Sorry, but Ive got some more carb questions...  I apologize for continuing to beg for help!

So here is the essence:

Car is a 56 sunliner, 292, ford-o, blue thunder intake, and mummert 270 cam, with 224 duration at .050.  (I mention that since it seems from what Ive read that this is a cutoff on some carbs - i.e. street demon 525 says for cams under 220 deg...)  Engine is healthy - compression was even when last checked (last year), new cap, plugs, wires.  Also has pertronix on a 57 dizzy.

Ive been working with 2 carbs - an edelbrock 500 and a holley 390.  Holley starts easier and has better throttle response (but still has some stumbling and spit back through carb at some points), but edelbrock runs better at wot (as would be expected with more cfm).

1)  Edelbrock carb pulls 11.5 in hg at idle, Holley pulls 7.5 at same RPM.  Does this make sense?  Not sure what this tells me or means, or if "thats just the way it is"

2)  I have been told to check the idle transfer slot on the Holley to make sure that I dont need to drill a small hole in the primary plates.  The transfer slots are covered... but JUST barely.  the throttle blades measure .065" thick, but the most they cover the transfer slot is .028".... just barely... is this enough, or would the hole in the blades help?  I dont want to drill them and have it be wrong... what are the consequences?  I do know that the idle screws dont have a lot of impact on the vaccuum... meaning that it takes a fair bit of adjustment to see a difference.

Right now the holley has 56 primary jets, 62 secondary jets, 3.5 power valve, and a 25 shooter.  Not sure if anything seems out of whack on that or if anyone has any recommendations?

Does any of this make sense?  I am getting really frustrated with trying to get a carb (ANY carb!) to run what I would consider right - easy starts, pulls evenly, without bogs or stumbles, with decent throttle response/mileage (is that oo much to ask???)  I think I wnat to try to get the holley dialed in first, as I dont drag race the thing, and the holley seems easiest to tune/adjust for me to learn on.

Please let me know if there is any more info that would help, or if anyone has any ideas/observations/different starting points?  Im really starting to regret the cam choice!

If the idle transfer slots in the Holley are almost covered, then additional bypass or holes in the throttle blades are not needed.  In fact, putting the holes in the blades at this point will add to any stumbling or hesitation issues the carb may or may not already have.  Recheck the secondary throttle blades in that they are seating closed and are both uniform in their closure.  As Pete suggests, sounds like a vacuum leak with the Holley.  Recheck the gasket flange crossways on the Holley with a straight edge.

I guess i never thought of the holley having a problem....I got it new, and it never occured to me there might be an issue.  I couldnt for the life of me figure out a logical reason why the vaccuum would be so dirfferent, and I guess there isnt one!
I will check the straightness of the bottom, and make sure all the bolts are tight before I put it back on, use a new gasket, and if it doesnt pull close to the same vaccuum, ill break out the carb cleaner spray to see if I can find a leak!

Also, do you guys really think its too much cam?  I talked to John Mummert, and he thought that this would be ok when I bought the cam... a little lopey idle, a little more power, but still useable/streetable w/ a fordo.  Thoughts?

I cant thank you all enough for the help.

Is that a Comp Cams item?  I know you have said you have had good luck with them.  Is that grind similar to an Isky RPM300?

Are you asking me?

     Either use oil (WD40) or ether and spray around the carb base, with ether, the idle speed will change when you hit the leak, with the WD40 you'll get white smoke from the tail pipes. Might check the hoses also, one could be in a different place that exposes a crack only on the Holley.   

      I'm not sure how you figure the cam will cause two different vacuum readings. That would be the subject I addressed. I'm sure your opinion on the cam duration is very important, but that's not the question here. Many of us here do not agree that a 224 degree cam ( @ .050) is too wild for a stret machine. In fact it's fairly mild. Mine pulls around 14 at idle in neutral at 700. There is no drive, it's a stick. 

      Noticed there's been no mention of initial timing here. Along with making certain the carb is correct, I'd suggest a bit more initial timing. Start around 12 degrees. You'll eventually have to close down the slot in the distributor to keep your total mechanical advance down to 36 to 38 degrees total.

