1972 250 - Cold start and acceleration problems - Page 3 - PeachParts Mercedes-Benz Forum

gmask · #31 03-01-2003, 11:27 PM

Quote:

Originally posted by rgnprof
The question is, if this is the problem what does it mean? How do I fix it?

I guess it means you need to replace the vaccum unit.. I may have an extra used set as I think they may not be available through the dealer anymore..but I'm not sure.

ctaylor738 · #32 03-02-2003, 09:43 AM

I was able to get a new vacuum advance unit from Mercedes for my '73 for about $25. There are also lots of places that sell Bosch (distributor manufacturer) parts.

I think you need to put together a plan of attack and start checking out these problems one by one.

- vacuum advance
- engine timing
- choke operation
- carb operation ( stuck secondaries)
- carb vacuum
- valve adjustment

The Haynes manual for the SOHC 230-280 has good instructions in all these areas. You can get it at www.books4cars.com

gmask · #33 03-02-2003, 02:06 PM

Quote:

Originally posted by ctaylor738
vacuum advance
- engine timing
- choke operation
- carb operation ( stuck secondaries)
- carb vacuum
- valve adjustment
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Wouldn't it be a good idea to adjust the valves first?? Then time the engine?

gmask · #34 03-02-2003, 03:50 PM

Here's some other places to get the haynes book..

http://www.amazon.com/exec/obidos/tg/detail/-/0856963461/qid=1046637995/sr=8-3/ref=sr_8_3/002-6426083-7046443?v=glance&s=books&n=507846

http://search.ebay.com/search/search.dll?MfcISAPICommand=GetResult&query=mercedes+haynes

ctaylor738 · #35 03-02-2003, 04:43 PM

Valves could go first, sure. My list reflected the original complaint of hard starting and poor running when cold.

rgnprof · #36 03-02-2003, 04:50 PM

Thanks to everyone, I think I am making some progress. I bought a timing light today and tested the vacuum advance, which works fine. I used a vacuum pump to test the diaphram and it holds vacuum. I did find, in the process, that the hose leading from the retard side of the vacuum unit and to the 2 way valve (emissions control?) on the passenger side firewal, and from there to the carb, had a hole in it. I fixed that and wen I hooked it back up, the car ran lousy. I had timed the car several months ago with a friend's help, but when I hooked up the timing light today - after fixing this hoe - it was way out of time. I adjusted the timing- per my plate under the hood - to 3-4 degree ATDC and it runs a lot better - maybe idles a little slow (600-700 rpm), but smooth. When I disconnect the retard vacuum hose from the vacuum unit and accel to 1500 and 3000 rpm the timing advances appropriately. So far so good.
Could this have been my problem?

Also, I started checking vacuum leaks at hoses, none except at the dashpot (throttle closing damper) on the rear carb. When I apply vacuum at the top outlet (which, by the way, connects to the same valves (emission?) on the firewall) it doesn't hold vacuum. Should it?

I am still working -got some parts coming this week. Hope to fix the air pipe that runs from the intake manifold to the air cleaner and is right between the two carbs. I remeber reading a post from earlier about it, but I don't remember and can't find it. I think this will make a difference with cold running.

I still haven't adjusted the valves - that is next and then I will check the timing again. Any thoughts? Thanks for the help.

Ryan

rgnprof · #37 03-04-2003, 12:58 AM

I have made some significant progress - synched carbs, adjusted idle speed, timing is set - but have a couple of questions.

1) I think I have some carb and emissions components from both 1971 and 1972 model (date of production was 7/71). For instance, my car has a low temp switch on the cylinder head, which is not supposed to be there on a 1972 model and the carbs do not have the float chamber venting valve, which is true for the 1972 model. Anyone else run into this? It sure complicates things.

2) How do I check the secondary vacuum diaphragms?

Thanks, Ryan

gmask · #38 03-04-2003, 02:22 AM

>>2) How do I check the secondary vacuum diaphragms?

