Back fire

[n/a]

hi,
i live in malta (europe) and have a 280 sl 1969. I have recently and
still am in the process of totally reconditioning my car. I have
reconditioned the engine (ie reboard it : new: sleeves/pistons/rings/chain/rocker arms/overhead camshaft/oil pump/
cylinderhead/main electrical distributor and coil and reconditioned my
bosch fuel injection pump). In spite of this huge effort on the engine, the car starts fine and drive fine until it is stil cold. When it warms up after driving it for a distance of 10 km and is driving at a low
speed around 50km/h without pressing the accelerator it backfires. This does not hapen when the car is under acceleration or cruising at a higher speed ~100 km/h. I have a hunch that it might be the timing but i am open to any suggestion. In addition what should be the correct timing for it?
thank you
john j borg
280 sl Malta

[Cees Klumper]

According to the tabellenbuch, 1969 edition, 280 SL/8 engines with normal compression should have Bosch distributor no. IFUR 6 0 231 116 051 (MB part no. 001 158 02 01) and timing should be 30 degrees BTDC at 3,000 RPM (with vacuum advance disconnected). You need to have a strobe light. Also check whether you have the correct and properly functioning spark plugs and spark plug leads (1K ohm resistance) and ballast resistor, coil etc. Sometimes the distributor itself has been replaced with an incorrect type or it is malfunctioning. You may want to consider replacing the points with one of the electronic alternatives, which has many advantages. Also, please check out the technical articles on this site, which contain many tips and a lot of info about (troubleshooting) engine performance. You should also check whether the cold start thermostat on top of the FI pump is operating properly: remove the small air filter on the side of the pump - when you start the car cold, there should be a loud hissing noise that should go away completely when the car warms up.
Let us know how you do.

[Naj ✝︎]

If Cees's checks do not cure the problems, I would check the valve timing. If the air was not bled from the chain tensioner when refitting, the vslve timing could be wrong.
naj

naj
hobel 65 230SL
jalopyno 68 280SL

[George Davis]

Someone may have to correct me here, but in addition to the 30 degrees BTDC at 3,000 rpm that Cees noted, I think timing at idle speed should be 3 degrees ATDC with vacuum connected.

Also, it sounds like the engine is running lean when it reaches normal operating temperature and the warm-up system is no longer enriching the mixture.  I suggest checking for correct linkage adjustment, as misadjustment can cause a rich or lean condition.

George Davis
'69 280 SL Euro manual

[Cees Klumper]

One more thing I thought of - Rodd Masteller had a rough running issue after replacing the small air hose going from the air cleaner top to the idle mix screw on the engine. It was not fitted tight enough. When he corrected this, his problem disappeared. So you may want to check that little hose.

white 1969 280 SL

[n/a]

i have now managed to get the car back from the mechanic he has adjusted the timing and trottle linkage connectors according to the haynes manual, and changed the points. it seems that the car performance has improved but unfortunately it still backfires slightly (low noise) upon decelleration. I am now looking into replacing the exhaust as its over 30 years old!

[George Davis]

Job,

I think a little popping and sputtering in the exhaust is normal when decelerating.  My '69 280 SL is a Euro spec car, runs very well, exhaust system is in good condition, and it always pops and sputters a little on deceleration.  I have read that this is caused by a temporary lean condition that occurs during deceleration.  Lean mixtures burn slowly, and often are still burning when the exhaust valves open.  This causes the popping.  I like it!  

George Davis
'69 280 SL Euro manual