New heads and intake, now engine bog off the line - Chevelle Tech
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post #1 of 33 (permalink) Unread Jun 15th, 19, 4:15 PM Thread Starter
Mike
 
Join Date: May 2016
Location: Ham Lake, Minnesota
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New heads and intake, now engine bog off the line

Hey fellas,

I have a 72 Chevelle with a 400 small block, stock bottom end, 350 horse 327 cam, .447" lift and 222deg at .050", aluminum heads with 200cc intake runners 64cc chambers 2.02 1.60 valves, performer rpm air gap, carter afb 625cfm, 1 5/8" long tube headers and 2.5" dual exhaust with flowmaster mufflers. It's still a 2.73 open diff and turbo 350 with 2000 stall.

Car was running awesome with no problems whatsoever. I swapped out the crappy small valve factory iron heads and performer intake for 200cc big valve aluminum heads and performer rpm air gap and the power increase was incredible.

Problem is now if I mash the gas from a standing start or even a rolling start the engine "bogs" for a second or two before it takes off. I already have the accelerator pump stroke maxed out and I verified I am getting accel pump discharge from both nozzles. initial timing is 12deg with another 24deg from mechanical advance for a total of 36deg. HEI ignition from performance distributors.

I'm thinking the carter just doesn't have enough of an accelerator pump shot for the new heads and intake. I don't know what the carb came off of originally but it was a freebie from a buddy. Car runs amazing part throttle and WOT once its past the initial "bog".

Anyone have any insight into what could be causing my bog issue? Any suggestions on a new carb for the engine?

Thanks guys.
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post #2 of 33 (permalink) Unread Jun 15th, 19, 4:59 PM
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jeff
 
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Re: New heads and intake, now engine bog off the line

Mike some tuning on the carb may help it as will a bunch more initial timing but you must watch the total.
18-22 initial timing.

Those heads have much larger runners than the old GM heads and low speed airflow will be much less so it is not pulling on the carb venturies near as hard so it goes lean.

Pump shot may not get you all you need.
Loosen the covers over the metering rods and rotate them so they cover part of the hole and snug the screw back down.
You should be able to see the power piston.

Now start the engine.
The piston should be down .. now just slightly blip the throttle like a quick 1/4 throttle blip the power piston should jump up instantly.
If it does not you need to install a stiffer spring under it.

It needs to be stiff but soft enough that the piston stays down at idle in gear.

Find that happy point.

Move onto the metering rods.
Factory on most of those 625's had a 70-47 rod with an .098" jet in the primary.
Try a 65-47 or even a 65-37 rod.

If no rods you could try a different jet like a .104" which will be marked 404.
4 signifies the jet is over .100" diameter.

I have chucked the metering rods into a drill press and sanded them down to the size I needed.
Yes you can chuck the top of the rod in the press or drill chuck if you do it correctly and it will spin nicely.

You are going lean as you know and more timing really helps in that situation as will more fuel.
Good luck.
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post #3 of 33 (permalink) Unread Jun 15th, 19, 7:12 PM
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Eric
 
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Re: New heads and intake, now engine bog off the line

The 12* initial is not helping your cause. Limit your total and get your initial up to at least 18/20

Your just jealous because you can't hear the voices!

1967 Olds Cutlass 468 BBC/T400
1963 Plymouth Fury 440/Torqueflite
2007 Ford Mustang GT (momma's car)
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post #4 of 33 (permalink) Unread Jun 15th, 19, 7:16 PM Thread Starter
Mike
 
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Re: New heads and intake, now engine bog off the line

I have another 4* in it so far and its already way better. I'll have to spend some more time tuning the timing. I did check the action of the power piston and it looks great. Rods are down at idle and they pop up as soon as the throttle is snapped open. I didn't check the diameter of the rods yet or the jet size but I will. Thanks for the help guys.
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post #5 of 33 (permalink) Unread Jun 15th, 19, 10:14 PM
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Eric
 
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Re: New heads and intake, now engine bog off the line

I would disconnect an plug your vacuum advance until you get the mechanical timing ironed out. You may need to install a stop.
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Your just jealous because you can't hear the voices!