This requires taking the top half of the carb apart and holding your finger over the vacuum holes in the choke throats and then seeing if the diapharaagms hold the vacuum once pushed in.

rgnprof · #39 03-05-2003, 08:59 AM

Got my 250 back together yesterday, and it runs great - best since I have had the car. List of changes - put a hose running from the exhaust manifold to air cleaner, fixed the air water separator tube between the carbs, fixed a retard vacuum leak running from the distributor to the emissions equipment, set the timing, synch the carbs (by ear using a hose) and adjusted the idle speed). Thanks for the suggestions everyone. I still have a cold start problem - 21 degrees here this morning - it kicks over just fine but it idles real slow. I don't think the fast idle is working when the car is first started in the morning - at least I know the idle is no where near 2000 - 2500 rpm. I'm getting gas - it just doesn't seem to want to idle fast enough.

Ryan

ctaylor738 · #40 03-05-2003, 09:34 AM

Great progress!

You adjust the fast idle on the front carb. You have to come up from the bottom with a small screwdriver and adjust the screw that contacts the cam on the choke counterweight.

240 Ed · #41 03-06-2003, 02:48 AM

Quote:

You adjust the fast idle on the front carb. You have to come up from the bottom with a small screwdriver and adjust the screw that contacts the cam on the choke counterweight.

If this is the adjuster for the cold fast idle on the chock, Do not attempt to do this adjustment when the car is running!! Because you need to turn the throttle open to get to the screw...

I think that this adjustment should not change during the cars lifetime, but rather that the electric choke mechanism is not working.

rgnprof · #42 03-06-2003, 12:04 PM

Ed, Thanks for the reply. I am a little confused about this fast idle issue. I am looking at the automatic choke assembly information in the Haynes Manual and they indicate that the marks on the housing and the cover should be aligned. Mine are not - they are both approx. 5 mm to the rich side, which is where they were set at the factory (on US cars with emissions control, which mine has). After a run-in period, it looks like they were supposed to be moved back. I am a little hesitant to adjust - I mean the car has been on the road for 30 years and 156,000 miles!!! Should I mess with it? Should I do this first before attempting to adjust the fast idle screw? I agree - I don't see how this screw could have gotten out of whack!

I think the electric heating coil in the bimetallic spring is working - at least the chokes open slowly when I turn on the ignition - slowly - but they open. Is there another way to test this? The rear carb has a 65 degree temp switch attached to it from the cylinder head and a 55 degree one from the thermostat housing(you might want to read an earlier post in this thread, because I think this car has some '71 carb components and some '72). I think I also need to adjust the choke valve plate - at least that's the next step in the Haynes manual before checking the fast idle. Thanks for any suggestions.

ctaylor738 · #43 03-06-2003, 01:37 PM

If the car is starting and warming up well, then I would leave the choke adjustment alone. To have it a bit rich seems OK given the winter temperatures that you are dealing with.

Maybe the cam or screw simply wore a little during the years.

240 Ed · #44 03-07-2003, 10:13 PM

Hmmm, gosh and golly, I am little bit stumped here...

Perhaps if you had a vaccuum leak, it would not run very well when cold...

You could try messing with the choke fast idle, but I would suggest that you count the number of turns you do, (and write them down!

), and see where that gets you.

If it doesn't work you should perhaps return them to where they were...

rgnprof · #45 03-08-2003, 10:21 AM

Chuck and Ed, Thank you for your replies. I will try and adjust today, counting turns etc. I'll let you know what happens. As for a vacuum leak, I can't find any. The only 'leak' (and I'm not sure it's really a leak) I have found is that when I hook a vacuum pump to the dashpot (throttle closing damper or vacuum regulator) on the rear carb, it does not hold a vacuum. I read a post from way back, that seems to indicate that this dashpot should hold a vacuum, but I'm not sure and I don't know if I am testing it correctly. Thanks.

Ryan