Great video... That's what's nice about the Holly HPXP… It has the holes in the Venturi's and is adjustable through the air cleaner threaded shaft… Pretty neat trick... Without the air cleaner shaft in you can adjust the airflow through those holes with a standard screwdriver. Thanks for all your information and informative video
You could always use the Rhoads variable lifters. They reduce the duration by about 10-20 degrees, then increase to full lift at around 2000-3000 RPM. I’ve used them for years, and they help a lot with your idle and low end
i took your advice and drilled tiny holes in my QF750dp and this helped tremendously. really appreciate the suggestion. my car has .500 lift and 302 duration. and for the life of me i could not get the idle under control. this tip along with locking the distributor advance out and setting my timing to 34* has it running like a completely different car.
I don't mean to offend, but drilling out the throttle blades on your carb is the VERY LAST THING anyone should do. Its reserved for basically radical race cars with big a stroker kit. What you should of done was attach an AFR gauge to your car, set your transfer slot to be square and measure your AFR at cruise(before entering main jets). Then dial in the IFRs to reach your desired AFR and reset the idle mixure. After that you crack open the secondaries until you reach your desired rpm and reset the idle mixure again. I wish carbs were more popular so this info could be more well known.
I actually like and follow your video posts on YT , but just gotta say drilling holes in butterflies is not the last thing to do. The AFR gauge is definitely helpful but most pro tuners for carbs don't rely on them because they only get it in the ball park and rely on reading the plugs. AFR monitoring was made for fuel injection. Plus the problem he had was the idle and an AFR guage doesn't help with that very much. Not trying say your advise is all bad or incorrect, but the last 8 carbs I put on put on customers motors had ALREADY came with holes drilled by manufacturers like AED, BG, BPD, and QF. Those holes when drilled to right size help idle circuits more then any gauges ever will with big duration cams.
@@djrowe10 hey man, I totally get it, but I'm positive he could have reached the same result by opening the rear throttle blades since it was a 4 corner idle carb(from what I remember in the video). If it was a 2 corner idle, then yeah, I would drill the holes, but since this was a 4 corner, I feel the holes were not nessesary.
There is something wrong with your combo or the tune if you had to drill the plates. The 280 Magnum is not what I'd call a radical cam in a 350 even with 1.6 rockers. The bump in rocker ratio hardly effects The duration. The 292 is fairly radical. I had the ISKY 280 Mega (485 lift/ 280 advertised duration/ 232 duration at .050/ 108 LSA). The ISKY being round on a 108 LSA gives it a worse idle with even lower vacuum levels. It was in a 9.8:1 350 with a 750 DP (only 2 idle mixture screws total) on a Performer RPM manifold plus 1 inch open carb spacer. Mine idled fine, no plate drilling needed. I'm betting most of your problem boils down to a combination or tuning error. Despite what Comp prints strictly for added sales the 280 Magnum really shouldn't be used with lower compression (9.5:1 or lower). Low compression = lower vacuum levels. If that motor got rebuilt with new semi budget pistons there's a good chance the top of the piston is sitting further down in the in the block than the stock ones did. That means even if they're flat tops you probably have less compression and combustion efficiency than stock. Sometimes being down in the hole greatly effects compression. If that's the case you are over cammed for sure and it will cause problems. If you have heads with big 72-76cc chambers (lowering compression) and are running flat tops, you are over cammed. In general if you have 9.5 or less compression you are over crammed. To me my cam choice was questionable at 9.8. I ended up installing a cam very close to the Comp 270 Magnum and boy did the low rpm drivability pick up. The 268High Energy/ Magnum is just a bit too weak for a muscle car with a 9.8:1 350 , 202 x 160 fuelie heads, 2500 rpm stall and 3.42 gears. Another possible issue is the distributor. Rather, how much base timing you are running. If it's a stock type distributor you very likely aren't using enough base timing. That is a tuning issue. You would have to limit the mechanical advance in that distributor to get there. You don't find advance stops in your general distributor recurve kits. That cam should be at about 18 or more initial. That means 18 whether you hook up vacuum advance or not. I had the MSD distributor and was able to change that.
My 600 hp 487 GTO required .100 holes. I started at .077. Works good now. I didn't make a video because it has already been done by people that know more than I.
