Team Camaro Tech banner
1 - 20 of 87 Posts

·
Registered
Joined
·
96 Posts
Discussion Starter · #1 ·
Just had my 600 Holley professionally rebuilt. Car is finally running pretty well, but as a final adjustment I wanted to check/adjust the idle mixture.

Now, I did notice this before the rebuild which is another reason I knew it need attention: right now the screws are both about 1 1/2 turns out but if I turn the idle screws in or out it makes no difference. I can turn both screws all the way in and the idle doesn't change at all.

Any ideas what might cause this, or is this normal?

Thanks.
 

·
Premium Member
Joined
·
1,245 Posts
Is it idling fat? First thing I'd do I pull it off, flip it over and check how much of the idle transfer slots are exposed. If too much (~ 0.02" is the spec) that will render the idle mixture screws useless as too much fuel will be bleeding from the slots and not the idle circuits. What you describe is the textbook result of too much slot exposed. As mentioned, assume engine is warmed up and choke off.
 

·
Registered
68 Base Camaro 355 Offy crossram Richmond Super Street close ratio 5 spd ivy gold 92K SoCal car
Joined
·
3,017 Posts
Since you had the same issue before carb was rebuilt verify your throttle linkage is adjusted properly and moves without any binding. Back the idle speed screw all the way off the stop and make sure the primary butterflies are fully closed. Also make sure secondary butterflies are fully closed. If secondaries are not fully closed they have a seperate stop screw you can adjust by taking the carb off intake and flipping it over. You will see it in the base plate.

Another way to verify engine is running on idle circuit only is with engine warmed up and choke off put a vacuum gage on the ported vacuum port. This is the port the distributor vacuum advance is advised by manufacturer to be connected (mfr advises, but many use full manifold port, so make sure it is the ported vacuum port you measure for this test). If your ported vacuum doesn't read zero the main circuit is active when it shouldn't be.

A few things can cause this but as already stated usually butterflies are open too far exposing the transfer slots in the base plate bores. Bad or damaged power valves can also cause this. If you had backfiring through intake/carb during your problems the power valve may have been damaged, particularly on older Holley carbs.
 

·
Premium Member
Ben - Delaware - 67 Camaro
Joined
·
641 Posts
issue is most certainly as stated above, transfer slot exposed by curb idle screw over adjustment. my guess is if you back off the curb idle screw the car will die. the REAL problem is your initial timing advance. make sure to have vacuum advance hooked up to full manifold vacuum. set base timing to 12-14 degrees, limit vacuum advance with limiter plate to add an additional 10 degrees. you should be idling with 22-24 degrees and then you'll be able to turn back your curb idle screw on the carb and have all the mixture adjustment available as intended.
 

·
Registered
Joined
·
96 Posts
Discussion Starter · #8 ·
issue is most certainly as stated above, transfer slot exposed by curb idle screw over adjustment. my guess is if you back off the curb idle screw the car will die. the REAL problem is your initial timing advance. make sure to have vacuum advance hooked up to full manifold vacuum. set base timing to 12-14 degrees, limit vacuum advance with limiter plate to add an additional 10 degrees. you should be idling with 22-24 degrees and then you'll be able to turn back your curb idle screw on the carb and have all the mixture adjustment available as intended.
Actually, once warm, I can get it to idle smoothly below 500 rpm with no vacuum advance. I didn't experiment to see how low it would go because that was irrelevant at the time. I was going to do some final adjustments when my ancient (1970's) timing light crapped out, and my vacuum gauge reads 4 pounds with the car not running. It reads a steady 20 pounds when running, so I don't know if the actual vacuum is 4 pounds lower, or if 20 pounds is accurate but this ancient (1960's) vacuum gauge simply won't go back down to zero for some reason. I bought a new timing light and a new vacuum gauge is coming tomorrow. Then I'll start over on these settings before I move on to checking the mechanical issues like the idle transfer slots being covered, butterflies open, etc. which others have mentioned. (None of which I have any clue how to fix, but some I, at least, can see visually.) Thanks.
 

·
Registered
Joined
·
96 Posts
Discussion Starter · #9 · (Edited)
Since you had the same issue before carb was rebuilt verify your throttle linkage is adjusted properly and moves without any binding. Back the idle speed screw all the way off the stop and make sure the primary butterflies are fully closed. Also make sure secondary butterflies are fully closed. If secondaries are not fully closed they have a seperate stop screw you can adjust by taking the carb off intake and flipping it over. You will see it in the base plate.

