Holley 1850 -2 Rebuild, Low MPG

[Pa PITT]

& My crazy question is .. Had you been running this carburetor, on this Bronco, Just before the rebuild ..
Meaning you didn't used to get 20 mpg on another car & Now that it's on your Bronco, iT'S GETTING VERY BAD MILEAGE ON YOUR BRICK.
MY 72 BRONCO only got about 4.5mpg ...Awful.

 

[seely]

& My crazy question is .. Had you been running this carburetor, on this Bronco, Just before the rebuild ..
Meaning you didn't used to get 20 mpg on another car & Now that it's on your Bronco, iT'S GETTING VERY BAD MILEAGE ON YOUR BRICK.
MY 72 BRONCO only got about 4.5mpg ...Awful.

Good point, I hadn’t calculated the mpg previously, but was surprised when I ran out of gas so quickly, as I previously could drive it a bit longer before having to fill up.

 

[pcf_mark]

Several good pieces of advice on this thread all of it good. When I get mystery carb problems I like to go back to basics and that may help us help you. The suggestions to just swap to another is good if you have one.

1) What is the engine vacuum as read on a vacuum gage? A stock truck should be around 16-18". Is it steady or does it wander back and forth between two number like 12-15? Does it wander slowly or dart back and forth?
2) If you disconnect anything connected to the carb including the vacuum advance and plug those connections and take off throttle linkage can you set an idle? Can you close the throttle blade with the idle screw and stall the engine? How many turns out is the idle screw to get it to idle nice?
3) If you install or remove the vacuum advance (plugging the carb port each time) does the idle rpm go up or stay the same?
4) What do the plugs look like? Not just the color but the ground strap. The color is the carb the ground strap tells you about timing.
5) Front and rear float level set correctly? On an 1850 I do not think there are site plugs you may need to take the bowls off to check it again.
6) With everything still disconnected how many turns out are the idle mixture screws to get highest vacuum?

This info should help us help you and you may stumble on your issue. Carbs are funny but relatively simple.

 

[seely]

I’ve done some more tests and have determined the issue is being caused by a vacuum leak leading to loss of power. I can hear a strong hissing noise when accelerating, which I think means air must be getting sucked in somewhere, messing with the air fuel ratio. I sprayed some starter fluid around the carburetor, especially the different vacuum points, but did not notice any difference that would narrow down where to look.

I am wondering if I may have tightened it down too much when mounting it, leading to a vacuum leak at the manifold. What would be a safe way to test this, and if my suspicion is correct, what would be a good way to resurface it?

 

[Broncobowsher]

I’ve done some more tests and have determined the issue is being caused by a vacuum leak leading to loss of power. I can hear a strong hissing noise when accelerating, which I think means air must be getting sucked in somewhere, messing with the air fuel ratio. I sprayed some starter fluid around the carburetor, especially the different vacuum points, but did not notice any difference that would narrow down where to look.

I am wondering if I may have tightened it down too much when mounting it, leading to a vacuum leak at the manifold. What would be a safe way to test this, and if my suspicion is correct, what would be a good way to resurface it?

Let me guess, you are running an open element air cleaner? Listening to the air sucking through the carb without the silencer of the factory style air cleaner. Normal stuff.

During acceleration manifold vacuum goes to its lowest. Vacuum leaks all but disappear since there is no vacuum left anymore. The open throttle blades are filling the intake with air. Vacuum in the intake manifold strongest during closed throttle coasting, second strongest vacuum is at idle.

 

[seely]

Let me guess, you are running an open element air cleaner? Listening to the air sucking through the carb without the silencer of the factory style air cleaner. Normal stuff.

During acceleration manifold vacuum goes to its lowest. Vacuum leaks all but disappear since there is no vacuum left anymore. The open throttle blades are filling the intake with air. Vacuum in the intake manifold strongest during closed throttle coasting, second strongest vacuum is at idle.

Thanks for setting me straight, I couldn’t remember if that sound was normal or not. You all can probably tell how new I am to all of this, but I appreciate all of your guidance and knowledge with all of this.

