1974jh5
Curmudgeon.
I got the throttle shaft seals but haven't put them in yet. We will see.
I won't give up on her, unlike my ex-wife.
I won't give up on her, unlike my ex-wife.
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REALLY bad flat spot off idle
- Thread starter 1974jh5
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- carburetors
1974jh5
Curmudgeon.
Aaaannnddd.... new shaft seals did not fix it. 
Although the idle speed screw had to go in almost 2 turns more to get the idle at the ~1200 RPM range, so yes they were affecting low speed operation. In fact, it seems to have gotten worse.
If I put my finger over the air bleed hole in the carb throat that feeds air to the pilot circuit, it kills the engine almost immediately. That would indicate to me that the pilot circuit is working. I went over it four or five times looking for vacuum leaks... nothing.
So this leaves pretty much only ignition. I pulled each wire one at a time, put a plug on the wire and grounded it to the engine. When all the crap starts I still have spark. I did notice that disabling the left cylinder made the engine run at a much lower speed, which I would expect but had no real effect otherwise. When I disabled the right cylinder, it started sounding like it was backfiring. Even though it's 'waste spark' I swapped the plug wires side to side just to see, it runs the same.
Anyone have a good used TCI box reasonable?
Although the idle speed screw had to go in almost 2 turns more to get the idle at the ~1200 RPM range, so yes they were affecting low speed operation. In fact, it seems to have gotten worse. If I put my finger over the air bleed hole in the carb throat that feeds air to the pilot circuit, it kills the engine almost immediately. That would indicate to me that the pilot circuit is working. I went over it four or five times looking for vacuum leaks... nothing.
So this leaves pretty much only ignition. I pulled each wire one at a time, put a plug on the wire and grounded it to the engine. When all the crap starts I still have spark. I did notice that disabling the left cylinder made the engine run at a much lower speed, which I would expect but had no real effect otherwise. When I disabled the right cylinder, it started sounding like it was backfiring. Even though it's 'waste spark' I swapped the plug wires side to side just to see, it runs the same.
Anyone have a good used TCI box reasonable?
mouser
XS650 Addict
got a viable TCI you can try? ..In 45 years of trouble shooting...every friggin thing..Is never overlook the obvious...ie. bad fuse! bad spark plug bad ground...don't matter if it is new! These aren't space shuttles...start at the moment your problem started..when it last ran well and what you had just done..
1974jh5
Curmudgeon.
That's the hell of it, I did ~zero~, I just rode it, noticed a quick stumble that went away, then the next time I tried to ride it it ran like ass and has ever since.
1974jh5
Curmudgeon.
I did just that today, the ZD1 Zener diode looked bad and from what I see in other TCI problem threads the 4 diodes for the pickup coil have been known to be weird. So I replaced all 5, it runs exactly the same. Anybody near Columbia SC have a TCI box I can borrow and plug in to see if that cures this friggin' thing?
Anybody near Columbia SC have a TCI box I can borrow and plug in to see if that cures this friggin' thing?
1974jh5
Curmudgeon.
I replaced the power transistior in the TCI box, still no change. I did some more testing, I ohmed the primary and secondary windings on the coil and they fell in spec, I have new NGK plug caps and they test good also. I verified ~700 ohms across the pickups, verified battery voltage at the coil, made sure the kill switch ohmed 0, verified battery voltage at the TCI module, verified a good ground for the module, etc etc etc. So I decided to see if the spark was going away when the engine begins to die. I did this one cylinder at a time (duh), on the left cylinder I had spark even as the engine began to die but it was a reddish weak looking spark, never any bright blue with a 'snap'. Then I went to the right cylinder and did the same thing, at idle I have the same weak reddish spark with occasional bright hot blue ones, when the engine begins to die I lose ~all~ spark on the right cylinder. Weird, since both cylinders run off of the same primary and secondary windings. But it's an indication of a bad coil so that's where I'm headed next.
1974jh5
Curmudgeon.
New coil today, it cleaned up the spark (now have blue instead of reddish) but it still runs like a bag of ass. Pull the choke out all the way and it revs up good, it can be ridden. Somewhere in all my crap ~100 miles away I have another set of BS34's, I guess that's the next step.
mouser
XS650 Addict
If it will run on choke, which in the case of our bikes an enrichment thing, not a true choke...then you are running lean. You could have a vacuum leak or plugged jets, or poor fuel flow.....petcock? ...fuel line. Plugged cap vent..
.When I got my bike...an 80 the very first thing I did was to throw away the 34's and installed 38's and manifolds...There is no comparison! IMHO
Keep after it....it is only a machine...and they generally work well.....good luck
And I know this will create chaos in the ranks...I only use Champion Plugs...in fact if anyone wants 10 pr of spare NGK plugs....like new!...they are yours...
.When I got my bike...an 80 the very first thing I did was to throw away the 34's and installed 38's and manifolds...There is no comparison! IMHO
Keep after it....it is only a machine...and they generally work well.....good luck
And I know this will create chaos in the ranks...I only use Champion Plugs...in fact if anyone wants 10 pr of spare NGK plugs....like new!...they are yours...
1974jh5
Curmudgeon.
Yeah, lean off the bottom was what I kept thinking all along with a side foray into possible electronics. If the other set of 34's cures it I will probably get a set of round slide Mikunis.
1974jh5
Curmudgeon.
Finally got my three extra carbs that have been in storage. Two of them are in too bad a shape to even mess with, the third one was in good shape but had been modified extensively to use with boost. The left side carb has been the worst offender with all of this crap, so I stripped off all the modifications and swapped this body with the original left side. It made things a LOT worse. So even though it's not fixed it seems I am on the right track. I am sick of messing with these damn things. Time for a set of VM34's.
Classic
rackin' up the miles
My money is on float level or clogged fuel line.
ProSimex
XS650 Addict
Those bs34's seem a little wimpy to use with you big bore kit. The VMs are pretty cool, but require some fucking around with to achieve awesomeness. Im pretty happy with the BS38s on mine. Dont even bother messing around with shitty worn out little carbs like those old 34s ou have
1974jh5
Curmudgeon.
It's a stock bore kit, when building this motor I purposely stayed conservative. (I plan to build another motor that will be whoopass: 277 rephase, welded crank, 750 kit and go with FI. Yeah it's ambitious and will be a long time coming but it sounds like fun!) It actually ran very well when it was all put together, I put ~300 miles on it then it went to crap. I think the progression phase of the carbs is clogged with fuel tank sealant but the problem is you have to destroy the damn carb to get to the passages. And I am also sick of screwing with them.
1974jh5
Curmudgeon.
Well, one set of 650 Central VM34's later it runs like a striped ass ape. 
I have stripped the jets etc from those 34's and dropped the carcasses in the recycle, they will go to that great carburetor resting place in the sky. 
I have stripped the jets etc from those 34's and dropped the carcasses in the recycle, they will go to that great carburetor resting place in the sky. 
rawdog12571
XS650 Enthusiast
i just read this thread and got alot of good info guys thanks...i just finished my build, bought it in fall and it ran like shitty at full throttle...pulled it in my house this winter and did a full carb rebuild and went with the heinken performance kit from mikes, oh and also changed diaphrams....so i had it inside and it reved nice and crisp but i had a nice winter day today and tokk it for its maiden voyage and once undr a load it runs like a turd...my buddy suggested coil which was news to me but what im really hoping is that it has a sprty tank with a cap that is ventless so i really hope that is the problem.....im gonna run it tomorrow and find out....this thread was helpful thanks guys...
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