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  #1  
Old 07-12-2002, 01:05 AM
Gympie
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Engine Cut-out @ 2K RPM

I'm back and am seeking your expert help once again. As an update I have rebuilt the Solex 4A1 on my '76 280S with a new kit and float. Since I have no access to a CO meter, I have read-up on as much as possible during vacation about quadrajet carbs, but mainly on the Rochester as it is the nearest to the Solex I could find. Have set-up the carb air/fuel mixture as per generally accepted rules for a Rochester. But am having a problem with the engine cutting out at 2000 RPM and the tach going crazy by going up to 3000 RPM then dropping down to 1000 RPM and dying.

The engine is setup as the following:

Dwell: 34 degrees
Timing: Advanced from 7 BTDC to about the sensor nipple on the wheel. About 15 degrees?
Vacuum: Can't get more than 11 in Hg. by adjusting the fuel/air mixture screws and idle screw. Best I can do without a CO meter.

Possible problem areas which I would like to bounce off you all:

1. Primary throttle plates to rich causing a transition failure at 2000 RPM
2. Timing set too advanced
3. Sever vacuum leak. Have noticed brake fluid around the vacuum booster. But don't know if it is or the master cylinder grommets. Also, possibly the trans vacuum modulator?
4. Imminent failure of the ignition computer.
5. Something else.

As always I appreciate your advise and time with my problem.

Good Day,

Gympie

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  #2  
Old 07-12-2002, 09:01 AM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,318
I don't see anything in your specs that would cause the problem you describe.

My suggestions:

Make sure you are getting enough gas from the fuel pump and that the fuel cutoff switch is working.

Try unhooking and plugging the vacuum from the distributor. I believe that there is a timing cutover that happens about 2000 RPM, and it may be malfunctioning.

Try to eliminate possible vacuum leaks.

- unhook and plug the line to the brake booster.
- unhook and plug the accessory vacuum behind the carb
- plug the modulator vacuum
- make sure you hooked all the vacuum lines up to the carb.
__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #3  
Old 07-13-2002, 12:27 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
I'd set the timing to factory spec -- 4 degrees after TDC with vacuum applied for the 73 model according to the Chiltons manual. This will at least allow somewhat normal operation. I suspect you are setting the timing too fast -- never ever set it "by eye" or "by ear" -- no way that is accruate enough.

Verify that the advance is working correctly -- you may have a vacuum retard/vacuum advance set up, and if the hoses are backwards, all sorts of strange things happen. Been there done that, on the '68 Ford wagon!

As for setting mixture -- the Quadrajet instructions won't be all that helpful, as every carb has a different "basic setting" (ie number of turns from full closed to operation position on the idle jets).

The best system for setting idle mixture without a CO meter I've found is the Ford (Holley) procedure: Get engine running, then open idle jet until no more speed increase is obtained (rich) -- slowly close jet (going lean) until a 50 rpm drop, then turn 1/8 turn rich. Repeat for other side, the fine-tune both again.

If the limiter caps are still in place, this will be much easier.

One other thought about loosing power at 2000 rpm -- is you main jet system working at all? Sounds as if as soon as you get off the idle circuit you are starving for fuel. Check the instructions and make sure you didn't leave a gasket out of the emulsion tubes or something similar, or set the float way too low.

Can't be too specific, as I have always managed to avoid this particular set up (the one 280C I looked at was too rusty!).

I've gotten the perception that these carbs are kinda cranky, but if you persevere, you will at least get a drivable car, I'm sure.

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #4  
Old 07-14-2002, 01:39 AM
Gympie
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Quote:
Originally posted by ctaylor738
I don't see anything in your specs that would cause the problem you describe.

My suggestions:

Make sure you are getting enough gas from the fuel pump and that the fuel cutoff switch is working.

Try unhooking and plugging the vacuum from the distributor. I believe that there is a timing cutover that happens about 2000 RPM, and it may be malfunctioning.

Try to eliminate possible vacuum leaks.

