Questions for Holley carb guys

[DC_Gearhead]

Carbs are like magic to me. So many variables that I don’t understand. And I don’t work on one but every few years so I forget everything between projects. I know the carb is too big but it is newish and I already had it.

Here are my facts:
351w with RV cam (unknown brand or specs)
750 Holley vacuum advance 4160 (Holley rebuilt)
65 jets and 8.5 power valve
Idle screws at 1.5 turns out
Manual choke
Edelbrock Performer manifold
HEI Chinese distributor with new plugs and wires
Vac hose for advance connected to carb base not on metering plate.
Timing set by ear since I have no needle at balancer.
Power brakes vacuum off manifold
New mech fuel pump, tank, lines and filter
Retorq
Checked for vac leaks with carb cleaner. Nada

Symptoms:
Idle surges a little (lean?)
Vacuum gauge bounces between 19 and 15. Connected to the manifold. Not good, right?
Adjusting idle screws does nothing really. When I screw to close, idle goes up as well as vacuum. This is bad, right?
Idles great and starts every time. Haven’t driven yet.

I’d love to get some help on this since I don’t even really know what to check first. I’ll be in my shop for the next 3 days so I could try things as they are suggested.

I know Holley tuners are like wizards. I also know I don’t have those powers and know when to ask for help.

Ben















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[SteveL]

Been there. I had a Holley for about 6 months and it would never stay in tune. I swapped it for a Carter afb and it was great for a dd. Same carb as the edelbrock performer but $100 cheaper. The edelbrock is smog legal☆. The afb is pretty easy to adjust for the non carb guru. I ended up swapping to a q-jet once my 72 was more dialed in for off road. As good as the afb works for a daily driver they dont do well bouncing around or at steep angles.

 

[rmk57]

Adjust the mixture screws to get highest vacuum then adjust the idle screw to get the idle where you want it. It would be nice to know where your timing is. Most non-stock camshafts like a little more timing, but being an unknown cam who knows. I always run ported vacuum on my stuff to.

 

[DC_Gearhead]

Been there. I had a Holley for about 6 months and it would never stay in tune. I swapped it for a Carter afb and it was great for a dd. Same carb as the edelbrock performer but $100 cheaper. The edelbrock is smog legal☆. The afb is pretty easy to adjust for the non carb guru. I ended up swapping to a q-jet once my 72 was more dialed in for off road. As good as the afb works for a daily driver they dont do well bouncing around or at steep angles.

It’s frustrating because you can teach yourself other stuff through logic. Welding, rebuilding motors, even electrical. But carbs need pixie dust or something....and I’m fresh out of pixies. It’s kicking my butt.


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[SHX669]

The experts say that 75% of carb problems are actually a vacuum leak or ignition issues.
That being said - remove the carb and flip it over , taking the usual precautions about spilling gas and etc. Look at the front bores - there's a vertical slot , this is the idle transfer slot . With the throttle plate closed this slot should form a square - as tall as it is wide. If not adjust the IDLE SPEED screw one way or another until it is. Make sure your choke hi speed adjustment is disengaged. Now - after this leave the idle speed screw alone - make a mark as to it's orientation so when you do mess with it you can put it back to where it supposed to be - did I mention after you "form a square " leave the flippin idle speed screw alone ? Both mine idle 700 -750 rpms this way .
Reinstall your carb and restart and then adjust the IDLE MIXTURE screws for the highest vacuum . If this doesn't fix your issue then you're going to have to find out what your initial timing is - generally 8*- 12* btdc .
If you still have an issue then remember what the experts say about 75% of carb problems.

 

[pcf_mark]

Did this just start happening or has it always done it? As pointed out above 75% of driveability "carb" issues are ignition or vacuum leaks. Without having the timing set right you are really wandering around in the dark but maybe we can help with that too. I would just buy the pointer it bolts on in about 10 minutes.

Things I would try to isolate the problem:
Adjust the idle mixture screws until you get the highest vacuum. Then go back and make sure they are both turned out about the same by gently screwing them in while counting the turns. Turn both side out the same amount. If you get one out 3 turns and the other out 1/2 you can get a hunting idle

Set the idle where you like it. Put a timing light on the engine on #1 and try to see the line on the balancer. You may not have a timing reference but look for the line. Watch the line as the engine idles and hunts up and down in rpm. If the line keeps moving around a lot you may have something in the distributor like leaky vacuum advance can or sticky weights.

Remove the vac advance hose and plug the port. Did the hunting stop? If so your advance can is probably wonky. If not put it back the way it was. In a stick shift vehicle I like manifold vacuum for the advance but not for an A/T it makes for a big drop in rpm from park to drive and then the idle gets unstable.

