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Thread: 750 HP Fine Tuning

  1. #1

    Default 750 HP Fine Tuning

    I'm looking to do some fine tuning on my Holley 750 HP 80528-2 carb. The car has always seem to run ok but have always thought that there is room for improvement. I've done some minor tweaking to the carb in the past to make it run better, jets, power valve and squirters. Recently I purchased a single sensor wideband to help with tuning in the carb. The wideband has helped a lot with telling me what is going on as far as the carb goes. I now have some data, however I'm not sure what needs to happen to correct the problem. If you look at the wideband log below, I'm cruising on a long flat road at 2,500 rpm in 4th gear and go to WOT. When I first open the throttle the afr goes to 21:17, then to goes back to 12:60. From there the AFR is between 13:0 to 14:0 till the end of the run, where it's between 14.0 to 15.0. I let out of it as I started getting close to the 15:0 mark, and I'm not sure why it kept getting leaner.

    Could you point me in the right direction for correcting the WOT spike and the lean condition through out the test run. Thanks!

  2. #2


    That initial lean spike can be cured by changing your pump cams/squirters. Need to know how you have your secondary linkage setup. If it is setup so the that the secondaries open in sync with the primaries, then you need to adjust both cam and squirters. Start with getting a more aggressive pump cams. If that is not enough, you have to get bigger squirters and then adjust your pump cams. If you have it setup so the secondaries come in after 50-60% primary throttle engagement then you need to make a couple of more logs. Start with removing the linkage to secondaries or just make a couple of hits making sure they don't open. See if the primaries are still showing this lean spike as well. Just want to make sure this lean spike is not caused by just the primary.

    The rising lean appears to be easy enough to cure with raising the secondary jet 2 sizes. If you have tried that, then go ahead and try to raise another 2 or 3 size. Let's just see if raising a total of 5/6 jet size higher is having any affect at all. If it is not having any affect, then you I would suggest you monitor your fuel pressure on WOT run. I would think FP is dropping or fuel supply is not keeping up with the demands of the motor. I think if that was the case it would really go lean. Make sure your floats are set right. It shouldn't matter since on a WOT run, you're measuring all available fuel circuits. But I would just check the log to see if you have a rising AFR while only running on the primaries as well.
    Last edited by 69cam; 09-29-2015 at 11:30 PM.

  3. #3


    69cam, thanks for taking the time to reply to my post. I'm thinking that the initial lean spike will require cam change, as I've already increased my squirter several sizes. The carb came with 31 squirters primary & secondary, but I've gone all the way up to 37s and still having the issue. The secondaries come in after 50-60% of the primaries being opened. I like your idea of disconnecting the secondaries, makes perfect sense. Make sure I have the primary side dialed in, before moving on to the secondary side. If I find that the primary side is lean as well, what cam would you put in next? At the point of changing cams, should I go back to a smaller shooter size?

    I did not make it to the local speed shop to pickup jets today. I thought increasing jet size would help fatten up the run, but wasn't sure how many steps up I should make. Currently I have 81 jets in the secondaries, so I will pick up some 83s & 85s, then test from there. Could the lean condition starting at the 5.5 mark be the main air bleeds being to large? I was only about 4,000 rpm, but I was moving down the road.

    I will post up some wideband logs in the next day or so, as soon as time allows. Thanks again for the advise!

  4. #4


    Just buy the whole cam kit. Relatively cheap, I think maybe $10. Keep in mind, you can flip the cams over and use the other side. Since you have an AFR gauge one way to get pretty close is to free rev it quickly. Sitting in your driveway/garage stab the throttle really really quick to full throttle. Do it so fast, that the motor does not break 5k on rpm. It may LEAN POP on you. When you rev it that quick and it goes lean, it will sound like a very small backfire. When you get it right, that lean pop will not be there anymore (on a warm engine). Pay close attention to your AFR gauge. If it goes lean (which it probably will) change the cams to the next size up. Your cam kit will come with a chart showing the aggressiveness of each cam. If I remember correctly, I think my 750 ended up with the blue one. Get it to where, when you stab the throttle, it goes rich with your current shooters. Then start back off your shooters. Optimally, you would like the AFR not to change, but realistically, get it to change as little as possible. (12.5 - 13.5 ish range)
    Could the lean condition starting at the 5.5 mark be the main air bleeds being to large? I was only about 4,000 rpm, but I was moving down the road.
    Think of the air bleeds as super fine adjustment. It takes a huge jump in air bleeds to make .5 - .75 change in your AFR. So get it close with the jets then you can fine adjust with the air bleeds if need be. Buying air bleeds 1 pair at a time, can get a little expensive quick when your trying to tune. I ended up buying a complete bleed kit & mains kit.

