715 cfm tuning

[gt350bp]

Been fighting a slight bog when cruising at 35 to 40 mph then try to accelerate hard. The engine is a 327 stroker with X 303 cam, Cobra dual plane intake and tri-y headers. I currently have 67 primary and 80 secondary jets. Using the #2 position green cam and a #28 shooter. (Elevation is 20 feet above sea level in the Daytona Beach area)

It has been suggested to change the 30 cc accelerator pump to a 50 cc pump and use the quick change secondary housing for ease of tuning the secondary opening. Using a 3/8" spacer and Hi-PO open element air cleaner.

The car originally bogged when starting out, but changing to the #28 shooter, green cam in the #2 position and better adjustment of the accelerator pump cured that issue. Just need to get the "mash the gas" bog adjusted out. Once the initial bog ends, the car accelerates hard up to 7,500 rpm.

Any help getting me turned in the right direction will be appreciated.

Thank you.

Don
gt350bp

[6s1802]

I finally gave up on trying to tune my 715. Found a 1960's 600cfm main body and Ford base plate with the correct linkage. Had it built with my Le Mans bowls and tuned by Wayne Richards right before his untimely death. Car runs great, no stumble and is very responsive and crisp.  Stock '66 G.T.350 4 speed. I've always said the 715 was too big for a 289.

[Greg]

Being that your engine is "hopped up" from stock, are you running the standard canister fuel pump and is it hi flow?  If so it would be interesting to see what your fuel pressure is under hard acceleration.  If it is lower than 3.5-3.8psi you have a fuel delivery problem.  What you describe is when you mash the throttle and once the fuel pump catches up then you accelerate on up to higher RPM's. 

Make sure you also don't have too much timing as well.  If all of that checks, it could be weak valve springs.   

Let us know what you find...

[acapulco350]

Holley Sniper

All your problems will go away

[gt350bp]

Worked the fuel pressure issues out with an electric pump and new Carter canister pump. Fuel pressure is steady at 5 1/2 psi on the gauge. I think the 715 will be OK for the 327 stroker? I think I'm close, as the bog is slight and it will pull hard once past the initial hesitation.

I'm hoping that the 50 cc accelerator pump or maybe a power valve change will solve the issue. Bought a #31 discharge nozzle and will try that on Saturday or when the weather clears.

Thanks for the input.

Don
gt350bp

[Greg]

Worked the fuel pressure issues out with an electric pump and new Carter canister pump. Fuel pressure is steady at 5 1/2 psi on the gauge. I think the 715 will be OK for the 327 stroker? I think I'm close, as the bog is slight and it will pull hard once past the initial hesitation.

I'm hoping that the 50 cc accelerator pump or maybe a power valve change will solve the issue. Bought a #31 discharge nozzle and will try that on Saturday or when the weather clears.

Thanks for the input.

Don
gt350bp

Don, I'm not sure I understand.  You are running an electric pump through the mechanical canister pump or you found an issue and switched to an electric pump?  You don't use both (electric and mechanical) they will work against each other.  One pulls (pump on the engine) and one pushes (electric pump as close to the the tank as possible. 

It is steady at 5 1/2 at idle or at WOT?

[EdwardGT350]

whats the date code on the 715?
65 has a bog that was corrected for 66.

[Drew Pojedinec]

Raise float level a fuzz, turn mix screws out 1/8 turn.

[gt350bp]

I'm running an electric pump to push fuel to the mechanical pump like the GT 350 Competitions. At idle, gauge is indicating 5 1/2 psi and under full throttle, it drops to about 4 1/2 psi +/-.

To start the car, I energize the electric pump and it pushes 5 1/2 psi to the carburetor and gauge. Turn key to start and car idles at 5 1/2 psi and cruises at
5 1/2 psi. Don't think I have a fuel pressure / fuel system problem as everything is new with less than 500 miles. It is a tuning issue to get the hesitation smoothed out. If I do a drag race start, car pulls hard in all gears. Only when running at a constant speed at about 2,000 to 2,500 rpm and mash the gas, do I get a hesitation, then a hard pull. Need to get the fuel delivery at that  instant to smooth out the hesitation.

[gt350bp]

Drew,

Will give that a shot!

No need for the 50 cc accelerator pump and adjust the secondary opening?

