Currently I have a built 360 Magnum motor in my 74 Duster. I have Magnum heads that have been mildly ported, a Comp Cam with 230* intake and 236* exhaust at .050 amd .544 lift on intake and .555 on exhaust,RPM air gap dual plane intake, with a Holley 3310 750 CFM vaccuum secondaries. The car has a 3200 stall converter in a 904 with TCI shift kit and a fresh rebuild of tranny,and 3.91 gears in a SG 8 1/4 rear end and Mickey Thompsons 275x60x15 ET street radials.

My problem is that the carb seems too rich. This combo was dynoed at 400 HP / 410 torque at the flywheel, but once installed in the car, there seemed to be too much fuel and has a strong smell of gas.

Is this too much carb for the motor? I have some that tell me to go to a Holley 4777 650 CFM, and the car will get better gas mileage and be faster at the track.

Which carb would you run and why? Thanks Moparts

check your vacuum ...and then check your power valve in the holley...

also...adjust your idle mixture screws..

check your float levels too...

i have a similar cam in my 73 duster and it runs great with a 750 vacuum holley..

Sounds like you have a carb. problem but it has nothing to do with the carb. size. Keep the 750 and fix/tune it.

I have checked the power valve and it is okay. Where should the level be in the bowls? What should I look for when checking the vaccuum? Where is a good starting point on the mixture screws? Thanks for the info.

adjust mixture screws 1/4 turn at a time for highest vacuum. then reset idle speed

what jets sizes are you running ?
check your float levels-with the engine idling gas should barely trickle out the bowl side screws

I have your twin...

I have a 360 Magnum in my '71 Demon. Stock bottom end with a 226/238 @ .050" with .520" lift. Eddy Air-gap intake, 727 with ~3500 converter and 3.91s. My best is 12.76 @ 104 mph. I am currently running an Eddy 650 right out of the box. I never even touched the idle screws. With that said, I am considering going to a 750.

""I have checked the power valve and it is okay.""

What is your in-gear vacuum reading with your foot on the brake (like you are at a stop sign)?

What number is your power valve?

I ran a holley 750DP on a mild 340 (stock 340 cam) w/ zero problems. It had great throttle response and I got 17mpg hwy... I would think a 650 might be too small for your application. The 750 vacuum is ok but it will require tuning as does any holley. If it were my car I'd probably get a used 750 DP, rebuild it and start w/ 72/80's a 6.5PV and 28 squirters...
FWIW, My 72 swinger had an almost stock 340, headers, J-heads and a holley 750 DP. It ran 13.80's on street radials and 3.55 rear.

the holley 3310 750 vacuum holley is an universal carb...it will need some tuning to make it work..and it will work great...

just need to check what number power valve u got...it needs to be something around a 6.5...and adjust the idle mixture...

the jetting will be close..


my 73 duster ran 3 tenth quicker will a 750 dp then the 750 vacuum...but driving every day i run the vacumm carb..

The power valve is 6.5. If the vac drops below 6.5 will this cause the PV to open? Where would I find step by step directions for setting this carb? Thanks

It sounds like the same problem I used to have with my Holley. If it has front and rear mixture screws, adjust them in all the way, then adjust out one full turn. Start the car and see how it idles and if there is still a rich gas smell. Make small adjustments to each one until the idle speed increases which means you are leaning out the mixture.

Thanks for all the help. What is the purpose of the PV and how does it operate? If manifold pressure drops to a certain point, will the PV open? The last time I checked my manifold pressure it was above 10 inches at idle. Don't know what it would be at WOT or part throttle.

behind the power valve is two small passages that provide additional fuel as a power enrichment circuit...

if u floor the gas....vacuum drops to zero...power valve opens and provides additional gas...

most holleys with two metering block in them ...the primaries will be about 8 to 10 jets leaner then the secondaies.

the pvrc provides that additional fuel....so u can cruise on a leaner mixture...then get more gas went u floor it...

http://www.mortec.com/carbtip1.htm

scroll down the page...has info on power valves.

http://www.youtube.com/watch?v=6vKlAnKdcB4

a holley video

http://www.youtube.com/watch?v=_QM0L9b4Q5s&feature=channel


another one about idle mixture..

http://www.holleytv.com/featured_landing.php?reset=true

""The last time I checked my manifold pressure it was above 10 inches at idle.""

Was this in neutral or in gear idle?

If you have an automatic, you can tune the carb to get it close in N, but then you need to finish tuning it with the car in gear and a vacuum gage hooked up.

The 750 will work fine but if the car is mostly a street car the 650 would be a better choice. I have a 360 with pretty much the same build (a little less cam) and have had 600, 670, 725 and 750 carbs on it. In order of preference its the 670, 600, 750 and 725.

Getting back to your rich mixture at idle. What is your initial timing? You should have between 15 and 20 degrees BTDC with the cam you are using. The magnum heads like less (32-34) total timing than open chamber heads.

With the overlap in the cam you are using the cylinders are not filling very efficiency at idle and you need to start the fire earlier to ensure all the mixture is burnt. Advance the initial timing in small increments until you get max manifold vacuum. You will then need to limit the timing in the distributor to keep the 32-34 number. If you have a MP distributor or an aftermarket one then you it will be easy.

Remember the first step in dialing in a carb is to get the timing right.

I think I'll stay with the 750 and try and fine tune it next week when I have some free time. Thanks for all the help.

If you buy a double pumper, go for the 650 because:
1 They're cheaper
2 they have the same baseplate as a 750
3 if you need more carb at a later date, buy the
new ported main body.