Low MPG

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mrbubbles

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I need some advise. I have a rebuilt 273 .30 over. I am running Edelbrock 1806 650cfm 4bbl, cam shaft slightly above stock, 355 gears with posi and a stock converter. I am getting about 8MPG:shock:. Does anybody have some advise on how to increase the mileage?
 
how rich is the carb running? whats the dist. set up like? you running vac advance? 3.55 gear will hurt ya some.
 
slow down or change the rear
seriously check your tire pressure, make sure the brakes are not dragging and give it a good tune up
 
abodyjoe said:
how rich is the carb running? whats the dist. set up like? you running vac advance? 3.55 gear will hurt ya some.
Click to expand...

not running huge jets. stock dist. vac advance is per chrysler spec.Carb tuned to edelbrock specs. PCV is hooked up correctly
 
What's your timing set at? My +.030 273 gets almost 20/hwy. I have 3.23's and a stock carb though.
 
The 355 gear hurts mpg. Stock points in a worn out distributor will hurt too. Electronic ignition will help but only when combined with a adequate charging system.
 
your MPG is horrible..my 360 with 4.10 gets 8-10 highway i believe
 
not running huge jets. not huge compared to what?

stock dist. vac advance is per chrysler spec. whats initial and total timing and when does it come in. whats it run for timing with the vac advance ?


Carb tuned to edelbrock specs. ????? screw what edelbrock says. what do the plugs say?
 
8 mpg is crazy for a 273 with 3.55s What color are the plugs?Will wheels spin by hand off the ground? Sounds like somethings dragging. If its not burning the fuel or that rich you would smell it. Sure cams not set a tooth off?
 
That carb is way too big for one. Is your vacuum advance hooked up?
 
In addition to the above, you sure some kid isn't visiting you after dark?

You sure you don't have a leak in the fuel system?

Engine running up to temp?

Is your odometer correct? You sure you know how to "figgur" mileage? (Hey jus' askin')

FIRST thing I would do is play with the timing. I'm big on checking timing marks. This is because, "back in the day," I used to find a fair number --seemed like a lot of Frauds-- which had "slipped the balancer."

So buy or build a piston stop


Remove no1 plug, make sure the piston is "down a ways" unhook battery ground, and install the stop. Gently wrench the engine over until it stops "on the stop." Make a temporary mark on the balancer directly under TDC on the tab. Do the same thing CCW. Now you'll have two temporary marks some distance apart. True TDC will be halfway between.

While you are at it, gently wrench the engine CCW, the CW, "feeling" for the slop in the timing chain. With practice you'll feel it. You should have NO MORE than 10* movement at the balancer, otherwise you need a new cam drive (chain.)

Regardless of what the above shows, I would advance the timing and play with it, advance it until it "pings" and back it off some.

You absolutely sure the vacuum is actually working, and that the mechanical advance is not stuck?

This is HORRID mileage, and your 355's are NOT what is the problem.

A real life example:

Back when, my 70RR 440 sixpack, with headers, an 800 Holley DP, Edlebrock, 3.54, 4 speed, and "hang on" air, driven "not all that careful" would get just SHY of 14 mpg at 70 mph.

Later, that same car got a 71 high compression 340 dropped in. Headers, otherwise stock, it would pull more than 17 on a trip on the highway. 3000 RPM at 70mph

ANY stock 318 that is in tune in an A body should be able to get at least that good and up into the low 20's.

I didn't think it was possible to get that bad out of a 273
 
Get your initial and ignition timing figured out FIRST!

Once you get that done, then start with your carb settings. Something really isn't right currently.

Is it the ede camshaft? What exactly is the cam?
 
crackedback said:
Get your initial and ignition timing figured out FIRST!

Once you get that done, then start with your carb settings. Something really isn't right currently.

Is it the ede camshaft? What exactly is the cam?
Click to expand...

