question for the carburetor gurus
11-09-2017, 09:44 AM
#1
Drifting
Thread Starter
question for the carburetor gurus
On my bone stock 1972 LT-1, the carb is list number 6239-1. I bought the "trick" rebuild kit for it, which includes all sorts of power valves, among other things. Reason: rough idle and misfire at higher revs. Ignition system has all been renewed with fresh plugs, new wires, and Pertronix LS1181 kit. Valve lash set to factory specs.
My engine currently develops 20.5 inches of vacuum measured at the PCV port, and verified at the intake manifold vac fitting. This with the un-rebuilt carb in place.
The rebuild kit includes many power valves rated at 6.5, 3.5, etc....but only one at 8.5 and only one at 10.5.
Here's the question: if I install the 10.5 in the primary metering block, and install the 8.5 in the secondary metering block, does this make sense to do? Or, should I get another 8.5 power valve for the primary?
I went ahead and installed the 8.5 and 10.5 per above and will try it out on the engine tonight. But what do you think will happen? Should it run ok? Or am I
My engine currently develops 20.5 inches of vacuum measured at the PCV port, and verified at the intake manifold vac fitting. This with the un-rebuilt carb in place.
The rebuild kit includes many power valves rated at 6.5, 3.5, etc....but only one at 8.5 and only one at 10.5.
Here's the question: if I install the 10.5 in the primary metering block, and install the 8.5 in the secondary metering block, does this make sense to do? Or, should I get another 8.5 power valve for the primary?
I went ahead and installed the 8.5 and 10.5 per above and will try it out on the engine tonight. But what do you think will happen? Should it run ok? Or am I
11-09-2017, 09:51 AM
#2
Instructor
You really need to drive the car with a Vac gauge hooked up and get the readings under very light load then begin with a PV 2" below that and work up until you get the best incremental increase (strip) or calibrated seat of the pants feel.
Milan
Milan
11-09-2017, 09:59 AM
#3
A 6239 is a 4150 vacuum secondary model
http://documents.holley.com/techlibr...al_listing.pdf
I'm no Guru but I know a PV is a vacuum operated jet. When it opens it is equal to about 8 jets sizes. They open at the marked number (3.5 inches, 6.5 inches, 8.5 inches, etc)
At what vacuum does your secondary become 50% and fully open? Transition from primary to secondary fuel should from the transition slot and the squirter, not the PV. As a secondary opens with low manifold vacuum (high ported) I would think you would want to use one of the low numbers (ie, 3.5) but does it need more fuel when secondary is partially or fully open? .
Don't just randomly start inserting parts. I would install an O2 sensor and figure out exactly what the engine wants for jets and PV sizes. The $15 bolt-on FiTech O2 bung is easy. That's what I did.
R
http://documents.holley.com/techlibr...al_listing.pdf
I'm no Guru but I know a PV is a vacuum operated jet. When it opens it is equal to about 8 jets sizes. They open at the marked number (3.5 inches, 6.5 inches, 8.5 inches, etc)
At what vacuum does your secondary become 50% and fully open? Transition from primary to secondary fuel should from the transition slot and the squirter, not the PV. As a secondary opens with low manifold vacuum (high ported) I would think you would want to use one of the low numbers (ie, 3.5) but does it need more fuel when secondary is partially or fully open? .
Don't just randomly start inserting parts. I would install an O2 sensor and figure out exactly what the engine wants for jets and PV sizes. The $15 bolt-on FiTech O2 bung is easy. That's what I did.
R
Last edited by Dynra Rockets; 11-09-2017 at 10:00 AM.
11-09-2017, 11:05 AM
#4
Had a 1976 L-82, 4-sp
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Royal Canadian Navy
To start, I would put the same size PV's back into the carb that came out and keep the carb stock jets etc. Are your secondaries fully opening when the gas pedal is to the floor?
11-09-2017, 11:49 AM
#5
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St. Jude Donor '05
put it to stock and start all over...know its an expense but the 02 kit is the way to go theres no guessing, if its misfiring at high rpm bet its more ignition or valvespring related
Seems every carb problem asked gets a response of fooling with the power valves 90% of the time they have nothing to do with it
11-09-2017, 12:02 PM
#6
Drifting
Thread Starter
Secondaries do indeed open under load when driven, but there is a definite misfire felt. OK, stock size is 6.5 I believe. I'll pop those in instead and see how she runs. Thanks.
11-09-2017, 12:05 PM
#7
Drifting
Thread Starter
put it to stock and start all over...know its an expense but the 02 kit is the way to go theres no guessing, if its misfiring at high rpm bet its more ignition or valvespring related
Seems every carb problem asked gets a response of fooling with the power valves 90% of the time they have nothing to do with it
Valvespring issue? Did a visual and appeared ok when I lashed the rockers. And the vacuum readings are rock steady...no flutter at all on the gauge. Wouldn't the gauge flutter if there was a valve spring problem? Or wouldn't I get a lower vacuum reading than 20 inches?
11-09-2017, 12:08 PM
#8
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St. Jude Donor '05
at higher rpm..if the springs are tired they could be floating. Sounds like youre on the right path one step at at time.
Had a misfire problem that baffled me even after checking it a few times found my #5 and 7 wires were crossed seemed ok until it needed throttle. One thing at a time..keep us posted.
Does your ign system take a full 12 volts or use a ballast?
