Hot Rods Chevy 235 gets shitty gas mileage

Well inline 6s and mileage has always been a joke played on the general public. But not to start any drama I won't go into it.

So it stinks of fuel when you are hard in it, that means it is running fat when you are hard into it. if it runs fat it is not running efficiently. First thing to look at is the carb setup or at least why it is not burning the fuel that it is sucking down the intake.

What are you using for ignition is going to be my next question? someone added a different cam shaft and carbs and exhaust but did the stock ignition stay in place? Hot engine requires hot ignition. I am not saying that you should switch to electronic or a modern ignition controller of some sort or even a mag, but at the very least a hotter coil and/or a dual point.

Next you want to tune or gear so that you are in the very bottom curve when you are cruising. Example: if your setup starts climbing the cam @ 3K then you want to be cruising @ 3K min, if you are building torque from say 3K to 5K then you are going to want to be cruising between 3K and 3.5K. Your engine runs most efficiently when it is in the torque band and when it is running most efficiently you are making best use of every ounce of fuel/air mix you are pumping through it.

9MPG is bad but I would not expect more the 14-16 out of it at any rate.

 

Dual Rochesters? The carbs MUST BE 216 sized to work well, this is according to a guy going by the handle Carb King. I have 2x2 Carter-Webers getting low teens in town, 18-19 65-75 highway. I don't think your 'hot cam' is set up for fuel mileage, also as you know "..getting hard into the pedal" doesn't do your mileage any favors. BTW, '69 3sp+od, 3.90 truck rear, can't beat it.

 

Well I'll throw my 2 cents in here, I have a 235 with a T5 and my mileage sucks as well. It has a 3.73 geared 70 nova rear end and the best I can get is 10mpg. I tried the pertronix, waste of time! Pulled out the whole distributor and tossed it in favor of a brand new electronic distributor. I have gone through the carb numerous times, and no matter what I do the mileage is still 10 mpg. I say the best fix is an engine swap, something in my future.....soon hopefully.

 

Great stuff, guys. Many thanks. I new to these inline 6's, so I appreciate the input.

The PO swapped out the points and added a Pertronix igniter conversion. That appears to work fine.

Beaner; the engine makes tons of torque at low rpm, it seems happiest in the low rpm range, below 3,000.

I have a stock intake manifold that I can swap in to see how she performs. I might try this just for shits and giggles to see if there is appreciable loss in power, and to see if the over-carburetion is a possible cause for the fuel odor.

Another question: these Fenton intakes are not mated to the exhaust header, like the stock setup is. It therefore stands to reason that the Fenton intake manifold doesn't heat up as much and therefor the air can't "hold" as much gas as a heated air mass can, resulting in an overly rich/saturated air/fuel mix. Is this a possible cause of the odor? And is this an inherent characteristic of these, and similar aftermarket headers?

 

Yes, there were Fentons that were mated to the bottom of stock manifolds, but they are rare. You are correct, must heat. Keep going, you can do this.

 

You're right, and the manifold setup was included in the sentiment (if not explicitly) when I said "worst possible setup" for gas mileage.

To me, it's a race setup.

 

They're more accurate carbs than the Rots. Better able to run in sync rather than progressive.

 

An overdrive would make you happier IMO. Gas mileage from an I-6 depends on low RPM torque and tall gearing. You have one & need the other.

Accuracy of carburation aside, I think this is a basic need unless you're willing to give up the low rear end.

 

Six cylinder log manifolds have poor fuel distribution anyway center cylinders vs end ones I can't see a progressive 2 carb set up being that much worse.

 

Two carbs can actually help, dump the Rochesters for Carters, since you're running two they should be a tad lower CFM, and tune them accordingly.

Should easily be possible to get that motor in the 15-20 MPG range.

 

Changing out the rear gears is pretty straight forward. What gear ratio do you recommend? I don't race or tug a boat, just around-town driving and highway cruises.

 

Hi. Definately go the progressive route with linkage. I compare progressive linkage on multiple carbs to having a single 4 barrel carb. If you do not drive around with your foot stuffed down you are only using 2 or the 4 barrels avalible & the 4 barrell gives pretty good milage. Then when you exceed the setting of the 4 barrels progressive linkage & engage the other 2 barrels the milage suddenly sucks.