    Your pretty close, in no time you will be tuning for efficiency while cruising & maximum power on WOT. That will get you to thinking about get the carb to work like 2 different carbs. It can be done with a little tweaking.

    Click image for larger version. 

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    Last edited by 69cam; 10-01-2015 at 04:15 PM.

  5. #5


    I was able to pick up some jets today, along with a pump cam kit. Looking at the chart you posted, I'm confused on which is the next biggest cam? The graph shows cam profiles, but not in order. Doing a little digging on the web, I came across this chart, which looks to go in order.

    I was able to get the car out this evening for a little testing. I disconnected the secondaries and headed out with the wideband. Here is a log cruising at 2,500 RPM give or take, then stomping the throttle. The AFR spikes up to 22.39 running just on the primaries alone.

    Currently, I have the pink cam 330 installed in position 1. I'm going to try the red cam 240 in position 1 and see how it responds. Is that a big enough jump or should I try a bigger one?

    One thing we have not talked about is the power valve. The car idles at 10", so I have a 4.5" power valve installed. I've read that there are two way to check for correct power valve size. Idle vacuum divided by 2, and idle vacuum minus 2. Which way is best?

  6. #6


    The graph shows cam profiles, but not in order. Doing a little digging on the web, I came across this chart, which looks to go in order.
    Don't worry about order just compare the color you currently have in there and the hole position it is in on the accelerator. You will notice the pump cams have 2 and 3 holes on them so that they can be oriented at different angles to provide different ramp rates. For example, the pink cam appears to have 4 different ramp rates based on which hole you have it mounted. Remember to readjust the accelerator pump linkage when you change a cam and double check the lever that rides on the cam it actually resting on the cam. You want the accelerator pump to react almost instantly when you touch the throttle. I have seen that when you change the cam the the lever might get a very small gap between the cam and lever. That will cause you fits and read a quick lean spike not matter what cam you put in there. Just be sure the accelerator pump lever moves when you barely touch the throttle.

    As far as power valves goes, that becomes a personal preference on where you want the power valve to come in at. Your fine on the 4.5 for now. Later when you get into tuning for efficiency is where the power valve will be of more essential. You will have your car running 14 ish while cruising and light acceleration. Light acceleration while having a lean idle becomes sensitive to tune. Usually cruise is 2400 rpm ish in 4th or 5th gear. if you just lightly accelerate in 4/5th gear it will naturally go lean into the 15 to low 16. That is OK for light acceleration but if you add any more throttle you have to get it to start going rich so it doesn't detonate. That's where the power valve can help you. Drive around with a vacuum gauge in the car and see what your vacuum is on that light acceleration and get a valve that will enrich right at that moment or very close to it. This is also where playing with air bleeds can help tremendously as well as you can fine tune your curve.

    Lastly, when using an AFR gauge don't get caught up on a specific AFR number. Think of it as your FUEL CURVE. obviously you don't wanna be in the 10s while idling or 18s while accelerating but you will never get it to be exactly a specific number for all driving scenarios. Just watch the curve.

    Your steady cruise looks really good. Very little fluctuation and when you stomp on it your curve has a slight rise to it. When you stomp it i personally wanna see low to high 12s. If it fluctuates .1 - .3 one way or another, I'm OK with that. Keep in mind, Different temps outside is going to change your curve. When it gets cold your curve will naturally go leaner. AIR is more compressed when colder. When it get hot it runs a little richer. This is one of the reason not to focus on a specific number, you will chase yourself silly trying to make that happen. (ASK ME HOW I KNOW THAT. LOL)
    Last edited by 69cam; 10-02-2015 at 08:43 AM.

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