The cam is
Summit Racing SUM-6901
 
Edel carbs are FAT out of the box so they "work" on alot of different motors, and "work" is subject to debate here. Get yourself a 65 FSM and look up the carb specs, or a rebuild kit for a 273 4bbl carb. It should show you rod dimensions and jet sizes but youll have to cross reference them to Carter numbers. Even a tuneup shop that has an analyzer should be able to get the carb in a little better shape than it is now, but jetting is a hardware issue. Does it smell rich, black smoke, choking drive-thru exhaust? try fatter rods to start, just a stage at a time. Edelbrock kits are not that expensive.
 
is that only around town driving? do you have the right speedometer gear for the 3.55 and your tire size? just curious, are you running headers? dual exhaust? what intake manifold?
 
You think you have it bad, I'm getting 15.5 mpg with a bone stock 225/auto.


Figure that out.
 
ahhh, slants arent exactly fuel efficient. i was always messing with mine trying to get more mpg best i got was 18 mpg with the super six 2.5" exhaust and recurved distributor . i put 3.73s in it expecting to kill that, i never corrected the speedometer but it really didnt seem to get any worse.
 
I've been helping Steve with his project so to answer some of the questions: the ignition is a new MP electronic ignition setup, new alternator, the carb was new when we put it on and have made a number of adjustments according to the guide that came with the jetting kit. We've messed with the primary jets, rods and rod springs to get a nice smooth acceleration and cruise. The speedo gear is the correct one for the gear/tire combination and the car has the stock convertor. The initial timing is approx. 8 BTDC but I don't know what the total is. Steve has the stock cast iron exhaust manifolds, stock 273 Formula S intake and Formula S 2.5" exhaust. The Summit cam specs are .441/.441 lift and 218/228 degrees duration at .050 lift installed at 112 degree lobe center. Nothing really that big. The next thing that we're going to check is to verify that the gas gauge is reading accurately. That would certainly mess up any mileage determinations.

Ken
 
physixx said:
The initial timing is approx. 8 BTDC but I don't know what the total is. ........ The next thing that we're going to check is to verify that the gas gauge is reading accurately. That would certainly mess up any mileage determinations.
Click to expand...

If you haven't modified the slots for the mechanical advance I'd guess your total timing is too high... I was running a stock replacement electronic dizzy on my 318 and one day I realized that with the initial set around 10-12* BTDC my total was well over 40* BTDC!! If that's the case I'm sure you're losing a good deal of efficiency from the engine having to work against itself. Vacuum advance on top of that probably just makes it worse in this case...

Also just checking, how are you measuring gas mileage? Most accurate way I've found is to fill up the tank, drive for a while and note the miles, then re-fill the tank and note how many gallons it takes. This way you don't have to worry about an inaccurate fuel gauge...
 
That's the plan for checking the gauge. I told Steve to do that about 3-4 times when the gauge reads approx. 1/4 full.

I've got to get the car to my house to get check total timing. We didn't do anything to the distrib. We just put it and set the intial timing with the vacuum advance blocked off. Hooked it back up and drove it. Makes sense to see what the total is. I set the timing on the race car for total advance if there is any, then lock it down.
 
My 2 cents... it needs in the 16-20 range for initial with that cam in that small of an engine. Might be a small cam for a 340, not for a 273
 
Joe... you crack me up "Carb tuned to edelbrock specs. ????? screw what edelbrock says. what do the plugs say?"

I feel the pain. I had to sell off my 06 wrangler I built up pretty nice because of the 10mpg's and suck up to a Minivan... dah
 
The MP distributors are good starting points, but I have yet to see one that was correct out of the box. In addition to the comments about the carb being too large, I think the advances need to be tweaked. IMO, I'd look at setting initial timing at 15°, and adjusting the stops to limit the centrifical to 20°. It should have one medium and one light spring... Make sure it doesnt have two mediums. Tune the vacuum can so it comes in as early as possible, and if it misses, preload the spring two turns until it stops. That should allow a lighter step up spring. Also make sure the accelerator pump shot is as small as you can. The car's light and there's gear in it. It shouldnt need a big shot. In otehr words, tune it better. Also verify tire pressures and the alginment.
 
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