curing hesitation
Thread Starter
Instructor
Joined: Apr 2005
Posts: 134
Likes: 0
curing hesitation
Hey guys, i own a 1987 corvette converted to carb and mech/vac ignition. I know this is a c3 forum, but i figure theres a lot more carb knowledge on here.
I got the car running 99 percent, perfect idle, cruise is also great,so is full throttle 2500 and up. I just have a stumble, if i floor it under like 2250 or so rpms. If im at higher rpms and floor it i have no hesitation or stumble.
Hers the basics of the car. 87 corvette coupe manual trans
stock Roller block, refreshed stock heads
Mild cam ( previous owner Dont nkow specs.)
Weiand xlerator intake manifold Imca legal version
holley 750 4160 Vacuum secondaries
hooker super comp headers 1 3/4 primaries dual 3 inch exhaust
Summit brand hei with adjustable vaccuum advance.
Running vaccum advance off ported vacuum
12-14 degrees initial timing.
fuel floats on carb set
The motor is running great everywear except when giving it throttle quickly at low rpms. At higher rpms i dont get this stumble no matter how fast i open the throttle .
Where should i start. vacuum secodary springs? Accelerator pump nozzles? Accelerator pump cam? Any recomendations?
thanks in advance.
I got the car running 99 percent, perfect idle, cruise is also great,so is full throttle 2500 and up. I just have a stumble, if i floor it under like 2250 or so rpms. If im at higher rpms and floor it i have no hesitation or stumble.
Hers the basics of the car. 87 corvette coupe manual trans
stock Roller block, refreshed stock heads
Mild cam ( previous owner Dont nkow specs.)
Weiand xlerator intake manifold Imca legal version
holley 750 4160 Vacuum secondaries
hooker super comp headers 1 3/4 primaries dual 3 inch exhaust
Summit brand hei with adjustable vaccuum advance.
Running vaccum advance off ported vacuum
12-14 degrees initial timing.
fuel floats on carb set
The motor is running great everywear except when giving it throttle quickly at low rpms. At higher rpms i dont get this stumble no matter how fast i open the throttle .
Where should i start. vacuum secodary springs? Accelerator pump nozzles? Accelerator pump cam? Any recomendations?
thanks in advance.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
This sounds like an accelerator pump problem. See if you can get a more aggressive pump cam in there. May want to see what size the shooters are. Maybe go one size larger.
Just as a test, unhook the secondaries. If you still have the bog it is the accelerator pump. If it goes away it may be the secondaries popping open too quickly.
Also, you may want to go to manifold vacuum on the advance.
I think you're real close. Keep at it.
BigBlockk
Later.....
Just as a test, unhook the secondaries. If you still have the bog it is the accelerator pump. If it goes away it may be the secondaries popping open too quickly.
Also, you may want to go to manifold vacuum on the advance.
I think you're real close. Keep at it.
BigBlockk
Later.....
Race Director
Joined: Sep 2002
Posts: 10,362
Likes: 5
From: San Diego - Deep Within The State of CONFUSION!
I don't think this is a carburetor issue. Isn't that intake an open-plenum manifold? It is designed for 2500+ RPM use, not street driving.
That's the reason that Edlebrock's performer series, and it's copies et al, have become so widely popular - the plenum is split. And in the Performer specifically, it is also divided into two different levels inside. A far cry from a wide open plenum.
That's the reason that Edlebrock's performer series, and it's copies et al, have become so widely popular - the plenum is split. And in the Performer specifically, it is also divided into two different levels inside. A far cry from a wide open plenum.
Thread Starter
Instructor
Joined: Apr 2005
Posts: 134
Likes: 0
Yah i know , its an open plenum single plain , and under 2500 its gonna be a dog, but shouldnt i still be able to floor it without the car wanting to die out.?
the c4 hoood line is extremely low, non of the good dual planes fitted. I figured cause its a manual , and relatively a light car, it would be ok to go with a single plain.
the c4 hoood line is extremely low, non of the good dual planes fitted. I figured cause its a manual , and relatively a light car, it would be ok to go with a single plain.
Thread Starter
Instructor
Joined: Apr 2005
Posts: 134
Likes: 0
Also this is my first american v8. Im used to imports,mostly small turbocharged engines. where your basically never below 2500 rpm in any circumstance.
I bought the vette non running. When i was looking for an intake manifold, And i saw 2500 and up rpm, I was like ahh who cares about below 2500, who drives that low anways.
But now i finnally get to drive the car, and i relize the vette is geared soo much taller in all gears , that moderate driving always leaves you under, 2500 rpm.
I bought the vette non running. When i was looking for an intake manifold, And i saw 2500 and up rpm, I was like ahh who cares about below 2500, who drives that low anways.