- unhook and plug the line to the brake booster.
- unhook and plug the accessory vacuum behind the carb
- plug the modulator vacuum
- make sure you hooked all the vacuum lines up to the carb.
Thanks for the suggestions. Will get to them this weekend, seems like when one car gets attention, the other one in the barn needs some attention. Funny, why are so many cars named after women?
My distibutor does not have a vacuum connection, but will check the other connections you suggested. Having only 11in. Hg., does not seem to correct for my engine. Have to track down the vacuum leak. Starting to think, or maybe I should not, that they float level is too high or low which is cutting off fuel flow. What do you think?
Noticed you have a 300E stick and am envious! This is my next Benz, but have been searching for a stick version whcih are pretty rare. Have been driving stick shift for most of my life and do not like automatic transmissions for many reasons. Also have been following your post on your stick conversion. Wish I could help, but my knowledge does not go to that depth.
Again, thanks for your post and advise. It is greatly appreciated!
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  #5  
Old 07-14-2002, 02:14 AM
Gympie
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Quote:
Originally posted by psfred
I'd set the timing to factory spec -- 4 degrees after TDC with vacuum applied for the 73 model according to the Chiltons manual. This will at least allow somewhat normal operation. I suspect you are setting the timing too fast -- never ever set it "by eye" or "by ear" -- no way that is accruate enough.

Verify that the advance is working correctly -- you may have a vacuum retard/vacuum advance set up, and if the hoses are backwards, all sorts of strange things happen. Been there done that, on the '68 Ford wagon!

As for setting mixture -- the Quadrajet instructions won't be all that helpful, as every carb has a different "basic setting" (ie number of turns from full closed to operation position on the idle jets).

The best system for setting idle mixture without a CO meter I've found is the Ford (Holley) procedure: Get engine running, then open idle jet until no more speed increase is obtained (rich) -- slowly close jet (going lean) until a 50 rpm drop, then turn 1/8 turn rich. Repeat for other side, the fine-tune both again.

If the limiter caps are still in place, this will be much easier.

One other thought about loosing power at 2000 rpm -- is you main jet system working at all? Sounds as if as soon as you get off the idle circuit you are starving for fuel. Check the instructions and make sure you didn't leave a gasket out of the emulsion tubes or something similar, or set the float way too low.

Can't be too specific, as I have always managed to avoid this particular set up (the one 280C I looked at was too rusty!).

I've gotten the perception that these carbs are kinda cranky, but if you persevere, you will at least get a drivable car, I'm sure.

Peter
That is one great statement about the Solex 4A1 as being cranky. Really like that term as I have called it many names from shistkpfoff to less friendly terms.

Have the dwell set to factory specs according to the 110 engine manual at 34 degrees dwell. Oh, let me apologize the beast is a '76 280S. I did not post this in my orginal plea.

Have the fuel/mixture jets turned out to about 1.25 turns to gain the best vaccum. Guess I should be adjusting for idle speed and vacuum. I'll give it another try. Starting to think the float is set too high and is flooding-out at a higher RPM. the exhaust is smelling pretty rich, but was not sure if it was the fuel/air mixture or what. I get a good idle speed, but even when the car is in neutral, pressing the accelerator, the engine balks at 2000 RPM. Starting to think the float is adusted too high. Thus, causing a too rich mixture at high RPM. What do you think?

Gympie
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  #6  
Old 07-14-2002, 01:26 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
I've never heard of setting mixture by vacuum, but then I've never worked on MB carbs before. I can check with Hans for pointers this week, need to chatter about what engines will fit in my "new" 280 SE with busted block. Note that while you have to "balance" the two sides of the carb, it's not the same as balancing a dual carb setup. The two needle valves may not be at exactly the same setting.

Dwell and timing are two different, although inter-related items -- chaning the dwell changes the timing. Set the timing to what the sticker under the hood says -- if you are significantly off, you won't get good vacuum.

