Advanced tuning woes

[lightfall]

I have been beating my head on a wall fine tuning my bike and I am wondering if my petcock is contributing to my woes. I attempted to T-connect my BS34 vacuum barbs to my stock petcock vacuum port but I am still having a hard time balancing perfectly. I can get my right cylinder to fire a consistent BA-BA-BA-BA-BA but the left cylinder in general will have a hiccup randomly spaced where the rhythm will be interrupted or offset slightly.

I can see a faint amount of vapor/fluid getting pumped through the clear vacuum line. It seems to deliver a bit more to the left than the right, which leads me to believe the petcock, if in perfect order, should not allow any vapor through. I set it to prime and plugged off both manifolds and it did not seem to make a difference.


I have gone through the carb bible and I have done and redone the dead cylinder method 100x. First I try to see if the air mix screw are correct, but I cannot get the idle to rise or drop with the mix screws even at 400RPM, it either shuts down when closed off or it runs the same at 1 turn or 4 turns. So I set it to 2.5 to be slightly rich because a 20 mile run showed a faint smoke ring on the plugs at 2 turns. I completely disassembled the carbs and cleaned about 10x and replaced all jets, screws, seals, measurements, everything to death. I replaced my manifolds, and I found no leaks. My diaphragms work well. The floats are correct, they seat. Pilots +1 Mains +2.

I set the left cylinder to just above dying on the idle set screw. Then I set the right cylinder to also just above dying. Then I go back and raise the idle up to about a thousand and then tweak the balancer screw a little more until the exhausts sound as similar as possible, which is really fairly close. I drove around making micro adjustments to the mix screws, to the carb sync screw in both directions in 1/8 turn increments and I simply cannot seem to get rid of the decel popping and I can't notice any performance difference with the mix screws or the carb sync. I was expecting to find a point where the mix screws would lean raise or rich raise the idle speed, no such point was found, it was off or on.

The bike pulls consistent throughout the power band. I get an annoying 3-6 pops on in gear deceleration from the left cylinder. The left cylinder also seems to make a more loud pop tone to me, though both exhausts are loud. I am using TCI ignition with the ultimate coil with 5k caps and BP7ES sparks which show good consistent powerful spark. I have triple checked my valve lash, timing chain guide adjustment. My compression is 150L 155R. I redid the top end with all the gaskets and everything seemed to be in pretty good shape. I know I will get a lot of shit for it but I am using velocity stacks and gordon scott open 1.5" pipes with new header gaskets. My new battery is fully charged to spec and the output is nice and high.

I am not sure if I am being unreasonable to expect a perfect BA-BA-BA-BA alternating in rhythm between the two exhausts, or if it is normal for it to run but have a somewhat irregular rhythm randomly and somewhat sparingly where a beat is slightly delayed. The bike performs great, but I am annoyed by the popping on decel and the non-perfect rhythm but I have no idea if that is within normal limits.

I wish I lived near an experienced member who could take a listen to my bike and look it over and tell me if it is tuned well or not.

 

[5twins]

My left cylinder has always had a little miss at idle. I don't know what it is, but the bike runs fine otherwise. Your popping on decel may be due to the mix screws being at 2 or 2.5 turns out. Try 3 to 3.5. The BS34s don't give much indication by the way they idle for mix screw settings. Like you said, it's either on or off. I think it's all the E.P.A. strangulation. I check the mix screw settings by using throttle blips. I watch for hanging RPMs or slow to return to idle, or popping, to indicate my setting is too lean. I set them just above where that stuff goes away. So basically, I turn the screws in until I get those lean symptoms, then back out until they go away.

 

[lightfall]

Thanks 5twins - If your well tuned bike does it for no apparent reason then I suppose I can give up the gun and just accept it as part of the bike knowing that I gave it an honest effort at perfection. It is good to hear you say that the BS34 is not very responsive when tuning the air mix screws, because I was convinced I was doing something wrong and I have never heard that sentiment mentioned elsewhere. I was not sure if I was being a meatball by approximating my settings as you described.

 

[5twins]

Yes, many of the '80s E.P.A.'d carbs are like this. The shops would set mix screws (if not capped over and even possible) using an exhaust gas CO2 "sniffer", not something most of us home mechanics have on hand. It's possible with your pipes and the stacks that you may need even bigger pilots (another size up).

If you want to experiment with fine tuning the idle circuit, start playing with the air jets that feed it. With them, you can achieve richness levels in between pilot jet sizes. For instance, if 2 sizes up on the pilots proves too much, but one size up is not enough, reducing the air jet size with one size up pilots will put you between one and two sizes for the richness level.

 

[XSLeo]

Put mufflers on it. The decell pops won't go away but you will not hear them as well.
Leo