Dellorto PHF help please.

RCGBOB44

RCGBOB44

Hero member
Location
Coulsdon,Surrey, sometimes Germany
Hi guy's, I need a little help.
The centre carb on my Jota is leaking petrol from the slots at the bottom of the venture, i.e, the engine side of the carbs.  Does that make sense?
I have stripped the carbs and checked the float heights and they all appear ok.  Now according to the Dellorto guide the float height should be 17.5 - 18.5mm from the base flange (bottom of the carb body) with the  carb resting on a flat surface. This seems a bit vague as the float can 'flop' towards the carb or away from it, which is the correct position?  I can find no mention of these two slots or the function of the holes at the end of the slots in the guide so am at a bit of a loss as why fuel is leaking from them, just on the centre carb.
Any help would be greatly appreciated.
 
Hi Rob,

check the washer under the float-valve seat is existing and of the right thickness (cross-check with the other 2). Bet it's either not there or the wrong one.

if you measure float hights like described in the book, the valve must be pushed completely in by the "tong" where it's held on the float. It must open right after you move it just 1/10 mm (or if you adjust it to 18mm, it shall move after 0,5mm movement latest of the floats).

Personally - even though this is a bit of a mess really... - I prefer to check the float hights with the carbs installed. this gives you an obvious result if they are closing or not... ;-)

 
Robert,

The float height should be checked with the valve JUST closing, ie, the spring-loaded plunger NOT compressed, only bearing against its seat.  While turning the carb through an angle, you can see when the float bears against the valve, turning further the float actually compresses the spring and drops a couple of 0.1mm.  You need to find the point before the spring compresses.

The seal at the inlet stub should be air tight, seems fuel is leaking past the insulating bushes.  Check the end of the inlet stub for damage, and that the carb is fully "home" on the stub.  Not unknown to find cracks starting from the slots...

piet
 
RCGBOB44 said:
These are the slots in the venturi, one has a small hole drilled in it, that the petrol is leaking out off.
Click to expand...

Sorry, understood you were describing earlier clamp-type carbs.

If fuel dribbles from that hole, float height is too high or valve is not closing, ie, overflowing.

I set PHB floats to 18mm.  The closer you get them all equal, the easier it is to adjust the carbs, idle and pick-up will benefit.

piet
 
RCGBOB44 said:
Thanks for the info Piat, Ill check my float height in the morning and let you know. This has only started after I replaced all of my needle valves.
Click to expand...

Did you check/adjust the heights afterwards?  You'd be very lucky if they were correct without adjusting, quite rare actually.  Alone the thickness of the sealing washer under the float valve body has an enormous influence on float height.

piet
 
Well, the saga continues. I rigged up a support for the bank of carbs so that I could measure the float height/level. This was done ensuring that the bottom of the carb was vertical so that the float measurement was accurate.  Long story short, the floats varied anything up to 21/22mm from the carb base which is way off the 17.5 - 18.5 stated in the Dellorto manual.  It took quite a bit of time but I reset, by bending the float arms, all of the floats to 18mm as suggested by the wise sages on this forum.
I rigged up my remote petrol supply from the roof of my man cave/garage and turned it on to supply all three carbs with fuel.  Just like before the two outers were ok but the centre leaked.
Once again I removed the float bowl and checked the heights of the floats , all ok.  Now the only thing I had done was to replace all three needle valve assembly's, that's the actual needle valve and the brass seat assembly it sits in.  I changed the new brass valve seat with a good original one, checked the floats again, re fitted the float bowl and filled the carb with fresh fuel.  Hey presto no leaking so I can only assume that the new brass valve seat is faulty and maybe has not been machined correctly.
Just got to refit the carbs to the bike with new fuel line (from Mdina)and see if she fires up.  I'll let you know.
By the way, thanks for all the advice chaps.
 
I had problems with 2 of the red tipped needle and seats same as you, flooding (in my case out of the float bowl vents), ended up putting the 40 year old original brass, black tipped, needle an seats back in, no more flooding issues. Others have had problems with them. Someone on this site, Paul H?, had a problem with them and smoothing back the sharp edges of the needle stem solved the problem.

Stu
 
In my opinion PHF's are not made out what there are met to be - I say better off with VHB's , Mikuni's or British AB's - Killian are an other expensive story. Did not spell things right there ;o)
 
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