Holley 650 hard stumble at 50% or more throttle

[3D FFR]

The car idles and drives fine. But as soon as I want to add a little bit of power and have to push past about 50% throttle you can hear+feel the power sower and if you push it to the floor it makes a horrible pop and fart sounds.

I'm using a mechanical fuel pump yet, and I'm thinking the carb could be starved for fuel? What do you think?

Specs: 650 double pumper 4150, 5.0 Ford racing crate engine, e303 cam, alum heads.

[hom-ade]

Is the popping coming from the exhaust or from the intake?

[Gumball]

A few things to check.

1) Do you have a fuel pressure gauge on the feed to the carb? You need around 4 - 6 psi to adequately feed a Holley.
2) Is your distributor advance working - i.e., are you getting advance to come in progressively as you increase the RPM with full advance coming on by around 2200 - 2400 RPM? Your static advance at warm idle should be around 10 - 12 btdc with it topping out around 34 - 36 btdc.
3) What primary and secondary jets are you using? Maybe not large enough to feed the engine as the revs pick up?
4) Accelerator pump cam maybe not enough?

Hope this helps to give you a few things to investigate.

Keep us posted on the outcome.

[hom-ade]

If it's coming through the exhaust the first things that come to mind is a timing issue, a valve is too tight, or you've got a vacuum leak. I've seen a wiped cam lobe work fine until you throw the power at it and then it starts popping through exhaust as well. Sometimes a broken valve spring will do it too.

My bet would be a timing issue, or a valve is too tight though...without knowing much on your set up.

[3D FFR]

Is the popping coming from the exhaust or from the intake?
Exhaust only.

Gumball, I have lots of things to look into. As I don't have a pressure gauge on the car nor know exactly what the size of my jets are.

hom-ade, I replaced a hose that was leaking last year, I might have to double check it.

Thanks everyone!!!

[GoDadGo]

A few things to check.

1) Do you have a fuel pressure gauge on the feed to the carb? You need around 4 - 6 psi to adequately feed a Holley.
2) Is your distributor advance working - i.e., are you getting advance to come in progressively as you increase the RPM with full advance coming on by around 2200 - 2400 RPM? Your static advance at warm idle should be around 10 - 12 btdc with it topping out around 34 - 36 btdc.
3) What primary and secondary jets are you using? Maybe not large enough to feed the engine as the revs pick up?
4) Accelerator pump cam maybe not enough?

Hope this helps to give you a few things to investigate.

Keep us posted on the outcome.

To add on to what Chris/Gumball just stated, you may have some issues with the power valves as well.
Holley's are known for having issues with power valves, even though they claim to have blow-back/back-fire protections in place.
The big question is: Did the car run well at one time or has it always had this stumble?

[PeteH]

Is this something that was good before and just started happening or is it a new build and happening right away? Presuming the ignition and timing are doing what they're supposed to, it could be a lean mix issue. Timing should be all in around 3000RPM at about 36degrees. If this is not happening, timing is too retarded at higher RPM and will lose power and possibly backfire. Be sure carb mount to intake is interfacing enough to achieve a good seal. I had a problem on an Edelbrock 289 performer manifold, it was too thin on the sides to seal properly to an Edelbrock 650 carb, causing a lean mix at higher RPM. A phenolic spacer corrected the problem. If the fuel pump is stock, you may have to try a higher volume unit with regulator setup. I have run a Facet electric pump from NAPA on a SB Ford 302 (Edelbrock 650) in another car successfully, and have one on the Cobra (670 Holley single pumper). The Cobra has only been go karted a short bit, so haven't wound revs up much yet.
Just in case, I have found that Pertronix ignition systems have to be within about .015" air gap, or they break up at higher RPM, misfiring and cackling thru exhaust. I had to file the mounting hole to allow it to swing close enough to achieve this tighter gap. Also ensure the distributor plate is not floating around from loose rivets, that can cause this same issue. Check the low tension ground wire in distributor as well, although that would show up at any RPM.

