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Exhaust blockage?

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NutmegCT

NutmegCT

Great Pumpkin
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On to stage two of my "repair the rolling wreck" adventure.

1960 Mercedes-Benz 190b. Four cylinder, gasoline, 4 speed manual. Very basic car.

The "engine won't start" problem was resolved when I discovered the ignition switch was faulty. Bypassed the ignition (hot wired the car) and engine starts and idles fine.

But the engine won't rev up. Slowly open throttle, nothing happens. No rpm change. Hold throttle open, engine dies.

Carb works fine; fuel system, ignition, timing, valves, points, etc. all test good.

Compression test shows 160, 160, 162, 160 psi. Spec is 128-142.

I test drove the car before purchase, back in October. Drove fine, but noisy (rusted out) muffler. One week after I drove it, the owner had the muffler replaced (Midas). Two weeks later I bought the car and had it shipped to me.

Exhaust system partially blocked? Any way to test without trying to pull the gol'dern manifold off? There's no vacuum test port on the manifold.

Thanks.
Tom
 
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One simple test would (presumably) be to check how much is actually coming out of the exhaust. Also, try restricting the carb intake (reducing air) what happens then?
 
Thanks JP. I do feel (and see) exhaust at the tailpipe. Exhaust looks and smells rich.

Slowly covering carb intake doesn't seem to change anything until it's completely closed - then it stumbles. But remember it's only idling.

Tom
 
Got one of those fuel pressure/vacuum gauges with the point metal tip?
Obtain some short 1/8" pop rivers (and appropriate too;).
Drill a hole before the muffler in the pipe.
Start the car, hold the tip against the hole, have someone attempt to rev it, see what the pressure is.
In the early days of cats and double-wall exhaust, we'd do this all the time. I cannot recall what the reading was to fail, but it was less then a pound. You peg the needle, you've got problems.
Then, do it behind the muffler, or wherever you've got a long run.
Where it reads good...the blockage is upstream from that.

If you can, just drop the pipe off the header, and run it.
It will be loud, but you'll know if it will rev or not.
Have you looked up the pipe with a light yet?
Someone jam a potato up there?
End of the pipe have any dents or misshapen edges?
Like somebody backed into something and kinked the pipe further up?

Seal the holes back up with the pop rivets.

Dave
 
Dave - thanks very much. My dense brain never even thought of checking pressure in the pipe itself.

Add a new piece to the puzzle: The symptom has been that the engine will idle just fine, but won't increase speed when throttle is opened.

Yesterday I opened (enriched) the idle mixture needle two more turns from spec. It's now *very* rich. Now the engine revs up and holds its speed.

Anyone want to make a diagnosis? Adding this to the way-high cylinder compression readings? Carbon build up in the cylinders, somehow requiring more gasoline to "burn"?

Thanks.
Tom
 
Tom, sounds like you are (somehow) over complicating things. That doesn't sound likely somehow - and certainly wouldn't come all at once. Are you by chance on any Mercedes forums? Not that there aren't experts here but local tribal knowledge is always the best.
 
Thanks JP. I actually belong to several of the M-B forums. Most of the guys there aren't familiar with the details of the carbureted engines. Also, I've found most of them either own their own shop, or take their cars to big-city shops for maintenance. Many of the "old car" owners seem to concentrate on how to get points at concours competitions. Last week's big discussion was on what color the bolts inside the glove compartment on a 1957 sedan should be.

I'm afraid my troubleshooting often appears as over-complicating things when I discover a new anomaly. I try to give the findings to help in diagnosis. I ask the M-B guys about one thing, and the few that reply just give random advice like "check the spark, check the fuel pump, rebuild the carb" etc. I've done all that. But actually tuning up an old engine, I haven't found anyone willing to help, who has much expertise.

I'll keep pluggin' away.

Thanks.
Tom
 
I know nothing about M-B engines, but the symptoms you describe are of carburetter trouble. I don't know how many jets it has, but if it were a simple type of fixed-choke carb, it'd have a slow-running jet, a main jet, probably an accelerator pump and a compensating jet.

