ET: Engine (Twin) Still Can't Start My Twin, Replaced Mag, But Not A Pop

[Jim Bush]

My recent experience with a good running bike that became a beast to start, 20-30sec run and big backfires, big enough to split a starter gear in half - came down to how the magneto bearing end housing with cam-ring was installed. I had taken the mag off to replace the capacitor - I pulled the end bearing / cam-ring housing off and then when I re-installed it, I had inadvertently installed it 180 deg out. Thankfully I had taken a photo of the mag before I removed it, so it became obvious when I looked at the photo (after the carnage).

The mag cap pillar bolt with the clip to hold the black cap on - I had placed at the bottom (right), when originally it was at the top (left). The short side of the cam ring is on the left (see pic). I rotated the housing 180, retimed it to 4deg BTDC (ATD closed) and perfect again.

 

[CarlHungness]

Carl, What you are seeing with the reduced travel on the ATD is the new ones have less overall range due to modern fuels not needing so much advance in these old engines.........It looks like you have got everything pretty much covered from what I've seen.......A couple of tricks I've learnt from working on these bikes when they are hard to start.........If after you have kicked the engine several times say 10 times or so........Stop and pull the plugs out........If the electrodes look dry, then you do not have enough fuel in there.......if they are wet, well obviously too much fuel. The plug check seems to be the only quick way I've found to really give you an idea as to what is wrong. If the plugs are wet, I generally blow them off with compressed air........reinstall them, then the engine should fire up within a couple of kicks at most.........The throttle opening amount is important.......generally these bikes like to start with minimal throttle opening, but if the mixture is quite rich then a half open throttle is what is needed........Kicking back on the K/s lever indicates the mixture is lean if all else is correct........Good luck with it........Cheers.......Greg.

Throughout my ownership the bike has started and often, because of my inept procedure, it has taken
multiple kicks, but never refused to start until my problems began. After replacing the B-TH coils it did
start and run rough, then fired and ran fine for 60 seconds with the Lucas, then nothing...and my journey
began. Each time, now, when it has refused to start, I've put in new plugs, 10-12 in all. The best I can think
of now is a camshaft has slipped on its moorings, if possible, and as stated if I take off the timing chest
and remove a cam, which I've done previously, have had every gear out of it in the past, I just don't know
what and how to measure timing and if or if not a cam has created the problem. The mixture should be the
same, I have replaced the old jets with new ones of the same size. What has me flummoxed is the one time
lately it started and sounded normal for a minute with the Lucas AND the old ATD so my installation procedure
had to be correct. When turning the plot over with the rear wheel in 4th gear, plugs out, it sure seems as though
the valves are in sync as they have been in the past. I'm tempted to re-time the thing at 8 degrees with the
ATD ears closed, but it seems that plus or minus on the degrees would have at least given me a POP which
I'm not getting. I've even wondered if the de-comp is allowing valves to stay open thereby not creating enough
compression. I 'backed off' the decomp a few turns. But now, I'm able to locate the 'short' stroke, and I nudge it
over TDC and sure seem to get a full swing in, without using the decomp and I'm not losing any compression. If and when I try to use the decomp, letting it go 3/4 of the way down, I must be late in doing so because it is not getting a full revolution in, and I can feel the plot swinging back at me a half turn or so. I've tried kicking it all the way to the bottom, THEN letting the decomp off, and THAT allows the engine to spin fully and I'm saying to myself, "That kick was surely enough to start the engine." I feel the same way when I don't use the decomp, and just find the right spot
to kick it through, so my combination isn't right yet. Hopefully I'll locate an aberration on a cam.

 

[CarlHungness]

My recent experience with a good running bike that became a beast to start, 20-30sec run and big backfires, big enough to split a starter gear in half - came down to how the magneto bearing end housing with cam-ring was installed. I had taken the mag off to replace the capacitor - I pulled the end bearing / cam-ring housing off and then when I re-installed it, I had inadvertently installed it 180 deg out. Thankfully I had taken a photo of the mag before I removed it, so it became obvious when I looked at the photo (after the carnage).

The mag cap pillar bolt with the clip to hold the black cap on - I had placed at the bottom (right), when originally it was at the top (left). The short side of the cam ring is on the left (see pic). I rotated the housing 180, retimed it to 4deg BTDC (ATD closed) and perfect again.

