carb swap help
Thread Starter
Racer
Joined: Nov 2008
Posts: 252
Likes: 0
From: Morehead KY
carb swap help
I have a 1989 corvette with a zz4 crate engine, 6 speed manual. My car runs good, cruises good, idles good and everything. However when i put it full throttle at 4000 rpm its like a governor cuts in or something. Its almost like hitting a wall, u can feel the acceleration of the car being cut off completely. It will rev past this 4000 rpm but it hesitates and does like crap trying to go above this. But if i go part throttle and dont floor it all the way it will rev past 4000 rpm.
My car has an 1984 corvette fuel pump with a bypass regulator installed and fuel pressure is at 7 psi. It has a 600 cfm 4 barrel summit brand carburetor. Intake manifold is weiand speed warrior. I also changed the fuel filter to make sure it wasnt clogged and re routed my fuel line coming from the filter to the carb to see if this was the cause and if it was getting adequate fuel flow, but it didnt help neither. So im kinda stuck, any ideas?
My car has an 1984 corvette fuel pump with a bypass regulator installed and fuel pressure is at 7 psi. It has a 600 cfm 4 barrel summit brand carburetor. Intake manifold is weiand speed warrior. I also changed the fuel filter to make sure it wasnt clogged and re routed my fuel line coming from the filter to the carb to see if this was the cause and if it was getting adequate fuel flow, but it didnt help neither. So im kinda stuck, any ideas?
Thread Starter
Racer
Joined: Nov 2008
Posts: 252
Likes: 0
From: Morehead KY
no cats, all cats removed, no precats no main cat.
Also the fuel pump is still run off the stock electrical connection. I dont think this has nothing to do with it, but im trying to rule out everything it cant be. To me it seems like fuel is being shut off at 4000 rpm, and i aint got a clue.
Also the fuel pump is still run off the stock electrical connection. I dont think this has nothing to do with it, but im trying to rule out everything it cant be. To me it seems like fuel is being shut off at 4000 rpm, and i aint got a clue.
Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
What carb? Vacuum activated secondary's? What main jet sizes?
From your description it sounds like no fuel delivery from the secondary barrels or the main jets are to small.
From your description it sounds like no fuel delivery from the secondary barrels or the main jets are to small.
Thread Starter
Racer
Joined: Nov 2008
Posts: 252
Likes: 0
From: Morehead KY
it has a vacuum advance distributor in it right now. i personally didnt change this out when my car was swapped over to carb, but know that the distributor was changed because the stock one wouldnt work with the carb since it was computer controlled. vacuum hose is connected to the distributor and was told that it was adjusted. however thinking about taking out the vacuum hose and plugging it and checking timing to make sure it is right whenever i can get me a timing light. I dont think the base timing is off bad if it is, it idles smooth and drives under normal conditions pretty good.
Thread Starter
Racer
Joined: Nov 2008
Posts: 252
Likes: 0
From: Morehead KY
heres link to carb on my car
http://www.summitracing.com/parts/SUM-M08600VS/
Last edited by mavidatt; May 11, 2012 at 09:36 AM.
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Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
According to the Summit web site your secondary jets should be large enough.
The distributor vacuum advance should not be a problem it is not functional at WOT.
Suggest setting the base timing first.
There should be sight plugs in the side of each float bowl. Remove them & check the float levels. The fuel level should be at the bottom of the sight plug thread. Have a rag handy in case the levels are high & fuel leaks out. To adjust the float: Loosed the large lock screw on top middle of the float bowl. Turn the large nut under the lock screw to change the position of the needle/seat. Turn the large nut counter clockwise to raise the fuel level. The adjustment is sensitive, a one flat change on the nut position = a big change to the fuel level in the bowl.
With timing & fuel levels set road test. OK? if so your good to go.
Problem persists? = loosen the 4 screws on top of the secondary diaphragm. Carefully break the seal between the diaphragm & top cover. Insert a small shim like a piece of matchbook cover between the diaphragm & top cover & snug the cover screws. The secondary's are no longer functional.
Road test.
Pulls past 4K rpm without issue = fuel blockage to rear float bowl or the carb is not functioning properly.
Problem persists = troubles somewhere else.
GL
The distributor vacuum advance should not be a problem it is not functional at WOT.
Suggest setting the base timing first.
There should be sight plugs in the side of each float bowl. Remove them & check the float levels. The fuel level should be at the bottom of the sight plug thread. Have a rag handy in case the levels are high & fuel leaks out. To adjust the float: Loosed the large lock screw on top middle of the float bowl. Turn the large nut under the lock screw to change the position of the needle/seat. Turn the large nut counter clockwise to raise the fuel level. The adjustment is sensitive, a one flat change on the nut position = a big change to the fuel level in the bowl.
With timing & fuel levels set road test. OK? if so your good to go.
Problem persists? = loosen the 4 screws on top of the secondary diaphragm. Carefully break the seal between the diaphragm & top cover. Insert a small shim like a piece of matchbook cover between the diaphragm & top cover & snug the cover screws. The secondary's are no longer functional.
Road test.
Pulls past 4K rpm without issue = fuel blockage to rear float bowl or the carb is not functioning properly.
Problem persists = troubles somewhere else.
GL
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
most carb problems are ignition related.
Know anyone in your area that can put your dist on a machine and check the curve/adjust the vac can? That will make a night and day difference.
Know anyone in your area that can put your dist on a machine and check the curve/adjust the vac can? That will make a night and day difference.
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
To the op. Do what you had suggested; disconnect the vacuum line going to the distributor, rev the motor to 3000 and read the timing advance with a dial back timing light. You should have 34-36 degrees total at that rpm. If the mechanical advance is not working, your upper rpm performance will be limited. When you plug in the vacuum hose, it should give you 15 or so additional degrees of advance.
Thread Starter
Racer
Joined: Nov 2008
Posts: 252
Likes: 0
From: Morehead KY
ok timing doesnt seem to be the problem. total timing is between 34-36 at 3200 rpm, so not the issue i dont believe. I tried replacing the ignition control module too it wasnt the issue. All plugs and wires seem to be good and are gapped properly.
Do have a question though how far is the secondary throttle plate supposed to open up. It barely opens a little bit when i move the lever before it feels like something its hitting a backstop. Also I have heard that the secondary throttle plate can be removed and works fine this way.
Do have a question though how far is the secondary throttle plate supposed to open up. It barely opens a little bit when i move the lever before it feels like something its hitting a backstop. Also I have heard that the secondary throttle plate can be removed and works fine this way.
