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Old 12-07-2010, 12:52 PM
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chevy 350 bogs around 3000 help

i have a 4 bolt main 350 with a mild cam the cam has a .454 lift dont remember all the specs. 992 heads 750 cfm holley headers motor is clean and was rebuilt from cam up. 4 speed trans. ok so i timed it got it running well and in first gear it will smoke the tires off but it seems when i dont pop the clutch cruise along at about 2000-2500 rpm and the push the pedal to the floor it bogs and sputters and at times back fires. it will do it in second gear as well. it idles well and will rev up fine some times and some times it will backfire and act like its gasping for air. i know when it backfires its a timing issue but i used a light and thought i got it spot on. any info would help. thanks guys

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Old 12-07-2010, 01:20 PM
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At how many degrees BTDC did you time it?

Could it be, that your accelerator-pump squirts in too much gas?

Do you have mechanical or vacuum secondaries? In case you have vacuum secondaries, maybe the open too early.
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Old 12-07-2010, 01:29 PM
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when i timed the motor using a light i put it at 4 degrees on the timing tab. the carb is a holley 750 cfm all mechanical. do u think it needs to be advance a lot more.. also how do you tune the accelarator pump down i think it does squirt to much fuel to fast
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Old 12-07-2010, 01:39 PM
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Did you mean 882 heads? If so with these heads, you are going to need alot more timing, like 12-14 degrees intial.
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Old 12-07-2010, 01:52 PM
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yep I would agree more timing, what is this cam, gears, weight/type of car, tire size?

Not knowing what cam you have I am gonna guess with .454 lift you are around 215 duration @ 0.050. I would recommend to do this first:

http://www.crankshaftcoalition.com/w...op_dead_center
http://www.crankshaftcoalition.com/w..._a_timing_tape

Set your base timing to 16 deg BTDC
Add another 18-20 mechanical advance by 2800 for a total of about 34-36.
Add vacuum advance limited to 12-14 deg on ported or manifold, try both and see what it does.

Check your fuel pressure after the filter, should be about 5.5 psi

Check your vacuum levels;
http://www.secondchancegarage.com/public/186.cfm

On the accelerator pump you can set the freeplay between the pump arm and the cam when at WOT, I think its 0.015 to get you close.
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Old 12-07-2010, 01:55 PM
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my secondary butterflies dont open either when reving it in neutral there a lot going into to adjsuting this holley carb
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Old 12-07-2010, 04:38 PM
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Quote:
Originally Posted by c_cham
my secondary butterflies dont open either when reving it in neutral there a lot going into to adjsuting this holley carb
Never will, they require load to open.
If it wont rev above 3K I would be willing to bet you are running out of fuel. Check the float levels, fuel pressure, fuel filter, twisted or pinched lines.
The above advice on your timing is also very good but your problem sounds like lack of fuel.
If you want to check if the secondaries are opening, you can put a paperclip on the diaphragm arm and take it for a ride. The clip will have moved if they opened.

One other item is try it without the air filter, lack of air flow will make it seem just like lack of fuel. I got burnt on this a while back, chased mt tail on a over restrictive air filter.
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Old 12-08-2010, 07:48 AM
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sbc timing question and carb question.....i am ready to absorb your knowledge

ok i have a 73 nova put a 350 4 bolt main and a holley carb 750 cfm. it has headers and the heads got redone just new seats and cleaned. they are 993 heads i think not 882. i put a new cam in it and broke it in... the cam specs are

Hydraulic flat tappet
Basic Operating RPM Range 1,500-5,000
Intake Duration at 050 inch Lift 216
Exhaust Duration at 050 inch Lift 216
Duration at 050 inch Lift 216 int./216 exh.
Advertised Intake Duration 272
Advertised Exhaust Duration 272
Advertised Duration 272 int./272 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.454 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.454 in.
Valve Lift with Factory Rocker Arm Ratio 0.454 int./0.454 exh.
Lobe Separation (degrees) 110
ok so the motor is all set and it was running like crap but i got it to run better...

what degree should i start out with for timing... i set at 12 but for my cam i have heard 16 would be better even though you cant see 16 on the timing tab....

ok second when i adjust my holley carb idle air mixture xcrew the driver set will drop psi when i turn it clockwise but whe backing the screw out (counterclockwise) it wont drop or even move it just sits at the high mark and you can almost take the screw out and nothing... the passenger side works fine tho.... info please i am a sponge
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Old 12-08-2010, 07:55 AM
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start by reading these links.

