Need carb tuning advice
12-13-2009, 02:38 PM
#1
Thread Starter
Join Date: Mar 2009
Location: 5.0
Posts: 39
Need carb tuning advice
Im new to carbs so I figuring it out as I go. Purchased a used 670 holley.
Current symptoms:
- Hesitation
- Low power
- Engine does not want to rev past about 3000 under load. When not loaded revs fine.
- Put new plugs in, ran about 20 minutes, engine died, eventually started back up.
- Plugs are fairly black and sooty (too rich?)
- Dug into the carb to determine what all the guy I bought it from had changed. Has very small jets on it, 54 on primary, 65 on secondaries. Stock is in the high 60's.
- Power valve was one size too big for my vacuum 8.5
- Squirter was a 37, changed to a 40 when i had really bad hesitation off the line.
Was going to go ahead and order a jet kit for the 70 - 80 range, and a 7.5 power valve and try again but if the engine is running too rich I would think that would make it worse. Thus I am a bit lost, wouldnt think I would want to go any smaller on the jets.
Current symptoms:
- Hesitation
- Low power
- Engine does not want to rev past about 3000 under load. When not loaded revs fine.
- Put new plugs in, ran about 20 minutes, engine died, eventually started back up.
- Plugs are fairly black and sooty (too rich?)
- Dug into the carb to determine what all the guy I bought it from had changed. Has very small jets on it, 54 on primary, 65 on secondaries. Stock is in the high 60's.
- Power valve was one size too big for my vacuum 8.5
- Squirter was a 37, changed to a 40 when i had really bad hesitation off the line.
Was going to go ahead and order a jet kit for the 70 - 80 range, and a 7.5 power valve and try again but if the engine is running too rich I would think that would make it worse. Thus I am a bit lost, wouldnt think I would want to go any smaller on the jets.
12-14-2009, 02:34 PM
#3
M.A.D. Motorsports
Join Date: Jul 2006
Location: Indiana
Posts: 8,014
Do you know when the last time the thing was rebuilt? Was the car running fine prior to installing the carb? Sorry I can't give you much info because I have never worked with a holley, just an edelbrock. Did the plugs turn black just from sitting around playing with it or from the drive? Do you have a vacuum gauge?
12-14-2009, 07:30 PM
#4
Thread Starter
Join Date: Mar 2009
Location: 5.0
Posts: 39
Engine ran fine before the carb swap, good compression on all 8 cylinders. Do have a vacuum gage, running steady15 in hg at idle.
Plugs seemed to turn black from letting it idle, warm up, and the 5 minute drive down the road and back. Thinking that means the plugs are telling me squat, no WOT, no real chance to get heated up during normal operation.
Basically what I have decided to do for now is change it to the stock jets and power valve and try again.
Plugs seemed to turn black from letting it idle, warm up, and the 5 minute drive down the road and back. Thinking that means the plugs are telling me squat, no WOT, no real chance to get heated up during normal operation.
Basically what I have decided to do for now is change it to the stock jets and power valve and try again.
12-15-2009, 11:59 AM
#6
Thread Starter
Join Date: Mar 2009
Location: 5.0
Posts: 39
Timing is set at about 16 degrees, figured I would take it down to about 12 when I get the new jets to give me a better baseline.
Idle screws are adjusted out about 1-3/4 turns. From what I understand they are supposed to be adjusted to acheive the max vacuum at idle and that seemed to be the spot although I didnt try to turn them out a whole lot further. Just seemed to start running rough as it went past that.
Idle screws are adjusted out about 1-3/4 turns. From what I understand they are supposed to be adjusted to acheive the max vacuum at idle and that seemed to be the spot although I didnt try to turn them out a whole lot further. Just seemed to start running rough as it went past that.
12-15-2009, 12:08 PM
#7
M.A.D. Motorsports
Join Date: Jul 2006
Location: Indiana
Posts: 8,014
12-15-2009, 04:32 PM
#8
Thread Starter
Join Date: Mar 2009
Location: 5.0
Posts: 39
OK now im pissed. The stock jets and the part # confirm this carb is a 570 not the 670 its supposed to be. Doesnt explain the problems I am having but this carb is not going to be anywhere near big enough for my application.
^thanks for the info.
^thanks for the info.
12-15-2009, 07:41 PM
#9
3rd Gear Member
Join Date: Jul 2007
Posts: 528
What are the rest of your mods?
Summit's carb calculator: Carb CFM = ci X max rpm / 3456
Example: a 302 with a max RPM of 6000 would need a 524 cfm carb (round up to 600 CFM)
This would be based on the engine operating at 100% volumetric efficiency, which it will never see. Most street engines run around 80% volumetric efficiency (race motors may see 95% or greater). So in theory, a 302 would only need something in the 450 CFM range. I don't know anyone who actually uses this formula to size their carb, so all of that mumbo jumbo aside, we need to know your mods. A 570 may be plenty for your car.
Summit's carb calculator: Carb CFM = ci X max rpm / 3456
Example: a 302 with a max RPM of 6000 would need a 524 cfm carb (round up to 600 CFM)
This would be based on the engine operating at 100% volumetric efficiency, which it will never see. Most street engines run around 80% volumetric efficiency (race motors may see 95% or greater). So in theory, a 302 would only need something in the 450 CFM range. I don't know anyone who actually uses this formula to size their carb, so all of that mumbo jumbo aside, we need to know your mods. A 570 may be plenty for your car.
12-15-2009, 07:46 PM
#10
2nd Gear Member
Join Date: Jun 2007
Location: IL
Posts: 480
Did you take it apart and inspect it before slapping it on there? Buying used parts isn't bad, but buying a used carb is sometimes a formula for headache like you may have on your hands now. That's one of the few things I won't buy used, granted new carbs can also come with problems.