hey all,
ive got a problem with the twin hitachi SU l20b in my off road buggy
stock l20b bottom end with l18 peanut head
motor ran fine for the last 15 years until i swapped in a bigger cam
unknown cam specs but its a stock regrind that took pretty sizable shims somewhere around 6mm if i recall correctly
the cam ran good in the old L18 the head and carbs came off, had enough power to spin 4wd tires in 3rd haha

it runs good around 3500rpm (at a guess no tacho) problem is at higher rpm it breaks down and splutters itself almost back to a stall and blows a lot of black smoke, also does the same at low to meduim revs under heavy load after a few seconds of running really good. is drive-able if you ease into it and put around. not really practical in large sand dunes seems to die just before the top haha
cylinders 1&2 seem to be running lean , 3&4 really rich (engine is mounted in the rear so #4 is towards the front)
new floats from Nissan, dizzy cap& rotor, leads and plugs
i have a feeling it needs bigger carbs but wanted too see others opinions before spending more $

the motor in question

I dont think you need bigger carbs.
What I would do is the EZ stuff first.
ck the valve lash
ck the distributor timming. and if still running pints make sure the dist shaft isnt wobbly.Bushing worn out. I carry a spare distributor and swap them in as its faster to trouble shoot.. Esp if you still run points and a condensor is bad. also ai seen the weitght or springs fly break fall off.
also a simple fuel filter could cause a cut out proplem in higher rpm. This happen just last week going up a hill on me. Tank get water down the fuel hose and rust the tank causing my idle jet to plugg up on my WeberDGV.

but since the rear carb affects cyl 3 and 4 seems like you got it figured out. I dont know nothing about SUs that they are hitting 40yrs old and maybe a Weber 38/38 is a better choice now(simple carbs). But lots of people like SUs.


also if running points ignition make sure you use a point coil with a ballast resisitor outwise coil will get hot and act like its cutting out also.

If motor was fine before the head swap then you must have installed soemthing wrong. Oil spindal? cam timming?