Holley 1850 -2 Rebuild, Low MPG

[seely]

Hey everyone, I recently did a rebuild on my Holley 1850-2 and after tuning it, my 75 seems to be running well. The only catch is, it has horrible gas mileage. I drove to and from my parents house a couple of times this week (about 15 miles each way for a total of 60 miles) and ran out of gas close to my home and had to switch to the front tank.

I’ve been tracking it since and calculated 3.5 mpg the last time I filled up. Any ideas on where to look to see what is causing this?

I’m this close to just buying a new carburetor and wiping my hands of this old one.

 

[Madgyver]

idle screw setting, accellerator pump leaks.. my 1st thought. 1850 is a good carb just not for the bumpy bumpy stuff.

 

[seely]

Some additional info:

I don’t see any gas leaking at the fuel pump or fuel lines/tank. After sitting for a couple days, the engine will crank right away and then die, almost like fuel is leaking down the barrels into the manifold. I then need to crank quite a bit before getting it started, like the fuel bowls are empty. I’m wondering if it is really getting 3.5 mpg on the road, or if it’s dripping gas while sitting and then burning it off on the first crank.

Relatively new to all this, so I’m not sure if this is feasible or not.


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[seely]

idle screw setting, accellerator pump leaks.. my 1st thought. 1850 is a good carb just not for the bumpy bumpy stuff.

Idle screw is at 1.75 turns from seated (on both sides). I put in a new accelerator pump diaphragm, how could I confirm a leak coming from it?

I don’t see any gas on the manifold underneath it.

 

[seely]

I did a little more checking and saw that the discharge nozzle doesn’t have any gaskets. I bet whatever was left of them dissolved when I did the chemical soak. I believe there should be two, one on top and one below on the nozzle. I am going to get new ones tomorrow, as I don’t believe any came in the rebuild kit.

Do you think this would cause my poor mileage from leaking gas into the throttle plates, or is there something more major that I should be looking for?


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[Tiko433]

I think you’re on the right track with the those gaskets, definitely fix what is causing the leak down. Then check what MPG you have

 

[bmc69]

I did a little more checking and saw that the discharge nozzle doesn’t have any gaskets. I bet whatever was left of them dissolved when I did the chemical soak. I believe there should be two, one on top and one below on the nozzle. I am going to get new ones tomorrow, as I don’t believe any came in the rebuild kit.

Do you think this would cause my poor mileage from leaking gas into the throttle plates, or is there something more major that I should be looking for?


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I can't see how missing gaskets on the discharge nozzle could possibly allow leakage from the fuel bowl.

The most common source of residual fuel leakage from the bowl of an 1850 is through the power valve.

 

[pcf_mark]

Find a friend with a wide band O2 sensor they are priceless when it comes to tuning. The price has come down a lot worth having if you want to understand what your carb is really doing.

Sounds like you already looked for the obvious leaking places from lines and fittings. Have you checked your dipstick? If it stinks of oil and the level is higher you may have a leaky fuel pump that is leaking fuel into the engine. Unlikely but that has been reported before.

On the carb side I assume you used the stock jets which I think are 64 or 66. That will control at a high level the fuel available to all the circuits. Generally if you missed a gasket or installed it upside down (hard to do) you will get poor performance and obvious leaks. With the engine running use a mirror and look down into the carb to see if it is syphoning fuel from anywhere other than the roubd boosters. For example if you are missing gasket on the accelerator pump nozzle you may see a stream of liquid / vapor coming from that area.

Also maybe double check your math. My stock tank was really tiny i think around 11 gallons. A 60 mile trip would use around half that tank or 5 gallons. That is close to 15 mpg.

 

[Broncobowsher]

Air cleaner off, look down the top while idling. Is fuel dripping off the boosters? Especially the secondaries? Garbage holding the float open will do that.
Choke opening?
How sooted up are the plugs? Are they all about the same? Maybe only one side of the carb is crazy rich and the other side is fine? (dual plane makes makes one side of the carb feed #1,4,6,7 and the other side do the others).
Did you touch ANYTHING with the ignition when you worked on the carb/things suddenly got worse

 

[seely]

Find a friend with a wide band O2 sensor they are priceless when it comes to tuning. The price has come down a lot worth having if you want to understand what your carb is really doing.

Sounds like you already looked for the obvious leaking places from lines and fittings. Have you checked your dipstick? If it stinks of oil and the level is higher you may have a leaky fuel pump that is leaking fuel into the engine. Unlikely but that has been reported before.

On the carb side I assume you used the stock jets which I think are 64 or 66. That will control at a high level the fuel available to all the circuits. Generally if you missed a gasket or installed it upside down (hard to do) you will get poor performance and obvious leaks. With the engine running use a mirror and look down into the carb to see if it is syphoning fuel from anywhere other than the roubd boosters. For example if you are missing gasket on the accelerator pump nozzle you may see a stream of liquid / vapor coming from that area.

Also maybe double check your math. My stock tank was really tiny i think around 11 gallons. A 60 mile trip would use around half that tank or 5 gallons. That is close to 15 mpg.

