Holley carb stumbles

Discussion in '1974 - 1978 Mustang II Talk & Tech' started by itsaMustangtoo, May 18, 2006.

  1. I have always used holly carbs ( don't know why) and have always had off idle stumbles, hesitations, flat spots, etc. I used to fix them with a different cam. My 750 vacuum secondary on my 347 stroker-cobra clone now has an off idle stumble/flat spot. I have rejetted it- changed the power valve ( 55) from 65. i got a book of cams bought the kit and spent an afternoon trying them out. I went full circle to no avail and put the stock one back in (orange)( which seemed best power wise) .I believe , without checking my notes that I tried the blue,white,red,and black ones. My question is this- should I change the squirter? Or am I headed down the wrong path? I am pretty much done spending money on her ( after the 347 stroker project) So a new demon carb is out of the question.:bang:
  2. I assume this carb is on a 347 ci Small block. What is your intake manifold vacuum reading at idle in Neutral and in Drive?
  3. Yes it is on an edelbrock rpm intake ( 7121) shelby style. Camshaft is --Comp cams 280 dur / 512" lift and 230 [email protected] .050. So it is healthy, and vacuum is 10" at idle. I will check it in drive and part throttle cruise.
  4. Holleys are the best performance carb made, other than the Race Demons, which are Holley clones, but improved versions.

    I'd expect more vacuum with a 230 dur camshaft. Unless you're only running 9.5:1 compression? Or you have the idle speed really low?
    What idle rpm in neutral?
    How much initial timing? How much total timing, and at what rpm does it come in?
    What method did you use to adjust the idle mixture screws?
    The cam isnt the only adjustment for the accelerator pump. There's pump squirter size, and also the position of the pump cam, too. (you can also flip the cam upside down for further adjustment possibilities)
    What method did you use to adjust the accel. pump arm? (the locking nut) If the car is idling, and you wiggle the pump arm up and down, you should just have a tiny bit of lash. If it's too loose, you'll have pump squirter lag, if it's too tight, you'll get a reduced pump fuel shot, which may not be sufficient.
    Lastly, a 750 Holley is somewhat too large for a 347" motor running relatively low compression and a [email protected]" dur camshaft, because it doesnt have the RPM potential to take advantage of the big carb, especially on a dual plane intake. You can bandaid the slight overcarb condition by playing with the accelerator pump adjustments to get rid of the off idle flat spot caused by poor vac signal.

    If you bought a new 650 or 700 BG Race Demon double pumper, the difference between that and a standard Holley carb is very noticeable. Even a Race Demon 750 would be fine, since those carbs are much more responsive to vacuum than the Holley carbs.
    If you're determined to use that 750 Holley, you can still get it to run very nicely with some patience.
  5. If you don't have a hotter torque converter with a taller stall, you'll probably need to crank up the RPMs a bit more at idle. Be sure to start off your power valve setting 1.5" below your idle vacuum reading in drive.
    You either have a rich or lean off idle condition. i'm guessing you may be rich given the size of the carb. Or you may not be pulling a good enough vac. signal as you come off idle and that would cause a lean condition. Try going 2 steps higher in squirter size as well as 2 steps lower and see what that nets. Leave everything else stock. Have you re-jetted? Are your floats set properly?
    Ten inches of vacuum is nothing to shout about, and i'm sure that with it not even sitting in Drive either. See what other readings you get and let us know. :nice:
  6. #1) After my engine math compression is 10.2:1
    #2)Idle is low 600-800 rpm
    #3) still sneaking up on timing- right now it is about 10 degrees initial- 34 total- where it exactly comes in I am not sure of- I will recheck
    #4) I always set idle mix with a vacuum gage.
    #5) zero lash on pump arm ( have new squirters to try #35 and # 28)
    #6) When I was running my old 74 mustang with a 289 with 351 w heads I had a 600 on it. I drove a vega with a 350 and 750 dp as well as my buddies chevelle with a 750 vac sec on a 350 ( the vega was pro built) So as I looked at these cars and the performance I decided to change my carb. I got a 735 cfm off a cobra jet ( wish I still had that carb) put it on and the difference was like night and day.
  7. Torque converter is only a 2000 stall. I will work on the idle. I have rejetted and yes the floats are set. I am going to try the squirters. Stock is .031" and I have .028" and.035" to try. It has been raining alot around here lately so I have not had it out much.
  8. I'm surprised the vac is only 10" with 10.2:1 and a 230dur camshaft. You might want to check the compression with a gauge to verify. Cylinder pressure is a good thing to know for certain.
    If you have that camshaft installed 8 or even 4 degrees retarded, that would drop your idle vac, too.

