Hood Scoop Slowing Down Car
 

Have a real brain teaser here. Have a 1000 cfm APD carb, snorkel hood scoop, no air pan or anything to seal carb to scoop. Car is footbraked and more often than not has a stumble AFTER I leave the line. Car initially leaves fine.

Last weekend the top of the snorkel broke off down track on the first run (after the usual stumble after leaving). 1.58 60ft

The rest of the day the car was amazing leaving the line and 60 footing. (1.54-1.55) Has never been that clean and crisp consistently. We didn't think anything of it. The air was fair (for us 3000 is good air) so we thought maybe it was just that.

Next weekend we replaced the broke snorkel with a new one and went racing. Air was bad to start the day (3600 at 9:30 am) Car stumbled two passes back to back. (1.62-1.65 60 ft) Got me thinking the hood scoop was doing something. Finished the next 2 rounds getting beat with an inconsistent car. 1.635 60 ft then a 1.654 60 ft.

Took the hood off for a time only. At this point the air was 4400 ft. Car went 1.606 in the 60 ft. did another pass to be safe and this time car went 1.598.

To me this says the car is fat down low in the first half second of the run and liked the unrestricted air with no scoop/hood. What's the first change? Seal carb to scoop? Different scoop? pump cam and squirter change?

Funny you post this, I’m going to try to run my car this weekend without a hood as well, I have a 1450 carb and I have alot of open area for the carb to suck in hot air, I would try what I’m going to do, try a clear lexan hood scoop with no hood and see how you do.


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Bowl vent tube slosh/siphoning from direct flow of scoop air, absent when it's gone?

If it puffs black smoke on the leave it's fat ifnot it's probly a lean condition.
Just my thought

GT

I did a bunch of testing on the snorkle scoop back in the 70s.

Do you have a "bleed vent" at the bottom rear of the snorkle scoop?

Tom V.

No I don’t believe I do

. Here’s the hood before new snorkel was put on. Hole size okay?

I noticed when the scoop was off I had a splash of fuel on the windshield on every launch. Sure it’s from the squirter

Could be. Could be out the vent tubes, and with scoop on it goes down the carb instead of on the windshield.

Lower floats a hair maybe? I can just see the top of fuel at the very top of sight window … just barely

I wouldn't if the engine is happy otherwise. A simple test is make some extensions out of fuel line and see if it helps do away with the stumble.(scoop on)

Here’s the carb I’m running

That looks like the vent tubes should be adequate. How close do those tubes come to the scoop when it's on?

About 5 inches I’d say. Carb barely sits under the hole in the hood.

I'd say it carb needs sealed to the scoop?
Too much (bad) air movement around the carb with it being open like that?

Plus the real benefits of the scoop aren't being utilized with it not being sealed to carb.

:confused:

With the larger diameter clear sight window, the fuel level should never be higher that centered in the sight window. Top of window is WAY too high.


Tom V.

x 2

GT