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Poor starting on new Amal and BTH Magneto

minivin

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VOC Member
I have a problem at the moment, having fitted a new Amal pre-monobloc and a new BTH twin spark magneto to my Comet, having finally got the bike started yesterday after a winter restoration I am struggling like hell to get it started.

It is running a brand new 9:1 piston, with platinum NGK spark plugs and it has really good compression, but I struggle to get it to fire and start. When I eventually get it started, it runs and ticks-over nicely, but not knowing either the ignition or carburation, I have no datum to start from!.

Having looked at the hints and tips book that comes with the Amal, I see I can play with the choke slide at different throttle positions to try out for weakness etc, but the first problem I have is that should this be done while riding the bike ie under load, or can this be done at stand still? Also, while I had the bike ticking over earlier, setting the choke to fully open or fully closed made no difference to the engines speed, which confused me when every bike I have ever owned, touch that choke lever and all manor of things happen!.

Starting could also be a case of weak spark at the magneto, what with very little obvious firing when I pulled a plug yesterday until after a few prods of the kickstarter, but is this a case of the BTH needing a few prods to charge the system?

Having two major assemblies that could be the problem has got me stuck at the moment, has any body else had issues with first commisioning on either the Amal or the BTH? jets wrong from manufacture on the Amal? recommended ignition timing from BTH wrong? There was no obvious pinking on 99 octane, but I'm open to any suggestions at present.......
 

Comet Rider

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VOC Member
Rob,
What ignition advance are you running?
When switching to twin plugs you may need to take upto 10 deg of ignition out of it fully advanced, also how much are you before TDC when retarded.

Secondly with the carb, kick it through with the valve lifter pulled, after tickling it slightly, to wet the bore


HTH
Neil
 

davidd

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VOC Member
It is so difficult to juggle so many things! You did not mention the heat range of the plug, but you might check to make sure it is hot enough just for getting the bike started. I would guess it is the carb. The lack of change with the choke is varied I think is your best clue. If you are setting it up according to the book it should start and idle. The cutaway, assuming it is stock, would not impact the start.

I use the twin plug BTH and I had difficulty on the initial start up. It turned out that the unit was not grounding reliably through the flange and studs. I attached an alligator clip as a ground and the problem went away. Because the spark is high frequency, you have to check the spark in a darkened shop. It does not show up very well. I am not familiar with the issue of the spark being delayed.

Nothing obvious springs to mind on the carb. You might send Greg Brillus a PM. Several months ago Greg mentioned that he was having trouble with the new 1-1/8" carbs, but none of the other new Amals.

I started my bike set at 34 degrees, but when I put it on the dyno to set the timing I ended up at 19 degrees. No starting issues either way, but I was using rollers. I ran two races with it at 34 degrees.

David
 
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Monkeypants

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David, when you shifted the timing to 19 degrees full advance, did the starting position then become 15 degrees after tdc or did you find a way to change the start position /full advance spread?

Glen
 

davidd

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David, when you shifted the timing to 19 degrees full advance, did the starting position then become 15 degrees after tdc or did you find a way to change the start position /full advance spread?

Glen

It is a great question. I have wondered about it myself, but I did not have enough time to answer it during the racing season. You can get the BTH as fixed, but I did not. When on the dyno I kept advancing it a few degrees at a time and it kept making more power. I reached the end of the slotting on the magneto body and had no Idea where the timing had ended up. I took the bike home and re-indexed the mag, but before doing so I strobed it to see where the timing was. It was at 19. I asked my friend, who was holding the strobe, to check the advance. He said it was not changing. To this day I do not know if I was just not letting the revs drop low enough or if my friend was not noting the change. With races coming up and no discernable problem, I stopped looking for answers.

I should also say that I was also busy figuring out how to time the BTH properly. I noticed that with the pin through the hole that marks the sparking point that I could move the crank 4.5 degrees in either direction. So I spent some time finding out with the strobe exactly where the spark occurred. It turns out that the spark occurs when the pin is inserted and then the crank is rotated 4.5 degrees in the direction of rotation. So, on my BTH, when the pin is tight in the direction of rotation, that is where the spark occurs.

So, I will have to check it again to see about the advance, but I think using a strobe is worthwhile as a check.

