Carbs or Ignition?

[JR Hammer]

So my XS (2FO) got a little shitty gas the other day and the pilot circuit got fouled. You know the kind? popping at idle crappy noisy running engine at low throttle, snappety pop pop BANG... So I stripped the carbs down a blew out every orifice and all the jets with carb cleaner poked out the jets with a wire bristle, rinsed it all in carb cleaner a 2nd time then reassembled, balanced vacuum, installed filters inline with tank... Real nice.

Then I took it for a 30 mile cruise around town, I hammered on the throttle a few times and everything seemed good... So I rode to work next day and now it's doing something else. Seems to run fine up to around 3k rpm then it gets choppy and sputters or stumbles under hard acceleration. Same thing from a dead stop, if I am at about 1/2 turn on the throttle or more the engine sort of lurches, like it wants to spin hard but then it doesn't.


so I stripped the carbs again, and halfway through I wondered to myself, "Is this the carbs?? Or could it be the ignition??"

pamco ultimate with iridium plugs gapped at 030. #137.5 mains/#27.5 pilots. Stock headers cut down to about 30"

 

[JR Hammer]

basically when the motor gets up to 4000 or so it starts acting like it's starving for spark or starving for fuel... I have no idea...

anyone have this happen?

 

[JR Hammer]

Bump... is this forum dead or what?

 

[gggGary]

Is it charging? Ever checked compression? How do the plugs look if you do a chop at speed?

 

[XSLeo]

If you had the carbs apart, chek the tops. If they arenot on right then there is an air leak above the diaphrams and they won't lift.
Leo

 

[JR Hammer]

Yeah XSLeo-- I think you might have something there. I had the carbs apart about 5 times and put them back together with the same gaskets because that's all I had on hand. So, I didn't mess with the diaphrams much but the bowl gaskets are dry and have been compressed too many times. Would an air leak in the float-bowl cause this symptom?

Yes it charges-- I am running a VR115 with stock alternator. Never checked compression. Plug chop i only have one set of good plugs so I read them but it's like reading the history of the bike since the plug change. 1st it was running on one cylinder, then I cleaned the carbs out. Next it ran fine but too lean in the right hole--- float level was off, so I opened the carb up again and adjusted the float. The I chopped the pipes and stepped the main jet up one size. It ran great for a little while but started doing this. Now the plug reads a nice light brown color with just a bit of soot around the ring but I can still see the white underneath from when it was running lean.

waiting now on a set of manifolds and gonna put the VMs on it.... probably get a rebuild kit for the BS38s and use them on the franken-bike I'll end up building with all the spare parts that keep showing up

 

[5twins]

VMs??? But you can't even sort your stock carbs? Good luck with that, lol. Float bowl gaskets have nothing to do with air leaks, they only seal the fuel in the bowl. Like I said, good luck with those fancy aftermarket high performance carbs, lol. I'm quite the carb guy and even I won't take that plunge.

 

[pamcopete]

JR...,

Your problems start at about the point where the ignition advance is supposed to be at max. If the advance rod is binding, then the engine will run out of enthusiasm at about 3,000 + RPM. Check your advance for binding like in this video:

Check your timing to see if it achieves maximum advance at about 3,000 RPM

 

[JR Hammer]

5twins---

if you ever get a close look at a VM carb and compare it to a BS carb you will see that the VM has less moving parts and therefore much less that can go out of whack. So if you are, as you say, "quite the carb guy" then I say, why not see how high you can jump? I may not have much experience but common sense tells me that tuning a set of VMs isn't going to be impossible as long as I follow the guide and have a few reliable sources to lean on if I need them. Honestly I don't even know if it IS a carb problem to begin with. I have cleaned them out twice and the problem remains so in my mind it must either be something simple that I overlooked in the fuel system or the problem is unrelated to the carbs altogether. These stock bs38s have 30,000miles on them and sat in dry storage fully drained for 3 years until this June.


Pete--- Thanks a lot man. I have been fiddling with that. my springs aren't real tight like that. The weights snap back on their own but it takes very little pressure to move them out.... still they dont seem to open all the way up til about 3500rpm and they do snap back but the idle sometimes changes mysteriously so I am always twiddling the idle screw. What would "running out of enthusiasm" sound like??

 

[pamcopete]

JR....,

It's more about how it feels than how it sounds. You reach a point where an increase in throttle opening doesn't result in an increase in engine speed.

If the springs are loose, then there will be some advance movement even at idle, so you would have to retard the timing to get it right at idle, thus using up some of the available movement of the advance so it does not reach the full advance position when it is supposed to.

 

[JR Hammer]

Ah---

I think it's a combination of the tired advance springs and some carb issues... It would be awesome if I could find the $8 spring set someplace nearby. It cost $7 to have someone take them off the self and another $5 to ship.

 

[scabber]

Did you ever check timing with a light ?

 

[JR Hammer]

yes, several times. Timing is dead to nuts on the mark at idle and goes past the advance mark under throttle.

 

[gggGary]

If the advance weights also have 30,000 miles on them it is rather likely they are worn, the symptom of worn advance weights is too much advance. Guess what the result of too much advance is? Hint there will be no compression on one cylinder.

 

[JR Hammer]

Gary,

I'm really not very experienced with these engines. On a classic volkswagen the advance comes from manifold vacuum line to the distributor. This idea of mechanical weight governed advance is new beans to me. When an engine runs advanced too much the plugs will read lean. So what would result in a dead cyl with no compression?? A big hole in the piston? a burnt out valve?? I'm throwing knives in the dark here because I dont really know for sure.

I cant be sure if they have 30k mi on the weights-- they are copper colored. not sure if that is OEM equipment or no.

 

[gggGary]

Yeah holed pistons happen...
Do a compression test?
A thumb on the plug hole would be enough if the piston is holed.
The stock weights are copper colored.

Read through this thread, several things should be of interest to you.
http://www.xs650.com/forum/showthread.php?t=11400&highlight=advance+weights+peen

 

[gggGary]

I'd be surprised if that Volkswagon didn't have a set of centrifugal weights in the distributor as well as the vacuum advance. That was pretty much the way all distributor/points cars were set up.

 

[JR Hammer]

Hmm never opened it up I just replaced the whole she-bang but you're probably right.

I checked compression today on the xs motor and it wasn't good. Really low in both holes So I'm getting ready to rebuild the whole thing... if it still has this problem then I will be PISSED. LoL the RAIN has begun so I'm not too upset. It was a fantastic summer-- albeit short

pricing out the gasket set and seals and whatnot right now. Probably get a new advance unit while I'm at it. I found an unused brand new head/rocker-box and a used XSperformance 750 jug/piston set on CL and I've been looking for an excuse to use it so here goes nothing...

any advice for the rebuild?? The bottom end has 30k on it... as of now I am looking at piston ring set, full gasket set, full O ring set, full top/bottom copper washer set, full oil seal set, new clutch fiber plates and springs, new throw-out bearing, spin filter mod.... can't think of anything else.

what should I do to the bottom end on an engine with this many miles?

 

[Patches]

Don't forget to check tour sump filter. If you haven't, it's probably torn.

 

[JR Hammer]

Patches,

thanks, I'm a step ahead of you I already had the new replacement sump filter--- I've actually been planning a rebuild for a while now.

So I'm considering the overdrive 5th gear and also new timing chain and new rod bearings while I'm at it.... and camshaft bushings... along with the gaskets, washers, seals, clutch kit, throw bearing, and a new advance assembly. I think that should about do it, can't really think of anything else... still open to suggestions