Is this normal MPG?

[crvc]

1967 bug, engine replaced with type-2 block, line-bored, 1600cc AA pistons/cylinders, heads given new valves and guides at machine shop, stock muffler. Recently got turn signal flasher problem fixed. I've been noting mileage for months, finally sat down with note book and calculator. It tells me for the past 6 months the bug has been getting between 14 and 18mpg on city streets at 7000ft altitude with steep hills. There is a fuel filter behind the right rear tire. With the tire removed I smell gas. It seems leaky but not to the point of leaving a drip or stain on concrete. It has a 34PICT 3 carb, 2-3 years old and covered with staining and gas odor if I'm close enough.

Any comments or suggestions?

I'm ready to trade it in for a pickup. If I have to live with 18mph might as well have a truck.

TIA,

kevin

[Marc]

Two things to do before your calculated fuel economy means squat.
1) FIX the fuel leak...it doesn't have to leave a puddle to be a problem, if you can smell raw gas something's not right. Check the bottom of the tank and all the lines for dampness; in humid air there'll often be frost evident on a "sweating" hose. The staining/smell at the carb is probably not cause for concern, but could indicate a float valve that's leaking by. Rich running typically leaves the inside of the tailpipes black & sooty ("tan" is normal); ensure that the choke is turning off fully and if a leaky float valve is suspected just change it...IIRC you've been down that road before.
2) Confirm the accuracy of your odometer, either by comparison to a GPS or by checking against highway mileposts the next time you take a trip. It's not a common problem on Standard speedos, but if the odo drive gear is slipping your indicated mileage will be reduced.

I've never spent a lot of time at your elevation, but considering the terrain I wouldn't expect to get more than around 25 MPG overall - that's still 50% better than you report.

[aussiebug]

So you are here too crvc?

This is a much shorter thread than on thesamba. Hope you read my post over there.

[crvc]

So you are here too crvc?

This is a much shorter thread than on thesamba. Hope you read my post over there.

Yes I read at all. Sometimes I get good advice on Samba but not always. Marc, on the other hand, knows all and has never let me down.

I replaced the fuel filter and don't smell gas anymore. I will get an assortment of main jets to see whether that helps as well.

Thanks,

kevin

[Marc]

Factory main jet sizing only varies slightly (mostly dependent upon whether the emulsion tube is the common 75Z or something different).
http://oacdp.org/wog72/022_k.png

A rule of thumb for main jet selection is to reduce the size by 6% for every 1000 meters above sea level...using that, one might expect to need a ~115 main at 7,000'.
But I wouldn't go that small right off the bat, since too lean will cause cylinder head temps to rise and can even cause misfiring/stumbling - to compensate, you might find yourself fanning the throttle pedal and mileage could even get worse. "Extractor" exhaust systems and gasoline with alcohol content (as is common nowadays) both call for slightly richer jetting than stock mufflers and 1960's gas, too.
Do check that emulsion tube size...I'd want bigger rather than smaller at your altitude (75 or 80, not 70 or 60).
Some folks also lower the float level slightly for high altitude operation...again, don't do anything drastic initially, but two (or at most three) total floatvalve gaskets might be worth trying.
Most cars run best if the accelerator pump discharge nozzle is aimed so that the stream just misses the side of the venturi - that gets the shot past the edge of the throttle plate as it cracks open for maximum effect (sometimes you can even back off on the discharge volume a little, which may help mileage).
I forget, what aircleaner do you have on there? A functional thermostatically-controlled one will give the best results for driveability when cold and economy when warm. I like the c1972 style with self-contained wax-pill thermostat, but the mechanically- and vacuum-operated ones can also work fine - they just have more to go wrong.

[crvc]

Stock oil bath circa 1973.

[Marc]

`73 were actually paper-element style, so I'd call it a `72 if it's the vacuum-operated type. Have you confirmed that it's working correctly and drawing warm air from under the RH head? This time of year, with a louvered hood, it should be doing that most of the time.

