I am trying to use a pair of SU from a 240 Z to replace the ones that came on my Fairlady Z (1970). After I had them bolted up the car was hard to get started, but once it did it ran like it was at WOT. I tried backing off everything, but it didn't help. :disappoin

Does any one know what may be the cause, and how to overcome it?

Start by making sure you've got both ends of the interior linkage capturing the linkage on BOTH of the carbs..... You'd be surprised how often that gets overlooked.

Are the return springs hooked up?

linkage was correct and the springs engaged. could it be that they are just too big? my Fairlady engine is only 2000cc.

Start by making sure you've got both ends of the interior linkage capturing the linkage on BOTH of the carbs..... You'd be surprised how often that gets overlooked.

Are the return springs hooked up?

The linkage was correct and the springs engaged. could it be that they are just too big? my Fairlady engine is only 2000cc.

Disconnect the upper ball end of the linkage from the throttle rod to the carb linkage, and see if the idle drops. When I converted my 260Z from flattops to SUs the linkage was too long and caused the same problem. Turns out I needed to use the thick carb spacers from a 240Z instead of the 260Z spacers. Changing the spacers increased the distance to the carb linkage and made the fit correct.

Anyway, if you disconnect the linkage and the throttle return to idle, you'll know if that it is too long and won't allow the carb throttle plates to close fully. Good luck!

Disconnect the upper ball end of the linkage from the throttle rod to the carb linkage, and see if the idle drops. When I converted my 260Z from flattops to SUs the linkage was too long and caused the same problem. Turns out I needed to use the thick carb spacers from a 240Z instead of the 260Z spacers. Changing the spacers increased the distance to the carb linkage and made the fit correct.

Anyway, if you disconnect the linkage and the throttle return to idle, you'll know if that it is too long and won't allow the carb throttle plates to close fully. Good luck!

Thanks for the suggestion. I had tried that, and it did not help. I even looked inside to see if the throttle plates were closed - they were. :disappoin

I did reuse the old spacers that came with the carbs (yes I did rebuild the carbs before I put them on). I am wonderiing if a vacume leak at the spacer plates could cause this, or as I stated earlier, could the larger ventrui size be the cause?

Larger venturis should not have anything to do with it. They might run rather rich, but shouldn't affect the idle speed significantly. Are you certain the throttle plates were closed, or were you seeing the pistons being down?

Larger venturis should not have anything to do with it. They might run rather rich, but shouldn't affect the idle speed significantly. Are you certain the throttle plates were closed, or were you seeing the pistons being down?

Thanks, that was the info i was looking for. I actually pulled the covers and pistons out to look at the throttle plates because i couldn't get a good look at them with a mirror.

I think I am going to try new gaskets on the spacer plates and try again. I don't beleive that a vacuum leak at the plates should would caus this problem, but I want to get these guys on the car again and see.

The twin 46mm 240 Carbs are well oversized for anything the L20A will ever need.

What was wrong with the HG38's you removed? They will power the L20A just fine.

Did you add the four studs so the 46mm units are properly bolted down and sealed, or are you still using the twin studs of the 38's?

As I recall, the inlet manifold is also smaller where the carb throat enters... this may create a big step the fuel will have issues with at higher RPMS.

Running rich should not be an issue, you can always let the needle out of the suction piston a mm or so to stick the needle into the jet a bit further, if required.

But the manifold opening, studs, and etc will make for a miserable running car. I have seen people do the same thing on L20B's with SSS carbs and eventually they go back to the 38's. In the L20B four cylinder's case, the real trick is to use the Fairlady Z jets and needles in the SSS carbs for proper fueling to around 110HP to the rear wheels (not bad for a four banger!!! Or an L20A for that matter!)

The twin 46mm 240 Carbs are well oversized for anything the L20A will ever need.

What was wrong with the HG38's you removed? They will power the L20A just fine.

Did you add the four studs so the 46mm units are properly bolted down and sealed, or are you still using the twin studs of the 38's?

As I recall, the inlet manifold is also smaller where the carb throat enters... this may create a big step the fuel will have issues with at higher RPMS.

Running rich should not be an issue, you can always let the needle out of the suction piston a mm or so to stick the needle into the jet a bit further, if required.

But the manifold opening, studs, and etc will make for a miserable running car. I have seen people do the same thing on L20B's with SSS carbs and eventually they go back to the 38's. In the L20B four cylinder's case, the real trick is to use the Fairlady Z jets and needles in the SSS carbs for proper fueling to around 110HP to the rear wheels (not bad for a four banger!!! Or an L20A for that matter!)

Thnaks, that is really great info. the reason I am rplacing the 38's is because of a vacume leak at the throttle shaft. I am assumming it is wear on the shaft. I know ztherpy can fix them, but replacement with these seemed less expensive.:nervous:

Hey Mardman, what color is your car?

Hey Mardman, what color is your car?

mine is orange - that bright orange that only datsuns can do

:classic: