Cylinder firing with hand over carb?

[JAG1]

80 xs650

Well it runs. Still about the same. The left side is not running as hot as the right (when sitting on the bike)

As per xs John he states


xsjohn
8-1 lover
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Join Date: Dec 2009
Location: NC USSA
Posts: 443
Re: carb problems
No............the left just runs colder than the right....and the jet needles have to be handled differently...as I well know..........after all these years of preaching .........and a 142.5 will just wash the whole thing out fast with those log needles provided by the epa......lots of fuel on the high end (142.5) and about none where you are at most of the time.............

xsjohn

Don't know how much truth there is in this statement. Seems like any engine shouldn't run hotter on one side than the other period. But I am new to these engines. Far different than all my Honda CB builds.

It looks as though I have the BS34 Mikuni carbs.
When the engine revs to about 3K in runs smooth, when it goes back down to 1200 rpms it shakes like crazy. Thought it may be a sync issue. I got the vacuum gauges out and was going to do it but i don't see a screw to adjust it. I know the tubes on the intakes is where to place the vacuum gauge bit can someone direct me in how to sync them? I read about a spark plug cap being removed allowing the engine to only work off one cylinder and doing it that way but it seems bizarre. Wanted to double check before I dove in.

Thanks again for all the help.

 

[JAG1]

Okay this is what is going on:

It starts on first kick
Seems to only run on one cylinder ( right side when sitting on bike )
Place hand over left carb jumps the rpms, hot air comes from exhaust.
Place hand over right carb - bike dies.

So it's the same as if someone has a bike that only runs with choke on and dies with choke off?

Me blocking air from left carb kicks the rpms up and makes that side get hot
Me blocking air from right kills the engine

So I either have an air leak ( which I tested for and found nothing )
Or still not getting enough gas in the carb

I'm stumped.

Swapping spark plugs doesn't make it follow to the other cylinder neither does swapping plug wires from one side to the other. Grounding plugs on engine also shows good spark

Only thing I'm left with is air/fuel I think


I dunno

Any suggestions out there?

 

[littlebill31]

What size air jet is in the carbs? The one in the bottom left hole of the carb in pic #1.

 

[gggGary]

Have you set float level and checked it with a clear tube?
Those look like 1980 brass float BS34s?

 

[gggGary]

The idle mixture screw holes are still plugged, you have not cleaned the carbs yet. Clean em set the floats, use the guide, then talk to us.

 

[JAG1]

Ive tried to determine the sizes of all the jets but the numbers are unrecognizable.

They do have the brass floats and the float levels are within range of what they're supposed to be.

All passages are clear. I submerged in carb cleaner for a few hours. Gone through with fine bits and blasted with compressed air

 

[gggGary]

You haven't removed the idle mixture screws, it has happened where carbs ran good without doing that but it's rare. Often the o-rings in there are shot. the choke picks up the mixture through a tiny orifice in the float bowl, if that's not clear it can cause serious running issues. Do you have the rubber plugs installed over the idle jets?
Is the diaphragm good, slow drop with the port covered? Several of the orifices are too fine for any drill bits I use single strands from automotive copper wire.

 

[JAG1]

Well I also use like a 12 gauge speaker wire. Pull bac some strands and make it big enough to go through the orifices.

To answer all these questions the best thing I know to do is take photos of everything and submit them in tomorrow.

 

[littlebill31]

Submerging will not clean them. You need to scrub and spray carb cleaner into a port and watch to see if it comes out the corresponding port. Plus, submerging rubber pieces is a no-no. The cleaner will degrade the rubber quickly and mess the carbs up. Little rubber pieces, such as O-rings MUST be removed before using any type of harsh cleaner. Especially the diaphragms, O-rings, pilot jet plug, float valve, ect. Every little piece needs to come out, checked and cleaned.

 

[5twins]

Sync is adjusted via the spring loaded screw on the linkage between the carbs .....

..... but since you haven't removed the plugs over the mix screws, that left idle circuit may not be clean enough yet. To properly clean the idle circuit, the mix screws need to come out. There's also a small o-ring on your mix screw that needs to be in good shape for the screw to function properly .....

Remove the block-off plugs, remove the mix screws (and all associated parts), clean the idle circuits again, sync the carbs, and you should be good.

 

[Davep]

Left carb, blocking the carb causes more suction to draw fuel in. Have you compression tested left cylinder? I had a similar isssue with a Z650 and the offending cylinder was low on compression, once it ran on that cylinder and was warm it ran and started fine. Right carb, blocking the carb up is obviously flooding the cylinder which is why the bike stops. It seems odd though that the cylinder running hottest is the right one as this also indicates a lean mixture as the cylinder is trying to burn air rather than fuel. Are all the diaphragms ok, particularly the left one?

 

[JAG1]

I found the problem was that the left intake manifold didn't have a gasket. Put one on. Bingo.

After that I sync'ed them.

Still can't seem to get it to idle well under 1500 rpms, and the handle bars vibrate bad.

I know these bikes have a vibration to it, but how much is too much?

With the cb750s 4 carb set up, one thing I'd always watch is the handlebars while syncing. When the vibration was gone I knew that not only were the vacuum gauges were right but it was running smooth.