Old school mechanics
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Mountain Pass
Joined: May 2003
Posts: 232
Likes: 16
From: Milwaukee USA
Old school mechanics
I know there is no exact answer here so looking for advice strategy. Here is what I have - 73 longbed with a 331 running this: Comcams Extreme High energy 218/224 cam with .493/.500 lift, eldelbrock rpm and holly 600cfm carb. 10.3. I'm picking up a vacuum gauge tomorrow - Im guessing at idle I should be about 15. Is that correct or close to correct?. My problem I'm ultimately trying to solve is I'm running very rich. My plugs sort of turning black and it smells rich. This is a brand new engine, not even hit the road. I have not done any tuning yet other than initial timing so the plugs don't surprise me but now its time to start to dial it in. Engine probably has like 2 or so running hours on it. I haven't done these things in like 40 years so I'm relearning everything. My plan is double check timing, hook up vacuum gauge and start turning idle mixture screws till I get to my highest point of vacuum and then run the idle screw in or out till I get to about a 700-800 idle if that holds with this cam.
Is this a realistic plan or is there better ways to go about this?
Thanks in advance
Is this a realistic plan or is there better ways to go about this?
Thanks in advance
Laughing Gas
Joined: Aug 2000
Posts: 1,005
Likes: 63
From: Eastern, Idaho
Yes do this. Your main jets might be too rich too. You can do the full throttle shut it down and pull a spark plug method to check your main jets. As far as vacuum at idle just tune mixture screws for the highest vacuum. Turn in until idle or vacuum changes, turn out until idle and vacuum changes and leave it somewhere in the middle. Try to keep idle speed adjusted down to what it needs to be and recheck mixture.
Cargo Master
Joined: Oct 2020
Posts: 2,675
Likes: 784
Something to check too is the fuel pressure. The Carter AFB/Edelbrock carburetors don't like to see as much fuel pressure as a Holley carburetor. This can cause the float levels to get too high and this will make it run rich. I can't remember exactly but I think that they are designed for around 5psi supply pressure.
That's also pretty much camshaft duration so it might be getting way into the transition slot at idle. This will tend to make the mixture screws unresponsive. The fix is so get a little bit more air somewhere else to allow the primary throttles to close a bit more at curb idle. You can try opening the secondary throttles just a bit or drilling a hole through each of the primary throttle plates. Starting out at maybe 1/16 inch to see if it helps.
That's also pretty much camshaft duration so it might be getting way into the transition slot at idle. This will tend to make the mixture screws unresponsive. The fix is so get a little bit more air somewhere else to allow the primary throttles to close a bit more at curb idle. You can try opening the secondary throttles just a bit or drilling a hole through each of the primary throttle plates. Starting out at maybe 1/16 inch to see if it helps.
Laughing Gas
Joined: Aug 2000
Posts: 1,005
Likes: 63
From: Eastern, Idaho
Good tip on the edelbrock carter carb having 5 psi or so but the guy has a holley. With a holley if the floats are set too high they can run rich or flood so check that too. Other issues I have had are float seats getting crud in them and causing them to stick open. if you have sight windows you should see the same result as floats set too high.
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Mountain Pass
Joined: May 2003
Posts: 232
Likes: 16
From: Milwaukee USA
I don't have sight windows on this carb but I'll talk with Holley on float levels. the engine has only 2 hours on it so I wouldn't think there is buildup/sludge/crud anywhere
Tuned
Joined: Sep 2014
Posts: 470
Likes: 104
With a Holley carb a damaged Power Valve can put you in an overly rich condition.
Holley's website goes through tuning your Holley carb.
I fought my 600 vac secondary Holley overly rich condition for a while.
get the throttle blades closed up, being in the transition slot will cause many tuning issues.
my 600 did not have external float adjustments or bowl sight gauge holes, I had issues with the floats needle and seats several times.
I jumped into a 450 cfm Holley and everything got better.
Holley's website goes through tuning your Holley carb.
I fought my 600 vac secondary Holley overly rich condition for a while.
get the throttle blades closed up, being in the transition slot will cause many tuning issues.
my 600 did not have external float adjustments or bowl sight gauge holes, I had issues with the floats needle and seats several times.
I jumped into a 450 cfm Holley and everything got better.
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Fleet Mechanic
Joined: Feb 2022
Posts: 1,370
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Definitely check your fuel pressure. I had a very expensive, shiny 110gph fuel pump from Holley that was delivering more than 10psi (max of my gauge) out of the box. Floated my needles and made it impossible to tune the carb. Lots of other similar experiences out there with stock replacement and performance fuel pumps.
If you get 15" of vacuum out of that cam you'll be doing well. Ditto on the transfer slot, but if you adjust it do so in very, very small increments. Opening that thing too much causes more problems than it solves. You basically want the opening to be a square shape or a very slight rectangle.
