Cam timing or tappet clearance? the wilder they get, the more sensitive they are to being put in correctly, and they need a lot more care and feeding, things like decent compression and spark...

my mte had same problem .... my ute could get up 2 160km stock with a12 in it but took a while 2 get there! i think its prolly got 2 do with gear ratio! of wear of motor!

Top speed has little to do with weight of the car. It is mostly about areodynamics, friction, and horsepower.

my 1200 is in a beach buggy that only weighs +-600kg so i dont see why it sould stop at +-140kph, i'll check my dizzy tomorrow to see if it's advancing fully.

the new zealand 1200 sss was quoted a top speed of 100mph a
acceleration standard 1200 as tested given in brackets
0-30mph in 4.6s (4.8s)
0-40mph in 6.2s (6.1s)
0-50mph in 8.5s (9.4s)
0-60mph in 11.9s (14.0s)
0-70mph in 15.2s (20.5s)
0-80mph in 21.1s (28.0s)
0-90mph in 30.6s (N/A) because it is near the top speed

standing quarter mile
1200sss (18.1s) std 1200 (18.8s)

A stock A12 will only push a sedan or coupe to 140 kph (88 mph) according to Nissan. Remember that speedometers can be way off, from the factory could be 10% off. Some A12s may have had a few extra horses due to production tolerances, so could go a liitle faster.

With a Weber carb, I would expect a few more HP and so be able to hit a little higher than 140, like maybe 142, but in any case not slower than the stock carb. Aerodynamics is the big thing here, and takes a lot more HP to go a little higher top-end speed.

I've had problems like this before, but I noticed it even earlier, 3rd gear dogging out at upper rpms...and then 4th and 5th would just stop pulling. For me it was the carbs (Dellorto's). First, they were out of tune...second, I had a float problem, third some junk had gotten into the fuel bowl and was clogging up some, but not all, of the jets. At the same time this would happen, my Air/Fuel ratio guage would start dropping off lean. Solution: electric fuel pump, regulator set to 2.5 psi, cleaned out carbs/jets, new fuel filter, and fixed the float problem (sticking low), oh and tuned it a little with larger idle jets. Problem was solved, but occasionally it still does it...I think it's the float still, sticking intermittently. If you are positive it's not your carb, do a good tune up, change cap/rotor/plugs/wires, check valve lash and timing, make sure everything is working like it's supposed to.

A few months ago I was driving my car around, and it just seemed like it wasn't as fast as it used to be, didn't rev as nicely, etc... occasionally I heard a little ticking at idle, but nothing noticeable. I randomly checked my oil at night in the dark and saw some sparking, and at the same time heard that "little tick" that it made intermittently...I had spark arcing to an electrical ground from the cap....then I grabbed one of the spark plug wires and got zapped. Lifetime warranty plug wires...crapped out. I bought a cheap set and it ran like it hadn't for months! What a difference. Good luck finding the problem. Even my stock A12/coupe would go over 95mph. Now I get mad and start wondering what's wrong if it won't get over 120mph....heheheh

i'm running a 1600 box, it pulls strong in 1st, 2nd and 3rd it's just 4th i have my problem, it wont go past 4200Quote:

Have you tried looking at your gear ratios you have lots of oohmf until you hit 4300 and after that it dies,what beatle box are you running on your buggy?

TRY holding your hand out at 140kph, wind is trying to push hand to rear of car. THINK hard about it DATO is not shaped like CONCORD , is shaped like BRICK but BIGGER.
For every kph you need X amount of KW(BHP)