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carb drain screws

Started by briandneville, October 26, 2020, 01:16:47 PM

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briandneville

The rear carb drain on my Vision has been dripping after running the engine.  After some irritation I was able to remove the screw, but had to use an easy-out so effectively destroyed the head (which was stripped when I started this project).  So ordered two new ones from Partzilla ($35, ordered a week ago with a 1-3 day ship but they have not yet been 'processed', whatever that means).  Meanwhile I found two drain screws in the box of goodies Fiat Doctor passed on to me, and put one of those in, but was old and rusty and the drip was still there.  So I swapped it to the front carb and put the one good drain screw I have into the rear, and it drips worse, all the time now.

Any ideas about what is going on? 

thanks!
How did it come to this?

Walt_M.

Have you checked the 'O' ring under the screw head? The very tip of the screw can be rusty as long as the taper is clean.
Whale oil beef hooked!

The Prophet of Doom


Set the petcock on PRI, put some clear pipe on the overflow point it upwards then crack the drain screw.  The fuel should up to 20mm below the bowl gasket.  Any higher and you have a bad float setting or a leaky float valve.  If it's lower then a leaky drain screw.

briandneville

Thanks Walt and POD...those are helpful tips.  O-rings appear to be fine.  I'll give the clear tubing tip a try.

How did it come to this?

briandneville

This problem has finally been solved.  After a thorough cleaning, I reassembled the carbs and filled the rear bowl with fuel to test the accelerator pump, and the drain screw was still leaking.  I took the carb to a marine carb specialist and he identified a small gouge in the seat where the drain screw seals.  He said 'it looks like someone's been messing around in there with a screwdriver', clearly thinking it was me (which it was not).  When I asked him how to fix it, he pulled out a tube of valve grinding compound and said put it on the tip of the drain screw, screw it in until it seats and then turn it around a few times.  I did this, twice, taking a total of 10 minutes, and then cleaned the bowl and drain screw area with gasoline and compressed air.  When I filled the bowl the leak was gone. 

This guy's advice was invaluable and I suspect might be helpful in other circumstances to someone.

After 48 hours the bolt covering the main jet started leaking very slightly, so I made a washer with carb gasket material for both front and rear plugs, and they've been leak free since.   On to the next challenge.

Here are some pics of the clean '83 carbs.
How did it come to this?

injuhneer

Quote from: briandneville on June 14, 2021, 12:26:04 AM
This problem has finally been solved.  After a thorough cleaning, I reassembled the carbs and filled the rear bowl with fuel to test the accelerator pump, and the drain screw was still leaking.  I took the carb to a marine carb specialist and he identified a small gouge in the seat where the drain screw seals.  He said 'it looks like someone's been messing around in there with a screwdriver', clearly thinking it was me (which it was not).  When I asked him how to fix it, he pulled out a tube of valve grinding compound and said put it on the tip of the drain screw, screw it in until it seats and then turn it around a few times.  I did this, twice, taking a total of 10 minutes, and then cleaned the bowl and drain screw area with gasoline and compressed air.  When I filled the bowl the leak was gone. 

This guy's advice was invaluable and I suspect might be helpful in other circumstances to someone.

After 48 hours the bolt covering the main jet started leaking very slightly, so I made a washer with carb gasket material for both front and rear plugs, and they've been leak free since.   On to the next challenge.

Another version of the problem is when a different screw is used and the seat gets crushed or deformed causing a chronic leak. The lapping action is a simple method to correct the problem. If available, a rod (steel or brass) with an appropriate point (taper, piloted seat, etc) works well. On rare occasions cutter is used to cut the seat out and replace it with a brass insert.
- Mike O
1982 Yamaha XZ550RJ