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Engine chokes and even stops when I step on the gas

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Hi Scotty,

Thanks for all your interesting videos from Tokyo, Japan. I am learning a lot from you.

My only car for the last 7 years has been a Mercedes 280S Sedan (W108 chassis with M130.016 naturally aspirated inline 6 engine, automatic transmission, has about 125,000 KM now) from 1971. I used to be a gear head in my younger days when I used to ride on different motorcycles, but when I quit riding bikes, I have totally given up that hobby and have been riding latest cars and left maintenance up to the dealership. That all changed when I purchased this Mercedes, which was a dream car for me for many years.
Since I acquired this car, I have been proactively changing different parts such as water pump, brake booster, carburetors, coolant hoses, engine and transmission mounts, alternator, wiring harness, a/c system, brake hoses, drag bars, steering damper, subframe mounts, etc. I also rebuilt the brake calipers, power steering pump and steering gear box. I replaced the mechanical fuel pump with Carter's electric pump and put a return type fuel regulator to keep the fuel cycling through all the time to keep the fuel system cool. The exhaust at one point became too smoky, but replacing the valve stem seals fixed that. The engine was quite clanky but adjusting the valve clearance totally fixed that. The car never died on me and I really enjoy driving it. As most work had been done by me with some help from a friend mechanic, the maintenance cost had been minimal so far. I purchased a used tranny which only had about 10,000 KM on it and had it totally rebuilt for about $2500 recently, which I plan to install soon, as the current transmission is a bit leaky around the shaft and gradually starting to slip, especially when I am accelerating on a freeway.

One issue I have with the car is when I step on the gas on a hot day when the car had been in idle for over 10 minutes or hitting the road after restarting after parking for up to 30 minutes or so, not enough for the engine to cool down. The engine chokes and sometimes even stops unless I step on the gas very very slowly. This goes away after driving for a 5 to 10 minutes and the coolant temperature goes down to about 85 degrees centigrade. I do see that the oil pressure drops on idle when the coolant temperature goes up to about 90 degrees centigrade. (I use either Chevron or Valvoline 20w50 mineral oil.) I replaced the carbs from Zenith INAT 35/40 to Weber 38/38, as the Zenith carbs kept on giving me air leak at idle when they got hot. I tried rebuilding the Zeniths but had far too many woes and just gave up. The mixture screws had been adjusted and seems to be fine, as the spark plugs don't get that dirty. The acceleration pump seems to be squirting enough fuel too. What could the problem be? The only thing I haven't adjusted is the float level. Thanks for you help.

Best regards,

John

5 Answers
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Those cars are tanks, built when a Mercedes was really something special. Sounds like you have it ready to run another 50 years!

Sounds to me like despite the fuel recirculation you have have a heat soak problem causing vapor lock. (That happens even on some fuel injected cars.) As others have mentioned you may also have to tune the Weber float level and jets for your engine, especially if the carb was not specifically set up for your car at the factory.

Thanks for your advice. I was worried about vapor lock too, so in order to check whether the fuel is flowing through, I attached transparent fuel filters in the fuel line going to the carbs and the fuel line coming out of the return of the fuel regulator going back to the fuel tank. I do see gas flowing through both of them. But of course, I won't be able to monitor them while driving so maybe vapor lock is happening while I am driving. The strange thing is that it works if I step on the gas really really slowly...

You might try the old hot-rodders trick of putting clothes pins on the fuel lines to break up vapor lock. Metal clothes pins are supposed to be best to help dissipate heat .

 

https://www.hemmings.com/stories/2019/09/20/clothespins-on-fuel-lines-fact-or-fiction

 

https://itstillruns.com/use-clothespins-vapor-lock-7971011.html

 

Hello, thanks again. The original fuel line was metal with braided cloth like cover but the former owner replaced it with rubber fuel hose. As this rubber hose was wearing out, I replaced it with the latest fuel hose from Bridgestone. Will clothes pins still work on rubber fuel hose? Thanks.

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Well certainly could be the float level just realized carburetors are always the problem in old cars like that if you can find an old mechanic like me who really knows how to work on those you might have them rebuild them and then adjust them with the gauge not just by ear using an actual flow gauge

Thanks for your reply. As this engine is a twin carb engine, I did balance the carbs using flow gauge and adjusted the idle mixture by checking the exhaust using air to fuel gauge. The only thing I haven't adjusted is the float level, as I took it for granted that it is adjusted when shipped out of the factory since they were new from Weber/Redline... I will take a look at the float level.

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The float level is a pretty critical adjustment on carbs.  So is the accelerator pump output, it needs to be properly measured.  Idle mixture screws, idle jets all have to be correct to function properly.

Thanks for the reply. I did measure the accelerator pump output using a plastic cylindrical gauge and looks good to me (between 7ml to 10ml). As the engine is twin carb, I adjusted and synchronized the carbs using flow meter. The air correction nozzles, jets, idle jets, emulsion tubes are all set to the ones that are for this engine. The only thing I haven't done is float level adjustment. I kind of took it for granted that the level should be fine as they came fresh from Weber/Redline. I will check and adjust the level. Thanks.

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When I got a Weber 32/32 specifically for my 22R, the idles and main jets were completely wrong and there was a small cotter pin stuck down one of the emulsion tubes.  The float levels were also, way off.  I guess they made it on a Monday morning.  I had to buy a jet kit to get it right.

Wow! I'm sorry to hear that... As for mine, I did take them apart before installing to check whether the jets were correct and they were the right ones. The thing about this particular Mercedes is that carburetors sit on top of the intake manifold which sits on top of the exhaust manifold. There is a butterfly valve like thing in the exhaust manifold which moves by a bimetal spring as it gets hotter and the heat of the exhaust is used to heat up the intake manifold. Since the fuel and air mixture is diverted 90 degrees to go to the engine, if the manifold is too cold, I guess it will condense the fuel so they did this. But this thing gets way too hot in summer... and the float bowl does get hot from the heat of exhaust manifold that is right under it. Thanks for the advice.

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Thanks, Chuck.  In this "modern" age of debatable engineering progress I forgot all about good old vapor lock. 

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