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Old 11-02-2010, 01:59 PM
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Holley 650dp too much for engine? bad street carb?

Sorry for the newb questions, I am fairly new to the game and I keep running into problems. Anyway, I have a holley 650 double pumper that I am having hesitation issues with, it is fine idling around and doesn't stumble when I ease into the throttle but as soon as I go more than half throttle it will bog down then come alive. My dad thinks that it is too much carb for the engine and wants me to put a holley 600 on it with vac secondaries as he says it will be better for the street. I still think it might be another problem such as the power valve, but I don't know how to check if that is the problem or see what I need to do. Here is my set up: 388 with comp cams xe274h, holley dominator intake, dart sportsman II heads and accel ignition with 14 degrees BTDC timing. The plugs are not fouling out but I have also noticed that after heavy acceleration when I let off it smokes like its running rich for a moment. What could be causing this hesitation? Thanks!

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Old 11-02-2010, 02:03 PM
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try using slower accelerator pump cams or smaller squirters. That is NOT too much carb, it just needs to be properly tuned.
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Old 11-02-2010, 03:24 PM
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Mine has excellent mech street manners, burn rubber on the fly. Changing carbs is not the answer. Rebuild what you have, it's easy and 40 bucks. I had the same problem, turned out to be a clogged fuel sock collapsing. Check your float levels, be sure you have enough fuel in the both bowls and your are not running out of fuel. Don't overlook any restrictions in the fuel delivery, particuly under load. You can do very little on the driveway. That carb requires a fair amount of gas. Squirt up to a 31's, dump the 28's. Set your idles mixtures to about 1 or more turns out. Check vacumn at idle, besure your choke is not engaging, check fuel pressure under load, vacumn can be 10+ at idle with cam, 6.5 power valve will be fine IF OPENING. It has to be delivery. (Is it mech or vac?) Replace rear tires after a few runs.

Last edited by aintmisbehavinn; 11-02-2010 at 03:37 PM.
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Old 11-02-2010, 03:41 PM
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The info you want is in the Knowledge Base on this site,
http://www.hotrodders.com/kb/carburetor-articles

A bog is to large of a delivery nozzle, so go the a smaller nozzle like turbolover recommended.

A backfire thru the carb is to small of a nozzle.

You can also do a Search This Site to find all the other posting on the same issue, or on the bottom of the page on your posting which gives a few of them.

I would advise to get a tuning kit for your carb which as the different cam's & nozzles.
I like using the pink one for a steady long shot.
A bad power valve cause's flooding.
Here is a test to check the power valve if it's bad,
If you expect a blown power valve, use this simple test.

TEST: At idle turn your idle mixture screws
(found on the side of the metering block) all the way in.
If your engine dies the power valve is not blown.
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Old 11-02-2010, 05:18 PM
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Much of the poor throttle response is from a lazy timing curve with not enough initial idle timing.

recurve the distributor for 20-24deg inital base timing at idle.
A short mech curve to 34-36deg at max advance peaking around 3000-3200 rpm.
(12-14deg mech advance curve) limit the advance curve to allow increased initial base timing.

Vacuum advance is for cruising aprox 8-10-12deg at hiway cruise.
Limit vac adv to around 15deg max at highest vacuum.
Use ported vacuum.
This curve with more initial at idle and a short smoother curve will work better.

The Comp XE274H-10 cam needs a 3000 stall converter.
A "stock" or 2500 stall is not enough.
For max performance a 3500 stall 10" is not too much converter.

Play with the accel shooters and different coloured cams.
Make sure the accel linkage is right without slack.

Use a 4.5" power valve.

Remove the carb, flip it over and reset the throttle blade idle transfer slot exposure at idle on both pri and sec.
It must be in the sweet spot or off idle response will suffer,
The extras idle timing allows better transfer slot exposure at idle.
Make sure the PCV is hoked up and functional. It is part the idle system.


What is the holley list number (on the air horn)? what is the pri and sec jetting.?
Make sure the air cleaner lid is not too close to the carb top.
Many drop base style air cleaners require removeing the choke tower to function correctly.
Consider upgradeding the carb center body to a cool Proform HP style 750 body.
May need fresh plugs. Gap at .035" Make sure the ground strap goes not extend past the center electrude when you regap it.. Trim if nessessary.

Get a O2 sensor/afr gauge to help tune the carb. When the idle/off idle transition/main metering is corrected the throttle response (and power) is much better.
Even a simple low cost narrow band type is very usefull for tuning.
You can read it with a good multimeter.

Keep your dad out from under the hood. The carb is not too big.
It's actually too small. The Proform HP carb body swap will fix that.
You gain improved air flow, improved fuel curve, swappable air bleeds,
Downleg style boosters. More Horsepower and torque.
Turns your carb into a "HP" carb at low cost.

