Using two different point’s plate and two different AAUs, I’m getting the following same results: • set both point gaps to .015 at the AAU scribe mark & rotating the back wheel forward..... • The points gap on the left opens wider at about 1/3 of the way through the high side of the cam face and then drops back to .015, before fully closing • The points gap on the right becomes narrower at about 1/3 of the way through the high side face and then opens back to .015, before closing
One points plate and one AAU are NOS. I was careful to not over tighten AAU bolt (5-7 ft/lbs), and also careful in removing the AAU so as to not bend it.
At first I was thinking I bent the AAU, but wouldn’t both points re-act the same way?
Anyone ever seen this before? I’m wondering if exhaust cam is not true?
Note: I replaced the timing cover due to a bodged OP test hole, and noticed the above points gap differences when reassembling. I have not restarted the bike yet. The bike was running fine on points for 550 miles prior to this. It’s highly likely it was doing this before….I just never checked before.
Points and the associated problems are why I have a Pazon in one and a Boyer in the other. I check the timing every couple of years or so. Can't help with your question, sorry. I gave up on the frustration of points 30 years ago.
1968 T120R 1972 T120RV Any advice given is without a warranty expressed or implied.
I would first make sure that the AAU is running true.That's the most important thing.
Lock the points plate down and see if the gap at one set of points varies around the high part of the cam as you turn the engine.If it stays within 0.002",it's pretty good.
It's easy enough to staighten the AAU if it needs improvement.Loosen the centre bolt to about 1/2-tight,and hit the cam with an aluminium drift.It will move fairly easily.Loosen and fully tighten the centre bolt and check again.
If you get variation in gap when the cam remains stationary and you move the points plate around,that would mean the counterbore in the timing cover is not Concentric with the camshaft axis.Even cam journal-to-bush clearance would have an effect on that. The range of adjustment of the points plate is not much,and not likely to have a big effect on points gap,even if there is some eccentricity.The amount of clearance the points plate has in the timing case is likely to have a bigger effect on points gap.If you push the points plate side-to-side,the points gaps must change a little.
When you use the eccentric adjustment to move each set of points individually,the points gap can change a little too.That would mean the eccentric adjustment locating the points is not perfectly Concentric with the camshaft axis.
If the points gap changes when you do a timing adjustment (either with the backing plate or the secondary eccentric adjustment),all you can do is re-set the points gap and try again.
The points gap is not 1/2 as critical as most people imagine it to be.The points must open and close.They must open enough to stop any arcing (0.003" would do that).They are meant to close for 160 degrees cam rotation.A bent AAU will have a bigger effect on that than a small change in points gap.Even if the points gap was 0.020",the 160 degree dwell angle won't change much.
Before '67,they only used 86 degree dwell angle on the points cam.Engines would still rev to 8,000 rpm.The 86 degree cam caused other problems,and some melted pistons.
“Lock the points plate down and see if the gap at one set of points varies around the high part of the cam as you turn the engine.If it stays within 0.002",it's pretty good."
I’m getting these measurements as I measure at 4 points on cam face with the points plate locked down. Using a 2nd AAU & points plate, I get approx the same. Definitely not .002 (The following is +-.001)
Left side: .015, .018 .018 .015
Right side: .015, .007, .002, .014 (It’s getting real close to closing)
“Even cam journal-to-bush clearance would have an effect on that.” When engine was rebuilt, didn’t take cam off of timing side bush.
Desco: my other bikes are EI...know what you mean.
That's unusual.The left side would be good enough (better than most). You'd expect a similar thing on the right side (higher gap at the centre of the lobe),but the opposite is happening.
The exhaust camshaft must be moving in the cam bush and moving away from the right set of points,at about the middle of points opening.You'd expect the cam journal to stay on the same axis as it was 180 degrees earlier.All the forces on the cam are much the same,and the direction of rotation hasn't changed.
I can only suggest that you "adjust" the AAU by pushing it over toward the point where it only shows 0.002" gap. You might end up with 0.015"-0.027"-0.015" on the left side, and 0.015"-0.011"-0.015" on the right side.
As long as the timing is right,it shouldn't be a problem.
Does it matter if left points are set at .015, and right are .017?
With these symtoms, what would you do if it was your bike?
It won't matter.It would still run fine with 0.020" points gap.The dwell angle will hardly change,and the points will still stay closed when they should.There's still enough dwell angle to fire a plug at 10,000 rpm.
Here's a thought.Try wedging the rear set of points open so the rubbing block doesn't touch the cam.Use a card or paper to separate the points. Then check the gap at the front points at different cam positions.It may not look so bad like this.It could just be that the cam has a generous clearance on the AAU spindle.If the rear set of points had more spring tension than the front set,the point gap would increase at the front;but only at the start and finsh of lift.The extra spring tension from the rear points would push the cam forward and take up clearance on the spindle,but this would only happen during the first and last 20 degrees of points opening. Chrome plating the spindle to reduce spindle-to-cam clearance would fix it.Evening up the points spring loads would help a little bit.
“It could just be that the cam has a generous clearance on the AAU spindle.” I think the two I have are as good as they get. I had an old one with perceptible movement to compare to.
