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

Started by agav, 02 December 2015, 02:40 PM

agav

Hi UTn_boy,

I recently installed a Petronix ignitor, and haven't been able to wire it correctly for the RPM meter to work. So I don't know the exact RPM but it could be a little higher than the 800-900. I'll see if that's something I can figure out today.

Regarding the air pop, I am assuming that the air filter adds a little bit of resistance to the air flow and adjusting the carb without it might lead to subpar results? Or in other words, is that why I never heard the pop when it was on?

I have checked valves a while ago. No points of course because of the Petronix. Plugs are new. Never done a compression test but I have a tester lying around.

Where do I find the intake vacuum on my car? On the carburetor? Is there a tesing port or to I have to insert a T piece with my vacuum tester attached?

Thanks!
1975 W116 280S in Topaz Brown - my project

UTn_boy

Well, we definitely need to know that RPM reading. 

I may not be able to help you knowing it has pertronix on it.  That set up has cause so many problems over the years on various model ranges.  When the box gets hot, the car goes to dying and missing. 

I imagine that the only reason you didn't hear the popping with the breather on is because the air filter muffles the sound, it doesn't restrict air flow enough to make this much of a difference.  If it does, then you'd probably have a clogged air filter. 

Go ahead with the compression test when you can.  Unhook coil wire, and floor the gas pedal each time you crank the engine.  This allows for maximum air flow through the engine during the test, which is necessary. 

When the engine is running, hook a vacuum gauge to any place on the intake manifold that you can find.  I usually use the connection that goes to the vacuum reservoir for the pneumatic door locks. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

agav

Thanks, will follow exact steps as below. Might be two days until I get to it. I ordered a little device on Amazon that'll give me exact RPM (optical reading from a reflector strip) so I do have that handy soon.

Shouldn't be hard to get rid of the Pertronix if need be for diagnosis. I put it in recently though, doesn't seem to make that much of a difference. Certainly not for this problem.

Air filter is just 2000 miles old, you are probabaly right then about the muffling effect.

Will have to dig around a little bit for the vacuum lines. Given that a big carb sits on top, I haven't even explored that whole system yet.

And another observation - I started her cold today and she did fine moving in and out of the garage. I expected it to be worse after me playing with the idle mixture.

Really appreciate the help!
1975 W116 280S in Topaz Brown - my project

agav

 Just to give everyone an update here. I couldn't immediately find the intake vacuum port, so I decided to pull the carb to have a peek under and couldn't help myself taking it apart for fun.

Turns out it is pretty badly warped. Images attached. When I stack the three parts up, there is a lot of gap in the middle. That'll explain where the leak came from... What's my best course of action here you think?
1975 W116 280S in Topaz Brown - my project

UTn_boy

You have only three options.  Send it to Apple Hydraulics in New York for rebuilding, find a good used one (much more difficult, as there were nearly 10 different versions of Solex 4A1, and you HAVE to find the correct one for your engine), or buy one already rebuilt from Mercedes for around $2,000-$3,000. 

One might try to straighten the top cover, but it's cast aluminum, and it'll break really easy.  Ideally, we'd want to lap the surface in figure eight patterns on a lapping plate until it's flat again, but that's nearly impossible with the pick up tubes in the way.  Maybe try straightening it the best you can with unconventional methods, but only after you've found another used one....so you won't be without parts.  After you get it relatively straight, you can double or triple up on the gaskets to take up what you couldn't straighten out.  No more than 3 gaskets, though. 

Fortunately, these Solex 4A1 carburetors are plentiful, so finding used ones won't be a challenge.  What'll be a challenge is being able to find one that isn't offered for sale by an idiot that thinks it's worth hundreds of dollars......because it's simply not.  It's a $30-$50 used part. Nothing more. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

UTn_boy

Well, scratch out buying one from Mercedes.  They're no longer available. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

agav

Thanks - that is helpful, if depressing that all my options are going to cost me a little fortune. First thing I'll try is put it back together with the new gaskets that I have, carefully use a silicone gasket in addition and see how far I can get it to work for now.

Long term – what do you think about a Weber conversion (http://www.jameng.com/products/38-DGES-Weber-carb-conversion-kit-for-Mercedes-280%2C280C%2C280S%2C72%252d76.html)? I can keep the stock air filter that way and can likely get it running decently for a smaller budget. And going forward, I don't have to fear warping.
1975 W116 280S in Topaz Brown - my project

gavin116

Hi Agav


I'm going to stick my neck out here, purists will hate me.  Have you ever considered some modern technology?  You could get one of those single point fuel injectors that replace a carb.  They are electronically controlled, so cold starts, hot starts etc. shouldn't be a problem, overall drivability and fuel mileage will probably be better as well as power output.  Here's a link to the type of thing I'm thinking about:


http://www.fuelairspark.com/fas/ez-efir-style-precision-cast-throttle-body-and-injector-assembly-2-injector-versionhtml/


It's just an idea meant in good faith.


