Misfiring?

drm1978

XS650 Member
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Hey Folks!

My wife's brother passed away suddenly back in December, and as part of the settlement of his small estate, she inherited the 1980 XS650 shown in the picture below. I've done little to it so far other than giving it new oil & filter, new air filters, new plugs, bleeding the brakes, installing the turn signals (for some reason, my brother-in-law had them in a box in his garage), and putting on a new front tire (the old one had a slow leak around the bead). Generally speaking the bike runs well, except that it seems to back fire occasionally from the left cylinder.

While idling, I occasionally feel a small gust of air emit from the nipple used for doing a vacuum balance between the two carbs (which I haven't done). This also happens in concert with the louder "POP" coming from the exhaust. Even more frightening is that I occasionally see a small "spark" exit from it (the nipple, not the exhaust!) which makes me think the intake valve isn't fully closed when the plug fires. By the way, there's a rubber cap and clamp on the right-side nipple, but it's missing on the left.

While checking the idle with my inductive pickup meter, I noticed that I got an unsteady measurement when connected to the left sparkplug lead, but a good solid measurement on the right.

Now, oddly enough, when I checked the timing using my inductive timing light attached to the left sparkplug lead, the mark appeared to be solidly on the 15 degree BTDC mark.

I have not adjusted the cam tensioner nor any of the valve clearances yet.

Any suggestions on where to start my troubleshooting?

Thanks!

Dave

Gambrills, MD

1980 Yamaha XS650SG
1993 Harley Davidson Ultra Classic
2000 Harley Davidson Low Rider
2012 Harley Davidson Sportster 883 Super Low
2015 Harley Davidson Heritage Classic
2021 Can-Am F3 Special


IMG_2976 (2).JPG
 
I would check the valve clearances and then inspect the carbs (BS34's). I am but a beginner so I would also suggest waiting for a more experienced member to reply. Thanks for joining.
'TT'
 
That uncapped left nipple originally powered a vacuum petcock. It looks like your original vacuum petcock has been replaced with a manual one, so you'll need to cap that open barb. As it sits now, you've got a giant air leak there and that's most likely responsible for the popping. It's also best to use a genuine rubber cap and clamp from Yamaha (both still available for a few bucks), they last and work best. I tried automotive vacuum caps and they would only last a month or so before they split.
 
I would check the valve clearances and then inspect the carbs (BS34's). I am but a beginner so I would also suggest waiting for a more experienced member to reply. Thanks for joining.
'TT'

That uncapped left nipple originally powered a vacuum petcock. It looks like your original vacuum petcock has been replaced with a manual one, so you'll need to cap that open barb. As it sits now, you've got a giant air leak there and that's most likely responsible for the popping. It's also best to use a genuine rubber cap and clamp from Yamaha (both still available for a few bucks), they last and work best. I tried automotive vacuum caps and they would only last a month or so before they split.

Here's a couple of pictures of the uncapped and capped nipples.

So, not to go off the thread, but what did the vacuum petcock do? Was the fuel flow from the tank turned on once a vacuum was pulled?

IMG_3122_LI.jpg
IMG_3126_LI.jpg
 
Yes, the petcock had a diaphragm on the back side that was "worked" by the vacuum pulses from the running motor. No vacuum, no fuel flow.

You can't run the bike with that nipple open like that. It's what's causing the backfiring and popping. If you still had the vacuum petcock, there would be a hose running from the nipple to the back side of that petcock .....

ManifoldClamp.jpg


NewVacLine2.jpg
 
Yes, the petcock had a diaphragm on the back side that was "worked" by the vacuum pulses from the running motor. No vacuum, no fuel flow.

You can't run the bike with that nipple open like that. It's what's causing the backfiring and popping. If you still had the vacuum petcock, there would be a hose running from the nipple to the back side of that petcock .....

View attachment 212799

View attachment 212800
Thanks! I'll fix it first thing tomorrow!
 
If the bike is still original, that will be an early version TCI module. In theory it shouldn't make any difference which spark plug lead you use for the timing light. 15 degrees for idle is good. The real test is on the road. The TCI module advances the spark between 1900 and 3100 rpm at which point the advance flattens out to 25 degrees plus whatever it is at idle (40 degrees btdc). That is determined by the two pickup pulses, 25 degrees apart. It should accelerate smoothly through those rpms. If it sluggish or lurches through those rpms it could mean that the TCI module needs a little attention. The good news is that version of TCI is completely repairable. If it is one of the later modules it becomes more problematic, but not impossible.
 
Thanks for the advice folks!

$11.43 got me a selection of vacuum system caps from Advance Auto, one of which fit perfectly! The bike runs much better now. No hint of backfiring and the idle is smooth.

I don't have plates for it yet, so I've been limited to driving it at or below 25mph around my neighborhood. Once I get plates, we'll see how it does on the open road.

Thanks again!

-Dave
 
One can DIY
A piece of rubber hose .available at gas stations the rubber hose for gasoline
comes in many inner diameters
In the open end a screw of the right tight size perhaps with permatex or silicone
A couple of steel wire loops or a hose clamp
But I dont believe you need it on either side the screew size has to be tight
 
Thanks for the advice folks!

$11.43 got me a selection of vacuum system caps from Advance Auto, one of which fit perfectly! The bike runs much better now. No hint of backfiring and the idle is smooth.

I don't have plates for it yet, so I've been limited to driving it at or below 25mph around my neighborhood. Once I get plates, we'll see how it does on the open road.

Thanks again!

-Dave
If they are the colored caps
1650891912544.png

they last "forever" you have no worries now. ;^)
 
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