Diagnosing rough idle – anyone interested in walking me through it?

[agav]

Hi everyone,

I am plagued with a rough idle on my 75 280S (Solex 4A1 carburetor). I recently replaced all rubber hoses after a coolant leak and really took a deep look at how things are set up. With that energy – would love to take a stab at diagnosing my rough idle / dying engine that I have trouble with.

Symptoms (not sure if all relevant):

• Car idles fine when cold, both in park and drive.
• After driving for a while she starts to kick a little when in idle and drive gear.
• When driving a little longer, I get a r really rough idle at stops to the point that she dies constantly. Moving the lever into park / neutral gives her just enough RPM to keep going, but she is almost dying constantly.
• The above is seemingly not directly related to engine temp (as measured by coolant temperature on dash) at least - with a freshly warmed up engine, the problems aren't immediately there. It takes a bit of driving until they show up. Doesn't mean the temperature doesn't have an effect over time on the carb for example - vacuum leaks maybe due to temporary warping?
• Cold idle is audibly high in park, which is the case since I gave it to a mechanic to "rebuild" the carburetor (which, looking at the gaskets, he didn't other than making adjustments).

I have developed this whole routine of, when getting to a halt, pushing the gear into neutral, wait for the light to turn green, then rev it up a little before I slam it into drive so I actually make it. Surely not ideal... 

[rumb]

Fuel filter?

[agav]

Thanks - should have mentioned. I remember from when I replaced all hoses - checked the fuel filter (which is in the carb) and it looked clean.

[agav]

Also, thinking of it, suspecting something along those lines, I replaced the tank screen (it was clean, but anyway replaced it with a brand new Mercedes part).

[polymathman]

Gotta look at several things in a orderly manner.

Check your timing.

Carb: An engine starts off cold and needs a richer mixture, so a choke is incorporated, sometimes manual, usually automatic driven by a bi-metallic spring. The engine reaches temp and the choke fully opens. Then the job at idle is taken over by the idle mixture, usually adjusted with a screw on the carburetor.

Sometimes dirt will partially obstruct the needle valve and mess with the mixture. Check this by gently turning the screw in, counting turns, until it seats. Back it off the same number of turns. Now start the engine and see if that makes a difference. Or you could take the screw out and blast the hole with carb cleaner.

Old school way to adjust is to turn the idle mixture screw for max speed, then back off the idle speed screw to get idle RPM, then go back and fine tune the idle mixture screw, going back and forth between the two.
Idle mixture is only at idle. If it runs rough at speed, then your problem is elsewhere

[agav]

polymathman, this is a great start, thanks.

Will start by checking timing.

Assuming that is fine, just one question on the procedure you mentioned for the carb. I am familiar with the choke valve - coolant runs through it to trigger it (I had a coolant burst hose there once, so learned the hard way what the parts are).

Does this sound like the correct approach: Drive the car for a while until I get the rough idle problems again. Then do the adjustments and tests you mentioned below, looking for any differences in idle?

Thanks!

[polymathman]

Been a while since I worked on a carb, but your description of the choke sounds good.

Idle should be adjusted with the engine at operating temp.

[UTn_boy]

Is your 280S a U.S. model?  If so, it'll have a float chamber vent valve on the top housing of the carburetor.  When/if this goes bad or leaks, it'll yield the problems you're having. However, if this is bad, you'll usually experience fuel starvation at high rpm's and throughout the acceleration range.

Also, there will be a purge valve mounted on the left inner fender well.  It'll have three hoses connected to it.  One from the charcoal canister, one going to the back of the carburetor (manifold vacuum connection), and a small one hooked to the top of it going to the front bottom of the carburetor at vacuum connection "B".  If the diaphragm ruptures in this purge valve, idle stabilization will be compromised. 

On the left side of the carburetor, there should be a vacuum operated dashpot that controls engine stalling when decelerating.  The vacuum line that goes to it is tied into connection "C" on the front bottom of the carburetor.  If the diaphragm in this dashpot ruptures, it'll create a large intake vacuum leak, and idle will be poor or not attainable, especially at operating temperatures. 

Also tied into vacuum connection "C" of the carburetor is the fuel return valve.  Only a few markets had this device integrated, but guess what?  It has a rubber diaphragm inside of it, too.  It will go bad, like the others, and cause yet another intake leak to atmosphere causing idle problems.  This creature is what screws on the the fuel inlet of the carburetor.  Kind of looks like an upside down hat. 