Nicely done: I think we've all been there/done that at one time. To stay away from cam replacement, you might go back to lower ratio rockers. That buys back some more idle vacuum in most cases. Maybe... a 1.52 ? But first, something that costs $0 [I assume you have hydraulic lifters]: try setting initial preload with only 1/3 turn past zero lash. [Beware of lifters bleeding down setting zero, as you can get more preload than you intended as plungers lose position rolling motor over - i keep my prelube tool handy]. Drive it, and see if things are better or worse. If you're happy enough to keep the cam at that point, then look into carburetor mods as needed - I like starting with 1/16 hole. It may go bigger, but it's so much easier after a good pilot hole is in place. The key is to stop sizing holes when idle mixture is right with about 1 1/2 to 2 turns out, and never add more than 1/32 each time you resize - easy does it. Having butterflies open too much at idle is evil: by exposing transfer slot too much, you rob signal from the idle circut as the two start balancing out, which kind of takes out your idle screw adjustments. And sometimes weird decisions get made, like... sizing up jets to compensate, and people wonder why the motor just isn't clean off the line/runs rich.
That is a very informative video. I would make one suggestion before drilling the holes though. As you mentioned the transfer slots on the secondaires sit higher, which allows you to open the secondaries a little more than the primaries, which allows more air and that's similar to drilling the holes.. In other words, as long as the secondary transfer slots are not over exposed, opening them is doing the same thing as drilling the holes in the plates.. I noticed when you pulled the carb off, the secondaires transfer slots weren't exposed at all so I would of squared them off first and then tried it.. If you did square the secondaires off before you put the carb back on, and also drilled the holes, that could be the reason the holes were to big (to much air from both sources).. I have a SBC with .670 lift and .308 duration and only had to open the secondaries to lean my idle out.. I just wanted to mention this to anyone watching before they try drilling the holes.
If not already done, recurving the distributor gives much improved throttle response- essentially bumping initial timing to 14-16 degrees and cutting back mechanical so both total no more than 36 degrees.
Intake runner leak in the lifter gallery had a vacuum leak and I had nothing but rich running finicky carb idle low then idles High so I replace the intake manifold gasket switch to a different brand and it cleared up things a lot
You can turn that screw so it's on the other side of the car so you can adjust it from the top on the secondaries... I have a race serious carburetor so my adjustments are both on the outside...
couldnt see a vacuum advance set up , run that and the extra vacuum advance over the initial timing will pick up the manifold vacuum , then you can reduce the idle , close the throttle blades , hopefully closing off a bit more transfer slot
Great Video, and a great idea....As a side note: If you're not already Roller cammed ... Roller lifter cam....use less duration due to more lift....but get similar air in as a bigger hydraulic lifted
Hate to tell you but the worst thing you can do is start the car and let it idle. Your exhaust will eventually rot from moisture if you can’t take it around the block a few times. Sitting at operating temp isn’t enough of a load on it. Just my opinion, you can do what you please. Cam has nothing to do with your richness, but your slots do along with your jets. You need to jet down the primaries for a start. Look at your spark plugs, they will tell you all there is to know how it’s running. I have my orig 780 cfm Holley with vacuum secondaries on my 70 Z28 and jetted the primaries from stock 70 to 68 jets. I left the secondaries with 76. Idle mixture screws are set with a vacuum gauge to the highest vacuum reading and total timing is 36*. I’m rarely driving in the city but instead in the country and am using AC R43 spark plugs galled at 0.035. I’m also running with points since my Crane XR-I died this past summer. Along with that I’m running with a MSD 6AL 6420 analog ignition box. I have a hyd roller cam in my LT1 with 11” of vacuum at idle. My cam specs are www.summitracing.com/int/parts/lun-20120121/overview My compression ratio is 10.2:1 with forged aluminum pistons. Rocker arms are set to 1/2 turn from 0 lash on my hyd roller lifters. Rear gears are 3.73. My factory Muncie is a M20 but I installed M22 gears in it cause I love the whine but like the ratio of the M20 better. I would dump the 1.6 rockers. Another thing, I would never run a double pumper on the street.
