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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.
 

greg brillus

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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.
 

Magnetoman

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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.
 

greg brillus

Well Known and Active Website User
VOC Member
If the mixture screw is wound fully in, that is fine.......it indicates there is either too much air bypassing the slide, or even a possible leak at the carb to manifold slip joint..........If it's an old carb, and the slide is a number 4 then it will almost certainly be too much ware in the slide/bore.......Now if the mixture screw was initially wound out from this, the mixture at idle would be too lean.......in which case if the mag had a weak spark when hot, then there is your hard starting issue........So my advice is to also look at the carb if you really want to tune the engine properly.........generally on the 229/289 carby's these days a number 4 slide is on the lean side anyway.
 

gregg-k

Website User
Non-VOC Member
Excellent news, Everiman. For whatever reason, I have not been getting message notifications the past few days, so I've missed the progress you've made.
Let me know if you'd like me to get in touch ...
... Gregg
 

Bill Thomas

Well Known and Active Website User
VOC Member
Gregg, You sound like a good chap to have on the Forum,
Are you a Club member ?, If not maybe that's why no notifications ?.
If He found ANY single cam , As Chris says, Would it not be easy to regrind the location slots,
We were given a Mag' off a Velo', "Should go the other way round " !!, To fit to a Comet,
I was away from home and thought we were stuffed !, But got in contact with a top Mag' Chap,
And He told us how to regrind it, That worked well.
Cheers Bill.
 

Chris Launders

Well Known and Active Website User
VOC Member
Gregg,to change the slip ring you have to strip the magneto and possibly re-shim the bearings, whereas the cam ring just slips in from outside. And as bill says modifying any single slip ring is easily done, I have done several V-twin ones for opposite rotation and also converting to manual adv/ret building mags for Broughs, easy with these modern slitting discs on an angle grinder.
 

robmfj

Website User
VOC Member
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.
DHL Drop, handle, lose or Don't hurry lads. good to hear they're improving!
 

Glenliman

Well Known and Active Website User
VOC Member
Good to hear that the problem is sorted.
A mag is a wonderful thing when it's repaired and working properly.
Nice to have some electrical freedom from the charging system, daytime anyway.

Glen
 
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gregg-k

Website User
Non-VOC Member
Exactly, Glenliman ... that freedom is something that's too often overlooked. A mag is completely self-sufficient, and hence you're not reliant on the dynamo, the battery or regulator. It's sensible to keep all those properly maintained in any case, but with a good mag, a bike's reliability (during the daylight hours) is not reliant on those components.
 
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