Gavin


In the meanwhile this may help and a German speaking friend.
1979 450SE "Mrs White"
2022 Touareg-R 3.0TSi Hybrid
[url="http://forum.w116.org/test-drive/my-first-w116-here-at-last/"]http://forum.w116.org/test-drive/my-first-w116-here-at-last/[/url]
[url="http://forum.w116.org/the-org/british-near-london-meet/msg97613/#msg97613"]http://forum.w116.org/the-org/british-near-london-meet/msg97613/#msg97613[/url]

agav

Gavin, thanks a bunch for the doc. I am actually German, so if anyone here has questions, go ahead 8)

Now as far as the purists go – I am pragmatic. This car is my hobby and my goal is to learn a ton - it's not that someone couldn't put the stock carb back in if they wanted.

Are there injection kits that fit this particular engine / intake manifold directly? What kind of changes would I have to make (like welding in an O2 sensor I assume)?

Thanks!
Agav
1975 W116 280S in Topaz Brown - my project

gavin116

Hi Agav
I don't have any specifics, but here's a link to someone who has used this system before.  Sounds like it was successful.  Not sure if the member is still active or not,but you could try make contact.


http://forum.w116.org/mechanicals/solex-4a1-efi-replacementgt-powejection/
1979 450SE "Mrs White"
2022 Touareg-R 3.0TSi Hybrid
[url="http://forum.w116.org/test-drive/my-first-w116-here-at-last/"]http://forum.w116.org/test-drive/my-first-w116-here-at-last/[/url]
[url="http://forum.w116.org/the-org/british-near-london-meet/msg97613/#msg97613"]http://forum.w116.org/the-org/british-near-london-meet/msg97613/#msg97613[/url]

adamb

Hi Agav

My Mercedes ownership started with a W123 250 that uses the same carb. During 3 years of ownership I learned to strip down and rebuild that carb. I had the original factory manuals and acquired a plethora of experience in the Solex. To cut to the chase, that carb is a pig. Great when new but a pain to maintain with age. Take the advice, upgrade to modern fuel mixing technology. You'll get much better performance most of the time.

Just my $0.02 as they say :)

agav

Very valuable 2 cents!

I think I agree - it's bent to the point where I have to think replacement with a practical option, not a stock carb. I will try and rebuild using the new gaskets and the high temp gasket paste. Nothing to lose really as long as am careful and not clog anything.

You mentioned fuel injection - what did you use?
1975 W116 280S in Topaz Brown - my project

UTn_boy

Webbers aren't meant to work on an engine like this.  They restrict air flow, and a smooth idle is not attainable no matter how one has it jetted. 

If you want to try a different carburetor, try to find a Holly.  Holly made a carburetor specifically for the M-110 engine some years ago, and it was a success.  There weren't many sold due to their high price, but if you dig deep enough, you'll find one. 

The TBI fuel injection is a splendid idea, but involves a lot of hacking of original parts and the addition of more wiring.  Again, it's a good system, and performs well when installed correctly.  However, even a pragmatic type of person, like yourself, has limits as to just how far you'll go when hacking and cutting is involved.  I try not to bring this set-up into any conversation because it gets really complicated really fast.  But, the beans have beans have been spilled, so to speak.  Choose wisely, I guess. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

agav

I spent a day working on the ild carb yesterday. My hopes are I can get her at least running again before I seek out the alternatives. A pic below. I used gasket sealant in addition to the gaskets, surely not what one would do with a working carb, but in this case I hope it helped fill the gaps created by warping in the short term.

I could not find a Holly conversion kit that is still sold, only the Weber. Does anyone have a link if it still exists?
1975 W116 280S in Topaz Brown - my project

gavin116

Hi Agav

I'm not 100% sure on this one, but I think you need a Holley 4160, but worth checking by email on the official Holley website.  From what I have gleaned off the Internet, it may be better to stay away from the Weber, seems to be lots of discussion about it not being compatible with the M110 I6.  Apparently a carburettor is not a carburettor...

Try these:http://www.summitracing.com/int/search/product-line/holley-4160-adjustable-float-carburetors and https://www.holley.com

Good luck.
1979 450SE "Mrs White"
2022 Touareg-R 3.0TSi Hybrid
[url="http://forum.w116.org/test-drive/my-first-w116-here-at-last/"]http://forum.w116.org/test-drive/my-first-w116-here-at-last/[/url]
[url="http://forum.w116.org/the-org/british-near-london-meet/msg97613/#msg97613"]http://forum.w116.org/the-org/british-near-london-meet/msg97613/#msg97613[/url]