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E: Engine Comet will not start when hot

everiman

Active Website User
VOC Member
Right, so I have been dealing with this pretty much since I have owned this bike, a 1954 comet. The bike starts easily when it is cold, choke slides closed 3 pokes on the tickler.
Once is it is warmed up it will not start until it has cooled down.
The bike runs great if I can manage to not kill the engine, power is excellent, no problems exceeding 60 mph.
There is no misfiring once off idle, but it will not idle for more than a few seconds.
The spark plug looks OK, a bit rich, but no different than my other bikes, Norton Commando, Yamaha XT500, both of which start easily hot or cold.
There is always spark when checked, hot or cold, magneto timing is where it should be.
It seems to be carburetor, everything else including cam timing has been checked and what it should be.
It is very easy to mess up the carburetor so that it will not start, hot or cold. The set up that enables cold starting is slide lifted about 1/16 or a bit less, air screw turned out 1 1/4 turns, anything deviating from this setting will not work hot or cold.
The carburetor is the original 229 amal, it has been thoroughly cleaned ultrasonically, all the fuel and air passages in the jet block are clean and open.
It is just about impossible to resolve the problem given that once the bike has started and warmed up, I try an adjustment, it dies, and then won't start again until cold, so back to square one.
My next move will be to replace the carburetor with something else, probably a Mark II, but before I do that I want to check with the forum to see if there is something I am missing.
 

Roslyn

Well Known and Active Website User
VOC Member
Not likely anything to do with the carb.
This is classic capacitor failure of an old magneto. Two solutions.
1) take the mag off and have it rebuilt
2) replace the mag with something modern

If you are not running a magneto what ignition do you have

Robert

PM me if you want and we can make contact. I am in Vancouver. I won't put contact details on the public forum.
My way of doing things is to alter one thing at a time even takes notes and photos along the way and taking time out before it starts to get to you I was on the edge of scraping a 1951 black shadow some time ago even a to the stage of putting a hammer though the case and getting the grinder out and cutting into little bits but found having breaks even taking a day off the problem helps an awful you will get there in the end but do it in little baby steps.
before rushing off to throw money away take a long look at the problem and write a check list then do one thing a day at the time you have a lot of support on this web site
 

Magnetoman

Well Known and Active Website User
VOC Member
I don’t have one, but could someone post a pic of one of those adapters with tubing to check level with a 229?
I hope these photos help with the general principle, as well as with the specific fuel level. These are from the carburetor on my 1928 Ariel. Burnishing the needle stopped it from overfilling (I believe it is the very narrow bright band that does the sealing), and with the level set where shown and it was a one-kick starter, happy runner, and non-fuel leaker, all the way across the U.S. in 2018.

PolishedNeedle.jpgAriel_carbfuellevel01.jpgAriel_carbfuellevel02.jpg

Amal_preMonoblocFuelLevel.jpg
 

MartynG

Well Known and Active Website User
VOC Member
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.
 

Attachments

  • premonobloc booklet.pdf
    1.5 MB · Views: 8
Last edited:

gregg-k

Website User
Non-VOC Member
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
 

LoneStar

Well Known and Active Website User
VOC Member
On a somewhat related note - is there any reason one couldn't fit a (common, inexpensive) K2F magneto to a Comet, using one output and ignoring the other, rather than the scarce standard K1F?
 

CollingsBob

Well Known and Active Website User
VOC Member
Full Disclosure..Gregg K rebuilt my twin magneto a few years ago. He carefully examines the mag upon arrival, photographing and measuring everything. He issues a complete report with recommendations. The process includes running it on his test bench for 8 hrs prior to packing & shipping. Impressive & thorough.
 

MartynG

Well Known and Active Website User
VOC Member
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.

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 "

In MPH 740 we are told that if you use a sight pipe (clear tube) the level must be just below the upper edge of the nut that screws onto the base of the mixing chamber to retain the jet block.
 

vibrac

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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
 
Last edited:

gregg-k

Website User
Non-VOC Member
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

Active Website User
VOC Member
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.
 

brian gains

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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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