Stumble and no power
 

The mechanic checked the car today. It seems the ignition is ok and he also set the timing right. The car idled better, but still wouldn't accelerate right. He tested the compression and it's 7.5bar all around. I have a 4-port 13B and a Mikuni carb. Dunno which type and I can't find it anywhere, but I think it's a 44phh.

When I hit the gaspedal it stumbles and wants to die, but usually picks up 1-2 seconds afterwards. When this happenes it does accelerate abit, but not very well. I also get this when slowing pressing the gaspedal.

I noticed there is no FPR, but would I really need it to solves this problem? Now there is a fuelline->carb->fuel returnline. Am I maybe loosing to much fuelpressure, because there is no FPR?

The carb itself doesn't seem clogged or anything. Maybe different jets is the solution? What fuelpressure does a Mikuni want?

BTW the imprint on the carb is 5428 2. Does anyone know what this means?