The Stovebolt Site Forums The Shop Area Engine Shop Carb tuning and troubleshooting

I was shocked recently to discover that my 1948 1/2 ton gets about 4mpg. I was expecting better than that.

Here's my setup: 235 bored 30 over, mild cam, offy intake with 2 Model B's and Fenton cast iron headers. The Model B's have been "rebuilt" with the standard kits you can buy at NAPA, but no work has been done on jetting or anything like that. The motor has about 550 miles on it.

Here's my questions. What's the most likely cause of such poor mileage? (Other than my foot, of course.) Power valve? Springs? Jets? Float level?

The truck runs great and has tremendous "gitupandgo". It has one minor problem. At just above idle, it tends to surge a little, which can make the truck jerk if you don't keep steady pressure on the throttle. It idles fine at 600RPM.

(Please don't tell me to buy the Webers from Stovebolt Engine Company or "Model B's suck". I'm interested in fixing what I've got, not listening to criticism of my choices.)

Here's a couple of pics of the engine so you can see the setup: pic 1 pic 2

I want to try and improve the mileage before I drive to Kansas City for the Midwest All Truck Nationals in September, so any hints, tips, or solutions are welcomed.

I am using two B's on my 216 one ton, I get about 16 mpg on my 35 mile daily commute, which is what I got before I put two on. Except it looks cooler, and goes faster. It has the 5.14 rear end in it.

I love that air-cleaner, where did you get it?

From reading California Bill, and my own limited experience working on my truck and driving it:

Are your carbs matched?

Are they in Syc?

Do the choke(s) come all the way off?

Are you using a choke in both?

Are the float levels right on?

Is your timing advanced enough?- I'm about 12 degrees ahead now, and not done...

Is your valve lash set right? (hot) Loose enough is important

Are your plugs clean and gapped right?

What do your plugs look like, after a pass? -like a 1/4 mile at or near full load in high, shut off ign as you clutch, no idle.

What's your vacuum at idle?

What's your vacuum while driving around?

Is the vacuum advance working right? Very important.

Are your points set right?- I see you are running electric ignition, I don't know about that.

What's your gearing. A taller rear end doesn't always mean better MPG.

More cam means less low end, which can lead to worse mileage.

Oh boy, will someone ever burn on me for the last two!!!!

I'm sure someone will chime in with more knowledge. Did you read the multiple rochester b tuning info on carbkings website? Its good info.

Do you have a copy of California Bill?

Paul,

That truly is horrible mileage..if it's accurate. What a beautiful engine. I see a fuel pressure gauge, that must be for looks right. I also think I see A/C and a hi-idle solenoid tacked onto the front carburetor.

You mentioned power valves, float settings and jets. Usually, if you have carburetion problems like the ones you mention, you will have secondary problems like fuel fouled plugs, diluted motor oil or a bogging engine with sooty tailpipe and smelly exhaust.

Are you running the maximum advance you can run? I would advance until it pinged uphill and back it off a smidgen. Make sure things like tires are properly inflated and the brakes aren't' dragging. I would run the hottest thermostat you can handle...a 180 degree maybe.

Those things sound silly but they do count. Is there any chance you calculated the MPG incorrectly? Are there teenage kids with siphon hoses in your neighborhood. I can't imagine a old Stovebolt guzzling one quart of fuel every mile!

Stuart

Wow, where to begin......

I hunted high and low for that air cleaner - I'll have to scrounge up the info on where I got it. It's aluminum, and I had it powdercoated black. I think it was O'Brien's out of Vermont, but I'll have to check.

Neither carb has a choke. The truck starts fine here in Texas and warms up fast, so no need for chokes. I don't know what my vacuum is, and don't know if the floats are set correctly, but the guy who worked on them is supposed to be good. (I was going to do the carbs myself, but a friend recommended him.)

I haven't fiddled with much of anything - the motor only has 550 miles on it. I'm just now trying to gather ideas about what to do. I haven't even looked at the plugs yet, but that was the first thing on my list.

The drivetrain - 5 speed with overdrive and 3.73 limited slip rear end. The engine runs about 2000 rpms at 45 mph in fourth and 2000 at 65 mph in fifth.

The ignition is electronic, so no points to worry about. Valve lash is set to stock specs .006 to .008 for intakes and .018 to .020 for exhausts. I think the cam was .450 or .454, don't recall. I'll have to look that up.

I'm not sure I know what you mean by "matched" carbs and "synched" carbs. I have custom made linkage that eliminates any variation in the way the throttles open, if that helps.

Stuart, I couldn't imagine it either.

I have an electronic speedometer. I checked it the other day with my Garmin, and it appears to be very accurate. I assume the odometer is as well, but I haven't done any precise measurement of miles driven. I simply wrote down the odometer reading each time I filled up. (I'm fixin' to fill up again.)

Yes, the fuel pressure gauge is strictly for appearance, although it is fully functional. Yes, I have A/C, but I haven't been using it yet - not hot enough here to need it yet (although it's getting there.) The truck is garaged and the gas filler tube has a locking cap on it, so I'm pretty sure no one is stealing my fuel.

