347 bogs down on acceleration

[VIRGIN MIKE]

My Blueprint 347 / tko 600 bogs down when trying to accelerate from low (1000 - 1500) rpm, first gear, idling along or off the line, easing out the clutch, like navigating a parking lot
Spec sheet starts at 2500RPM - shows about 200 ft/lbs - is there not enough torque at low RPM or do I have a mixture/timing issue. It really comes to life once once you get going but is a pia in stop and go situations

[edwardb]

EFI? Carb? If so, what type?

[VIRGIN MIKE]

EFI? Carb? If so, what type?

holly street warrior

[Papa]

Are all unused vacuum ports properly capped?

[mkassab]

My guess is your carb needs tuned by a pro that knows carbs. May need new/different jets?

Mark

[edwardb]

I'm not familiar with that exact model. But a quick search confirmed it is a vacuum secondary style. That used to be a common discussion on the forum but has died down I'm guessing because of the wide use of aftermarket EFI. Many, including me, had the exact symptoms your describe with a vacuum secondary carb. These cars are light, high power to weight, normally manual transmission, all making them less than ideal to get the proper vacuum signal for the secondaries. My very first build I started with a Holly Street Avenger. Common at the time. I never could get it to run right. Bogged down out of the hole no matter what I did. I tried everything I could find, including all the different springs, jets, etc. Granted I'm not a carb expert. But when my experience was similar to others seemingly without a solution, I replaced it with a Quick Fuel SS-650 double pumper and bog was gone and the engine lit up. I'm not saying your carb can't be adjusted to be better, and maybe even resolved. Or maybe it's something else. I suspect others will chime in and say theirs works fine. I can only cite my experience which was similar to others. Good luck.

[F500guy]

I have a BPE 427 with a holley carb, really bogs out of the gate. I suspect it is a rich excess fuel bog. Holley has a known issue with rich low end. There are some post about what needs to be done to fix it. My engine as delivered was not optimal for timing either, so if you have not checked that, good place to start.

[ggunter]

Most of the time you will find you need a larger accelerator pump discharge nozzle. When I had my 347 in the car it had the same bog. I went form a #30 stock to a #36 and it went away. The accelerator pump has a lot to do with getting the car off the line. A bog is usually a symptom of lean. A stutter is usually rich. There is a difference in sound and feel. The vacuum secondaries have nothing to do with low end rpm. They only come in above 3500-4000 rpm depending on what spring is in the diaphragm. Like was said above make darn sure there are no vacuum leaks. Spray some either or flammable brake clean around the carb base and listen for rpm increase. If it changes, there's a leak. Summit sells a Holley tuning kit with various jets, springs and nozzles. It's around $80, but a good tool if you're carb tuning.

[J R Jones]

Most of the time you will find you need a larger accelerator pump discharge nozzle. When I had my 347 in the car it had the same bog. I went form a #30 stock to a #36 and it went away. The accelerator pump has a lot to do with getting the car off the line. A bog is usually a symptom of lean. A stutter is usually rich. There is a difference in sound and feel. The vacuum secondaries have nothing to do with low end rpm. They only come in above 3500-4000 rpm depending on what spring is in the diaphragm. Like was said above make darn sure there are no vacuum leaks. Spray some either or flammable brake clean around the carb base and listen for rpm increase. If it changes, there's a leak. Summit sells a Holley tuning kit with various jets, springs and nozzles. It's around $80, but a good tool if you're carb tuning.

When I raced a GT350 in SCCA I modified the Holly 715 (square bowl). It had mechanical secondaries that I removed and installed a primary to secondary gear drive. That means both throttle shafts were syncronized idle to WOT. The kit included a larger accelerator pump and a four port pump discharge nozzle. Two a-pump tubes extended from the front of the primary throat to the rear throat to "squirt" all four venturies simultaneously. There was no bog.
I would not do that on a street application and I can not attest to a benefit on the track.
We vintage raced a GT40 MKIV with 427 side oiler with (2) four barrels. All four throttle shafts were syncronized like the Shelby. It was very hard to drive in traffic, no bog.
jim

[Norm B]

What cfm is your carb? Summit Racing has an online calculator to help you choose the correct size of carb for your intended use. I stuck a 347 turning at a max rpm of 6500 in the calculator and got 550 cfm for street and 700 cfm for racing. Too big of carb will cause low rpm issues with insufficient air velocity to pull fuel through jets. Can be compensated for, to a degree, by increasing the accelerator pump output.
Before switching to the Sniper I had a 600 cfm Edelbrock on a 302. Fought with it regularly.

Norm

[VIRGIN MIKE]

Timing! the lock clamp for the dizzy was lose and the dizzy had rotated about 15° counterclockwise; to about 0° at idle

[GoDadGo]

Timing! the lock clamp for the dizzy was lose and the dizzy had rotated about 15° counterclockwise; to about 0° at idle

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