[kvinmn]

I have a 68 Cougar with a 390 (X Code - 2bbl) FE engine. According to the previous owner, it has the following:

3500 miles on the rebuild
.040 overbore and about 10:1 forged pistons
GT or RV cam (he can't recall the specs)
balanced crank
mildly ported/relieved heads
interceptor valve springs
double roller timing chain
Edelbrock RPM Performer intake
Demon 750 'Race Demon' carb
MSD Ignition with MSD Distributor
stock exhaust manifolds with 2 1/2 in pipes through Flowmaster mufflers

I have to admit it doesn't run so good. I reset the timing from 12 to 18 degrees BTDC and it improved a bit. There is no vacuum advance to disconnect.

Does it seem normal? Is there any way to get a better handle on the cam specs? Considering the lack of real rumble in the idle I figure the cam has to be fairly mild.

It hesitates with anything other than a mild acceleration. Even then it seems to run rough until it idles for a few seconds and then settles down again. On hard acceleration it gags and coughs.

Doesn't it sound like there is too much carburator for the engine? Anyone have suggestions?

Thanks...

 

[modifiedmark]

I dont think that a mechnacal only dist is the hot setup for a street driver, thats why it run a little better when you advanced the timing.

A 750 is not to big for the 390 but a " Race Demon" probably isnt suited best either. The accelorator pumps are probably calibrated wrong for you. I got a bad taste of Barry Grant (who makes the Demon Carbs) when I was racing.

I also witnessed a friend destroy a 410 chevy race motor a couple of weeks ago with a borrowed race demon. Just my guesses. mm

 

[b0nes421]

Demon 750

The Demon 750 isn't too much for your 390 to handle. My cousin is running a Demon 750 w/vacuum secondaries on his 289 with an RV cam. His dad decided that he could make all of the necessary adjustments without reading the manual that came with that monster and ended up screwing everything up! It took my father, my cousin and me what seemed like an eternity to correct whatever it was that my uncle did. The car ran really rough at idle and seemed to choke if my cousin tried to jump on it, all because of that damn carb! Actually, it wasn't the carb's fault, it was my uncle... now that we have everything tuned my cousins '66 Stang is runnin' with the big dogs... that little 289 is a screamer now that the Demon 750 is good to go!

I would suggest making adjustments to the carb first, at least that was my experience with the Demon carbs, but then again we built that engine ourselves and knew the carb was the problem. Kinda hard when you have to take someone else's word for what was done to your engine.

 

[kvinmn]

Yeah....I know. However, he has talked with me numerous times to try and remember as much as he can about what went into it.

The sad part is he screwed with the carb too.

I'm thinking it is the source of my woes along with the exhaust manifolds. I'll probably pull the carb this weekend to check the butterflies and then set the idle screws back to the factory setting (1 turn out).

I just wish it would get a little warmer here (Minneapolis) so it would be easier to work on...brrrrrr

 

[modifiedmark]

kvinmn, Go ahead and check what you want but the idle screws are just that IDLE screws. Wont change the symptoms you describe. Once the butterflys open the idle screws are pretty much out of the equasion. Also, how in the world do you think its caused by stock exhaust manifolds?:1poke: That aint it either. mm

 

[kvinmn]

I guess I should have been more clear...the manifolds aren't part of the hesitation problem, just a part of the whole lack of performance issue.

I made reference to the idle screws just because I thought if they were way off and the butterflies were partially open at idle isn't it possible it would be too lean at acceleration and potentially be the source or the hesitation???

I am really rusty at this tuning stuff so if I am being more stupid than usual someone please help!!!!

Ken

 

[kvinmn]

Did a little more tuning work today...

Retared the timing to 13 degrees and it's performing pretty well.

The hesitation disappeared...after I filled it with a tankful of 100 octane racing gas. Now I have an uneven idle...

The fun never ends!

 

[calcat]

Vacuum secondaries make sure the engine only gets as much fuel as it needs. If the Race Demon has mechanicals, that is another story. Try going down two sizes on the mains and use that as a baseline. It sounds like you may need a smaller accelerator pump or cam as well.
just make one change at a time and note the difference.
I'd also switch to a vacuum advance unit. You can pick up a rebuilt unit dirt cheap and a recurve kit is only $25.

 

[kvinmn]

The Race Demon has mechanical secondaries.

Most everyone I have talked to thinks the carb is too much for the engine. According to the tech at BG Fuel the carb should have .076 primary and .083 secondary jets. I havent re-jetted a carb in a long time so I am a bit worried about tearing it apart.