But now i finnally get to drive the car, and i relize the vette is geared soo much taller in all gears , that moderate driving always leaves you under, 2500 rpm.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i think you need to work on your timing before you start screwing with the carburetor. lots of improvement can be made through correct timing settings.....what kind of vacuum is your engine pulling and what vacuum cannister are you using?
edit ok i see the vacuum cannister you are using....why not switch to a fixed unit that is matched to your engines vacuum?
edit ok i see the vacuum cannister you are using....why not switch to a fixed unit that is matched to your engines vacuum?
Last edited by bobs77vet; Feb 28, 2006 at 10:04 PM.
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Thread Starter
Instructor
Joined: Apr 2005
Posts: 134
Likes: 0
I wanted to make sure of the timing as suggested so, i got some timing tape, and a recurve kit. Im hitting 36 degrees timing by 2500-2700 rpm with Vac advance disconected. Car has alot better low end after doing this, but still has that hesitation if i floor it at low rpm.
So still with that hesititation. I upgrade my accelerator pump nozzle size, from .031 to .040. Seemed to help abit , but i still got the hesitation. definately not as bad. But still there.
Should I try a bigger nozzle. Or am I never going to get rid of the stumble with a single plain intake manifold?
So still with that hesititation. I upgrade my accelerator pump nozzle size, from .031 to .040. Seemed to help abit , but i still got the hesitation. definately not as bad. But still there.
Should I try a bigger nozzle. Or am I never going to get rid of the stumble with a single plain intake manifold?
Instructor
Joined: Mar 2002
Posts: 239
Likes: 0
From: Fredericksburg VA
Your nozzle size is already quite large, as is your carburetor CFM rating. What model # is the carb? Also, does it go then stumble, or stumble then go at lower RPM?
You should try this as part of the initial setup:
- Set primary throttle blades such that 0.020" to 0.025" of the transfer slots are exposed below the bottom surface of the throttle blades. This is extremely important. Try to leave them there. Set idle RPM with the secondary blades, and retune curb idle mixture screws (both the same turns out from seated).
- Then, if it goes then hesitates, go back to the 0.031" nozzle. If it instantly hestitates try the green pump cam in #1 hole (lots of gas early). Make sure pump cam follower is riding on the cam at curb idle. Slightly bend cam follower if necessary to give gas out the nozzle on slightest movement of the throttle shaft.
- Use full vacuum to vac advance can. Plumb brakes to rear of carb, and PCV to front of manifold.
- Leave jets and secondary spring alone for now.
You can put some chapstick or petroleum jelly on the gaskets so they wont stick to the carb metal.
You should try this as part of the initial setup:
- Set primary throttle blades such that 0.020" to 0.025" of the transfer slots are exposed below the bottom surface of the throttle blades. This is extremely important. Try to leave them there. Set idle RPM with the secondary blades, and retune curb idle mixture screws (both the same turns out from seated).
- Then, if it goes then hesitates, go back to the 0.031" nozzle. If it instantly hestitates try the green pump cam in #1 hole (lots of gas early). Make sure pump cam follower is riding on the cam at curb idle. Slightly bend cam follower if necessary to give gas out the nozzle on slightest movement of the throttle shaft.
- Use full vacuum to vac advance can. Plumb brakes to rear of carb, and PCV to front of manifold.
- Leave jets and secondary spring alone for now.
You can put some chapstick or petroleum jelly on the gaskets so they wont stick to the carb metal.
Tech Contributor
Joined: Aug 1999
Posts: 14,375
Likes: 6,374
From: At my Bar drinking and wrenching in Lafayette Colorado
Rodney is right on. Getting the transition slot exposure set to .020" and getting equal airflow through the secondaries at idle is key to curing the hesitation. Drop back to the stock nozzle size when doing this so you have a good baseline. Then, bump the jetting up 2 sizes from the stock size if you still have an indication of hesitation, keeping the secondary jetting 8 sizes larger than the primary. Run straight manifold vacuum to the vacuum advance - not ported vacuum.
Pro
Joined: Sep 2004
Posts: 595
Likes: 0
From: Boston MA
Originally Posted by lars
Getting the transition slot exposure set to .020" and getting equal airflow through the secondaries at idle is key to curing the hesitation.
Which condition would cause the hesitation?
If the transfer slots are open too much or closed too much?
Thanks,
Joe
Last edited by 68coupe; Mar 6, 2006 at 04:16 PM.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Has this carb been fiddled with? Bubba is very creative and is known for dumping a soft spring and removing the check ball from the vacuum motor to "improve performance." A 3310 (model 4160) is a very soft carb right out of the box and this behavior would be very unusual for a properly functioning carb. This low-speed behavior but absent at high speed indicates your carb has been fiddled with.
