MatraSport Forum

I've just been to check my car and it appears that it is using twin Dellorto DHLA45s!!

No wonder it is using so much fuel.

I spoke to the garage that fitted them, Sports Cars Unlimited, and they claim that, after much testing, the 45 is best one to use.

Not sure myself. When I floor it at low revs, it doesn't like it and will only take full throttle at higher revs.

I guess I really need to get them serviced and on a rolling road.

Anyone else on 45s?

Nope.. 40mm here.. they work fine.... as long as you keep one the throttle.... cruising it tends to spit and misfire... think the fuel pressure is wrong. 

Unless the float is overflown, fuel pressure has no effect on a carb as the fuel mixture is entirely flow controlled.

My Murena 2.2 was also on 45s, also from SCU. When I had the new engine dyno'd they decided it was better on 40s (they did say why but it went over my extremely non technical head :)). Still trying to get the video.....

I'll try to explanation from what I know - using a large bore carburettor allows more air to pass through, but the air speed (especially at low throttle) will be low compared to one with a smaller bore.  It's like when a river narrows in, the water flows quicker. Air speed is important for getting the fuel out of the main jet and into the venturi, with low air speeds it simply does not work well enough. For this reason, the carburettor has the progression holes in the side connected to the idle fuel line that allow fuel to enter the base of the venturi at low throttle openings where the main jet isn't working well, but all this has to mated to the engine.

Another thing is that the inlet ports in the head are 40 mm diameter, so I think right on that 45s would be a strange choice since it would only generate turbolence when the different bore sizes meet?

- Anders