Do you have a tachometer in the car. That is a good indicator of how you are doing as well as a vacum guage. The higher vaccum you can cruise around with the more eonomical you are. Remember the old eonomizer guages of the 50's, just vaccum guages.

If you do not have a tach you can figure your best setup for eonomy with compromizes between gear ratio, tire diameter, rpm by using this formula - The 336 is just a constant multiplier in the formula.

336 X MPH X gear ratio divided by tire diameter.

For example with 600/16 tires with a diameter of 28.3" & 411 rear end =
336 x 65 (mph) x 4.11 divided by 28.3 = 3172 RPM. That milage will suck.
for same tire with 373 rear end -
336 x 3.73 x 65 MPH divided by 28.3 = 2878 Rpm. Better milage but still sucks.
For same tire with 355 rear end -
336 x 65 x 3.55 dived by 28.3= 2739. Better but not great milage.
Or sub a 750/16 rear tires & get -
336 x 65 x 3.73 divided by 31.14 = 2616 RPM.
Or use a 355 rear end with 750 tires =
336 x 65 x 3.55 divided by 31.14 = 2489. Much better milage.
The 355 is a passanger car ratio avalible for those years and not sure if it will fit truck rear end.
OR SLOW DOWN A BUNCH.

This is the stock setup for my 46 Chevy coupe -
336 x 65 (mph) x 4.11 divided by 28.3 = 3172 RPM. That milage will suck.

My 46 chevy coupe has the 411 gears & with the new 235 going in it I have the 355 gears to install soon & will go to the 750 rear tires. It has the clifford dual intake with 2 - 2 bbl Webber carbs with progressive linkage, bored 0.060, a 264 grind cam, tube headers & HEI. I will not race this car but making it my daily driver & with this set up it will give good milage & have enough power to run a Vintage Air unit in this Texas heat.
With the new 355 & 750's I should get -
336 x 65 x 3.55 divided by 31.14 = 2489. Much better milage.

My Mechanic is going real slow lately but with some luck should have it on the road soon & will do a thread on how everything is going then.

Jimmie

 

No tach in this truck yet. I plan on installing one very soon though, like as soon as the UPS man shows up with it. I don't drive this thing above 55 mph, the motor screams at that speed. Honestly, I drive it like an old man most of the time.....nice and slow and take in the scenery. But when I get the itch and stomp it, the fumes remind me to knock it off. lol

 

I've got no clue about the specifics of your car, nor what's available. I was happy with 3:90 gears and a 3 speed with OD in my P15. She got up to 85 mph handily with a near stock flathead 6 & cruising at 70 was nice. Without the OD, 50 MPH was a much happier speed. Your car is lots lighter & maybe less aerodynamic (but don't quote me there please!) so maybe 3:50s would be OK.

 

In my mind the hot setup would be 3 dual sidedrafts on an I-6. Worked well for Jaguar

But the more butterflies and shafts you have, the bigger and faster your air leaks will develop from ordinary wear. That means staying syncronized gets more and more difficult as the carbs wear.

A good single carb is the most reliable and durable & will stay in good tune the longest. It can't produce the most HP, but so what?

My ex-neighbor had a '48 chevy pickup with a single small progressive 4 BBL on an aftermarket manifold, and was very pleased with it. He drove that truck almost daily too. Unfortunately I don't remember the specifics except I believe the manifold was Edelbrock.

 

Ill trow in a little more input as i was able to find some of the rescources i used when i put together my set up.

This is the article for the rochester carbs and how to get them to work
http://www.inliners.org/tech/tech4.html

And this article is an explination why you need heat.
http://www.inliners.org/tech/why_you_need_heat_to_your_inlet_.htm

Those 2 articles were very helpful to me when i built my setup. I know alot of people here and other sites say to trow the rochesters away and use carters(yet the rochesters still fetch good money for just being labled junk...) I had to go through some internal matching on the carbs to make sure they are right. I run the progressive linkage which probably helps too(ran an offenhauser linkage for a while and it was junk). The heat to the manifold is needed for sure. Its easy to hook up the intake to the exhaust manifolds(no need to buy the kit, i ended up building it myself).

