Nino1984

I am sure you have all had your fill of the Holley stumble/bog/hesitation issue, but this one has proven to be unsolvable so far. The car is a 1984 TA with the L69 HO motor. I bought the car about 4 years ago and it was basically stock and ran great (please, no comments that I should have left it that way). I hated working on it because there was so many wires and hoses in the way. I'm old school, and like my engine bay clear of crap. So, I removed all of the emissions stuff, including the cat, EGR, EFE, AIR, etc. I also removed the computer and harness. I installed a Non-computer HEI distributor and mounted a Holley 4150 that I rebuilt, including the geo bracket for the AOD TV cable. I kept the stock GM manifold, and used a square bore-to-spread bore adapter to mount the carb.

I am not a master mechanic, but I'm pretty good and can usually figure anything out on an older vehicle. So, now I essentially have a stock 305 with 1970s era fuel and ignition delivery systems. My problem is an off-idle bog that I get whether I'm driving the car or just flipping the throttle while in park.

I tried a few adjustments on the carb, but could not solve the issue. So, assuming I either missed something on the rebuild of the Holley, or had a bad carb core to start with, so I ordered a rebuilt Holley 4160 off of E-bay and installed it. Guess what? Same exact problem, only worse. So now I'm thinking timing? So I gave it more timing, which helped very little, so I gave it more timing, that caused it to start pinging.

Here is what i did today....
I pulled the vac advance hose and plugged it. Then I turned the curb idle way down. I gave it 12 degrees of initial timing and locked it down. Next I adjusted the accelerator pump so I had zero free-play and checked the after-travel with a feeler gauge. Next I turned the mixture screws all the way in, then out 1-1/2 turns. I connected my vacuum gauge, and then adjusted the mixture to maximum vacuum (about 19 HG). I brought the idle up to 750 rpm in drive. I reconnected the adv hose.

So, now I have the timing right where I think it needs to be. I have the acc pump adjusted. I have the idle mixture and idle speed adjusted. With the engine off I flip the throttle and I can see a strong shot of fuel coming from the squirter. When I start the car, it starts right up, choke is working fine, runs great on high idle, kicks down to idle nice, but if I stab the throttle, it flattens right out, even in park. It acts like a lean condition. In fact, if I put my hand over the choke horn and flip the throttle, it solves the stumble. So, that makes me think it's lean.

After all these adjustments, I took it out on the road again and when I jab the throttle it bogs or even stalls if I don't feather it. I pulled over and added more timing, but that only helped very little. When I added even more timing, it started to ping. I have fresh gas in the car.

This is where I gave up. I have basic tools, but no oscilloscope or propane vac leak detector. I did check for a vac leak using a can of carb cleaner, but found no leaks. The new carb is a 80457 4160 with electric choke and the stock .31 squirter and stock jets. I'm ready to take it to a shop that has all the right test tools, but I just can't believe that I can't find the issue myself. What am I missing or overlooking? Sorry for the long description, but wanted to give all the details. Any thoughts before I send it to the shop?

I should add that it runs great at 50 mph except that first off-idle stab. The total timing with centrifugal and vac is 36. Also, this cannot be a fuel pump or filter issue, because, like I said, it runs out fine on the highway.

sofakingdom

iTrader: (1)

https://www.thirdgen.org/forums/carb...ey-tuning.html

pancherj

Only once in my life have I used an adapter to go from spreadbore to square bore (on a 1975 factory manifold) and the results were less than stellar. I gave up on it pretty quick and went to a Weiand Stealth.

For you, Go through Sofa's tuning and see if it helps. You may need to up the squirter size to get rid of that hesitation. Make a drastic change (not one or two numbers) to see if it helps. Another issue can be the spring on the pump arm. If the spring is worn out and old, the spring may compress instead of activating the pump. The slightest movement of the arm should cause fuel to come out.

Nino1984

The oddity here is that I have the same issues with both the 4150 I rebuilt and the replacement 4160 I bought as a rebuilt. I thought about increasing the squirter size, but thought that the stock .31 would be more than adequate for the small displacement 5.0 litre L69. Is it realistic to think that I would need a .34 or larger squirter on a basically stock 305?

JamesC

iTrader: (5)

Note in the link above that I didn't change squirter size (31 in my 4160) to eliminate the hesitation/bog. Sofa's sticky is at the top of the page.

JamesC

sofakingdom

iTrader: (1)

THIMK... in a PERFECTLY WORKING carb, the accel pump almost isn't even necessary. Its sole purpose is to "cover up" the lag in fuel delivery from the main systems, when the throttle is opened suddenly, and the carb doesn't deliver fuel quickly enough to keep up with the air. If you get those other systems REALLY RIGHT, almost no pump shot at all is actually needed.