Another way to verify engine is running on idle circuit only is with engine warmed up and choke off put a vacuum gage on the ported vacuum port. This is the port the distributor vacuum advance is advised by manufacturer to be connected (mfr advises, but many use full manifold port, so make sure it is the ported vacuum port you measure for this test). If your ported vacuum doesn't read zero the main circuit is active when it shouldn't be.

A few things can cause this but as already stated usually butterflies are open too far exposing the transfer slots in the base plate bores. Bad or damaged power valves can also cause this. If you had backfiring through intake/carb during your problems the power valve may have been damaged, particularly on older Holley carbs.
Thanks. Throttle linkage is good. Why would any of these things—like the transfer slots being exposed—have developed... (other than the backfires damaging the power valve, which are recent.)
 

·
Registered
68 Base Camaro 355 Offy crossram Richmond Super Street close ratio 5 spd ivy gold 92K SoCal car
Joined
·
3,017 Posts
Transfer slots become exposed when the idle speed screw is turned in too far causing the throttle plate to open more. Eventually the plates open far enough to expose the tranfer slots and you begin transitioning from the idle circuit to the main circuit and idle mixture screws are no longer able to be effective.

Floats set too high can also cause idle mixture screws to be ineffective. Check your floats are set correctly on your float bowls you should have brass float level sight plugs. Should be able to remove them and just see the fuel level at bottom of hole without it running out of the holes. Does your carb have 2 or 4 idle mixture screws? Some have "4 corner idle" as it is referred to when primary and secondary fuel blocks have idle mixture screws.

So linkage is good, check float levels, check vacuum at idle when you get your new gage, make sure your timing is set like Ben suggested, then check your idle vacuum full manifold port and ported vacuum ports. Disconnect and plug PCV vacuum hose and check if any effect.
 

·
Registered
68 Base Camaro 355 Offy crossram Richmond Super Street close ratio 5 spd ivy gold 92K SoCal car
Joined
·
3,017 Posts
Pic shows an 830 annular booster Holley baseplate. You can see the transfer slots on all 4 bores with throttle plates open. You can also see the secondary throttle plate stop screw I mentioned in my original response. Small straight slot screw driver adjusts it after removing carb. This is an old NASCAR legal carb. All 4 throttle plates have small hole drilled to allow additional airflow at idle. Your carb, like most, probably has solid plates.
Automotive tire Art Rim Wood Bicycle part

White Black Automotive tire Bicycle part Auto part

White Light Automotive tire Automotive design Motor vehicle

Wheel Automotive tire Motor vehicle Alloy wheel Automotive design
 

·
Premium Member
Joined
·
2,839 Posts
Thanks. throttle inking is good. Why would any of these things—like the transfer slots being exposed—have developed (other than the backfires damaging the power valve, which are recent.)
When your idle mixture screws have no effect, it's almost always a symptom of too much transfer slot exposed, throttle blades open too far, most likely your fronts, that's why it's being mentioned.

Pull the carb off turn it over and set front and back throttle blades to make the transfer slots look like a perfect square.
then always adjust front and back idle speed screws evenly.
as well as idle mixture screws,
the mixture screws should almost always be set the same turns out. unless you have an engine dyno and O2 sensors on each header tube to balance it, which most of us don't.
 

·
Registered
Joined
·
96 Posts
Discussion Starter · #13 ·
Transfer slots become exposed when the idle speed screw is turned in too far causing the throttle plate to open more. Eventually the plates open far enough to expose the tranfer slots and you begin transitioning from the idle circuit to the main circuit and idle mixture screws are no longer able to be effective.

Floats set too high can also cause idle mixture screws to be ineffective. Check your floats are set correctly on your float bowls you should have brass float level sight plugs. Should be able to remove them and just see the fuel level at bottom of hole without it running out of the holes. Does your carb have 2 or 4 idle mixture screws? Some have "4 corner idle" as it is referred to when primary and secondary fuel blocks have idle mixture screws.

So linkage is good, check float levels, check vacuum at idle when you get your new gage, make sure your timing is set like Ben suggested, then check your idle vacuum full manifold port and ported vacuum ports. Disconnect and plug PCV vacuum hose and check if any effect.
Thanks. I believe the floats are set tad too high on the front and two tads too high on the rear. I'll look at those again. I will also check the transfer slots to see what they look like. From what I understand the slots should look small from the top with the butterflies closed? And would look rectangular if the butterfly isn't closed all the way? I may have spoken too soon about the throttle linkage...The throttle rod was disconnected for a long while a while this winter while the carb was off and I was messing with distributor cap, coil, coil bracket and wires. I can imagine that may have gotten spun around a time or two..or three. That could potentially keep the butterfly open when throttle is off could it not? On a side note: I've never had one on there, but I've seen photos of Holleys with two return springs—one back to the coil bracket and one forward to the thermo housing. Is that ever necessary/helpful? Thanks again.
 