“Try wedging the rear set of points open so the rubbing block doesn't touch the cam.Use a card or paper to separate the points.” Here’s some Pictures. On the opposite side of either the high or low spot of the point opening, the closed point does not touch the cam face. Due to picture angle, it’s kind of hard to tell, but the gap between white heel and cam on right side is larger than the one on left side…which is the gap referenced in above post. The green magic marker spot on cam is the start of the high/low spot. It's the same spot on both AAUs.
Notice how the right side point gap has decreased from .014-15 at scribe mark down to .005-6.
note the gap of heel to cam on right point is > left point in above picture
#481585 - 03/16/132:48 pmRe: Points gap differences on AAU face on 68 Bonnie
“I suppose you've checked the obvious,and the AAU is not rubbing against the points plate.” Not till I read your post. It’s close on the high side of cam, but couldn’t find any rubbing point all the way around.
I tried this also. Last weekend, I taped a plastic tie to case w/o points plate to be flush at scribe line. At the computer marked red spot it bent the tie. Got the opposite effect on other side. Only tried this on one AAU though. The cases are a matched factory set.
It is normal for heel to not touch cam on lowside..right?
As a previous poster has said, I'd be checking that nothing is bent.
My old man had a trials-trim 5TA which was plagued by an odd misfire: this turned out to be due to the AAU etc not being square/true, so the points gaps were inconsistent.
In theory, you could repeat the cable tie check on the corners of the head of the cam retaining bolt: they should all just clip the tie as the engine is rotated. If half the points of the hexagon touch and half don't, then that tells you the bolt's not straight.
I recall the 5TA was fixed by a judicious tap to straighten things out.
Strobe timed both cylinders to full advance(just under 3000rpm). Bike ran just as good as it did prior to changing timing cover.
After bike cooled down, I took some measurements of points’ gap across the AAU face. (allow .001 +- for operator error) Left point: scribe .016, .018, .013, .013, .013, .013 Right point: Scribe .020, .012, .005, .008, .014, .018
Given these screwy numbers, can’t explain why it ran well, nor have I figured out why the disparity in measurements.
Your cable tie touching test demonstrates to me that the breaker cam is not rotating on a true centre with the camshaft.
Assuming here that the camshaft doesn't have excessive play (3-4 thou max?) in the bushing.
The way it seems fine on one set of points, but is wierd on the other, suggests the cam being slack on its shaft, and moving when spring forces dictate. Can you move the cam side to side, and alter the breaker gap significantly?
You have strobed it, but that will only have been for one of the cylinders I guess, what's the other like?
#483045 - 03/25/1311:29 pmRe: Points gap differences on AAU face on 68 Bonnie
Dave….I agree the “breaker cam is not rotating on a true centre”…at least with the points plate. Both points are timed to 3000 rpm, bike runs nice, and idles smoothly.
As mentioned prior, got the same effect with a NOS AAU, so I’m not really sure what is causing it? I couldn’t detect any movement in the cam…but that doesn’t mean it isn’t. I didn’t remove timing cover to test it.
Pete R’s statement explains it best (to me): “The points gap is not 1/2 as critical as most people imagine it to be. The points must open and close. They must open enough to stop any arcing (0.003" would do that). They are meant to close for 160 degrees cam rotation. Even if the points gap was 0.020", the 160 degree dwell angle won't change much”. This describes my bike on points.
Someone else summed it up this way to me: “Screwy sh*t like this is why everyone runs EI.”
It tells me that the exhaust camshaft is not rotating on a fixed axis. When the points open,the exhaust cam is always being held downward and forward by valve spring load,so there's no variation there.About 80 crankshaft degrees later,there is no valve spring load on the cam so it can move about easily.About 45 degrees later,it's loaded again and becomes steady.
I don't think there's any great problem.The difference between sides at the centre of the lobe is about 0.012".It's a little low at the centre on both,but more on one.
You could bend the AAU over toward where you get the low spots,and even turn them into high spots. You'd still get the 0.012" difference.One could be 0.015",0.015",0.015" and the other would be 0.015",0.027",0.015".
There must be about 0.008" front-to-rear movement of the cam in its bush for this to be happening.That's more than normal,but not necessarily a problem.The way I see it,the bush just holds the cam up and stops it falling on the ground. If you had the engine apart,you'd fit a new timing side bush and measure the cam journal to see if it's round.It's still running OK,and doesn't appear to be causing any harm.
Yeah timing scatter...I've seen this many times on automotive engines with worn points distributors. Usually causes a steady random misfire at idle. A high performance engine might suffer from loss of performance. Some install an aftermarket electronic points conversion ....But if the bike runs ok for you then no big deal.
650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
when i worked at shollys cycle shop in the 70s we had a timing light with 2 pick ups, it would show two strobe lights, when the two became one both cylinders were timed alike, i would guess two timing lights would achive the same results
Pete R…Thanks for your input & your explanation on points. Going to ride the bike, and see how the points do over some miles.
In hindsight.....if I had put an EI on the bike instead of points, never would have noticed this. With the majority of these old bikes on EI nowadays, wonder how many of them would measure up like mine did?
Dick, Hillbilly, John…appreciate your comments… Timing wise, the timing line advances smoothly up to 3000rpm on each cylinder and idles perfectly....go figure.