I've now finished fitting the twin carbs to my 2.0 type 4. They've been stripped, cleaned and prepared as per the Bentley manual. It starts and idles nicely and revs fairly freely but on the road it's very hesitant between idle and higher revs and gets bogged down easily when pulling away (it's an auto). I've set the float heights, gapped the butterflies, set the mixture screws to 2.5 turns and balanced them with a snail air flow meter. I've also correctly set the valve clearances and ignition timing. The carbs are 34 PDSIT 2/3 The balance pipe between the 2 manifold stubs is fitted as is the 3 way idle circuit pipe, the vac port on the LH carb is blanked and the distributor is connected to the RH. It idles fine and runs fine at higher revs, it just boggs down between the two. Any thoughts
Watching this thread with interest....suspect its a accelerator pump issue... it's that hesitance between pressing throttle and engine actually doing something.
Is your vac advance working on the dizzy? Following from above, can you adjust the pumps on those carbs?
On an auto, there's an adjustable spring/lever on right hand carb. Mine was too far in and was causing a leak around diaphragm. Now further out and has hesitance...think it's trial an error to get it right.
yes the nut just after the spring lets the pump draw more or less fuel ready for the next press of the pedal
Thanks for the info, I'll have a look at the pump adjustment. It certainly looks to squirt fuel into both carbs when I open the throttle. I'll also check the vac advance with the strobe @F_Pantos , I disconnected it when I timed the engine so I'll refit it and see if there's any advance when I open the throttle.
Take a look at https://www.ratwell.com/technical/DualCarbs.html. You have a total of five air/fuel circuits. An extra idle circuit on the left that feeds through the balance tubes so its idling on a centre carburettor..then a couple of less used idle circuits left and right, then main carburettor chokes left and right. Could be its adjusted to idle on the left/ right rather than centre idle circuit and thats producing a flat spot..
Quite a good set up guide attached, if you ignore the vacuum retard parts that only seem to be US carbs/dizzies, misses accelerator pump set up too but think that's in Bentley
Whether this makes any difference I don’t know but it seems unusual to have the distributor vac connected to the right hand side. Mine is a German spec auto. If yours is US spec things get more complicated.
I don't remember ever seeing a vac advance connection on the RH carb. Maybe whatever it actually is, it's not providing the correct signal. I would try connecting your vac advance to the LH carb where it's supposed to be.
Interestingly my RH carb has what looks like an insertion point for a vac pipe but has never been drilled through. It might be possible to slip a hose on this but I’d get no vac advance. With a timing light and vac disconnected and blocked I get a max mechanical advance of about 32 degrees, but then with it connected it’s about 45 degrees. This is using the LH carb.
Should max out at 46 degrees at light load, drop back to 7 degrees at idle. Its why you need a ported takeoff from inside the carburettor- its positioned so the throttle plate blocks it off at idle, but as soon as you come off idle the vacuum starts to build, but it drops off again at wide open throttle. If you connect the vacuum advance straight to a manifold vacuum it will immediately pull another 12-14 degrees of advance at idle once hooked up. The revs will rise a lot immediately, so you wont see slow idling.
Here you go.. vac advance on a right hand solex.. this set came off the SA import Sent from my SM-G973F using Tapatalk