Thinking you need to dink with squirters is about a 99.999% CERTAIN indicator that the carb isn't properly tuned to begin with; and is similarly about 99.999% CERTAIN NOT to make the carb WORK RIGHT, even if it successfully covers it up sometimes.

In point of fact, Holley accel pump systems are ALMOST ALWAYS larger than necessary; and once the carb is properly tuned, can usually be reduced DRAMATICALLY. Which in turn, can significantly improve gas mileage at the same time it makes the car run better, esp in a stick car around town where you're on & off the gas repeatedly every single time you move the car.

Pablo

iTrader: (1)

What pump cam are you using and what position is it in?

I have a similar combination, 4150 650dp, performer RPM, 1 inch open spacer, stock LB9 with headers etc. My combo is very crisp with a 31 squirter and blue pump cam in the #1 position. I have a 28 squirter and blue pump cam on the secondary.
It sounds to me like your issue is in the primary pump circuit. If you haven't done so, I suggest trying a blue pump cam in the #1 position.

I'm assuming it will pull just fine if you ease into the throttle. If the problem is only when you are stabbing the throttle it's pump shot. Especially on a tight converter which is sort of like a stick in gear.

Pablo

iTrader: (1)

This hasn't been my experience. I think holleys are more appropriately tuned for something with a loose converter and/or a lot bigger lungs. A small 305 on a tight converter or a stick isn't going to hit the boosters with as much signal, especially on an open spacer.
The 28 squirter was too small with a blue pump cam in my case (which is is more aggressive than the pink cam that most DPs come with). I confirmed this with not only seat of the pants but with datalogging the stock narrow band O2. It would clearly drop on when stabbing the throttle. Powervalve is a 9.5 which shouldn't really matter a whole lot anyway, vacuum drops to 0 immediately on a quick throttle hit.

sootie007

Im no expert but have tuned several Holleys in the past ...I am thinking maybe "possibly" you have too much shot -too quick so maybe you ought to try going the other way with it - the only way I know to test that is for you to buy a smaller pump shooter which should lean and spread the shot out over a longer time frame...you said if you feather it - it responds better ....so that equates to less fuel over a longer time frame .....

If that doesn't work another angle to explore is I would try is bringing your idle speed down from 750 if the car-cam will allow it.....I have seen high rpm idle burn off acc shot too quick also and cause issues ? You have 19" of vacuum so that is great .

Also there is an adjustment called "squaring" the idle transfer slot. You may have the butterflies set too far past the transfer slot - so the throttle transition isn't getting detected by the carb properly and you are left with these type issues. Do some reading on squaring the transfer slot on the internet . You just simply adjust the butterflies until the transfer slot looks like a perfect square and use that as your tuning starting point FWIW .

pancherj

Absolutely! It isn't about the size of the engine. It is about air speed, how torturous the airpath is and wall wetting. When the throttle cracks open rapidly, the airspeed drops. The pump shot covers up that hole by making more fuel available. This fuel is coarse in droplet size and most likely hangs around on the port walls and floor (not suspended in the air stream).

I do agree with Sofa that once the other systems are tuned correctly, you will most likely require less squirter volume.

Pablo

iTrader: (1)

I have virtually the same engine combo and a 31 is just right. Same rear gear, only difference is that you have an auto.
If you are to the point where a pump shot is big enough to cause such an abrupt stumble, you aren't just too big, you are way too big and you will see black smoke. Manually choking it with your hand and seeing the results you're seeing is not consistent with overly rich.
IMHO an out of the box holley dp is tuned for a larger engine with a looser converter. The pink pump cam is designed to give you shot beyond an elevated stall speed (think, greater throttle position) and stronger vacuum signal due to the increased displacement. The size of the engine does matter in that it creates the airspeed. If it didn't you could put a 1050 dominator on your 305 and have it run great. When you crack the throttle you go from low pressure to high pressure and as was touched upon, the fuel condenses on the port walls. The pump shot is there to cover this and the lag the fuel experiences exiting the booster (air moves faster than fuel). I think you may just need a more aggressive pump cam such as the blue one I recommended earlier. I'd be surprised if you needed bigger than a 31.

If you swap to a smaller squirter, I bet it gets worse. Could be wrong though, maybe my car is just way different somehow.

