low RPM troubleshooting

[n/a]

Hello!
My '71 280SL(european,manual)starts OK, and everything's all wright when cold.With warm engine, idle speed is normal(~800 rpm) and constant, but low rpm driving is irregular ("hiccups") at half-load(i.e. in 2nd or 3rd gear).This does'nt sound like I got an ignition trouble,but rather fuel injection problem, as if there was no more engine flexibility. With higher rpm, engine just runs smooth.Someone told me this could have to do with igntion coil.Tuning the FI pump by a specialst(Bosch) did not solve the problem.
What do you suggest?  Are these symptoms of a worn-out FI pump?
Is it normal to hear a little "hiss" when decelerating?
Thank you for advice!!

[Naj ✝︎]

goldSL,
Check the ignition timing. At idle without vacuum it should be 8 degrees bTDC but WITH vacuum it should be 3 degrees AFTER TDC.
It has a vacuum retard distributor and if the vacuum system does not retard the timing, you will get these hicups.
The 'hiss' could be an air leak which may lean out the fuel mixture and cause similar problems - unless the vacuum pipe is not connected??
naj

naj
'Kloines Scheisserle'
65 230SL
68 280SL

[Ben]

Maybe you have a vacuum leak on the manifold somewhere. Check all pipes and connections for security, maybe the vacuum advance is tube is damaged or disconnected ?  Also the pipes and connections at the other end for Brake Servo etc ?

The fuel injection systems are p-retty much unburstable. They either work or they dont. It sounds more like an ignition fault to me !  Check everything is in spec before replacing anything, DWELL, timing etc !  And check those leads !

Regards,
Ben in Ireland.
'64 230SL 4sp.