Stop and Go Traffic with Carb Car

Sweet. Good Dad!

 

What octane are you using? It's hard to find anything higher than 91 at times.

 

Kind of an unusual use case as you noted. One thing that might help here would be having both R1 and R2 points working (so the idle timing would be more retarded) -- are you running an "R1 only" type ignition system or is it stock? Also, you haven't done something crazy like eliminated the fuel return line have you? Or eliminated the carb-to-manifold insulators?

 

Sounds like overall the car is running pretty good so you've got a bit of experimentation to do to get to that last 5%. I would recommend only making one change at a time and being really careful about noting how things change. Maybe make some videos of it loading up and also it running normally so you have a good idea of where you're at.

Anyways, I agree it sounds like it might be loading up a bit. It sounds like the carbs are synced and set up well, but maybe the idle jet is a tiny bit rich and causing the plugs to foul a bit after idling for several minutes. You could try leaning it out and noting changes. Another thing you could try is going one heat range hotter on the spark plugs to see if the hotter plugs can burn off the fuel a bit better.

Curious what you find out!

 

it's the carburetors loading up the plugs. What happens in stop/go situations is the pump jets are getting constant little jabs of fuel when stopping and going, causing a frequent overly rich mixture which the plugs can't burn off, which in turn loads up the spark plugs. Using two MSD boxes (one for each bank) resolves this issue as the spark is so strong the loaded up plugs don't make a difference. You'll need a tach adapter too (MSD 8910 version)

 

The Pertronix unit is what I would call an "R1 only" ignition set-up (i.e., a single thing fires each bank's coil), and you are missing the R2 points (and the throttle microswitch that activates them at idle).

I have a hard time believing this as the return orifice is just a small opening so the return flow is small compared to what the engine is ingesting at any reasonable speed. It's like saying that you can drive continuously at 60 MPH, but, if you go 70 MPH, the fuel pump will burn up . Did you have a Hardi pump fail, and this was the recommended "fix"? Many carbed 308 are running around with Pierburg or Hardi or Facet pumps with the return in place AFAIK. Is this a diaphragm type Hardi pump or a rotary vane Hardi pump? Do you have a Hardi part number?

Could be, but not having the fuel return is just crazy IMO, and the easiest 1st thing to correct to see if it makes any difference -- if F didn't think it was necessary or required, they would've been happy to not include it.

 

Seems to me a mid engine carb car with no cooling air flowing across the motor could be prone to fuel percolation without a continuous flow fuel system.

 

Agree with @Cameron Henlin guidance: Only change one thing at a time and monitor/test. Maybe a dumb question, but @cbmcdona: Do you know when the issue started... What, if anything, was changed? My first order would be @Steve Magnusson guidance, re-plumb the fuel return to the right tank. Also, depending on fuel available in your area, I would run 91 Octane Non-oxygenated fuel.

 

I respect everyone's experience and knowledge of the diagnosis and countermeasures of Caleb's issue.
Please explain to me why 8:5:1 compression requires only regular gas instead of premium 91-octane or better.

 

Octane is a measure of a fuels ability to resist self detonation under pressure thus is is directly reacted to compression ratio. There is no more energy in higher octane fuels. Always best to use the octane level specified in the car's manual. Using higher octane in just wasting money.

 

After reviewing the Hardi website, I can see why they recommend not having any fuel return flow path. Being a solenoid diaphragm pump, it has internal mechanical switch contacts to "oscillate" the solenoid plunger so having a return flow path will put more cycles more quickly on those electrical switch contacts. One advantage of the Hardi pump is that it has good suction (lift) behavior so is self-priming. Maybe this is important on the Porsche application that it seems designed for, but that's not something needed on the carbed 308 fuel system layout where the fuel tank outlet is at the same level as the fuel pump inlet so gravity does the priming:

https://www.hardi-automotive.com/en/products/technology/

So, although the Hardi 8812-3 specs match the carbed 308 specs well, put me down as not a fan, and think that you'd be better off with a rotary vane electric motor pump like the stock 308 fuel pump -- JMO.

 

Did these cars exhibit the problem when they were new? If not, and assuming the carbs are correctly jetted/set, I'd suggest the problem is with the aftermarket changes.

 

My thoughts exactly. Especially with our current government gas that is more prone to vapor lock at low pressures thanks to mandated additives.

 

Even if it doesn't help this problem, I'd say that is the right move, and restoring the fuel return is a very good thing to do.

If a US version 1977 308, what's the status of your fuel evaporation system? Is it still intact and unmodified, or not?

 

I don't think that it would make much difference (if you mount the pump vertically), but, yes, in that orientation, inlet at the bottom would be my choice as it points the outlet in the direction that the outlet hose needs to go (up). However, I don't have any experience mounting that pump on a 308 so really have no suggestion how it should be mounted, and will rely on you to post some photos about what you found the best orientation to be (and I wouldn't rule out diagonally). My top priorities would be having as little (or none) of the hoses extend below the bottom of the chassis frame and good hose routing (short lengths, no tight bends).

 

Stumbled across this picture that I had saved of someone putting that Pierburg pump on their carbed 308:

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Not saying that this is the best, or only, way to do it -- just posting it for reference (it does seem to have the input on the bottom, but the various plumbing connections aren't too clear on the output side).