75 280S Backfiring through Carb and tailpipe

[1971 280SE]

Hello all, Car is running like crap barely stays on.. The car will only start if car if distributor is turned full advance.. I would like to check to make sure distributor is in right place. I believe my engine is the M110 DOHC.. Do any of you have any idea what size the crank bolt so I can turn the engine to line up timing marks?

[Rick76]

27 mm

[1971 280SE]

Thank you fpr that information :-)

[Frank Reiner]

First steps:
Adjust valve clearances
Check Compression

Then ignition, then fuel

[97 SL320]

What is the history of the car? Was it running fine then suddenly ran bad?

Random popping from the intake can be caused by lean fuel mixture / spark jumping under the distributor cap from tower to tower - from wire to wire / plug wires swapped between 2 cylinders.

Random popping from the exhaust can be spark jumping under the distributor cap from tower to tower - from wire to wire / plug wires swapped between 2 cylinders.

Distributor caps usually don't " crack " , what they get is " carbon tracks " on the inside that provides a conductive path to other cylinders at the wrong time.

If this uses ignition points, the gap may be way too small ( as in nearly non existent. ) .

Does this engine have electronic ignition / D jet ? If electronic ignition, what sort of trigger does it use? I've seen magnetic pickups in a distributor wired backwards / reluctor wheels in the wrong rotational position causing the spark to occur when the rotor is between towers.

[1971 280SE]

Hello there, no car was running crappy before. I sent the Solex 4A1 carb to a highly recommended re builder as I thought that was my problem but I installed it and it was the same thing.. I have a Pertronix installed in the car. The car will not even turn over unless the distributor is turned full advance. I will try to bring number 1 cylinder to TDC and see if the rotor pointing towards the #1 cylinder on the cap. Checked firing order and it is correct

[BWhitmore]

Make sure when bringing number 1 cylinder to TDC that it is on the compression stroke.

[1971 280SE]

will do..thanks

[97 SL320]

The trigger for the ignition might be in the wrong position relative to the rotor. This will cause the spark to jump the the prior / next cylinder.

Do you have a spare distributor / parts to turn the pertronix back to original. ( making sure the trigger is clocked properly on the pertronix wold be good as well )

[1971 280SE]

Well today I brought #1 up to TDC and rotor was pointing in corrext spot...checking compression next..

[1971 280SE]

Checked compression..All cylinders between 150 and 160psi.. Took out a spark plug while still connected to dizzy cap..Cranked car and spark was not in proper place..It was going between center insulator and body of plug..not in the electrode area.. Plugs in here are fairly new..My friend had this car before I did and changed them before selling to me..They are Bosch W7DCO plugs.. They had gas on all of them.. Cleaned them up and sanded a tiny bit..same result...I think I may get NGK's for it..Car has only like 66,000 miles on it..my friend was 2nd owner... Hopefully she will run again..

[97 SL320]

Did you check that the distributor trigger is in the proper place? Rotor position is only part of the picture.

[ScooterABC]

Just to clarify - when you are turning the distributor to full advance you are turning the distributor itself, right, by loosening the allen bolt on the side under the distributor? There is also a fine-adjust on some distributors that has a screw going down and a pointer that points to different ticks on a plate. Just want to rule that out...

[1971 280SE]

Hello all..thanks for all the replys..Yes I am turning the distributor..but theres no allen bolt..its a 10MM socket to take off..must have been replaced at some time..also im a little confused as to what a distributor trigger is...do you mean the Pertronix? As far as the pertronix goes theres one bolt and a pin to install and they both in correct place

[1971 280SE]

Hello..I was wondering if anyone can tell me whether that Ignition module mounted to left fender would cause little or no spark if it goes bad.. my friend thinks its just for Tach signal...I am unsure of exactly what it does..