Poor running - very smoky!

[Oggers]

Gents

Velo Viper with a Monobloc 376 carb which I have just overhauled. Put the carb back on and the bike is now a pig to start and when it does, it misfires amd smokes very badly. Ran fine beforehand

Everything was blown through, all jets correct, but obviously the carb came off and was put in all manner of positions - inverted etc.

My thinking

Stuck float - but it seems free - worked fine beforehand.
Float Needle valve stuck in the seat or not operating

A pointer perhaps. When the engine was running - badly - I turned the petcock off, let it run badly and after about 20s or so, it picked up revs and began to run far bettter for about 5 or 10s before dying of fuel starvation presumably.

All clues gratefully received!

 
 

[R]

Blue smoke - or black smoke.  ?

If its black smoke, then for some reason its flooding - badly - and the black smoke is like its got the choke on, and then some.
As the fuel level comes down to where it should be, it runs how it should be set.

Don't Monoblocs have that little dot on the float cover there, that indicates the correct fuel level.
Sounds like you need to work towards getting that set correctly.
Obviously something has gone awry with the fuel float/needle setup.
Hopefully you can correct that just by taking the float cover off, and working what needs to be done.
have fun !

[Oggers]

R

Yes indeed. Was thinking about it last night and came to a similar conclusion - flooding of some sort. Thing is I never touched the float/needle set up, but yes, I think the answer is in there somewhere!

[R]

I once pulled something like a blade of grass out of the fuel pipe.
The fuel needle/seat didn't like that one little bit.

It only takes a chunk of rust or debris out of the tank to upset the apple cart,
esp if there is no screen to keep chunks out of the fuel system.

[Oggers]

Right

I think I have cured it. For information sharing purposes and for others, I did as follows.

I checked the level with a clear pipe attached to the pilot jet and held up the other end to the pip on the float bowl cover. Fuel level was spot on.
Checked out the (brass) float, the needle valve and the seat - all good. Cleaned them all up - again!.
Checked main needle seating, the jet block, cleaned out main jet - all good

No joy- thing won't start at all well, and when it does, it smokes worse than a Chinese factory.

Brainwave - I had another look at the pilot jet, all good, and screwed it back firmly into the recess. It now starts and runs as before - result! What I think was the cause, is that the pilot jet end was not mating fully with the sealing face within, allowing petrol to pass through and flood the mix. Lesson learned. It also now appears to idle OK. We live and learn.

[iansoady]

Good result. I thought that was well known but obviously not - I'veseen other people make the same mistake. If the jet isn't fully home petrol can go round the outside and give the sort of effect you were seeing.

[Oggers]

Ian

When I removed the carb to clean it, I did remove the pilot, and screwed it back in as far as I thought necessary without wishing to damage it. Clearly it wasn't enough. It did need to be screwed in pretty firmly - far tighter than I would have thought. I'd estimate it only took another 1/8 turn or so, and it really took some torque for such a small screw - perhaps the seat isn't great?

   

[Rex]

Maybe that's why it's common to come across old Monoblocs where the pilot jet has been screwed in tighter than a very tight thing, and then the subsequent owners/bodgers have muellered the top of the jet while trying to remove it.

[Oggers]

Rex

Yes - I have seen similar on other carbs. The pilot screw slot broken off! Mine really did need what I would consider excessive torque to seal it. I wonder if a small O-ring would help at the mating faces? 

[john.k]

I see some comment that Burlen have stopped using plastic needles in Monoblocs and use brass instead,as the bit of extra weight cures needle sticking .

[iansoady]

I nver liked the plastic needles and always change them to viton tipped brass ones.