Nino1984

Pablo, Sofaking, and the rest,

You have all given me some great ideas and some great places to start. Thanks. I ordered a .34 shooter and will try that when it arrives. I also have a .28 shooter, and I will try that as well. Those two things should tell me, depending on if one makes it better, if I have a lean or rich condition. I suspect it's lean, as I have no black smoke, and as I stated, I can eliminate the problem by placing my hand over the choke horn. Remember, this thing bogs in the garage in park or neutral. BTW, this is not a DP, it's a vac sec carb. I've rebuilt and set up a lot of Holley carbs, but never experienced anything like this before. Any other thoughts? Could it not be the carb?

pancherj

I didn't realize it was a vacuum secondary carb (I should have read closer). I never recommned to anyone to use Holley vacuum carbs if they are interested in maximizing performance. You want the secondaries to open as quickly as possible (without bogging the engine). Unfortunately, you can't get enough pump shot from the single primary squirter to cover up the opening of the secondaries and primaries. What you are left with is opening the secondaries slower and living with less acceleration. In addition, the vacuum carbs generally require a bigger squireter and more agressive pump cam. In the end, I don't see any better gas mileage. On my personal IROC, I routinely pull down 14MPG with 4.10 gears, 3600 stall and a 750 double pumper. The 3310 was about the same.

Nino1984

Thanks Pancherj. I don't think the vac sec has anything to do with it. The stumble happens long before the sec circuit would open. In fact, it stumbles in neutral or park sitting in the garage.

sootie007

Nino how did you make out - did you get it tuned out yet ?

Nino1984

Nope. I pulled the carb off to double check the secondary throttle plate position/adjustment. I understand that the proper adjustment is 1/2 turn from closed on a motor with good vacuum (say 14" or over), or a square transfer slot on low vac motors. The adjustment seems correct. I also pulled the front bowl off to check for crap in the metering block. Apparently even factory rebuilts can have issues like that. I've yet to put it all back together and start from scratch with my adjustments.

sootie007

" I also pulled the front bowl off to check for crap in the metering block. Apparently even factory rebuilts can have issues like that."

Y - I can concur I had a brand new Barry Grant 750 that had a chunk of thread cut debris in it...it ran great for about a week after I put it on then seemingly overnight ran absolutely terrible. I pulled it apart and found 2 chunks of machining debris that apparently got up in the needle and seat . Put it all back together and it ran great for 2 years solid until I sold it . I have heard horror stories from a speed shop owner I know who told me stories of carbs coming in with venture boosters horribly misaligned , Choke plates machined wrong and sticking on the sidewalls etc etc....so unfortunately its a crap shoot a lot of times on quality .

dlinger

iTrader: (4)

I hesitate to post this as you had same problem with two carbs. I had one that had too much clearance the in check ball in the accelerator pump.

skinny z

Remember that the shooter doesn't affect the amount of fuel being delivered. Just the duration of the shot. The amount and timing of the delivery is controlled by the cam. (I'll exclude the 50cc pump). For the record this about WOT from idle (or cruise).
What shooters were installed when you first started and found this problem?

Nino1984

Stock .31 shooter.

skinny z

The most telling aspect is that it stumbles in neutral.
Can you physically see fuel being discharged from the shooters (I'm sure you can but I had to ask)?
Stock or near stock pump cam?
The vacuum secondary operation is confirmed and that it's not allowing the butterflies to open when they shouldn't?

gbeaird

iTrader: (1)

I experienced a 'throttle move stumble' on a brand new 4150 I installed on our 5L LS1 in our Firebird. Since I used the Hawk's 3rd Gen swap headers, and I had O2 sensor bungs, I got a FAST dual AFM to help with tuning. I noticed that even with it running well, any time I opened the throttle any amount, the car would go very lean for a moment, resulting in a 'bog' at throttle movement. That should be accelerator pump. I noticed that the accelerator pump did indeed move when the throttle was moved, but that it was already partially-depressed even at idle.

I shortened the lever bolt until the head of the bolt just rested on the lever and all the slack in the mechanism was taken out. I set it so that any movement in the throttle would squirt fuel. THIS took that bog out of the engine when the throttle was moved.

So if you have a AFM, verify your bog is caused by too rich or too lean conditions. With a brand spanking new Holley carb, I suspect you're too lean on throttle opening.

Damon

Is there ANY chance you just have a weak ignition and/or shot plugs/wires? I am NO fan of adapting a Holley onto a QJet manifold, but even that disaster shouldn't act the way yours is acting.

Snap-throttle conditions are THE HARDEST on the ignition. If it's weak, that's where it'll usuall falter first.

Two different carbs with the same problem? I'm just not buying the magic on this one.