1967 Olds Cutlass 468 BBC/T400
1963 Plymouth Fury 440/Torqueflite
2007 Ford Mustang GT (momma's car)
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post #6 of 33 (permalink) Unread Jun 16th, 19, 9:18 AM
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Rick
 
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Re: New heads and intake, now engine bog off the line

As Jeff said, the runner volume changed your air speed. It will be a juggling act between carb and timing tuning. Just takes time. I would eventually move closer to a 700 cfm vacuum carb.
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post #7 of 33 (permalink) Unread Jun 16th, 19, 10:32 AM
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Re: New heads and intake, now engine bog off the line

Quote:
Originally Posted by MNmalibu View Post
Hey fellas,

I have a 72 Chevelle with a 400 small block, stock bottom end, 350 horse 327 cam, .447" lift and 222deg at .050", aluminum heads with 200cc intake runners 64cc chambers 2.02 1.60 valves, performer rpm air gap, carter afb 625cfm, 1 5/8" long tube headers and 2.5" dual exhaust with flowmaster mufflers. It's still a 2.73 open diff and turbo 350 with 2000 stall.

Car was running awesome with no problems whatsoever. I swapped out the crappy small valve factory iron heads and performer intake for 200cc big valve aluminum heads and performer rpm air gap and the power increase was incredible.

Problem is now if I mash the gas from a standing start or even a rolling start the engine "bogs" for a second or two before it takes off. I already have the accelerator pump stroke maxed out and I verified I am getting accel pump discharge from both nozzles. initial timing is 12deg with another 24deg from mechanical advance for a total of 36deg. HEI ignition from performance distributors.

I'm thinking the carter just doesn't have enough of an accelerator pump shot for the new heads and intake. I don't know what the carb came off of originally but it was a freebie from a buddy. Car runs amazing part throttle and WOT once its past the initial "bog".

Anyone have any insight into what could be causing my bog issue? Any suggestions on a new carb for the engine?

Thanks guys.
Get rid of the Carter (same as the Eddy's) those thing have a problem with the sec air valve coming open too soon, to confirm this take off the sec linkage on the pass side take the car out and get on it, I am willing to be the bog is gone. If you must run one of the Carter/Eddy carburetors the get a Thunder carb with the adjustable air door, same as the Q-jet setup or just try a Holley VS.
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post #8 of 33 (permalink) Unread Jun 16th, 19, 10:48 AM
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Kerry
 
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More initial timing and a larger squirter for the carb are what you need.
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post #9 of 33 (permalink) Unread Jun 16th, 19, 1:10 PM
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Chris
 
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Re: New heads and intake, now engine bog off the line

I agree, more initial 1st. That thing should blow the tires off at will PROVIDING the rest of the combo/tune up is correct.
Ask me how I know.
The 406 in my sig now resides in my sons 67 C10 with smaller headers, carb and big single with a 1" open spacer and will do rolling burnouts in 3rd gear (M20 muncie)
Timing locked out at 34* (ran 37* in times in my sig)
No starter retard,switch etc. Starts every time. Hot or cold with a factory starter.
Tune that thing! It will humble much larger engines.

73 nova,3515#(all steel)Pump gas!(91octane)406ci N/A, best 1/4,1.45 '60,[email protected],10.6 @ 122.38, short shifting,NO tuning!

http://www.youtube.com/watch?v=ng86PsGUcTY
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post #10 of 33 (permalink) Unread Jun 16th, 19, 8:22 PM
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Re: New heads and intake, now engine bog off the line

"initial timing is 12deg with another 24deg from mechanical advance for a total of 36deg".

"HEI ignition from performance distributors." Run of the mill ignition, at best.

For a 400, initial is just fine, they have a long stroke, and 12 for one of them is like 16 for a shorter stroke 350.

