Poor gas mileage stock 300 six and C4

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F6 lathead
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Poor gas mileage stock 300 six and C4

Post #1 by F6 lathead » Tue Jan 09, 2018 5:15 pm

I have a 67 F100 with a 300 six, just rebuilt the carb, which is a Ford F1 with a manual choke. I replaced the spark plugs gapped at 45, plug wires, distributor cap, rotor, new points gapped at 25 degree's. The truck has a C4, and I am unsure of the rear end ratio. I adjusted the fuel mixture to where I got a smooth idle, I adjusted the dash pot screw per the instructions in the kit. I have had some leakage or squirting out of fuel around the dash pot gasket when I have had the air cleaner off and was revving the motor by hand. I tightened the screws around the dash pot and the leaking stopped. I am only getting around 8 mpg with this set up right now. I believe these motors got around an average of 15 from what I remember. I have read on here to check for vacuum leaks so will do that next. In checking the spark plugs before I replaced them I noticed that only one of the plugs looked as though it had the correct burn, the others looked as though they were wet. So I think that I am running rich but not sure how to rectify that? The motor is completely stock, no modifications at all. The exhaust system leaves something to be desired as the tailpipe is gone from the muffler back , but I don't think that is an issue. I also just did a transmission service as well and I drive the truck conservatively. I have a 71 F100 with a 460 and a C6 and get 12 mpg so this is unacceptable to me. Any ideas would be greatly appreciated.

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Re: Poor gas mileage stock 300 six and C4

Post #2 by wallen7 » Tue Jan 09, 2018 5:56 pm

In a stock 1967 motor the plug gap should be .035 and the point gap should be around .018 to .021 and give you a dwell reading of 32 - 36 degrees if my memory is correct. Timing should be set on 6 - 10 degrees BTC with no vacuum on distributor. Check the balancer and make sure the marks are correct, these are prone to slip after age.

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Re: Poor gas mileage stock 300 six and C4

Post #3 by F6 lathead » Tue Jan 09, 2018 7:59 pm

ok I will check that info thanks, I also took a look at my vacuum advance and discovered that it was not working/moving at all while applying vacuum to it. I checked the vacuum line from the carb for leaks and there were none. I removed the dist cap and moved the breaker plate by hand and found it to be stuck and would not move in either direction. I used some WD and some electrical cleaner and now it works by sucking on the vacuum line, and I think that there is not a hole in the vacuum advance diaphragm. I took it out for a test drive and it seemed to respond a little better. I'll Keep working on it.

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Re: Poor gas mileage stock 300 six and C4

Post #4 by Lazy JW » Wed Jan 10, 2018 9:50 am

Make sure that the heat riser in the exhaust manifold is not stuck; it should move freely and the spring should keep it closed until it warms up.

I'm not sure if that distributor is a Load-a-Matic or the newer dual-advance type. Regardless, the advance mechanism needs to be able to respond to the appropriate signals.

Also, check the exhaust for partial plugging.
"The White OX" 1974 F-350 300-6, Stock single exhaust, Carter YF, T-18A, Dana70 w/4.11, Flatbed dually w/dump bed. "Where no oxen are, the crib is clean, but much increase is by the strength of the ox" (Proverbs 14:4)
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Re: Poor gas mileage stock 300 six and C4

Post #5 by F6 lathead » Thu Jan 11, 2018 6:41 pm

I will check the heat riser, also just put a new exhaust system on it, 2" all the way back with a free flowing 30 inch glass pack. The distributor is a single advance unit. I not sure about the info that I was given earlier on the point gap/plug gap. I checked the timing and it is right on 10 degree's btdc. The truck runs good until you get up to speed (60mph) and then I am feeling some stutter? just did a complete tune but still just 11 mpg, as far as the advance goes, should it hold when putting vacuum to it. ie. when I have my tongue over the vacuum line, right now it bleeds off but will move the breaker plate.
thanks for the help

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Re: Poor gas mileage stock 300 six and C4

Post #6 by pmuller9 » Thu Jan 11, 2018 6:56 pm

Is this "Load-O-Matic" distributor without mechanical advance?
Last edited by pmuller9 on Fri Jan 12, 2018 11:37 am, edited 1 time in total.

