450scl sluggish starting

I recently purchased a 1979 450slc. It runs very well but does have a few minor glitches. It takes a few seconds to start when cold. It is very consistent about how long it takes to fire up depending on how
cold the engine is. Also the engine bogs down if you give it a small amount of throttle. As long as you push the pedal past the first inch or so then it runs fine but I have a feeling these symptoms are related. Beyond routine maintenance does anyone have an idea what might cause this?
Thanks in advance.
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I'd suspect the cold start system which provides additional fuel for cold starts and initial running - up to about a minute. Basically it consists of a temperature sensor (cold ON, hot OFF) a thermo-time relay (ON longer with colder temps) and an electric fuel valve (cold start valve). All of these must operate if the additional fuel is to be delivered into the intake manifold. The trick is to know where they are located on the engine - find the cold start valve and work backward via the electrical connections to it.
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When standing in front of the car looking at the engine, the cold start valve is on the very front, right corner of the intake manifold. It has a fuel line and some wire running to it. It's a small upright cylinder held in with two screws.
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I replaced the cold start valve and this did not make a difference. The car still takes about 3 full seconds of cranking before it will turn over when cold. Are my expectations for this engine too high and this is normal? Additionally, it still 'sputters' on acceleration unless I push the pedal down pretty far and then it seems to run (take off!) OK. These symptoms may or may not be related but it seems suspicously like a fuel issue. I plan to replace the fuel filter next but would appreciate any input.
P.S. I also replaced the spark plugs with Bosch Supers...no go.

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Gee... cold start valve... not! Don't spend anymore money...I would enrichen your fuel mixture a bit first... I think your fuel mixture is a bit too lean.
The stuttering of acceleration is usually due to your warm up regulator... they do go out of whack and needs to be adjusted... only a few mechanic knows how to do this.
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Thanks for the info Tiger...I will look into that. Just to clarify the problem, the car starts immediately once it has warmed up but the 'sputtering on acceleration' happens regardless of engine temperature. Does this also sound symptomatic of lean fuel? Can you tell me a bit more about how the warm up regulator works?
Thanks again!!

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Sputtering every time when you step on the gas regardless of engine temperature and every single time you step on the gas from stop? If so, I had the same problem... It wasn't lean mixture... it had something to do with the valve.
What did I do in attempt to solve the problem and worked? I changed the engine oil to synthetic and add a quart of Marvel Mystery Oil... never overfill your engine. Then I added some Marvel Mystery Oil to the gasoline as per instruction... The sputterins disappeared after 15 minutes of driving... try this and see if it work.
How did the sputtering occur on my car? I tried a different fuel injection cleaner... perhaps too concentrated... and it was bad sputtering. I did all other obvious check before I tried above method.
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