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Old 02-06-2008, 12:45 PM
350c10
 
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SBC 350 won't accelerate

My sb 350 is dying out under hard acceleration It has a edlebrock jpreformer rpm top end kit. Idles smooth runs smooth until I try to accelerate up hill or merge at freeway speeds. I love my truck. I just wont drive it like this and can not figure it out!

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Last edited by OneMoreTime; 02-06-2008 at 12:56 PM. Reason: Other.
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Old 02-06-2008, 01:11 PM
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what kind of specs on the motor?what carb?and where is your timing at?sounds like a bad filter?or the carb is emptying before it has a chance to refill
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Old 02-06-2008, 01:19 PM
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Its a 600 cfm edelbrock carb running 8 degrees btdc, 0.98 primary 0.95 secondary. Specs on the motor are stock cam, and rpm heads I have had a hard time finding the specs off of edelbrock's website. The number I pulled off of the heads is 6089 and doesnt give me specifics.
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Old 02-06-2008, 01:49 PM
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Quote:
Originally Posted by j_sirnio
My sb 350 is dying out under hard acceleration It has a edlebrock jpreformer rpm top end kit. Idles smooth runs smooth until I try to accelerate up hill or merge at freeway speeds. I love my truck. I just wont drive it like this and can not figure it out!
hey man, i had restored a 72 monte carlo, it did the same thing. i found out that the fuel line to the carb. was getting hot and the carb. was sucking it close. i tried new fuel line with a heat grade. and increased the pressure on my fuel pump. and it worked.
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Old 02-06-2008, 02:11 PM
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I dont think thats it. My fuel line is some nice thick 3/8 hose. Im sure it has to be something with the carb or vacume advance. I am running 15 hg at an idle and 21hg at 2500 rpm. This problem seems to come up eveytime I lay some good rubber for a few blocks.

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Old 02-06-2008, 02:49 PM
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youre running a stock cam and getting only 15" of vacuum at idle?
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Old 02-06-2008, 02:52 PM
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Not many ignition issues cause one to die out, the issue is obvious its running out of fuel, this could be to many things. A fuel pump going south, the sock in the fuel tank collapsing, clogged fuel filter, float level too low or vapour locking. Take it for a test drive and hammer down on it, when it goes dead ease to the side of the road and pull a spark plug, if its white its dead lean indicating its running out of fuel. Under hard acceration the engine is pulling in lots of fuel and air, when the fuel supply cant keep up you know the rest of the story. On another level, is the vac advance connected to a manifold full time vacuum source? if not it should be. in most cases SBCs like 12 degrees BTDC and full vac advance at idle which will give it around 24 degrees, this will give it better throttle response.
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Old 02-06-2008, 03:02 PM
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Sounds like a vapor lock from that edelbrock carb. Those carbs tend to vapor lock easily when the engine gets hot since they are made of light weight aluminum. Try a thermal spacer under the carb. Or buy a real carb like a Q-jet or Holley.

would also be a good idea to put a fuel pressure gage in right before the carb, so you can see if it has fuel pressure issues when it acts up.

low vacuum at idle indicates an incorrect a/f mixture. adjust the idle mixture screws for highest idle (highest vacuum). should be getting 18 or 19 inhg at 750 rpm with a stock cam. Low ignition timing can also lower vacuum.
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Old 02-06-2008, 03:07 PM
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Thanks now I just have to pull my metering valves out of my intake. Dont you hate that.
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Old 02-06-2008, 03:35 PM
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Now that my metering rods have returned to the proper position underneith the hood. Vapor lock sounds to be the issue. Fuel pump is as great as she has ever been. Im not a huge carb guy I am all diesel. The vac advance port is the one that goes to the distributor right? at an idle that one sits at 2hg of vac. and the other one sits at 15hg at an idle. at roughly 2000 rpm ( i really dont know i cant see my tach from under the hood) both sits at 20-25 hg's never had issues with it until now and the vac is the same as its always been.
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Old 02-06-2008, 03:42 PM
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hows your exhaust, could it be plugged?
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Old 02-06-2008, 03:59 PM
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Quote:
Originally Posted by j_sirnio
The vac advance port is the one that goes to the distributor right? at an idle that one sits at 2hg of vac. and the other one sits at 15hg at an idle. at roughly 2000 rpm ( i really dont know i cant see my tach from under the hood) both sits at 20-25 hg's never had issues with it until now and the vac is the same as its always been.
15inhg of vacuum at idle will not cause any problems. However, with a stock cam, that does indicate something is not set up right. You could be getting better mpg and have better throttle response with more tuning.

There are two vacuum sources, ported and manifold. The port that reads 2inhg at idle should been the ported source. However, it shouldn't read anything at idle, unless the throttle at idle is opened too far (past the idle circuit). This makes the idle very rich and makes the vacuum drop.

Remove the vacuum advance line, rev the engine up to 4000 rpms, and set the total mechanical timing to 38 degrees. then let the engine idle down and read where the timing is at idle. This should be your new base timing. Then plug in the vacuum advance into the manifold vacuum port (high vacuum at idle) and it should add another 10 to 15 degrees at idle. If it adds more than 15 degrees then get an adjustable vacuum advance canister from summit racing (accel makes them).

When using the manifold vacuum port to the distributor (high vacuum at idle), it will increase the idle rpm a lot, so then you can lower the rpms with the idle screw, and adjust the idle vacuum to the highest amount with the idle mixture screws. FYI, lowering the idle with the idle screw will close the throttle blades into the proper idle range and the ported vacuum source will read zero. don't forget to adjust the idle mixture screws for "best idle, highest rpm, highest vacuum".
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Old 02-06-2008, 05:40 PM
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You have a fuel volume delivery issue aggravated by carb,pump,fuel lines hot soak.

Check the rubber hose connecting the fuel line to the fuel pump inlet. Often this short rubber fuel joint collapses under high flow demand WOT and hot temps.
Get a mechanical fuel pump mounting heat shield kit. (Seals It) and a carb heat insulating gasket or wooden Edelbrock carb heat spacer. Get an Electric fuel pump and mount it at the back near the fuel tank to pressureize the fuel feed line to the mechanical fuel pump. Carter #P4594 electric fuel pump works very well. Eliminates vapour lock. No regulator is required.
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Old 02-06-2008, 06:55 PM
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Could It possibly be a bad vacuum advance unit?
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Old 02-06-2008, 07:49 PM
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Quote:
Originally Posted by j_sirnio
Could It possibly be a bad vacuum advance unit?
A faulty vacuum advance would not cause the vehicle to have an acceleration problem.
Vacuum advance adds additional spark timing while you are cruising along at part throttle/high manifold vacuum with low load on the motor such as on the hyway. it is not used during relative hard acceleration or engine load, such as merging onto a hiway or climbing a hill.
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