       The reason for this is that the camshaft with more than stock duration won't see as much compression as the original at low RPM's. It will have quite a bit more at mid to higher revolutions. This normally means more initial advance is required to help cover the "flat spot". The down side being that you'll have to limit it on the top end to keep it from advancing too far. If you do a search on this site you can find a number of discussions on this topic. If you cannot find anything wrong with the Holley, you might try returning the carb to te original specs, and retest it. You may very well discover thet the Edelbrock works better for your application.  

DING DING DING!!!!  We have a winner!  I cant believe it.  I checked the screws that mount the base plate/flange to the main body and they were really loose.  Tightened them up, used a new carb/manifold gasket, and all is right with the world!  Was able to lean out the idle circuit by over 1 turn, pulls 11.5" vacuum at 500 revs in gear. 

Guys, I have been fighting this problem with this carb since I put it on last summer.  I have spent a ton of $$ and time on power valves and jets, etc.  trying to fix the problems.  I bought the carb new and it never occured to me that there was something so simple wrong with the carb.  I can not say THANK YOU enough to everyone who has participated on this thread!

I still think there is room to improve driveability a bit with some tweaking, but at least now it is responding properly to tuning/adjustments, etc. 

Next questions; I currently have a 4.5 power valve in there, and its running ok.... how will I know if I need to move up (i.e. to a 5.5, which is what they say I shold use with that vac. reading)?

And yes, I need to adjust the timing as well.  I thought that I had it properly indexed to TDC on the damper, but the car isnt acting like it is.  Per the timing light, its at like 25 deg initial advance, but the car starts/runs well; in fact it feels like it could use a little more advance, and I dont hear any pinging (although the exhaust is loud, so not sure how audible it would be?)  Even under WOT next to a concrete divider I dont hear any pinging...  Is there a good way to calibrate TDC w/o pulling the heads?

THANKS AGAIN to everyone for the help!  I really appreciate it!

Ron

Thanks again for all of the guidance everyone!  And thanks for the quick tip on timing!  Im assuming the spoke should line up with the pointer, correct?

What are the symptoms of a too small power valve?

If you feel a slight hesitation or stumble during part throttle acceleration where the power valve is still closed but the engine is wanting the additional fuel, then a quicker opening power valve can sometimes help.  If you have 11½ inches of vacuum with the tranny in drive, then the factory supplied 6½” power valve is typically more than adequate.  You can potentially jump the power valve up to a #85 (8½” vacumm opening) and have it come in slightly sooner but you’ll also see a slight decrease in fuel economy to go with the slightly earlier opening 8½” power valve but idling characteristics will remain the same as with the #65 power valve.

Is the only downside to too high of a power valve decreased mileage (assuming it isnt to the point of opening at or right off of idle)?  Could I possibly pick up a little more power at WOT with a higher PV?

As Gary mentions, the full throttle performance will not change with the different size power valves.  But there is another tuning opportunity that exists in this area and it’s in the size of the PVCR (power valve channel restriction).  If the engine likes more fuel under full thottle but fuel mileage is suffering with the larger jet sizes to get that better full throttle performance, then the PVCR can be enlarged appropriately so that the jet size can be kept smaller, but full throttle fuel mixture is increased.  This allows the smaller jet sizes to be used for cruising while a larger PVCR will allow for additional fuel in those situations where the vacuum drops below the power valve rating.  Just keep in mind that making the PVCR larger is easy but backing back up to the smaller sizes is considerably more difficult.

Ummm.... not really sure what calls for the attitude, but at least get your facts straight and read the issue and other parts of the thread before you get on your high horse.  My engine is a 292, not a 312.  I never cleaimed that it was "built" for a high performance.  If you do the math, the 390 CFM should be right for an engine of that size at 5500 revs, assuming 85% VE, (which is optimistic).... Ill do the math for you; it calculates out to needing 394 CFM based on those variables.

MOREOVER - if you read the beginning of the thread, I have an edelbrock 500 cfm & a holley 390 (do you approve of the edelbrock?).  the POINT of this thread, as stated, was that I wanted to see how good I could get it to run with the 390, as that SHOULD be enough carb and give a little better throttle response.  Once i got it running as good as possible on the 390, I am going to work on the edelbrock a bit more and from there figure out which one I like better. 

And also FOR THE RECORD, the FOM doesnt ALWAYS start in 2nd gear... if you go to WOT from a dead stop, it will drop into 1st and function like a regular 3 speed auto.

And as for cam recommendation, it was after several long conversations with John Mummert, whom I trust.  he seems to know a *little bit* about these engines.

