QuestionMr Fitzcharles,
According to your profile one of your areas of expertise is USA Midgets. Although I don't own a Midget I just bought one of its "sisters"-- a ’69 Austin Healey Sprite MK IV, 1275 engine (original box-type Sprites are hard to come by).
Anyway, A previous owner(s) replaced the original SU carbs with a Weber DGV 32/36 two barrel. The problem is I cannot get the engine stay on without the manual choke almost fully engaged; the engine will simply not run unless the barrels are almost closed off by the choke plates! It's weird! As soon as I back off on the choke and disengage the choke plates increasing the volume of air into the barrels the engine becomes unstable and simply stalls out. Of course, while the choke is engaged it’s racing like crazy (over 2000 RPM). The car may have been sitting in storage for a while so I tore the carb apart, purchased a basic weber rebuild kit, cleaned it good and reassembled. In the process I changed the needle valve assembly that controls the float and accelerator pump diaphragm (only because these parts came with the kit so I assume they tend to wear out quicker??).
I also changed the fuel filter and added a fuel stabilizer to a fresh fill-up of gas. But it didn’t do any good. The car is behaving exactly the same way as before the rebuild. Since it’s behaving the same way as before I assume that I didn’t make any screw-ups when I rebuilt it.
I adjusted the idle mixture screw and the throttle idle adjustment screw every which way and followed the instructions as best I could to set idle but the result is the same. The carburetor seems to be locked in this “choke cycle”, for lack of a better way of putting it. And even though it seems to be a carburetion issue I changed the plugs just to be sure that it's getting a good strong spark.
Have you heard of any problems associated with installing aftermarket non-SU carbs (especially weber) on the 1275 engines of this particular year? My understanding is that Weber is one of the best designed carbs on the market (so I've read).
If it's a vaccuum problem everything appears to be pretty tight as far as I've checked. I made sure the bolts holding the manifold to the head were tight although I didn't apply a torque wrench to it (and I'm not even sure what the torque would be for that application. I'd have to look it up).
Is it possible the engine isn't getting enough fuel? My line of thinking on this is that in absolute terms even though there's not enough gas entering the carb, in relation to the volume of air it will stay running provided the choke is engaged and limiting the volume of air. But once the volume of air increases, then the relative lack of fuel isn't enough to keep the engine running. Again, it's just an educated guess on my part. Sorry if I sound like a first-year auto tech student but in a way I am a student!
I’m stumped! Any ideas what this could be? I admit that I’m a novice and I apologize if I didn’t use quite the correct jargon in explaining my problem but as an owner of two Miatas (that I love also), carburetors are a whole new world for me although I admit that I’m really enjoying it! I loved tearing apart that carburetor. It was fascinating! Now if the car would only idle properly!
Also, I want to check the fuel pressure but most of the diagnostic equipment available at the auto parts stores are calibrated for the modern fuel-injected cars which seem to run at a MUCH higher pressure than these terrific old cars. Bottom line is I can't find any equipment that's properly calibrated to measure pressures in the 1-8 PSI range rather than 30 or 40 PSI! Is it true that most modern cars have fuel pressures that are that much higher? Again, please excuse my beginners ignorance!
Any advice you could provide would be most appreciated.
Thanks!
Dave
AnswerHi Dave,
To start with no engine can run at 2000 RPM without a lot of air going into the intake. If you have the choke on, you have restricted the air. unless you have the other throttle plate open at the time.
As far as fuel pressure you should not have more than 1.5 to 3 PSI Max. Ask to look at a vacuum gauge. Most vacuum gauges had a section for carburetor fuel pressure on them. You need to test intake vacuum anyway. With throttle closed and cranking you should see good vacuum, which I am guessing you will not on your car. Just a guess.
There is nothing wrong with a two barrow down draft Weber for mild street cars, but it will not out perform the twin SUs that were on the car. Anyone who told you it would, didn't know anything about SUs or what they were talking about.
Howard