    I'd try idling it up to 900 or 950. Higher vac, better response, especially with the larger carb.
    You can probably get more initial and 36-38 deg total timing in without it pinging, depending on what heads are on it. (this would give you the higher idle speed, without adjusting the idle screw, and would likely give you much better response, too.)
    The power increase you got from the larger carb was more likely due to a better state of tune on the 735 rather than because it was bigger, especially since 600s are designed less for performance and more for general factory replacement use. But as I said, the 750 will work well once you get rid of the flat spot(s) and get it jetted right.

    Wait to install the larger squirters until you fiddle with the other stuff, squirters should be the last effort if all else fails. There is still the pump cam position to mess with, too. I've always had the best results from installing the pump cam with the highest peak positioned just before the pump arm, so that the instant the throttle is moved, the highest peak hits and gives it an immediate shot of fuel.
  9. Well , after last night I am convinced I need a new vacuum gage. I had 8" @ 900 rpm and 5" in drive, and 15" at around 1500 rpm. I would think @ 5" my power brakes would not work and they work fine. Cam is installed straight up. I did idle it up to 900 rpm. I am going to check compression when I pull the plugs for a reading( jet change) but the starter is a good indication it's all there (grunts).I have tried all the cams in all the positions around the 17-18-18.5 and 19 cc positions. The squirter is my last ditch effort. I put the .035 in last night and things were better, BUT it has a mild flat spot( lays down ) when easy on the throttle. I am assuming the flat spot is too much fuel ,so I am going to try the .028 squirter which makes sense since I jetted it down.

    Has anyone had this problem? I remembered last night when it happened. Back when I had my 600 on I would develop a stumble ,change the cam, while driving she would sneeze back through the carb( blow the power valve) and be all better. WELL last night my 750 sneezed back through and my stumble was gone( I do not think the squirter had anything to do with it) SO WHAT THE HECK IS IT AND WHY? I figure it is the air bleeds getting gummed up ( fuel varnish) whatever. The demons and other carbs come with changeable screw in bleeds.

    Piss on it I am ready for fuel injection.:bang:
  10. When they pop through the carb, it often kills the powervalve. With a popped powervalve, the main jets will be getting fuel sooner, (sooner, as in, even at idle)
    When the carb pops it's either way off ignition timing or more likely, a lean condition.
    I did a lot of experimenting with my Mustang over the years, back when I was learning to tune Holleys. I even tried running the huge 50CC accel. pump housing that I took from a 500cfm 2barrel carb. The motor didnt have any flat spot, even with a huge custom-made pump cam that I installed with the big housing. Adding too much fuel off idle didnt seem to bother the motor much at all, nothing like starving it of fuel will.
    Flat spots are almost always caused by a lean condition. You would have to put a LOT of fuel down its throat to make it flatten out, such as when the needle and seats stick and fuel starts pouring out of the vents.

    Guessing off the top of my head, a 10.2:1 302 with 230dur camshaft installed at zero would have at least 14" of vac at 900rpm.
    Since you said that the flatspot went away after it popped through the carb, I'm going to assume that the powervalve you had in it wasnt correct, and was starving the motor at partial throttle. If this is the case, then your motor was making too much vac for that particular powervalve, and was keeping the pvalve shut or mostly shut when it shouldve been wide open and letting fuel through to the primary jets.

    There's a spring inside the powervalve, keeping the fuel port open, and the engine provides the vac required to resist that spring, and cause the fuel port to close when vac is high, such as when it's idling.
    (The primary jets receive fuel via the powervalve fuel port, so when that valve closes, fuel is cut off to the main jets, or mostly cut off.)

    The instant you touch the throttle, the vac drops, and the fuel port inside the powervalve should snap open from the spring pressure. The different ratings on the powervalves indicate the vac required to force the spring to close. So a 6.5" p-valve will remain closed in any vac of 6.5" or higher, a 10.5" will take much more vac to close, and a 4.5" will take much less vac, and so on. The Holleys come with a 6.5" pvalve, and it generally works ok in most street performance engines. The idea of the powervalve is to only give the main jets fuel when the motor needs it, not at idle.