And another problematic area.  The dreaded thermostatically controlled auxiliary bypass choke.  (In short hand, known as the TN choke)  This device  aids in warm start up's.  It essentially draws fuel from the float chamber and directs the fuel into the primary mixing tubes and mixes with the additional air from the air ducts at the outlet bores.  The mixing ratio is set at the factory and usually doesn't need adjusting.  What normally happens is that that thermostat inside quits working.  Consequently, too much air can be drawn in while running causing.......you got it!  Crappy idle. 

On the back of the carburetor is a pull down delay mechanism.  It essentially disallows the engine from potentially  stopping immediately after start up, and also has some control over the choke.  It keeps the choke at the specified air gap and keeps it there until the choke is no longer needed.  It, too, has a diaphragm inside of it, and when it goes bad, erratic behaviors will present themselves upon cold start up's, and can also cause a no idle problem, but usually on a cold engine only. 

The area between the top cover of the carburetor and the intermediate part, (where the float bowl and float is) is the infamous area where warping will occur.  In 50% of cases, two gaskets can be installed to remedy the excess gap between the two housings.  When this area warps, excess air AND fuel can be drawn in to areas in which it isn't needed, and can also make the carburetor starve itself of fuel in more drastic cases of warping. 

So what are we supposed to do if a Solex  4A1 exhibits all of the aforementioned problems?  Well, if you're curious and have a good mechanical mind, you'd buy a book and go through it and hope for the best.  For those of you that depend on your mechanic, then take note.  Most mechanics today have no idea what a carburetor is, much less how one works.  The older mechanics are usually one of two mindsets: 1) it's been too long ago, I don't remember, or 2) Get rid of that piece of junk and put a webber on there.  Disparity sets in, and the car owner is left dangling, and feels like they have no other choice but to listen to their mechanic.  It doesn't have to be this way. 

There are several places that can successfully rebuild these carburetors, and can do a good and proper job.  One place comes to mind....Apple Hydraulics.  They're rather expensive, but it'll be less expensive considering the alternatives.  One can also buy a rebuilt carburetor from mercedes.  They're about 1,000 Euros, or $1,200 U.S. dollars.  I've bought two over the last few years for customers' cars.  Apparently, Mercedes has rectified the warping problem, and both customers have had zero problems since they were installed. 

So there are options.  If you're a "do it yourself" kind of guy, then buy a book, set a side a whole weekend, and have a blast.  It's a wonderful carburetor when everything is correct, and is a complete blast when set up on a high compression M110 engine.  Both BMW and Rolls-Royce/Bentely used the Solex 4A1 back in the day, as well.  It's a very common carburetor, and parts are pretty easy to come by.  The reason so many of the U.S delivered carbureted engines had carburetors that failed prematurely was probably due to all of the EGR, secondary air injection, etc, that raised engine temperatures, and engine bay temperatures, as well.  A bad place to be for a cast aluminum carburetor.  Sometimes you'll see where people have over tightened the breather where it attaches to the carburetor, but that damage is usually repairable. 

My reply will either help, or scare the hell out of you.   Do follow Polymathman's sound advice, too.  Making certain that the ignition Timing, points dwell angle, valve lash,and compression are in check, and 100% certainty that there are no vacuum leaks is absolutely necessary before we start playing with carburetors and mixtures. 

[polymathman]

UTn_boy is right.

Before all the electronic stuff, you used to replace the points, condenser, plugs somewhere every 3,000 ~ 6,000 miles. Then you would set the timing and the LAST thing you would do would be to adjust the idle mixture and speed.

I don't miss that at all !!

[Jenno1981]

I had the exact same issue and my remedy was exactly the same! I don't miss my heart thumping at the traffic lights thinking the old girl was going to stall at the critical moment.

I posted the same question on here and got a mixture of responses.  There probably are other things to check first but I decided to have a go at the fuel mixture (mine is on LPG).  I let the car warm up and got the revs to an acceptable level then adjusted the mixture (screw) until it leveld out and the idle was perfect.  Took a bit of trial and error but I have to say now she just purrs at the lights, no stumbling.

So, the right way very much well to check other points or you can cut to the chase.

Getting it sorted was such a relief and made the drive so much more enjoyable!

Good luck

Jenno

[agav]

All, this is wonderful advice! Thanks! Yes - love to tinker and looking forward to rebuilding things myself. What literature do you recommended for this specific Solex?

