E: Engine Comet will not start when hot

[greg brillus]

Brian's question about fuel running from the top of the jet block locating slot would normally indicate the float level is too high......However if the float needle is not seating well enough, the fuel will continue to rise causing the same condition. This also happens when a bike is left on a side stand and the taps are left on, or are leaking internally......... The upper red sealing washer in the picture above, is a standard Amal part probably around 40 to 50 thou thick.......I have seen home made ones where the ID was too small, and this effectively blocks off fuel delivery to the pilot jet........a nice one to catch out those who aren't aware.......Note also the the large plated cap nut that holds the jet block to the main body comes in several different heights........so the wrong one will upset the float height.......This applies to the 276 and 229/289 carby's.........A bit more useless info.

 

[vibrac]

In that picture of the carb crossection last pic in #55 you can see how the excess lapping of the float taper I mentioned in #51 would change the level

 

[everiman]

Update, Magneto replaced, comet starts when hot.
Magneto arrived Thursday (24th) I ordered it Monday (21st), which is amazing, as I am in Canada and the magneto was in UK, this was all through the ebay international shipping program, available only to UK and US sellers apparently. This is an ebay service, the seller has to choose it. Ebay handles all the border fees and shipping costs up front, no surprises when the delivery guy shows up with extra charges. The package arrived at my door delivered by DHL, which in my past experience is the worst by far package delivery company operating in my area, so all credit to ebay.
I had the new magneto on the comet the same night. I compared both on the bench turning by hand and could detect no difference in spark quality.
Once on the bike the the new one worked, but it took me a long time to sort out the carburetor. The only way it will idle and start is with the air screw in all the way.
I just returned from a ten mile ride today (Saturday), and the bike started after being shut off but still hot, so it seems the magneto was the problem, but the carburetor was probably not helping, it is very sensitive to the slightest adjustment.
So all's well etc., and thanks to all for all the input.

 

[Sakura]

Update, Magneto replaced, comet starts when hot.
Magneto arrived Thursday (24th) I ordered it Monday (21st), which is amazing, as I am in Canada and the magneto was in UK, this was all through the ebay international shipping program, available only to UK and US sellers apparently. This is an ebay service, the seller has to choose it. Ebay handles all the border fees and shipping costs up front, no surprises when the delivery guy shows up with extra charges. The package arrived at my door delivered by DHL, which in my past experience is the worst by far package delivery company operating in my area, so all credit to ebay.
I had the new magneto on the comet the same night. I compared both on the bench turning by hand and could detect no difference in spark quality.
Once on the bike the the new one worked, but it took me a long time to sort out the carburetor. The only way it will idle and start is with the air screw in all the way.
I just returned from a ten mile ride today (Saturday), and the bike started after being shut off but still hot, so it seems the magneto was the problem, but the carburetor was probably not helping, it is very sensitive to the slightest adjustment.
So all's well etc., and thanks to all for all the input.

Well there's a surprise (but not to me)!

 

[Magnetoman]

Magneto replaced, comet starts when hot.
I compared both on the bench turning by hand and could detect no difference in spark quality.
it took me a long time to sort out the carburetor.
so it seems the magneto was the problem,

Not so fast there to conclude it was the magneto. From what you wrote it could be the magneto, it could be the carburetor, or it could be both.

 

[LoneStar]

Glad to hear the Comet is sorted.

I have to dissent about Ebay's "Global Shipping Program", though. Its main benefit appears to be to Ebay shareholders, with shipping rates to the US about four times that of Royal Mail. I always request sellers to use Royal Mail instead, and have never been charged any import fees or duties (Canada may differ).

I agree that DHL can be fast; I ordered a cylinder head from Conway and it made it across the pond via DHL in a few days, like your magneto. Not sure if Ebay's program always uses them; if so the speed might justify the expense if one is in a hurry.

 

[Sakura]

Not so fast there to conclude it was the magneto. From what you wrote it could be the magneto, it could be the carburetor, or it could be both.

I think we all know it was the mag, be it windings or condenser, it matters not which.

 

[Magnetoman]

I think we all know it was the mag, be it windings or condenser, it matters not which.

I'm sorry, but no we don't know this. You clearly want to believe it, but the facts known to us simply don't support a definitive conclusion. After the owner spent a long time working on the carburetor and installing a different magneto, the bike works. Given this, it could be the magneto, the carburetor, or both. That's why when diagnosing a problem you should proceed by changing only one thing at a time. If you change two things and the problem is fixed, you don't know which of those two things (or both) fixed the problem.

p.s. he also wrote "The only way it will idle and start is with the air screw in all the way." That clearly indicates a problem with the carburetor. That said, there isn't enough information to know if the carburetor was the source of the original problem, or if it was the magneto, or both.

 

[Sakura]

I think the simple fact of changing the mag and it now starts when hot is sufficient proof. A carb problem may make it harder to start when hot but invariably they will start. A duff mag will not allow starting when hot until it has cooled down. That's simple experience with many bikes that won't start when hot.

 

[Magnetoman]

I think the simple fact of changing the mag and it now starts when hot is sufficient proof.

No, it most certainly isn't.

A carb problem may make it harder to start when hot but invariably they will start.

No, that's most certainly not the case.

But, his bike is working again and there are no additional new facts to address, so I've nothing more to add to this thread.