Carbs leak only when running

[Furious D]

Carbs leak only when running-SOLVED

I did a search first but didnt find anything like this:


Today I put my carbs back on for the third time. Everything sounds good, decent idle, no popping, but both carbs leak fuel only when running. shut down the engine and it stops which leads me to think its not the float level.

Original problem was that it would not idle, lots of pops when you gave it throttle. I was told when I got the bike the carbs had been rebuilt but never synched. I took them apart, they were clean, but I went thru every passage just in case.


Any ideas? Its an 80 special with 34mm Mikuni CV's.

 

[Sundie]

Does it have a vacuum petcock? When you put it to prime, do they start leaking again? If so, it's your float pins. Replace them.

 

[weekendrider]

+1 on the petcock. Another possible leak could be the o-ring on the float needle seat?

 

[Furious D]

Yep, it leaks when you switch it to prime, stops when its switched back,not running.


Are you talking about the float hinge pins? My tired brain is trying to figure out why it only leaks when running or in prime.

 

[xsjohn]

Put a bit of "fine" steel wool on the end of a q-tip and push it in the float seat and spin it say 15 times......polishes the hole in the seat and takes the sharp edge off.....and you can replace the o-ring on the float seat or use some thread wound next to the oring in the groove and the end hidden to puff the o-ring a bit..........and the floats should be set a 27.25mm on the 80's with brass floats........make sure there is no fuel in the floats.......

xsjohn

 

[Furious D]

Thanks to all,

XSJohn, I will try the steel wool trick tonight after work .


I think this is the last time I am going to take this set of carbs off. The reason I picked up the bike so cheap is that it was plagued with carb problems and the previous owner became tired of throwing money at it. I'll give it one more try then start looking for a different set.

Any suggestions on what to replace them with ? I know a lot of people dont care for the later 34's.

 

[Sundie]

Yep, it leaks when you switch it to prime, stops when its switched back,not running.


Are you talking about the float hinge pins? My tired brain is trying to figure out why it only leaks when running or in prime.

No, the little pin that shuts off fuel to the bowls when the floats come up. The guy circled in the image.

If they're not stopping the fuel from entering, the bowls will overfill.

To test if this is your problem, take the petcock vacuum line off (so that fuel won't flow), block off the vacuum barb on the carb, then try running the bike. If it runs fine, turn the petcock to "prime" (which will flow fuel into the bowls) and see if it runs crap. Then turn the petcock back to "on" (which will stop the fuel again because of no vacuum to the petcock) and see if it settles again.

If yes to all of the above, you've found your problem, and it's easily fixed.

 

[Sundie]

+1 on the petcock. Another possible leak could be the o-ring on the float needle seat?

Not the petcock, but the float needle? If the petcocks aren't flowing when the bike is off in the "on" position, and are when in the "prime" position, they're working correctly...?

 

[weekendrider]

+1 on checking the petcock in the prime position. As per your instruction.
furiousD, fwiw these are pretty good carbs once you get them sorted. I wouldn't junk them, till your're sure they are junk. And if you do junk them I'll pay shipping.

 

[bluebikerblan]

Dear Mr. Tired Brain....Please wrap around this.

With vacuum operated petcocks the gas flow will always be off if the bike is not running and/or the petcock is not in the "prime" position because; vacuum is not being provided from the operating (running bike) carburetor which holds the diaphragm valve in the petcock open to let the gas flow. ("prime'" is just a position which by-passes the vacuum valve)

Therefore, a leak within the carburetor can only occur when..... A. The petcock is in the "Prime" position, or..... B. The bike is running (supplying vacuum) to hold open the petcock diaphragm shut-off valves.

Copish? Best, Blue

 

[Furious D]

Ok, I have the petcock thingy figured out. It took me a while to figure it out, I'm on my last day of a 10 day,10 hour day work-week and my brain goes to mush after 8 days.



Anyway, i took the carbs apart today but really didnt find any major problems. I checked the float levels, and gave everything a quick cleaning with carb cleaner and compressed air. I did replace the o-rings on the float seats. They were a little loose and brittle.


I'll bolt them back up tommorrow and let everyone know how it turns out.

Thanks to all for your help

 

[Sundie]

I did replace the o-rings on the float seats. They were a little loose and brittle.

That probably it right there. How was your needle tip? I can't recall if it's a solid brass needle or has a rubber/viton/something not brass tip.

 

[bluebikerblan]

Go Furious.....you're on a roll now and in good hands with Sundie & XS John on your side.
You're not the first guy baffled by the vacuum petcocks! Best, Blue

 

[jayel]

wouldn't hurt to check the float hinge itself, float should pivot freely with no rock or sideways cocking, have heard some reports of loose hinge to pin causing problems, easy enough to squeeze the hinge down to a slip fit

 

[Furious D]

Fired it up today, No leaks. Must of been the o-rings on the float needle/seat assembly.


Thanks again to everybody. I just need to do a little tuning on the carbs and wire up the tailight and I'm on the road.

 

[weekendrider]

You might add SOLVED to the subject line of your post. That way someone else having flooding problems may find it (the thread) easier.

 

[xstom]

Nice thread I have had exactly the same problem. Now to make time and fix it. This is a great forum!

 

[BillM]

The son-in-law and I just picked up an '81 XS last Saturday. Got it home and it runs and rides fine. Started taking stuff off and check this and that and found that the carbs are leaking fuel into the airboxes. The foam rubber around the filters are being destroyed by the fuel. Wondering if this could be the same problems? We have not torn anything apart that far yet. Bike doesn't flood out. Idles fine.

 

[bluebikerblan]

Yep, it leaks when you switch it to prime, stops when its switched back,not running. Are you talking about the float hinge pins? My tired brain is trying to figure out why it only leaks when running or in prime.

Because your petcocks are only supplying fuel to the carbs when either A. The selector is set to "PRI"me or B.The selector is set to either "ON" or "RES" and vacuum from a running engine is holding a vacuum operated diaphragm valve/s in the petcock are open (supplying fuel). That's way the starting drill is to first set the petcock selector to "PRI"
start the engine, switch the petcocks to "ON".....Prime bypasses the vacuum diapraghm valve.

Pls really read and study the carb guide.....

http://www.amckayltd.com/carbguide.pdf

You likely have some problems with your floats. float needle valve & adjustments which results in fuel dumping through the vents into the carb throat and out into the air filters.

Not uncommon with aged, dirty, carbs & fuel. BOL...Blue