Haven't looked at increasing the advance yet either. I'm just starting to look into the problem.

I was expecting to get about the gas mileage that Mr. Lang is getting. I really never checked the mileage early on, because I was having too much fun driving it and I knew that the speedometer/odometer wasn't accurate anyway. I calibrated the speedo a little while ago. After the second fill up I chalked it up to writing something down wrong. Now I know it's accurate. I just don't know why it's so low yet.

Here's the info on the two fillups; mileage 471.8 and 508. Total miles driven; 37.2. Gallons on the second fillup; 9.847. Mileage; 3.78.

Yeah, it's horrible.

Matched carbs, means the tags match, so you know the jets and venturie are the same. Sync, is measuring the flow through each carb with a manometer, a unisyn, to be sure that both are doing their job the same. They get in sync with the linkage loose.
Here is the link to a great article. Jon, the Carbking posts here often. He's extremely knowledgeable, and a nice guy- I spoke to him on the phone one day. Its a great read. I found it very helpful when working on my truck. I had originally followed the instructions that came with my offy manifold, but they were wrong about the mixture and such.

http://www.thecarburetorshop.com/RochestermultipleB.htm

A vacuum reading will tell you alot in a situation like this. It's like a window inside the manifold.

Some of them "hotrod" cams have lash specs of their own.

Don't forget too that the Canadian gallon is larger than the US one. The same AD gas tank is 17 US gallons and 13 Canadian gallons.

Neither carb has a choke, but the front carb appears to have an electric energized throttle sensor. Is that correct? If so, pull the wire and see what changes. Maybe it's holding the throttle plate closed in full choke position. If that's not what it is,what is the device for?

Another source for tuning tips is the "Multiple Rochesters" link in the "Tech Tips" link (about half-way down the sidebar on the left side) in the Inliners website.

Hoyt

DrewP, the solenoid is to increase the idle when the A/C is running. It's pretty much standard on anything that runs A/C.

Chris, and that would be an imperial gallon, correct??? That’s got to put you down around 13 mpg (US). Yet, that is still quite amazing for a 1-ton with a 5.13 gear running dual carbs.
The best I’ve ever gotten with my stock ’50, 3600, 216 with a 4.10 and stock sized tires is 13-14 mpg (US). Wish I know your tuner.
Gear ratio? In my experience that always makes a difference, It’s simply a matter of rpm turned per miles traveled.
More cam? Valves open longer therefore more time for fuel and air to be pulled in, result, more power which equals poorer mileage.

Paul I to find it hard to believe that a 235 would run for very long pouring that much fuel through it and still not have the plugs fouled out within a few miles. Lets just apply a little common sense here, an Indy car engine averages 650 to 800hp and is pulling a 1550lb car around a track at an average of between 170 and 230mph. It will burn just under 2 gallons (US) of methanol, a little better on ethanol and where they running gasoline that figure would be somewhere between 3 to 4 mpg. A NASCAR operates in about the same horsepower range on gasoline but has to pull nearly twice the weight, 3400lbs., however they are running at average speeds of 100 to 200mph, their mileage is about 4mpg. Now does it make sense that our 216 pumping out 80hp and pulling 3300 to 4100lbs. at 40mph is going burn 4gallons per mile?

Stu, I’ve experimented with advancing my stock 216 so far that the starter would not even turn it over with out kicking back and the idle was so rough that I thought the engine was going to break a mount and it still would not ping out on the road with 87octain E10 when it was lugged down in forth gear. That’s with stock compression by the way.

Well in the time that it has taken me to research all this, the tone of this thread has changed a bit. Now I see we have over drive with a 3.73 gear, which makes it all the more unbelievable. Paul a rough guess from my end would be that you haven’t sampled enough fill ups to get a real average. There is just no way that your getting that sort of mileage with that set up.
Denny Graham
Sandwich, IL

Not an imperial gallon, a US, 3.78 liters american gallon.
Much of my commute is at 30-35 MPH, at which speed I run at 19 inches of vacuum. So the old beast is hardly workin'.

It drives my Dad crazy when I use american gallons, not british!

My old Pontiac only got about 10MPG. But the fuell line was leaking. Boy, fixing that sure helped!

Paul,

Does your oil smell like gasoline?
I've had fuel pumps go bad and pump gas into the crankcase.
I can't hardly believe that you could get 500 miles if this was the case, though.

regards,
Leon

Since it you said it really runs great other then the off idle surge tells me you are on the high side of the jetting. If it surged all the time and ran crappy but still got 4 MPG, I would think it was lean. You need to step down a lot more and open the carbs more if they are lean. The off idle might be the power valves opening to soon or not at all. Vacuum will still be the same as if it has one carb, but the airflow thru each one will be about half. The lack of airflow can really mess with them.

You will need to pull the tops off each carb and take a look. There were a ton of model B's made so you have to be sure you have a matched set. Jets, floats, ventures, power valves, etc.....

I have a Uni-syc gauge I could loan you. These let you set the throttle valves just the same so each carb has the same airflow. Then you set the throttle linkage.

