Sachs engine: running/Carb adjustments

[pklinck]

I have a 1973 penton 125 B engine sachs motor.  It is running lean just after a rebuild.  In fact, the cylinder is a bit dry.  The bing Carb. following specs are as:  a 140 main jet, a 40 idle jet, a 2.73 jet needle had been installed in 2011.  The needle is at the 3rd position according to the manual.

Observations: After a test ride, I noticed the plug was very dry. I took the cylinder head off and confirmed the cylinder walls were also very dry (though no damage).
1. I lubricated the cylinder walls and put the head back on.
2. I also changed the jet needle from 2.73 to 2.76 for a test ride.  3. The air adjustment screw is set at about 1/2 turn out from the stop (as a starters)but I did not adjust due to this running issue.
Result; for the first few seconds, the engine reved normal, sounded normal, etc..  I came back after a test ride and found that the top of the cylinderhead around the plug was moisture or wet but the cylinder walls were dry again and power down, etc..

Comments:  the bike seemed to run normally at idle speed but once you gave it gas, it was flat and not smooth or uneven sounding.  The ignition is vance smiths with an NOS motoplat spark coil, black lead. Timing is a bit before the BTDC mark, according to the penton manual. It appears to spark normally when checked. What do you think is the problem?

peter klinck

[DKWRACER]

Intersting topic, try raising the needle, can't hurt to go a bit richer, if it sounds as tho gasping when cracking the throttle...slide cutaway could be an issue too
Tom Brosius

[brian kirby]

Air leak.

Brian

[pklinck]

Brian,

Could you give me some sources, sites of all possible air leaks?

peter klinck

[brian kirby]

Could be many places. The mag side main seal, base gasket, connection between the carb and manifold, center case gasket, and it could even be from porous metal in the case casting. Even a small air leak can cause jetting to be off and can drive you crazy trying to jet out.

Brian

[pklinck]

t20s/Brian,

Yes, this bike ran ok before in 2011 as i did new jetting replacements.  I had coil and float problems which are had recently been corrected.  But after the rebuild, the cylinder and head and piston are different while the carb is the same.  When boring the different cylinder, I also resurfaced the head because I noticed a leak after testing.  

Do you think I should try and double up the cylinder base gasket for testing for a leak?  Crank seals are new.  Again, note that i changed the jet needle from 2.73 to 2.76 and it ran wet in the head but not on the cylinder walls after inspection.

peter klinck

[pklinck]

Brian, t20s et al,

I think I solved the problem after a conversation with Kip.  Since I recently planed the cylinderhead, I decided to measure how much had been planed off prior to my having this new (to me), used head compared to another same cylinder head and discovered that it had at least 20 thousands (overtime)planed off. This means that the compression was to high if I understand it.  Thus, not allowing the fuel mix to get down the cylinder walls, even if I richened the mixture to a 2.76 jet when it became sopping wet in the area around the plug in the cylinderhead.

Solution: Add another base gasket, as Kip mentioned, which is about 20 thousands thick and give it a test ride.

peter klinck

[gooddirt]

WHAT[?][?] I run heads with 1.5 mm to 2.0 mm milled off of them. And you dont want the air and gas mix to escape down the sides on upon compression that what the rings are for.

[brian kirby]

I dont think the "wetness" of the cylinder is an accurate way to measure jetting or compression. Before you go too much farther you need to do a leakdown test on the engine and verify that it does not have an air leak. Just because its got new seals and gaskets doesnt mean they cant be leaking.

Brian

[firstturn]

You better read what Gooddirt and Brian wrote......

Ron Carbaugh

[pklinck]

Brian/Gooddirt:

Fair enough your comments.  I will however, start with the base gasket as I source of an air leak.  As mentioned, this previously jetted carb ran find on the same cylinder, though re-bored.  The head has been planed with a new head gasket.  I'll double check if the carb clamp to cylinder is tight.........

I think though it is obvious and strange that the head port was moist and the cylinder walls were dry.  Also as mentioned, I adjusted the carb to run richer, put 2-cycle oil on the cylinder walls and the same result; a very wet cylinderhead with a dry cylinder wall when checked the second time by removing the head to verify.......

Does the throttle cable going into the carb slide on top have to have a rubber boot so as a source of air leak?  I never bothered with wrapping the end of the cable going into the top of the carb with tape.....

peter klinck

[gooddirt]

no air from carb cap rubber..... rings will wipe dry the sides of barrel  .

[pklinck]

Gooddirt,

Could you explain in more detail your comment just above?  There is a rubber ring seal underneath the cap on top of the carb. while the metal threaded ring screws down to snug the assem. down.  I would assume this is to prevent any air from entering the top of the carb.??  let me know this effects a dry cylinder wall condition while the inside dome of the head is moist with mixture.....

peter klinck

[brian kirby]

The carb will not leak air through the top, those seals and boots are to keep water/dirt out, not air. The carb can leak around the sides of the slide if it is too worn, or at the clamping surface where it mates to the manifold. The rings will wipe the cylinder dry each time the piston moves up and down in the cylinder. The dry cylinder thing is not something you need to worry about, if the engine runs at all it is getting enough for lubrication purposes.

Brian

[pklinck]

Gooddirt, , t20s, Brian, DKWracer,

I found 2 problems in the end of this mystery which were solved and no, it was not a carb problem since the bike ran fine in 2011 with all new parts in the carb, adjustments, etc..

1.) A.) When I made the first new bore 54.5mm with a new cast piston:  the piston (from ebay) apparently was for an A sachs motor not B engine and the rings must have not been correct to the piston.  At .004 clearence, the ring end gap was .025 thousands.  As a result, the cylinder walls were dryer than normal and the compression was 75 lbs.
 B.) I re-bored again to 54.75 and selected a new forged Wiesco piston from Buehner's Supply and left a piston clearence of .003 thousands and the compression was over 125 lbs.
2.) After then re-testing, the new piston/bore installation, I found that the bike ran well at idle speed, but when I gave it gas, the power was flat.  I checked the sparkplug and it was wet (unlike before).  I changed the spark coilwith a red lead wire (with diode) and it ran poor the same as before.  Then I took a known coil and stator combo from another running 73' penton  125 and tested again.  The bike ran normally.  I believe the output in amps by the stator was the problem especially since there was a new re-bore and the compression was high.  alias!

peter klinck