. That would make the carbs '77 or older, unless that old type slide was swapped into a '78-'79 carb set. That might happen because the '78-'79 slide/diaphragm assemblies are very hard to come by.

Early Slide.jpg
 
I’ve not had much luck confirming their year as I can’t find photos that look exactly like mine e.g. showing the vacuum ports. But I think they’re 78-79 era. However, in support of what you’re saying: they do have the slide as per above and I think the needle didn’t match the 78-79 spec. I think it was from the earlier model. They were jetted to 78-79 spec.
 
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