poooooor mpg??

[752dbl02]

I rebuilt my carb to cd's jetting rx about a month ago, and am still getting horrible gas mileage (13 mpg).

No fuel leaks from the tank ( I just had it boiled and relined).

Timing is spot on, plugs were just replaced ect.

Any ideas?

Thanks,

Chris

[Guest Anonymous]

I rebuilt my carb to cd's jetting rx about a month ago, and am still getting horrible gas mileage (13 mpg). No fuel leaks from the tank ( I just had it boiled and relined). Timing is spot on, plugs were just replaced eTC. Any ideas? Thanks, Chris

It seems like this topic is a FAQ on each auto board. I know I have seen it here, and it should be in the archives. That being said, with fuel mileage being that bad, something is very wrong with your means of measuring your mileage OR your car OR your driving style.

Measurement

I recently read on this board where the FAQ'er complained of using too much fuel as determined by the faulty gas gauge. I also know of someone who frequents this board that failed to realize his odometer was barely working making it seem as though he was going through tanks of gas without putting on any significant mileage. What kind of mileage did you get before you rebuilt the carb?

Car setup

CD's patented jetting is merely a starting point and cannot take into account all the variances found in 30+ year old engines. Sure the floats are set correctly and the needle valve seats properly? Are those jets in the right place? Replace the accelerator pump diaphragm? Engine reaching operating temperature of 170 degree F? Your timing is spot on - but what spot? Factory recommendation is a good starting point, but no guarantee of maximum power or economy - especially if you have other modifications. Running straight 50W instead of that multi vis?

Ever try pushing your car - by yourself? How difficult is it? Is it possible that your rippled sidewall slicks have extremely low tire pressure? Are the brakes fully releasing? Is the front end aligned?

Driving Style

Are your brake lights on more than usual? If you ride the brake, even unknowingly, your mileage can suffer greatly. Is every stop light a different drag race? Do you only drive uphill?

Those are my ideas. hth

[FunElan]

I'm on the same page as you 75dbl02, I'm just going to replace the carb (weber 32/36).

[c.d.iesel]

13 MPG with what kind of driving ?

on highway trip - measured after 200 mile leg?

or just putzing around local roads ?

Climate ?

compression?

timing set how? and to what spec?

how accurate is your speedo ? odometer ?

[robsanab]

Chris, I've had the same issue... i was getting 24-28 mpg on an old 32/36 with a surging idle due to leaking air around the throttle shaft... I decided to do the CD setup with a brand new carb. Now I can't seem to break out of 16-18... and it's not from putting my foot into it more. Even though my engine is well kept I doubted myself, so this last time i took it to my mechanic (a pro bmw/mercedes mechanic and 2002 racer... so he knows what he's doing) for timing, valve adjust etc. Everything is tip top and my gas mileage has not improved. Again I'm not a racer... and lately especially I've been driving for MPG, not speed. This is stock motor in excellent shape... electronic ignition, tii manifold, good exhaust system etc.

I agree with everyone that says there are a lot of variables... but I had a good reference point, a stock jetted, albiet old, 32/26 carb. I thought I'd at least keep the same gas mileage as the leaky carb, but that hasn't been the case.

As mentioned, I'm hoping that this jetting is still a good starting point and then just pay the labor for my mechanic to hopefully tweak it from there (I don't have the time or patience right now) OR I've got another jetting recipe from Carl Nelson of La Jolla Independant that sounds promising.

I'll let yall know the results once I get around to it.

[Guest Anonymous]

i was getting 24-28 mpg on an old 32/36 with a surging idle due to leaking air around the throttle shaft... I decided to do the CD setup with a brand new carb. Now I can't seem to break out of 16-18. . . . I had a good reference point, a stock jetted, albeit old, 32/26 carb. I thought I'd at least keep the same gas mileage as the leaky carb, but that hasn't been the case.

As mentioned, I'm hoping that this jetting is still a good starting point and then just pay the labor for my mechanic to hopefully tweak it from there (I don't have the time or patience right now) OR I've got another jetting recipe from Carl Nelson of La Jolla Independent that sounds promising.

This remains an unsettled debate no matter how may times someone says "trust me" and "just do it." I take no sides in this matter since I am using a Kuglefischer. But if you are referring to Carl's recommended formula, it is different that the CD perscription. I think it has been addressed on this board on several occasions. http://www.bayarea02.com/features/carburetor/carl_nel.html "The jet package that the carb is delivered with is not good. Switch to f7 emulsion tubes, and switch the air correctors (between the primary and secondary) -- this is 160 primary 170 secondary. Change the main fuel jet to a 130. Leave the secondary fuel jet at 140. Use 55 or 60 main and secondary transition jets. This works great for most near stock cars. You can use the late 2bbl air cleaner, modify the bottom of the air filter and fit an adaptor, and it looks stock. This combination works great and delivers great flexibility. This combination also would be an upgrade for the late 72 and on 2bbl Solex."

I do not have a clue what the delivered jetting might be for this carb or if it changes every Tuesday. But it should be obvious that one size does not fit all, unless you just need to move the car around a small parking lot every now and then. Without the use of a lambda sensor, I'll bet Toby would tell you the leaner is usually the better. Just like timing the engine by ear, when you start to hear pinging, back it off a bit and you are at least near the target. If your initial premise is correct (that your worn carburettor created a lean running condition), you proved that lean could be better as far as fuel economy. Obviously, driveability can suffer.

If you do your homework, you will find the idle jet size can dramatically affect most of the engine's normal operating range. Carl oddly describes them as "secondary transition jets" but I think this is a misquote. In my experience, unless the main jets are disgustingly large or the accelerator pump leaks like a sieve, "60" is the magic idle jet number. And, unless you plan to push the engine (which most of us do) you may as well jet the carb for a VW. I am not recommending this. But remember, when things are very lean, it is not as easy to fire the mixture, so the ignition system must be functioning well and it could not hurt to widen the plug gaps to fire those extra lean mixtures.

[752dbl02]

Thanks to all of you guys who gave your good advise.

I will have to play around with the jetting i suppose.

But i did measure the mileage on the highway, as i just made the trip up to Athens, GA and got about 160 miles out of a tank. ((

robsanab, def post some results of your jetting changes. Part of my problem is, i wouldnt know what to change mine to in hopes of still keeping the good performance that the jets provide now, and achieving my ideal gas mileage.

Thanks agian,

Chris