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Re: Poor gas mileage stock 300 six and C4

Post #7 by bubba22349 » Thu Jan 11, 2018 10:13 pm

F6 lathead wrote:I will check the heat riser, also just put a new exhaust system on it, 2" all the way back with a free flowing 30 inch glass pack. The distributor is a single advance unit. I not sure about the info that I was given earlier on the point gap/plug gap. I checked the timing and it is right on 10 degree's btdc. The truck runs good until you get up to speed (60mph) and then I am feeling some stutter? just did a complete tune but still just 11 mpg, as far as the advance goes, should it hold when putting vacuum to it. ie. when I have my tongue over the vacuum line, right now it bleeds off but will move the breaker plate.
thanks for the help


Yes the advance should hold steady if it bleeds off then the vaccum can is bad and needs replacement. This is likely a big reason your MPG is down. Good luck :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Poor gas mileage stock 300 six and C4

Post #8 by Lazy JW » Fri Jan 12, 2018 9:53 am

F6 lathead wrote:...The distributor is a single advance unit...


Then it is probably a Load-a-Matic, which requires the proper Spark Control Valve (SCV) to be working correctly on the proper carburetor. On a vehicle of that vintage it is impossible to tell from this distance if all of those things are correct at this time. More details and pictures would be helpful.
"The White OX" 1974 F-350 300-6, Stock single exhaust, Carter YF, T-18A, Dana70 w/4.11, Flatbed dually w/dump bed. "Where no oxen are, the crib is clean, but much increase is by the strength of the ox" (Proverbs 14:4)
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Re: Poor gas mileage stock 300 six and C4

Post #9 by F6 lathead » Sat Jan 13, 2018 6:04 pm

Thanks folks for the response, I'm getting a little disappointed with the mileage so far and I want to drive this truck as a daily driver, so I will replace the vacuum advance, and see if that helps. also is there an area that I can lubricate so that the breaker plate moves more freely? The carb that I just switched to is for a manual vehicle as it only has the one accelerator pump as opposed to the dual set up for the automatic tranny. I am not sure now about the distributor being a load a matic, it has vacuum advance and not sure about the weights?

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Re: Poor gas mileage stock 300 six and C4

Post #10 by F6 lathead » Sat Jan 13, 2018 6:06 pm

also the heat riser is rusted and barely movable. I loosened it as much as possible by working it back and forth and I think it is in the open position which I think is better than it being closed or partially closed.

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Re: Poor gas mileage stock 300 six and C4

Post #11 by F6 lathead » Sat Jan 13, 2018 6:33 pm

also again I just checked the new spark plugs for how they are burning and they all have a nice light brown burn to them so I think the carb is tuned right so maybe the vacuum advance will help I will keep you folks updated, and thanks again I like coming here for some ideas and all the experience and interest in a common subject.

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Re: Poor gas mileage stock 300 six and C4

Post #12 by F6 lathead » Sat Jan 13, 2018 7:59 pm

hey there Lazy J, I just read about what you were talking about with the Spark Control Valve, I think if I understood correctly that I need the carb that has the SVC, which I don't. would you happen to know what carb that is?

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Re: Poor gas mileage stock 300 six and C4

Post #13 by bubba22349 » Sun Jan 14, 2018 12:31 am

The 49 state 1967's Distribitors should all have a centrivical and vacuum advance (first year was 1966 with Calif emissions) so no SCV needed on the carb. Some pictures would help though to confirm that since parts get changed over time. If the breaker plate dosent move easy it is time to pull the Distribitor out and disassemble it for a good cleaning and lube of all the parts, or you could replace it with a rebuilt Distribitor. Good luck :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Poor gas mileage stock 300 six and C4

Post #14 by F6 lathead » Sun Jan 14, 2018 4:22 pm

ok, so here are a couple of pictures of the distributor that is currently in the truck. I am thinking that this is the Load A Matic? could someone advise if they know what it is. My understanding is that if it is not the Load A Matic then I don't need a carb with the SCV, currently I have a Autolite clone that does not have the SCV and has only one accelerator pump on the side and not two. I know that this carb is for a manual trans and I have an Auto. I am learning more all the time. Ha! The vacuum advance diaphragm on this distributor is suspect and I will be replacing it. thank you for the input.
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Re: Poor gas mileage stock 300 six and C4

Post #15 by THE FRENCHTOWN FLYER » Sun Jan 14, 2018 7:41 pm

When you spin the distributor shaft do you see weights spinning beneath the breaker plate? If yes then it is not a Load-o-matic and you are good to go.
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Re: Poor gas mileage stock 300 six and C4

