Carb issues: choke on runs for a few seconds and dies

[JRay77]

I have a 75 with a 78-79 2FO motor. Carbs have the stock pilot and jets.(135 27.5) Slide needle clip is at 3rd slot. Float height was measured when I cleaned and rebuilt carbs. I also used a metric grease zerk and some clear tubing to check height (attached to bowl drain plug) and it appears fine. I'm running a PMA and the basic PAMCO ignition. The PMA has a key way and appears to only go on one way. The Pamco is the same. I've checked in this order

Cam chain tensioner=ok
Valve settings=ok
TDC= I used a homemade piston stop made out of an old spark plug. In natural rotation (counterclockwise) I went slowly until rotation stopped. Wiggled the valve rockers to assure this was TDC on this cylinder. Then I marked on the side cover. I backed off from the piston stop to remove. I then put the stop in the other cylinder and rotated slowly in the natural rotation until it stopped. Wouldn't half way between these two marks be exactly TDC?
Timing=ok firing close to 15degrees before TDC (used a timing light)
Carbs= not ok. Haha!
Carbs I've started the air fuel mixture at 1 1/2 out and 2 1/4 out and several combinations. It will on full choke, run for a few seconds and shit out. I've done the slide needle diaphragm check by holding my thumb over the moon shape hole and released the slide. Works great! Diaphrams are working fine. No holes. Brass floats I inspected when I had disassembled, and seem fine. I'm trying to run pod filters. Notice if I hold a hand over the carb (where filters go) I feel suction and the bike will run until I remove my hand. I don't want to damage the diaphrams so I don't do this long. It's almost like there isn't enough suction to draw fuel and no "chitty chitty bang bang" Any help or suggestions would be greatly appreciated.

Rock On!

 

[figure8]

"Isn't enough suction", have you checked compression?

 

[JRay77]

Yes I've checked compression. It's been awhile but I recall it being in the 130's in both cylinders. Forgot to add that to my list. There's a lot of suction with my hand over the carbs. I can feel it. Fuel will enter with my hand over but when my hand is removed the bike will stall. It's almost like not enough suction with filters on to get fuel.... Where to adjust?

 

[figure8]

What filters are you running? And your positive the diaphragms are good?

In regard to your other question, yes...

 

[JRay77]

Yes, I'm sure. I've done the tumb test. Also, I inspected them when I cleaned the carbs and installed new pilot, jets, gaskets, etc.

 

[figure8]

So stock filters or pods? Any chance to exhaust?

 

[JRay77]

Far as air filters, cone shape clamp on type.
Obviously I'm not real techy so this forum is a pain in the arse to use. I would post videos and pictures but I need to figure out how.

 

[JRay77]

Open dual exhaust that runs underneath a little past the motor.

 

[figure8]

Did up your jets?

 

[JRay77]

Yes, I had went up on the jets. Main and pilot. Main 140 Pilot 30. I was still chasing my tail. It was running fine before with stock jets until I had charging issues. This is when I went to the PMA and PAMCO setup with stock carbs jets.

 

[figure8]

Come shape filters and open exhaust indicate the need for an up in the jets. At least 1-2 in the pilots, 2-3 in the mains.

 

[JRay77]

I'll go back. I still have the up size jets. Why did it run fine before with stock jets? I dunno but I'm starting to think these XS650's are junk. Why is it that a 35+ yr old Honda or Kawasaki isn't this difficult? Hell Honda even had a snap style to remove carb bowl. Nice! There's a reason xs shit is cheap on Craigslist. Best days of owning an XS, the day you buy it and the day you sell it! Ok, rant over. I'm just pissed not being able to figure this simple machine out.

 

[figure8]

Patience here, xs's are super reliable when maintained properly. You need to play with the carbs, floats set correctly?

 

[peanut]

Nice! There's a reason xs shit is cheap on Craigslist. Best days of owning an XS, the day you buy it and the day you sell it! Ok, rant over. I'm just pissed not being able to figure this simple machine out.

sounds like thats the reason you've not cured the problem right there ...you've given up already and are blaming the bike ! Maybe you should buy a new bike

Hondas and Kawas and Suzis of the period all used the same cv type carbs and they now need exactly the same level of maintenance and repair to run reliably as do the XS series.

The most common cause of poor running and starting is air leaks ...tiny minute air leaks on the carbs and both sides of the rubber inlets. Thats where I would start after I had fitted the 30 pilot jets .

You need to adjust the air screws individually on each carb not turn them both out out the same number of turns. Turn a screw in or out 1/4 turn then blip the throttle and wait 30 seconds or so for the engine to settle down adapt to the change you have made .

When you rebuild the carbs make sure you set them in sync properly . The easiest way to do this is to blow each carb like a trumpet. You can feel very accurately by the back pressure when the carb butterflies are equally open and in sync.

Check both carbs all over for tiny air leaks especially the choke body gaskets .

Personally I apply Yamabond 1104 to both sides of all gaskets especially the rubber inlets . It may be hard work to remove on rebuild time but it eliminates any chance of air leaks period

 

[JRay77]

Thanks guys. I'll be in the garage soon. I understand that these old CV carbs operate on the same idea but the XS is the only one I have problems with for some reason. I have two KZ 650's and a 77 CB 750 SOHC which is my favorite.

 

[peanut]

wow ! thats a very nice stable you got there

Once you have got the carbs sorted they should be as reliable as the other bikes, provided they receive a little periodic maintenance.

Getting them set up and working correctly is a steep learning curve if you are not familiar with cv twin carbs especially if we fit foam filters and straight through silencers etc but if we are methodical and meticulous those BS38s come right in the end.

I struggled to get mine running right for months without success and I eventually ended up shoving the bike in the garage for 2 years never ridden ! I eventually took it out and really got stuck in to those carbs and in the end she ran like a scalded cat.

This is how rough my bike went before I put it away for 2 years in frustration

 

[weekendrider]

Thanks for the ride peanut. The ye-ha @ 6:18 caught me off guard but was good for a chuckle.

 

[XSLeo]

www.amckayltd.com/carbguide.pdf Have you read this? If not it might help.
Leo

 

[peanut]

Thanks for the ride peanut. The ye-ha @ 6:18 caught me off guard but was good for a chuckle.

you're welcome
It was my first ride on a bike for 10 years so I was kinda excited and anxious at the same time How do you like our tiny roads ?

 

[Brassneck]

Sounds like it's too lean and/or air leak...With your 75 frame (and tank, I assume manual petcock) and the 78/79 engine/carbs...have you checked the Vacuum ports have been capped at intake Manifolds?