 

[pcf_mark]

Get a vacuum gage on it that will tell you a lot. Plus they make tuning a carb much more precise.

https://www.harborfreight.com/fuel-...O2w9kSO3JIxcWUihlIYKjfWZMUeT7i_oaAoKIEALw_wcB

 

[Bitch'nBronco]

I know you said you checked the tank, but have someone follow you and watch for fuel sloshing out around turns. The rubber hose that went around the spout on my tank looked good visually but was loose enough (the hose deteriorated on the inside) to let gas run past it when I was making right turns, when the tank was full.

 

[seely]

Hi everyone,

I finally had the time to do some more tests and looked the primary bowl over, top to bottom making sure everything was tight, gaskets on correctly etc.

I put it back in and the idle mixture screws are still not killing the engine when fully seated. I can hear a slight hissing sound go away when they are both turned in, so I think the idle pathway is working correctly, but I seem to be getting gas into the manifold somewhere else.

When I took the carb off, I noticed the rear two pathways in the manifold were dark, as though the fuel might be coming in from the rear barrels. I can’t see any fuel dripping when I look down the barrels, but something is getting fuel down into the manifold.

The last thing I tested was the idle speed screw, thinking it might be too high and is bypassing the idle pathway, but realized I could turn it out almost to where the screw was out and it was still idling. This, I feel, is the most definitive proof that something is seriously wrong with the fuel flow into the manifold.

Any ideas what to look at, especially in the rear float bowl?

 

[1970 Palmer]

Pull the carb.

Place a trouble light under the rear throttle plates. You should see "no light" anywhere around the plates when the secondary is closed. It should close completely with just the diaphragm spring pressure. Check the secondary throttle plate stop screw adjustment. If you have the secondary sticking, hung up, twisted shaft, fast idle cam stuck, improperly adjusted throttle plates, etc. you will pull fuel and air through the secondary and have the symptoms you previously mentioned (mixture screws not sensitive). Still idles with idle screw back off.

While the carb is off, remove the power valve and inspect for a blown diaphragm. At the very least check what size valve is installed currently.

Dump the fuel out of the carb, into a metal pie pan. When you remove the primary float bowel, place the cork gasket into the gas in the pie pan. This will prevent your cork gaskets from shrinking up when they dry, and you can reuse them.

Did you ever check the manifold vacuum? Is the correct power valve installed for the given manifold vacuum of your engine? You will need to look up the correct power valve for your engine vacuum, and altitude. See Mike's Carbs, or the Holley sites.

What's the initial and total ignition timing? Do you have a vacuum advance on your distributor? Do you have a dual vacuum (advance/retard emission) style advance? Plug the manifold vacuum line to the distributor. Run the ported vacuum to the advance side of the advance can. The ported vacuum is the fitting on the side of the mixture plate, not the one under the throttle plate.

Note every one of the above will affect your idle mixture, and idle speed adjustment sensitivity, AND fuel consumption.

 

[EPB72]

So I'd look at the secondaries as 1970 palmer suggested...but if the carb is still on and in running state then you can check it while at idle .idle speed screw backed all the way off.. what RPM are you at ???
You can take a couple of damp shop rags ball them up and you can block off both secondary bores and see if there's any rpm drop or stall out'''

 

[Broncobowsher]

Also check the fast idle screw isn't holding the throttle open beyond what the normal idle screw is.

 

[seely]

Alright, after doing some more troubleshooting, I fogured out the secondary metering plate was not toght enough. I was using a flat head screwdriver since I didn’t have the right head for the screws, but realized trying not to strip their heads, I wasn’t getting them tight enough.

I got the correct, clutch head bit and was able to get them quite a bit tighter, and then reset the floats, since I suppose, fixing the leak changed created a problem where my floats right now set to high. I was able to tune the idle mixture, screws, idle speed screw, and float settings and the carb is behaving like it should. Idle, accelarator, and everything seems to be a lot smoother.

I’m going to drive it around and see if there is any noticeable imrpovement with the mileage and report back. Thank you to everyone who is able to not only help me diagnose, but also learn a whole lot about my carburetor. Hopefully, I’ll be able to report back some good news!


Sent from my iPhone using Tapatalk

 

[DirtDonk]

Hey, a great report so far!
Sounds like a success, even if your mileage doesn’t go up drastically.
But we are looking forward to that next mileage report.