Long story short, I was getting about 17mpg highway with a 351C 2V in my F100. That was with a Screwed up 2100 carb, and a comp 260 cam.

I swapped to another 351C 2V with a edelbrock 600 carb, performer dual plane intake, and 2172 performer cam.

Mpg has dropped to 10 highway. That isn't gonna fly with me.

Could it be that the bigger carb and cam killed it that bad? It's really not that big a cam?

Today I swapped the metering rods and jets to get as lean as I can on cruise. Hopefully that will help. But if it doesn't I'll have to take more drastic measures.

 

New distributor?

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No, same distributor.

Maybe a vacuum leak? I haven't found any, but...

 

Here's the cam specs side by side

Current
486 intake lift/ 512 exhaust lift
282 intake duration/ 292 exhaust duration
112* lobe separation

Old (good milage cam)
484 lift
260 duration
110 lobe separation.


Is that longer duration what killed my mileage???

 

Might have to go through and check your base or initial timing, springs/weights, total timing and finally, make sure vacuum advance is set right. That's real important for mileage.

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Yea that's all good. Running about 15* initial timing, holding a good 17-18" vacuum at idle.

 

Maybe check it with a light and see what all in timing is at cruise. Sure the vac advance is working?

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I'll check it out, it was working fine before.

Something is definitely wrong though. I would expect this milage from a hopped up big block, but not a mild small block cleveland.

 

Could running too lean be the problem? I just did the math of the useable area of the rods and jets, and I'm running pretty gosh darn lean compared to stock calibration.

Stock calibration uses a
.100" jet with a .070" rod, makes for .0040 sq. in. Opening

My calibration was
.095" jet with a .070" rod, makes for .0018 sq. in. Opening

It ran ok like that. No drive ability issues that I noticed...

I swapped to bigger (.075") rods
.095" jet with a .075" rod, makes for .0012 sq. in. Opening

There are now lean drive ability issues. Large flat spot and no power until power circuit hits

So obviously I've hit the very lean end of things. And was running pretty lean anyway.

 

.0018 and .0012 are wrong, should be .0032 and .0026

 

Swapped back to 100 jets and the 075 rods. Just a hair richer than my original setup. It's back to driving nice.

I wonder if I should go to the stock edelbrock setting, 100 jets and 070 rods. That would be even richer. But richer means more fuel will go through and that's my problem. I've got too much fuel going through.

 

. You swapped entire engines? From maybe a 9:1 compression ratio 1970 351C to a 7:1 compression ratio 1971-1974 engine? New engine definitely a 351C and not a 351M? (look the same externally except for block height/manifold width)

. Problem with choke not opening at the same rate?

. Too heavy a right foot now?

. Would you go to a bigger cam if not planning to use the power/right foot?

. Thermactor not opening on end of air cleaner intake tube (if equipped)?

. Calculated MPG incorrectly/math mistake?

. 17 MPG sounds high for your previous setup... was that overall from a tank of gas? Or just a steady 55 MPH cruise? Do you have a MPG computer on the truck?

. Did you change the cam advance, retard, straight up when installing it? Is there advance or retard ground into either cam?

. Compression check to verify all valves are able to seat completely when lifters on cam base circle?

. We mainly look at the durations at .050" lift, what are they on both cams? (OK, Comp 260 @ 212/212, new cam about 214/224?) 112 new LSA usually helps improve MPG over 110 old LSA... cams don't look to be that different...

. 4 bbl. carb. sometimes get better cruising MPG than a 2 bbl. carb. because of smaller, more efficient throats/venturis/butterflies/jets in primaries of 4 bbl. carb...

 

Yea I swapped entire engines, compression ratios should be damn close. Both engines are built basically the same way.

It's a Cleveland, no ifs ands or buts

Old motor didn't have a choke, new motor got a manual choke. It's open.

My foot did get a little bit heavier with the new power, but just cruising it's still bad

Both engines are using the exact same open air cleaner.

17mpg is what I was able to get on the highway, cruising at 60-65 with an autolite 2100.
However that carb had serious lean issues.

Cam timing is straight up on both.

The cams are pretty close except for the duration and exhaust lift.




I was playing around with the carb yesterday. Swapped some jets. I think I may have had a stuck float. But not sure.

I drove it around town more yesterday and the gas guage didn't drop as quickly as it had been. Actually kinda slower than expected. So somehow I think I may have fixed the problem, but I haven't driven enough to know yet.

 

Your new cam is pretty big. Is there a noticeable differance in idle? I would guess that the new cam would drop at least a few mpg simply due to it's opperating rpm range over the idle to maybe 4000 rpm opperating range of your old cam.

 

It idles nice. No loping.