Post #16 by bubba22349 » Sun Jan 14, 2018 7:48 pm

Yes that is indeed a Load O Matic Distribitor! Since your carb doesn't have the SCV to operate the LOM this is a real problem. You best choice considering that the Distribitor needs work and the vaccum can is bad would be to upgrade to the later point Distribitor (ask for a 1968 to 1974 rebuilt unit and turn yours in as a core) it would give you better performance and economy. If you wanted to go even better you can swap in the DuraSpark II electronic too. You other choice is to repair your current Distribitor and the get the correct carb with a SCV that works with the LOM. In the below link is article on how the LOM system works and also has a picture of Autoliie 1V 1100 carb with SCV looks like. Good luck :thumbup: :nod:


ci/Loadomatic.html
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Poor gas mileage stock 300 six and C4

Post #17 by F6 lathead » Mon Jan 15, 2018 10:01 pm

Thanks guys I did a little more digging after that post of the pics and found that it is indeed a Load A Matic, I ran through all the options that you came up with Bubba, and I am thinking that I may run with a later year points distributor. If I did this would I use the vacuum off of my current carb for the later distributor? I am also throwing around the idea of the DS II or an HEI recurved by Bill Ambler, as I have an Offenhauser intake, and Holley 350 that I may eventually bolt onto to this motor along with a cam. Great job guys and thanks again.

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Re: Poor gas mileage stock 300 six and C4

Post #18 by bubba22349 » Tue Jan 16, 2018 7:30 am

F6 lathead wrote:Thanks guys I did a little more digging after that post of the pics and found that it is indeed a Load A Matic, I ran through all the options that you came up with Bubba, and I am thinking that I may run with a later year points distributor. If I did this would I use the vacuum off of my current carb for the later distributor? I am also throwing around the idea of the DS II or an HEI recurved by Bill Ambler, as I have an Offenhauser intake, and Holley 350 that I may eventually bolt onto to this motor along with a cam. Great job guys and thanks again.


Yes with either the late point type or the electronic type Distribitors you use the vacuum port (a ported vacuum) on your current carb. I been running the DSII conversions on all my Ford six'es for many years it's a much better ignistion. Good luck :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Poor gas mileage stock 300 six and C4

Post #19 by Lazy JW » Tue Jan 16, 2018 9:51 am

It's mighty tough to beat a Duraspark 2 ignition. I literally haven't looked inside the distributor cap on my White Ox for years; it only goes a few hundred miles per year though.

That said, I have driven many hundreds of thousands of miles on breaker point ignitions; they run just fine but do need regular maintenance.
Joe
"The White OX" 1974 F-350 300-6, Stock single exhaust, Carter YF, T-18A, Dana70 w/4.11, Flatbed dually w/dump bed. "Where no oxen are, the crib is clean, but much increase is by the strength of the ox" (Proverbs 14:4)
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Re: Poor gas mileage stock 300 six and C4

Post #20 by woodbutcher » Tue Jan 16, 2018 3:54 pm

:D Love the DS2 system.On my second setup.First was in my 74 F00 with a 302.Current is in my 84 F150.Have put over 200K miles on the two systems with never a failure,with one exception.Lightening struck a power pole about 10 feet from my truck,and the ESD killed the ignition box.If you use the DS2,put an extra ground wire from one of the mounting bolts/screws to the fire wall or intake.Can`t have too many grounds.
Good luck.Have fun.Be safe.
Leo
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Otto von Bismarck

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Re: Poor gas mileage stock 300 six and C4

Post #21 by F6 lathead » Tue Jan 16, 2018 9:51 pm

Thanks guys I will be looking for a DS2 to run along with my performance stuff later on. Thanks again another problem solved and I will keep you posted.

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Re: Poor gas mileage stock 300 six and C4

Post #22 by F6 lathead » Thu Jan 25, 2018 9:22 am

Just got my carb in the mail, they sent me the wrong one! They sent a non SCV carb instead! but I think its a blessing in disguise. I will now get the 68 and later single vacuum distributor and my problems should be solved for the most part at least on the spark and fuel side anyway, so now when I decide to put on the Offy intake and EFI exhaust, I will have the correct distributor unless I go electronic. I will keep you posted as to what the mileage is looking like after installation.

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Re: Poor gas mileage stock 300 six and C4

Post #23 by THE FRENCHTOWN FLYER » Thu Jan 25, 2018 6:20 pm

you win!
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

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Re: Poor gas mileage stock 300 six and C4

Post #24 by F6 lathead » Thu Jan 25, 2018 8:11 pm

thanks TFF

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