That's a small cam...I have 580 lift cam,,1.7 rockers.. same issue..runs rich at idle.. Check your vacuum in manifold..maybe lower the power valve to a 3.5...jet it down a little to ...my pro street 78 GrandLeMans half baked big tire car is much more aggressive sounding....I have been thinking of doing the same,, drilling holes in the plates..but not sure I want to go there.. I drag race the car.....there are other ways better ways to correct this problem..... Blown power valve will cause this problem as well...bench test carb and set float on bench... I've been researching this problem quite extensively..... There are a number of options some better than others... When checking your manifold vacuum take that and divide it by half..in drive. That's what you should put in it for a power valve...and yes if your idle circuits open too far you will expose too much of that square and cause a rich problem at idle.....low initial timing will do the same thing it'll act like it's running rich because it doesn't have enough timing to burn the fuel... My distributor is locked out I run a crank trigger I set my timing at 36° locked... As other suggested you should raise your initial timing....good luck with it hope that drilling them holes helps out but I'm just not ready to do that to mine yet.. but I may end up doing it...
The Pony car will ALWAYS outshine the modern muscle car, that third pedal makes it so, if you're set on changing the cam, you might consider the Extreme Energy Series from Comp Cams, I have had a few of them now and to this point, I like the Extreme Energy Hydraulic Roller (XE294HR) a bit better than the flat tappet 305 Magnum that I had for 30 years, used 1.6:1 rockers for both. You might regret detuning the car, the performance is likely what drew you to it in the first place. Having owned a '73 Camaro since 1984, we have a lot in common, I just increased the horsepower and torque in the 377, cannot bring myself to take power away...
You can turn that screw over introverted, so it's on the other side of the car so you can adjust it from the top on the secondaries... I have a race serious carburetor so my adjustments are both on the outside... And you should adjust your idol mainly off your secondaries not your primaries.. I'm no carburetor expert not at all just what is my two cents....now I have a 66 Tempest 406 Pontiac that carburetor never gives me any trouble never runs rich smaller cam of course....
your duration has nothing to do with your overlap... second what initial timming are you running? a 280 magnum isn't such a large cam that u should have these issues with ...
Great video... That's what's nice about the Holly HPXP… It has the holes in the Venturi's and is adjustable through the air cleaner threaded shaft… Pretty neat trick... Without the air cleaner shaft in you can adjust the airflow through those holes with a standard screwdriver. Thanks for all your information and informative video
You could always use the Rhoads variable lifters. They reduce the duration by about 10-20 degrees, then increase to full lift at around 2000-3000 RPM. I’ve used them for years, and they help a lot with your idle and low end
i took your advice and drilled tiny holes in my QF750dp and this helped tremendously. really appreciate the suggestion. my car has .500 lift and 302 duration. and for the life of me i could not get the idle under control. this tip along with locking the distributor advance out and setting my timing to 34* has it running like a completely different car.
I don't mean to offend, but drilling out the throttle blades on your carb is the VERY LAST THING anyone should do. Its reserved for basically radical race cars with big a stroker kit. What you should of done was attach an AFR gauge to your car, set your transfer slot to be square and measure your AFR at cruise(before entering main jets). Then dial in the IFRs to reach your desired AFR and reset the idle mixure. After that you crack open the secondaries until you reach your desired rpm and reset the idle mixure again. I wish carbs were more popular so this info could be more well known.
Now this is sensible.
I actually like and follow your video posts on YT , but just gotta say drilling holes in butterflies is not the last thing to do. The AFR gauge is definitely helpful but most pro tuners for carbs don't rely on them because they only get it in the ball park and rely on reading the plugs. AFR monitoring was made for fuel injection. Plus the problem he had was the idle and an AFR guage doesn't help with that very much. Not trying say your advise is all bad or incorrect, but the last 8 carbs I put on put on customers motors had ALREADY came with holes drilled by manufacturers like AED, BG, BPD, and QF. Those holes when drilled to right size help idle circuits more then any gauges ever will with big duration cams.
@@djrowe10 hey man, I totally get it, but I'm positive he could have reached the same result by opening the rear throttle blades since it was a 4 corner idle carb(from what I remember in the video). If it was a 2 corner idle, then yeah, I would drill the holes, but since this was a 4 corner, I feel the holes were not nessesary.
There is something wrong with your combo or the tune if you had to drill the plates. The 280 Magnum is not what I'd call a radical cam in a 350 even with 1.6 rockers. The bump in rocker ratio hardly effects The duration. The 292 is fairly radical.
I had the ISKY 280 Mega (485 lift/ 280 advertised duration/ 232 duration at .050/ 108 LSA). The ISKY being round on a 108 LSA gives it a worse idle with even lower vacuum levels. It was in a 9.8:1 350 with a 750 DP (only 2 idle mixture screws total) on a Performer RPM manifold plus 1 inch open carb spacer. Mine idled fine, no plate drilling needed.
