84 5.0 w/carb conv.not cooperating/HELP!

nitrocapps · Jun 2, 2005

Hi, working on an 84 5.0, pulled the CFI & TFI, replaced it w/ Duraspark and a carb. Bypassed the hi pressure pump, using the lo pres. in tank pump, runs 5 psi (2bbl autolite, new rebuilt). HERE'S THE PROBLEM! Idles way too high(1200 rpm)despite the idle screw being set as low as it'll go. Power is weak at best. Mixture screws seem to have no affect on anything. And engine continues to run for several seconds after being turned off. Have not found any vacuum leaks. Any help with this would be hugely appreciated! Thanks!

txstang84 · Jun 3, 2005

how's your timing set? it should be about 12* btdc with the vacuum advance, pulled, and hose plugged...carb, hmmm, autolite 2bbl? what is it orginally for? whats the longest you've had it running for? is that carb equipped with elec choke? if so, the little assembly in there may be sticking causing idle to go up.

honestly, go buy a 4bbl intake and carb setup...

PuterAmI · Jun 3, 2005

txstang84 said:
honestly, go buy a 4bbl intake and carb setup...

Agreed.

It sounds as if you have a serious vacuum issue.

crazypete · Jun 3, 2005

It's a vacuum leak guaranteed!!

Has to be. What you said about mixture screws was a dead giveaway when coupled with the high idle. An intake leak is as likely a culprit as the carb. Spray carb cleaner around the base of both the carb and the intake.

I was shocked but last week, I found out that I basically had a leak around the entire base of my holley. All I can say is RTV like a madman. Gaskets are crap, RTV rules. Smear it real real flat first with your finger so its a thin film and then bolt down.

Stang8URMPRT · Jun 3, 2005

Why the fuuuuuuuuuuuck would you be running a 2 barrel carb on a 5.0? You should be running atleast a 575-600 CFM 4 barrel. What were you thinking?

crazypete · Jun 3, 2005

No way man.....the 6000rpmx302/3456= 524 cfm

The magic formula says 525 cfm is the right size for a low revving factory motor. Alas, the hollies only come in 2 barrel versions for the 500 cfm versions but they seem to have a 390 cfm 4 barrel. Weird. 2 barrel is easier to tune and less stuff to go wrong. Has it's advantages. OOOOODLES of torque with a small 2 barrel.

Ridiculous throttle response.

Stang8URMPRT · Jun 3, 2005

Oh yeah! I forgot! That's why the 84 4 barrel was FACTORY 575... Hum... I guess your theoretical calculator is way closer.

nitrocapps · Jun 3, 2005

Thanks guys! I was planning a 4bbl swap soon, so I'll get that done, that should solve any vacuum leaks from the manifold! Thanks again,
Nitrocapps

nitrocapps · Jun 3, 2005

Hi, It's at 12deg. Originally, carb was for 67 Cougar w/ 289. It has no problem staying running. I've had sit there idling for hours! It just drives like crap. No power, can't pick up revs when it's in gear.

PuterAmI · Jun 3, 2005

crazypete said:
Alas, the hollies only come in 2 barrel versions for the 500 cfm versions but they seem to have a 390 cfm 4 barrel. Weird. 2 barrel is easier to tune and less stuff to go wrong. Has it's advantages. OOOOODLES of torque with a small 2 barrel.

*looks at my 2-4V intake*
That is why 390 is a good size.

2V carbs are less fuel-efficient.

Always use a laminated insulator gasket between carb and intake.

txstang84 · Jun 4, 2005

12* is about right for the timing, but hey, like the rest of the guys are saying, good probability for vacuum leak...

whenever you decide to go with a new 4bbl and intake, just read up on what exactly you want-anyone can suggest the best setup, but ultimately, your decision should be based on how you drive. If you want some good low speed torque and strong midrange and aren't too concerned with the high rpm's, then go with a low riser intake, and a little 600cfm vac sec holley (if you can deal with a holley-bad luck with me), if you can trade off a slight amount of low end for some more midrange and top end, go with something like a weiand stealth, or edelbrock rpm...you get the idea. personally, i'm running an rpm intake with a 600cfm edelbrock, it has good low speed torque, and plenty top end pull.

you could also consider swapping out the cam while you have off the intake...you'd still have to pull the front cover and and valve covers off...but flat tappet cam kits are dirt cheap for your pre-roller block, and pretty much anything you buy would be an improvement over the stock cam...if you were to choose that option, just keep the lift kinda low, the stock rockers don't handle high lift too well

...and about rtv....keep it simple-it's a gasket sealer-not a gasket maker, the more you smear on, the more of a PITA it will be to remove it when you pull that part off again. when properly used, gaskets seal just fine by themselves, i just use the rtv to keep gaskets in place during assembly

Killer50stang · Jun 4, 2005

txstang84 said:
how's your timing set? it should be about 12* btdc with the vacuum advance, pulled, and hose plugged...carb, hmmm, autolite 2bbl? what is it orginally for? whats the longest you've had it running for? is that carb equipped with elec choke? if so, the little assembly in there may be sticking causing idle to go up.

honestly, go buy a 4bbl intake and carb setup...

