Sachs Ignition Timing & Seizure

[Ernie Phillips]

My son's '72 six day had a bad outing at Barber.  The stator plate screws failed during the Cross Country race causing a DNF.  We towed the bike to the pits hoping to be able to patch things up.

With the stator screws sheared in the case and no way of extracting them in the field, we decided to swap engines.  We had a spare fresh bottom end but no fresh top end so we pulled the old top end and installed it on the good bottom end.  In the process we found a cracked piston.  Luckily we had a new piston & rings.  Since we had problems with the Motoplat ignition we were forced to use a hybrid CR125 internal rotor set-up.  (This is what was originally on the bike and had the stator screw failure)   I had no timing reference marks so I guessed.   The bike seemed to run fine in MX practice.  However, in moto 1 the bike partially seized (I think).   Back in the pits, it ran fine.  I told Christopher to take it easy in moto 2 and no WFO on the straights.  The engine seemed to get tight again.  (I haven't ridden the bike at race pace.  I'm just reporting what a 16 year old is telling me.  I know what a bike that is about to seize fells like.  He, on the other hand, only knows something ain't right.)

My question is:  If the ignition is too advanced could this cause these problems?  Otherwise the bike seems to run fine.  No noticible detonation/ping.  No changes to carb or jetting.  Just fresh bottom end and piston & rings.   Running 93 octane with Yamalube 32:1 (our standard mix).  What else am I overlooking?  Help!


Ernie P.
Chattanooga, TN

[linglewn]

Ernie,
Yes, advanced timing can make it run hotter, and eventually seize. Also, did you check the deck height when you put the top end onto the new bottom end? Could be you ended up with not enough clearance and therefore extra compression.

Nelson Lingle
73 Jackpiner
71 DKW 125

[Ernie Phillips]

Nelson,  I did not check the deck height.  But I will.  I think I will pull the top end and check clearances everywhere.  A new piston in an old cylinder (out of round?) could be compounding the problem.  Thanks for your input.

Ernie P.
Chattanooga, TN

[brian kirby]

I rode the bike in practice also and now that I think back on it, I didnt rev it really hard but I did notice how snappy and strong it was on the low-mid range. That could have been from too much advance, which will make it very snappy in the midrange but after it gets hot and heat soaks could lead to a seizure.

Brian

'72 Six Day (on loan from Ernie P.)

[Ron]

Hi Ernie,
Another possiibilty may be an air leak at the base gasket or the ignition side crank seal. Pressure check it before you take it apart.
Sucking air would lean you out and make it sieze.
32:1 Yamalube perfect!
RonW

[PentonRestoration]

Speaking of ignition timing, back in the 1970's when I replaced my point ignition with the electronic unit on my Six Day (motoplat I think) the timing was WAY off.  When I kicked over the engine, it popped loudly and started sluggishly with a strange sound.  I popped it in gear and let out the clutch only to have the bike go backwards with a groan. The situation didn't register in my mind so I let the clutch out again and again it moved backwards.  The engine was rotating in the wrong direction.

Jerry
1972 125 Six Day
CRF150
CRF230
CRF450X

[Ron]

Hi Jerry,
I was pulling ribbon after race one day and my motor nearly stalled. I grabbed the throttle to keep it running while I pulled the ribbon and when I let the clutch out, it felt like I was stuck on a rock. When I looked down at the wheel it was turning backwards.
I shut it off, kicked it over and took off.
They'll run backwards when the lower rod bearing is worn out. It lets the piston dwell at TDC to long.
RonW

[paul]

Ernie, I would put a mark on the engine case and stator plate to show you where it should be once you have correct timing Checked. That way its simple to see when trouble arises. I have never seen an engine that was advanced that did not burn a hole in the top of the piston. Check the bore/piston clearance and also the ring end gap for proper tolerances. Let us know what you find. Got a +.060 piston and rings ifn you need them. Let us know what you find. Tootallpaul

[Ernie Phillips]

Guys,  Thanks for all the ideas.  Last night I retarded the timing and rode the bike.  It just seems sorta doggy to me.  However this bike (6B) is running 54/14 gearing where my bike (5A) is 60/13 so that may be contributing to my impression.  We are out of time for digging deeper into the engine and are headed to New Blaine.  Vance Smith has a reconditioned Motoplat for me that I know how to set properly.  So, we will give that a try Friday afternoon.  I offered to let Christopher ride my bike at New Blaine but he says he will stick with his.  Cross your fingers ...

Ernie P.
Chattanooga, TN

[Merlin]

..........if your timing was too far advanced you would/should have had a crater in the dome, most likely you have a fuel delivery problem from the tank to the carb allowing the bowl to dry up, a petcock or  tank vent problem??........

[Ernie Phillips]

Merlin,  Although as you point out it could be a fuel delivery problem, it turns out to be an ignition/timing issue.  I installed a PVL and set the timing properly.  No other changes -- all is well.  As far as piston damage, I just don't know.  I have not pulled the head but we raced the thing pretty hard last weekend and it ran great -- those Sachs engines are tuff!

Ernie P.
Chattanooga, TN

[Merlin]

Hhhhmm??

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[Ernie Phillips]

Merlin,  Great call!  I should have dug deeper. Thanks -EP

Ernie P.
Chattanooga, TN