sugguestions on fixing carb (?) issues

[RPC3]

Hi Guys,

Fall season wrapped up with a little riding, most of my time was focused on my wedding and some other house projects. Bike is put away for the winter in my shed, but accessible to be worked on, which I'd like to do.

When I last worked on my bike it was to fix some carb issues I was having, and they were never really solved. Through some trouble shooting on the forum it was determined that my throttle shaft seals were bad. I replaced them with OEM seals, but my bike still seems to have fuel issues AFTER the bike is warmed up and has been ridden for 10-20 minutes.

A short recap on my fuel system/updates:
1978 standard, bs38 carbs, stock tuning with aftermarket mufflers (installed by prior owner, similar to stock but a hair shorter), main jets are 145s, idle jets are stock. New fuel lines, fuel filters in place. I've set cam chain tension, valve height, timing per manual (95% this is all set). Carbs were completely rebuilt and gone through TWICE, with compressed air and new gaskets/seals, including the throttle shaft seals. New springs on the mechanical advancers, advance rod freshly greased, advance timing happens within acceptable spec. Bike is completely re-wired with a new harness, new battery, new regulator/rectifier. Only old components left in my ignition are the stock coils, stock condensers, and stock points. Carbs were bench synced, then balanced with a homemade manometer, I did not have issues balancing them. Charging system is perfect (brushes, rotor, battery - all working great).

I fully understand that there is a CHANCE I've done some of my maintenance incorrectly, but I did it very by the book, researched the forum extensively, asked and received help multiple times, and believe I followed the correct order of operations.

So back to my current issue - the bike will idle cold perfectly well at 1,100-1,200 rpm, it does not backfire, it does not hiccup or have any flat spots when riding for the first 10 minutes. After the bike is warmed up the idle will start to climb a bit (wants to sit around 1,400-1,600 rpm), it will occasionally die when I come to a stop in first gear and pull in the clutch, and ONCE WARM it appears to still have an air leak by my throttle shaft seals. The bike is low miles (12,000) and a New England bike, so it seems like a worn out bore where the shaft seals sit would be unlikely, but I'm sure it isn't impossible. Aside from idle, even when warm, the bike seems to drive perfectly fine - though it has felt "faster" in the past, meaning I think my performance is probably suffering due to either the air leak or inadequate combustion. I will *rarely* get some popping on decel, but it is not the norm.

Quick side note - I did make the mistake of getting into a discussion about this with a non-xs650 person, he was convinced my diaphragms looked shot (they seal and they don't have any tears - but I think he was saying the rubber didn't look as pliable as he's used to seeing. I haven't seen many posts addressing diaphragms as being a common issue from being "tired", so I'm not sure if his claim is legit or related to my problem).

My question is - what would you attack next? I really wanted to put in a pamco in with an optical advancer, but I don't want to ignore the carb problem. I'm sure the pamco would help (along with the new coils, lack of condensers, lack of points, and perfect advancing), but it'll be easier to tackle the carb issues first. I'm expecting to hear people say my carbs are shot, or probably that I've done something incorrectly (I'm not claiming to be an expert mechanic, but I can follow instructions as good as anyone and I go slow and careful). Since we've already gotten snow a few times this year I don't think my next drive will be until early May, but I'd like to get some of the work done over the winter months. If this was your bike, what would you do next?

 

[Brassneck]

I am not an expert on carbs...but I left my choke on and forgot to turn it off, and had similar issues...perhaps yours is stuck? Once off, the carbs worked great.

 

[NONclow]

Has the tuning been performed on a cold motor? Because the setttings will change when warmed to operating temp.....

 

[DogBunny]

As the bike warms up it burns gas more efficiently, so it is completely normal for the idle to rise. Just back out the idle stop screw during your ride as the idle speed rises.

 

[peanut]

I think NONclow has a good point. you need to set the idle when the engine is hot.

You say that it idles fine at c1200rpm when cold ...presumably this is when the choke has just been taken off and the engine is not fully warmed up yes ?

