Well i decided to get rid of the computer controlled crap and go old school. ot myself a quadrajet m4mc carburetor and a vacuum advance distributor. Hooked everything up, set the timing about where the old hei unit was (20 degrees).

The car drives great lots of power, but i'm only getting 10 mpg and the exhaust out the tail pipe smells kinda rich. I also notice a small stumble upon acceleration from a dead stop. what could be causing this?

Also the bottom plate to the new m4mc was missing a idle mixture screw so i swapped the bottom plate from my e4mc 307 carb. could that be causing these problems?

 

what carb did you get, they're not all the same as far as calibration goes. swapping the throttle body from an e4m onto an m4m doesn't sound like a good idea. there probably passage issues, at the very least.
what is the advance curve like for the distributor?


bill

 

Heres the carb number is case it helps (17058253) says it came off a 1978 olds 403.
I compared the base plates and they were exactly the same, every hole and passage-way was in the same place.

I have the APT screw set to 1.5 turns from lightly seated

To be honest i dont know anything about advance curve or how it works or any of that. all i know is the distributor came off a 1977 olds cutlass with a 350.

 

Also thought i should add that the 10 mpg is highway with light acceleration (just cruising) even with cruise control on its still 10 mpg

 

Carb needs rejetting to match a 307? A 403 would naturally require more fuel...
Glad the power level rose though!

 

It has a olds 350 now no more 307.

 

My bad... But still might need a small rejet since a 403 would require more fuel flow than a 350. Should be an easy jet swap (I hope)...

 

Okay thanks i'll give it a try since i have an extra spare carb that came off a 79 chevy 350. I'm assuming these jets should be about right. (i hope)

 

Okay just changed out the jets the ones installed were 73 jets with 55p rods. And I installed 71 jets with 51m rods. (from a chevy 350 m4mc carb.)

What do you guys think? Is that good or should i go smaller? The engine is a stock 1970 350

 

Install rejeted carb and report back with results.

 

fuel mixture curves are quite a bit different between brands, a 350 chebby and a 350 will be very different. then, there's the power piston spring..... the letter following the rod number denotes the type of taper, this correlates to the power piston spring used. and the spring is also selected on the vacuum that the engine develops at different throttle opening angles. generally, chebbys used a different spring than olds, or the other marques, especially in the m4m series.


bill

 

Just got back from the test drive. Drove 35 miles used roughly 2 gallons so i think its good now. Won't know for sure until I refill the tank which will have to wait cause now I'm flat broke. But thanks for all the help guys

 

If that mileage is correct, then you're good!
At this point I would be happy even despite the brokeness!

 

Well i think i need to go either a jet size smaller or rod or both. I may have gained 2 mpg i'm averaging 12 highway right now.

Can you guys give me some rod and jet combos that would work for my 350 and give good fuel economy? I dont expect much but i figured if i can get atleast 15-18 mpg highway i'd be happy. Thanks.

 

Over the years, as I disassembled Q Jets, I wrote down the metering rod/jet combo's. I noted a '76 Cutlass 350 as having 67 jets and a 41J primary metering rod. On the secondary side the metering rods were marked CV with an H metering rod hangar.

The power tip on the primary metering rods is .026" with the exception of rod 50D which is .036". That one was apparently used in the 455 motorhome engine. If you can find one, pick up ROCHESTER CARBURETORS by HP Books. My latest copy is 1986 and it has tons of usefull Q Jet info and charts.

When making changes, calculate metering area first then adjust rod/jets to acheive the percentage change you are looking for.

One other thing. As you've effectively eliminated the computer, does your torque converter lockup function still work. It's controlled by the ECM. Without it, you're going to take a 15 to 25 percent hit in fuel mileage alone.

 

Wow cool thanks for the info. Gonna try that rod and jet combo next. And I dont have lock up anymore i'm running a th350.

 

Well i'm trying to understand primary rods and jet sizes and this is what i come up with correct me if i'm wrong but, if i were to install the 55 rod in the 71 jet would it be leaner then the current 51 rod and 71 jet?

I was thinking about it, that if its messured in thousands of an inch a 71 jet is smaller then a 73 jet and a 55 rod is bigger then a 51 rod correct?

Sorry if its a little confusing, This is basically the first old school carb i've ever messing with. I've always dealt with the old E4mc and fuel injection on my ranger.

I need to figure this out quick otherwise i'll be forced to reinstall the ccc carb and dist. and figure out the carb. problems on that. Or worse have to take the olds off the road and drive the ranger again .

 

The 55 metering rod will leaner than the 51 as it takes up more space in the jet. You can easily calculate the metering area with the old formula: pi (3.1416) times the radius of the circle squared. The radius of the cirlce being one half the diameter.

Calculate the area on the 67/41 combo from the 76 Cutlass first as a ballpark. Then duplicate it as best you can with whatever assortment you have at hand. This will determine light to part throttle area. Remember, the power tip is .026" so full throttle area will be a different figure.

 

Here's what I think the problem may be;

The CCC throttle plate that you're using doesn't have the hole drilled for getting vacuum to the power piston of your non CCC carb, therefore the piston is always 'up' and is at the full rich position 100% of the time.

 

Okay cool atleast i somewhat know what i'm doing now gonna try the 55 rod tomorrow see if it helps any. Apparently a 67/41 combo is more rich then my current setup of 71/51.

 

I think you hit the nail on the head. Just compared the throttle plates and indeed the e4mc plate does not have a hole for the vacuum under the power piston. I'm gonna drill a hole for it tomorrow since i cant reuse the m4mc throttle plates i'll just modify the e4mc plate and make it work.

 

Be aware you may have to punch a hole through the gasket too.

 

Thanks for the help guys i think that last repair fixed it. I drilled a hole in the throttle plate for the power valve and now the exhaust isnt rich and it runs great. Gotta go to harbor frieght today so perfect chance to see what the mpg is now.

 

Cool! Keep us posted on mileage, power, and fun factor!

 

Well mileage seems to be better getting roughly 15-17 mpg. Power is amazing, theres a big difference in power compared to when it was using the CCC crap. And a lot more fun to drive, i can actually spin the tires from a dead stop again .

I do notice when i'm idling the exhaust is still fairly rich not as bad as when it was burning my eyes though. Thinking i might need to adjust the idle mixture screws a little bit.

Also I think the well plugs are leaking (common problem i heard) could that cause the slight rich condition? Gonna JB weld them as soon as i get the chance.

 

the well plug leakage issue affects primarily 1965-1967 quadrajets. it is possible that yours are leaking-but pretty unlikely. the leaking well plugs manifest themselves after the car is shut off and sits for several hours. the engine is difficult to start. a slightly rich idle condition is much more likely to be your calibration(primary rods, jets, and power piston spring) and idle mixture screw settings. i would leave your well plugs alone.


bill