[hom-ade]

Ah yes, could be power valve as well. If the car used to run great, and just started having issues..id lean towards a vacuum leak. Whether it'd carb to manifold, manifold to motor, or a vacuum line that's cracked or leaking by. You've got some stuff to check! Lots of possibilities unfortunately

[3D FFR]

So what you're saying is I need a Fitech 30003 system to fix it :)

At full throttle it's never felt perfect, but its much worse after sitting for the winter. I have no issues getting the car to a higher RPM, I just can't use all of my throttle to reach that point.

I scheduled a time to take it to a speed shop here in town next week (DCA Fabrication (https://www.google.com/maps/place/DCA+Racing+%26+Fabrication+Inc/@43.86962,-91.235638,606m/data=!3m2!1e3!4b1!4m14!1m6!3m5!1s0x87f953414bffafd 5:0x5abd258073e54869!2sOnalaska+High+School!8m2!3d 43.8764232!4d-91.2273048!3m6!1s0x0:0x23651e9802618f77!8m2!3d43.8 696199!4d-91.2338222!9m1!1b1)). I called him and shared all of your feedback, which gave him a very good idea on where we can start looking at getting it running again.

[hom-ade]

Sounds like DCA will take care of the headache for you then with no worries. You could go fitech if you want to, I opted for fast ez efi 2.0 set up with their distributor set up that allows you to adjust the timing from the touch screen. Seems like a good unit. I've heard good things about fitech though as well but have no personal experience with them.

[GoDadGo]

So what you're saying is I need a Fitech 30003 system to fix it :)

At full throttle it's never felt perfect, but its much worse after sitting for the winter. I have no issues getting the car to a higher RPM, I just can't use all of my throttle to reach that point.

I scheduled a time to take it to a speed shop here in town next week (DCA Fabrication (https://www.google.com/maps/place/DCA+Racing+%26+Fabrication+Inc/@43.86962,-91.235638,606m/data=!3m2!1e3!4b1!4m14!1m6!3m5!1s0x87f953414bffafd 5:0x5abd258073e54869!2sOnalaska+High+School!8m2!3d 43.8764232!4d-91.2273048!3m6!1s0x0:0x23651e9802618f77!8m2!3d43.8 696199!4d-91.2338222!9m1!1b1)). I called him and shared all of your feedback, which gave him a very good idea on where we can start looking at getting it running again.

Buy a rebuild kit and go through the carb or have a qualified shop rebuild rebuild and tune it for you.
If it sat all winter, it is likely booger-ed up (Technical Term For Varnish & Stuff In The Jets & Bowls) especially if you were running Ethanol Laced Fuel.

Good Luck & Hope You Are Motoring Soon!

[first time builder]

Put a quick fuel Carb on it !

650 might be a little small.
Kenny

[TomH33]

Increase jet size and checK accelerator pump change to more flow. Fuel line size may be small. Power valve my be incorrect. Tray a 750CFM

[3D FFR]

Put a quick fuel Carb on it !

650 might be a little small.
Kenny It's a 5.0 crate from Ford racing, the spec sheet reads it should be only 340hp. So the 650 carb should be in the ballpark.

[bobl]

I bet you have crud in the secondary jets. You could try disconnecting the secondary linkage to prove that, without spending any money. If so then the carb just needs to be disassembled and cleaned. If it ran fine before winter storage and not afterwards it's very likely a carb problem.

Bob

[Blue MK3]

Here's another possibility. The same thing happened to me several years ago. Read these links and consider it.

http://www.volvopenta.com/SiteCollectionDocuments/Penta/Recalls%20and%20technical%20campaigns/Carbureted%20gasoline%20engines%20recall.pdf

https://forums.holley.com/showthread.php?17773-Poor-Quality-Holley-carb-issues-porosity-amp-flaking-on-metering-block

If your carb is about ten years old, it's possible you have one of the faulty ones. Holley issued a recall, first on carbs installed on Volvo Penta marine engines and later included ALL Holey Carbs of that period. If you find the flaking and/or corrosion, no amount of cleaning or rebuilding will solve it. The metal they used had problems and the corrosion would soon return after cleaning. Don't ask me how I know.

The only solution is to replace the faulty carb. I went with a QF 650 DP and have had no problems in the past four years since making the swap.

[hom-ade]

Disregard my posts about valves being tight and such. I wasn't aware that the car had been okay at one time. For some reason I thought this was a brand new engine build. More than likely it's something with your carb as others have said.