Glad you found the problem. I've had just two minor kickbacks in the few hundred times I've kicked it in the past three weeks and am positive my mag is in perfect shape.

 

[DucATIRadeon]

I think what you are saying is not to use my spark plug stop that sets the plot at 35 degrees, rather go
through my drill of TDC, then back it up only 8 degrees with ATD closed. Your photo shows the ATD fully
open, (ears held by a Ty-Wrap and weight on the ATD to take out the slack, correct?) It had been suggested
I follow what you've said, only to use 4 degrees BTDC, not 8. The writer noted I may get some pinking out
of the engine with such a setting. Then too I understand it is not unheard of for the cam pinion to slip on the
cam proper and at the moment I don't have the knowledge to check if that's what happened. We know for
fact the new ATD has 1/4" separation between the ears, and the original was 5/16". I have never re-checked
timing in past years when I installed the ATD (or the B-TH). I have always put the ATD on, ears blocked open
set at 39 degrees, light flickering, never used a bolt in the earth brush hole previously to hold the armature, now
I do and it's bitch to finger tighten it and keep the light OFF..light wants to go ON every time I finger tighten.
This time after setting the new ATD and no result, I re-installed the piston stop after getting to TDC, backed
it up to allow room for the stop, put it in, put piston against it, looked at the magneto and the SHOE sure
seemed to be in the original place I set it for my now 35 degree mark. The 'mark' is just damn close to the
old 39 degree mark, my old pencil mark still there. Thus it seems as though the shoe is at the 35 degree mark
where I set it originally with the ATD ears blocked open. Shouldn't the shoe be many degrees away from that mark
(in a clockwise direction) if in fact the ATD advances the mag?
I checked the timing to see: Did the ATD slip on the mag shaft, am I certain I set it at 35 degrees. The ATD
did not slip far as I can tell, it went on perfectly, using ONLY the bolt, NO socket to set it as I had done in
past years. So I'm asking just why in the Hell the bike started last week, ran great for 60 seconds and quit.
I got an ultrasonic cleaner today, took off carbs, cleaned them, and once again, no result. No pop, no kick back.
I'm getting spark, took out rear plug, held it on the Britax bar, kicked it and can see spark.
Have had the pipes off 7-8 times, drain oil, take off timing chest, and on and on...will do it all again, but have to
get some knowledge on how to actually check the cam timing to see if something has slipped on a shaft.
Really appreciate the suggestions. I am supposed to be practicing my oxy welding aluminum and haven't
fired the torch in a couple of weeks.

hi Carl,
yes correct that's what I'm saying. and yes I mentioned at the bottom of my post that the picture is with ATD full advance and set with crank at max advance.

TBH, given the remark from Greg Brillus about less gap with new ATD devices, if the ignition is set as per "the manual", the ignition timing when kicking the bike is 35-20=15degrees. that's a lot.
so with a new ATD, id set the ignition from 8degrees BTDC and let it be rather than setting it up as per "the manual".

about your timing question:
my very first attempt to start my rapide early this year (I've only got it 'bout a year give/take few months) I noticed the nr1 cylinder was sucking and halfway blowing back. on inspection I found the markings to be 2 teeth out of phase with the large idler but not shifted on the actual camshaft.
on the other Vincent website (www.thevincent.com) there is a nice picture with an explanation that helped me a lot, and a sentence in the works instructions on the positioning of the first dot relative to the cam.
ive included a screenshot from my rapide diary (databook coving all since purchase), with added the blue lines on how the cams should be. when you take off the timing chest cover and setting the ignition, give this a go, it doesn't require any measurements but good eyesight.
take note of the slot in each camwheel: the rear is aligned with the blue line, the front is aligned vertically down (or parallel to the blue vertical line).

 

[fogrider]

Carl, there are lengthy posts on here about cam timing, various methods possible, but the quick and easy way to get a rough idea if the cams are where they should be is " valves rocking ".
On the non compression stroke, both valves should be an equal amount open at TDC, ie they should rock around TDC. ( OK folks, 4 before ! ).
To be more accurate, with your timing disc on, the inlet valves should be fully open around 100 deg after TDC, give or take a few degrees depending on which cam . 97 typical.
These checks will show if you're in the right field. If not, back to the forums for the lowdown on cam timing.

