Monobloc 389/37 Leak

[Brucie64]

After developing a fuel leak from the main jet cover nut gasket (whilst the bike was running at idle) I thought I would treat my carb to a little service.

Upon strip down I noticed that the Float chamber needle seemed to be stuck thus I assume allowing unlimited fuel supply to the carb. To sort it, I therefore duly purchased a new complete gasket set and float chamber needle seating kit from Burlen fuel systems. However once I had replaced all the parts and switched on the fuel tap to my horror,  I watched the fuel now spurt out through the tickler plunger instead.

Am I right in thinking that this can only happen if either the float itself is stuck in the fuel chamber or the float level needle is still stuck thus allowing the fuel level to reach the tickler plunger at the top of the chamber?

Should the tickler plunger not act as a one way valve and only allow fuel to be released if it is depressed into the fuel chamber body? 

[LJ.]

I've read somewhere that this is a common problem but just cannot remember what the fix is. Not sure if the fix was mentioned here or on BritBike.com? I've a suspicion that it was Alec who told of the correct fix for this...

[rocket man]

hi Bruce i had the same problem with my carb  it was the float
its a new 389 and the floats made of plastic  and its molded in
2 halves then plastic welded together and the joint was very rough
so i got some fine  emery  paper and sanded the joint till it was smooth
it worked it stopped leaking hope this helps

dave

[Brucie64]

I think I have sorted out the issue after reading some other posts. It seems a fibre washer should only be placed under the float chamber needle housing if you have a new Monobloc carb as the carb body has a recess built into it to accomodate the washer.

If you have an original carb (as I have) you should leave out the washer from the housing as they weren't originally fitted as standard. I fitted the washer to the housing as instructed and as such the viton needle was not able to shut the fuel flow off properly...hence it running out of my tickler!

I intend to correct the issue tonight and will let you know how I get on.

We live and learn! 

[dpaddock]

Two things:
     1. The Main Jet Cover Nut is a taper joint with the Main Jet Holder in my Monobloc; there is no gasket.
     2. A Float Spindle Bush is needed in order to keep the Float from drifting away from under the Float Needle and allowing the Needle to come away from the Seating. This is NOT shown in the "Hints and Tips" publication by AMAL (publication List No. 102/3), but is on the exploded view on page 82 in Bacon's "BSA Twin Restoration". I use a short piece of PVC flexible tubing as a substitute.

David

[tombeau]

I had a new plastic float that was a tight fit on the spindle.
I spotted it when I put everything together, but it could have caused problems.

Cheers,
Iain

[beezalex]

David, I think Brucie was talking about a gasket between the float valve body and the carb body that wasn't supposed to be there.  This is a common mistake that I think everyone makes when they put a monobloc together for the first time (I know I did). 

Good point about the spacer bush, though.  That's gotta be in there or you're hosed.

[Brucie64]

Many thanks all for your inputs, Alex was right in that I was talking about the fibre washer that sits (or doesn't as the case maybe) under the float needle housing and the main body of the carb. Once I took mine out the leaky tickler stopped leaking (original monobloc's only). The washer should be used on new monobloc's for some reason.

Anyway now I just have to figure out why my bike stutters now as you open the throttle more than say 5%, it feels like the choke is on but it isn't as far as I can see.

[dpaddock]

What, pray tell, distinguishes an "original" from a "new" Monobloc?

David

[Brucie64]

Hi David,

From what I have read in this forum (and I am no expert as you can tell), you can still buy new Monobloc carbs from people like Burlen Fuel Systems, these I am told have a groove under the float housing to accomodate a fibre washer.

My carb has 03/65 stamped underneath, again I am told this is the date of manufacture (March 1965), it doesn't have the fibre washer recess and hence I presume it shouldn't be used as it will alter the viton needle sealing height.