Hitchcocks Forum

Hello,
I have a question about my 376 Amal carb. currently residing in my 1955 Velocette MAC 350.
Please help, this is doing my head in!

I have two carbs both 376/48. The newer carb was bought new from Amal and ran fine until the pilot jet became gummed up and I had to drill it out and re-tap (2BA) to accept a new jet. Unfortunately, it never idled properly afterwards as the drilled hole was off centre and allows unmetered air in. This carb still runs fine off idle (1/4-full throttle).
In an attempt to fix this I bought a used body and transferred over everything including jet block and slide. This carb idles nicely but will not run. It's very lumpy and will not rev cleanly.
I bought one of Amal's recon kits in desperation, but that made no difference - good idle but poor running performance.
Reinstalling the original carb and everything was back as before - poor idle and good running performance.

What's going on? Any ideas?

Regards,
Chris Hopkins

If the pilot jet blocked due to old fuel it may be that the pilot air passage's are blocked these are the two small hole's you can see at the back of the carb try blowing them through with carb or brake cleaner
Also worth checking are the two tiny drillings at the engine side one in front of the rear edge of the slide one behind

On the replacement carb give all of the drillings a blast of brake or carb cleaner and while you have it in your hand check the mounting flange is flat you can use wet and dry taped to a kitchen worktop to take out a slight warp
Hope this helps Dai

Hi Chris
Just had a look at my chart's
Anal monobloc 376/48
Velocette MAC
Pilot 25
Slide 3.5
Main 200
Needle jet 0.11 = 106 I think
Needle groove 3

Then again I bet you knew that already
Cheer's Dai

Have you checked the float levels on both carbs?
Depending on your float needle and its seat, the early ones did not have a fibre washer under them.
If you fit one, it will raise the level too high.
I think the newer viton tipped float needles require a washer..?
If you don't have a float bowl cover with a perspex cover, you can take the pilot jet cover nut off and fit a clear tube over the end of the pilot jet to see the fuel level.
The little pip on the float bowl cover should be your level.

OP joined 21 sept at 10.08
Posted 10.10 about his alleged VELOCETTE woes
Not been seen since
Time waster

Hi Brian
Don't you just love em?
I thought he would have been back as motor and carb added up !
But guess not
Oh well cheers all Dai

Many apologies all. I had some technical issues this end.
So, to clarify. I'm not interested in the idle circuit of my newer carb. I buggered that drilling out a gummed up jet to re-tap 2BA and fit a new one. It does still work nicely on the road.
The second hand carb is the one I want to get going. It has all the correct (new) bits as you suggest Dai. They all work fine in the newer carb but not in the second hand one. It's a pig on the road.
One carb has the thick flange o-ring and thus no float chamber feed pipe washer. The other has the seal and thin flange o-ring.
The float is plastic and I've used a viton tipped needle valve and a plastic one.
Cheers,
Chris

Welcome back see I said I thought you were OK

To be honest I have been having murder's with the new e 10 fuel bike doesn't like the new e5 either

I have gone up to a 40 pilot jet from a 30 gone from a no4 slide to a 3.5 gone down to a 106 needle jet from a 107 needle position I have gone for number 3 slot main jet down from 260 to a 240 the float needle I have changed from brass to aluminum
It's better but still needs some tweaking

With your used carb could it just be too worn to work or is the mounting flange warped ?
Would it be worth having it sleaved ? (spelin)

Anyway good luck with it

Cheers Dai

Thanks Dai. Mounting flange is OK. Sleeving might sort it. Difficult to see where else it can wear. Are the classic symptoms of a worn slide poor performance off idle?
Chris

It will be the carb body that will need sleeving (spelin)
When very worn it will let the slide rattle about allowing air past

I have never had a monobloc sleeved but whenever I have gone too weak with the jetting it will not pull at all literally under any load it will refuse to run

It's this that makes me think air is getting past the slide

Another common mistake people make is screwing the pilot jet into the cap then screwing both jet and cap into the carb as a unit
The jet has to go into the carb body then fit gasket washer over the jet then the cap goes on last

Hope at least some of this helps Dai