Holley 670...can't get rid of the massive lean spot

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travis
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Holley 670...can't get rid of the massive lean spot

Post by travis »

I'm just about out of ideas here...

I've got this 670 Holley Street Avenger vac sec carb that has always had this bad lean stumble since I've owned it. I've tried it on various engines between an estimated 200 to 450 HP...it doesn't matter. Jetting, power valves, squirters, float levels settings...nothing has helped. Today I tried a whole variety of different pump cams...best I can do is move the bad lean spot around a little bit. What is crazy is that other than that bad lean spot, it runs great...noticeably better than any of the other 600 edelbrocks or variety of 600 VS Holleys that I've tried on this particular engine. Its essentially undriveable...lightly roll into the throttle from a dead stop and it takes off for a few feet, then falls on its face and usually dies. Punch it from a stop, it launches, stumbles, then runs awesome from there on out.

Can I disconnect the secondary's vacuum pod, so the secondary's won't open? I'm starting to wonder if maybe the secondary's are flopping open or something. Whatever secondary spring that is in there now is stock...its one of the few things I haven't touched yet. This has the quick change vac pod on it. Isn't there supposed to be a check ball or something in there that keeps the secondary's from flopping open?

Engine is a very mild 351w that pulls a rock solid 19" idle vacuum at 700 rpm idle. Currently the carb has 66/71 jets, 8.5" PV, and a blue pump cam on position 1. 14x4" open air filter, shorty headers and dual 2 1/2" exhaust with Super Turbo's...so no restrictions there. Other than the dead spot, it starts, runs, and drives excellent.
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mt-engines
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Re: Holley 670...can't get rid of the massive lean spot

Post by mt-engines »

have you tried different secondary springs? or advance curves? whats the cam specs? compression ratio? base timing? cubic inch? torque converter stall?
travis
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Re: Holley 670...can't get rid of the massive lean spot

Post by travis »

mt-engines wrote: Thu Jan 14, 2021 3:01 am have you tried different secondary springs? or advance curves? whats the cam specs? compression ratio? base timing? cubic inch? torque converter stall?
The secondary springs are the only thing I haven't touched yet. I'm going to try that in the morning...maybe start with the stiffest spring first.

Like I said, I've tried this on multiple engines, but currently it is on a basically stock late '80's 351w (factory FI engine out of an '88 or '89 Bronco iirc). Lightly ported heads with a 3 angle VJ, stock 206/221@.050 HFT cam installed 4* advanced, 115 lsa. 8.5-1 compression. I've moved the timing and curve all over the place...it doesn't help at all. Currently its 12 initial, 36 total, all in by 3000, plus iirc 15* vacuum advance on manifold vacuum. Stock iron 4bbl intake with a 1" open spacer...spacer is required for linkage and fuel line clearance. Stock converter, C-6 trans, 3.55 gears, approx. 4400 pounds.
travis
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Re: Holley 670...can't get rid of the massive lean spot

Post by travis »

Interesting...I do a search and can't find squat. Make a post, and the "similar topics" below pulls up multiple posts about this exact carb. It appears this is a well known problem...
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Re: Holley 670...can't get rid of the massive lean spot

Post by novadude »

I would check out the accelerator pump arm. Sometimes the bend in the "knee" is wrong, limiting total travel. Also, make sure the arm is riding on the cam and not on the clock spring.
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Re: Holley 670...can't get rid of the massive lean spot

Post by ssregal39 »

travis wrote: Thu Jan 14, 2021 3:12 am
mt-engines wrote: Thu Jan 14, 2021 3:01 am have you tried different secondary springs? or advance curves? whats the cam specs? compression ratio? base timing? cubic inch? torque converter stall?
The secondary springs are the only thing I haven't touched yet. I'm going to try that in the morning...maybe start with the stiffest spring first.

