170220 FORUM

Boards => Technical => Topic started by: twigren on July 15, 2019, 08:24:40 AM

Title: Starting problems (MB 170S sedan)
Post by: twigren on July 15, 2019, 08:24:40 AM
Dear friends,

After some test runs with my newly renovated car I decided to do some additional tuning.
Main reason was a very bad idling and I started to rotate the distributor, adjusted air-screw at carburetor etc.
As I couldn't really solve the main problem I took some additional measures and had both carburetor as well the as distributor off for cleaning and reviewing all details.

When everything assembled again suddenly the engine doesn’t even start.
Starter works fine but the engine doesn’t work. Instead, when starter stops there’s a back-suction through the carburetor and gasoline is spitted out (no air filter mounted.)
No back-fire occurs, but simply a ”cough” with gasoline upwards through the carburetor.

Here is what I have tested and verified:

1. Fuel system (from tank, through filter, through pump into carburetor and the needle-valve closes correctly)
2. Coil (ohm measured and seems to work fine)
3. Spark plugs (tested and verified I get a spark outside the cylinder)
4. Breaker (breaker adjustment set according to manual, 0.4mm)
5. Engine position (upper dead position for cyl #1 is set)
6. Compression test (done on cyl#1 when setting the cylinder position).
7. Battery OK (recharged several times)
8. Carburetor (acceleration pump works fin, gasoline is pumped into the chamber)

Have tried many things but need your help. :-)
Would appreciate any ideas or thoughts on why engine and despite all ingredients (fuel, air and spark) doesn’t start.

Thanks in advance!

MB 170S Sedan, 1950

Title: Re: Starting problems (MB 170S sedan)
Post by: Scott Montoney on July 15, 2019, 06:11:36 PM
I'm not sure what to make of it. . .
From all that you have described, fuel is making it into the cylinder and the spark plugs generate a spark outside the cylinder head, The only conclution I can come up with is that the timing isn't correct.
I would re-check that the distributor isn't 180degrees out of position.  #1 piston at TDC and Distributor rotor is at #1post.

I don't know what the 170 distributor drive looks like compared to my 220, but I recall several years ago when I had not seated the distributor all the way down into the drive and it "slipped" bringing the engine to an immediate halt.

Other than that, I'm out of ideas.
Title: Re: Starting problems (MB 170S sedan)
Post by: Ted on July 17, 2019, 10:21:37 AM
Are you sure about the TDC of #1 cylinder? The piston would be at the top twice, once for compression and once at the end of the exhaust move. So, it has to be the right top position, from the two.
Title: Re: Starting problems (MB 170S sedan)
Post by: twigren on July 17, 2019, 10:35:32 AM
Thanks for your reply! I have recheckef all that. Have also tried to rotate 180 degrees just for testing.

@Ted. Thanks for reply! I verify TDC by attaching a compression
-meter to cyl #1. When it shows compression, I know both valves are closed and compression stroke is underway.

Title: Re: Starting problems (MB 170S sedan)
Post by: twigren on July 17, 2019, 10:38:21 AM
Developing story:

1. Condensator checked and also tested with another one
2. Coil checked and I have tested two others, with no result
3. Have tested to rotate distributor 180 degrees, with no result

...getting out of ideas 🤔
Title: Re: Starting problems (MB 170S sedan)
Post by: Scott Montoney on July 17, 2019, 03:23:35 PM
Hmmmm. . . . When the spark plug(s) are removed, are they damp with fuel?  In other words, fuel is being drawn into the combustion chamber?

If yes, and spark is confirmed as yes, and the timing of both is correct, there should be ignition.  I'd say one of those three must be missing.
Title: Re: Starting problems (MB 170S sedan)
Post by: Ted on July 17, 2019, 04:13:30 PM
Why don't you try another ignition coil? Two Sundays ago a vintage car would not start, no matter what was tried - a whole crowd of clever guys gathered, it had well visible spark, had fuel, had excellent timing... After a day of checking this and that and everything and replacing part after part - plugs, wires, condenser, etc etc.... someone took out the "good working coil" and threw in some old spare coil - and the engine started instantly, worked like a charm! The puzzling part was that the coil showed no problem whatsoever, it belonged to that car and did work before that, still there was very good visible spark, but actually somehow did not work that day! Try it, it is a very easy check to wire another coil, worth trying.

Also, did you try another set of spark plugs? Sometimes there would be a perfect spark in the open air, but very bad in the chamber under pressure. A friend has a plugs tester and strange enough, excellent plugs /tested in the open/ prove to be total loss under pressure /there is a small glass on the test chamber and the spark is observed/.

Another idea - recently a 50s Renault would not start, until we found out a shortage in the breaker. No visible signs of any kind, no sparks, burns or smoke, all seemed right, just would not start until that was fixed.
Title: Re: Starting problems (MB 170S sedan)
Post by: GF on July 18, 2019, 11:14:40 PM

Is the engine turning over fast enough to fire?  A weak battery or a failing starter could cause slow turning of the engine.

I would also check compression in each cylinder.  Your initial bad idling could be due to a bad head gasket.  If you installed a new gasket it could have been damaged during installation.
Hope this helps.

Title: Re: Starting problems (MB 170S sedan)
Post by: Ted on July 20, 2019, 03:08:18 AM
A little joke, from yesterday: a friend called me, a 170V engine would not start. Restored years ago by someone and stored ever since. Long story short, after checking this and that and finding out that the distributor is not rotating when the engine turns, removed the front lid /thinking that the composite timing wheel has lost its teeth/ to find out the wheel actually having fallen off the camshaft... Some genius assembled it without the big bolt holding the wheel to the camshaft!!!!  ::) :o :o ;D