Take your timing light and point it at your hand and watch the strobe it should be even and consistent. Take the timing light pick up off #1 and put it on the next cylinder and point it at your had and watch. Repeat for each cylinder - you are basically checking the soundness of your engine ignition system wire by wire. If the light is inconsistent (flash-flash-pause-flash-pause-flash etc) you may a bad wire, plug, cap, coil. If it is only wonky on one cylinder swap the wire from a "good" on to the "bad" and see if it fixes. If so replace the plug or wire. If it is inconsistent across several wires your coil or cap are probably bad.

If you have too much timing you can impact the carb tuning because you expose the transition slot as someone pointed out above. But that will get you effect the transition as you pull away from a light. Buy the pointer, set it to 12-14 with the vac advance disconnected.

You may have a vacuum leak under the intake manifold or along the end seals. Google or search this site for 302 intake vacuum leak there are about a zillion guys who had that problem. When I had it my plugs oiled up like bad valve seals. If you pull your plugs an done or two look messy I would think about intake gaskets.

Finally to the carb. Idle circuit is really simple with very few parts to fail. Take out the idle mixture screws careful not to lose the cork seal and shoot a jet of carb cleaner into the hole. It should squirt out into the throttle bores and may come out of the air bleeds on the top of the carb. Spray carb cleaner into the air bleeds on top of the carb it should also trickle out of the idle mixture screw hole. Down in the throttle bore you could get some build up but I have never really seen that. Idle bleeds usually get filmed up from poor tuning and sitting a lot.

If you can take a few pics of your carb sometimes we can spot something helpful. That carb is pretty solid and reliable once set up.

My guess from 3,000 mile away is your Chinese distributor is giving up the ghost and you are getting random mis-fires.

 

[DC_Gearhead]

The experts say that 75% of carb problems are actually a vacuum leak or ignition issues.
That being said - remove the carb and flip it over , taking the usual precautions about spilling gas and etc. Look at the front bores - there's a vertical slot , this is the idle transfer slot . With the throttle plate closed this slot should form a square - as tall as it is wide. If not adjust the IDLE SPEED screw one way or another until it is. Make sure your choke hi speed adjustment is disengaged. Now - after this leave the idle speed screw alone - make a mark as to it's orientation so when you do mess with it you can put it back to where it supposed to be - did I mention after you "form a square " leave the flippin idle speed screw alone ? Both mine idle 700 -750 rpms this way .
Reinstall your carb and restart and then adjust the IDLE MIXTURE screws for the highest vacuum . If this doesn't fix your issue then you're going to have to find out what your initial timing is - generally 8*- 12* btdc .
If you still have an issue then remember what the experts say about 75% of carb problems.

Ok. Done. It was way off. No slot showing. Putting back on truck now.

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[DC_Gearhead]

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[DC_Gearhead]

The experts say that 75% of carb problems are actually a vacuum leak or ignition issues.
That being said - remove the carb and flip it over , taking the usual precautions about spilling gas and etc. Look at the front bores - there's a vertical slot , this is the idle transfer slot . With the throttle plate closed this slot should form a square - as tall as it is wide. If not adjust the IDLE SPEED screw one way or another until it is. Make sure your choke hi speed adjustment is disengaged. Now - after this leave the idle speed screw alone - make a mark as to it's orientation so when you do mess with it you can put it back to where it supposed to be - did I mention after you "form a square " leave the flippin idle speed screw alone ? Both mine idle 700 -750 rpms this way .
Reinstall your carb and restart and then adjust the IDLE MIXTURE screws for the highest vacuum . If this doesn't fix your issue then you're going to have to find out what your initial timing is - generally 8*- 12* btdc .
If you still have an issue then remember what the experts say about 75% of carb problems.

Ok. This was a big help. The idle mixture screws actually work now. It has a little high idle but I was able to get the vacuum up to 20ish, but still not steady. It shakes between 22&18. The surge seems to be gone as well.

I’m going to try and figure out my timing, get it set and come back to carb.

I’m going to hunt again for vac leaks as well.


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[DC_Gearhead]

Did this just start happening or has it always done it? As pointed out above 75% of driveability "carb" issues are ignition or vacuum leaks. Without having the timing set right you are really wandering around in the dark but maybe we can help with that too. I would just buy the pointer it bolts on in about 10 minutes.

Things I would try to isolate the problem:
Adjust the idle mixture screws until you get the highest vacuum. Then go back and make sure they are both turned out about the same by gently screwing them in while counting the turns. Turn both side out the same amount. If you get one out 3 turns and the other out 1/2 you can get a hunting idle

Set the idle where you like it. Put a timing light on the engine on #1 and try to see the line on the balancer. You may not have a timing reference but look for the line. Watch the line as the engine idles and hunts up and down in rpm. If the line keeps moving around a lot you may have something in the distributor like leaky vacuum advance can or sticky weights.

