My car runs way rich at idle, even when fully warmed up...to the point that noxious fumes enter the cabin when sitting in traffic. The carb is a Holley 750 double pumper, #4779. Assuming the idle mixture screws are set correctly, what else could cause this? Blown power valve maybe?
Carb Too Rich?
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List the whole combo if you would...including timing, vacuum #'s if you have them.
Some motors just stink no matter what though.
1973 vintage 2-bolt 454 block, bored .060 and align honed
Balanced rotating assembly from Mike Lewis (Wolfplace) – SCAT forged crank, h-beam rods, SRP forged pistons
Moroso 20451 7-qt oil pan with Moroso pump & pickup
GM 781 casting heads, modified with 2.19/1.88 valves, otherwise as-cast (no porting)
Isky hydraulic roller cam (thanks to Mark/VORTECPRO) #396275/284
228/238 @ .050, lift .553/.578, 112 LSA, installed at 109
Isky lifters & springs
Manley pushrods
Comp Pro Magnum roller rockers, 1.7 ratio
Edelbrock Performer RPM 2.0
Holley #4779 750 double pumper
MSD Pro Billet distributor with MSD 6AL
Carter 172 fuel pump with -8 AN line from tank to carb
Summit headers, 1.75” primaries, 3.00” collectors, 2.5" muflers/tailpipes
TH400 with Coan 11” converter, stall 2400-2800
3.42 rear gears
Mechanical timing is 16* initial, 36* total. I am also using vacuum advance, but haven't measured how much advance the vacuum canister is adding. I have not measured idle vacuum either, but this can idles very smooth, so I'm sure the vacuul numbers would be fairly high.
Not that base timing is the issue being at 16 but even thats a little retraded IMHO for your setup.
That cam can use at lest 18-20 deg base timing so bump it up to at least 18 deg base timing for 38total befoe getting into dialing in carb better .
Also,dont forget to idle motor low like 600 rpm when setting base timing to stop mech adv from partially activating with too hiof idle speed when setting base timing.vac adv needes to be unhooked/plugged when checking /setting base & total timing too.
But Idle vacuum being decnt can still be an issue if wrong power valve is in carb and throttle plates partially out of idle circut to obtain good idle speed and power vlave is partiall open at idle speed and motor is getting some ofits idle fuel from main circut with prim plates out of idle circut. Thats esp possible if not running a vac adv for additoonal timing at idle and somewhat retarded base timing like 16 deg when it needs 18-20 deg base timing. The additonal 4 deg base timing can allow you to back off prim throttle plates further back down into idle circut when you do that . So chk idle vacuum in gear with auto trans only after after you bump up the base timing to at least 18 deg if not 20 if you setup can handle 40 deg total with 20 deg base,the 18/38 should be ok but 40 total may push it too far into ping unless low comp.
Then if for example idle vac in gear is 12-14" the what comes stock in most holleys 6.5 p-valve in prim side would be fine.
Holley recs approx 1/2 idle vac in gear for prim p-valve so if for ex a motor has 9" idle vac in gear then i would try maybe a 5.0 or 5.5 p-valve in prim side.
But not 4.5 half of 9" idle vac like holley rec's,thats just too much on the lean side for fuel calibration needing your foot to be in it too hard before getting additional fuel to the motor when it needs it under hard load.
BTW,you did already try to lean out idle mixture to see if that would help,correct?
Also,if motor uses some oil that can also add to stinky exhaust fumes at idle.
Wrong float lvl and too high fuel pressure can also cause righ idle as can bad/shrunken carb gaskets allowing fuel to be sucked in from where it sould not be sucked in or even a warped or cracked metering block can cause rich idle too allowing fuel in where it should not be.
Dirty idle air bleed can also cause rich idle to clean them out with gumout spray.
Then if you get all the above squared away-adv timing /verify all else is ok with carb correct p-vlave etc and it still idles rich then maybe its time to try different idle air bleeds to lean out idle some which dbl pumpers are notorious for idling rich.
Scott
That cam can use at lest 18-20 deg base timing so bump it up to at least 18 deg base timing for 38total befoe getting into dialing in carb better .
Also,dont forget to idle motor low like 600 rpm when setting base timing to stop mech adv from partially activating with too hiof idle speed when setting base timing.vac adv needes to be unhooked/plugged when checking /setting base & total timing too.
But Idle vacuum being decnt can still be an issue if wrong power valve is in carb and throttle plates partially out of idle circut to obtain good idle speed and power vlave is partiall open at idle speed and motor is getting some ofits idle fuel from main circut with prim plates out of idle circut. Thats esp possible if not running a vac adv for additoonal timing at idle and somewhat retarded base timing like 16 deg when it needs 18-20 deg base timing. The additonal 4 deg base timing can allow you to back off prim throttle plates further back down into idle circut when you do that . So chk idle vacuum in gear with auto trans only after after you bump up the base timing to at least 18 deg if not 20 if you setup can handle 40 deg total with 20 deg base,the 18/38 should be ok but 40 total may push it too far into ping unless low comp.
Then if for example idle vac in gear is 12-14" the what comes stock in most holleys 6.5 p-valve in prim side would be fine.
Holley recs approx 1/2 idle vac in gear for prim p-valve so if for ex a motor has 9" idle vac in gear then i would try maybe a 5.0 or 5.5 p-valve in prim side.
