Stop and Go Traffic with Carb Car

So I swapped in the Pierburg pump this weekend and opened up the return line. In short the car drives much worse. It will run ok for a short time at low revs and low load. But if you try to accelerate the car sputters and acts like it has run out of gas. It seems the Pierburg pump isn’t supplying sufficient pressure with the return open. I checked flow out of the return and the flow is really good. I’m wondering if the return flow is too much and now I don’t have enough pressure to fill the float bowls. With the Hardi pump and return blocked, the car ran terrific except stop and go in car pool. Now it can’t even make it two miles down the road without sputtering and shutting off. I had bought the car as a non runner. At the time it had the original Corona pump and the return line was blocked. Thoughts?

 

No safety relief plumbing. All of it was gone when I got the car. Mine goes from the tank -> filter -> Pierburg pump -> carbs -> return to other tank. Do you think I need to find a way to built the safety relief. I tried measuring the pressure down at the pump and with the return open it’s only 2 PSI. I’m thinking the outlet on my last carb that goes to the return line might be too big. 2 PSI at the pump seems low. Lots of flow out of the return line but very little pressure.

 

That’s good to know. I’ll leave that part alone then. I checked the orifice just now. Couldn’t get my caliper in there with the airbox on but the orifice looks big. By eye, it’s at least half the diameter of the inside diameter of the line. So probably 4-5 mm or so. How small should this orifice be? 1-2mm? I also noticed that if I take the line off the pump itself, the flow at the open pump seems nearly the same as flow out of the return line so it seems the orifice is not restricting the flow. There is also no change in tone when turning the key on after the floats fill. The pump doesn’t slow down or change tone as if it’s working harder or under pressure. I’m guessing I need to restrict the return flow. As I don’t have a different orifice, I found an in line restrictor from Riva racing. It allows different carb jets to be screwed into it to change the orifice size. Do you think this might work with a 100 pilot jet screwed into it? That would give me a 1mn orifice for the return. I could also use a 150 if you think more flow is a better idea.

 

This helps. So I’m guessing the restrictor from Riva with a 150 pilot jet in it should be ok then. It would give me the 1.5mm orifice the GT4 specifies. I’ll keep you posted once the restrictor arrives. The joys of buying old cars and not knowing what was done to them in the past.

 

Quick question. Would you recommend adding a fuel pressure regulator between the Pierburg pump and the carbs as well or would having a properly sized orifice most likely solve the issue? I know Webers are pretty sensitive to pressure. I was looking at getting the one from Holley that has a range of 1-4 PSI and is preset at 2.7. Or is this just unneeded complexity in the system?

 

Thanks! That’s what I was thinking. I’ll go without the regulator and just to the 1.5mm orifice. I’ll keep everyone posted how it goes!

 

Well this is frustrating. I put the restrictor in the line today with a 140 pilot jet and it is equally as bad as without the restrictor. I investigated if it was a venting issue by putting a vented gas cap on it which did not help. In every instance the car would die at anything over 3k rpm under load. Light load or low rpm it was fine so it seems there is not enough fuel to keep it going when the return was blocked with the Hardi pump it was at least happy besides the car pool. Now it won’t even go a few blocks down the road with the return in place. It is extremely frustrating.

 

Pump voltage seems ok. It stays over 13V and never stops running. I may pull the fuel filter temporarily and see what that does. It’s a brand new filter but maybe it’s causing a restriction. I’m not sure if the Hardi does better with the restriction of the filter before it than the Pierburg. I’ll see what it does without the filter and if it’s better I may replace it even though it has less than 100 miles on it. I’m losing my mind I think lol

 

I agree. This should be one of the simplest fuel systems ever. I’m not so sure why it is giving me such a headache. The only other thing I can think of is a mounted the pump horizontally instead of vertically. It seemed to fit better this way and kept the lines tucked up a bit better. However I’m not sure if it performs better in a vertical orientation. That is the only thing different from the install picture you posted as far as I can tell besides the safety bypass being gone on mine.

 

Ok I just read something else interesting. The Pierburg installation instructions say the paper fuel filter must be installed after the fuel pump with only the fine mesh filter before. The standard routing on the 308 puts the paper filter before the pump which is what I did. I wonder if the pump is running into issues getting fed on the suction side under high demand?

 

Carb floats should be fine. Rebuilt carbs with floats all adjusted to within spec. With the Hardi pump installed and no return the car was absolutely fine under load and could run to 7k no problem. The only issue it had then was in the car pool line. Now with the return open I never can seemingly get enough fuel. Even with a restrictor in place I’m only at 2.5 psi at the pump. I would think it should be at closer to 5. The Pierburg instructions say not to put the filter before the pump. So I have a feeling the filter is being a restriction for some reason. I’ll try to swap it this weekend and at least see what it does. Worst case is I put the Hardi back and deal with the slight issue during car pool as the car ran amazing that way otherwise

 

This is really helpful information indeed! I’m betting this is my main issue. I’m hoping to swap the lines around this weekend. I will let everyone know how it goes.

 

Thanks! When I was running the Hardi pump my plugs all looked a nice light brown across all 8. It only had issues in stop and go idling. The major issues arose with the Pierburg pump. I guess first step is to change the lines around and if that doesn’t do it, dive back into the carbs.