Another thing ill share(not sure where you are located) but i lived in california and when i moved to texas and started driving the truck outhere i had a constant overflow problem with the carbs. After countless adjustments, taking the carbs apart 10 times making sure its not a mistake i made, i ended up finding out that the etheanol in the gas was giving me all the problems, etching the float needle causing it not to slide smoothly causing overflow issues. To solve that problem, i polished the needle again and now run a little marvel in my gas and havent had a problem since. I know the o.p doesnt have this issue but ill just trow that info out there too just in case someone else is having those problems.

 

pork and beans said it best , make sure your in the torque range of the cam as torque is the sign of best efficency of fuel used .

I wonder if the 2 carbs are dropping the vacuum numbers down so low that the enrichment circuit ( economizer ) is not shutting down and making it run rich , I know on the race car I work on we have to put a real low vacuum power valves in if we run duals quads other wise it will try to flood out at idle and mid range due to the ammount of airflow because they are not progressive

 

This is the procedure I didn't have time to describe earlier. (Am I still in the right thread? )

I'd put a vacuum gauge in the car, from the most appropriate manifold vacuum port, to help as I was tuning the carb.

I'd have a "T" fitting on it, with a hose as a second vac port. I'd cap that hose with my thumb as I held it on the shifter.

While driving I would watch the vac gauge for max cruise vacuum, then while cruising there uncover the port to create a tiny drop in vacuum. If the RPM went up I knew to back off on the power valve or increase the economizer action to reduce fuel at that RPM.

This also helped to adjust the accelerator pump action.

I also just listened to the engine as I did this. You'll know when it sounds happy.

This method can also be used to diagnose idle and off-idle issues, as that "tiny" vacuum leak at cruise is a huge vacuum leak at idle. You feather your thumb over the hose to add or subtract air from the mix, while driving to determine if you're lean or rich at any particular range.

I also used a small air valve to do just that.

That valve came in handy to lean out the idle from the driver's seat, when climbing to high altitudes. From Fresno (+300' elev) to Huntington Lake (+7500' elev) is too much for my Holly 1920 to handle without adjustment, and fouling at idle above 6000' was an issue with either the Holly or the Carter.

 

I have a 261 in my '51 pickup. It's stock, though whereas yours is not. I have a Fenton header with split exhaust and a Holley 94 from my flathead stash. I drove the truck for several with a saginaw 4 speed and 3:54 in the rear and got around 15 in town and 18-20 on the road. I just changed to a t-5 and the MPG is up by quite a bit...a solid 19-20 average city/highway now.
The biggest improvement I made was tossing the Rottenchester and going with the Holley. My mileage went from the same 9 your getting up to 15 within one tank of gas and the performance jumped right up, too. The only reason I replaced one vintage carb with another, aside from the difference in design/quality/performance, was that I'm a cheap old fart and have plenty of 94s that are already rebuilt. It's a good carb and works very well for me but might not for some one who doesn't know them well.
However, Langdons sells 4 bolt Holley set up that will adapt to your intake that would make a huge difference for you.

 

Had a guy back in the mid 70's build a '54 235 with the Offy intake and used two of these: http://www.allcarbs.com/detail.php?pid=462&gb=1&ctgn=10&stt=0 Pinto Carbs kind of a Holley/Weber worked real well after some dialing in and claimed it was using a lot less gas.

 

These are great tech tips, thanks everyone! The lack of heat on my intake manifold always bugged me. I suppose I should fabricate a heat transfer mechanism between the intake and exhaust manifolds as a first step. Whether this is the source of the problem or not, I can't imagine that it would make things worse. I'll do this first and report the results. Thanks again, much appreciate the advice, guys!

 

The exhaust heat crossover in the intake manifold may actually serve a couple purposes. It mainly provides a source of heat around the base of the carburetor and the intake manifold itself. This improves cold driveability by helping to keep the air/fuel mixture atomized. Heating the base of the carb also helps to prevent carburetor icing when driving in cool, humid weather.

Depending on the engine and carb design, the heat crossover can also provide the heat source for operating the automatic choke.Though as mentioned above, for everything to work properly the crossover can't be plugged with carbon and the exhaust heat riser valve needs to be working freely.

