Rough running

[maton_acoustic]

Hi, I'm having issues with my 1978 Datsun 260z. It's been sitting for 5 years during welding repairs and what not. Managed to get it to pass an MOT today but it's back firing and stuttering and generally doesn't pull well at all. The carbs are hs6's from a Volvo which the po installed. I've installed an xr700 electronic ignition kit and a ps90 coil. New plugs and leads. New fuel line and filters. Changed the su needles and jets and float valves and springs. The float levels aren't adjustable. Fuel bowls have been 3/4 full when I've checked them. I've set the timing to 10btdc with vacuum advance connected and it does advance when I blip the throttle. Checked for vacuum leaks and compression is good. The dash pots do seem to leak oil quickly though.

When I adjust the car by the colour of the fuel mixture using a colourtune the mixture nuts are around 3 flats out and if I lift the carb lifting pins it sounds like it wants to stall and rpm decreases. I've tried the car at 12 flats out and it does seem to run better and smell less, engine rpm stays the same when I lift piston. I can get it to idle ok But still similar when driving. All and all quite contradicting to me.

Im doubting wether these carbs are suitable for the vehicle at all. I'm trying to find a specialist at the moment who could take a look but there's a lack of them here. 

Any ideas appreciated

Edited August 19, 2016 by maton_acoustic

[siteunseen]

Backfiring usually means it's a little rich.  Have you checked to make sure the chokes aren't hanging up, are they shutting off and moving freely?  Sometimes the the push/pull cables get loose and they won't shut them completely off.  It may be in that linkage.  Just guessing, good luck.

[SteveJ]

The HS6 carburetor was used on B18 & B20 engines. Since this was a dual carburetor setup on the 1.8 & 2.0 L engines, I have to wonder if they can flow enough to make the car run right. If someone cannot provide you a solution that allows you to keep the current carburetors, you might want to buy some round top SU carburetors from the US.