Remove the vac advance hose and plug the port. Did the hunting stop? If so your advance can is probably wonky. If not put it back the way it was. In a stick shift vehicle I like manifold vacuum for the advance but not for an A/T it makes for a big drop in rpm from park to drive and then the idle gets unstable.

Take your timing light and point it at your hand and watch the strobe it should be even and consistent. Take the timing light pick up off #1 and put it on the next cylinder and point it at your had and watch. Repeat for each cylinder - you are basically checking the soundness of your engine ignition system wire by wire. If the light is inconsistent (flash-flash-pause-flash-pause-flash etc) you may a bad wire, plug, cap, coil. If it is only wonky on one cylinder swap the wire from a "good" on to the "bad" and see if it fixes. If so replace the plug or wire. If it is inconsistent across several wires your coil or cap are probably bad.

If you have too much timing you can impact the carb tuning because you expose the transition slot as someone pointed out above. But that will get you effect the transition as you pull away from a light. Buy the pointer, set it to 12-14 with the vac advance disconnected.

You may have a vacuum leak under the intake manifold or along the end seals. Google or search this site for 302 intake vacuum leak there are about a zillion guys who had that problem. When I had it my plugs oiled up like bad valve seals. If you pull your plugs an done or two look messy I would think about intake gaskets.

Finally to the carb. Idle circuit is really simple with very few parts to fail. Take out the idle mixture screws careful not to lose the cork seal and shoot a jet of carb cleaner into the hole. It should squirt out into the throttle bores and may come out of the air bleeds on the top of the carb. Spray carb cleaner into the air bleeds on top of the carb it should also trickle out of the idle mixture screw hole. Down in the throttle bore you could get some build up but I have never really seen that. Idle bleeds usually get filmed up from poor tuning and sitting a lot.

If you can take a few pics of your carb sometimes we can spot something helpful. That carb is pretty solid and reliable once set up.

My guess from 3,000 mile away is your Chinese distributor is giving up the ghost and you are getting random mis-fires.

I’m going to try to catch the only motor tuning shop and in town before the close and by a pointer for timing. Then I will get out the timing light. You have a bunch of good trouble shooting ideas above.


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[B RON CO]

Hi, you need to clean all the gasket debris from the bottom of the carb, and intake manifold.
The bouncing needle can be from a sticky valve, a burnt valve, a valve adjusted to tight or a vacuum leak at an intake runner. If this is causing your problem you will never get the carb adjusted.
I would get the carb back on, but don't hook the vacuum to the distributor, just plug the port at the carb. This will eliminate the distributor vacuum can from the issue.
There is good information on reading a vacuum gauge on line
Good luck

 

[DC_Gearhead]

Hi, you need to clean all the gasket debris from the bottom of the carb, and intake manifold.

The bouncing needle can be from a sticky valve, a burnt valve, a valve adjusted to tight or a vacuum leak at an intake runner. If this is causing your problem you will never get the carb adjusted.

I would get the carb back on, but don't hook the vacuum to the distributor, just plug the port at the carb. This will eliminate the distributor vacuum can from the issue.

There is good information on reading a vacuum gauge on line

Good luck

The heads shouldn’t be the issue. I just had them rebuilt with hardened seats, new springs, and beveling. I replaced a couple valves too. They should be in good shape. Could the bouncing needle be because of the new rebuild? Not broken in yet?

I’ll jump into the distributor and timing this evening.


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[SHX669]

Glad you're making progress - I had a minor brain fart - that's Idle TRANSITION slot not TRANSFER slot - as in transition from idle to off idle.

 

[DC_Gearhead]

Glad you're making progress - I had a minor brain fart - that's Idle TRANSITION slot not TRANSFER slot - as in transition from idle to off idle.

You can call it whatever you want.


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[Seventee]

Sounds like you got your throttle closed enough now that the idle mixture screws are working. However there is one other item to look at for anyone who is troubleshooting this. There is a stop screw for the secondaries on the underside of the carb. If this is set too deep, the throttle plates won't fully close and it's possible to idle through the secondaries. This usually show up when you cant get idle speed down, or mixture screws are unresponsive.