What IS needed, is 10 more degrees for the vacuum advance, set with a positive stop on the V/A pull pin, and connected to full manifold vacuum sourcing, bottom of the front of the carb (Carter junk will e on the DRIVER side of the carb, NOT THE PASS SIDE.

12 INITIAL + 10 VACUUM ADVANCE, ON FULL MANIFOLD VACUUM, will give you the 22 deg/BTDC @ IDLE, the engine will enjoy it, and will still start easily. It will also allow the throttle plates and idle speed to be lowered to a reasonable level.
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post #11 of 33 (permalink) Unread Jun 16th, 19, 11:02 PM
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Mike
 
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Re: New heads and intake, now engine bog off the line

Thanks for posting Dave.
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post #12 of 33 (permalink) Unread Jun 17th, 19, 10:30 PM Thread Starter
Mike
 
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Re: New heads and intake, now engine bog off the line

Quote:
Originally Posted by Dave Ray View Post
"initial timing is 12deg with another 24deg from mechanical advance for a total of 36deg".

"HEI ignition from performance distributors." Run of the mill ignition, at best.

For a 400, initial is just fine, they have a long stroke, and 12 for one of them is like 16 for a shorter stroke 350.

What IS needed, is 10 more degrees for the vacuum advance, set with a positive stop on the V/A pull pin, and connected to full manifold vacuum sourcing, bottom of the front of the carb (Carter junk will e on the DRIVER side of the carb, NOT THE PASS SIDE.

12 INITIAL + 10 VACUUM ADVANCE, ON FULL MANIFOLD VACUUM, will give you the 22 deg/BTDC @ IDLE, the engine will enjoy it, and will still start easily. It will also allow the throttle plates and idle speed to be lowered to a reasonable level.
Dave Ray, any recommendations for a carb upgrade?
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post #13 of 33 (permalink) Unread Jun 18th, 19, 12:10 PM
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Kerry
 
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Location: Lloydminster, Alberta
Posts: 803
Re: New heads and intake, now engine bog off the line

If you are going to do a carb swap I would look at a Quick Fuel HR780 or SS780. These over the 770 street avenger because they already have the secondary vacuum tube, and changeable IFR, emulsion, air bleeds, PVCR.
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post #14 of 33 (permalink) Unread Jun 18th, 19, 1:11 PM
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Re: New heads and intake, now engine bog off the line

H, O, double L, E, Y, that's HOLLEY, and make sure the carb has what pockets outlined just above, makes the carb fully tune able.
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post #15 of 33 (permalink) Unread Jun 18th, 19, 5:56 PM
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Re: New heads and intake, now engine bog off the line

Quote:
Originally Posted by MNmalibu View Post
Hey fellas,

I have a 72 Chevelle with a 400 small block, stock bottom end, 350 horse 327 cam, .447" lift and 222deg at .050", aluminum heads with 200cc intake runners 64cc chambers 2.02 1.60 valves, performer rpm air gap, carter afb 625cfm, 1 5/8" long tube headers and 2.5" dual exhaust with flowmaster mufflers. It's still a 2.73 open diff and turbo 350 with 2000 stall.

Car was running awesome with no problems whatsoever. I swapped out the crappy small valve factory iron heads and performer intake for 200cc big valve aluminum heads and performer rpm air gap and the power increase was incredible.

Problem is now if I mash the gas from a standing start or even a rolling start the engine "bogs" for a second or two before it takes off. I already have the accelerator pump stroke maxed out and I verified I am getting accel pump discharge from both nozzles. initial timing is 12deg with another 24deg from mechanical advance for a total of 36deg. HEI ignition from performance distributors.

I'm thinking the carter just doesn't have enough of an accelerator pump shot for the new heads and intake. I don't know what the carb came off of originally but it was a freebie from a buddy. Car runs amazing part throttle and WOT once its past the initial "bog".

Anyone have any insight into what could be causing my bog issue? Any suggestions on a new carb for the engine?

Thanks guys.
Maybe I can look at it for you next month, I live in the Twin Cites. Are you going out to the Summer Cruise this year with your car?
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