Now, since your engine idles OK, I think we can assume the slow-running jet is OK, and since the compensating jet is designed to admits air when the main jet would deliver too much fuel, I doubt that's the issue. If the accelerator pump wasn't working, the engine might stumble while accelerating, but would get there eventually.

So, though you've said your carb works fine, I'd bet on a blocked or obstructed main jet.
 
Roger - thanks very much. I actually rebuilt that carb (cleaned, new jets, gaskets, checked passages, etc.) and found no change on my engine. Then I took the carb to a guy who owns a car just like mine (same model, year, engine). I put my carb on his engine. Engine (and carb) worked perfectly for starting, idling, and acceleration. So I tend to think it's not a carb problem; the other guy feels the same.

I've read here on BCF many times: many "carb" problems are actually electrical problems. My next step is to get a new coil and see if that makes a difference. Original coil was a Bosch, no ballast required. Maybe the enriched mixture fires during acceleration where the normal mixture doesn't, because of failing coil. (?)

With new plugs, new points, excellent timing, and excellent fuel flow, this seems a good path to follow for now.

Thanks.
Tom
 
Yeah, many are...and oppositely, too.
Did you try de-carbonising the combustion chamber?
Just for grins?
Got an old empty anti-freeze jug?


Is that a single barrel Solex on that unit?

You've checked the spark intensity (again, for grins)?

Dave
 
Thanks Dave. Could you give me some details on that de-carbonizing process?

Solex PAITA 32 carb; primary throat and secondary throat. Secondary has two butterflies; one begins to open when primary is fully open. Second butterfly in the secondary throat opens by manifold vacuum. And an oil-filled damper keeps the secondary from closing too fast.

Spark is bright blue when tested at idle - but I never tested it under load because I can't move the speed above idle for very long. I'm pinning hope on replacing the coil and the distributor condenser (external, not the one with the points inside).

Engine starts and idles with normal idle mixture needle setting, but won't rev up unless I enrich the idle mixture way too high. Maybe a symptom of a failing coil, but the Bosch blue coil (made in Germany) tests good (bench test) on both primary and secondary. Maybe just fails under use?

Weird.
Thanks.
Tom
 
Hard to do if you can't rev it.....but even a little...warm engine, slowly pour a bunch of plain water down the primary.
Water turns to steam, blasts all the carbon out the tailpipe.
I've done a LOT of them (engines, not MB's).
Condensor craxxing out?
 
I use a generic spray bottle to send MISTED water down the induction hole. But as Dave noted, you need some revs to get it to work well. I'd not attempt it 'til you solve the non-revving issue. Higher compression wouldn't be the culprit, methinks.
 
Thanks gents. As I know the carb is ok (works on another car just fine), it's *gotta* be electrical (right?).

So I'm going to replace the coil (hey, it's only 50+ years old), the points and condenser (about five years old). Still can't figure out why the compression is so high - and so uniform.

When's the last time you saw one of these: Bosch blue coil, 0 221 119 02 stamped on bottom.

View attachment 24693
 
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Actually I think I may have one of those Bosch coils laying around somewhere.
 
Does your distributor have an advance, vacuum or centrifugal. May need to clean advance base or find lack of vacuum. Idles fine but will not pull revs! Also, FYI , JC Whitney has an exhaust vacuum/ pressure test for low bucks ( catalyist tester.
 
Thanks Larry. Centrifugal and vacuum advance actually work fine. The carburetor vacuum connection is for ported vacuum only; vacuum from that connection is present only when butterfly is open.

I'm still thinking ignition ...

Tom
 
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IT'S FIXED!

After nearly a month of troubleshooting the problem (hard to start, rough idle, no acceleration, etc.) I found the problem. The connection on the side of the distributor (wires to coil and condenser) was loose inside the distributor. It's usually a simple problem, right?

Discovered it when I replaced the condenser this morning.

Engine now starts immediately, idles smooth, and accelerates when the throttle is opened.

View attachment 24777
 
Woohoo :banana: :banana: :banana:

Isn't the saying , 90% of carb problems are ignition?
 
Sure glad that's over!
 
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