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Discussion Starter #1
I am having some issues with the carb on my '77 400 big block in my duster.

The carb is a 750 cfm Holley 3310 4 bbl with vacuum secondaries.
-main jets are #72
-6.5 power valve
-green accelerator pump cam
-.025 accelerator pump nozzle
-Quick Fuel Technologies secondary diaphragm housing

I got it used and rebuilt it when I rebuilt my engine ~5 or 6 years ago. Rebuilt everyting back to the factory specs listed in my holley carb manual, except for the aftermarket diaphragm housing. The problem is a lack of torque off-the-line with this carburetor and a very slight "flat spot" when applying very light throttle from cruise. When heavier throttle is applied from cruise, there is no hesitiation.

It does NOT bog off the line, although bottom end torque is not spectacular. I had chalked this up to the engine combination UNTIL I installed an older, used 600cfm edelbrock performer carb on the car to see if that affected the slight hesitiation from cruise. When I installed the Edelbrock carb, this thing suddenly turned into a tire-burning torque monster, much to my surprise.

This tells me that there is definitely something wrong with my Holley, but I don't know where to look. I would like to fix the holley and put it back on since it provides noticeably better midrange and top-end power over the smaller edelbrock carb. Does anyone know what might cause such a problem? Thanks!
 

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I am trying to compare what I have used in the past, and I think the .025 accelerator pump may be a little bit on the small side, I think the smallest I was able to ever use on anything was a .028 on a 440, and .032-.036 going to small blocks (big engine, small squirter, small engine, bit squirter).

I take it you are running about 9 inches of vacuum? That's where the 6.5 would be functional. Steady vacuum, divide by 2 and add 2.

What kind of mileage do you get with the engine as it is?

Where are the air bleeds as far as turns out goes?
 

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Discussion Starter #3
Thanks for the reply. I am running 9" of vacuum in gear at idle. With the car in neutral and the idle at ~700 rpm she pulls 13-14 inches. I have not calculated mileage, but I went through half a tank driving probably 70-80 miles; much of that driving was running 70 mph on the interstate with the motor turning over 3000 rpm. The rest was me stomping it and winding it out through first gear.

I am not sure about the air bleeds; I have not touched them other than cleaning them with carb and choke cleaner.

I have not messed with the accelerator pump since I have had this cam in it (the cam I am running now is the Summit cam I have talked about in other posts; I replaced a much rowdier comp cam with the summit cam), but with the old cam (comp xe 275 HL) I ran everything from a .025 squirter up to a .040 squirter. I also bought a pump cam kit and tried various different cams; I also checked adjustment of the accelerator pump arm several times.

Last year I ran #74 main jets for pretty much the whole summer, and this spring when i pulled the plugs they were black. That's when I put the #72 jets back in it and since then the plugs look normal.
 

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You can tell if the jetting is too rich or lean by taking the needles on the side of the metering plate and adjust them both to 1.5 turns out to start with, then when she is warmed up and idling, turn in each side quarter turn at a time, rich will let you get all the way down to bottoming out, lean won't let you get past a quarter turn in each side and actually run better when they are taken out past the 1.5 turns out. Sorry, I said air bleeds, meant fuel bleeds, it is the 6210s that are air bleed, not fuel bleed, sorry. Also, if you can turn the fuel bleeds all the way down or to within a quarter turn of all the way in, power valve can be blown and dumping fuel all the time, so something to check.

To the power valve, it is the correct one, so that isn't the issue unless it is blown (see above).

Now you have one of those special adjustable secondary vacuum canisters, right? Have you shut it down so see how the primaries themselves run without extra vacuum opening up the secondaries to see how the bottom end does without them opening?
 

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Discussion Starter #5
I have not messed with the secondary adjustment in a while; when I did, I did not notice a big difference either way. I do have a stock diaphragm housing and a spring kit in the garage, however, so I am thinking about putting that on the carb temporarily and seeing if that makes a difference at all.
 