Like I said, I've tried this on multiple engines, but currently it is on a basically stock late '80's 351w (factory FI engine out of an '88 or '89 Bronco iirc). Lightly ported heads with a 3 angle VJ, stock 206/221@.050 HFT cam installed 4* advanced, 115 lsa. 8.5-1 compression. I've moved the timing and curve all over the place...it doesn't help at all. Currently its 12 initial, 36 total, all in by 3000, plus iirc 15* vacuum advance on manifold vacuum. Stock iron 4bbl intake with a 1" open spacer...spacer is required for linkage and fuel line clearance. Stock converter, C-6 trans, 3.55 gears, approx. 4400 pounds.
I have the similar quick fuel 680 on a 351W 9.7:1 220 220 cam. It has an adjustable vacuum pot and I had to tighten it up a bunch to get rid of the lean spot. I think I landed on a white cam.
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Re: Holley 670...can't get rid of the massive lean spot

Post by rebelrouser »

Messed with lots of Holleys, don't remember list numbers or models but generally a couple things I have done is, on the accelerator pump, you must remember the pump cam and nozzle size while they play a big part in how much enrichment you get, the most important thing is the spring rate on the pump arm. Since you can not compress a liquid, when you mash the throttle, the spring compresses, to keep the linkage from bending, and the rate of the spring then pushes the pump shot in to the throttle body. I use shims under the pump arm spring to raise this pressure and get a good pump shot. I don't know where Holley is buying those springs, or how the select the rates, but I have seem many weak ones in the last couple years. The other thing is matching the power valve opening point, and sizing the power valve restriction. As well as the idle air restriction, been fixing a lot of the old Holleys, by taking the time to drill and tap, to install replaceable idle bleeds. The other gorilla in the room is if you are running pump gas is what a petroleum engineer on a forum told me several years ago, he said " we don't make gas to run in carburetors anymore, we are not worried about carburetor driveability" If you look at the OEM engines, the fuel injection pressures keep going up, as the evaporative emission regulations go up as well, so they don't want the fuel to evaporite as easy for emissions, but it has to evaporate to run in an engine, so as the nozzle pressures increases on the injector we maintain driveability and fuel mileage, Diesels have been doing the same thing.
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Re: Holley 670...can't get rid of the massive lean spot

Post by travis »

I’ve been reading through a bunch of different posts here and on other forums. A couple things stand out. 1...apparently there is something screwy with the primary metering block, and a lot of people have fixed the issue by replacing the metering block either with a fully adjustable block and farting around with the idle air bleeds, or by using a regular 1850/80457/3310 metering block (which I have on hand). 2...some have claimed to have cleared up the off idle hesitation by using a considerably smaller squirter to extend the pump shot duration. The thinking is that on a heavy vehicle the pump shot gets used up before everything can catch up. That’s exactly what my situation feels like...once the pump shot is used up it falls on its face (dead lean).

Looks like I’ve got a few more things to try
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Re: Holley 670...can't get rid of the massive lean spot

Post by skinny z »

Off idle lean spot at what amount of throttle input? Flooring it? Gradual acceleration?
The comment regarding the power valve is one that reflects directly on the remedy for my 670 Avengers lean spot. I used an in-cabin vacuum gauge as well as an AFR gauge (with wide band O2 sensor) for tuning.
I found that under certain accelerating conditions (varying degrees of part throttle input) the AFRs would go lean and engine vacuum was at a point above the opening spec of the PV. I went from a 10.5 valve to something like an 8.5. (IIRC).
After that swap, when accelerating, I could observe the AFRs getting richer once the vacuum reached the PV setpoint.
No secondaries involved at this point.
Further to the comment above, I opened up the PV channel restriction to the recommended maximum.
The car was night and day to drive.
FWIW. All the experimenting with the accelerator pump did very little. Mostly because in its stock trim, it's suited to the application because the application was nothing out of the ordinary. Just another 400 HP SBC.
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Re: Holley 670...can't get rid of the massive lean spot

Post by PRH »

I’d pull the carb off and see where the primary blades are in relation to the transfer slots.

Sounds like there may not be enough added fuel as you transition from idle to the main circuit(blades open too far at idle......or no slot showing at all).

I see you read thru some other threads where an increase in IFR was mentioned.

I would make sure the blades were properly positioned at idle for the correct amount of t-slot exposure, then a slight increase in IFR diameter, then increase the P/V to a higher value.
Float level can sometimes help here too.
Try raising 1/4-1/2 a turn above “normal”.
Somewhat handy with a die grinder.
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Re: Holley 670...can't get rid of the massive lean spot

Post by rustbucket79 »

Is the check ball in the secondary diaphragm “pod”? If it’s missing the result can be what you are experiencing. I’m reading that your issue is wide open throttle and how the secondary’s open. (Too fast)
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