Doctor! Here are my symptoms

[MBdude]

OK, My newly acquired 1969 280SL is bringing me much pleasure and only a few surprises.
My single major problem will cause me to take it to an expert mechanic unless an expert forum member can give me some suggestions.
When I start it up cold in the morning ( it is 35-40F here in Idaho) it runs fine and accelerates to 5000 RPM with no problem..smooth, no misses, perfect.
After only a minute or so when the heat gauge starts to go up it starts to miss over 5000 RPM, then 4500 RPM, then 3000 RPM as the temp gauge reached to 180F.
Then a loss of power plus it backfires and pops at full throttle.
The problem seems to be getting worse every time I drive it.
I pulled the fuel tank filter and it was clean.
I changed the fuel filter that also looked clean.
The tank is very clean.
New gas with a little Sea Foam.
Plugs are clean and look a little grey/white probably on the lean side.
Wires are fine.
Rotor and cap look fine.
Haven't checked the timing but it was checked when a mechanic installed a Pertronix points conversion a few years ago so it should still be OK.
Haven't checked to see if the vacuum advance is working but the lines look OK.
Hate to think it has anything to do with that nasty FI pump since it looks like a mystery and full of engineering wizardry.
Thanks for any suggestions before I drain the bank account at the MB dealer.

 

[Benz Dr.]

It's runnig too lean.

[Garry]

MB dude,

If you were a Full Member,   ($30) all would be revealed in the Technical Manual and you would be able to do the Linkage Tour video presented by Joe Alexander another of our esteemed members that would fix this problem and save you a lot of money getting someone else to do it.

Garry
Membership Administrator

[ja17]

Hook up a fuel pressure gauge and monitor the fuel pressure as the engine warms up. Yes, sounds like a lean running problem. Don't rule out the ignition coil however. Get back to us with the results or any questions and we will proceed!

[MBdude]

OK, here is what I did this morning:
1.  I read the forum comment where it said if I joined as a full member my car would run better.
2.  I joined as a full member using PayPal.
3.  I went out to the garage and started my 280SL and let it warm up.
4.  It still runs the same.
What did I do wrong?
Did I miss a step?

[J. Huber]

The journey of a 1000 miles in your Pagoda starts with a single step... you are on your way.

The next ones will be to go slowly and methodically over what you hear and read from the Gurus (you have the attention of two of the best already)

I am confident you will get there soon enough!

[Benz Dr.]

OK, here is what I did this morning:
1.  I read the forum comment where it said if I joined as a full member my car would run better.
2.  I joined as a full member using PayPal.
3.  I went out to the garage and started my 280SL and let it warm up.
4.  It still runs the same.
What did I do wrong?
Did I miss a step?

Yeah, you forgot to send me $1,000,0000.00 

Try turning the barometric compensator out a half turn. You will have to get it loose enough to turn first. I helped a friend of mine get his running right over the phone recently so maybe you should call me on this. It's easier to explain all of this over the phone than it is to do it here.

[stickandrudderman]

MBDude, I congratulate you on the funniest post on here in a long time!

Dan: Whoa there boy! You don't want to be encouraging novices to start fiddling with the barometric compensator until they've gone through the basics. They're only going to confuse themselves. You and I (and other too) know that the barometric compensator can be used as a diagnostic tool but that's because we have a comprehensive understanding of what is in reality a very simple system that looks very complicated.


What's happening, by the sound of it, is that your cold start device is working, which is a good thing.
The cold start device adds more fuel when cold and lessens it as the engine warms up. The symptoms you describe indicate that when the cold start device is active, the additional fuel is allowing the engine to perform. As the cold start device reduces its input, the amount of fuel being delivered is no longer sufficient.
Do what your 30 buckeroos has entitled you to do and take the linkage tour then report back.
It could simply be a matter of a few clicks of the pump adjustment.

[Benz Dr.]

MBDude, I congratulate you on the funniest post on here in a long time!

Dan: Whoa there boy! You don't want to be encouraging novices to start fiddling with the barometric compensator until they've gone through the basics. They're only going to confuse themselves. You and I (and other too) know that the barometric compensator can be used as a diagnostic tool but that's because we have a comprehensive understanding of what is in reality a very simple system that looks very complicated.


What's happening, by the sound of it, is that your cold start device is working, which is a good thing.
The cold start device adds more fuel when cold and lessens it as the engine warms up. The symptoms you describe indicate that when the cold start device is active, the additional fuel is allowing the engine to perform. As the cold start device reduces its input, the amount of fuel being delivered is no longer sufficient.
Do what your 30 buckeroos has entitled you to do and take the linkage tour then report back.
It could simply be a matter of a few clicks of the pump adjustment.


[/quote

I keep forgetting how long I've been here so I often don't bother to look at how many posts someone makes.  :)There are far more lurkers than there are posters around here. I know quite a few of them....

I agree that it would appear that the auxilary air valve is working. The engine is getting extra air and fuel when cold but as the valve closes it runs lean of fuel. Changing the setting on the idle mixture knob would have no affect at higher RPM's and this engine needs a lot more fuel to run properly.

I've been setting mixture ratios using the compensator for a couple of years now with fairly good results. Even freshly rebuilt IP's sometimes need a bit more or a bit less fuel to obtain optimum performance. Of course, you would want to go through all of the other systems first such as ignition timing, throttle linkge adjustments, ignition componets, valve lash, and checking your cold start valve before you try anything else. All of these systems need to work in harmony with each other or the engine will not run properly.

I also very much agree with stick that , '' If you don't understand what you're doing, don't $#^@* with it! ''

 

[ctaylor738]

Well, first of all, don't expect much from a dealer unless they have a tech who is actually familiar with mechanical fuel injection.

With a Pertronix, you should have a Bosch blue coil with 12 volts (no resistors) to the 15 side. 

Agree with Joe that fuel delivery should be checked after the engine warms up and starts to crap out.  You might find it easier to do a fuel volume check instead of pressure. Lots of posts and stuff in the T M on how to do that.

Welcome to the group.

Cheers,