I'm betting most of your problem boils down to a combination or tuning error.
Despite what Comp prints strictly for added sales the 280 Magnum really shouldn't be used with lower compression (9.5:1 or lower). Low compression = lower vacuum levels. If that motor got rebuilt with new semi budget pistons there's a good chance the top of the piston is sitting further down in the in the block than the stock ones did. That means even if they're flat tops you probably have less compression and combustion efficiency than stock. Sometimes being down in the hole greatly effects compression. If that's the case you are over cammed for sure and it will cause problems.
If you have heads with big 72-76cc chambers (lowering compression) and are running flat tops, you are over cammed. In general if you have 9.5 or less compression you are over crammed.
To me my cam choice was questionable at 9.8. I ended up installing a cam very close to the Comp 270 Magnum and boy did the low rpm drivability pick up. The 268High Energy/ Magnum is just a bit too weak for a muscle car with a 9.8:1 350 , 202 x 160 fuelie heads, 2500 rpm stall and 3.42 gears.
Another possible issue is the distributor. Rather, how much base timing you are running. If it's a stock type distributor you very likely aren't using enough base timing. That is a tuning issue.
You would have to limit the mechanical advance in that distributor to get there. You don't find advance stops in your general distributor recurve kits. That cam should be at about 18 or more initial. That means 18 whether you hook up vacuum advance or not. I had the MSD distributor and was able to change that.
My 600 hp 487 GTO required .100 holes. I started at .077. Works good now. I didn't make a video because it has already been done by people that know more than I.
Nicely done: I think we've all been there/done that at one time. To stay away from cam replacement, you might go back to lower ratio rockers. That buys back some more idle vacuum in most cases. Maybe... a 1.52 ?
But first, something that costs $0 [I assume you have hydraulic lifters]: try setting initial preload with only 1/3 turn past zero lash. [Beware of lifters bleeding down setting zero, as you can get more preload than you intended as plungers lose position rolling motor over - i keep my prelube tool handy]. Drive it, and see if things are better or worse. If you're happy enough to keep the cam at that point, then look into carburetor mods as needed - I like starting with 1/16 hole. It may go bigger, but it's so much easier after a good pilot hole is in place. The key is to stop sizing holes when idle mixture is right with about 1 1/2 to 2 turns out, and never add more than 1/32 each time you resize - easy does it.
Having butterflies open too much at idle is evil: by exposing transfer slot too much, you rob signal from the idle circut as the two start balancing out, which kind of takes out your idle screw adjustments. And sometimes weird decisions get made, like... sizing up jets to compensate, and people wonder why the motor just isn't clean off the line/runs rich.
That is a very informative video. I would make one suggestion before drilling the holes though. As you mentioned the transfer slots on the secondaires sit higher, which allows you to open the secondaries a little more than the primaries, which allows more air and that's similar to drilling the holes.. In other words, as long as the secondary transfer slots are not over exposed, opening them is doing the same thing as drilling the holes in the plates.. I noticed when you pulled the carb off, the secondaires transfer slots weren't exposed at all so I would of squared them off first and then tried it.. If you did square the secondaires off before you put the carb back on, and also drilled the holes, that could be the reason the holes were to big (to much air from both sources).. I have a SBC with .670 lift and .308 duration and only had to open the secondaries to lean my idle out.. I just wanted to mention this to anyone watching before they try drilling the holes.
Yes I agree with what you say, opening the secondaries without overexposing the transfer slots would have made a difference for sure.
Took the words right out of my mouth.
I made that same mistake back years ago, because I saw the holes in the blades of a dirt track car. big big cam lol. That's learning the hard way. Lol
You need to get the butterfly plates with the holes in them , more air flow. 😎✌
If not already done, recurving the distributor gives much improved throttle response- essentially bumping initial timing to 14-16 degrees and cutting back mechanical so both total no more than 36 degrees.
Hi,respect from car flippers country Lithuania,Mike if you messing with tuning,you probably should buy a wideband O2 sensor with Air/fuel ratio gauge.
Your right, but I am too cheap to do that!
@@PhotomikesGarage hi,respect from Lithuania,if you runing lean,you may damage your car engine,and thats will be realy expensive.
Intake runner leak in the lifter gallery had a vacuum leak and I had nothing but rich running finicky carb idle low then idles High so I replace the intake manifold gasket switch to a different brand and it cleared up things a lot
Hi, thanks, that's something to look into.
