Carbs are off again. What now?

[Its been a long time]

Latest installment. Put the carburettors back on the bike yesterday and tried to start on the electric starter. No dice, nothing, cranking but no fire. Measured the battery voltage after three attempts and it had dropped to 12.2v, too low.

(When running, the charging system is strong, there's no shortage of voltage while running. With the lights on it's sitting at around 14.2v at 2 to 3000 rpm.)

Charged the battery again today and at 12.6v decided to give it a start. To get the best possible spark, decided to try the kick starter. Started 1st kick

I have taken a video and I hope someone can give me an insight into the regular "bop, bop, bop, bop" misfiring sound from the right side cylinder. Too rich? Too lean? Something else?

Left side sounds fine. The motor cuts at less than 2000rpm. I am sure that's because the right side cylinder isn't pulling it's weight. There is a fair bit of smoke from the right side exhaust. Left side is clean. I think the smoke is more white than blue so it looks like unburnt fuel rather than oil.

There's a link to the video below and I would appreciate input on likely cause of the right side uneven running. Apologies about the noise of the keys jangling. For information, the mixture screws are set both sides 1.5 turns out and all jetting is stock 1977.

Appreciate thoughts on where to look next. Thanks.

https://photos.app.goo.gl/Dq7jmnJuvHvc9Fib7

 

[5twins]

Sounds like it's only running on one cylinder? Did you check the plugs, and is the right one all black, maybe fouled?

 

[Its been a long time]

Sounds like it's only running on one cylinder? Did you check the plugs, and is the right one all black, maybe fouled?

Thanks for the thoughts. Plugs are new Denso and there's big fat sparks.

It's definitely firing on the right side but it sounds like every other compression stroke rather than every stroke. Hence white smoke which I think is unburnt fuel. I don't think it has run long enough for reliable plug readings?

I haven't experienced anything quite like this before hence my asking for any ideas here.

Current thought is to turn in both pilot screws (presently 1,5 turns out) to 1 turn and try. If not successful, turn out to 2 turns. See what happens.

(Inlet rubbers are both tight, not suspecting those). Thanks.

Edited to add - It was running really rough on the right side before I did the throttle shaft seals. I had hoped the new seals would solve the issue. But it definitely has made no discernable difference.

 

[5twins]

Any way to verify timing is correct? I'd also check carb sync and float bowl levels (float setting). I don't think the mix screws being off a little would have that big of an effect and make it run like that.

 

[Its been a long time]

Any way to verify timing is correct? I'd also check carb sync and float bowl levels (float setting). I don't think the mix screws being off a little would have that big of an effect and make it run like that.

I agree about the mixture screws but I'm grasping at straws really.

Float heights are set at 25mm as per this thread. Static timing is spot on, checked three times. Can't get it running reliably enough to double check with a timing light. Starting 1st kick suggests things are close. But not close enough. Carbs were bench synced with a 0.005" feeler gauge.

Plugs -

Left and right as seen -

 

[Its been a long time]

Another thought. Though both slides pass the finger over the vent drop test, I think I'll swap the slide assemblies left and right. Easy enough to do and it'll confirm whether the diaphragms are in fact both good. I think they are, but it's an easy check and I am out of ideas.

 

[5twins]

It seems reluctant to rev up much past 2K RPMs, almost like the timing isn't advancing.

 

[Its been a long time]

It seems reluctant to rev up much past 2K RPMs, almost like the timing isn't advancing.

Hmmm. I think that's me not revving it above that actually. I have revved it to around 3k but with the lower rev range being so rough I haven't explored beyond 3k revs. I will try tomorrow. Being on my own I haven't had a timing light on it yet to check the advance. Need three hands for that.

 

[Wingedwheel]

That’s exactly how mine was behaving. I had those carbs off and apart a dozen times. Finally changed out the carb body and got results. Not saying that’s your problem but unless it migrates when you change those diaphragms, I’d say it’s a possibility.

 

[Its been a long time]

That’s exactly how mine was behaving. I had those carbs off and apart a dozen times. Finally changed out the carb body and got results. Not saying that’s your problem but unless it migrates when you change those diaphragms, I’d say it’s a possibility.

Thanks. Did you ever work out what was wrong with the carburettor body that you changed out?

