rocky383

This is my first carbed car. I'm learning slowly but how carbs work is making more sense all the time. I have an Edelbrock 1407 (750cfm).

The first actual component change I did was put in silver step up piston springs to stop bog on acceleration. Vacuum drop was not enough to allow the step up rods to raise with the weaker stock springs.

Then I changed my plugs and they were black, so today I put in 073x052 rods making cruise and power modes each one stage lean. The car seems to accelerate more readily down low up to around 3500-4000 rpm. The tires spun when pulling onto streets from a stop in regular driving (not hot rodding) when they didn't before.

There still seems to be a lack of power up top. I know it's impossible to say exactly what will fix this over the net. But, what would you suggest I do next? Should I wait to see what the plugs look like in a month or two before going leaner? Could the secondary metering system be at fault and how do I check that?
BTW, I'm getting terrible gas mileage. About 10mpg city.

Thanks.

:hail:IROC-Z

Pablo

iTrader: (1)

I actually think you should go with the even weaker springs. That is a seriously small cam for that 383
that 750 comes with like .113 primaries and like 104 secondaries right? something like that? Id step them back a bit. What i find has really helped me is my AFR gauge. It didnt help me tuning my FI but it really works with the carb. Granted, pay no mind to it at WOT but alot of the stuff you tune for at part throttle to get at 14.7 translates into good wot performance (course the secondaries are a whole 'nother story)

as far as the plugs being black, when did you pull the plugs? after shutting it down at idle? if thats the case, then that only means your idle is tuned rich. no fuel exits out of your primary metering at idle
and even at cruise i think most of your fuel is coming from your idle circuit. If i were you Id lean it out substantially. Remember that carb comes calibrated from the factory for something a little hotter than what youve got

as for the power up top, the cam is not going to give a whole lot of power up top. But if anything, I think youd need to go a little leaner than what that carb came out of the box with.

course you can do the right thing and put in a 234/244 in it and quit wasting that 383 on that tiny cam

Damon

Terrible mileage can be either running too rich or not having enough timing advance.

Check timing first. Set the initial timing to say, 8-10* BTDC for starters with most non-computer controlled distributors. If you have a computer controlled distributor, that's your problem right there. Once you ditch the computer controlled carb the ECM goes into limp home mode and gives a pathetic advance curve. Dropping in a non-computer controlled HEI replacement (like the $150 Summit HEI) will make a BIG difference in both torque and top end power. Don't forget to hook up the vacuum advance- it can improve mileage by better than 5 MPGs in some cases!

Then do the carb. You're on the right track. If you got black plugs, you're runnin rich. Check simple stuff first. Is the choke opening all the way? It should be fully open (vertical) after 10 minutes of running (assuming an electrical choke). If not, find out why and fix it (usually, you're not getting a full 12V to the choke element, FYI).

Remember that your carb is running about 80% of the time on the idle circuits during normal driving. You wanna run the idle as lean as possible without causing a miss or hesitation. With the engine FULLY warmed up (20 miles of normal driving) you want to lean the idle mixture screws as far down as you can go. To the point the engine begins to stumble a little. Then back 'em off 1/2 turn. THe idle mixture screws are on the front of the carb, down low, pointed up at a 45* angle. Make sure you adjust them EVENLY! Not 3 turns out on one, and one turn out on the other. EVENLY. If it doesn't want to run right with an even number of turns on each you've got internal carb problems.

NOW you are ready to start playing with the PRIMARY jets and rods. The 750 is jetted rich from the factory, not designed for fuel economy (as you have found out). You may have to go quite a bit leaner than the factory calibration. I have found it VERY HELPFUL to simply disconnect the secondary side of the carb while doing tuning oefforts on the primary side. Keep playing with it until you get it right everywhere from idle to WOT up to 4000 RPMs on the primaries only. Don't worry about anything over 4000 RPMs for now.

If you have a transitional stuble that won't go away try playing with the accelerator pump adjustment. Putting it on the hole that gives the most pump action is a good place to start, then back it off one hole at a time until a stumble returns, then move it back one step.

Now that you got the primary side set up well, turn your attention to the secondary side. Everything that applied on the primary side will also apply to the secondary. Jets, rods, etc. You can also play with the rate at which the secondaries open on these carbs. You want them to get open as quickly as possible short of getting a hesitation or stumble.

That should get your started. Good luck.

rocky383

Thanks for the great info. I'm gonna have to wait on a new cam (money and time).

Timing's at 10*BTDC. HEI Pro Billet MSD.
Choke's good.
I leaned out the idle til it stumbled and almost died, then richened it up until it smoothed back out in park and in gear. Idle park is about 900rpm.

That's a good idea about disconnecting the secondaries while tuning the primaries. That's where I'm headed.

:hail:IROC-Z

Pablo

iTrader: (1)

damon, thats an interesting idea about disconnecting the secondaries to tune the primaries

im going to try that

one thing though, what about a setup that gives pretty crummy vacuum signal on the low end.. like mine for instance, I was under the impression that youd just jet it pretty rich on the primaries until the airvalve opens on the secondaries at which point there is enough airflow to get fuel out of the boosters. Would tuning like that make the car overly rich on the top end? Would tuning it like you advised rocky make it right on the top end but still leave it soggy on the low end? can I run a 2 hole carb spacer with just open plenum for the secondaries or is that just outrageously dumb because if the primaries dont have a good signal neither do the secondaries.. then again at high rpm the motor really comes "on" and the plenum floor of a torker II is pretty close and flat.. dig?

Id appreciate some advice from ya, i consider you one of the true gurus on the board

rocky383

would 1.7 rockers improve the cam situation enough to notice?

:hail:IROC-Z