Misc: Carburettors 1952 Comet Carburettor New Float Level.

[Matty]

After around 60 years and the possible effects of new additives to petrol I thought it about time that I changed the brass float and needle of what I assume is 276 carb (though Paul Richardson's book says my carb should be a 229F, whatever that is). I was also trying to cure a problem I have always had, in that the plug is always black, no matter what grades I have tried over the years - I presently use an NGK 6 grade which is pretty soft, but even this runs black and sometimes makes the bike difficult to start.

I bought the new float and needle from VOC spares and they sent a float kit for a 276 for the Comet, and on the same order I asked them to send a suitable needle jet and taper needle. They sent these last two items for a 289 and I assume they know that these will be OK for my carb.

I changed the float etc and the bike ticks over very well as it has always done - though the pilot jet setting is, and never has been very sensitive to set up and seems to work OK over a very large range of adjustment.

It was a bit more difficult to start and spat back when cold with the new plastic float, but ran well when hot. However the exhaust pipe has blued up, suggesting it is now running much weaker. The plug colour has also changed to brown now suggesting that the mixture had improved. I have checked the ignition timing and ATD which are OK

So to investigate, I have made a little adaptor to screw into the drain screw on the float chamber with a clear tube attached to be able to see the fuel levels in the float chamber.

With the old float the fuel level is roughly 1.125 inches below the top face of the float bowl without its top on, but is 1.25 inches, plus a bit, below with the new float, though the difference is probably a bit more like 3 sixteenths of an inch between the levels - so this could account for the weaker mixture.
(the top of the float chamber is normally on when the levels are taken, the top of the float bowl has just been taken as a datum line)

I am surprised that this small level change has made such a great effect on the plug and exhaust but practically no effect on the slow running which I did not have to reset. I have also not taken the bike much over 60 mph after the change and the performance seems the same as usual, but the exhaust has blued up. Does anybody have any ideas on what might be happening please ?

Does any one know what the fuel level should be in the float chamber please for this original carb, because I seem to be "stuck between the devil and the deep blue exhaust pipe"?

I have also changed the main needle jet and slider but these have had little effect on the running which seems the same with the old or new float when the engine has warmed up. For some reason the new slider was vey tight in my old worn carb which "clinked at slow speed" as the air pulsed past. I had to lap in the new slider by several thou. before it would move up and down and it is now a beautiful fit but has not really affected the low rev end of the running in any way - the bike has always ticked over very slowly and evenly.

Hope someone can help with the expected petrol levels and can explain why I seem to have either a mixture which is too weak or plugs which are always black.

Matty - Had the bike since 1956 and it has always been used pretty regularly.

 

[greg brillus]

Hi Matty, Your method of checking the float level is fine but the clear tube should be held next to and along side the idle mixture screw where the level should be at or slightly bellow the base of the mixture screw thread. The float level will have a large effect on the running as the higher the level the easier it is for the fuel to be drawn in and burnt as a rich mixture. The level you find can be adjusted by shimming the float on its needle with small washers if necessary. The slide needles are different from the 276 and 229/289 carb's as the 276's needles are shorter, so this will affect your mixture as well....i would think running a 289 needle in a 276 carb would make it run lean.....your issues may stem from one problem trying to compensate another, hence the odd running and plug colour. These carburretor setting are quite sensitive, even a worn needle jet (usually a 106) will make it run rich as well.....if you replace the needle jet, and use the correct slide needle...and set the float level to the right height, your engine will be much happier. Perhaps buy your Amal parts from someone who knows what the parts are before they send them........A never ending problem......Cheers and good luck......Greg.

 

[A_HRD]

Matty,

As ever, perfect advice from Greg.

A std Comet had the 229 1-1/8th bore carb. Not a 276 (1-1/16th) (but these were used on Rapides).

If you are sure you have got a 276 on your Comet and you want to stick with it, you must replace the needle and needle jet with 276 items. The 229s items are different (longer for one!) and if fitted will cause all manner of problems.

The float-chamber components are identical on both carb types.

I am surprised you had to lap in the slide! Somethings sounds mighty suspicious to me…..

Peter B
Bristol, UK.

 

[nkt267]

How can you shim the float/needle position when the clip slides into a slot in the top of the nylon float? Are you using the correct slot as there are usually 2 on the float needle,though if I remember correctly the lower one may be too low.
You can get different length jet block covers(bottom bit of carb on side float carbs) to get the correct level,as Greg says by checking it against the pilot screw.You can even rotate the carb to get the correct level at the pilot screw (it can be very deceptive just by 'eyeing up').Possibly Greg meant shimming the float chamber where it bolts on to the carb. John

 

[Matty]

Hi All Thanks for the good advice.

The plot thickens a bit however, because I put the new float back - to check the levels again and the bike now seems to run very well !!!

I must admit that it seemed to run OK for a week or so when I first changed the float, slide, needle etc. but the problems appeared with the onset of the cold weather, so weak mixture seemed to be the cause.

I now think that some piece of "grot" had got in somewhere and with draining the petrol from the carb. for my tests etc. it has been flushed away !!

The petrol level is correct with respect to the pilot screw with the new float and I can confirm that my carb is 1-1/8 bore and the proper one (the bike had only been used for 2 years when I bought it, so I would be surprised if it had any non standard bits.).

Can any one confirm please that the needle and needle jet for a 289 sent by VOC spares are the correct ones for my Series C Comet carb.

By the way - the plastic float has a clip which is designed to fit into the lower slot on the new needle - the upper slot is a fit for the old brass float type of clip. If you use the upper slot with the plastic float it hits the tickler before the petrol is cut off and just floods. Also because the float clip is non adjustable it is not possible to change the float level by shimming.

Thanks again - and I hope this is now put to bed, but I may add more to the thread if mixture problems persist or the exhaust glows bright red !!

Matty