Yes, she is a US car. Yep, she has the charcoal canister and the purge valve. And another thing to mention: She hasn't always done this, the problem has gradually started to get worse over the last 500 miles or so (don't drive her that much, she is a weekend car).

Unfortunately another mishap over the week, hence my delayed response. The starter stopped working and after a couple of successful "taps", it died entirely. Imagine the pleasure of driving in stop-and-go freeway traffic with super rough warm idle AND knowing that if she dies, I won't be able to start her again.  So I had AAA come down and pick her up where she died and a mechanic will replace the starter today. As much as I really, really enjoy mechanics and solving problems, I wouldn't have been able to even get her into my little garage work space.

That being said, I am going to follow everything you guys suggested. This should be an interesting project - I love learning while trying things out.

I'll report back when I have her back in my own garage.

[UTn_boy]

The best literature I can recommend is the M-110 Engine repair manual.  There are two volumes, and then there is one single thick book.  Either way, both versions will have the same information.  The carburetor section is about 100 pages long and may seem overwhelming, but if you follow the procedures and take the time to rig up the necessary tooling, you'll be ok.  It's pretty "cook book" regarding the type of reading/repair, but is necessary.  After you've done one Solex, the manual you won't need so much....other than to refer to specific data that we normally wouldn't retain.  Numbers, measurements, etc. 

What parts that are still available for the carburetor are rather pricey.  The float chamber vent valve diaphragm isn't available by itself or in the gasket kit.  You actually have to buy the whole vent valve.  The same goes for the TN choke diaphragm, and the last one I ordered was around $300.  I think you'll need to prepare yourself more for how expensive it'll be more than you will of actually doing the job.

If the top housing and/or intermediate housings are warped past being able to use two gaskets to take up the gap,, then you'll have to source a used one that's in better shape or send the whole carburetor to Apple Hydraulics in New York so they can resurface the mating surface flat again.  At that point, one might as well pay them to go ahead with the rest of the rebuild. 

If you need any help locating parts etc, yell at us.  We're usually goo for a part number or two.     

[agav]

Just got her back. Brand new starter (what a difference in cranking, I didn't even notice how bad the old one was).

I'll work on it over the weekend and see how far I get diagnosing with your advice above. My manual contains a decent amount of info on the carburetor and I also have a long PDF I found in the manuals section of w116.org.

Will report back!

[agav]

Hi everyone. Job and travel have interfered with the little time I can spend on the car these days. I did drive her a little bit yesterday to get her to temp and then started lookin into timing and idle mixture with mixed results.

First, timing seems way off. Hooked up the light and attached it to spark plug on one. Timing in idle says 30 degrees on the wheel but if I push it down to the recommended 7 degrees the engine dies. 30 degrees on my engine is what sounds best intuitively. So I wonder if the wheel might have slipped?

Then I started playing around with idle mixture. I pulled the air filter off because otherwise it's impossible to reach the screws (no idea who came up with that layout  ). When the airfilter is off, there is a clearly audible knock or plopping noise coming from the engine. After adjusting both barrels for a while just by listening to revs and the plopping noise, I got it to a point where the engine felt very good - no noise in idle, stable revs with warm engine.

However, when I put it into gear, it dies quickly unless I get it rolling quickly und driving, meaning it dies when idling in gear. I managed to pull her into the garage with a little luck before she died again and gave up for the day.

Any ideas?

[UTn_boy]

First of all, where are your RPM's at idle?  In park with no accessories on, you should have 800-900 RPM.  In gear, around 600-700 RPM.  If the idle is higher than this, then your centrifugal advance flyweights will fly out and advance your timing, which will give you a false reading.  Consequently, trying to set the ignition timing under these conditions is impossible. 

The popping you were hearing is referred to as a "lean pop"....this means that there is too much air getting in a mixture that is supposed to be stoichiometric, or 14.7:1.  this means that there are 14.7 parts air to 1 part fuel.  Altering this ratios us unfavorable for proper running.  In addition, air leaks will increase RPM, and if the leak is bad enough, it'll be impossible to get a correct idle RPM. 

I think you have warpage in the carburetor.  It's not uncommon, but very unfortunate. 

Just so we're clear, you have checked valve lash, point dwell, plugs and wires, compression, etc?  And how many inches of mercury if your intake vacuum pulling?