78 cb550 carb leak... HELP

C

curby

New Member
I'm getting close to the end if my build and getting it on the road. They are PD46 carbs. carb 1 leaks out of the overflow tube after awhile. I'll ride it and park it and no leak. turn fuel off and petcock, come back hours later and fuel is coming out overflow tube.

I check float height, float needle, float valve and everything else.

I'm thinking the floats are filling with fuel but i take them out and soak them in gas and they don't sink.

Any opinions?

also, are the float valves replaceable on the PD's?
 
Check to see if the brass drain tube is cracked, if you're riding it around the level is lower than if its off and the bowl fills up again
 
You said your turning your petcock off too? If you've got ur pet off and hours later your leaking, your petcock isn't working right. Anything out your over flow is usually float related. Could be a combination of two problems. Good luck
 
I can rule out the petcock as it sat on my bench with fuel in it for weeks and not a drop.

As for the overflow tube, i can't rule out yet but every time i take the goals off after it leaks it's still full. and wouldn't it leak all the time?
 
That's a weird situation for sure. This is one thought that popped into my head after your last post, check your float pins. Possibly while your running your bike they could (your floats) move to a certain level that causes no overflow while bike is consuming fuel. After bike is turned off it could be possible that they (or just one more likely) is getting stuck in a position that would cause over flow out your tube. As for leaking all the time. No it would only push out excess gas. If your petcock is shut off, it will only leak gas out of an over flow until it reaches a certain height in your bowls. This height is controlled by your floats. Hope that helps? Post a pic of you carbs with bowls removed I may be able to point something out. Also are you measuring your float height to 12.5mm or 14.5mm?
 
If you have a center stand park it bike on it, turn the gas on, fill the bowls, shut the gas off and walk away for a bit and come back and see if it is leaking.

If not then set the bike on side stand and see if it leaks then. I bet I does and I bet its because that float either gets cocked and lets whats left of the fuel in the lines into that carb bowl. Or you have your Floats set incorrectly. I've had issues myself with stuck floats on my 550 and on a 500 I used to have years ago.

Good rule when parking a bike either way is shut the fuel off while the bike is still running say a block out or at the end of the driveway. Let the bike run sorta dry so to speak each time you park it. That allows the floats and the float valve pins to relax some, which allows them to last longer.
 
Ive tried floats at both 12.5 mm and 14.5 mm.

leaks on both center stand and side stand.

this is my 3rd build and i've never came across an issue that has stumped me like this.

I did notice the jets don't match up with honda specs. 42 slow jet and a 90 main which those are factory cb500 jets.

mind you this was a barn find. the guys son "tried" to get it running but couldn't figure out why the fuse box was melting. ::)

before
IMG_1267_zps57367836.jpg


after
PhotoJul1583119PM_zps509e5a2d.jpg
 
Can you change them on the PDs? If so, how do they come out? And I can't find replacements anywhere.


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I don't think you can change them on PD (a) models. By float valves we are talking about the copper ring area that the float needle sits on, right?
 
That's the part I'm thinking he's talking about. It sits flush to the body of the carb. No way to take it out.

These are not mine just a googled image. I don't see how they would come out.

y4uga7et.jpg



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If they are changeable i will change them.

I've taken a float needle from another carb and it still leaked and the other one did not.


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The carb on the right hand side. Everything is removed from that one that you can remove. Except for the fuel/air screw on the bottom of pic. There's only one over flow on this rack of carbs. No matter where you have a problem it will (should) only over flow out of #1. I'm scheduled to take a set of PD46's next Thursday for a full rebuild and paint. I'll take pics of every step and post them while I do them. I hope it will help. I still think it's one of your float pins or one of your float needles and your just not noticing yet.
 
I have to say your wrong on one over flow. Each carb has it's own overflow. These are my carbs after rebuild.

mu7e3u6a.jpg


maybe this will help

ScreenShot2014-07-24at101012PM_zps769dfe99.png



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If you swapped needles and the carb still leaks then the only thing left is the valve SEAT. Have you looked at it with a light and magnifying lens? Maybe polished with some tooth paste and a cue tip?
 
One thing that I caught when I did my PDs:

If you didn't replace the float valves they need to be cleaned, by that i mean taking brake cleaner to the spring loaded plunger in them and make sure they spring back quickly and freely. If those are gunked up then no matter how good the floats are set they'd always be open (or closed)
 
Have you checked that your floats actually float in gas?
The float moving freely in the bowl? Blow into the "in" line, then turn the carbs over, you should not be able to blow any more.
My vote is the needle/seat combo. They are matched and should never be put back in different positions. Also it doesn't take much to gunk them up.
 
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