TT carb on 500 single

[robert.clark]

Hi out there,
I have seen a number of articles on carb wear recently and would like to sound you out before making any changes. I have covered about 4000 miles with the TT carb fitted with new parts, having a good set up and no issues. On petrol.
Slowly but surely over the last say 500+ miles I have noticed the engine overrun at tick over together with some fuel leakage from the body of the carb (mid way up forward facing).
1st - I have made adjustments to the adjustable needle control and this did not change the overrun.
2nd - I removed the carb completely gave a thorough clean and passageways etc and fitted with new joints. no difference.
3rd - I checked the float bowl and parts and everything appeared as it should be.
I have no previous experience of this occurring before - is it possible that this is simply a symptom of wear between the needle and the needle jet and lowering the needle by one notch is the answer.
Actually that's my next step but someone may be able support the theory or any other suggestions.
Rob Clark

[nevhunter]

Can you check the fuel level in the float bowl?. I always do this first. Nev

[adrie.degraaff]

The level must be center airscrew.
It sounds like the floote needle is worn but you have a new one in?

The centerfeed TT is better than the sidefeed.

[robert.clark]

Thanks for the reply. Float bowl - OK it's a bottom feed and what you suggest I have considered. And I can't be sure if the needle was a new item.

Do you think a new replacement is necessary or simply a dab of fine grinding paste and re-bed of the existing faces? as there are 2 tapers to consider?

I can't see much material would be removed in this way that would affect the float level. I'm happy to replace but considering the options so needle and needle valve seating together?

Again thanks both for responding
Rob

[david.anderson]

Robert
The needle and seat can be ground with valve grinding paste but it is very often ineffective as the paste is often not fine enough and the needle does not seal properly afterward. Repeated lapping in this manner used to be common practice and will alter the float level. A better method is to tap the needle onto the seat with some gentle hammer blows while rotating the needle. But first check to see if the needle is sealing on the seat by leaving the petrol turned on. If the carb doesn’t flood then that is not the problem.
The change in your idle could also be as a result of a worn carburettor body or slide. I have seen amal carbs having considerable wear after a similar mileage to yours, particularly where no air filter is used. The wear is worst when a zinc pot metal slide is used with the zinc pot metal body (which has been amal practice for years). A brass slide or an anodised slide greatly reduces the wear and extends the life. And of course the continual movement or warping of the carb body due to over tightening also results in air leaks around the slide. I would check the slide fit in the body and if lose at all try a new slide.
David

[robert.clark]

Dear David. First you were correct with the float bowl lid off it was obvious that the bowl was over filling.
2nd time to hold my head in shame - I carried out the revised seating with gentle hammer taps - the clue here is to select a sensible punch that fits exactly with the bottom of the tapered needle - bugger that was my mistake I didn't have a sufficiently pointed one around - I can only just admit that my choice was poor and used a phillips head punch - well I distorted the thing sufficiently well to prevent it bedding ever again and despite some clever file work I spoilt it - I know .
3rd - I purchased a new one - it's fitted and after marking with engineers blue have a nice bed about 1/16 inch wide about 1/8 inch below the widest part of the taper. Tested and the float level appears to be correct - obviously around the correct level so I have to rely on the genuine Amal parts being correct. The needle compared with the old.
4th - Strangely there was still some leakage from the drilled hole in the small boss just above the adjustable pilot needle. I have adjusted this in to a point where there is just a slight weep. The engine is a 1936 exposed valve single port. It started once maybe a minute and nothing since so i have sparks and obviously fuel so I have some further investigating to do. It felt crisp and right but not warmed up enough.
5th - I afterwards thought that maybe a chrome cleaner solution or brasso would have been an option for the bedding of the needle.
6 and finally I know the slide was new at 4000 miles so maybe that's next on the shopping list.
Thanks for the input.

[nevhunter]

The float needle will seat itself if it leaks a bit, usually. You might have to set the bowls down a bit or put a small fibre washer on top of the float, unless it is the type with the spring clip soldered In place Nev

[Dave.Barkshire]

Just a thought about carb wear... my bikes drink a splash of Castrol R (after shave) in the petrol to make them smell nice but it also helps the carb to last forever.

[robert.clark]

Update on this belated topic just to let members understand the carburettor issues were consequential to the main fault I found.

I removed the 1936 single port head and found really extreme inlet valve guide wear. Exposed valve gear and grease lubricant to the valve guides over 5000 miles were no substitute for continous clean lubrication. Excess fuel in and around the carb I put down to the poor seating of the inlet valve. The slow fall off in compression over 500 miles would appear to support this.

The valve guide bores in the head were not true and have now been machined oversize. New valve guides have been fitted made from Colsobro a copper nickel silicon alloy. It has high wear resistance and heat conductivity, and the lubricity prolongs valve stem life.
Re-machining of the valve seats cleaned up well and ready to build. It's taken over a year to get this work done and at £65 per guide a little extreme. Re-assembly planned in the coming months.

[paul.jameson]

Extreme wear on the valve guide on an exposed valve engine can be caused by the rocker not being central to the head of the valve. Assemble the engine with some engineer's blue on the top of the valve stem. Turn the engine over. The mark should be central to the stem. If it is to front or back of the stem you should be able to adjust this by varying the height of the tappet and compensating for this by altering the length of the pushrod. Rather than dismantle the pushrod you may well be able to get a satisfactory result by swopping pushrods and cam followers.

Are you using the oil mist feed pipe to the inlet guide? Did you drill the colsibro guide to allow this feed to work?