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Old 05-17-2019, 04:48 PM
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Tuning Holley 750 Blower Carbs w/671 sbc

I'm running two Holley 750 blower carbs, on a 671, with 5-6lbs of boost, on a fresh 355 sbc, Comp Nitrous HP .502 / .520 lift roller. 18 degrees of initial timing 35 all in. Brodix 200cc heads. I have tuned the carbs, set the jets 67/77 with a 6.5 PV, and managed to get the blower surge out of the idle circuit using the idle mixture screws. I have found that too much idle fuel creates the surge (these carbs have 4 corner idling (8) screw adjustments) About 14 lbs. of vac at idle.

It idles at 1100 rpms in park, 950 in drive. She's running smooth with a slight rich mixture. The plugs have a clean white electrode with a black ring. My problem shows up when the engine gets hot, 200 degrees, and I'm in stop and go traffic. The idle drops off 600-700 rpms, and the car wants to stall. I keep it running using the pedal and maintaining 1000 rpms. There are no vac leaks since all the hoses are new. Once I get going the idle will return and she'll be fine at the next stop. Any ideas on what is happening inside the carbs?

My second problem is the stumble when I jump on it, but I am dropping the nozzles from 31 to 28 and we'll see if that improves. I think it floods.
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Old 05-17-2019, 05:31 PM
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Carbs aren't the cause....it is high blower case/rotr and engine temps make the inlet air less dense, making the carbs appear to go rich but actually the carbs ae staying the same and incoming air is thinner.....once you get moving and get some air volume going through the blower case and rotors cool off and things return to normal.

Stumbles are more typically lean hole rather than rich bog, especially on blown engines
31 nozzles aren't even that big even on a 750 carb on a naturally aspirated engine.

What pump cams are on the carbs??
Pink cams are slow rate cams right off idle and can lead to a stumble.
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Old 05-17-2019, 06:53 PM
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Thanks for the info on the air temperature in the blower body. I wasn't thinking in that direction.

I am using white cams for the pumps. I started with 35 nozzles and heard a backfire pop so I thought it was too much. However, it was early on and I was using 70/80 jets at the time. I had way too much fuel and that would have added to the problem. (Totally black plugs). I replaced the pump diaphragms and springs. I can try 35's again now that I have the jetting close 67/77.
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Old 05-17-2019, 09:50 PM
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Backfire pop in the carb/intake is never too much fuel, it is the result of a big lean hole in the fuel curve if that backfire was up through the intake/blower.
Too rich bangs only show up in the exhaust/muffler outlets.



.
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Old 05-19-2019, 10:24 AM
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The 35 nozzles (tube type) are installed and work better but I still have to roll on the throttle. To make the jump to 37's Holley recommends 50cc pumps (updated cam) and hollow drilled nozzle screws. Cha Ching! (like these HP blower carbs aren't expensive enough, haha) I'll try the 37's with the hollow screws on my existing 32cc pumps first. I'm looking to eliminate a low speed stumble. The motor climbs nice.
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Old 05-19-2019, 05:03 PM
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Have you tried the white pump cams on the second hole?? Cam is a little bigger and a little sooner that way.
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Old 06-03-2019, 08:41 AM
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I discovered that a dirty needle was the cause of the fouling spark plugs at idle, and hard starting when warm problems. It was not totally noticeable when the motor ran 1500 rpms or higher. One of the 4 needles was sticking due to dirt. The bowls looked clean but the sticky needle was flooding the engine at idle and after the motor was turned off (residual fuel pressure caused gas to pour into the engine until it bled down after I shut it off).
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Old 07-07-2019, 03:52 PM
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After solving the dirty needle (cleaned it) I decided to install an AFR gauge. Placed the sensor in the header collector. Now I'm running a 75 primary jet, 81 secondary and 6.5 PV. and 35 nozzles. AFR readings of 12.5 at idle, 11.0 WOT, and 13.8 - 14.0 cruising at 45-50 mph 2K. I've tested 76 and AFR drops into the high 10's, 82's and again too rich. I've tested 74's and my 45-50 cruise AFR is 14.7-15.0..too lean. My idle mixtures are richened just to the point where blower surge stops. (1/12 turn at a time.) This could be as good as it gets. I installed Autolite Racing AR-52 plugs.
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Old 07-08-2019, 05:14 PM
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I should add that at 50mph, or 2000 rpms at a light cruise, the AFR will hit 14.7. Any increase of power and the AFR drops immediately to the stated 13.8-14.0. As I increase speed AFR averages drop and I think this is due to the secondary's suppling fuel. Above or below 50, AFR begins to drop. I'm not sure what it all means but the plugs look great and I will put a few miles on this set up.
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Old 07-11-2019, 06:55 AM
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I think you should go with the 50cc pumps and cam. I think there is really only 1 cam that works with them it’s way bigger. That’s what I ran on my Willys. The AFR was a wise choice. I found that unless I was “getting after it “ that you run mostly on the intermediate and transition circuits as you use very little throttle opening. Tuning the air bleeds and feed restrictions work this area. My carbs didn’t have adjustable air bleeds or metering blocked with adjustable feed restrictions. I purchased a pair from Quick Fuel. They didn’t have a good blower application at the time so it was up to me to learn to tune them. I got a whole range of feed restrictors....they aren’t cheap for a nearly invisible item. Like panning for gold. For what they cost. It doesn’t hurt to put a vacuum gage on the carbs and manifold. You generally have a difference.