Dont' forget, if he's got locked timing, it would be around 35* or so.

txstang84 · Jun 4, 2005

very true killer-didn't think about that aspect...

maverick0716 · Jun 4, 2005

crazypete said:
No way man.....the 6000rpmx302/3456= 524 cfm

The magic formula says 525 cfm is the right size for a low revving factory motor. Alas, the hollies only come in 2 barrel versions for the 500 cfm versions but they seem to have a 390 cfm 4 barrel. Weird. 2 barrel is easier to tune and less stuff to go wrong. Has it's advantages. OOOOODLES of torque with a small 2 barrel.

Ridiculous throttle response.

Are you trying to say that a 2 barrel will make more power and torque than a 600 cfm 4 barrel? Your right, that IS ridiculous! Or by your method, a 302 with H/C/I that revs to only 6000 rpm will be better off with a 525 cfm 4 barrel? Oh, what were the guys putting on 650 dp's thinking!.....As you can see I've always been skeptical (to say the least) about the engine/carb calculators.

txstang84 · Jun 4, 2005

well, the calculator does work....for someone not looking to maximize they're performance potential

crazypete · Jun 5, 2005

Dude, I went through 3 carbs and the smaller it got, the better the car ran!!

The calculator IS right. A lot of people totally forget that driving a car is NOT like sitting on a dynometer roaring the engine and looking for a bigger "peak" number. There are a lot of variables that make those peak number car suck while the less than peak number cars shine. If you choke off the top a tiny bit, you gain air velocity which equates to a good vacuum signal and that, my friend, is what carbs live on. Open it up for "maximum flow" and the air charge doesnt change direction and velocity quite as fast. What do you get? Good top numbers but a nasty bog when you hit the gas and only OK pull down low. I've gone through all this. A 670 is too much for a "stock engine". My 40-p headed 302 loves the 570 over the 670 as well. I'm n ot armchair racing here. I actually have 3 hollies and tried them all out and the winner is the small 570 vacuum sec carb.

Now.....

With a stick and not an auto, you can rev up first and bypass some of the bottom end funkiness. But with an automatic, I need to make **** happen at 700 rpms.

txstang84 · Jun 5, 2005

it's probably a foregone conclusion most of the people replying to this thread know very well that velocity makes the carb work...venturies love airflow, otherwise, they don't make that wonderful fog of go-go juice drop down the runners.

but, like i mentioned to nitro in an earlier thread-you have to find what works best for you-not the next guy, not MM&FF, not 5.0 and super fords, not the guy down the street, etc. what works for you doesn't necessarily work for the rest of us, if that little 570 works on your car-great! i have a 600 that works like a champ, but with the comp 270 magnum cam in mine, i still had to put in lower vacuum springs to make the damn thing idle-and even with that lopey cam, i still have GREAT low speed torque...btw, what all did you do to bring those other two carbs you installed into tune? what kind of cam do you run? what kind of compression? all those variables play into induction choices

and if those peak number cars suck, then their driver/owner probably needs to setup the rest of the car to match the engine combo...and i'm pretty sure that the majority of people replying here also are WELL aware of the fact that setting up the engine for peak power does not always mean great driveability....those are the compromises people make to pursue their hobbies.

Killer50stang · Jun 5, 2005

txstang84 said:
very true killer-didn't think about that aspect...

Just something I wanted to point out. Are you running locked timing?

maverick0716 · Jun 5, 2005

Sorry if I sounded like a jerk, I didn't really mean to be. It's just I had a friend go through this exact same thing not long ago. Someone reccomended a 390 cfm 4 barrel to replace his 2 barrel ('70 Maverick with an '82 GT engine). We installed it with an Edelbrock Performer 289 intake and Holley 390 cfm vac. secondary carb. I thought there would be a huge gain, but there wasn't.....you actually couldn't even tell anything was done to the car until about 3500 rpm then it picked up a bit.

crazypete · Jun 5, 2005

Cam is stock 91 roller cam. Compression is slightly lower than a standard gt-40p headed 302 since I did valve reliefs and chamber polishing. Port matched lower intake. 4.56's, AOD with 3 k stall and very light car.

The bog happens when you are sitting and tap the throttle 1/2 the way in: in far enough to signal a sudden high load situation but not so far that the accelerator pump gives a full shot. I've tried the most aggressive pump cam on the more aggressive position and it doesnt really make a big difference so it's not mixture I dont think. It got better going from the 600 DP to the 670 vaacum sec and better going from the 670 vacuum to the 570 vacuum so it must be aircharge velocity. I did all the standard stuff like floats and mixture screws, jets between 54's and 72's and the 9.5 power valve always. I'm too lazy to wire in my mixture guage but that would be the next step for me.

Oh yeah and between the 2 holley avengers and the 600 dp race carb, I've not ever had a working spark advance port.

84 5.0 w/carb conv.not cooperating/HELP!

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