Visit The Dirt Forum for More Information

Author Topic:   2bbl Question...
outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 20, 2005 01:56 PM  
hey guys, it's been a while since i've been on the forum. i've been doing a little asphalt racing and they have a 4bbl cast iron intake or 2bbl aluminum intake rule. i'm running a 5.0 ford with a victor jr. and a holley 4412, turning about 7000 rpms. the engine does not like wide open throttle. it sputters when the throttle is wide open, even sitting in the pits. i don't believe it's a fuel delivery problem. i think it's a mixture/atomization problem. i have a 6.5 power valve and 77 jets. it idles fine and sounds good at part throttle. engine ran good on the dirt with a 4bbl. any ideas?

[This message has been edited by outlawstock17 (edited October 20, 2005).]

way2fast72
Dirt Freak

Total posts: 303
posted October 20, 2005 04:37 PM  
I would check to be sure you arent over centering the butterflys at WOT,does the car run right if you barely back out of the throttle?

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 21, 2005 08:32 AM  
i checked.....it's not that. it does seem to run better when i back out of it a little though. i'm wondering if there isn't enough air volume coming through the carb to keep the mixture atomized in that huge plenum without the turbulence of the butterflies being angled a bit. i have a cheap adapter that centers the 2bbl over the plenum. anyone else? where's KP?

[This message has been edited by outlawstock17 (edited October 21, 2005).]

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted October 21, 2005 08:49 AM  
I'm right here, 17, just trying to weigh all your info and "assume" the rest. You mentioned the engine was built for a 4bbl...cam choice, overlap, duration, etc could be magnifying the turbulance and reversion under the 2bbl in that big open plenum. I've not seen that symptom happen at only WOT, tho.
Perhaps your symptom is really an extreme lean condition popping up at WOT.
You didn't mention the compression ratio, or much about the ignition system. Whatcha got there?
Also, can you give us any info on what (if anything) has been done to the carb? Was it modified by anyone? If so, what was done?

Keep the info coming, we'll get ya straight.

KP

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 21, 2005 09:04 AM  
compression is 9.5 to 1. cam is an isky solid .540 lift, 254 duration @ .050, 108 degree lobe centers. i rebuilt the carb to stock specs except for the jetting. stock jetting is 72, i think. ignition is stock ford duraspark. autolite AR32 plugs gapped at .035" do look a little lean on the left bank. the right bank looks good. i'm going to change the plugs to get a better reading. i think i'll fatten it up to 81's as well. it seems like when the engine gets hotter, the problem is less severe. it also sounds real bad when i get off the throttle going into the corners. no one can hear the WOT miss in the pits, but i can hear it inside the car.

[This message has been edited by outlawstock17 (edited October 21, 2005).]

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 21, 2005 09:12 AM  
if it matters.....i'm running on a high speed, high banked 1/4 mile asphalt track. we're turning laps in the 12.90 to 13.10 range. a lot of g-force...

magician227
Dirt Full Roller

Total posts: 55
posted October 21, 2005 10:03 AM  
There's your problem 17. The fix is easy as pie.....just get that hard black stuff off the track, groove your tires, and all is well.

Other than that, I dont know squat about carbs.

magician

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted October 21, 2005 10:08 AM  
Other than having a cam that's not optimum for 2bbl use, the primary things to check for are firstly the obvious such as a missing air bleed(s), plugged emulsion bleed holes in the metering block, a loose or very crooked booster(s), especially on the left side.
This is also assuming there isn't something else suddenly going on such as an intake gasket leak into the lifter valley, a vacuum leak, etc. Also, a severely worn throttle shaft on the throttle side can cause it to go lean more on that side than the other.
I'm also assuming that the throttle linkage is not loose on the shaft itself, and there isn't a bent or twisted shaft either. This would cause you to think the throttle is wide open when you view it manually, off the motor, but the thing could move around erratically when in use.
Let me know if any of this helps, and we'll keep suggesting areas to check. It's kinda tough without actually seeing the carb in person.

KP

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 21, 2005 11:14 AM  
i've checked all of the obvious things(vacuum leaks, bent/loose throttle shaft, booster problem). none of these seem out of normal.

i just pulled all of the plugs and the back one on each bank looks a little "sandblasted". i put in a new set of plugs and fattened it up to 81's. i'm going to blow some compressed air into the air bleeds to see if there is some trash in there or something.

thanks KP....i'll post back after the race this weekend. the car is already fast. i was 2nd last weekend, but i need the engine perfect to have a legitimate shot to win. it's tough giving up 50-100 cubic inches on the chevrolets....fortunately this is not a horsepower track or i would be in trouble.

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 21, 2005 11:23 AM  
quote:
Originally posted by magician227:
There's your problem 17. The fix is easy as pie.....just get that hard black stuff off the track, groove your tires, and all is well.


heh, heh....i used to think that too. if you ever get the chance, you should try the asphalt. it is challenging. it took me about 3-4 weeks to get the car handling well. i added sway bars, changed rear springs, took some of the toe-out away, changed the tires and removed a bunch of rear weight. 44% rear weight is nice on the pavement...

99What
Dirt Roller

Total posts: 5
posted October 21, 2005 01:37 PM  
We had this problem on a chevy 350 with a 2 bbl and fixed it by putting a Flame Thrower 50,000 volt coil and module on it. Problem solved. Just my 2 cents

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 23, 2005 04:12 PM  
no difference. still sputters under WOT. it's definitely not lean now though...

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted October 23, 2005 05:01 PM  
Are ya sure it's not a valvespring problem?

Got time to send the carb to us? If it's in the carb, I'll find and fix it.

KP

outlawstock17
Dirt Forum Champ
Total posts: 1363
posted October 23, 2005 06:15 PM  
yeah, i'm sure it's not a valve spring problem. at first i had the wrong gear when i switched from dirt to asphalt(too high) and it still sputtered under WOT running 5200 rpms. the spring pressures are 130lbs. on the seat and 340lbs open. plenty of spring pressure to turn 7000... this problem cropped up when i went to the 2bbl. i bought a new demon circle track 2 bbl, but it had some issues(one of the boosters was dumping fuel uncontrollably and the vent tube was cast closed) so i sent it back to barry grant. i'm waiting for them to send me another one. if the demon cures the problem, i'll send you the 4412.

about what does it cost to re-work one?

[This message has been edited by outlawstock17 (edited October 23, 2005).]

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted October 23, 2005 07:36 PM  
The basic clean/refresh is only $75. Full blueprinting is $150 (still pass all guage checks). There's other levels above that, but we won't need 'em.
KP

Back to the Archives