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  #1  
Old 06-06-2017, 05:18 PM
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350SL high speed problems

I have a 1972 350SL with 3.5 liter engine and it won't rev above 4000 - 4,500 RPM. I've checked all the usual things and replaced points, plugs, set the valves, checked the timing chain, and adjusted the throttle linkage. Spark plugs look normal. Fuel pressure stays right at 30 PSI and ignition distributor is advancing fully. Looks like it has new trigger points and maybe a new throttle position switch. Engine pulls OK up to about 4,000 RPM and then it just stays there and won't go much higher. I'm getting 40 degrees of timing advance so it shouldn't be a problem with that. Seems to be missing at higher speeds and then it goes flat.

Any ideas would be helpful.
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  #2  
Old 06-06-2017, 07:08 PM
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Do a parts # search of the ignition controller, aka "switchgear". If a unit from a 6-cylinder engine has been installed the rev limiter will cut off the 8-cylinder engine right where you are experiencing it.
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Old 06-06-2017, 07:58 PM
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Quote:
Originally Posted by Frank Reiner View Post
Do a parts # search of the ignition controller, aka "switchgear". If a unit from a 6-cylinder engine has been installed the rev limiter will cut off the 8-cylinder engine right where you are experiencing it.
We hooked up another box and it did the same thing. I don't know if it was from a 6 cylinder though. I'll check that out. Thanks.
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  #4  
Old 06-06-2017, 09:08 PM
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350SL high speed problems

Stupid question: Does it have a rev limiter rotor in the distributor?


And is this a d-jet or a k-jet we are talking about?


Edit:
And will it rev above 4500 when not under load?


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  #5  
Old 06-07-2017, 10:39 AM
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should be a Djet
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  #6  
Old 06-07-2017, 12:15 PM
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Quote:
Originally Posted by meltedpanda View Post
should be a Djet


Yeah. I missed the 1972 in the first post.

Fuel pressure would be worth testing only if it can exceed 4500 when not under load.

If it can't exceed 4500 even when not under load, I'd question the ignition system. Since it's a points system, it's possible that under higher RPMs the points aren't closing due to the points floating (not closing fast enough). The opposite can happen at idle which would be the points not opening at the lobes if the rider has worn down. This can cause stalling at stop lights when the RPMs get low while some float will cause the points to open at higher RPMs. So if there was some service to the points recently done, it's possible the gap is too large. But if you haven't messed with them, and before you could go above 4500 RPMs, but not now, it's unlikely to be the points. At that point I'd question the coil and ICU.


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  #7  
Old 06-08-2017, 12:52 AM
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I'm guessing rotor or vacuum advance problems at the distributor.
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  #8  
Old 06-08-2017, 10:57 AM
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Quote:
Originally Posted by fonzi View Post
Stupid question: Does it have a rev limiter rotor in the distributor?
BD:

This may very well be the cause; a weak or broken spring in the grounding slide in the rotor would exhibit the described symptoms.
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  #9  
Old 06-12-2017, 12:55 AM
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I had the wrong coil in mine (too weak) and it would not make power past half throttle, or run at higher rpm (don't know the exact rpm, as it has no tach). It was super weird, as there was no drama, no noise, no hesitation, just after half throttle there was no power.
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  #10  
Old 06-12-2017, 09:43 PM
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There is no need for a rev limiter rotor. But if anyone wants a brand new one, I have one

I have same car. What I would look at, is the centrifugal advance system. The weights can get stuck. BTDT.

Otherwise, check the carpet under the pedal and the bearings on the fore/aft linkage rod.
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  #11  
Old 06-12-2017, 10:21 PM
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I had one of these old EFI motors doing this same thing, long time ago, but I seem to remember that I had set the dwell to spec with a dwell meter after replacing the ignition points and it just wouldn't rev past 3500-4000. After double checking everything and finding nothing I decided to just set the points with a feeler gauge and it started to come around, I think I adjusted them from a base value of around .016" and just opened them up a couple thou at at a time until it ran best.
Also, make sure that the engine to chassis ground cables are clean and tight, take em off to clean and restack. Also had one that had a high resistance ground wire in the trigger point assembly, I was able to clean and restake that ground point and it fixed the car.
hope it helps.
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  #12  
Old 06-13-2017, 09:10 AM
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Quote:
Originally Posted by Junkyarddawg View Post
I had one of these old EFI motors doing this same thing, long time ago, but I seem to remember that I had set the dwell to spec with a dwell meter after replacing the ignition points and it just wouldn't rev past 3500-4000. After double checking everything and finding nothing I decided to just set the points with a feeler gauge and it started to come around, I think I adjusted them from a base value of around .016" and just opened them up a couple thou at at a time until it ran best.
When I still had points, I would set them to 0.014" with feeler gauge, then check with dwell meter once running. It always came out close to specs. Pertronix eliminates that problem!

When car would not rev, it was the two items I already mentioned. Stuck centrifugal weights and worn out linkage shaft bearings. Latter can be checked by manually opening throttle plate. Centrifugal weight requires distributor to be dismantled. We are supposed to lube them down center of shaft where there is felt pad.
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  #13  
Old 06-22-2017, 08:17 PM
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What Pertronix System would you suggest for a 1975 Mercedes SEL 450
( where to buy one )
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  #14  
Old 06-22-2017, 08:20 PM
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What Pertronix System would you suggest ( where to buy one ? )
I have a 1975 SEL 450 and nothing but trouble with the trigger points .
TKS

Quote:
Originally Posted by Graham View Post
When I still had points, I would set them to 0.014" with feeler gauge, then check with dwell meter once running. It always came out close to specs. Pertronix eliminates that problem!

When car would not rev, it was the two items I already mentioned. Stuck centrifugal weights and worn out linkage shaft bearings. Latter can be checked by manually opening throttle plate. Centrifugal weight requires distributor to be dismantled. We are supposed to lube them down center of shaft where there is felt pad.
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  #15  
Old 06-23-2017, 11:38 AM
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Quote:
Originally Posted by aluminum View Post
What Pertronix System would you suggest for a 1975 Mercedes SEL 450
( where to buy one )
The link below describes type of Pertronix to use and how to install it on an M117 Djetronic. There is a link at bottom of post #1 that relates to the 1975 450SL.

However, I don't know anything about the 450SEL. I read that it was changed to Kjet in mid 1975. And some had a 6.9 engine? In any event, the Pertronix links are only valid for 1971-1975 cars with M116/7 engine.

Bear in mind that vendors and even Pertronix may suggest a later model of Pertronix, but that requires different HT wires and reports on it's use on early Benzes have been mixed. I have had the 1885 model on my car for many years just as a points replacement and it works fine. Others have successfully used it to also replace the MB switchgear (ignition module) as outlined in link.

Pertronix Install with and without Switchgear - Mercedes-Benz Forum
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