74 Still Breaking-up

4 SPEED

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Joined
Jul 21, 2009
Messages
78
Location
New Jersey
OK here's the deal.
74 still is breaking up on both sides under load.
Starts and runs fine until I give it 3/4 pedal or more.

New plugs and wires
New HEI distributor/set timing
New fuel filter and new gas

Carb is HP 700CFM DP
Turned mixture screws in and engine died, means power valves ok

I give up Any ideas welcome
 
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I did a image search on that, and I see a primary choke plate, but nothing on the secondary, if you open that secondary too much all at once, I would think it would bog like crazy man, just as if the secondary flapper spring on a QJet is too loose.....

I assume the primary place closes all the way when cold, and is pulled open maybe 1/4" or so upon starting.....

that's all I know of....:beer::drink:
 
Isn't a 700cfm DP a little much on anything smaller then a big block or ultra high revving small block?

I would think a 600cfm vacuum secondary would be better for a street car that isn't heavily modified.
 
Isn't a 700cfm DP a little much on anything smaller then a big block or ultra high revving small block?

I would think a 600cfm vacuum secondary would be better for a street car that isn't heavily modified.

This is not a stock 350 and I have been using this carb for 5 years with no problems.
Also, vac secondaries are better for auto trans.

I am thinking something is not right inside this carb.
 
OK here's the deal.
74 still is breaking up on both sides under load.
Starts and runs fine until I give it 3/4 pedal or more.

New plugs and wires
New HEI distributor/set timing
New fuel filter and new gas

Carb is HP 700CFM DP
Turned mixture screws in and engine died, means power valves ok

I give up Any ideas welcome

Floats set correctly? Secondary needle and seat not plugged with some junk? Secondary jets? Past the carb, what RPM are you looking at? Could the valve springs be losing their pressure and floating the valves? Hydraulic cam? Lifters getting bad? Cam?

Did this just start happening after the tuneup or an ongoing problem? In a later post, you mentioned you've had this carb 5 years- 650-700CFM is not too bad for a 350. A 650 is not enough for 6500-7000 RPM-(runs out of air about 6k) 750 is just about right for 7000+, but a little much for low end. A 700 would be an "in the middle" carb.
 
OK here's the deal.
74 still is breaking up on both sides under load.
Starts and runs fine until I give it 3/4 pedal or more.

New plugs and wires
New HEI distributor/set timing
New fuel filter and new gas

Carb is HP 700CFM DP
Turned mixture screws in and engine died, means power valves ok

I give up Any ideas welcome

Floats set correctly? Secondary needle and seat not plugged with some junk? Secondary jets? Past the carb, what RPM are you looking at? Could the valve springs be losing their pressure and floating the valves? Hydraulic cam? Lifters getting bad? Cam?

Did this just start happening after the tuneup or an ongoing problem? In a later post, you mentioned you've had this carb 5 years- 650-700CFM is not too bad for a 350. A 650 is not enough for 6500-7000 RPM-(runs out of air about 6k) 750 is just about right for 7000+, but a little much for low end. A 700 would be an "in the middle" carb.

Hi,
Engine red line is 5600 and has a mild cam, not sure what kind and has roller rockers.
This problem started all of a sudden that is why I don't think it is valves or cam.
When I picked my car up from the body shop this problem started.
They had my engine out and did remove the carb and manifold.
This is why I am thinking it's a carb problem maybe some crap go in the carb.
They did replace a diaphram for a accelerator pump because iof a gas leak.

Seems like the car runs go until I give it enough gas to open up the secondaries.

Today I will check for vac leaks around the carb and manifold.
Also, when the shop had my car they only put in plugs.
I added the distributor after the problem started.
I really thought that was the problem.

Thanks for the input.
 
OK, if anyone is interested I finally found out what was making my 74
break-up under a load.
This is after I replaced the distributor, set timing, new plug wires, new gas and re-gapped plugs and also went over the Holley 700 cmp, dp.

The problem was a KINK in the 3/8 rubber gas line that comes from the fuel pump to the carb.
When the guy in the body shop put my engine back in he replaced the rubber gas line with a different type of rubber line.
Very soft and wound up with a kink in it.

Shit happens.
 
OK, if anyone is interested I finally found out what was making my 74
break-up under a load.
This is after I replaced the distributor, set timing, new plug wires, new gas and re-gapped plugs and also went over the Holley 700 cmp, dp.

The problem was a KINK in the 3/8 rubber gas line that comes from the fuel pump to the carb.
When the guy in the body shop put my engine back in he replaced the rubber gas line with a different type of rubber line.
Very soft and wound up with a kink in it.

Shit happens.

From what I understand, in New Jersey it's legal to go throttle the guy who did that (pun intended :))....
 
Rubber line from the pump to the carb? OH NO! It's not tough to bend a piece of 3/8 brake tubing and get rid of the rubber fire starter.
 
Bset I've got. And it's changed 2 or 3 times since this picture was taken. Now, it's all blue anodized -AN fittings with a proper T. This was a repo L-88 fuel line that fit like shoes on a fish.

PICT0026-1.jpg

3/8 brake tubing, a flaring tool, and a bender and some fittings, it's done. At the fuel pump, there needs to be a pretty tight bend, but there is a hi-flow 90* fitting that makes that tight bend not needed. The rest is just a trial and error fit.
 
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Bset I've got. And it's changed 2 or 3 times since this picture was taken. Now, it's all blue anodized -AN fittings with a proper T. This was a repo L-88 fuel line that fit like shoes on a fish.

PICT0026-1.jpg

3/8 brake tubing, a flaring tool, and a bender and some fittings, it's done. At the fuel pump, there needs to be a pretty tight bend, but there is a hi-flow 90* fitting that makes that tight bend not needed. The rest is just a trial and error fit.

Thanks for the picture.
How did you attach the filter to the 3/8tube?
I have 3/8 metal tubing coming out the pump up a foot or so then to rubber and filter then back to rubber then back to tube to the carb.

Also,
The problem I was having with the fuel supply was the 3/8" rubber tube from the frame to the pump, it had a kink in it.
From what I am finding out now is I need to purchase a preformed S shaped hose for this application otherwise the pump will collapse the tubing.
 
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Then filter has -6AN male fittings at each end. I just dropped the nut and seat over the tube and flared the tube. That filter is (IIRC) an Edlebrock part. It's been replaced by a Russell with a replaceable element. Same basic plan. Cut and flare the tube.
That picture was taken in 2009. Since then, the distributor has been changed, the carb is now a Demon, and the fuel lines have been completely redone. They actually look like they are supposed to be there.
 
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