I have a 91 crx si with a b18b swap converted to obd1 with bolt ons of h/i/e/crank pully/blox intake mnaifold/fidanza flywheel/ exedy clutch stage 1, and I am getting horribly gas milage since I did the motor swap! I get like 260 to the tank if that! I have replaced the fuel filter about 7000 miles ago, what should I check or do to fix it something got to be wrong to get that horrible gas milage?

 

That doesn't sound that horrible. about 26 mpg. Is that all highway or city.
I get just around 30mpg w/D16a6 all city driving.

Things you can check...
O2 sensor
IAT sensor
ECT sensor
FPR
Thermostat

 

How hard are you driving it? If you're stomping on the gas, it'll go faster. City /HWY like smogman said? I usually get around 300 +/- 50 out of a tank depending on those two factors. Otherwise you can try checking your injectors and the sensors mentioned.

 

I do stomp on it a little, How would I check the sensor and injectors?

 

curious to know how the thermostat effects milage? ithink mine is stuck open and im getting bad milage compared to what i used to get, about 650-700km a tank now i get about 400-500km a tank but with a little bit more city driving then i used to do.

 

i get about 30mpg average on a completely stock b16 in a crx hf but i drive it normal so yours doesn't seem to bad considering you step on it and all the upgrades

 

Stuck open T-stat will keep your car in open loop(Not responding to sensors) longer. Alot longer. Because the ECU is thinking the motor is still cold. During this time you are running off a set fuel map which is rich.

This does lower your mileage by two to three mpg. My mileage has increased by three mpg's since I did the T-stat. And the engine warms up in three blocks instead of three miles.