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Thread: FLAT SPOT

  1. #1
    Senior Member RACING JUNKIE
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    FLAT SPOT

    What is the cause of a slight flat spot (mid range RPM) only in the primaries? I think the primary float is set ok, PV is fine, fuel @6.5PSI (for street), etc... fuel delivery is Edelbrock high vol. manual, Holley reg w/ bypass, QF 950 DP. What I am I missing?? power hits a flat spot about 4500 in the primaries, seems to go away when hitting the secondaries.

  2. #2
    Senior Member EXPERT BUILDER
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    i had a problem like that,try alot of things,last thing i tried was blocking the bypass off & that took care of the trouble.good luck

  3. #3
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    Thanks!! I had not given any thought to that yet... but the bypass is fairly new, that could be a problem. I will giver a shot. Thanks again!

  4. #4
    Senior Member RACING JUNKIE
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    It's on a street car?I would look at the powervalve size,Rigsby.Make sure it matches 1.5" below your manifold vacuum at idle at operating temperature.Just my $0.02
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln

  5. #5
    Senior Member RACING JUNKIE
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    Ok Olds, my Vac is right at 5" at 1200/idle. The PV is wrong then... I think the stock PV is 4.5 which is the one I believe in the carb right now.I have a 3.5 too which I will install probably today. I think that you have a good point... + elevation here is about sea-level. The other thing I found yesterday was the NEW Holley reg (w/bypass 4-9psi) is leaking a little at the base of the pressure adj bolt... I teflon taped the bolt & it stopped. It is my understanding that the top of the reg should be dry.... the diaphram is fine (no holes) WTH????? Yeah, it's a street car... I guess.. It's more like a drag car w/slicks & license plates. The state of Oregon would not call it a street car... Lol!!

  6. #6
    Senior Member RACING JUNKIE
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    Took it apart, you were right Olds... the new carb has a 4.5. So now I know that this is a problem... I have a 2.5 & 3.5, so will start w/ the 2.5 & see what it is like. Probably have to get a ne regualtor AGAIN too... man, 3 reg's in less than a year trying to get the right one, then it is defective!!

  7. #7
    Senior Member RACING JUNKIE
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    If your vacuum is that low then you might be better off with no PV.My expereince has been that rowdy-engine cars with auto trans,with that low of manofold vacuum don't really need one.A track only car doesn't need one at all.And manual trans cars,especially ones that are geared high,usually do need one,unless it is a real rowdy motor.JMO. What kinda setup you got?
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln

  8. #8
    Senior Member RACING JUNKIE
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    Well Olds, as you can probably guess... I am still learning. This is the most built eng i have ever had... all the rest were SBC & mild, w/ no solid cam. So, I did not need to do much on them. This is a 454 w/ a 396 ss crank (427) all roller solid Luatti, 12.5 pistons (but much port & polish on heads making about 10.2 comp) cast iron heads (angle plug) w/ bigger valves, quick fuel 950hp on a team G, flowing from hooker super comps. I did not build it- so specs on some things are unknown. Its all MSD 6al, sumped tank w/ all 10 & 8an tank to carb. It was all drag car when i got it, but i have brought back to the street some. It's 69 Camaro... 6pt cage, all autometer guages, with a stock dash. Still has racing harnesses. I think it is about the most impractical car I have ever owned... but when I am on the street, I have not ran into anyone that could give it a run!!! Still has 488's in a Dana60. It still has the cutoff switch on the back even. Damn it's fun!! I work on it more than I will ever drive it though. Anyways, I want to take to the track this year & see if it will go 11's... if I can get this carb right. Anyways, your tips are always apprecaited.... you helped me set the valves right (first solid lifter adj i have done). What is the disavantage of blocking the primary PV if I am on the street?

  9. #9
    Senior Member RACING JUNKIE
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    Olds: tried the 2.5 last night, it got better. Not totally gone however. I am wondering about the accel pump now... tell about how they work, should I work on the accel pump/cam to ensure I am deliering enough fuel throughout the primary range??

  10. #10
    Senior Member RACING JUNKIE
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    The whole point of the powervalves are to give you adjustment in the midrange circuit.Cater,Edelbrock and Quadrajets accomplish this with stepped metering rods,which you probably already know.Since Holley doesn't have anything restricting their jets,they use the PV's to do this.All carbs have and idle,cruse and WOT circuit.The PV is the cruise circuit so to speak.If your car is in need of extra fuel(undr load) tha manifold vacuum will drop,allowing the spring pressure to vercoma the vacuum and allow extra fuel in.If you plug the PV off,you have to compensate by jetting up alltogether,which is usually a waste of fuel.A race only car only needs to idle and run WOT,so you chuck the PV 'cause you don't really need it,and to remove something that could fail,IMO.So,the real disadvantage is burning extra fuel,but it can also cause drivability problems.As far as the acc pump shot,if it is out of tune,it will act up immediately when you stomp it by over-fueling the motor or leaning out.Over-fueling will usually act like a stumble/flat spot and leaning out will usually stutter and pop.Feels kinda like a set of points that are sticking.I don't know which cvam is on your pump shot,but usually it takes a real touchy car to have problems out of that.I mean,you can sometimes find definite improvements there,but usually doesn't cause major drivability issues.I would start with size 31 squirters front and rear.That seems to have worked on just about everything I have ever tuned.If you think that your pump shot is "running out" too soon,use a cam with a longer ramp.Look for a longer one,not a taller ramp.Also,for a street driven car that hits bumps and potholes and such,a pump shot that is adjusted woth no lash will dribble a little fuel every time it gets shook.I like to put a little bit of slack in mine,not much,just enough so that it's not tight.A 4-speed car,though,will just about need the pump shot to come on instantly,since a clutch will put more af a strain on the engine than a torque converter will.This might sound dumb,but if your flat spot is primary only,it might just be lean.What jets are you running front and rear?It's best not to stagger them more than 2 jet sizes.If you remove the powervalve,you will have to jet up 5-10 sizes,depending on the car.I don't know if I answered any of your question,but my fingers are sore from all this rambling.Talk to you later
    "The problem with quotes on the Internet is that you cannot confirm their validity"- Abraham Lincoln


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