If you get 15" of vacuum out of that cam you'll be doing well. Ditto on the transfer slot, but if you adjust it do so in very, very small increments. Opening that thing too much causes more problems than it solves. You basically want the opening to be a square shape or a very slight rectangle.
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Mountain Pass
Joined: May 2003
Posts: 232
Likes: 16
From: Milwaukee USA
Tuned
Joined: Jan 2019
Posts: 364
Likes: 91
All good advice ^^. When you get your vacuum gauge,report findings. Vacuum might be low enough to have the power valve open at idle which definitely causes rich condition. If you had to tighten the idle screw to keep it running with a decent idle,like mentioned,you may have exposed the transfer slot which causes fuel to be drawn from the main circuit. Like Dave says,in this case,more air from the secondaries is the key. Don't know about yours but Holleys have a secondary adjustment screw under the base plate, rear LH side. Tiny screw,can't see it from the top.You have to remove the carb to adjust.
Fleet Mechanic
Joined: Feb 2022
Posts: 1,370
Likes: 706
^THIS, YES. Forgot about that. The power valve was causing another "too rich" issue on our engine which has a large cam. The engine makes about 11-12" of vacuum and the included power valve (7.5 I think?) was opening at idle. I switched to a 4.5 valve and it cleaned up the problem. Holley/Qwik Fuel will tell you that the rule of thumb is that the power valve should be rated for half of what your idle vacuum. If there's not an exact match, round down.
FTE Legend
Joined: Nov 2015
Posts: 30,912
Likes: 4,123
From: Angier, NC
Holley's are a vary easy carb to dial in.
Check the float level and adjust before making any changes to the carb. I dont thing the fuel PSI is too high or the carb would be pushing fuel out the vent tubes.
It has idle mixing screws / mid power valve / high speed jets and that is how you need to tune it.
Idle if you get over 800 RPM you might be into the mid / PV / main jet area and the mixture screws will not make a change when adjusting.
I have not had any issues with the slot being uncovered unless the idle was set too high and as said the screws dont work.
To check the PV as a starting point it is half of the vacuum reading at idle in gear if auto.
Then adjust up / down from there a point at a time, you can go half points if you want but full point works most of the time.
The main jets as someone pointed out you want to do a full throttle blast and then shut the motor off and cost to the side of the road, done where its safe.
Pull plugs and see what they look like and adjust up / down as needed.
Check the float level and adjust before making any changes to the carb. I dont thing the fuel PSI is too high or the carb would be pushing fuel out the vent tubes.
It has idle mixing screws / mid power valve / high speed jets and that is how you need to tune it.
Idle if you get over 800 RPM you might be into the mid / PV / main jet area and the mixture screws will not make a change when adjusting.
I have not had any issues with the slot being uncovered unless the idle was set too high and as said the screws dont work.
To check the PV as a starting point it is half of the vacuum reading at idle in gear if auto.
Then adjust up / down from there a point at a time, you can go half points if you want but full point works most of the time.
The main jets as someone pointed out you want to do a full throttle blast and then shut the motor off and cost to the side of the road, done where its safe.
Pull plugs and see what they look like and adjust up / down as needed.
Fleet Mechanic
Joined: Feb 2022
Posts: 1,370
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I hear ya about fuel being pushed out of the vents, and that eventually happened which caused me to check the fuel pressure, but it didn't start that way and there was about a week of time where I was spinning my wheels trying to figure out what was going on. I'm guessing the failure started slowly before it spun out of control. That's why I suggested checking the fuel pressure. It's easy to do and at least he'll have peace of mind if he finds out that it's good.
FTE Legend
Joined: Nov 2015
Posts: 30,912
Likes: 4,123
From: Angier, NC
I hear ya about fuel being pushed out of the vents, and that eventually happened which caused me to check the fuel pressure, but it didn't start that way and there was about a week of time where I was spinning my wheels trying to figure out what was going on. I'm guessing the failure started slowly before it spun out of control. That's why I suggested checking the fuel pressure. It's easy to do and at least he'll have peace of mind if he finds out that it's good.
If I did not have the air filter off and the fan blowing fuel back at the windshield I would of never known the pump was putting out too much PSI, it was new why should I right
Well the gauge got up to 18 PSI before the vent was pushing fuel out and I shut it off. The 2nd new pump of a different brand was good but I have seen other posts of the same thing so was not shocked when it happened.
So yes would not hurt to check PSI even with a new pump.
Dave ----
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Mountain Pass
Joined: May 2003
Posts: 232
Likes: 16
From: Milwaukee USA
Wow - lots of details (million thanks). I'll need to go slow as much of this is stuff I forgot about 40 years ago. This weekend I'll get the gauge hooked up and do some readings and see where I am at.