Thread: Changed carbs and engine felt dead

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I changed carbs from my edelbrock 750 mech secondaries to a Holley 600cfm vacuum secondaries. There was a huge difference in part throttle acceleration. It was much much slower. Full throttle acceleration was good but it stopped pulling at 5500 instead of 6000. Should I play with timing more or scrap the 600 and worry about fine tuning my 750.

if i pulled off a carb. that made my truck much much more slower, i think id put it back on. why did you pull it off to start with?

Most all of the threads I read said 600cfm would be plenty for my engine, plus I have been fouling plugs so I figured the smaller jetting would cure that. The power was gone. It felt like it lost 75 ft/lbs of torque down low and plenty of HP up top. I think I am going to try to rejet the edelbrock 750 to lean it out a little. Any thoughts on what size jets or what else I can do?

you cant have both worlds. a 350 with a 750 will pull in the high rpm range. i cant imagine how going to a 600 would kill throttle response. i run 600's with 62 primaries . you'll get quicker throttle response with a lean tune. even using edelbrocks formula a 350 only needs 600 cfm a 6k rpm. tune the 600, you'll get better millage and a cleaner burn for street use. that 750 will wash down your cyclinders and cause ring failure.

First of all you went from mech to vacuim secondaries, first mistake in my openion. You cut the volume of air as well at the same time. Try smaller jets in the 750 or hotter plugs.

Originally posted by DADNOVA
First of all you went from mech to vacuim secondaries, first mistake in my openion. You cut the volume of air as well at the same time. Try smaller jets in the 750 or hotter plugs.

Hotter plugs?Let me guess.You think he will burn more fuel this way.

Tried 3 different plugs and all have been getting fouled. I dont think the heat range is going to help with the fouling, at least it hasnt so far. Should I bite the bullet and take it to a professional and have them tune the carb and set my timing?

unless you have some good hp the 750 is to big. lean run better than rich. you should be tuning the 600. JMO

Originally posted by joewee350
Tried 3 different plugs and all have been getting fouled. I dont think the heat range is going to help with the fouling, at least it hasnt so far. Should I bite the bullet and take it to a professional and have them tune the carb and set my timing?

That was the point I was trying to make.You can try all the heat ranges you want.Sounds to me like you have to much carb.

I have a TH350 with 2500 stall. Rear gears are 3.42 with posi. With the 750 it melts the tires through first and well into 2nd. With the 600 I had to stall it up quite a bit and then hammer on it and it left about half the peel. I know this is a crude measure of power but I dont have a dyno or 1/4 track. So Is everyone saying that there is no sense in trying to rejet this carb to lean it out. I should scrap it and buy a 600?

I'd go with the Holley 600, but if you're running a spread bore manifold I'd run a short four hole spacer to square the plenum opening. Holley's don't transition to well on spread bore manifolds. Go to www.summit.com and check out the Holley kits. There's a kit that has different accelerator pump shooters and cams to tailor the accelerator pump which IMHO is the way to go. I'd also consider the secondary diaphram kit which makes it easy to change when the secondaries come in. You can also opt for the kit that changes your 4160 to a 4150 by replacing the metering plate with a metering block for the secondaries.

the edelbrock 750 afb works on vacuum .so dose the holley you can play with secondarys with spring assortment and get the quick change secondary spring kit

depending on your cam i would look at the powervalve also in the 600......whatever your idle vacuum reading is go two sizes smaller in the power valve......ie: idle vacuum inches equal 8 run a #6 power valve........and also as pat mcarthy suggested altering the seconday springs......nolley sells a kit that has several springs in it.....test and tune........once working proporly you will see a noticable difference......

IMO a mech. sec. is out of place on the street unless its pro-street with serious mods. If the 750 was vac. I'd say tune it. It would work great. Low speed torque and hi speed horses. The reason I think you dropped so much power is that you have power valve problems, way too small jets, or too strong of vac. spring. Either buy a vac. 750 or tune the 600 The 750 tuned right will make more power. You can tune it yourself, just takes a little tinkering.

I bought a timing light and set my timing for starters. Set it at 15 initial and 36 total and it loped harder and ran much stronger but still the black smoke. I changed jets on the edelbrock 750 from .113 to .110 on primary and from .107 to .101 on the secondary and there is no smoking problem and it revs nicely to 6000. Thanks alot for the responses.