Set the timing up some, get it to idle as best as it can using only the distributor. Turn it till it speeds up, using a vacuum gauge here set the timing for the most vacuum. If it starts hard, back it off a little. We have better gas now then way back when, so you stand way more timing then the books call for.

I also have a air/fuel ratio gauge that sticks in the tail pipe. Its real handy for setting the idle adjustments. I have rigged it so I could drive around and check the mixture. Mine was way rich, then I went way lean. It was easy to see on the gauge.

:et me know if you need to borrow them, Joe.

I would think that your plugs will tell the story. I haven't been trying to get an exact reading on my mileage but I have no drivability problems and my plugs are a light tan color so I am happy with it.

maybe you have a small gas leak some where..

I dont see how any bad tune up would get that bad of gas mileage..
mine would get up to 20 mpg but with out the extra carb, ...

good luck,, gas is going up so you will have a large gas bill driving the 1,000 miles ...

My truck was way lean and ran awful. I had followed the directions that came with the manifold, calling for 3/4 turn opening on the idle screws. And opening the throttles a whole bunch so it would idle. It ran after a fashion. After opening the idle screws a turn and a half, I was able to close the throttles a bunch. Made all the difference.

I sure hope this straightens out for you. It looks so good, it should run that way as well.

Paul,
In your fourth post, if I understand you correctly, you are basing your conclusion that you have poor milage on the one fill-up where you took 9+ gallons on thirty something miles. I would keep checking.

Perhaps during the previous fill-up you didn't really fill it to the top, so on the one fill-up where you did your calculation the tank took the 9 gallons. Keep checking and let us know. My guess is that you are really getting double digit mileage.

lightholder, you gotta remember that Paul counts in zero's and one's.....lol

good point though, fill that sucker up to the top, run it till it is almost empty on both hiway and city and then do the math

should not be any reason at all that you cant get at least 17..

or we could make a deal on my rod,, I know for sure you can get 20 on the hiway at 65 to 70 mph and probably 17-18 in town

I've been thinking about what could possibly put me in the < 4mpg range, and the only think I could think of, with the truck running good, was a leak. Either a slow leak all the time, or the fuel bowls overflowing into the manifold and then the gas cooking off after I shut the motor down. (And I have smelled gas in the garage after driving it.)

So, today I made a run down to a truck stop that had a Cat certified scale, so I could weigh my truck. (It weighs 3400 pounds, btw.) I filled up at a station in Richardson before I left. Then I filled up at the same station when I got back. I drove normally, for me, reaching speeds of 72 once, running in fifth gear at 65 most of the time. (Over 70 gets a bit "hinky" - the truck shakes.)

My odometer readings were 543.2 and 555.1 respectively, and I bought 2.405 gallons of gas. However, I know darn well that that trip was more than 6 miles one way (the highway exit numbers tell me that!)

So, since I had both addresses, I checked mapquest. Total distance was 30.44 miles - 12.67mpg.

What I can't figure out is how the speedometer could be so accurate yet the odometer is clocking about 1/3rd the mileage. I'm going to have to check the manual, because I may have missed a setting somewhere. (It's electronic.)

Anyway, the mileage isn't great, but it's nowhere near what I thought it was.

I haven't had time to read all your posts, so I'll do that now and comment on anything I need to. Thanks for everyone's input.

Joe, since I now know that I will be driving my truck to KC, I might just borrow those tools from you and return them to you at KC. I saw a manometer online for about $50 last night, so I might just go ahead and buy my own. Let me think about it.

Some of the other suggestions, about fiddling with the timing, making sure the carbs are matched, etc. will definitely be tried over time. I guess the first thing I need to do is figure what the heck is wrong with the odometer and remember that I need to change oil every 1000 miles until I do. :-)

I finally dug up all the paperwork on my truck.

The cam: valve lash is 0.012, per the cam manufacturer. Max cam lift: .28843. Net valve lift: .43264 I bought the cam from Patricks. It's a 254 grind, solid lifter cam, with .425 lift.

I got the air cleaner from O'Brien Truckers. It's a cast aluminum, finned, oval air cleaner. (I think it was 19".) It's a two piece air cleaner. The base had to be modified very slightly to align to my carbs. I had the top powdercoated black.

your mileage still seems a tad low..... check it on the hi-way for a couple of hundred then you will know..

yours might be pretty close with the 3:73 gear... 14 around town..



you need to find why your truck shakes at 70,,, should not do that.

I drove my as fast as 82 with the stock front end, it would go straight as an arrow..

Thanks, joker. I'll have to take you for a ride so you can advise me.

Are you coming to the show next Sunday?

Sunday I will be in El-reno...if you are speaking of the Sunday coming up.. why not make the El-reno meet, it is not to late.. you will enjoy it...would be a good test for your mileage chart plus you will have some great mechanics there that love to tinker, I am not one of them..lol

if not when and where is it? I could also advise you from following behind you.......lol... I am just saying it should not be shaking at 70... probably just a wheel out of balance, my ranger shakes just a tad at 66-68 then it clears up.. I too need to get it checked out..

I've committed to the car show in Addison that raises money for cancer survivors. Andrew posted about it here.

Just curious.
Were you able to find out why the odometer was reading wrong?

regards,
Leon