If your floats are too high, you will see fuel flooding over the top of the vent tube into the carb. You will get black smoke and smell rich, too! A leaking float valve seat (scored, improperly installed, etc) will do the same thing reguardless of float setting.

I've read the rebuild instructions for the Solex, and I think there are some cork gaskets that must be soaked before installation, or they crack and leak fuel, but I don't know if that would cause you the problems you have.

Make sure you don't have a vacuum leak, set the ignition timing to factory, make sure you have good plugs, points, wires, cap, and rotor (if you haven't already), and try to re-set the mixture by the idle drop method. The carb can appear to be running rich due to poor ignition, and you can adjust it untill the end of time with no improvement until you get the ignition problems or vacuum issues under control -- my Mom's old Ford Fairmont was the same way -- never could get the idle mixuter right until I discovered that the vapor control canister purge valve was leaking, causing a serious vacuum leak via the PCV system. Got that fixed, and suddenly it ran right!

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #7  
Old 07-14-2002, 08:22 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,318
I have been thinking about this, and I wonder if the flaps over the vacuum-operated secondaries are opening prematurely and causing a huge loss of vacuum.

Did this all start after you rebuilt the carb?
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #8  
Old 07-15-2002, 02:01 AM
Gympie
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Would sincerly thank you all for sticking with me on this. Worked on the the 280S today for awhile and still am scratching my head. Took of the carb cover to see what is what with the float. Found it was adjusted too high. Fuel was soaking the cover gasket an leaking into the primaries. Reinstalled the cover and torqued down the bolts.

When restarting, it ran like a gem up until it reached 175 degrees temp. Then started to hiccup. Will idle in Park for about 20 seconds @ 800 RPM, then drop to 650 and pick back up to 800 almost on schedule as Germans would. :-) Checked the vacuum and was getting a steady needle at 11 in Hg. Then drop to 5 in Hg during the coughing period.

Decided to go back to basics which are fuel/air/spark. Put some Heat in the tank and ran it for awhile. But did not solve the problem.

Brought back the timing to book value of 7 degrees BTDC. No change.

Checked the points, cap, rotor which had a little tracking but not excessive. No tarnish on the points. Oh BTW would like to change to a breakerless ignition, but this could be another post. Plug wires were checked and no crackes found which have no more than 2K miles on them. Plugs are Bosh Supers set to gap. No change.

Have not checked the fuel pressure regulator as yet nor checking the ballast resistors. Will keep you up to date.

Again, I appreciate your time and expert advise.

Good Day,

Gympie
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  #9  
Old 07-15-2002, 03:02 AM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Gympy:

Check all the vacuum lines, especially the rubber ends, on the pollution control stuff. You may have a switchover valve that causes a vacuum leak when you reach operating temp. I can't give you a diagram, maybe someone else has one, that would let you see what is what and check. Dollar to a donut you have a bad line and it opens up, or shuts something off when it shouldn't when the temp goes up.

I know there is a vacuum switchover for the distributor -- goes from retard to advance when engine is warm. I'm sure you DON'T have a mixture control as this isn't an electronic carb. You could also have a bad temp sensor, but I don't know which one.

Keep at it, sounds like a leak rather than a carb problem!

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #10  
Old 07-15-2002, 08:06 AM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,318
psfred is right.

There is a timing cutover at 175 that may be introducing a vacuum leak. Double check that everything is connected correctly and that you have no leaks in any of the connectors.

The manual also says that at 175, the choke is completely diabled by the TNS.

I don't recall if you said whether or not your car has idle shutoff solenoids on the carb. If one of those is malfunctioning it could cause the idle to drop.
__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #11  
Old 07-16-2002, 01:59 AM
Gympie
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Thanks for the suggestions and help. Decided today to go back to step one, taking off the Solex to see what is what. It is easier this way to check all the vacuum connections off the carb.