Stalling and unstable/rising rpm at idle could indicate that something is changing when the engine is getting hot, rather than some setting or jet being incorrect ,otherwise it wouldn't idle so evenly when still cold.

You have obviously checked everything meticulously but something is still evading your attention somewhere.

Have you checked carefully for air leaks around the inlet stub manifolds when the engine is hot ? I use a liquid gasket to both sides of my stubs so that I can be certain there will be no leaks. Over-tightening the carbs can cause the stub faces to distort and create inlet leaks.

Have you checked your flow rate from the tank ? let it flow into a container for 30 seconds and use a measuring jug and check it is to spec.
Next time you run it up to temperature try removing the tank filla cap to make sure its not a tank breather issue

 

[gggGary]

Sounds like a thorough methodical attack, good on you!
A few might look into's;
You didn't mention petcock R&R. Vacuum petcocks can leak fuel into the manifold.
Points wear out from lack of lube, condensers frequently (usually) die of old age. Yours are well past their born on date. I have found improved starting and running when I swap in a Pamco and e-advance.
Have the floats been hot water bubble tested? New float valves?
Choke rubber seals known good? how does the slide drop test do?
Timing tested/set on BOTH cylinders?
As mentioned BS38s do not have a fast idle setting with the choke, it results in using a compromise idle setting or a short section of rubber tube on the throttle stop screw for easy (and frequent) finger adjustments.
Sad but true; the first thrill of performance gets lost with familiarity. A good running engine doesn't seem as powerful after you have been riding the bike for a while. This is true even with "stupid amount of power" class bikes.

 

[5twins]

I think your jetting is probably off as well. A 145 main is very big for only one mod. That's 4 sizes larger than the stock 135 your carb set came with. You probably need only one size up on the mains, 2 at most. Along with that, the needles will probably need to be leaned a step. That will create a flat spot just off idle and require a larger pilot. So, my recommendations would be one up on the mains to a 137.5, needles leaned a step, and one up on the pilots to a #30.

 

[chizler62]

I have no experience with those carbs, but IMO carbs are carbs.
When cleaning carbs compressed air isn't the way to go. You need to push a wire from a wire brush thru the jet and jet emulsification holes then spray cleaner thru and finally air. Just because you see daylight thru them doesn't mean that the jet isn't restricted in some way.

My uneducated guess would be that you didn't drill out the low speed fuel screws. This passage is most likely blocked. Also could be a reason for "light" deceleration popping.

I'd drill, with a 1/16" bit, the brass plugs being careful not to go more than a few mm's deep. If you go too deep you can hit the head of the screw and turn it in damaging it. Take a common sheet rock screw or the like and screw it in until it feels snug. Again don't allow it to go too deep otherwise you'll turn the screw. Grab head of screw with a large pair of pliers for leverage and yank. The brass plug will pop out.
You'll now see the fuel screw. "LIGHTLY" turn it in untill it bottoms, again lightly, counting the turns exactly in 1/8th increments. Make note each side will differ. Remove screw, spring, washer and rubber oring seal. You'll see debris on the tapered part of the screw. Scrub it off with a common green emery cloth. Reclean pilot jet and all the passages in the system.

When you reassy carbs turn the fuel screws in LIGHTLY till they bottom then turn them out exactly to the original position.

Once the running and idling at a normal operating temp you can do an "idle drop" if you have a 90 deg screw driver.
Turn first screw, either side, OUT a full turn. Then basically you turn idle screw IN by 1/8 increments waiting 10 seconds between increments to see if the idle drops. When Idle drops but not stalls turn screw back out half turn and leave it. Do other side.
This should be after carb synce

 

[5twins]

You're referring to the BS34s. These are BS38s. They have no brass cap over the mix screw, just a plastic cap that is easily popped off to allow full adjustment. They have an o-ring and spring but no little washer.

 

[RPC3]

Thanks everyone,I've got some homework to do on my end. I'll dig through the list of suggestions and start testing /ruling things out.