'72 250 Zeniths (INAT 32/40's) - Hot Start Problem - Page 2 - PeachParts Mercedes-Benz Forum

mbdoc · #16 08-16-2007, 08:15 AM

Many of those cars did that when they were NEW!

I remember having to install a "hot start" kit so they would be re-start in HOT weather. That kit allowed the secondary vacuum diaphram to be pulled open durning hot re-starting!

As far as the heat riserd go just remove the "cardboard" tube & block off the air intake where it connected to the air filter housing.

todds · #17 08-16-2007, 11:56 AM

By heat riser, I think most were referring to the exhaust diverting flaps/weights/springs in the manifold under the carbs. I think what you are talking about is the heated intake air duct which goes to a "stove" on the exhaust manifold from the intake horn. That is a great point to bring up, both of these, if malfunctioning, would make heat soak and hot starting miserable.

My 67 had zero hot start issues in rather hot n' humid weather here before the conversion to webers from the zeniths. (although it had plenty of other issues) Perhaps that fuel return is more important for some of us than others, especially with the stock mechanical fuel pump. I understand the gas picks up a lot of heat from the engine at that point.

My car has the dreaded weights and springs and manifold flaps, but the air intake stove flap is a manual valve instead of vacuum controlled thankfully. I'm not sure what your scenario is ryan. Mercedes seemed to retain these manifold flaps for so many years, (back thru the pontons at least) I wonder why they never chaned the design since they freeze up so easily and cause so many issues when malfunctioning. I know what they are SUPPOSED to do theoretically but I'm sure they could've developed a more fail-safe system to do the same thing. I'm tempted to retro-grade to updraft carbs ;-)

rgnprof · #18 08-16-2007, 05:49 PM

Thanks for the replies. The chokes seem to work fine, the carbs are balanced and I haven't pulled the plugs yet since the rebuild.

Accel pumps work fine. I think my problem is related to timing and advance issues and cooling/heat issues.

As far as the cardboard tube that connects from the exhaust pipes to the underside of the air filter housing - I don't have it connected right now. I usually connect it in the winter. I have never really checked the air filter valves etc.

My heat risers, as mentioned previously, are closed, blocking off the hot air.

I still don't know how to really check the fuel return valve.

I am waiting on a tach to check my timing at 3000 and 4500 rpms - my current one only goes up to 1600. I will post some further findings later this week.

rgnprof · #19 08-18-2007, 05:14 PM

I checked the air intake pre-heater valve in the air filter assembly today and it is NOT working properly. The thermocouple is supposed to close the flap to any more warm air (above 82F) - which is coming from the exhaust pipes near the firewall and connected to the underside of the air cleaner assmebly via a cardboard hose (which is currently disconnected). Once the flap is closed, air enters only from the end of the horn of the air cleaner, where the air is a little cooler.

My flap is always open to the warm air from the exhaust and the closed to the fresh air that travels in through the horn of the air cleaner assembly. The pin that extends from the thermostat will pull out, but it won't depress the spring and hold the flap in the closed position.

Is this fixable? If not, can you still get this part? I can't find it anywhere online. If I can't fix it or find it, what is my best bet? Could this have anything to do with my hot start problem?

ryan

Chas H · #20 08-18-2007, 06:04 PM

It could be difficult to find that part. They were going bad 25 years ago. I think you can live without the carb heat and it probably contributes something to your problem. Block the valve shut and try it.