 

[lee_812d]

The new ATDs may have less range but you probably can't assume it is the correct range, you need to measure it. The 10 degrees I mentioned was an example, it really needs to be ~15deg. Unless you know the range you can only really install it closed at ~4deg to get the bike started but it might then only idle ok, you won't know what is happening at higher throttle.
You mentioned 1/4" gap, so for 15deg range you would need R of ~0.957", or as R is fixed and the gap can be changed, for R=1" gap would need to be 0.261".
It's probably easier to measure the ATD manually by turning it.
EDIT - of course you can also determine the range close enough by measuring it in the advanced/retarded positions on the bike with the degree wheel.

 

[vibrac]

If the valves rock at TDC and you are ignition timing on the closed valve stroke and on the correct cylinder...
the mag and carbs have been checked many many times Then
With all the work you have done it must be something unusual and not a normal fault

 

[bmetcalf]

I have to confess that I hadn't been able to start my Twin for a few months, so I have watched these threads. It did run briefly, but had no power and barely made it to the top of my sloping driveway. My last step was to take the mag off, clean the oily slip ring carefully, and clean the earth brush and recess. One pickup brush spring was stretched out and I was never sure I was getting it into its recess while installing. I happened to have some spare springs and installed one. That made the difference! It started readily, but I haven't ridden it yet (30C here today) to see that all is well again.

 

[Alyson]

fingers crossed....

 

[CarlHungness]

Am happy to report my Twin now runs, starts, idles better than ever before thanks to my visit with
journeyman Vincent man Glen Bewley. Of course the natural question is, "What was wrong?" and not
sure that can be answered other than 1) My starting technique; 2) A professional job of adjusting, cleaning
carbs. I visited
Glen at his impressive shop in Tennessee and thought I had prepped the bike to be worked on..more on that
in a moment.
Glen was able to start the machine on 3rd kick as it was and I waited a few seconds for it to die. It didn't.
But it also didn't sound great. So up on the lift to check timing, all adjustments, etc. Turns out my timing was
OK, so Glen fiddled with carb adjustments, and after re-starting, it was still giving us a bang-bang..Ba-bang-bang
at low RPM's. So off with the carbs and into his ultra-caustic solution, then more adjustments, and on and on.
I had timed the thing half a dozen times prior to taking the bike to him, and thought, "Well, he's got some
extra B-TH's on the shelf, so maybe he'll want to install one of those." Thus the last time I put it together, and for
the first time in my life, I just hand screwed the exhaust nuts on, and left the muffler off, and just slid it on to start
the bike. I finally mentioned to Glen the exhaust nuts could be 'a little loose' and Glen noted with some
real clarity, "Why didn't you tell me so?"
After tightening up the nuts and some more air screw/idle screw adjustments, the bike started per normal, and
idled better than it ever had. I started the machine several times and Glen then said, "OK, don't even think about
touching the throttle, just kick it." It started and immediately went to idle. I've never trusted my own idle, especially
at a stop light, so have always kept the throttle just above idle for fear of having it die at an intersection.
From what I can tell, my starting technique with my old B-TH was all wrong, yet got the plot started time and
again. And after watching Glen start the bike I attempted to clone what he was doing, finding any compression, and
kicking and releasing. I don't always get 'a full swing' in as he does, but when I do, the machine starts. I weigh about 160 lbs, and even with strong legs, I use more body weight than leg strength, thus my kick (and decomp release point) hasn't been enough to give it spark enough to fire it. I now, after all these years, more fully understand what is
needed to get the bike fired.
I still don't know why it died when I got it started with the Lucas in my shop, and why I couldn't re-start it, but
I'd have to admit that I just wasn't getting a full swing through, as the Lucas must require. Then too, the carb
adjustments he performed were the result of knowledge I didn't have.
So for now, I'm about as happy as one can be. Nevertheless, my B-TH has been sent back to England and I
should have a report on it this week. I don't know if the 'factory' can really assess if it is in need of repair or replacement, but my sense tells me the plot would be a little easier to start, dead cold, with the B-TH than the
Lucas. Once warm, as noted, it's just waiting to fire.
A trip through Glen's shop tells one he speaks fluent Vincent from machining broken, worn parts to welding aluminum and bringing the 'it's no good' back to life. I couldn't have found a better person to go over the machine and sure enjoyed the trip to the beautiful countryside.