Are you running a vac advance ignition? You should, it will help clean up the idle. I think Crain makes an easy to adjust unit. I had an MSD but when it failed I had a tough time replacing it. You can get GM points dist ones but the length of the advance is too long. I made my own limiter. Then there aftermarket adjustable ones. I went through a couple of them before I went back my home made madevone.

Eric is right on your hot idle issues. That blower case gets really hot. I had a little trouble with this too but solved it with a bigger and more efficient fan and shroud. Doing it again I would oversize the radiator and run two powerful electric fans. Try to keep it at 185 in traffic. I used Maradyne fans. They were nice to deal with. Don’t forget those big fans take a huge amount of power. Alternator needs to be 100 amp or better. I had the local Starter/generator shop tune my alt so it had a bit more power at lower speeds. The big fan, lights and AC strain the belt big time. Get the best one and a spare.

I also went to an aluminum water pump. It’s been a long time so I don’t remember brand. Possibly Edelbrock. It never gave a problem.

Last edited by bentwings; 07-11-2019 at 07:02 AM.
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Old 07-16-2019, 02:22 PM
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Thank you for the info on the air bleeds and IFR although my metering blocks would also have to change for me to adjust the IFR jets. I do have adjustable air bleeds and I will read up on changing them.

I have my fan coming on at 210 degrees and it will bring it back down, but the motor is cruising at 185-195 on a warm day.

You are correct that I am using very little throttle at 2000 rpms and this is the spot where I am running a little lean (14.7). If I reduce the Idle Air Bleeds to richen the mix at Idle+. I think the idle mixture screws could then be backed out to bring the AFR back up. My timing is good and I am using a 6BTM with timing retard by MSD, no vac advance. I think I will be adding the 50cc pumps, but for now I'm recording the AFR in order to fine tune all the jetting and try to pin point ratios throughout the rpms. It's tedious and there is a lot to understand on these carbs.
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Old Yesterday, 06:07 AM
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To conclude this thread: The Holley's were lean during cruising, rich upon acceleration and good at idle. The Air Bleeds corrected this uneven fuel curve. Low speed bleeds were reduced from 75 to 68. less air brought cruising AFR from 14.7 to 13.6. High speed bleeds were increase from 34 to 42 which brought the AFR up during acceleration from 11.0 to 12.2. A benefit from these changes was noticed in acceleration and increasing accelerator pump size was not necessary. When I was cruising lean and hit the throttle, the squirt was just never enough. By richening the cruise AFR the squirt was sufficient with a 35 nozzle and a 30 pump. I plan to continue fine tuning these Holley's and the fuel curve (air bleeds, jets and squirters), but I will conclude that the addition of the AFR gauge was very important. The motor is running, like a bat out of hell, and the exhaust is so much cleaner. Blower surge is gone. Thanks for everyone's comments they really helped me look at the whole picture when tuning these amazing carburetors. These Holley's really can be tuned to perfection. It just takes a little time and study.
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Old Yesterday, 12:12 PM
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Glad to help out. I’m sure you learned a lot in this exercise. Once you get the Holleys working correctly they are great carbs.
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