"lurching"

[p.smitty]

I just had a total, stock rebuild done on my original 72 3.0cs engine. We put new Webers on it and it runs great under load (accelerating). At constant RPM it "lurches". We have checked carb jetting and the electrical system has also checked out.
Could we be using the wrong plugs? What plugs are recommended these days for this application?

Thanks,
Pat

 

[61porsche]

Your

mixture is incorrect and the carbs need adjustments. (most likely lean - by your statement that it runs good when accelerating.)

It would be helpful to know what jets and settings that you currently have. If you can, pls. distinguish at what rpm what symtoms occur. I.E., 1200 rpm, 3000, or idle as that may get you a specific fix or suggestion.

But before anything, confirm your timing and that it's rock steady and not pittted or burned points.

 

[jmackro]

We put new Webers on it and it runs great under load (accelerating). At constant RPM it "lurches".

What kind of Webers: sidedraft DCOE's or downdraft DGAV's? When you say you "checked carb jetting" do you mean that you checked that jets were installed, you checked it against _____ reference, or what? As 61porsche asks, please specify what the jetting is.

Also, can you clarify what you mean by "lurching"? Improperly jetted DCOE webers will result in a loss of power in the 3,000 rpm range as the engine transitions between the idle and main circuits - might that be what you are describing?

I am sure the problem isn't the sparkplugs, and since the Webers are new, it seems unlikely that something in the ignition suddenly changed while you were installing different carbs.

 

[p.smitty]

lurching

Thanks for the response guys.
Sorry, I put 3.0sc in original post. Wrong car, this is for a BMW 3.0cs
The car runs unsmoothly at most all RPM's when held at a constant throttle. Most noticeable at slow speeds between 2-3000 RPM
The Webers are downdrafts DGAV's 32 primary, 36 secondary.

both carbs
main jet= 140
air correction= 160
idle= 60
emulsion tube= F50

The work was done a Sea Level and I live at 2200 ft. I just drove it over a 3500 ft pass. No difference
in results or driveability due to altitude changes.

Pat



Will check points when I get home. Timing is right on the mark.

Pat

 

[adawil2002]

What is your fuel pressure?
May just need to increase it, if you don't have a gauge, drive, increase fuel pressure, drive, repeat, until symptom stops. Mark the regulator where you started in case that does not solve it. Worked for my CSi.
Are you running an electronic ignition module?
Mixture should be set slightly on the leaner side for thinner air at high elevation than sea level.

 

[61porsche]

Increase

your main jet size to 145s from stock. Not enough fuel for today's gas.You won't have that funky jerking at a small expense of fuel economy.

Temporarily, you can slightly close your butterfly ( less air=more rich) and adjust the mixture screw OUT 1/4 to 1/2 turn (as needed.) which is what you have to do with Zeniths since larger idle jets aren't available. What can be done with Zeniths is to decrease the drilled idle air opening on the casting which is the equivalent of changing ( going smaller) the emulsion on a weber.

happy trails.

 

[Arde]

Had similar symptoms on same engine/carb setup and could no hold constant speed/RPM. Car wanted to go faster or slower but not constant. Fixed with larger main jet I recall. Can check the exact number, it has been a while.

The problem may have been latent since PO converted to Webbers but it only became noticeable after converting from automatic to 5-speed.