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Old 11-13-2004, 02:37 PM
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Join Date: Feb 2003
Location: Oklahoma
Posts: 1,016
Update on my 250

I haven't had much time to tinker with the car lately, but I have noticed a couple of problems probably related to some of the work I have done over the past couple of months (valve stem seals, vacuum modulator - per previous posts).

The smoking on start-up is still there and I am still at a loss - but I have noticed a coolant loss. I don't see any dripping but I had to add a quart, or so, today to the radiator. Could the smoke be coolant, and if so, what are the causes (head gasket??)?

I also have a very noticeable hesitation upon the initial acceleration, with a backfire from the carbs on occasion. The hesitation occurs when I first step on the gas, both from a dead start and from a coast. In the past, this has been due to accelerator pump problems but I checked this and I have a good stream of fuel from the accelerator pumps in both carbs. I checked the spark plugs and found #1 and 2 to be in good shape, 3, 4, & 5 to be a little oil fouled and wet and #6 dry and sooty (carbon fouled).

Can anyone piece this together and give me some advice - thanks!

RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE
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Old 11-13-2004, 07:03 PM
300SDog's Avatar
gimme a low-tech 240D
Join Date: Jan 2003
Location: central ky
Posts: 3,602
Coolant smells sweet when it burns, but I dont think your startup smoke is coolant. And I'd probably start by checking valve clearances. Self-torquing adjust nuts can lose their grip after 120k miles or so.
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Old 11-15-2004, 08:52 PM
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Join Date: Jul 2004
Location: OKlahoma City
Posts: 180
The hesitation and backfiring sound like possibly a result of improper timing. This could also cause fouled plugs...

When you say the plugs were "wet", do you mean with oil, gas, or coolant?

Lack of coolant in your radiator could be the cause of many things, such as a small leak from the radiator hoses, or even a leak in the hose to your coolant overflow bottle and/or hose.

The "puff of smoke" you are getting at start up... what color is it?

1967 Mercedes 250S w/ 350 Chevy V8, stock transmission and rear end.
1968 Mercedes 250S w/ 229 Chevy V6, 350 Turbo Transmission with shift kit, and 1981 Corvette rear end.
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Old 11-16-2004, 08:25 AM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Without ruling out a head gasket problem, which could definitely cause the symptoms you describe -

If you are getting hesitation and a backfire, the you probably have a vacuum leak somewhere. Carb base gaskets, throttle shafts, or warped top plates are the usual suspects. Your spark plug symptoms point to the rear carb.

The water pump can leak around the shaft seal and this leak can be hard to detect.

Didn't you mention some suspicions about your new valve seals? They're the usual cause of start-up smoke.
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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Old 11-18-2004, 10:01 AM
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Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
Smile 130 type of engine.

Own a couple of these. Spray a little wd 40 around the base areas of the carbs as i have found the carb insulation gaskets tend to shrink unevenly with age. If you have leaks engine will pick up with the wd 40.If they turn out to be a little leaky I just get out my gasket punch set (under 10.00) and make eight new thin gaskets and install them with the old spacers and seems to catch the problem. Someone else may have better ideal though.The reason I do not buy new spacers is the cost. Sometimes have foolishly left gas in the system too long without using cars (years) and it has turned to semi-gum inside the carbs. Used to take them apart dip them and kit them in the old days. But with lack of time and cost of kits,chemicals,manifold gasket,as found it easier for me to plull the manifolds to get rear carb nuts. Decided had to be a better way. Well this may be cotroversial but here goes. On the other hand may be general knowledge. I really do not know. I knew gas engines run fine on laquer thinner.The thinner eats residue gum like crazy. Remove fuel line under car before fuel pump. Run a quart of thinner up into the carbs. First of course run carbs dry. continued.
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Old 11-18-2004, 10:22 AM
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Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
Smile dirty gummed up carbs.

shut engine down and let the carbs soak in thinner for about 15 minutes if really gummed. Just hook fuel line up again and start engine and the gum is gone. It works for me and even if that was not the problem the car always seems to run better after. You have basically cleaned the carbs. Seems to help eliminate lean mixture problems. Oh yes, the reason to run the car until engine quits is that there is a return line from the carbs and when they are pretty empty the thinner of course fills the bowls first before dumping excess to tank. Just feel bad about all the carbs that I used to take apart, clean and kit when i feel today it was not neccessary. Have never had a problem doing this except wondered about plastic floats ability to take the thinner in some designs. If anyone can think of any pitfalls let me know as have not run across anyone that uses this proceedure. Also not talking down by inserting all the instructions as I know very little myself. Maybe the guy asking the question has even less experience than myself. Remember buyer beware. Now if only I knew the base chemical to run through and purge some of my diesels.
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Old 11-18-2004, 10:59 AM
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Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
Smile sooty sixth plug-coolant leak

The smallest vaccum leak at the end of the intake manifold close to firewall. (there is a vaccum takeoff there) will cause real mixture problems with the sixth cylinder even to the point of that cylinder not firing and should be checked. Of course check heater core for coolant leak if you have not already as these cars are not babys anymore anythings possible. Have you checked for bubbles in radiator with engine running? How about borrowing a coolant system pressurizer and have a look around? Let us know what you find on this thread if possible.
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Old 11-18-2004, 09:04 PM
300SDog's Avatar
gimme a low-tech 240D
Join Date: Jan 2003
Location: central ky
Posts: 3,602
Originally Posted by barry123400
Now if only I knew the base chemical to run through and purge some of my diesels.
Cant speak for diesel fuel base chemical, but Lubro Moly Diesel Purge can be bought off the shelf. Best technique is to disconnect feed and return lines then route them both into 1 qt. plastic juice container. Add a couple cans of Diesel Purge to the container full strength and run it through with warm engine idling. It will recycle over and over again until it all gets used up in the combustion process, generally about 30 minutes per can of DP. It's good stuff that really works and should probably be used once each year. Raises hell with clouds of smoke too as it cuts grime at injectors and precombustion chambers.
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