4,900 rpm maximum engine speed

[pch2021]

  Greetings pagoda fans,
     I am sorting out a low mileage 1966 250se coupe, european version, with virtually no options. This is a nice car with a lovely body and 74,000 miles.
  This car runs very nice and quiet. Recent items include:

new thermotime switch
new cold start injector
new fuel pump
fuel tank
spark plugs [Bosch w5dco, from Mercedes-Benz]
rotor

valves have been set, and igntion timing is happy.


My issue is as follows: Under any sort of driving, as my rpm's increase , the closer i get to 4,500-5000 rpms, the lless power I have. As my engine hits about 4,900 rpm, the power is gone, and the rpm's will not increase at all. I have enrichened the mixture, both by reshimming the the WRD and , when that made no difference, the  rack enrichment was turned up.   I turned the screw outwards to richen. Is this right?

  Anyway, the engine will balk a bit in the 3000 rpm and up range. I have followed the linkage tour and it helped  me quite a bit also, but still  to no avail.

  I am quite willing to send the pump to Pacific if needed but I want to make sure I haven't ignored any small details first.
Thank you so much , in advance!
Pierre

[jeffc280sl]

Hi Pierre,

To add more fuel via the rack turn the rack screw CCW.  I suggest you mark the BC and then spin it CCW in 1/2 turn increments and then go for a test drive.  Wedge a piece of rubber hose against the BC to hold it steady.  This will also enrich the mixture and its easier then adjusting the rack.  Repeat this step a few times and see if you notice an improvement.  If you can get it running the way you want just add a shim to take the place of the CCW turns.

[ja17]

Hello Pierre,

Make sure the rack in the injection pump is not stuck. You can remove the cover on the front end, then thread in the correct metric machine screw and you should be able to move the rack freely. The rack should spring back to the rest position on its own when it is released.
Keep us up to date.

[pch2021]

dear sirs,

forgive my ignorance, but BC is...? (:

   Herr Alexander,
The rack doesn't seem to be binding. It seems to move well, but how far should it open? What is the maximum movement of the rack? Do I have to pull the pump out to do this?

[knockmacool]

Barometric Compensator, everybody knows that  
Good luck Pierre

[menesesjesse]

If your distributor is worn you may have possible floating with your points.  Something to check while your looking.
Jesse

[twistedtree]

Does the engine limit at 4900 RPM both with and without load?  If so, I think you have an ignition problem, not a fuel problem.  Either way, it sounds more like and ignition problem to be.  I'd be checking/replacing points and condensor and checking the distributor as previously suggested.

[ja17]

Hello Pierre,

There is a small cylindrical cover on the front end of the injection pump. This can be accessed and opened with the pump on the car. It covers the front end of the passage in which the rack travels. The rack on a 250 series has a threaded hole in the end of the rack. So you can thread in a metric screw and actually move the rack back and fourth and observe the rack to see if it renurns and moves when the throttle is opened. This can all be done with the IP on the car. You may want to look at the injection pump information. There  should be a photo with this cover. I have a 250SE IP on my workbench so if you need an picture let me know.

Good advice  from Jessie, ignition problems can be elusive and seem like something else.  A faulty coil can also give the symptoms you describe. You may want to quickly switch out the ignition coil if you have one laying around, to rule out this issue.

Carry on !

[pch2021]

 Herr Alexander,
  A picture would be amazing!

   So, for starters, here is what I'm going to do
:
check barometric compensator

Check travel of rack screw

do final check of ignition timing- as a sidenote, I have set up the ignition timing, but I am not above making a mistake.

Joe, do you think I gntion timing is also a possibility? The car would not red-line initially, but after doing the linkage tour, it will reach 6,300 with a little stumbling.

[pch2021]

  I have a feelings that I will find many solutions here, as I service a few of these cars. Becoming a full member is a small price to pay.

[ja17]

Hello Pierre,

The linkage tour was a good place to start.  Do the inexpensive things first. A fresh set of NGK BP6ES spark plugs can do wonders. These are a non resistor spark plug which is correct for these cars. Bosch no longer supplies non-resistor spark plugs. Even if the plugs in the car are fairly new, tuning and adjustments can get them a little queezy. This is an inexpensive check off, and your local NAPA store can get them for you. Take a good look at the old plugs when you remove them also.