    You need to get a good vac gauge. Like I said, leave the factory squirter and pump cam in place, and try to work out the flat spot by other means first. If you had a 6.5" powervalve in there and it was leaning out off idle, try a heavier one, like a 7.5 or an 8.5. If I'm right about your motor's high vac, a 6.5" is too weak for it. That thing will be staying closed until you really start to get on the throttle.
  11. Thanks for the info. I really appreciate it. The 5.5 power valve was installed due to the suspected faulty vacuum gage reading. I will probably put a 6.5 back in. But I was under the impression that the main jets worked all the time and the power valve only popped open to give x-tra fuel during that low vacuum condition. Thus if you have a healthy cam with low vacuum you need to lower the power valve so that it does not open too soon. One last thing, my new 750 should have power valve blow out protection? Right?It also popped back through right after I started it , so it wasn't running very long or even idleing yet. OH , I tried the .028 squirter and it was worse. I put the .035 back in with the orange cam in pos.1 , the flat spot is small so I think I can live with it for a little while . What would be a good 650 to get if I was to come into some money?? Are the demons that good outa the box?
  12. The Race Demons I've worked with are insane right out of the box.
    I couldnt believe the throttle response and power increase when I bolted one onto a 340 in an old Dodge Demon(heh). It felt like a whole other car. My Mustang runs pretty well with a 750 Race Demon on it, too.

    I would say a 700 Race Demon would work perfectly on your motor. A 650 would work nicely, too, but you can get away with more cfm with the Demons, due to their greater sensitivity to vac change and higher bore velocity, and the motor will probably benefit slightly in the mid and upper range from the larger flow.

    Did you make sure the idle jets were adjusted exactly even for both sides? I always close them, and then turn them both out 1.5 turns(counterclockwise) You want the highest rpm attainable from leaning the idle mixture, then turn them out an extra 1/4 turn, to make them slightly richer than optimum. It cools the motor at idle, and also helps to eliminate that flat spot off idle, since the motor has more fuel initially to play with.

    Edit: Yes, a motor with a very aggressive camshaft won't make enough vac to close a higher rated powervalve, so it would run rich at low speed/idle. My 332" makes 7" of vac at 1300 idle speed, so a 6.5" rated p-valve would just barely stay shut at idle. The instant the throttle bores opened a crack, just enough to drop the vac half a point, that valve would be open. (I can't remember if I have a 5.5 or a 6.5 in there)
  13. NICE Read! :nice: Filling in holes and gaps in my tuning knowlege.
    However, like itsaMustangtoo, I too was under the impression that the main jets worked all the time and the power valve only opened to give more fuel during a low vacuum condition. If you had a healthy cam with low vacuum you needed to lower the power valve so that it didn't open too soon. At least that was the impression had gotten over the years. So am I all bas ackwards now? (this read is good data for me as my tunnel ram car still runs like dog do do)
  14. To tell you the truth, you guys could be right about that, I can't remember whether the valve supplies all of the fuel to main jets, or most of it. I only remember that the motor really runs lean if the powervalve doesnt open, so I never re-jetted until I had the correct powervalve in there and I was sure the mains were getting enough fuel.
    I learned to tune them by fiddling with the carbs over the years, so I don't have any reference books handy.
  15. I do not think I need a race demon ,and have my eye on a 650 mighty demon ( it's always about cost isn;t it? ):rlaugh: No seriously I am still hesitant about the mech sec., so what about the speed demon? I know all out performance a double pumper is the way to go, but I am not going to race this thing. I swear my 347 gets way better gas mileage than the 302 I took out. But that may be cause it has so much torque that I don't have my foot in it all the time:D

    Dano- I had a tunnel ram on my 289/351w headed mustang II but it was a single 4 barrel top. What are you running on top of yours?
  16. You can plug the power valve port( close it off) you just need to increase jet size to compensate.
  17. If your pulling 5" @ idle in drive your starting point in valve would be a 25. A higher numbered power valve could easily explane why you have stumble when coming off the idle circuit.

    The power valve by-passes the main jets.

    Holley carbs have had power valve blow out protection for over a decade.
  18. Ah ok, a 2.5. Yes.
  19. But the motor will idle rich, without the valve to control fuel at idle.
  20. Yes, and I've still seen blown powervalves.