Last edited by F-BIRD'88; 11-02-2010 at 05:31 PM.
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Old 11-02-2010, 07:37 PM
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Quote:
Originally Posted by IntrstlarOvrdrve
Sorry for the newb questions, I am fairly new to the game and I keep running into problems. Anyway, I have a holley 650 double pumper that I am having hesitation issues with, it is fine idling around and doesn't stumble when I ease into the throttle but as soon as I go more than half throttle it will bog down then come alive. My dad thinks that it is too much carb for the engine and wants me to put a holley 600 on it with vac secondaries as he says it will be better for the street. I still think it might be another problem such as the power valve, but I don't know how to check if that is the problem or see what I need to do. Here is my set up: 388 with comp cams xe274h, holley dominator intake, dart sportsman II heads and accel ignition with 14 degrees BTDC timing. The plugs are not fouling out but I have also noticed that after heavy acceleration when I let off it smokes like its running rich for a moment. What could be causing this hesitation? Thanks!
Is the car automatic or manual trans? Is it a big car or small car (lbs.)? What is the gear ratio?
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Old 11-02-2010, 07:50 PM
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It is a 79 el camino, 3.73 gears auto trans with 2500 stall. Oh also vac advance is not hooked up, with it hooked up the car won't idle below 1500 without dieing in gear.
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Old 11-02-2010, 07:51 PM
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Tee off at the fuel inlet at the carb, run a line back to the firewall and up past the lip of the hood. Mount a 0-15 fuel pressure gauge on the cowl with duct tape or tie wraps or whatever you have to use so you can see the pressure gauge through the windshield while you are driving. It's only temporary, just until you get the pressure under control, then you can remove it. This is the only way you will know what the fuel pressure going into the carb is while you're driving. Holleys will tolerate a maximum of 6 1/2 lbs of fuel pressure before the pressure overpowers the needle and seat and blows raw fuel into the intake manifold, so get the pressure under control, like 6 psi. Some guys will say that they are running a mechanical pump that says it is compatible with the carb. BULL HOCKEY. I have seen stock-type mechanical pumps that put out 10 psi.

Many of you youngsters have grown up with electronic fuel injection, where more pressure is more horsepower. It's not like that with a carburetor. More pressure than 6 psi on a Holley blows fuel past the needle and seat and loads the motor up with raw fuel. Engineer for 5 psi on an Edelbrock, Carter AFB or Quadrajet. They are more sensitive than a Holley.
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Old 11-02-2010, 08:13 PM
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Quote:
Originally Posted by IntrstlarOvrdrve
It is a 79 el camino, 3.73 gears auto trans with 2500 stall. Oh also vac advance is not hooked up, with it hooked up the car won't idle below 1500 without dieing in gear.
recurve the distributor according to my post. Use ported vacuum.
Fix the vacuum advance rate too. You have it all comming in at idle, as it sits now.
It will not idle correctly until you recurve the distributor and get the right converter.
Get the right torque converter 3000 stall mininum 3500stall best.
The converter you have is too tight.
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Old 11-02-2010, 08:20 PM
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Quote:
Originally Posted by IntrstlarOvrdrve
It is a 79 el camino, 3.73 gears auto trans with 2500 stall. Oh also vac advance is not hooked up, with it hooked up the car won't idle below 1500 without dieing in gear.
Okay- does the carb have 4 equal size barrels or are the front ones a lot smaller than the back ones... like on a Q-Jet carb? Holley made a 650 DP mechanical seondaries. It was Q-Jet replacement carb model 6210.
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Old 11-02-2010, 10:16 PM
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is the intake a street dominator ( which is a dual plane intake) or strip dominator ( a single plane intake ) if it is a strip dominator , you need more fuel ! have someone follow you when you go for a test drive and tell them too watch if they see black smoke come out your pipes when you acellerate , black smoke is too rich, you'll have to lean your carb out through the acellerator pump cams and jets, but if there is no black smoke she is running too lean on acelleration,go to a 50 cc rear acellerator pump (Holley 50 cc pump part #20-11 ) ( Holley trick kit part # 37-910 ) and change your front and rear acellerator pump cams, and go to a bigger acellerator discharge nozzle . 6.5 power valve should be ok for your set up.to be sure check your manifold vacuum , i think there should be a chart to go by for power valves. tune your car like F bird88 said, ( do your timing and recurve first then do your carb) more base timing and a dist recurve , and adjust front and rear throttle plates. what kind of exhaust are you running on your car, you need a free flowing exhaust to go with your set up
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