You can turn that screw so it's on the other side of the car so you can adjust it from the top on the secondaries... I have a race serious carburetor so my adjustments are both on the outside...
couldnt see a vacuum advance set up , run that and the extra vacuum advance over the initial timing will pick up the manifold vacuum , then you can reduce the idle , close the throttle blades , hopefully closing off a bit more transfer slot
Check the ignition timing to see what timing is running at idle and total advance timing. Should be around 36 degrees total timing.
Slightly larger than 1/32, did you drill 3/64?
I did this on mine
You need it converted to 2 idle circuits.
Great Video, and a great idea....As a side note:
If you're not already Roller cammed ...
Roller lifter cam....use less duration due to more lift....but get similar air in as a bigger hydraulic lifted
Hate to tell you but the worst thing you can do is start the car and let it idle. Your exhaust will eventually rot from moisture if you can’t take it around the block a few times. Sitting at operating temp isn’t enough of a load on it. Just my opinion, you can do what you please. Cam has nothing to do with your richness, but your slots do along with your jets. You need to jet down the primaries for a start. Look at your spark plugs, they will tell you all there is to know how it’s running. I have my orig 780 cfm Holley with vacuum secondaries on my 70 Z28 and jetted the primaries from stock 70 to 68 jets. I left the secondaries with 76. Idle mixture screws are set with a vacuum gauge to the highest vacuum reading and total timing is 36*. I’m rarely driving in the city but instead in the country and am using AC R43 spark plugs galled at 0.035. I’m also running with points since my Crane XR-I died this past summer. Along with that I’m running with a MSD 6AL 6420 analog ignition box. I have a hyd roller cam in my LT1 with 11” of vacuum at idle. My cam specs are www.summitracing.com/int/parts/lun-20120121/overview My compression ratio is 10.2:1 with forged aluminum pistons. Rocker arms are set to 1/2 turn from 0 lash on my hyd roller lifters. Rear gears are 3.73. My factory Muncie is a M20 but I installed M22 gears in it cause I love the whine but like the ratio of the M20 better. I would dump the 1.6 rockers. Another thing, I would never run a double pumper on the street.
That's a small cam...I have 580 lift cam,,1.7 rockers.. same issue..runs rich at idle..
Check your vacuum in manifold..maybe lower the power valve to a 3.5...jet it down a little to
...my pro street 78 GrandLeMans half baked big tire car is much more aggressive sounding....I have been thinking of doing the same,, drilling holes in the plates..but not sure I want to go there.. I drag race the car.....there are other ways better ways to correct this problem.....
Blown power valve will cause this problem as well...bench test carb and set float on bench... I've been researching this problem quite extensively..... There are a number of options some better than others... When checking your manifold vacuum take that and divide it by half..in drive. That's what you should put in it for a power valve...and yes if your idle circuits open too far you will expose too much of that square and cause a rich problem at idle.....low initial timing will do the same thing it'll act like it's running rich because it doesn't have enough timing to burn the fuel... My distributor is locked out I run a crank trigger I set my timing at 36° locked... As other suggested you should raise your initial timing....good luck with it hope that drilling them holes helps out but I'm just not ready to do that to mine yet.. but I may end up doing it...
The Pony car will ALWAYS outshine the modern muscle car, that third pedal makes it so, if you're set on changing the cam, you might consider the Extreme Energy Series from Comp Cams, I have had a few of them now and to this point, I like the Extreme Energy Hydraulic Roller (XE294HR) a bit better than the flat tappet 305 Magnum that I had for 30 years, used 1.6:1 rockers for both. You might regret detuning the car, the performance is likely what drew you to it in the first place. Having owned a '73 Camaro since 1984, we have a lot in common, I just increased the horsepower and torque in the 377, cannot bring myself to take power away...
You can turn that screw over introverted, so it's on the other side of the car so you can adjust it from the top on the secondaries... I have a race serious carburetor so my adjustments are both on the outside... And you should adjust your idol mainly off your secondaries not your primaries.. I'm no carburetor expert not at all just what is my two cents....now I have a 66 Tempest 406 Pontiac that carburetor never gives me any trouble never runs rich smaller cam of course....
Put stock rockers on
your duration has nothing to do with your overlap... second what initial timming are you running? a 280 magnum isn't such a large cam that u should have these issues with ...
Old fuel maybe
Hair transplant?