The speed shop I bought the 8920 tach adapter from recommended a local mechanic that specializes in FE engines. I think I am going to let him have a look at it and see if he can make any suggestions...I'll keep everyone posted.

Oh yeah, one more thing...this morning when I was leaving I noticed a flood under the car. The fuel pump started leaking. It was really painful to watch that 2.49 per gallon stuff hit the ground...

Ken

 

[1974 cougar]

you have two things wrong that I can tell with out looking at it. The 750 race demon and no vacum advance. No 390 should have a 750 on it. 650 tops, holley and nothing but a holley. No Barry Grant crap. It's coughing and gaging couse that demon is doing what it's built to, dump the gas down into the heads at full force during wide open stomps. It would be great if ya were dragging it but on daily street use, that is just way too much. A 650 with vacum secondary is what you want. A little fine tuning and with help from vacum advance and she purr like a kitten and with some jet swapping you can pull some gas milage out of that 390 as well. With some work 20 mpg is not out of the question. And yes it can be done.

 

[Cougrrcj]

Drivability/tuning problems

1974 Cougar is correct.

For any kind of street use you will want to get rid of the mechanical advance only distributor and get one with a vacuum advance. That is step one.

Step two is to sell off the Race Demon and get something with vacuum secondaries. They are much more suited to street duty. The factory GT390 Mustangs and Torino/Fairlanes used 600cfm vacuum secondary Holleys. That should be a good start for your engine as well. Even the 428CobraJet came with 'only' a vacuum secondary 735cfm Holley!

Chances are that if you sell the Race Demon and the MSD distributor, you'll have enough to finance getting the right parts you need!

 

[kvinmn]

Ok, I understand the reasoning behind replacing the carburator. That part is pretty simple in my mind - however, can someone explain the difference between a vacuum advance distributor and a mechanical one like the MSD?

I'm definately interested in streetability since I don't plan on racing it at all. However, I still want to have big block power on tap when I want to use it.

I want it all....

 

[modifiedmark]

Yeah thats what I was trying to tell him! But I still say a 750 is fine if its a vac secondary. mm

 

[kvinmn]

Since it has an Edelbrock RPM Performer, would an Edelbrock 600 be a good fit?

Holley 650?

Maybe I should just consider fuel injection

 

[Logan]

Mechanical advance distributors can be tough to dial in, as you're stuck with one rate of advance throughout the rpm range of your motor. Your pick your amount of advance and rpm you want it all in by and pick the springs and stops to match that curve.

Vacuum advance distributors are alot easier to dial in as they rely more on the vacuum from your motor to tell it how much advance it needs.

Me, I was able to get my MSD mechanical dialed in perfectly with a little work. (Get the spring kit from MSD if you don't have one)

 

[1974 cougar]

Since it has an Edelbrock RPM Performer, would an Edelbrock 600 be a good fit?

Holley 650?

Maybe I should just consider fuel injection

I've never had any good luck with Edelbrock carbs on a ford. They make great intakes and all but I feel they fall very short for Carbs. I like holleys, Holley's run great on fords and easy to fine tune. 600, 650, either one would be fine. Fuel Injection would be cool. I would love to multi port on my 460 for my cat but then you have to deal with the wiring harness and all of that and that is one area i really struggle with. You coukld get the max out of your engine in horse power, torgue, gas Milage and emmisions with FI. It would be the way to go.

 

[rbaney]

I happen to agree with Logan on this whole distributor thing the mechanical advance may take time but it will work just fine the vacumn advance is much easier but that doesn't mean the other isn't useable for general street usage. Also I do believe depending on what that 390 has in it a 750 with vacumn secondaries should work out just fine as well just tuning time will be needed .

 

[modifiedmark]

Eldebrock, carbs are the way to go in my opinion. Never seen one give any trouble, no gaskets below the fuel level=no leaks!!!

Very easy to change metering rods and springs and not much harder to change jets, but mostly just bolt them on and forget them.

I've run Holleys for years, everything from 500 2bbls to 850 alkys and though they have there advantages, trouble free is not there best one.

Carbs dont care what brand motor they set on, Chevy, Ford or Dodge. There all just air pumps to the carb. mm

 

[kvinmn]

So, I have ordered the advance kit from MSD and plan on working with it this weekend.

The remaining question I have is:

Should I bother with re-jetting the Speed Demon 750 (mechanical secondaries) down or just sell it and put a 600 cfm vacuum secondary carb on it?

 

[modifiedmark]

If your not racing the car I belive you will be much happier with the vac secondaries. I myself would go with a 750 but if your looking for some economy and not top horsepower a 600 would be a very crisp carb for you. Your choice, holley of eldebrock.mm