hesitation / misfire under light load

[Neil Thompson]

Help please!. my 280sl Auto, with warm engine I have a hesitation/misfire under light load between approx 2500 - 3000 rpm before changing gear. It is hardly noticable performance wise and can be driven through, but is more noticable with the 'burbling' noise from the exhaust on light, steady acceleration and also slight 'popping' on overrun. This is very similar to Naj's post of 1993 ( http://www.sl113.org/forums/index.php?topic=177.msg605#msg605 ) Naj, did you sort it or do you still have the problem??

I personally feel the problem is timing related as following my timing chain renewal I adjusted the ignition timing to 30deg BTDC @3,000RPM - STUPIDLY I didn't check the ignition timing before chain renewal and noticed that I retarded the ignition timing. This gave me a slight lag/flatspot on takeoff. I have advanced the timing slightly in stages on the roadside without a timing light and the flatspot has gone and the misfire less evident, Benz dr recently posted 38deg BTDC timing, is this correct as I think it will reduce the problem but I don't want to cause engine problems/overheating.

Any help appreciated

FYI, electronic ignition, new plugs, copper leads with all good 1kohm caps (swapped to eliminate) new fuel filter, clean air filter etc, etc...

Neil

[stickandrudderman]

Try pre-loading the injection rack by lengthening the control rod. This will introduce other problems but will help in the diagnostic path in that if the partial load hesitation is gone it proves that you have a fueling issue.

[Neil Thompson]

thanks Stick, will have a look tomorrow, does this make it richer or leaner?

Neil

[twistedtree]

Depends on how you adjust it.  Normally the IP lever and throttle lever start to move at exactly the same time so air and fuel are regulated in tandom.  If you adjust so the injection pump lever moves before the throttle flap opens, you will be enriching the mixture.  If the throttle opens first, it will be leaner.  You can experiment with both, but my money is on an enriched mixture solving your problem.  Do let us know what you find.

[wwheeler]

You say you just installed a new timing chain and now the problem is just showing up?
Are you sure the cam is in time with the crank? It is also possible the IP pump slipped time during the change. Ask me how I know! The IP and distributor run from the same sproket. So if the distributor was off after the change, there is a good chance the IP timing is off as well.

[Neil Thompson]

Thanks Wallace, nothing slipped when the new chain was fitted, 3 of us gently kept things tight and fed the chain round as per posts on the site. All marks were noted before and afterwards and were as they started and the car struck up perfectly. The problem only showed up after I set the timing electronically afterwards during routine tinkering etc., but I noticed I did retard the timing a few degrees but foolishly I didn't note the ignition timing with a strobe before and after the chain change.

Neil

[dwahi]

Looks like few of us having this issue all at once!

i have exactly the same issue, now identified to be the warm air device (?) ie the choke is not closing when warm, mixture being rich and the car not driving as it should. It went low on power, a lot of hesitation and burbling and popping. Its going in to the garage for further diagnosis but 99% it this device which can happen as some of our cars are not driven too much...

Ill feedback once I hear from the garage

[Neil Thompson]

Stick - good advice, I double checked my ignition timing and slightly adjusted. I also checked the idle co., a little rich but stable and not enough to worry about. I then had a look through the linkage tour on the site and checked a few things all looking good except the length of the fuel injection pump control rod - this was a few mm longer than recommended in the linkage tour so, against your advice, shortened the rod 4mm, off down the road and the problem was exaggerated and the car was awful to drive, I pulled up on the roadside and adjusted it the same the other side of original and tried again. Perfect!,, no hesitation or burbling, car drives great.

I'm not sure of the side effects you mentioned but I can't see any!, no doubt this is the area that now needs fine tuning as I've only guessed at the setting, what's the next step?, please.

Neil

[twistedtree]

So that means you were running lean.  The correct fix, now that you know what's wrong, is to restore the linkage to it's correct geometry, and make the mixture adjustment in the injection pump.  But, before doing that, I'd suggest a thorough check for vacuum leaks which is the other way to get a lean mixture.  Spray WD-40 of the like around the manifold to head joint and listen for changes in RPM.  Disconnect and plug each vacuum line to check for changes, especially the vacuum line to the brake booster.  Old hoses and boosters are prone to leakage.

[Neil Thompson]

Thanks Peter but if I had an air leak would it not show on idle? My idle is perfect and smooth.

Neil

[twistedtree]

Thanks Peter but if I had an air leak would it not show on idle? My idle is perfect and smooth.

Neil

Maybe, but maybe not.  Idle mixture gets set with the air bypass screw, and in doing so you would be compensating for any vacuum leak.  That would mask the leak at idle, but it would still surface at other load points.  Hoses, brake boosters, gaskets, etc can deteriorate a lot over 40 years and start leaking even though they may look to be in good shape.  You will be chasing your tail trying to tune the engine with a vacuum leak, and things like injection mixture screw adjustments (what you need to do next) are a PITA to access, so you only want to do it once.  At this point, it seems just as likely (perhaps more likely) that your IP is set correctly and you have a vacuum leak.

Anyway, I could easily be wrong.  This is all about making educated guesses, experimenting, and the process of elimination, and lots of paths can lead to the right answer.

[Neil Thompson]

Thanks Peter, wise words I'm sure. At the weekend I will isolate the brake servo and have a spray around the joints just to eliminate these parts.

Neil

Had a little spare time tonight and disconnected and isolated hoses as suggested, also sprayed wd40 around gasket areas and no changes in revs at all. Had a good run with steady driving, driving hard through the gears and town work and the car runs and sounds a treat. I think I'll leave well alone til after the European Event in a couple of weeks and see how it goes.

Thanks for everyone's input

[Eminent]

Ill feedback once I hear from the garage

Dwahi never get his car back from the garage... 

[dwahi]

  I did and sorry forgot about the post! It was re tuned with the sundries changed too, now running as before not sure exactly what it was...