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That may be an idea, along with determining if the secondaries are running rich or lean with the fuel holes in the secondary plate. Think of it this way. If the smaller carb gave better signal for the bottom end, see if shutting down the secondaries can get the primaries to get the engine to come alive on the bottom end first, then adjust the secondary opening to respond to it afterwards, that way you can tell for sure the problem is not from the front half of the carb. Vacuum secondaries can be difficult to set up properly because you can't tell when it is opening, so adjustments need to be small and incremental to make them work properly. Bottom end can drop out if they open too quickly but over-rich on the primary side can cover that up, something that needs to be verified. Once set up, great.

When you are looking at the carb when the engine is running, do the secondaries try to open or barely blip at the rev, or do they move a lot? Also, there is an adjustment for the secondary blades to be just short of touching closed, right on the edge. There is a little set screw located under the baseplate edge that sticks out under the vacuum diaphram location. Carb has to be removed in order to adjust, but I don't think you are there yet.
 

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Discussion Starter #7
The secondaries do NOT open when the engine is revved with no load. The secondaries definitely do open when there is load placed on the engine however; I tested this by attaching a twist tie to the secondary linkage just below the housing and then making a full throttle pull through second gear. When I popped the hood, the twist tie had been pressed down the linkage.

I am wondering if this is a problem that has been present since before I got this carburetor. I don't recall the engine ever being a torque monster, even after the rebuild, and this is the only carb I have run on it up until now. It has just always felt a little lazy off the line; but as soon as I installed the edelbrock, it is like a completely different engine down low.
 

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I am suspecting the secondaries are opening too quickly. Possible they are actually killing the bottom end by opening too soon and the front of the carb is actually running rich to compensate for this. Slow down the secondaries for starters, see how she works with a 2bbl basically, then start softening up the secondaries to see what happens.
 

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Discussion Starter #9
I will probably get a chance to work on it some on friday; I will definitely troubleshoot the secondaries again. Just out of curiosity, how can you tell if the power valve is bad with the carb off the engine? I don't necessarily think it is, but might as well check everything while I am at it.
 

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Pull it out of the metering plate and suck and blow on it. You should not be able to get it to pull any air, but still move in and out on the side that does not have the knob sticking out (that's the vacuum side that closes it with vacuum and allows it to open if the vacuum drops, fuel goes in the intake, and the vacuum it makes will close it). Popping through the carb tends to blow them after about the fifth or sixth time.
 

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Discussion Starter #11
So I did get a chance to work on it a little yesterday. I did not get the Holley carb back on the car yet, but I discovered some interesting things when I went through it.

For starters, the power valve is NOT actually a 6.5. For some reason I thought that I had checked it out and that it was a 6.5, but I guess I must have been thinking of the spare 600 Holley I have in the garage. In actuallity, I cannot tell what this one is because it does not have numbers stamped on it like they normally do. However, I pulled the 6.5" power valve out of the spare 600 holley I have and the spring on the 6.5" valve feels much stiffer than the spring on the power valve that was in my 750. The 750 is supposed to have a 6.5", so I am assuming it was swapped out before I got it for use with a big cam.

Regardless, I installed the 6.5" valve from the 600 holley in the 750 and we will see if that makes a difference

Also, the adjustable diaphragm housing has a drawback that I had not thought of before: when you close the passage more to slow the opening rate of the sedondaries, you also slow the CLOSING rate of the secondaries since air cannot escape as quickly. This does not seem to be the case with a standard housing, as the restriction does not change so the secondaries still close quickly depite opening later. I'm not sure if this is a contributing factor yet, but I hope to find out soon.

I set up a stock diaphragm housing with a purple spring (one step lighter than stock; holley says on a 400 inch engine it should be full open by 6000 rpm) that I may swap on if I can't get the results I am looking for with the adjustable one.

I plan on putting the carb back on the car on Monday and doing a little troubleshooting at that time; I will post again after that.

Do you think that the power valve opening at a much lower level of vacuum could be contributing to the problem?
 

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Yes, because if it is opening too quickly at lower rpm then you are running too rich and the secondaries then have to try to compensate for the bog, which, if you are then trying to open the secondaries to offset, you are then dropping more vacuum and falling on itself as it is. The idea with the bigger cam is to make the engine pull harder to get things to open accordingly, too soon, as you experienced with the 600 being more responsive on the bottom end, works better for the bottom end, but doesn't make for good top end. Slow the 750 down a little bit more and it should respond better. The secondary closing slow, I think, is overcome with the linkage itself, shutting them down manually, which is how they function properly. Since you have the carb off, check that secondary screw I talked about to ensure they aren't opened too much. Linkage-wise, you should be able to have cruise throttle opening without the secondaries being able to automatically open, about 20percent throttle I think, any extra automatic vacuum opening before then will run a lean situation, which you are then compensating for with the primary side, jetting and stuff.
 