[aussiebug]

Marc,

Just read your post above, and was interested in your altitude comments. I've always understood you needed about one jet (2% fuel) smaller per 5000' - less change than you indicate (6% at 3000' is three jet sizes smaller).

And your comment about a larger air correction jet is interesting. My understanding is that a larger air correction jet would lean off the middle rpm (airflow) range but leave the bottom and top end running richer, so I would be reluctant to change that - the main jet on the other hand alters everything from roughly 1500rpm upwards, which is what you'd want for the thinner air at altitude.

And I think a one-size smaller idle jet (55 back to 50 for example) might be needed above about 5000', otherwise low speeds would be still rich.

Your comments would be welcome.

[crvc]

Hmmm,

I took out the main jet and put in the new one. I don't remember the number. But holding the two jets up to the light I saw a big difference in the bores, the old one being markedly larger. Now looking at the old one with a magnifying lens the number is barely visible but I think it says 115.

Driving the bug for the first time since changing the jet, it drives horribly. Going up hilly inclines it has no power. There's a ten-mile long 8% grade outside my office that I'm sure the bug would not be able to climb. In the past I was able to reach the peak before needing to shift into 3rd gear.

I also added a second paper gasket to the carb top. Now that the engine has warmed, I'm going to try driving to the top of that steep incline just to see what happens.

kevin

[crvc]

So that experiment failed. I couldn't reach the peak, even in second gear. Back home I fiddled with the two left-side adjustment screws but didn't get much change. Then I noticed the bug idling a lot faster than normal. Spraying the carb top with engine starter the idling slowed abruptly. So the two gaskets are causing a vacuum leak?

Probably need to remove the carb again to go back to the old main jet. And order an in-between size. Not much fun in below-zero wind chills.

kevin

[Marc]

No one ever proposed doubling the carb top gasket...even if you managed to find two that were identical, the added thickness would RAISE the float level, the last thing you'd want to do. Use one gasket (check the holes against those in the top to make sure you choose the correct one). If you want to experiment with lowering the float level, add a gasket between the float-valve body and the carb top.

If the old main jet is in fact a 115 (and hasn't been drilled/reamed larger at some time) and is "markedly larger" than the new one, what the heck did you put in?

[Marc]

Rob, as I stated earlier my personal experience with high-altitude jetting is limited (I had to deal with the opposite problem when racing buggies inside the stadium in Vancouver BC where the roof was held up by increased air pressure) but the 6%/1000m thumbrule is the only one I've heard of. As for the air correction, its effect is predominately at the upper end - but the size and location of the emulsion tube holes and the diameter of the tube matter too - all I was saying was that it wouldn't seem logical to use a small A/C where the air is thin.

http://356carburetorrescue.com/altitude.html

http://docs.engineeringtoolbox.com/docu ... ensity.png

[crvc]

I ordered a 127 main jet. Taking the old jet out it's hard to read the number but with a magnifying lens I think says Solex 115. That done, it drives horribly; No ability to climb even mild grades.

In my junk drawer I found an old pict 34 with a 120 main jet. Tomorrow I'll replace the 127 with the 120. Taking the carb out so many times I've stripped the threads on the two mounting studs. Using a Permatex thread repair kit the carb is sitting under a heat lamp for now.

kevin

[Marc]

You should be able to change the jets with the carb in place. I think a legit 120 should be in the ballpark; 127.5 is stock for most applications at sea level.

[aussiebug]

You should be able to change the jets with the carb in place. I think a legit 120 should be in the ballpark; 127.5 is stock for most applications at sea level.

I'd concur with Marc on this - 115 would definitely be too small (running lean) and 120 might be around the right size for the main jet. If it's still running rich at low speeds, try a one-size smaller idle jet - reduce from 55 to 50 for example.

Yes, on the 34PICT card you should be able to change both the main jet (bottom of float bowl) and the idle jet (right side) with the carb still in the car.
On some other carbs (H30/31 I think) the main jet is not "straight" in the float bowl but angled up, so that needs the top of the carb to come off to get at it.