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Old 12-18-2007, 08:00 AM
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Need help tuning a holley

I have a 68 camaro with a 454 bored .030 over with 9.5-1 comp. The heads are GM large ovals 820 is the last three numbers on the head. Mild port job. Cam is a Comp Cam XE274H 230/236 @.050, 110 lobe. Intake is a Edelbrock performer RPM. The problem I am having is that at idle the car will actually burn your eyes and choke you down from being so rich. The carb is a Holley 3310-c, with #70 jets and a 3.5 power valve. I changed the valve out from a 6.5 on a whim hoping that would help. not. I recently bought a vacuum gauge and checked my manifold vacuum. It is at ten. I then adjusted the mixture screws until i got the highest reading @ 10.5 but it is still extremely gassy smelling. To get any response on the mixture screws on the idle I have to run them in 1/2 turn from bottom. Timing is at 15 initial and 35 total. fuel pressure is at 6. any advice will be appreciated. Thanks Toby

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Last edited by 68ss; 12-18-2007 at 08:21 AM.
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Old 12-18-2007, 12:10 PM
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bump

moving to top
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Old 12-18-2007, 02:18 PM
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I have the same problem with a 4160 600 cfm on a 350 so I hope some one knows.I checked and rechecked float level and mixture adjustment,transfer slot exposed etc. to no avail I'm at the point of trying smaller jets but it is awful cold here so it may have to wait.
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Old 12-18-2007, 02:34 PM
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Have you checked to see how much of the transfer slots you have exposed at this point below the butterflies?

Also what happens if you go in further with the idle mixture screws?
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Old 12-18-2007, 04:58 PM
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check to see if fuel is comming out by the rear throttol plate if it is yall may have the wrong gasket on the get plate on the rear of the carb. tech guy called them transfer slots
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Old 12-18-2007, 05:06 PM
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Have you tried an Edelbrock carb?
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Old 12-18-2007, 06:53 PM
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Holley

I just replaced a 4160 650 cfm carb last week that was doing the same thing yours is. I had it gone though by the local carb guru, but it had no effect, car ran fine but loaded up like crazy at idle, fouled out all the plugs. Finally, I got a used 4150 670 (Street Avenger) cfm from a local guy, tore it down myself and cleaned it up. Put it on and all my troubles went away. Maybe its something about those little carbs?

Last edited by DeputyDawg134; 12-18-2007 at 06:54 PM. Reason: spelling! :pain:
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Old 12-18-2007, 06:56 PM
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1/2 a turn out on the idle screw usually means a carb thats too big .... but a 3310 with a 70 jet shouldnt do that id have thought. A 70 jet in that seems a bit small.

Have a look down the carb whilst its idling and look for extra fuel coming out of the booster leg and pooling on the butterflies. If there is check the power valve is ok. When you pull out a power valve look for a ripple in the rubber diaphragm. If there is its stuffed as it should be taught. Power valves hate backfires...we all know that but also water or a drop of sweat will ruin it.

If its ok but fuel is still pooling then its opening rating is too low and you need to change it anyway. If i ran a standard DP 8.5 power valve in my engine itd be open most of the time. I run a 10.5.

To find the right power valve youll have to rig a vacuum gauge that you can see whilst driving and select a power valve that is shut whilst cruising but opens on acceleration when the vaccum signal drops.
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Old 12-18-2007, 08:01 PM
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i have solved this problem on several cars. first check:
float level, if it's too high you could have fuel dribbleing from the venturi boosters, you will be able to see it by looking down the carb barrels while its idleing.

check power valve(s) you said you replaced it. 3.5 is pretty low, drive the car normally with a vacuum gauge hooked up and put a power valve rated 2-3" lower than your lowest vacuum reading, probably around more like 5-6".

now if everything else checks out here's the fix i use. tech guy is right on, with a healthy cam sometimes the throttle blade need to be cranked open enough to expose the fuel transfer slots to get it to idle. this draws extra fuel from the transfer slots along with the idle circuit-result, way to rich. thats why you can almost close the idle adjustment screws(you said 1/2 turn from closed).
you need to get the throttle plates closed far enough to get off the transfer slots and back on the idle circuit.

1-set your idle speed, don't worry about it being rich.
2-remove the carb without changing the idle adjustment
3-turn carb upside down and remove throttle plate
4-look at the position of the throttle plates from the bottom/engine side.

intermission

OK, i just went out to the garage to look at a holley throttle plate- there will be a small hole in each bore, one a 10 oclock and one about 2 oclock, these are the idle fuel feeds.

you will see s slot in each bore, one about 11 oclock and one about 1 oclock, these are the idle transfer slots--if those are showing below the throttle plates that is where your extra fuel is coming from.

you need to get the throttle plates closed more to get off the transfer slots, the way to do that is to drill a small hole in each of the PRIMARY throttle plates (start with 1/8", you can go bigger if need be but once you drill the holes too big you can't go smaller) 1/8 to 3/16 will usually do the trick. most people who know this trick just drill the holes at 12 oclock, i drill them at 10 and 2 oclock about 3/16 from the outside edge so they are right in front of the idle fuel holes. with these holes brilled in the plates you will be able to close the throttle plates more and get off the transfer slots and back on the idle circuit. when correct the idle mixture screws should be around 3+ or - and the over richness should be gone.

