1978 GS1000: carb #3 leaking from overflow hose

[superawesome]

Hi all,


in preparation of my GS1000's 40th birthday on April 1st I gave it a good carb rebuild. I disassembled the carbs (stock VM26), cleaned all the metal parts in an ultrasonic cleaner, replaced gaskets as well as O-rings (cycleorings.com).
After reassembling everything I have carb#3 dripping fuel out of the overflow hose when on prime. Had that issue before, which is one reason for the rebuild.


I did fix the petcock last year and it has been working since (and as it is only leaking when on prime I guess it still works).
Also compared fuel levels between carb #3 and #4 with a clear tube, and they are equal height (set the float height to 24mm on all carbs), about middle of the upper rim of the float bowl.


I strongly assume the float needle/seat need to be replaced. I did see a "ring" on all the float needles but could not feel any cavity there. I did not check the seat though but it has gone through US cleaning.

Are there other possible causes? What do you think?

As it only happens on prime, I am a bit reluctant to pull the carbs again to fix it...


Thanks

 

[Nessism]

Fuel traveling from the tank to the carbs via the vacuum line because the petcock failed?

 

[Big T]

I'd go with the float needle/seat idea also.

Another issue can be the brass overflow tube in the float bowl is cracked, but, since it only happens on prime, I'd rule that out

Since it only happens on prime, I'd order the parts (as so to be ready) but live with it until it happens more often

 

[superawesome]

Hi,

since the problem got worse rather quickly after a short test ride (first one in a while), I went to my local dealership to order a new needle/seat assembly.

Pulled the carbs yesterday (in what felt like world record time), looked at it today and could not find anything irregular on the needle/seat. Blowed into the fuel tube to test, but all the valves seemed to seal tight.

Then Mr. Big T's comment came into my mind

Another issue can be the brass overflow tube in the float bowl is cracked, but, since it only happens on prime, I'd rule that out

and I filled some water into the float now. You guessed it, started dripping after a minute.

Looks like there is a 1.5cm hairline crack in the overflow tube.

I guess those float bowl are not available anymore. Is there a way to seal it?

Thanks

 

[steve murdoch]

Plenty of threads on overflow tube repair.
Here is a good one from Ed.
https://www.thegsresources.com/_for...wl-Overflow-Tubes&highlight=overflow+and+tube

 

[superawesome]

Ok, got the soldering iron from the basement and tried to fix it. I am not sure if the crack goes all the way down, but tried my best to fill it with solder. I filled some water in for testing, and hopefully no more leaks.

 

[Big T]

If you were in the States, I'd just send you another float bowl. I'm sure someone in the EU or GB has a spare

 

[superawesome]

Hi,
I managed to mount the carbs yesterday evening, and went for a short night ride through the neighbourhood to make sure everything basically works. Still got some backfires, but I guess those boots need to settle in (as it is quite cold and I had to bend them quite a bit to get the carbs in). Float bowl kept tight. I will keep looking for a spare one, just to be prepared.
We had some fresh snow the day before so the settings was beautiful. Stopped and took some pictures for you to enjoy

I did a bench sync of the carbs but still need to further tune them. #3 is quite hot compared to the other cylinders. Is there a dedicated thread on this, or is the BikeCliff's VM carb rebuild guide the way to go? Having some problems to tune by ear since first time doing this.
UPDATE: Ok, did some reading, and my question rather is: At what point does the Morgan Carbtune (that I purchased recently) come in place? Do I do the "highest idle" first, then use the carbtune to sync, then do plug readings?

My problem is, I do basic work in the underground parking, and the workshop I use for bigger tasks is about 40 miles away. I don't want to go that distance with a way too lean #3 cylinder...

 

[hannibal]

I received some good advice on this. Hook up the Carbtune and sync the carbs. With the Carbtune still attached, set the air screws for highest idle. The highest idle should register as higher vacuum on the Carbtune. After adjusting all four carbs, recheck the carb sync with the Carbtune and resync if necessary.

 

[superawesome]

Thanks hannibal, exactly that's the info I needed!
My pilot fuel screws were all about 3/4 turns out, I guess I'll take that as a base setting.

 

[Chuck78]

I have a hefty amount of Mikuni VM float bowls with cracked overflow tubes! I was running so short, that when we rebuilt one of my friends bikes, I had him get out is soldering torch to preheat the tube, after sanding it extensively with Emery cloth, then we fluxed it up and used a soldering iron to solder the inside or rather inner float bowl portion of the outside of the tubes. It's still working good to this day! That was two years ago!
I also did this on the choke tube on a very very beat up neglected GS550 that I have the fortune of having to rehabilitate back into life a couple times over! Not my bike, but I am it's personal mechanic!

For future reference, I fill the float bowls with water, and use a rubber tipped air compressor blow gun nozzle and put it over the bottom of the tube where the hose connects, then I put my finger over the top of the tube, and I blow air through the tube which is sealed off with my finger on the other end, and then I look for bubbles on the brass tube in the water. I do this on every VM carb rebuild now.
I'm not sure the specifics of the size of a water molecule versus the size of a gasoline or ethanol molecule, but they may not be the same, so gasoline may leak through easier than water, so I always do the pressurize air test. I got that theory from doing plumbing for a living, natural gas molecules are smaller than atmospheric air molecules, so you definitely have to use higher pressure to find a leak with atmospheric air when you are testing a gas pipe system in a building.

 

[superawesome]

Well I did not have a compressor at hand to test, but it seems my soldering skills were enough to keep the fuel in (2 days and counting...).

 

[chuck hahn]

I use a piece of vacuum hose on the nipple at the bottom. Fill with water and hold a finger over the top hole of the tube and blow in the hose. Even the tiniest crack will make bubbles.

 

[superawesome]

I started with fuel screw 3/4 turns out, air screw 1 1/4 turns out, on all carbs. I did hook up the carbsync, plugged the vacuum tube, and it fired right up.
#3 was a bit out of sync, but I corrected that, see before and after pics (later read that #1 and #4 can be a bit higher, but I will consider that next time). Runs well now.

before:



after:


Naturally I lost one of the carb top screws during the process (looked for more than an hour, but could not find it anywhere), so I need to get a replacement *sigh*

Unfortunately my time was limited so I could not do highest idle air screw tuning yet. But I did order a Colortune, just in case, and out of curiosity.

 

[hannibal]

Looks good!

 

[Big T]

Getting closer, spring is here