http://www.corvette-restoration.com/.../Timing101.pdf

http://www.bob2000.com/carb.htm
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Old 12-08-2010, 08:05 AM
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doesnt answer the idle air screw i know all about timing just asking for info thanks anyone else maybe have a type of answer for me
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Old 12-08-2010, 08:20 AM
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Quote:
Originally Posted by c_cham
doesnt answer the idle air screw i know all about timing just asking for info thanks anyone else maybe have a type of answer for me
What do you mean by "the driver set will drop psi....", and the "high mark" thing, I have no idea what you mean.
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Old 12-08-2010, 08:25 AM
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I replied to your previous post and will repeat the same info. Verify TDC and make a timing tape for your damper:

I would not state that you know all about timing or you would not be here

http://www.crankshaftcoalition.com/w..._a_timing_tape
http://www.crankshaftcoalition.com/w...op_dead_center

This necessary so that when you do set your timing you will know that it is correct.

In a nut shell,,,

Then with the new timing tape you can go for a base setting of 16-18 BTDC to start with. What type of distributor?
Have the mechanical advance worked so that it will add another 16-18 for a total of 34-36 at approx 2800 RPM
Use a limiter plate on the vacuum advance so that it will not add more than 12-14, an adjustable vacuum can is also nice because it allows you to set the range in "wg engine vac that the vacuum advance operates through.

A higher base timing and full vacuum advance at idle (manifold port to dist) will help to ensure that you are on the idle circuit of the carb at idle. Your mixture adjustment senario seems OK but I don't understand what you mean by psi, I suppose you mean "wg that the vacuum gage is reading? At any rate when you back a mixture screw all the way out it should not have an effect passed a certain point so I don't see a big issue here. Once you get you timing more advanced at idle and close down your base idle adjustment (closing primary throttle plates) you will have better luck with your idle mixture adjustment. Turning the screws all the way counter clock wise will do nothing passed a certain point.
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Old 12-08-2010, 09:53 AM
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Quote:
Originally Posted by Custom10
I replied to your previous post and will repeat the same info. Verify TDC and make a timing tape for your damper:

I would not state that you know all about timing or you would not be here

http://www.crankshaftcoalition.com/w..._a_timing_tape
http://www.crankshaftcoalition.com/w...op_dead_center

This necessary so that when you do set your timing you will know that it is correct.

In a nut shell,,,

Then with the new timing tape you can go for a base setting of 16-18 BTDC to start with. What type of distributor?
Have the mechanical advance worked so that it will add another 16-18 for a total of 34-36 at approx 2800 RPM
Use a limiter plate on the vacuum advance so that it will not add more than 12-14, an adjustable vacuum can is also nice because it allows you to set the range in "wg engine vac that the vacuum advance operates through.

A higher base timing and full vacuum advance at idle (manifold port to dist) will help to ensure that you are on the idle circuit of the carb at idle. Your mixture adjustment senario seems OK but I don't understand what you mean by psi, I suppose you mean "wg that the vacuum gage is reading? At any rate when you back a mixture screw all the way out it should not have an effect passed a certain point so I don't see a big issue here. Once you get you timing more advanced at idle and close down your base idle adjustment (closing primary throttle plates) you will have better luck with your idle mixture adjustment. Turning the screws all the way counter clock wise will do nothing passed a certain point.
The post I have quoted above is a very good post as to what the OP should be looking for.

I would emphasize using the full manifold vacuum sourse.

Set/check your timing with the vacuum advance dis-connected and plugged.

Re-connect after you have set the timing.

Re-adjust your idle speed with the curb idle screw after hooking up the vacuum advance hose. The idle speed should be below 800 RPM (In DRIVE if an automatic or NEUTRAL if a standard shift). Anything above that will introduce some mechanical timing and also will allow the throttle plates to be open to the point where the idle slot is more exposed and the air/idle screws will have little or no function because you will be out of the idle circuit.

A typical reason for the air/idle screw(s) not yielding any changes when being adjusted is because the throttle plate(s) are open too far. This will cause the idle circuit to be by-passed and the air /idle screws will have little or no function. When the the carb is removed from the intake and the throttle stops are fully off the high idle cam, the throttle plate(s) should be closed and the opening for the idle slot should look like a small square. If that slot looks like an elongated opening the throttle plates are being held open too far and the air/idle screws will not function.
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Old 12-08-2010, 04:08 PM
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Quote:
Originally Posted by c_cham
my secondary butterflies dont open either when reving it in neutral there a lot going into to adjsuting this holley carb
He is not running vacuum secondary. He's running a DP and DP 750 do not like running low numeral gear ratio.
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Old 12-08-2010, 04:08 PM
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Duplicate, similar threads are merged. Double posting won't get an answer any faster.
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