freshly rebuilt engine starts and runs great except stumbles when warm

[AllenF]

Hi Folks,

I have just brought to life a rebuilt 129 motor in my 66 230SL and it started and idles beautifully.  I didn't have to shave the head and it all lined up properly. I installed one of the Crane optical trigger systems, and while it's possible that that I have this mal-adjusted I am assuming I have it right given that it fired up and idled so well.  It runs great in warm up mode but once it gets hot it misses and stumbles and even occasionally backfires a bit as I rev it up.  It smooths out nicely at higher RPM's, but there's no load on it yet as i haven't installed the interior and can't drive it. The timing is set at 30 BTC at 3k, but it doesn't seem to make much difference to the rough and bumpy issue as it rev's up if I change it a few degrees one way or the other.   It has a fresh red coil and matching resistor. 

I completed the linkage tour, and none of the settings were off really after the rebuild.  I checked the fuel injectors and flow prior to buckling it all down and they all had decent patterns and flowed well.   I haven't attempted to adjust the mixture just yet given that the pump/engine ran OK prior to the rebuild but given that it performs so well at start up I am wondering if it's lean.  I though folks attempted to enrich them on weak engines but I am not sure about that.

It almost feels like the way a vacuum leak feels on a carbureted  engine, but there are no leaks that I can find on this engine.   

If I rev it up slowly, it's smooth as glass. 

I will post a video of the problem but my first attempt had the file too large.

Any ideas?  Thanks in advance!

Allen

   

[Benz Dr.]

It sounds like it's too lean. A fresh rebuild may require a different mixture setting due to increased compression but every engine runs a bit differently anyway so that's not always the case. Did you have your IP rebuilt when you did your engine?

[AllenF]

thanks for the quick reply.  Nope, I didn't rebuild the pump as the car ran pretty well prior to the rebuild.  I don't have any history on the 129 motor but the pump was not the original 230 pump.  I rebuild it because of the usual head issues and the crank bearings were pretty bad.  Sounds like I need to check the CO settings.

[Tyler S]

It does sound like a lean condition. You can do the "split linkage test" to confirm if it clears up by adding additional fuel to the mix.
However, just an FYI. When I had the crane system in my 129, it did not like the red coil setup. Resistance was too low. I ended up putting a new 1.8 ohm coil back in. Also make sure you have the red power wire for the crane box hooked to +12v ignition side of the ballast resistor, NOT the coil side.

[Benz Dr.]

30 degrees BTDC isn't enough timing. It should be more like 38 degrees with about 8 degrees at idle.

 Which engine do you have? You are calling it a 129 which is a 250SL while a 230SL has a 127 engine. If you do have a 129 engine, do you have the correct injection pump on it? I believe it should be a R15 pump and you might have a R11 pump installed from a 230SL engine. Each engine has its own unique IP and they won't interchange.

[AllenF]

Thanks for the guidance all.  I will check these items and report back.  This is a 250SE engine that was in the car when I bought it.  Sadly this car fell into the hands of some folks who were clearly not familiar with the 113.  They stuffed the 250SE motor in along with its motor mounts, and reshaped the hood to fit over it.  They also modified the shift linkage and mechanism, along with several other odd modifications.   So, there's plenty of forensic work going on here.   I can't get back out there tonight but you gave me plenty of items to check.   It did run fine before the rebuild (usual top end smoking and sub 150 compression) but it was smooth.

Allen

[RonDwyer]

My guess is your vacuum on the distributor is frozen.  Did you pull the breaker plate, clean it and lube with molybdenum spray? 

[ja17]

Check the fuel pressure when the engine is warmed-up.

[Benz Dr.]

Thanks for the guidance all.  I will check these items and report back.  This is a 250SE engine that was in the car when I bought it.  Sadly this car fell into the hands of some folks who were clearly not familiar with the 113.  They stuffed the 250SE motor in along with its motor mounts, and reshaped the hood to fit over it.  They also modified the shift linkage and mechanism, along with several other odd modifications.   So, there's plenty of forensic work going on here.   I can't get back out there tonight but you gave me plenty of items to check.   It did run fine before the rebuild (usual top end smoking and sub 150 compression) but it was smooth.

Allen

My 230SL also has a 250SE engine and it was that way when I bought it. My impression is that it's perfectly placed between a 230SL and the 280SL in terms of power and fuel economy. It sounds like they didn't know that the motor mount arms are different on the SL and left the sedan units in place which are too high making the valve cover hit the hood. That can be fixed - too bad though.......

[AllenF]

Hi Folks,

Thanks for all the tips so far.   The split test suggests that it's not running too lean, but i need to pick up a tach to get a precise measurement.  To my ear it's picking up a couple hundred RPM and I am not sure I trust the tach in the car.

I put points back in it to eliminate the Crane kit but no difference. 

The engine is a bit of a Frankenbenz possibly but since it ran prior to my rework I made an assumption it would run OK after...

The block appears to be 250SE 129 980
The head is 129 018
The pump appears to be an R18 if I am reading it right.
The advance is working and it looks like I am getting several degrees of advance at the time the throttle opens
I did enrich it a few clicks but no improvement

I am going to pick up a fresh set of points just to be sure to eliminate that, and probably go back to the old coil just for good measure. 

Joe, I will look into the fuel pump pressure tip.  Thanks for that.   


I posted the video on youtube, but not sure when it will be live.
https://www.youtube.com/watch?v=iXj6uEwQMSM&feature=youtu.be


Thanks again all for the tips.  Very eager to get this back together!
Allen
 

[Tyler S]

Just a few comments.  The R18 pump is the correct IP for that engine. Was IP timing set correctly when the engine was put together?
If you have put points back in, you cant use the red coil and balast resistor.
Adjusting the IP enrichment knob on the back of the pump only affects idle mixture, not full range. You may need to shim the baro compensator to add fuel for full range enrichment. Do the split linkage test again, but do it at say 2000 rpm. One more thing to verify is valve adjustment. If they are too tight, the issue only gets worse as the engine heats up.

[AllenF]

Thanks, I will double check the valve settings.  On the split test, how do you do that at 2000RPM?

[Tyler S]

You can disconnect the IP socket rod from the cross shaft ball. Then hold the two together without snaping it back on. Rev the engine while holding tension. Then you can push down on the rod going to the Ip to add fuel, or down on the cross shaft arm to add air. Its a bit of a juggling act but you can practice with the engine off first.

Another way is to put some sort of shim under the throttle plate/arm linkage stop screw to bring up the rmp. Then perform the split test as usual.

[AllenF]

Thanks, so should the rpms jump up about the same amount with a small change in opening movement of the throttle?  Like 150 to 200 rpm' ish?

Thanks again. 

Allen

[AllenF]

Update:

I adjusted the idle mixture a bit more lean and the barometric compensator and that cured its ill.   I will make some finer adjustments once I get it out on the road, but for now, thanks to all!

Allen 

[Tyler S]

Thats great! Make sure the engine is at full operating temperature when you adjust IP settings. Otherwise you will be chasing your tail as the warm up device will skew your adjustments/settings.