BACK TO THE POINT - I would like again to thank those who have contributed their knowledge & expertise to help me trouble shoot a carb problem that I never would have found.  Ted, regarding your power valve restriction comments - is that a matter of opening the port on the PV itself, or does it involve opening a passage in the carb body?

THANKS AGAIN to those who have been very helpful in sorting out the 390 carb!

Ron

     The answer depends on which 390 you're messing with. Probably the passage. There were some power valves made that had "picture window" openings to pass enough fuel on large carb applications. Most have a series of holes drilled, while those had larger square passages, thus the name. I would really be surprised if you needed to enlarge the PV passages but I've been wrong before! The fact that no one else has had to do this makes me wonder if there's not another issue. For what it's worth, almost all Holleys come from the factory with slightly rich jetting, on purpose. That includes the power valve circuit.     

Thanks for the reply!  I think I will not mess with it much beyond putting the 6.5 PV back in and trying it.  It really runs pretty well now.... the higher PV should help smooth it out a little under acceleration, if i understand correctly.  I may also try leaning the main jets a little, as Im sure that need has changed a bit since I fixed the loose baseplate, etc. and solved the vaccuum leak.  I initially went up from a 51 to a 56 or 58 main jet when i was chasing the main problem.

Overall the carb is working pretty darn well now.... and best of all it responds the way that it should to tuning changes, which makes it much easier to sort out/fine tune.

Next up after this one is put to bed is to start working on the Edelbrock.  Car doesnt like to idle nearly as well with the edelbrock, and it is a little harder to start.  Throttle response isnt quite as crisp either.  Ill need to do some reading up on how to tune that one.  Alternately I was contemplating a demon jr. 525 - can you use holley jets, PV, etc. on the demons?

Ron

   I vote for the Demon unless somebody gives you the Edelbrock. Lotsa guys like them, but I am extremely happy with my Demon. I think Ted runs one also and will tell you the same thing. They do use Holley style Jets and power valves. The biggest problem with the Demons is that they work and don't leak! Kinda boring!

I actually already own the edelbrock - it was the first carb I bought with the blue thunder, but it always seemed a bit "sloppy" on throttle response, so i tried the holley.

More folks I talk to seem to endorse the demon jr.  I need to figure out how it would sit/fit under the original air cleaner.  I gutted the oil bath part and converted it to a hi-po paper type filter, but I like it to appear mostly stock for shows, etc......

Ive seen rebuilt ones on line for not tons of cash....   any idea if they work as good as new?

Thanks again for the help!

Gary, you are right again!  it is amazing how much knowledge is on this site....

So a while ago I used a whistler to try to confirm TDC was correct; but as mentioned, timing light said it wasnt quite right.  I just checked per Garys recommendation, and sure enough the marks are wrong!  I never would have known, as the whistler I used obviously wasnt precise enough....so I think timing for right now is ok, but I will have to fix that over the winter so that I can really get it dialed in properly....

THANKS!

Yup.... im thinking that if/when I get this fixed, id likely put a small mark in the pulley even with the 0 deg mark. 

so in researching, they say the 525 is for cams less than 220 deg duration @ .050.  Should this deterr me from making the switch?  Also, any thoughts on demon vs. demon jr?  I think the Jr has a single inlet - would be my preference with smaller float bowls, etc. if it would perform as well for my application...

Thanks!

The Road Demon Jr. uses a jet plate in the secondary side in lieu of standard jets.  Not a big deal as the carb is jetted reasonably close on the secondary side as supplied.   If stepping up to an equivalent sized Road Demon instead of a Road Demon Jr., then you’ll require a dual feed fuel line but you’ll also have conventional jets in the secondary side if you like to fiddle with them.  I’ve run both the 625 Road Demon and 525 Road Demon Jr. on my 272 with the Isky E4 camshaft and the 525 carb definitely gets my vote.  Previously to using the Demon carbs, I ran a myriad of Holley and Carter carbs with a 500 cfm Carter carb being the carb of choice until using the 525 Demon Jr.  The air cleaner requirements for the Road Demon Jr. are similar to those for a Holley List #1850 (600 cfm).  I also used the same air cleaner for the 500 cfm Carter for what its worth.

t-bird. Let us know how the holley 390 cfm works after the original power valve is reinstalled.