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Weird sync issue..

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schrader7032
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Re: Weird sync issue..

Post by schrader7032 »

I went out for a spin on the /7 and got to thinking! :) I think I have an idea of what is happening. I think the left slide in the carb is sticking and not raising up as it should. Sort of like this:

- rotate the throttle just a bit which increases the RPM and the amount of air needed by the cylinders
- the right slide is rising as it should be the left slide stays down
- but the engine is still needing more air because the piston is going up and down faster
- that additional air has to get underneath the stuck slide which increases the pressure...the venturi effect
- finally, when the differential pressure between the underside of the slide and the part above the diaphragm gets big enough, the slide finally lifts up.

So that's something I would check. I believe my "chopsticks" method might show that the slides are behaving differently. If it's bad enough, you might be able to just see it with the naked eye. At least try raising each slide by finger with the engine shut off...any differences?
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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skyler.robbins
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Re: Weird sync issue..

Post by skyler.robbins »

I like your thinking Kurt!
Both slides move easy and the same with the bike not running.
I was thinking maybe the new spring that I installed in the diaphragm area was "maybe" holding it up.. so I removed both springs from the carbs, but same thing.

However!

I switch the carbs from the left to the right.. luckily I have just enough throttle cable slack and I added a small chunk of fuel line to make this happen.
Now I haven't fully given it the proper warm up and such, but the carbs sound and feel better.
When installing the carb sync tools, they both react similar... granted not a smooth/slow increase in numbers, but the same non the less.
I'll take it for a spin in the neighborhood tomorrow like this to see if it's any better.

Getting closer to problem solved guys!
Thanks for the input of course!

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skyler.robbins
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Re: Weird sync issue..

Post by skyler.robbins »

I'll do what you're saying tomorrow.. and post a video.

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srankin
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Re: Weird sync issue..

Post by srankin »

So, If I am to read your latest post correctly, you switched the carbs to opposite sides and they are working better? From what I am reading, you left the cables attached and had enough slack to do the swap, so any difference in performance is do to moving left to right? St.
Owner of a 84, R80RT and 78, R100RS

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skyler.robbins
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Re: Weird sync issue..

Post by skyler.robbins »

Correct St,
I moved the carbs to opposite sides.
Reattached cables and fuel line.
Gave it a start and this idea is working so far, but I'll really check it out today with proper warm up and ride through neighborhood.

I'm not sure what this is accomplishing other than if there is something wrong with the left carb, left inlet pipe, left cylinder etc etc..

I'll keep everyone posted..

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skyler.robbins
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Re: Weird sync issue..

Post by skyler.robbins »

UPDATE:
Same results on my carbs.
A buddy of mine has some spares of the same carbs.
I borrowed them from him.
They have been sitting for a long time so I gave them a clean up and such.
One thing I noticed that was different from mine are 2 things.
The air/fuel mix screw is made of steel not brass.
Also.. this could be the reason here.. where the inlet of the carb is, there are 2 super tiny holes in the bottom of the flange part. I watched a guy on YouTube describe this and what they are for. My carbs only have 1 of the tiny holes. My friend's carbs have the 2 holes.
I installed his carbs after doing a clean up on them and BAM!
Bike runs great! No popping.. both carbs respond exactly the same with sync tool!
The air/fuel mix screw is 3/4 out like it should be.. I messed around with it and that's where it likes it.
On my carbs, I was like 1.5-1.75 out depending of the carb.
So..
Tomorrow I'm going to take the new guts out of my carbs and install them on his to see if it still runs good.
If so, then perhaps I have an issue with one of my carb bodies or that 1 hole vs 2 hole thing could be the culprit?
Either way, I know the bike runs great with the carbs I borrowed.

Thanks again gang for hanging in there with me..

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skyler.robbins
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Re: Weird sync issue..

Post by skyler.robbins »

My carb.. one hole
My carb.JPG
Borrowed carb.. two holes
Extra carb.JPG

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srankin
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Re: Weird sync issue..

Post by srankin »

LOL, I can't argue with results, if changing the carbs solved the problem, go for it. I have never heard of the one hole two hole thing nor even noticed the holes you show in any of the carbs I have rebuilt or handled. That of course does not mean a darn thing, as my ignorance is vast.

I do find it strange Bing made the carbs with the difference. If you can please post a link to the You tube video, I would like to see it, I am having a hard time comprehending what these two holes are for and why two versus one would make a difference?

The steel versus brass screw is a kind of moot point unless of course the steel is rusted.

Anyway, post us results of your rebuild of the two hole carbs. Not to be a pompous jerk but I wonder if you were not missing a vacuum leak in the original carbs. Of course, not knowing the mileage on them or the pre rebuild condition, they could just plain have be shot and ready for the bin. One of my original carbs on my RT started acting up with a vacuum leak around the butterfly flap shaft and unless I could find someone to machine an insert and rebuild the butterfly shaft, it was not useful. I was lucky it was found by a leak test. So now I have two "newer" carbs. Sometimes a total replacement is needed, despite rebuild. At least, as you say, you can transfer the new rebuild parts over to the newer carbs.

Remember let us know how you make out. St.
Owner of a 84, R80RT and 78, R100RS

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schrader7032
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Re: Weird sync issue..

Post by schrader7032 »

I tend to agree with ST. I don't remember seeing those holes on my /7 carbs, which are different numbers and later carbs. It's been some time since I've been inside my carbs. It's hard to tell from the images, but the holes almost look like they were done after the fact. It would be interesting to know where the holes go in the carb. You could get some idea by spraying carb cleaner into them and see where the fluid comes out.

Those holes are after the butterfly. The only purpose them might serve is to support fuel flow in the idle circuit, when the butterfly is essentially closed. I do see other stock holes in the bore for that purpose.

BTW...were the butterflies removed as part of the carb rebuild?
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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skyler.robbins
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Re: Weird sync issue..

Post by skyler.robbins »

Thank again guys..
Here is the link to the video of the Bing carb rebuild.. the person explaining everything is PHD Engineering Professor for a MIT robotics school or something like that.
The entire video is great!
The part where he starts explaining the 2 holes is at the 29:00 mark..

https://youtu.be/dzDUE5FmlL4?t=1747

I'm going to double/triple check that I do/don't have that second hole.
The one part of the carb that I didn't rebuild was the butterfly shaft.
Everything feels fine there, no side movement, no air gaps, smooth operation, checked for air leaks.
That has been the one thing on my mind since I didn't go in there.

Thanks Gang!

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