·
Registered
Joined
·
644 Posts
Go to the Holley site for help. Carburetor Technical Support - Holley You will find pictures and tune up / adjustment info there.
Two springs are not necessary, but is a safety addition in case the original one spring breaks or comes unhooked.
The throttle linkage rod can be adjusted to wide open throttle. Make sure the bell crank from the gas pedal is not hitting the back of the intake and preventing the front throttle plates from closing. If they are not closing, your idle circuit will not function properly.
 

·
Registered
Joined
·
204 Posts
Make sure you exhaust every possible carb, vacuum leak, ignition timing possibility. Drilling holes are an absolute last resort and rarely necessary unless you got a full-on race setup. Usually when it's done, it's just masking the real issue. Heck, even do a compression check on all the cylinders. Even that could cause the engine to need so much idle adjustment to expose transfer slots.
 

·
Registered
Joined
·
96 Posts
Discussion Starter · #17 ·
Make sure you exhaust every possible carb, vacuum leak, ignition timing possibility. Drilling holes are an absolute last resort and rarely necessary unless you got a full-on race setup. Usually when it's done, it's just masking the real issue. Heck, even do a compression check on all the cylinders. Even that could cause the engine to need so much idle adjustment to expose transfer slots.
• 1969 350/300 internally stock.103,000 miles
• Rebuilt 4150 Holley 600 cfm
• New points, condenser, coil, and cap
• Compression—all cylinders test 150-155 except one (#5) at 145
• Float height at primaries and secondaries adjusted to correct levels.
• No vacuum leaks detected
• DWELL 29º
• Timing 14º BTDC (I'm not 100% positive the distributor vacuum canister does anything. No change at idle with vacuum connected to canister or without. Is there a way to test this?)
• Idle 600 rpm in drive (engine will actually idle as low as 375 rpm—not well, but will run without dying.)
• Manifold vacuum steady 20 in park, steady 17 in drive while idling at 600 rpm.
• Ported vacuum 0 at idle
• Car idles smoothly, accelerates quickly without any lag or misfires, and pulls strong to the yellow line—best in years
• Exhaust still smells mildly rich, idle screw adjustment does nothing

I have not yet removed the carb, I wanted to do everything else, short of that, first. Using an isolated bright light shining down into the manifold thru the wide open carb secondaries I can see a sliver of light when primaries are slightly opened, but when tested, it appears that the primary butterflies are fully closed with the throttle at rest...but I'd like to try that test once more.

All that to say that everything appears correctly adjusted but STILL the the exhaust smells slightly rich and the idle mixture screws have absolutely no affect on idle or vacuum even when screwed all the way in.

One theory suggested is a damaged power valve, possibly from the previous backfires.

Any suggestions?
 

·
Registered
Joined
·
96 Posts
Discussion Starter · #18 ·
I have read several posts on other forums like Team Chevelle (related to this subject—that generally give the same advice as you guys have given me) that suggest jet sizes in a stock 600 Holley are too large and the power valve too low a number. It is suggested to start by dropping the jet size by 2or more and getting a power valve of a bigger number.

Although I don't believe this is related to my current problem… does anyone know what is the stock primary jet size in a stock 600 cfm 4150 Holley? Can I tell by looking at the jet?

I believe the stock PV is a 6.5, and if it is indeed damaged and I need to change it anyway… just curious.
 

·
Premium Member
Ben - Delaware - 67 Camaro
Joined
·
641 Posts
• Timing 14º BTDC (I'm not 100% positive the distributor vacuum canister does anything. No change at idle with vacuum connected to canister or without. Is there a way to test this?)

• Manifold vacuum steady 20 in park, steady 17 in drive while idling at 600 rpm.
• Ported vacuum 0 at idle



do you have vacuum advance hooked to ported vacuum? if so, try switching to full manifold vacuum. you may have to put a limiter plate on vacuum arm beneath cap to limit the amount of vacuum advance provided to around 10 degrees. as stated earlier, your engine is really going to want 22-26 degrees at IDLE. this can be achieved by an initial base timing of your 14 degrees with added vacuum advance giving an additional 10-14 degrees. ported vacuum source for vacuum advance does nothing for you at idle. i'm still confident if you went to full manifold vacuum and targeted the numbers provided you'd be able to realize the adjustability of the idle mixture screws
 
1 - 20 of 87 Posts
Top