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Author Topic: Weak engine and mysterious behavior  (Read 396 times)

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Offline twigren

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Weak engine and mysterious behavior
« on: February 02, 2020, 08:01:34 AM »
Dear friends,

I have been struggling with various engine problems the past time.
Basically everything under the hood has been gone through, in detail.
Several issues have been worked with and components have been renovated/replaced.

The engine now starts perfectly and runs smoothly, on IDLE (!).
Responding quite fine as well as the accelerator/throttle kicks more fuel in.

However, when put into gear and load increase, the behavior is very strange.
It's extremely weak, coughs and when load increase more it dies.   
Starting things up again, engine starts directly but the problem repeats as soon as the smallest load is added..

I have checked:
- Ignition timing (calibrated based on UDC and checked by strobe-light on cyl #1).
- Breaker contact (measured. But as things runs smoothly on idle it cannot be an issue)
- Coil (as it works on idle it cant be poor. Resistance measures are also OK)
- Compression (which shows more or less exact the same numbers for all cylinders)
- Mixture screw, on carburetor (2 to 2.5 laps unscrewed versus the "dead mode")
- New spark splugs
- Wires checked and absolutely no issues.

In regards to the spark plugs, there is obviously a problem as they (all four) turn very black/grimed)
I'm using a new set of NGK B6HS., as recommended by another 170 owner.

So, the question remains - How can the calibration for idle-run be so perfect, but at the same time so bad when load is added?
Obviously I am missing something out here... ?!

Thanks in advance!!
/Thomas
 











Thomas Wigren
Mercedes-Benz 170S, 1951

Offline Scott Montoney

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Re: Weak engine and mysterious behavior
« Reply #1 on: February 02, 2020, 06:53:34 PM »
Just out of curiosity, what is the specified ignition timing for that engine?
Also, have you verified that the mechanical and vacuum advance is working correctly in the distributor?
"Gertrude" a.k.a. "Troidl"
1952 - 220 Cabriolet B

Offline twigren

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Re: Weak engine and mysterious behavior
« Reply #2 on: February 08, 2020, 03:40:54 PM »
Thanks Scott!
I think you are right. I changed my vacuum adjuster (rebuilt my original one which I replaced earlier by another one, OEM from a VW car...). Together with more tuning on air-intake at carburator the engine runs better. However not really perfect and the work goes on...
Have been out on the roads again today. A good day 👍😀
/Thomas
Thomas Wigren
Mercedes-Benz 170S, 1951

Offline Scott Montoney

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Re: Weak engine and mysterious behavior
« Reply #3 on: February 09, 2020, 06:22:01 PM »
For what it's worth, here is a story I hate to admit, about a mistake I made once when setting the timing on my car.
I don't know what the specification is for your car, but mine stated in the shop manual, 5 degrees AFTER Top Dead Center.
I set the timing to that and had similar results to yours plus my engine ran very hot.
The problem that I induced was that the timing was to be set to that value STATIC, not running and with a timing light.
When, I set it exactly like the shop manual said, with a test light bulb/volt meter, it ran MUCH better, and cooler.
When I then hooked up the timing light, it turns out the setting, while running, was actually in the 10-15 degree BEFORE TDC range.
So. . .  just be sure your timing value is appropriate for the measurement method you are using.

"Gertrude" a.k.a. "Troidl"
1952 - 220 Cabriolet B

Offline Owen

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Re: Weak engine and mysterious behavior
« Reply #4 on: March 16, 2020, 03:58:17 AM »
Hi Thomas,

Not sure if you've fixed your problem or not, but I did have a similar issue with my 170S a few years back.  I eventually traced it back to a leaking valve in the fuel pump, attached to the timing case of the engine.  Although it ran just fine under no load, it went to pieces under load.  It's quite easy to check, remove the fuel line from the carburetor, hold you finger over the line and hand pump the primer leaver on the fuel pump.  Finger pressure should not be able to hold back the fuel if the valves and diaphragm are in good order.  Might be worth a try....

Owen

Offline Marc1340

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Re: Weak engine and mysterious behavior
« Reply #5 on: March 16, 2020, 06:38:41 AM »
Hello Thomas

Just an additional point of verification to try to help you :

- you adjusted your ignition using the strobe light: ok
but
the reference mark on the engine flywheel is it well placed ?

For be sure you must also check that your control point on the steering wheel is in accordance with the TDC cylinder 1

Best regards

Offline Marc1340

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Re: Weak engine and mysterious behavior
« Reply #6 on: March 16, 2020, 09:15:20 AM »
Hello Thomas

Just an additional point of verification to try to help you :

- you adjusted your ignition using the strobe light: ok
but
the reference mark on the engine flywheel is it well placed ?

For be sure you must also check that your control point on the engine flywheel is in accordance with the position of  TDC cylinder 1

Best regards

Offline twigren

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Re: Weak engine and mysterious behavior
« Reply #7 on: April 07, 2020, 01:49:47 PM »
Thanks Marc, Owen and Scott!

Things are solved but it was a tough one.
Had issues with  spark plugs (leaking/low compression in two cylinders), ignition timing as well as air-leak in carburator.
Anyway. The engine runs ok now 👍

Thanks again for your support

/Thomas
Thomas Wigren
Mercedes-Benz 170S, 1951

Offline Scott Montoney

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Re: Weak engine and mysterious behavior
« Reply #8 on: April 08, 2020, 11:22:17 AM »
Thanks for the update.  Happy it was a positive outcome.
"Gertrude" a.k.a. "Troidl"
1952 - 220 Cabriolet B