I think there's three small pilot mixture holes at the top of the carburettor bore, adjacent to the throttle plate. I am 100% sure I get carburettor cleaner spray and air blowing through them. But I am wondering if one out of the three very small holes is partly blocked?

 

[Wingedwheel]

I never figured it out because once I got the set working correctly I just moved on.all my passages were clean and I shined a light through to check. I basically just classified the one body as junk.

 

[Its been a long time]

I never figured it out because once I got the set working correctly I just moved on.all my passages were clean and I shined a light through to check. I basically just classified the one body as junk.

Thanks for that. Problem is - I have no way of knowing if I swap out one 48 year old carburettor body for another, whether the replacement one is any good. I could be buying someone else's scrap carburettor body.

I am starting to think whether or not I'm throwing good money after bad and I would be advised to buy a set of new VM34 Mikunis? I don't want to do that.

But I have already been around that exact same loop with ignition systems. I ended up buying a new electronic ignition kit in the end after wasting money on the old points system.

 

[Adamc]

If you can collect I’d be willing to let you borrow the set from Taffy to eliminate / prove the possibility of the carbs being at fault.
Ads..

 

[Its been a long time]

If you can collect I’d be willing to let you borrow the set from Taffy to eliminate / prove the possibility of the carbs being at fault.
Ads..

Thanks very much for the offer. Appreciated. Let's see how a third attempt at cleaning the right side carburettor goes this week. The bike starts first kick, can't be much wrong. One would think

(Noted the suggestions about poking around with some thin copper wire).

 

[Wingedwheel]

Although I never really looked too hard for my LH carb issue I’m kinda thinking there may have been a “micro” crack or distortion in the case. Could have been dropped, who knows like I said I got a box of parts with no known history and built up from that.

 

[Its been a long time]

Although I never really looked too hard for my LH carb issue I’m kinda thinking there may have been a “micro” crack or distortion in the case. Could have been dropped, who knows like I said I got a box of parts with no known history and built up from that.

Thanks. I'll know more tomorrow after the right side carburettor has been in a ultrasonic cleaning bath for a while. I'll also have a poke around with a strand of copper wire. Nothing to lose doing that.

 

[Its been a long time]

Removed the carburettors again this morning. Stripped the right side carburettor to remove all the brassware, ready to go in an ultrasonic cleaning bath.

Being hyper critical since there's an unknown issue in there, I noticed the needle jet/emulsion tube with the O ring might be an issue. The emulsion tube along with it's O ring was very easy to remove. Pulled it out very easily with my fingers. The O ring that has been in there about 7 or 8 months hasn't taken a set at all. I have a suspicion that it's possible for fuel to be pulled up the outside of the emulsion tube and past the O ring. That's not going to be helpful.

Seeing as I'm shooting in the dark here, has anyone any experience of such a situation? Thanks.

Edited to add - Irrespective of the cleaning I'm doing tomorrow, I am going to fit the emulsion tube back in with Loctite retainer. After cleaning, I'll refit the emulsion tube and O ring held in place with the float bowl for an hour to cure before refitting the rest of the brassware.

 

[Its been a long time]

Right side carburettor put through ultrasonic cleaning this morning. Followed by blowing compressed air through all the passages to validate cleanliness. All seems good. Will rebuild the carburettor this afternoon. Going to install the loose emulsion tube and O ring with Loctite retainer. Taking no risks of fuel bypassing the emulsion tube due to looseness.

 

[zrx1100]

Right side carburettor put through ultrasonic cleaning this morning. Followed by blowing compressed air through all the passages to validate cleanliness. All seems good. Will rebuild the carburettor this afternoon. Going to install the loose emulsion tube and O ring with Loctite retainer. Taking no risks of fuel bypassing the emulsion tube due to looseness.

i'd keep the loctite away from the inside of carburators for risk of particulates, etc

 

[Its been a long time]

i'd keep the loctite away from the inside of carburators for risk of particulates, etc

I hear you. But if I don't Loctite the emulsion tube in place, the carburettor body is probably scrap. It doesn't fall out of the body under it's own weight, but not far off, it's loose. I am probably 95% confident a loose emulsion tube is the root cause of the rough running. I'll know soon enough.