Ferret Carb
 

Could anyone give me some info on tuning the carb on a Ferret. The carb is new, put on by previous owner but never run. I have set the slow idle metering screws? The two on the bottom back 1 1/4 turn out each, any other settings to be done? I also set up the choke controle and lubed the cable. Also looking for a couple parts, anyone have spares?

Is carb actually new, or a rebuild?

They will usually idle well with the two idle adjust screws out about 1 1/2 turns, if everything else is in good nick (ignition timing, plugs, wires, coil, points, condenser, compression).

You should see the idle speed noticeably drop and get rougher as you turn the screws in to 1-turn out and out to 2-turns out if everything else is OK.

The only other external adjustment is the screw on the throttle lever which raises and lowers the idle rpm.

What problem are you having? Just idle or all through the rev range?

You can find the manual for the carb by googling "solex_40nnipcarb.pdf"

Malcolm

The carb has Marcus instead of solex on it so I'm assuming its new. It has never been run since the carb was put on so it was not set up. As far as I know this Ferret hasn't run since the military released it in 2007. What about the altitude adjustment? Does that make much difference in how it runs? Right now I will be happy if I can get it to run for more then 15 seconds.

Carb troubles.
 

I have one on my trailer now that seems to idle fine with just the fuel in the carb, but will flood if I add a fuel line. It won't rev up and seems to need some tinkering. Is there a manual that shows step by step what needs to be done to adjust the carb to "factory" or end user settings? I am thinking if there was something that said- set screw #1 back off 1 and a half turns, set screw #2 back off 2 turns..... etc. Robin you may have some thoughts? Lots of experience tuning these....

Steve,
As you say a private owner fitted the carb, check it is a 40NNIP and not a 48NNIP. The 48 was for some B81 engines, also the 40NNIP was fitted to 8 cyl engines of certain Marks and jetting would vary between 6 and 8 cylinder engines. If the wrong carb was fitted, this could be the problem.

If you can, make a note of any numbers on the carb and around the base flange (if any) and post them on here.

I have linked to the manual above which shows the basic settings.

I found there are a few things that make these carbs run rich or flood.
The fuel needle valve that is closed by the float to maintain a constant level in the carb could be stuck partly open, so when you connect it to a pressurized supply, it passes too much fuel. This can also be because the floats are sticking on the pivot pin. Sometimes tapping the carb gently will free things up.

Someone has adjusted the float level too high. I have found the 3/4" level specified in the manual is a bit too high and the floats can hit the cover gasket before they close the needle valve fully.

If you are using an electric fuel pump in place of the stock mechanical pump, they often supply fuel at far too high a pressure, which overwhelms the needle valve and floods the carb.

Every one of the four or five carbs I have rebuilt has had the accelerator pump diaphragms leaking. I suspect this is an ethanol problem. The leak is directly into the intake manifold and makes the engine run really rich. It also allows the float chamber to drain down overnight. So a good check for this is, after the engine has been shut down for a few hours, look down through the carb with the throttle plates open for a large puddle of fuel sitting below the carb in the manifold.

Malcolm

The numbers on the carb are as follows
40NNIP
RE16434
849

The other side has
Marcus
British
Pat'd

Thanks Steve,
40NNIP is correct type, the only specification number I have is the Solex one, which Marcus would use as well, that is S1682 for a Ferret. The RE number is a Rolls Royce stock number. Those carbs used on 8 cyl engines could be considered identical, but I believe jet sizes differ. Will have a delve later on and let you know sizes for a Ferret.

Hi Steve,
The jets in a Ferret carb are as follows ( sizes are marked on the head face so no need to remove ):
Main jets (2 off, either side of economy valve); 190
Starter jet (1 off, between the two mixture screws): 180
Aux. or Pilot jets (2 off, above economy valve); 55

Richard, the main jets are 130 as shown in this link and also what I have seen in a few carbs. 190 is the air bleed (correction) jet for the main circuit.

http://www.ferret-fv701.co.uk/mnuls/...40nnipcarb.pdf

Steve, FYI, I have worked on two Ferret 40NNIP carbs that also had RE16434 stamped on them.

Malcolm

Thanks for the correction Malcolm.
I was going through the British Army parts list and noted them down wrongly, well spotted. Well look like Steve's carb is correct then from your confirmation of RE number.

Looked back at the parts list and the correction jet and main jet were wrong way around. Just shows you cannnot to rely on official publications.

The two carbs stamped RE16434 had 22747 and 19195 stamped underneath.

Two other Ferret 40NNIP carbs I worked on were stamped RE14549. They had 5537 and 6264 stamped underneath.

I have to think these are serial numbers, so Solex made a lot more RE16434's than RE14549's? I didn't notice any difference between the two RE #'s.

These were all Solex carbs, with no sign of the "Marcus" designation Steve refers to. What is that? Did Marcus make Solex carbs? Is that why the "849" number is so low relative to the above numbers?

FYI, as you can see from the 40NNIP number in the photo below, the 40 and the NNIP were cast into the carb body, and the 0 was stamped.

Malcolm

Attachment 78265

Just a wild guess, but maybe the venturi can be factory changed for a bigger different size, but this model has a 40mm bore???

Malcolm,
Marcus Engineering took over the manufacture of these carburettors in later years. It was they that resolved the inherent problem with the 48NNIP carb fitted to the CVR(W) Fox, with a redesigned base plate and accelerator pump linkage. Nearly too late as the Fox was near the point of being withdrawn.

My carb.......
Attachment 78284
Attachment 78285

carb 2
Attachment 78290