David
 

Martyn Goodwin

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As I understand it the "choke" lever on a AMAL Pre (and post) monoblock is not a true choke. What it does is alter the mixture but it does not affect the throttle position in any way - so when you use it you should not expect the engine speed to significantly change. As to timing for an "easy" start. After much investigation and experimentation I have found that if easy starting is the objective you should adjust the static timing so that the motor fires between 4 and 6 degrees Before Top Dead Centre when the ignition is fully retarded. Unfortunately, as many others have discovered, it is almost impossible to set static timing with the "new" semi electronic BTH units, for they do not produce any output till they are spinning at around 250 rpm; coupled with the fact that the locking pin can have a variation of plus or minus 5 degrees. Your best bet - if you want to persist with the BTH is to set it up on the locking pin and when using a timing wheel to verify the position. Then start experimenting by very slowly advancing the timing. If it starts to kick back when you try starting you can be sure that the static full retard timing is more than 10 degrees before TDC so then start retarding it bit by bit till you get back to 4 to 6 BTDC. This can take an awful amount of time. For me - I "retired" the BTH and had the original Lucas magneto professional rebuilt with modern internals that I then installed in place of the BTH. This allowed me to be certain of the static timing at startup and I now have a very easy starting and sweet running motor. Good Luck!
 

craig

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What I found as a Comet mechanic/owner/rider was the start technique had to be developed. My start routine is as follows -
1. turn on fuel taps, both sides
2. tickle carb until it pees or dribbles on the engine (very important)
3. Kick engine thru slowly until you hear engine slurp at least once. Use lifter to get past compression
4. Kick start in earnest and it should start first kick.
At first I did not want fuel to spill on the engine and dynamo area, all new and tidy, it could catch fire, it will get dirty, but you need to get fuel going, that was my issue.
I never use a choke to start or run, even in very cold weather on a Comet...............Vincent twin a different story.
Buy a 10 box of spark plugs Champion N5C or the like, throw away every used plug you own, carry 2 new plugs with you at all times already gapped to .020, never waste time or kicking effort trying a doubtful plug. Throw all doubtful plugs away immediately. After 1000 miles and 50 cold starts you will have the bike tuned and requiring less plug attention.

Good luck, let us know what you find
Craig
 
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greg brillus

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Hi there Minivin, Ok first off...as David Dunfey has pionted out in an earlier post, the twin spark BT-H will not need so much overall advance due to the burn in the cylinder occuring much faster, so as David said you may only need 19 or 20 degrees. If the mag has variable timing, then on start and slow running it's most likely the timing will be somewhat retarded in the region of 10 to 14 degrees past TDC this will make starting an issue. If it is a fixed timing mag, then it will start but be inclined to kick back. On the carb side of the equasion, I have fitted a few of these new 229/289 Shadow/Comet carb's and have found every time that the slow running is always too lean, requiring more use of the choke slides than normal just to keep it running. If the choke is lifted too early, the engine tends to cough and spit back through the carb.....the cure i've found is to fit a number 3 1/2 slide and an air cleaner ( which helps, and should be used anyway...) if the 3 1/2 slide is unavailable, i just machine 30 thou off the bottom of the original number 4 slide, reassemble, and then the engine is much happier.....and easier to start. This i have found on 7 new 289 carbs fitted to a Comet, a Rapide and two Shadows.....perhaps it has something to do with our modern fast burning poor atomising fuel available to us these days. Original leaded fuel seemed to last forever, but this modern junk is good for about 3 months at best....Oh, my wife just loves the smell of stale fuel on my hands if i walk in the house.......NOT...!!! Cheers for now......Greg.
 

hadronuk

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My experience with my Rapide and other British bikes is that with everything else correct, the key to good starting is the tickover setting. The setting that gives the best tickover when hot is likely to be too lean for easy starting, so the setting is a compromise. Old Amals seem to be bad at supplying a combustible mixture at cranking speeds, so generous flooding helps, but if it doesn't start fairy quickly, there is of course a good chance of sooting the plug and making starting impossible.
I have also found that using new parts does not guarantee the correct fuel height. Note that the fore and aft position of the float chamber affects the fuel level, because the front carburettor has a slight updraught. The most forward position gives the highest fuel level for the front carburettor. (The rear cylinder of a twin is the opposite; due to the rear carburettor having about 5 degrees downdraft, so the most forward float chamber position gives the lowest fuel level). I eventually made a modified banjo bolt with a small brass stub soldered in so I could attach a clear plastic pipe and observe the fuel level directly.
 

vibrac

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I have a twin spark BTH I think 34 BTDC is a good place to start my Racing Comet is at 32 but its higher compression my carb is a Gadrner carb not reccomended for kick starts The float level is aproximate the low down running is poor but she runs like smoke where it matters. but that is of little help for you, me? I have a foxley roller started
 
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