It was actually 60 miles when I ran out of gas in my 11 gallon tank. I then drove with the front tank and after 15 miles, the gauge was showing that it was low. Filled up with 4.5 gallons, and which is where I calculated the mileage.

I’ll try to check down the carb with a mirror and pull the pluga to see what’s going on there. Thank you everyone for the tips!

 

[Oldtimer]

Did it take 11 gallons to fill the main tank, or did the pump stop sucking before tank was empty?

 

[MS73HD302]

Transfer slots set up properly? My 600 barely takes 1/2 on idle screws mine also acted like this when the bass check ball retainer came lose in the accelerator pump housing… run good till you hit the throttle then would die.

 

[seely]

Did a bit more troubleshooting and squirters are working well, I don’t see any noticeable leaks, but the idle screws are not killing the engine when both are seated, which is telling me the idle circuit is being bypassed. I took the idle speed screw out as far as I could without the engine stalling, and even at a lowrr RPM, no idle circuit.

I might pull the front bowl and triple check that no fuel is getting past the power valve or jets, but last time I checked it, everything looked good. Any other ideas on where to look?

 

[seely]

Did it take 11 gallons to fill the main tank, or did the pump stop sucking before tank was empty?

The fuel gauge was bottomed out and I filled up close to 11 gallons when I took it to the pump, but I appreciate the tip!

 

[Broncobowsher]

Check your oil. Really. With enough gasoline in the oil the PCV can provide enough vapors to keep the engine fueled at idle. How do you get that much fuel in the oil? bad fuel pump.

Pull the sight plugs on the fuel bowls (both of them) and see if fuel is running out. Needle and seat not stopping fuel will fill a bowl and drool fuel down the engine.

 

[seely]

Check your oil. Really. With enough gasoline in the oil the PCV can provide enough vapors to keep the engine fueled at idle. How do you get that much fuel in the oil? bad fuel pump.

Pull the sight plugs on the fuel bowls (both of them) and see if fuel is running out. Needle and seat not stopping fuel will fill a bowl and drool fuel down the engine.

I actually just did an oil change and have recently installed a new fuel pump. I did double check the rear sight plug and saw that the float was too high. I adjusted it so that fuel is not coming out of thw sight plug until I rock the car back and forth. I took a video of the carb and it doesn’t look like anything is leaking.

I’m wondering if my mileage problem might be a misfiring spark plug or something ignition related and am thinking of double checking my timing and spark. This wouldn’t explain the idle screws not doing anything, but I’m not sure where else to look.


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[1970 Palmer]

I actually just did an oil change and have recently installed a new fuel pump. I did double check the rear sight plug and saw that the float was too high. I adjusted it so that fuel is not coming out of thw sight plug until I rock the car back and forth. I took a video of the carb and it doesn’t look like anything is leaking.

I’m wondering if my mileage problem might be a misfiring spark plug or something ignition related and am thinking of double checking my timing and spark. This wouldn’t explain the idle screws not doing anything, but I’m not sure where else to look.


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You have already been given some good ideas to check.

Here's some more.

Pull the carb off of the manifold. Turn it upside down (over a pan), and check the rear throttle shaft adjustment. Make certain that the secondary throttle plates are fully seated when the shaft hits the adjustable threaded stop. The stop is very sensitive even a 1/4 to a 1/2 turn. If the plates are hung open just slightly, the idle air will enter the secondary side of the carb and you will lose the idle air screw adjustment sensitivity. If held open it's just like a big vacuum leak.

While the carb is off pull the primary bowel and take a look at the power valve for a blown diaphragm. A leaking power valve will give many of the same symptoms you have stated.

Check the spark plugs for the color burn, and which cylinders have a different color. The color will tell you a lot about where to look for your problem. Check initial timing. Make sure you have "a working" vacuum advance ignition system. Same by checking your mechanical advance ignition system.

What's the air filter look like?

It only takes a few minutes to make these system checks.

 

[DirtDonk]

Is your speedometer correct? Do you have larger tires and stock gearing?
Any changes to correct the speedometer and odometer for this tire size difference?

Even though your fuel math sounds a little fuzzy, it shouldn’t make that much difference. So you may very well be getting that horrible mileage.
But the only way to really tell for sure precise, fuel mileage is to put a known amount of fuel in one tank, note the odometer reading, then drive until the tank runs out.
if you have for example 5 gallons in that tank and you only get 20 miles (accurate miles) then you know you’re getting 4 miles per gallon. But these numbers are only good if your odometer is correct and you have a an exact known quantity of fuel.
Having nothing to do with the reading on the gauge.

 

[seely]

I agree, I calculated the mileage with a short distance after I ran out of gas, 15 miles in 4 gallons. It’s very possible that something else was going on that contributed to that number in such a short distance (possibly the gas pump I used detected a full tank later than the last time I filled it).

I am going to keep driving it and checking what others have suggested and will report back on anything new I discover.

Thank you everyone for all of the different things to consider, I love this community for this very reason!

 

[Madgyver]

For me I would just swap carbs with a good running one to find out differences. It is an option for me as I have several running rigs and carbs to mess with.
a 650 double pumper would be fun to play with but not so good on the bumpy bumpy roads.