I find myself having to look this thread up every few years, so I thought I'd bring it over from the other forum as it's proved very useful.
Posted by user: Thrilou
...Under certain situations.
We all know that worn advancer weight stop tabs can cause the timing to be too far advanced, but what to do when your timing is good on one side and too far advanced on the other?
This is the question I asked HERE.
No matter which advancer I selected, there was this discrepancy at full advance.
I emailed Mark Paris (AKA Hondaman) and asked his advice. Below is the lengthy correspondence I had with Mr. Paris about a month ago:
Bryan said:
Hi Mark,
I have one of your ignition units for my CB350 twin and we have corresponded a bit in the past.
I am having a problem with my ignition timing unrelated to your unit. I am hoping you could shed some light on this problem or at least offer some advice to mitigate the issue.
Here it is in a nut shell:
When both cylinders are timed exactly dead on at idle, the left side and right side end up at two dramatically different timings at full advance.
In more detail:
Sometime ago I noticed signs of detonation in the right cylinder (aluminum deposits on spark plug!). When I checked the timing with a timing light, I found that at full advance, the right side was advancing past the furthest timing mark by about half of the distance of the entire advance range... well past the furthest allowable advance time. The left side only advanced to the middle of the advance range... odd.
I suspected the advancer unit, as they are reported to wear at the advance stops causing too much full advance. I measured some other advance units I had laying around and chose the one that had the least distance between the stop tabs. I timed it statically and then timed idle dynamically. Right side is exactly in the middle of the timing marks but the left side is now a bit retarded from the first full advance mark... About the same degree of discrepancy between the right and left that the original advancer showed, just less advanced overall.
No matter what I do (and I am setting the dynamic idle timing spot on) the timing at full advance is drastically different between the right and left.
I reinstalled the first advancer and tried to sort of split the difference by advancing left side overall (rotating the timing plate) and the retarding the right side over all (moving points) thus resulting in full advance times that are both inside the full advance range. The problem is, the idle timings are off (advanced on the left and retarded on the right). This makes for lousy response from off idle to 3000rpm or so. It does run fine once the engine is at full advance (4000 on) which seems to make sense... The bike also stalls regularly after sitting at idle under these conditions.
I know some people suggest setting the advance timing to where you like it and letting the idle timing be where ever it ends up, but I can't see how this is a good idea. This seems like a poor way of dealing with the problem.
Perhaps having differently timed idles will result in different demands from the carbs... I haven't spent any real time dialing them in after changing the timing settings... perhaps this will yield better results.
Should I try advancing or retarding the overall timing so that right and left are within the full advance letting the idle times remain the same relative to one another (albeit, a bit advanced or retarded from the ideal setting)?
What could cause this problem? Given the design of the advancer, with identical idle timings, I can't see how I could be getting two different advance times (the points cam should be in the same position for both rubbing blocks, correct?).
Have you ever observed anything like this before? Any advice or solutions?
I apologize for the lengthy email... I realize you are a busy guy, so If you don't have the time right now, I completely understand.
Thanks in advance.
Bryan
Hondaman said:
This is actually pretty common (even back "in the day"), and it comes from 2 things.
First, the thin washers at the base of the rotating cam on the advancer are often gone: they were plastic on the 1970 and later 350. These are .004" thick (0.1mm) washers that were used to shim the base of the cam flat and to make the clearance between the top bolt's washer and the end of the cam about 0.004" or so. Also, over time, the pivot area inside the cam has worn, adding extra clearance that lets it rattle back and forth. This tends to make the L cylinder late at full advance and the R cylinder follow the points plate position.
You can test yours easily for excessive clearance at the top bolt, and shim it at either the top or bottom to take up some of the slack. The ID bore is a tougher issue, and it came from the bike sitting for a long time, getting a little rusty, then getting run, so it sanded the ID of the cam into the oval shape it has now.
I've given some thought to making refurbished units like these for the Fours, although they did not wear as badly. It would consist of making a sleeve to fit over the shaft and fill in the wear, much like on the end of a crankshaft on a common V8 engine where the front seal leaks. It may become a project later this year? In the meantime, try the shims, making some from brass sheet that you can get at Ace Hardware in varying thicknesses.
