E: Engine Comet will not start when hot

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vibrac

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Just a note on the lapping of the float needle A little thought will reveal that over zealous use in the lapping will result in a higher float level. Remember unless you have had the carb since new, how many have lapped the taper before you?
 

Viny4

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A bad condenser will show itself will bench testing using a 3 spark gap test at 500 rpm. When cold it should give an uninterrupted spark. Heat the mag with a heat gun , the spark will become intermittent. It's very ovious. Mag testing without proper testing equipment can be iffy.
 

Roslyn

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Observation - Banging in the exhaust when trying to start suggest the ignition timing is possibly way out.

First Principles to be attended to in order for a Comet:

1. Valve Timing - set the camshaft position so you have EQUAL lift of the exhaust and inlet valves between 5 BTDC and 3 BTDC. 4 BTDC is ideal. Ignore marks on the timing gears - use a degree wheel and measure everything
2. If you are using the timed breather, it should close between 5 ABDC and 10 ABDC
3. Valve clearance is zero when stone cold BUT you must be able to rotate the pushrods
4. Magneto points between 0.012" and 0.015"
5. Spark Plug gap 0.018" with a Lucas magneto
6. Ignition Timing for easy first kick starting 4 BTDC fully retarded and for reliable running without the risk of modern crap fuel killing the motor no more than 34 BTDC fully advanced.
6. Carb adjustments - see attached guide and note that the pilot jet needle should be approx 1 1/4 turns out from fully closed.
7. If the carb is worn (most likely) have it professional rebuilt or replace it.

If you need more info on how to do any of these things - just ask.
change my pre monbloc as it had a 1/16 play in the slide bit worn 70 years old so put a new monobloc carb on my bike now i run three types of carbs on all of my bike.
 

Magnetoman

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From MPH 737 "The correct 276 float level is just below the hole that exits at the top of the base cap so that fuel does not flow out of it continually"
The hole visible just above top of the base cap of an assembled carburetor is the one through which I've inserted a drill bit in the jet block of a Shadow carburetor in the first photograph.

PreunitCarbLevel01.jpg


The needle jet screws into the jet block so this photograph shows where the height of the fuel level would be on the needle jet if it were at almost the height of the hole.

The ridge at the bottom of the jet block determines its position with respect to the main body, so the second photograph shows where the mixture screw is located with respect to the same hole in the jet block as shown in the first photograph.

PreunitCarbLevel02.jpg


I tried to compensate for parallax as best I could when taking this photograph. The hole in the jet block is approximately even with the bottom of the head of the adjusting screw.

Shown in red in the third photograph are the positions of two fiber washers whose thicknesses affect the height of the fuel.

AmalStandard3.jpg
 
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gregg-k

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Regarding LoneStar's question, if you plunked a K2F in place of a K1F, you would have to deal with the un-used second spark ... ignoring it would mean that spark energy would have to be dissipated inside the mag. Yes, the "safety gap" would fire, but with it not being intended for continual use, you would soon etch the brass sector of the slip ring.

The cheap and dirty way to do it would be to replace the mag's twin cylinder slip ring with a single cylinder slip ring, and plug the hole for the second HT pickup. That way the second spark would fire the same plug on the exhaust stroke, just like a "wasted spark" configuration on a twin.
.. Gregg
 

everiman

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Great stuff everyone, FYI, the cams were timed by me with a degree wheel, the tappets have zero clearance but the push rods turn. When the fuel tap is opened and left open without using the tickler, the bottom of the carburetor and the hole to level to stay dry, so if anything is out, the float is too low. The last time it ran, it started and ran, I forgot the fuel tap was turned off, it died and I have not been able to start it since. I bought the rebuilt the comet magneto that was on ebay uk, the mag was supposed to have been rebuilt by C H Day, and is supposed to have a 3 year guarantee, so if nothing else, I will have a good spare magneto :) It will take about a month to arrive. I will update after it gets here.
 

everiman

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Everiman, I am located in Ottawa and am intimately familiar with the Lucas K-series mags. Full disclosure: I restore Lucas mags and dynamos as my business.
Why not ship your mag over and I'll inspect it for you?
.. Gregg
What's your contact info?
 

brian gains

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taking a slight detour from the main thread:
in Magnetoman's third photo of a section through the carb, do the washers in red have a specified thickness, is final fuel level a suck it and see approach , I guess the fibre washers are prone to some compression ?.
I have the , what seems usual state of affairs, where once assembled fuel weeps out of the top of the slot through the threads of the lower carb body seen in photo 2, once I'm motoring the issue is no concern. Is it a case of 'they all do that' or indication of a poorly assembled / badly adjusted carb'?.
 

Chris Launders

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The cheap and dirty way to do it would be to replace the mag's twin cylinder slip ring with a single cylinder slip ring, and plug the hole for the second HT pickup. That way the second spark would fire the same plug on the exhaust stroke, just like a "wasted spark" configuration on a twin.
.. Gregg
Surely it would be easier to replace the cam ring with a single one, making sure which pickup is still firing before blanking the other off, one of the firing positions must still line up as you cannot change the flux positions on the armature.
 
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