Make sure the main fuel filter is clear.

Take each spark plug wire and check it from one end to the other with an ohm meter. Leave the ends on so their resistors will also be checked also. Check the coil wire also. Make sure the ends are not separated from the wires, sometimes this happens under the boots and it is not obvious. If the car has those aftermarket carbon supression wires on it replace them. Anyway the plug wires should all be fairly close to the same ohm reading. If you get any that are way higher, then the wire or resistor ends are bad and should be changed.

An ignition coil can also cause a high rpm miss and work just fine at lower rpms. Do not just add any ignition coil to the car. It must have the correct ballast resistor to go with it. You may temporarily use another coil for a test with the existing balast resistor.

I will send you a pic of the IP rack access latter. Then we will get into the "split linkage CO test" to see if you have an injection mixture problem. I would not be surprised to see one of the fixes above solve your problem.

[pch2021]

Joe,
I have correct Bosch w5dco plugs in the car. They were sourced through Mercedes and are fairly new [less than 50 miles]. I am a stickler for using correct equipment- You and I talked long enough at Star tech so that you may have noticed that, but all of my basics- fuel filter, ignition componentry, etc. have been renewed when necessary, and tested for failure. One thing that would be useful is the means by which you test the coil. I'd like to know your Method.

  Anyway, my plug readout was good- no fouling, and no signs  of the plugs overheating due to lean operating conditions.

[pch2021]

  I did the split linkage CO test- the engine seems to do the following:

  at idle, if I increase the fuel input separate from the intake, the car idles down.

If I increase air , without fuel, the car idles up slightly, and then idles down.

  As I rev up the engine, at 3000 rpms, the engine will bog down if I increase air, and will increase speed if I add fuel.

  At 5,000 rpms, No matter what I do, it starts to bog down. If It's really happy, it'l stumble up to 6,300, but after a few se3conds, it will bog down to 5,100 with no load.

No matter what my inputs arem in the higher rpm band, the fuel and air quantiies seem to be balances, but It seems that the motor has to have more and more fuel as the rpms increase and is simply not getting it, no matte rhow much fuel I give her.

[ja17]

Hello Pierre,

Yes, the correct Bosch non-resistor spark plugs are fine, but they NLA in many places as old inventories deplete. Our group has has outstanding results from the NGKs. I like to use original parts also in most cases. On some modern Mercedes cars a single shock absorber (hydraulic strut) is over $4,000.00 each list! With prices like this on consumable items, owners will be forced to look at aftermarket products.

Different coils have different resistance values, and if the coil is intermittent, a simple resistance check may not show the problem. You can put the engine on a scope and try to identify the high rpm miss or just swap the coil and take a test drive (my choice). Is the coil original ? or an upgrade? Even an old non-transistorized ignition coil from a domestic car can work for a quick test. Do not leave it in after the test since the ballast resistor will probably not be correct.

A fuel volume and pressure check should be perfomed if the problem persists after the coil test.

[twistedtree]

Can you drive the car with reasonable success, even though you can't attain high engine RPM?  If so, I really think you have an ignition problem, not a fuel problem.  It takes very little fuel to red line an engine in neutral - way less than even modest driving around.

[reggie]

i once had this - turned out to be a blocked exhaust (muffler)
Reg

[pch2021]

  Turned out to be an ignition coil issue. Just for fun, I pulled out my ignition analyzer again, But I realized I needed to put the engine under a greater load to really see the issue, as the car was redlining well. I made some longer connectors to the analyzer and took the car for a drive.

   In the higher rpm bands, the needle did some funny things. I wasn't quite sure of what they meant, but with NO load on the engine, my dwell was ok. Now, as I accelerated the motor under a load, I was getting the same numbers, but the needle just bounced alot more, almost like there was sort of disruption in the current. The points were new, and the wire to the coil from the distributor was new, so I tried another coil.

   Bam. Problem solved. Nice analyzer readings, too.

So I ordered a new coil, resistor and cover from Mercedes. Thanks to all the great info on the forum, that car ran so well I thought it was a new car.