 

Voodo
If most of your power is down low then tall gears is a good choice. Depending on how tall your tires are I would think that a gear in the 3.0 to 3.26 range would suit you well on the highway.

That said we have yet to deal with your rich running problem. it could be that you do just have too much carb. If you are pulling too much air in you will have to pull a lot of fuel to compensate, perhaps a smaller CFM carb would be your easiest solution. But you will still have to deal with jetting, if you are running carbs that are meant to be run as singles they are just going to be too fat for your setup. It does not matter if you run less CFM or more CFM if the carb that is meant to run by itself and feed your engine has a buddy the setup is going to be too fat and you will have to jet accordingly. Does that make any sense?

There is a good book that can be purchased through Summit Racing if you are playing with Rochesters. Look here: http://www.summitracing.com/search/...re/brand/hp-books/title/rochester-carburetors
it is full of tuning info for performance and mileage with these carbs.

 

And there's the bug...those things always rust up and clog. I eventually put mine on a cable linkage next to the choke. People would ask if I had dual carbs when they saw 2 choke cables.

Something else stockers use to achieve good atomization in cold weather is the manifold heat stove & pipe to the intake snorkel. Without that my 60's Ford would ice up going over the Rockies in winter. I remember sticking my pinkie in each venturi to clear the ice, then having it ice again within a few miles.

Anyhow, heat to the intake air, when it's needed, is a good thing for mileage and performance, where we normally think "cold air intake" for performance reasons.

 

BLASPHEMY! Hot rod status is based on the number and size of these potmetal gas pizzlers.

Oooops! 'scuse me. I was overcome by emotion.

What I should have simply said is:
"He has too much carb for his current state of tune."

 

OH, yes...the smell of raw gas, if from the air filters, is commonly caused by double-carburation.

I don't mean dual carburetors though.

Double-carburation is where you stick your foot in the gas faster than the engine can deal with it, and the intake mix enters the cylinder, then bounces right back out the intake valve, then some of it sucks back in again to burn, as the engine struggles to accelerate against a huge drop in manifold vacuum.

So the engine's literally spitting the gas back in your face.

This condition is exacerbated in engines with short intake and short exhaust, and doesn't clear up until the vacuum starts building again. Drag racers suffer this and so often have to launch at higher RPM to get off the line without a bog.

 

We call it Holley Fog ( you can see the fog hang over the throats at idle )

 

OK, so I had some time to play today so I figured I should deal with the lack of heat in my Fenton intake. Took off the carbs and the intake. Yep, it's got the plenum on the bottom and through the log, but the PO left it open and unused. Here's a pic of the underside. The plenum runs along the log as evidenced by the raised boss.


I grabbed a sheet of 1/8" steel and fabbed up a closure plate to bolt up to the bottom of the open plenum chamber. Nothing fancy or complicated. The circled hole is where I welded up a steel elbow to pipe to the exhaust.


Here it is with the elbow welded up, bolted and gasketed to the Fenton intake. I made a gasket from high temp sheet that I had laying around, and some Permatex goop for good measure. I think 'goop' is a technical term.

I installed the intake and carbs, with new base gaskets for good measure, and piped the elbow to one of the Fenton exhaust header ports with 3/8" diameter copper tubing and compression fittings. Time will tell how the copper stands up. It's what I had on the shelf, so I gave it a shot. Here it is all back together.



Fired it up and got it good and warmed up around the neighborhood. The intake plenum now gets freaking HOT! So hopefully this McGyver setup actually works and helps the situation. Ignore my hack piping job. Plumbers, your jobs are safe! Driving impressions: i might be crazy (wife votes yes), but the gas stench seems to be GONE! I stomped on the loud pedal many times testing it, and the smell is no longer present. I dunno if this solves that issue, but so far so good. A huge thunder storm rolled in which put a halt to my driving today. So far I've driven it 20 miles, no gas stench, and the throttle response appears to be more betterer. And on downhills, it doesn't seem to "pop" any longer (something it always did when I backed off the loud pedal). Is it related? Who knows, but so far so good. I'll see how the fuel mileage changes, if any, and will report back. With my luck, it'll get worse. lol.

 

LOL

I call it air pollution.