disclaimer-this assumes the rest of the carb is operating correctly and is not a band aid fix for another problem.

sorry for the long post but i wanted to explain it as clearly as possible.
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Old 12-18-2007, 08:08 PM
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oh, and guys, it has nothing to do with the size of the carb or the size of the jets, this happens as a result of camshaft size and intake vacuum at idle.
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Old 12-18-2007, 09:07 PM
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?? For Techron

Friend of mine has similar problem 383, Vortes heads,Comp cam largest for these heads without mods. Dual Exhst, Headers, VictorJr. Intk, 750 Speed Demon carb, If we uncap the headers it's perfect. Hookup the exhaust and it will choke a mule, with raw gas fumes, Idling or cruising. Should we restrict the idle circuit with small wire or do you think the butterfly trick will work better? Thanks
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Old 12-18-2007, 09:39 PM
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Don't drill any holes in the primary butterflys. A better fix is to adjust the secondary butterflys just a little bit open. Then you can close the primaries more and cover up more of the idle transfer slot. The adjustment screw is on the passenger side on the bottom of the carb, very difficult to get to. It's a little set screw that an arm connected to the secondaries rests against. It keeps the secondaries from closing tight and binding. You may have to remove the carb to get to it. It's a slotted set screw, 5mm I think. You have to have a very tiny screwdriver to get to it without removing the carb. If you take the carb off go to the hardware store and get a replacement with an allen head, then you can adjust it with the carb installed. It only takes a very little bit of opening the secondaries to fix that problem. Also you should connect the vacuum advance can to the full manifold vacuum port and not the timed port. That will help a lot to get a good idle without the gassey smell.
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Old 12-18-2007, 09:57 PM
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I dont think you should need any mods on that carb for your setup. Just needs to be fixed or tuned. Techron goes into good detail so ill leave that. Besides this great forum get some Holley books and read it all.

SSedan....get a Holley ... but first check the exhaust system is clear. If the (old) muffler internals collapse the debris blocks the muffler outlet real nice..
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Old 12-18-2007, 10:24 PM
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Quote:
Originally Posted by SnottyAust
I dont think you should need any mods on that carb for your setup. Just needs to be fixed or tuned. Techron goes into good detail so ill leave that. Besides this great forum get some Holley books and read it all.

SSedan....get a Holley ... but first check the exhaust system is clear. If the (old) muffler internals collapse the debris blocks the muffler outlet real nice..
He's tried straight pipes, no muffs, pipes to the rearend etc. Anything but open headers and it's a gas chamber. LOL. I think the idle fuel circuit is just setup for open headers and needs restricting slightly. I've checked the fuel transfer slots. There not uncovered. It'll Idle very low, no problem. The mixture screws do nothing also. ???
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Old 12-18-2007, 10:56 PM
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Quote:
Originally Posted by Will H
Don't drill any holes in the primary butterflys. A better fix is to adjust the secondary butterflys just a little bit open. Then you can close the primaries more and cover up more of the idle transfer slot. The adjustment screw is on the passenger side on the bottom of the carb, very difficult to get to. It's a little set screw that an arm connected to the secondaries rests against. It keeps the secondaries from closing tight and binding. You may have to remove the carb to get to it. It's a slotted set screw, 5mm I think. You have to have a very tiny screwdriver to get to it without removing the carb. If you take the carb off go to the hardware store and get a replacement with an allen head, then you can adjust it with the carb installed. It only takes a very little bit of opening the secondaries to fix that problem. Also you should connect the vacuum advance can to the full manifold vacuum port and not the timed port. That will help a lot to get a good idle without the gassey smell.
will, i know about the secondary adjustment screw for the butterflies, that will work too. i'm NOT saying you are wrong. i'm going to explain why i prefer my method of the fix with big cams and low intake vacuum (the OP mentioned 10"). by opening the secondaries you are letting air in at the rear of the carb but the idle fuel is coming from the front of the primaries. my reasoning is this will tend to make the rear cylinders IDLE (not run off idle) lean and the fronts tend to idle richer (fuel distribution at idle). by drilling the holes in the primaries at 10 and 2 oclock you are letting the air in right where the idle fuel ports let the fuel in, getting better atomization and more even fuel distrubution. i have no proof of this like EGT readings of every cylinder at idle but it just seems logical to me.
i was a mechanic in the 70s, 80s and 90s and have rebuilt 100s of carbs of all kinds, built hi-perf carbs and fixed carb problems after other shops couldn't so i know carbs.
any carb that is built and adjusted properly can be a good carb (except a few wierdo old ones-SUs and carter YFs come to mind) but i prefer holleys and except for 2 EFI cars everything i have has a holley on it.

Will's fix is a good one to try first because if you are not satisfied with it it is reversable, it's not a mod it's an adjustment. my fix in the post above is a mod for a car WITH A BIG CAM AND LOW IDLE VACUUM and is not reversable, but it works good on a car with low idle vacuum that needs all the help it can get.

i agree with hooking the vacuum advance to manifold vacuum and not port vacuum, if you have ported vacuum change it, it will help a lot with idle quality.
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