HM
B said:
Mark,
Thanks so much for your reply.
I removed and measured the parts that you mentioned.
There definitely is play at the points cam top bolt as you said. .012"
to be exact. I do have a shim that sits under the points cam. It is
metal, however, and measures .011" I actually have three of the shims,
all measuring the same.
I checked the roundness of the advancer shaft OD (where the points cam
rotates) and the points cam ID.
Shaft OD measured .589 and cam ID measured .590 (I measured all the
way around, top and bottom). .001 difference and no detectable
out-of-roundness... seems like as good as it gets.
I will make a shim measuring .008 and place it under the cam to take
up the slack and see if this helps.
Thanks again for your help. I truly appreciate it.
Bryan
HM said:
Let me know how it turns out?
Thanks,
HM
B said:
Mark,
Today I shimmed the points cam as you suggested. Timing at full
advance is now spot on!
I have to admit, I couldn't see how a few thousandths of lateral play
could affect things so dramatically. Out of roundness (which wasn't an
issue in my case) makes sense to me, but I was somewhat incredulous
that this shimming would totally fix the problem.
To be specific:
I used a spare steel points cam shim, super glued it to the end of a
damaged points cam (something to hold on to!), filed it down from .012
to .007, removed the shim and cleaned it.
with the extra shim installed, I have .004 of clearance, cold, and
this brought the timing on the right side back from over advanced to
the middle of the range, identical with the left (or near identical,
as far as I can tell without a mark to line up...).
This is a great relief!
Thanks so much for your help.
Would you mind if I shared this with the guys over at the Hondatwins.net forum?
Bryan
HM said:
I'm glad it worked! It's been a long time since I did those, but it was a common problem because the shims on many were plastic, and broke off in use. It's the tabs where they lock into the advance weights where the looseness makes the cam tilt: the upward pressure of the points feet (90 degrees apart) cause it to tilt upward as it rotates the high side down, which shifts the timing late for the "L" points. So, when the "L" points get set up first (per Honda's instructions), the "R" points end up advanced. The shim presses the cam down into those slots on the weights and holds the alignment better.
Can be frustrating!
HM
The only thing I can't figure out is the "plastic washer" thing... I've never seen a plastic washer in any parts fiche (although I suppose no parts fiche shows an exploded view of an advancer unit) or heard mention of it until now (but what the hell do I know? Certainly not as much as a man who's been working on them since they were new). Nevertheless, shimming the lateral play on the points cam solved the problem. Whether done the way I did it by filing down a spare steel shim or constructing one from scratch, since it's a quick and easy experiment, it's worth a try if you're experiencing this problem.
Be sure to accurately check for out-of-roundness of the point-cam shaft as Mr. Paris suggests. If this type of wear exists, it seems reasonable to try and find a good used unit since repairing this kind of damage is a bit more difficult.
I hope this is helpful to everyone. Be sure to check your timing on both sides and watch for signs of detonation!
User Sikeclass responds:
Came across this thread, and I'm hoping it's the answer to my problems. Might be the answer for a few others as well. Been chasing what seemed like a lean condition for years, even when dialing everything to the point of being sooty black rich. The best way I've seen it described was, "it feels like fish nibbling on the line". That's on a good day. On a bad day, it feels like blatant misfires/backfires. Never thought to compare the advances. Left was always good, at idle and advance. Right was always good at idle, and never checked the right advance until I saw this thread. It ended up being significantly past the advance marks. Totally makes sense now.
To update the thread, I've taken a picture of advancer apart.
The pieces are arranged in stacking order. The washer thrilhou is talking about, is the thin washer that slides on the pipe portion of the advancer body. (Sorry my terminology is all off.) It's shown, just behind the advancer body, and the removable cam part slides on top of it, and is then locked down with the washer and bolt.
My washers were both made of very thin metal. I had a spare advancer, and noticed that one washer was much thinner than the other, to the point of being flexible. Both advancers had a bit of slop, but when I stacked the washers onto one, it removed most of the up and down movement in the cam. So this is an option if you can get your hands on another advancer. I'd already ordered some 0.004 shim stock from Amazon, before I got the bright idea to stack.
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Originally Posted by Middletons