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Discussion Starter #13
I had a chance to work on the carb some more yesterday and today. I closed the passage to the diaphragm all the way and that definitely improved the performance off the line, but there is still a very slight stumble on throttle tip in from cruise. Just for kicks, I installed the stock diaphragm housing with the purple spring, and the bottom end is still good, but did not help the stumble.

For the record, Holley says this spring should begin opening the secondaries around 1650 rpm and reach full open around 6000. 1650 is higher than i typically cruise at, so I don't think the secondaries are the problem.

This stumble is weird: it is only under light throttle tip in from cruise; moderate or abrupt throttle application does not produce a stumble. Also, it does not stumble off idle. I drove it yesterday and today; it was cooler outside today, and the stumble was slightly more pronounced. Sounds like a lean condition, maybe? I don't know what is causing it though; it is below the rpm when my secondaries are supposed to be opening, the power valve is the right size, and I have like double TRIPLE checked the accelerator pump linkage clearance.

I also stepped up to a .032" discharge nozzle from the .025" nozzle i was using; this did not affect the stumble.
 

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OK, so the stumble could be a rich condition, too. The accelerator pump cams you have, try swapping it out for one that is as mild as possible and work up to more agresssive one. A light stumble with light acceleration point may be giving too much fuel, but when you step up to medium and heavy throttle, the spring on the accelerator pump and the squirter itself will allow only so much fuel to actually enter, thus enough air is pulled in with the fuel. How does the exhaust smell? Hopefully it doesn't smell rich. What is the setting for the fuel needles on the side of the metering block? 1.25-1.5 turns out I hope?
 

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Discussion Starter #15
The idle mix screws are out 1.25 turns. I adjusted them using a vacuum guage and found max vacuum to be about 1.25 turns out. The exhaust does not smell noticeably like raw gas, and the plugs look normal.

I hadn't thought about the stumble being a rich condition, but that actually makes a lot of sense. It only does it during situations where the accelerator pump is being triggered slightly but there is not a significant vacuum drop. I have a pump cam kit, so I can play with that some and see if that makes an improvement. I wonder, then, if I should put the .025" discharge nozzle back on?
 

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Yeah, start with the .025 since there was no change with a larger squirter.
 

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Discussion Starter #17
Okay, so I put the .025 squirter back on and replaced the green cam with the white cam in postition 1. This seems to have solved the problem for the moment; I took it for a several mile cruise outside of town and did not notice the stumble anymore. The white cam was the first cam I tried because it delivers a smaller volume of fuel overall and has a less agressive lift curve. Time will tell if this is the right cam or if I will have to try a few others. Thanks for the help!!
 

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I do have to say, electronics can make the difference in some things, mathematically controlling things and the like. Carbs can be a bit of a challenge at times, just takes a little bit of calculating mechanically, so to speak.

You are welcome. Now you can start running lighter springs at the secondary side, see if it picks up there, and when it starts to stumble again, back it off one spring.
 

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Discussion Starter #19
So the carb has still been acting up depending on weather conditions, but I finally figured out definitely what the problem was and fixed it.

Turns out the throttle body to main body gasket that came with the rebuild kit when I rebuilt it was not the right one; the holes in the gasket for the thottle bores were smaller than the actual throttle bores. I had probably between 1/16" and 1/8" of the gasket sticking out into the throttle bores just above the transfer slots, effectively blocking flow over the transfer slots and preventing vacuum from properly pulling fuel through them as the throttle plates opened. I put the correct gasket on it and the problem is totally gone.

I just can't believe I didn't see that before now.
 

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Yeah, one expects the parts themselves to be correct so you can troubleshoot the actual part as a whole instead of something within it. Not like it is the first time something stupid had someone chasing their tail and not knowing it. Glad she's fixed.
 
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