But not 4.5 half of 9" idle vac like holley rec's,thats just too much on the lean side for fuel calibration needing your foot to be in it too hard before getting additional fuel to the motor when it needs it under hard load.
BTW,you did already try to lean out idle mixture to see if that would help,correct?
Also,if motor uses some oil that can also add to stinky exhaust fumes at idle.
Wrong float lvl and too high fuel pressure can also cause righ idle as can bad/shrunken carb gaskets allowing fuel to be sucked in from where it sould not be sucked in or even a warped or cracked metering block can cause rich idle too allowing fuel in where it should not be.
Dirty idle air bleed can also cause rich idle to clean them out with gumout spray.
Then if you get all the above squared away-adv timing /verify all else is ok with carb correct p-vlave etc and it still idles rich then maybe its time to try different idle air bleeds to lean out idle some which dbl pumpers are notorious for idling rich.
Scott
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11,295 Posts
Mike, running manifold vac, if not that may help.
That cam doesnt have alot of overlap, I bet it idles real nice and smooth. Other than maybe running manifold vac and trying to run the mixtures in for max vac, not mich else you can do. Some motors just stink...at least mine does. Are your mixture screws responsive? May want to try another carb if you can...you would be suprised at the difference another carb could make.
That cam doesnt have alot of overlap, I bet it idles real nice and smooth. Other than maybe running manifold vac and trying to run the mixtures in for max vac, not mich else you can do. Some motors just stink...at least mine does. Are your mixture screws responsive? May want to try another carb if you can...you would be suprised at the difference another carb could make.
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5,065 Posts
Could also be as simple as a float level set too high, it has an effect not just on how much gas is in the front fuel bowl but also how many emulsion holes get covered up IIRC. If above doesn't do much, try dropping the float level a little at a time also.
Thanks guys, I'll investigate those items as well.
When I first bought this carb 10 years ago, I had it on a 350 with WAYYYYYY too big a cam, and I screwed around with it quite a bit trying to get it to idle properly. I may have jacked something up in the process.
When I first bought this carb 10 years ago, I had it on a 350 with WAYYYYYY too big a cam, and I screwed around with it quite a bit trying to get it to idle properly. I may have jacked something up in the process.
Mike,carb is 10 yrs old?
If you havent rebuilt it in all that time or at least recently holleys had big issues with the older style gaskets they used yrs back shrinking /cracking causing a multitude or running/tuning issues .
So if your carb has not been freshened up in yrs you 1st need to at least diassemble/clean it up/reassemble with new gaskets needle & seats/reset float lvls -idle mix and go from there.
If not you cold be chasing your tail forever on this thing eventually giiving up for a new $300 carb when all this carb needed was a good tune up sort of speak with a $40 kit and a little dialing in.
And BTW,if you decide to freshen it up let us know what the prim & sec jetting is along with power valve/s in prim and sec side if theres a p-vlave in sec side.
I suggest that becasue if the current fuel calibration setup is obviously too rich we can suggest slightly leaner jetting and or different p-valve before you get it re-assembled to save you some time.
scott
If you havent rebuilt it in all that time or at least recently holleys had big issues with the older style gaskets they used yrs back shrinking /cracking causing a multitude or running/tuning issues .
So if your carb has not been freshened up in yrs you 1st need to at least diassemble/clean it up/reassemble with new gaskets needle & seats/reset float lvls -idle mix and go from there.
If not you cold be chasing your tail forever on this thing eventually giiving up for a new $300 carb when all this carb needed was a good tune up sort of speak with a $40 kit and a little dialing in.
And BTW,if you decide to freshen it up let us know what the prim & sec jetting is along with power valve/s in prim and sec side if theres a p-vlave in sec side.
I suggest that becasue if the current fuel calibration setup is obviously too rich we can suggest slightly leaner jetting and or different p-valve before you get it re-assembled to save you some time.
scott
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5,065 Posts
^^^Bingo^^^ (Scott's always on the money...)
I've had it apart in the past for jetting and PV changes, so it's got the expensive blue gaskets (I forget what they are called) on each side of the metering blocks. The only other large gasket I can think of would be the one between the main body and the throttle body.
I will probably tear it down and go completely through it over the winter though.
I will probably tear it down and go completely through it over the winter though.
Ok,the newer style blue gaskets should cover the gasket issue but dont forget to too look closely at needles/seats for wear & replace if in question along with floats for issues too while your at it and to report back to us the fuel calibration setup/jets p-valve when you disassemble it just in case its it too rich so we can catch it prior to reassembly.
Scott
Scott
Cool, thanks. Also, FWIW, both floats have been replaced. Both of the original style plastic floats eventually developed pinholes, filled with fuel, and sank, stranding me on the side of the road with severe flooding. Both have been replaced with the old soldered brass floats, scavenged from an older carb.
as others have said, floats too high can make for a stinky garage....
I set mine so that I have to nudge the fender pretty darn hard for fuel to slosh out the sight hole.
set the idle mix to the highest vacuum ensuring you set the idle speed down with each idle improvement. My Super Budget 454 has an 800 DP and 226*/108 LSA cam. After some more adjustments yesterday, it now idles at 14" neutral, 800 rpm.
I set mine so that I have to nudge the fender pretty darn hard for fuel to slosh out the sight hole.
set the idle mix to the highest vacuum ensuring you set the idle speed down with each idle improvement. My Super Budget 454 has an 800 DP and 226*/108 LSA cam. After some more adjustments yesterday, it now idles at 14" neutral, 800 rpm.
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