31 replies to this topic

[HUBBA.HUBBA]

This a vid i have put on youtube just after I have put a brand new float needle assembly on. I just flows out as if there is no float in there at all! www.youtube.com/watch?v=fFTqtV8kpTE

I set the float level again to just less than 1mm to the dip in the float using a straight ruler edge. 

 

[attachment=185359:carb float.jpg]

[HUBBA.HUBBA]

Not seen this on a Metty or Mini that I recall, but s'pose it's an option?

 

That's different to mine

[Ethel]

..and any I've seen. Hole's in the right place though and most , if not all, have a blanking plug where the drilling would be on the manifold flange.

[HUBBA.HUBBA]

I've just swapped all the float chamber components inc jet and adjustment assembly and in the other carb it all works fine. I also put the bits out of the standard HIF and it still doesn't work.

The sprigot as mentioned earlier, is all ok.

I noticed in the shaft where the jet assembly goes there is a small hole there. Anyone know what that is for? Just wonder if it is coming from that.

[Icey]

I've just been out and stripped mine down and it seems like it can only come down to the float needle and/or seat.

 

I plumbed the fuel inlet into an airline and it would happily seal against moderate pressure - you can also do this with the float chamber open so if the needle seats and seals you can fill it with water (or fuel) and see where it's coming from.

 

Edit:

 

I noticed the hole as well bit it seems to be used for other purchases - there is a blanked drill-out in the casting so I'm assuming it's not involved.

Edited by Icey, 28 September 2016 - 05:34 PM.

[HUBBA.HUBBA]

I've just been out and stripped mine down and it seems like it can only come down to the float needle and/or seat.
 
I plumbed the fuel inlet into an airline and it would happily seal against moderate pressure - you can also do this with the float chamber open so if the needle seats and seals you can fill it with water (or fuel) and see where it's coming from.
 
Edit:
 
I noticed the hole as well bit it seems to be used for other purchases - there is a blanked drill-out in the casting so I'm assuming it's not involved.

Thanks for having a look. Ive changed the float needle and seat 3 times now and doesn't work, but works in another carb. Tried a different float and still no joy.

[Icey]

Thanks for having a look. Ive changed the float needle and seat 3 times now and doesn't work, but works in another carb. Tried a different float and still no joy.

This is what I tried to work out. How else could fuel get out of the jet if the float was sealed? The only other route I could see - other than something more dramatic, for example a crack in the casting - was if the thread around the seat was damaged allowing fuel to bypass the needle.

 

So if this was my carb I'd flip it upside-down, take the float cover off and either do as I did earlier (plumb in an air line) or connect the funnel again. The weight of the float should stop the fuel/air and show how/where else it's getting in.

 

I'm really quite interested in seeing what's causing this. I'm half way through the rebuild of some bike carbs (Keihin, 4 of for a flat-4 engine) and have been cursing them in the name of the HIF for it's simplicity, I'm starting to doubt that now!

[burchy35]

If your carb is fitted with a lean off pipe pull it off the front nipple of the carb and you should be able to blow down the pipe which is still connected to the side of the carb. It should be free of blockage down to the float chamber. I still think the float chamber is pressurising when the fuel is pumped in
. The air has to go somewhere when the fuel enters.
It's always worth doing the T piece trick if you haven't already tried it.

[HUBBA.HUBBA]

Tried stripping out the jet assembly ( just incase it was catching the float somehow)and the valve seals fine. Put the seat for the jet back in but left the jet out and it still seals ok, but as soon as put the jet in it flows out the top again. As if is syphoning.

[HUBBA.HUBBA]

If your carb is fitted with a lean off pipe pull it off the front nipple of the carb and you should be able to blow down the pipe which is still connected to the side of the carb. It should be free of blockage down to the float chamber. I still think the float chamber is pressurising when the fuel is pumped in
. The air has to go somewhere when the fuel enters.
It's always worth doing the T piece trick if you haven't already tried it.

You know, you have just made me think. My lean off pipework is blanked off. That sprigot you were taking about before. Could that be linked to to my boost/vacuum pipe to equalize the pressure?

[burchy35]

That outlet spigot next to the fuel intake can be teed into the pipe that runs between the fuel regulator and plenum (T piece trick). I would first just pull that blank off the spigot next to the fuel intake and turn the pump on. Fuel should not come out of that as it's above the height of the fuel in the float chamber. It should let the air out of the float chamber so the pressure doesn't force the fuel up through the jet. You have nothing to loose apart from a bit more fuel.

[Icey]

Tried stripping out the jet assembly ( just incase it was catching the float somehow)and the valve seals fine. Put the seat for the jet back in but left the jet out and it still seals ok, but as soon as put the jet in it flows out the top again. As if is syphoning.

 

So the float is working but the bowl can't fill enough to close the valve, or more pointedly when it fills to the point of covering the jet any further fuel is forced out. So it does seem like a vent rather than valve issue.

 

If nothing else but for my own curiosity, I'll open my carb back up and/or try and dig out the diagram of the internal passages.

[Ethel]

Other than the float chamber not venting at all, it's hard to see how on a non running engine.

[burchy35]

Luckily when I had the same symptoms as this I had another carb to compare and thats when I noticed that the vent pipe next to the fuel pipe was shorter than fuel inlet. I pulled it out to the same length and everything was fixed. If the bung on the vent pipe was removed or the T piece trick installed and fuel still flows out then it's obviously not a chamber pressure problem. It was blocking off the internal passageways through to the plenum and lean off. I had searched every forum at the time but couldn't find an answer.

[HUBBA.HUBBA]

Good news. Read an article on turbo minis about the the tee trick. It transpires the gasket between the carb and plenum didn't have the hole to pressurise/vent the chamber. Apparently a common problem as the turbo gasket has more holes that the air filter gasket. Took the plenum and gasket off and it works with a manual fill. Going to make a new gasket, put it all together and test with the pump.
Thanks for the advice everyone, it provoced me to look outside the box ( especially the " The air had to go somewhere when the fuel enters" comment)

Edited by HUBBA.HUBBA, 29 September 2016 - 03:34 PM.