W200 318 misfire, I put my old 2bbl carb back on, the new Chinese one didn't work properly, now out of storage it's has developed a misfire at about 4000rpm foot down on petrol,
no misfire on LPG, took carb in bits found no problems. Beginning to think ignition problem under load. Any ideas?

 

That sounds like a classic spark plug breaking up. Remember, no AC plugs in these motors. AC will do just what you're saying.

 

Hummmmmm, I run A/C Delco R44XLS and dont have an issue... I must be lucky....

 

As for the missing issue,
You could be headed in the correct direction..... CNG and LPG systems need to have the Ignition systems in top notch shape or in very good working order...
Also, make sure the gas valve is opened all the way,,,, and the LPG pressure regulator is working properly and the system is sealed so it is getting the proper amount of fuel to the system...

 

I remember when I worked at the Dodge dealer we had a car in there that was popping, skipping and backfiring under a load. It had a fresh tune-up with AC plugs by the owner. The service manager who was really good said yank those ACs and replace with Champions. We did and off it went purring. This scenario over the next few years played out over and over. This was then of course back in the 70s and 80s. Maybe things and plugs have changed. It seemed to affect the big blocks more. I was running Autolites in my 1968 Mopar which ran ok. I changed to Champion J12Y and again it smoothed out and runs much better.

 

 

Thank you, I thought so, I will get a fresh set, they have been in 8 years but only 6000 miles.
doing body work on cab at the moment and fitting a 16inch electric fan to the front of radiator, it gets quite hot in warm weather parking the caravan on site 😀

 

Not saying that i have never ran into those kind of issues before as well,,,, Also dont want to come off that i'm correcting you..
And I do agree, Spark plugs can be finnicky, I do agree with that..
But like i said, I must be the lucky guy with the usage of A/C Delco spark plugs,

Cheers

 

The AC plugs of that time 70s 80s certainly could be different from the ones made today. No worries as to correcting me or suggesting something. I learn something new everyday thanks to this forum and good people like yourself. Take care!!

 

yep today the plugs are made in china by an eight year old.