Carb Conversion Motorway Running Problems

[ULTIMA69]

Hi We have reacently fitted a 1380 motor with a HIF 44 carb into a Mini Sprite SPI. The high pressure fuel pump for the fuel injection has been replaced by a standard electric carb compatible fuel pump, and the tank replaced by one out of a carb car. We are still using the same line (one of as no return for carb) to take fuel to the front of the car. The car runs really well, until you run down a motorway / cruise over 60mph for a bit of distance, and then it suffers from massive lack of power (fuel starvation?). If you stop and then restart its fine or if you keep below 55mph its fine. We have replaced the fuel pump with a new one, fuel flow/pressure 4psi? at the carb with car static looks more than adequate. Does anyone know if the original fuel lines have restrictors or any none return valves in them which may cause problems? Or anyone had similare with this set up? Dave

[guru_1071]

dave

have you checked to make sure the tank is venting? - if not (and those later cars did it in a complicated way) then it may be that a vacuum is forming on a prolonged run and is defeating the fuel pump.

a way to check would be to force the car to do it and have the tank key in the lock ready, jump out and whip the cap of quick and see if you can hear a noticable 'hiss' as the pressure eqalises.

if you can you could probably try driving it a short distance with no cap on to see if this helps (caps can be either vented or non).

if the tank isnt the issue then id guess its a carb problem - hif38s are trouble on bigger engines when running at constant speeds - particually if pancake filters are used, but a hiff44 should be able to cope with a 1380 fine

[jagracer]

Is the pump up to the job, you need flow rather than pressure, I would think with a Mini you wouldn't have much problem but it's worth a check. You may have some crap in the float needle, the fuel level too low or some other problem with the carb.
Start with the simple stuff first such as mentioned with the tank vent and go from there.

[ULTIMA69]

Ok I will check tank is venting. Then have a look at carb. Dave

[Scrooloose]

Surely if it's a fuel starvation issue, then it will be related to engine speed, not road speed.

Try driving in a lower gear for a while, but at the same RPM that you see above 60 mph and see if you get the same problem.

Alternatively, just sit still with your foot on the gas!