Had a misfire problem that baffled me even after checking it a few times found my #5 and 7 wires were crossed seemed ok until it needed throttle. One thing at a time..keep us posted.
Does your ign system take a full 12 volts or use a ballast?
11-09-2017, 12:12 PM
#9
Drifting
Thread Starter
at higher rpm..if the springs are tired they could be floating. Sounds like youre on the right path one step at at time.
Had a misfire problem that baffled me even after checking it a few times found my #5 and 7 wires were crossed seemed ok until it needed throttle. One thing at a time..keep us posted.
Does your ign system take a full 12 volts or use a ballast?
Had a misfire problem that baffled me even after checking it a few times found my #5 and 7 wires were crossed seemed ok until it needed throttle. One thing at a time..keep us posted.
Does your ign system take a full 12 volts or use a ballast?
11-09-2017, 12:22 PM
#10
Had a 1976 L-82, 4-sp
Member Since: Mar 2011
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11-09-2017, 12:48 PM
#11
Drifting
Thread Starter
Sometimes the throttle cable gets stretched or the gas pedal rod gets bent over time which prevents the secondaries from fully opening. Checking this by just flicking the throttle fully open at the carb is not verifying they are fully opening by using the pedal. It's happened to me and I got a lean condition, lack of power, "misfiring" symptons as you described at wot. It was a bent pedal rod preventing the secondaries opening all the way.
11-09-2017, 12:51 PM
#12
Drifting
Thread Starter
Another dopey question: I've read (a lot) that we're supposed to take the idle vacuum reading, divide that by two, and round up in order to determine the proper power valve.
If these cars develop 20-inches of manifold vacuum at idle, dividing by two yields a PV rated to open at 10 inches. So why did the factory install 6.5's in these? Is my formula wrong?
If these cars develop 20-inches of manifold vacuum at idle, dividing by two yields a PV rated to open at 10 inches. So why did the factory install 6.5's in these? Is my formula wrong?
11-09-2017, 01:03 PM
#13
Had a 1976 L-82, 4-sp
Member Since: Mar 2011
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Thats just a rule of thumb. To precisely determine the correct PV's for your setup, you need a vacuum gauge and afr meter and drive the car at various loads and log the info, analyze and select PV required. No easy steps to get it right so maybe follow the thumb rule.
11-09-2017, 01:09 PM
#14
Holley has no idea what cam you have installed in your car. They assume that if you are buying a Holley that you have modified your engine with an aftermarket cam (that provides less vacuum).
See this thread from earlier today (post #11),
https://www.corvetteforum.com/forums...r-upgrade.html
See this thread from earlier today (post #11),
https://www.corvetteforum.com/forums...r-upgrade.html
11-09-2017, 04:57 PM
#15
Drifting
Thread Starter
Holley has no idea what cam you have installed in your car. They assume that if you are buying a Holley that you have modified your engine with an aftermarket cam (that provides less vacuum).
See this thread from earlier today (post #11),
https://www.corvetteforum.com/forums...r-upgrade.html
See this thread from earlier today (post #11),
https://www.corvetteforum.com/forums...r-upgrade.html
Last edited by Tonio; 11-09-2017 at 05:26 PM.
11-10-2017, 09:50 AM
#18
Drifting
Thread Starter
So I put the carb back together using the 6.5 PV's. Bolted it all up, started the car, idled REALLY BADLY and now I have raw gas POURING out of the venturi boosters on the primary side. Lowered the float level a bit with the engine off, restarted it and it's still pouring out. Took the primary bowl and the primary metering block off, removed the power valve to inspect and found that I installed 2 of the gaskets under the power valve. They are paper thin, but could this possibly cause fuel to pour out of the boosters? I can't imagine it, but....? Called it quits for the night and will go at it again tonight. I haven't tried adjusting the float level while the car runs.....only because it won't stay running with this condition. I'll crank the floats down a lot this time to see if the condition stops. Argggg.
Last edited by Tonio; 11-10-2017 at 09:53 AM.
11-10-2017, 10:17 AM
#19
So I put the carb back together using the 6.5 PV's. Bolted it all up, started the car, idled REALLY BADLY and now I have raw gas POURING out of the venturi boosters on the primary side. Lowered the float level a bit with the engine off, restarted it and it's still pouring out. Took the primary bowl and the primary metering block off, removed the power valve to inspect and found that I installed 2 of the gaskets under the power valve. They are paper thin, but could this possibly cause fuel to pour out of the boosters? I can't imagine it, but....? Called it quits for the night and will go at it again tonight. I haven't tried adjusting the float level while the car runs.....only because it won't stay running with this condition. I'll crank the floats down a lot this time to see if the condition stops. Argggg.
"pouring" is a lot of fuel. Either the float seat is sticking open or you forgot to put the squirter transfer tube back in.
The PV gasket is usually pretty thick, about .040-.050"
The PV has nothing to do with idle.
11-10-2017, 10:55 AM
#20
Drifting
Thread Starter
Edit: just googled it. I don't think this applies to my carb. Mine is the list# 6239-1. Factory carb for the '72 LT-1. No transfer tube. Besides, the gas is not pouring out of the accelerator pump squirter. The gas is pouring out of the venturi boosters....both of them on the primary side only. Secondaries are fine.
Yes, it's "pouring" out. Not a drip, an obvious pour, at idle and acceleration. The only thing I can think of is the float level is way too high. The bleed vents are clear, I made sure of that.
Last edited by Tonio; 11-10-2017 at 11:29 AM.
