1983 MB 200 (Carb) surging every 3 seconds to a lower RPM then up again!

[Haroundb]

As said in the title, the car surges every 3 seconds as if it is going to die but it doesn't. This is noticeable when in idle and gear in D position. Not very much noticeable when gear in N or P. Why this odd behavior, and how can I fix it?

 

[IAN MAC]

Sounds like its wanting to pull away.

 

[tom7035]

This is known as 'hunting' and caused by incorrect fuel/air mixture. I'm not sure what carburetter is fitted, but if it has a neophrene diaphragm, this is probably split.
Certainly sounds like a carburation fault.

 

[Ultymate]

If it's a stromberg carb then as the above post says it may well be the diaphragm that's gone, they're cheap to buy and you need'nt go to the dealers for one. The only downside on the Mercs and Volvos that had these carbs is that if the mixture needs adjusting after renewing the diaphragm then a special tool is needed :???:

 

[Haroundb]

If it's a stromberg carb then as the above post says it may well be the diaphragm that's gone, they're cheap to buy and you need'nt go to the dealers for one. The only downside on the Mercs and Volvos that had these carbs is that if the mixture needs adjusting after renewing the diaphragm then a special tool is needed :???:

Which tool you are talking about? Can you please elaborate more?

 

[Ultymate]

If it is a Stromberg carb the mixture is adjusted moving the needle up and down in the piston of the carb this is done by means of a two part tool the inner part is a long allen key and the outer sleeve has two notches which lock into grooves in the top of the piston to stop it rotating whilst making adjustments with the allen key. If you are going to attempt this care has to be taken as there is only limited movement and screwing it too far renders the assembly U/S and trying to do it without said tool will shred the diaphragm.It may be worth fitting a new diaphragm first and seeing what the engine runs like as the mixture won't always need adjustment

 

[Ultymate]

Here's a picture of the tool in a kit from www.tooledup.com the one I'm on about is the one at the top of the box nearest the lid hinge
I've just read your original post again and you say the idle is fluctuating, this is not usually symptomatic of a punctured diaphragm, usually it leads to poor or no acceleration at all but the engine will as a rule idle perfectly all day, so you may need to be looking for a different problem