 

[jckkys]

There are endless books detailing Holley carb tuning. My favorite is Holley Carburetors and Manifolds by HP Books. Then there's Holley Rebuilding & Modifying by S-A Design. Both cover all your issues and any potential issues. With proper information and a few tools like a vacuum gauge, timing light, and an engine analyzer for a tach, you can deal with most problems. You're starting at a disadvantage with the 700 CFM center float. Good Holleys for a 351 are smaller. The List#4548 450 CFM is the best I know of for 351s. It carries a Ford part #DOPF-9510-U. The 700 you have, flows more air at idle then you want. My 351 idles smoothly at 550 RPM using a 480 CFM 4100. The center float bowls were designed for road racing. Not an early Bronco activity. The #4548 has side floats, that work better off road. The transfer slot square window is easier to achieve with a properly sized carb. Holley by the way, calls it a transfer slot in all their technical guides.

 

[DC_Gearhead]

Sounds like you got your throttle closed enough now that the idle mixture screws are working. However there is one other item to look at for anyone who is troubleshooting this. There is a stop screw for the secondaries on the underside of the carb. If this is set too deep, the throttle plates won't fully close and it's possible to idle through the secondaries. This usually show up when you cant get idle speed down, or mixture screws are unresponsive.

Do you think it is worth pulling and checking that? I’m going through my timing excises after i eat some turkey. The bouncing vacuum gauge really adds some validity to Mark’s comments above about timing or a miss. I need to investigate that. I always assumed it was just carburetor issues. The surge is gone and now the idle mixture screws are responsive now. Being new at this, I’m not sure how “responsive” the idle mixture screws should be.

If it is worth pulling the carb, what is that procedure called, so i can google? “Secondary stop screw adjustment”?

Edit : I think mine may be good. I just watched the Holley how to and it said don’t touch this adjustment unless you have a big cam with low vacuum. It said to verify the flaps cover the groove and when I had if off yesterday, I did see that the vertical groove was not visible when the flap was closed. I adjusted the primaries to have the square and saw the secondaries didn’t have a visible groove at all when closed. You can kind of see it in the pic below.


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[DC_Gearhead]

There are endless books detailing Holley carb tuning. My favorite is Holley Carburetors and Manifolds by HP Books. Then there's Holley Rebuilding & Modifying by S-A Design. Both cover all your issues and any potential issues. With proper information and a few tools like a vacuum gauge, timing light, and an engine analyzer for a tach, you can deal with most problems. You're starting at a disadvantage with the 700 CFM center float. Good Holleys for a 351 are smaller. The List#4548 450 CFM is the best I know of for 351s. It carries a Ford part #DOPF-9510-U. The 700 you have, flows more air at idle then you want. My 351 idles smoothly at 550 RPM using a 480 CFM 4100. The center float bowls were designed for road racing. Not an early Bronco activity. The #4548 has side floats, that work better off road. The transfer slot square window is easier to achieve with a properly sized carb. Holley by the way, calls it a transfer slot in all their technical guides.

I do appreciate the advice, but I have read a lot and come up empty handed. Plus I love the discussions on the forum. The back and forth helping each other. The conversation could help someone else in the future. Plus those books don’t say the “why”. I need to know why, when I am doing something. Maybe me stumbling though this tune job will make someone feel confident to pull their carb. We need more people in this hobby.

I know the carb is not optimally sized and it is not perfect, but I’m trying to get what I have working. This is the carb I had on the shelf from another project (1970 Challenger) and don’t really want to waste it. If I can get it working, I’ll keep it, if not....well....I guess I need to go buy another one. From what I have read lots of people use this 4160 carb on 351w with success. I made the jet power valve adjustments based on books and the Holley forums. From what I have read, all should be good. It is one of the most common carburetors out there.


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[rmk57]

You shouldn't have to mess with the secondary throttle blades unless you have a low vacuum type camshaft.

Mixture screw adjustment can vary quite a bit. My 514 BBF is sensitive to a 1/16 of a turn with 950 HP Holley. I put on my little Quick fuel 750 and I can have a 1/2 turn adjustment on it. Depends on how the metering is set up from the factory. From my experience Holleys normally run on the rich side.

 

[DC_Gearhead]

You shouldn't have to mess with the secondary throttle blades unless you have a low vacuum type camshaft.



Mixture screw adjustment can vary quite a bit. My 514 BBF is sensitive to a 1/16 of a turn with 950 HP Holley. I put on my little Quick fuel 750 and I can have a 1/2 turn adjustment on it. Depends on how the metering is set up from the factory. From my experience Holleys normally run on the rich side.

Something strange that I noticed was that the driver side was sensitive to about a 1/16 turn but pass side didn’t make much of a difference until a 1/2 turn.

It was running rich until I switched out the jets. I went from 67s to 65s. Jury is still out on whether that was a good choice. Haha. Haven’t been able to drive it yet, but running better in the garage.


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