All connections are solid into each oriface and all interconnections are good as well. Did find a mistake in the vacuum booster connection where one line was attached to the wrong port. The Solex I'm working with has no "D" connection drilled in the cover which goes to the vacuum booster. Going to have it drilled and piped due to the Secondary butterflies are sticking at full open. Hopefully the shaft can be rebushed, polished to cure the sticking full open condition.

To my surprise when taking off the carb a pool of fuel was found in the intake. Now I am really puzzled! The float level is adjusted correctly, but why the pooling of fuel? Going back to the engine manual, reading each page, I'm starting to think it is the fuel return valve is not limiting the pressure or the float chamber valve is not working correctly not allowing the float to equalize pressure causing the overflow. Dunno, any suggestions would be appreciated. By-the-way, if anyone knows what group the float chamber diaphram belong to, please let me know. Called two MB dealerships and both could not find what I was talking about. Think the 280S is older than both of the dude's I talked to as they acted as it I was trying to sell them some muffler bearing grease. :-)

On the last rebuild, I did not replace the bottom gasket, or insulator gasket which is between the Solex and intake manifold. This is suspect was causing the vacuum leak, knock on wood.

Also engergized both fuel cut offs and both are retracting fully. That is one thing to rule out. Have also checked the fuse for each and neither have blown.

Well, now I'm up to getting the Secondary shafts refurbished and if I can find it a diaphram for the float chamber valve. Any suggestions would be greatly appreciated and do thank you for your help so far. Want to sell this car and get a Stick 300E like ctaylor738 has ;-) .

Gympie
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  #12  
Old 07-16-2002, 08:10 AM
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In my parts catalog, all of the Solex stuff is "Group 7/1." I unfortunately do not have the big one with the part numbers for the engine. There is a diaphragm-looking thingey in the diagram.

There is a used 4A1 on e-Bay!

Dan Penoff (dpenoff@tampabay.rr.com), who hangs out on the mercedes-veterans list described that same exact problem with the fuel puddle in the intake manifold.
__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #13  
Old 07-16-2002, 12:04 PM
Gympie
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Thanks, I'll give Dan a shout today.

Gympie
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  #14  
Old 07-27-2002, 01:42 PM
Gympie
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Same story, different page......

Well I’m back at the old 280S again after a couple week hiatus. As an update, have done another rebuild on the Solex. Took it to a carbmiester and had the secondary shaft gone over as it was hanging a bit. Also this time replaced the insulating flange, which I neglected to do last time and replaced the fuel return/pressure valve. After a little coaxing, it started and idled right at 800 RPM. But then the old problem came back once again, at 175 degrees temp; it started to hiccup once again.

Am now going trough the process of checking the emission system with all the temp switches, switch valves, vacuum lines and relay box. What a chore! Have found a temp sensor that is located on the passenger side of the engine that according to the service manual closes at 60 degrees C, which is 140 F. This switch controls the EGR along with the switch valve-giving vacuum to the EGR at temps above 140 F. The electrical portions checks out fine, but have not done the tests while the engine is running as yet.

My question is, would a faulty EGR valve cause this situation of irregular idle at operating temperature? As my car is considered Vintage, I no longer have to have the emissions checked. What would be the advantages, or disadvantages in disconnecting the EGR from the system?

Thanks,
Gympie
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  #15  
Old 07-27-2002, 04:43 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,318
My 280C had an EGR valve installed when I first got it, but the line that went to the exhaust was not connected to anything. In fact, there was nothing on the exhaust to connect it to. Then I looked at the valve, which was completely plugged, which was good because the entire bottom of it had corroded away. I debated about this and then went to the dealer and bought a plug for the threaded hole in the intake manifold behind the carb where the pipe screws in.

This is a BIG plug - like 25mm. It is either #16 or #18 on the Group 14/1 picture. It needs to plug the hole where #61 in 14/2 goes.

I have not noted any problems attributed to the missing EGR valve, but since the engine was dead when I got the car, I have no benchmark.

And are you in luck if you want to replace it! Check this out on e-Bay:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=1846254282

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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