The carburetor is the latest generation as sold through Branda Performance. Not an original 65 or 66.

[Drew Pojedinec]

I've never had one of the 3259 replicas, so I dunno how they are set up.

I've never needed to use a 50cc pump on anything that didn't already come with one.
Seems like the issue is you are on the idle circuit, punch it, and have a lean spot before the mains come online.
To close the gap, you can use a slightly smaller high speed bleed, even raising float level.
1/16 of an inch will make the booster flow sooner

I suggested what I did because it is free 😂


Could it be the secondaries flopping open?  Maybe, but generally speaking 715's are slow to open.
Original 3259 used the short vs diaphragm and an 8.5 PV, but again, that requires more effort. Plus I doubt the boosters are flowing at 30mph.
Last two things that can make a bog are jacked up mechanical advance and a really tight converter.

Anyway, just tossing out ideas

[gt350bp]

Drew,

Thanks for the suggestions. It sounds like I do not need the 50 cc accelerator pump, so I'm going to turn out the mixture screw as suggested an 1/8 of a turn to start. May increase the shooter to a #31 as the next adjustment before raising the float.

Weather has mot been cooperating here in Central Florida, so hoping we get a few hours of clearing on Saturday. Will keep you informed

If this does not work, may try to find someone who can do air / fuel check on a dyno to try to speed up this process and dial it in?

Don
gt350bp

[Royce Peterson]

Generally most "carburetor problems" are ignition related. What is the timing set to? What is the fuel being used? What is the engine's compression ratio?

Often folks retard the timing in order to use cheap 92 octane fuel. Advancing the timing to factory specs results in severe "pinging" or spark knock. A side effect is a severe off - idle bog and higher than normal engine operating temperature.

Been fighting a slight bog when cruising at 35 to 40 mph then try to accelerate hard. The engine is a 327 stroker with X 303 cam, Cobra dual plane intake and tri-y headers. I currently have 67 primary and 80 secondary jets. Using the #2 position green cam and a #28 shooter. (Elevation is 20 feet above sea level in the Daytona Beach area)

It has been suggested to change the 30 cc accelerator pump to a 50 cc pump and use the quick change secondary housing for ease of tuning the secondary opening. Using a 3/8" spacer and Hi-PO open element air cleaner.

The car originally bogged when starting out, but changing to the #28 shooter, green cam in the #2 position and better adjustment of the accelerator pump cured that issue. Just need to get the "mash the gas" bog adjusted out. Once the initial bog ends, the car accelerates hard up to 7,500 rpm.

Any help getting me turned in the right direction will be appreciated.

Thank you.

Don
gt350bp

[gt350hr]

   Don ,
      Why not increase the primary jet size to 68 or 69? Drew's suggestion to raise the float level causes a similar reaction ( richening) to the mixture. "Stumble" is not always solved with the squirter or accelerator pump. Today's 10% alcohol laced fuels need richer calibration because of it.
  Randy

[pbf777]

If this does not work, may try to find someone who can do air / fuel check on a dyno to try to speed up this process and dial it in?
Don
gt350bp

     I would recommend staying away from the "chassis" - "inertia" - "wheel" or whatever you wish to call it dyno services.     

     First and foremost this process is not well suited for recreating your real-world part to transitional throttle event as experienced, though with some "fishing" with an experienced operator one might be able to find it, sort of, therefore it is probably unlikely that any real benefit in the arena of your concern would be realized.             

     We have repeatedly over the years (decades now) looked into the acquisition of one of these dynos, but after in-depth study with several manufacturers, each time we walked away.  The issue here is realizing that we would have to spend a lot of money, and then participate in the general misrepresentation of what they are capable of, and how they are often utilized for the purpose of reaping income; or otherwise then after making the investment, then finding ones' self talking many of these potential customers out of spending their money.         

     The Holley carburetor is not that complicated, and if reasonable for the application intention, with some study and consideration of function most anyone of reasonable faculties should be able to make for good function.  I have always been a believer of the best tuning will be accomplished in the vehicle, the owner/operators' butt in the seat making the observations, then through if only trial and error, executing the tuning process him or herself, and then drawing their own conclusion of if the effort produced results of better or worse have been had.                         

      After all, it's just another event, in the enjoyment of the ownership of an old car...................right!       

      Scott.