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holley 1920 carb
https://slantsix.org/forum/viewtopic.php?t=25276		 Page 1 of 2
Author:  newport77 [ Mon Oct 08, 2007 10:42 pm ]
Post subject:  holley 1920 carb
Guys-

I have a 73 Scamp with a 225 and a Holley 1920 carb.
I've been trying to figure out why my gas mileage seems to be lower than it should be. i.e. when I first got the car, I could get 24 mpg on a highway trip. Now I get about 20 on a highway trip. I have checked everything I can think of tune up wise, and already rebuilt the carb once a while back.

I've been looking into the power/economizer valve in the carb to see if that is the source of the issue. I see that it has a little vacuum passage that comes to the top of the carb and pulls up on a diaphragm, which pulls up on a rod, that releases the power valve.

What I have noticed is that the vacuum passageway that leads to the power/economizer diaphragm goes to two different vacuum ports on the bottom of the carb. One of the ports is much larger than the other.

However, what I have found is that the carburetor base gasket covers over the larger of the two vacuum passageways on the underside! I have no idea if it is supposed to be this way or not! Would anyone know, by any chance?

The other, much smaller opening isn't directly covered by the gasket like this.

The thing is, I am wondering if a lack of a strong vacuum signal to the economizer diaphrgam could be causing the power valve to be slightly on all the time, and therefore causing more gas to be used than it should be.

Thanks for any thoughts!
-Scott
Author:  emsvitil [ Mon Oct 08, 2007 11:04 pm ]
Post subject:  ethanol
in the gas..............
Author:  1969ronnie [ Tue Oct 09, 2007 5:45 am ]
Post subject: 
hi, ethanol based fuels. you need more ethanol fuel to get the same amount of energy btu's than reg gas. some reported cases up to 30% less mileage with ethanol. 24 mpg vs 20 mpg sounds about right assuming everything is in tune with your engine. ron
Author:  newport77 [ Tue Oct 09, 2007 6:42 am ]
Post subject:  really?
Putting ethanol in the gas really reduces the fuel mileage by that much??

I remember wen they were putting MTBE in the fuel in the winter time, that was supposed to have reduced fuel economy, but I didn't know putting ethanol in (which was in the summer, I don't know if they do it year round or not) would reduce fuel economy also. At least it doesn't seem like it on my other cars.

So it reduces your gas mileage and you pay more per gallon as well... wonderful.
Author:  SlantSixDan [ Tue Oct 09, 2007 6:48 am ]
Post subject: 
Starting in '73, the holes in the carb base gasket don't serve as vacuum ports to the carburetor, so that's probably not it. Do you lose MPG with ethanol in the fuel? Yes. Do you lose 20 percent (20 vs 24 mpg)? No, you lose between 3 and 10 percent, depending on the fuel blend and driving conditions. Something else is the matter. You may want to look more carefully at your distributor advance mechanisms to ensure they're working right, check for timing chain stretch, make sure your speedometer calibration is accurate (because this affects the odometer reading and therefore the "M" part of your MPG calculations!), and make sure the choke and/or thermostatic air cleaner damper aren't staying in the "cold engine" positions too long.
Author:  newport77 [ Tue Oct 09, 2007 7:03 am ]
Post subject:  thanks
Thanks Dan.
I think you're right- something else isn't right.

I thought it was weird that some of the ports at the bottom of the carb were covered over, but if that's how it's supposed to be, well, OK then.

I'll have to check my odometer, but if it is off, I don't think it's by much. It seems pretty accurate.

I am certain that at least the base timing is correct (actually a little advanced, which should help mileage). I know the vacuum advance is at least operational. Whether it's accurate, I don't know.

How would I find out what the advance curve for the mechanical advance and the vacuum advance would be?

I will say this- the car runs just fine. There's no driveability problem that would point to a particular area to look at.
-Scott
Author:  Aggressive Ted [ Wed Oct 10, 2007 7:04 pm ]
Post subject: 
I have used a dial in timing light which is very quick to do or a stock timing light but you need to file extra 10 degree marks into the balancer.

Unplug the advance and check the mechanical timing, then plug in the advance and check them together. You can then subtract the difference to see how much vacuum advance your getting.

I like to check it at 2000 and 2500 rpm. I cruise at 2500, 60 mph in my Swinger.
Normally I never see 3000 rpm unless I am having fun passing a Mustang. :)
Author:  newport77 [ Wed Oct 10, 2007 7:26 pm ]
Post subject: 
OK, I'll try that. Thanks.
Author:  newport77 [ Tue Oct 16, 2007 8:58 pm ]
Post subject:  update...
OK Guys-

To recap, my Scamp's gas mileage has dropped off from 23-24 highway to 20-21 highway, and I was trying to figure out why.

Here is what I have checked based on your suggestions, and my own thoughts as well.

-main jet in carburetor is #612 as is factory spec

-power valve is pulled up fully at 12 in Hg (checked with carb off car using vacuum pump)

-thermostatic air door and choke are fully off when engine is warm

-odometer is accurate- actually reads 0.2 mile higher than it should for every 10 miles driven (meaning my gas mileage estimates are actually higher than they should be, not lower).

I checked my vacuum and mechanical advances.
If anything, I have more advance than I need, not less.

The vacuum advance holds vacuum and it starts working at 7 in Hg, maxes out at 11 in Hg.

Factory spec is 0.5 - 3 degrees of vacuum advance at 7 in Hg. I get 7 degrees.
Factory spec is 7-10 degrees of advance at 11 in Hg. I get 17 degrees.
17 degrees is the maximum vacuum advance I get.

Mechanical advance:
factory spec says at 2000 distributor rpm (which is the same as engine rpm right??) I should have 12-14 degrees of advance.
I get 15.5 degrees.

My base timing is 2.5 degrees instead of the factory 0 degrees.

So with everything hooked up properly at 2000 rpm, my total advance is 35 degrees, instead of factory spec 19-24 degrees. Having MORE advance than you should won't hurt mileage, will it?

The car pulls 17 in Hg at idle in drive (650 rpm), and 20 in Hg at 2000 rpm (in Park). I did not check for timing chain slack (not sure how w/o pulling the cover) but I assume I would not have readings like this if the timing chain had slack.

Here is something weird- and I don't know if it's just coincidental or if this could really happen. Checking back through the records I have kept on what my gas mileage is, it seems like my mileage dropped off after I adjusted the valves on the car. They had been loose, and I re-set them so they are perfect. Could that somehow have had an effect??

I don't know what else to check. Cap/rotor/plugs look fine.
Thanks for any thoughts!!
Author:  SlantSixDan [ Tue Oct 16, 2007 9:18 pm ]
Post subject: 
Pop off the distributor cap and manually rotate the engine in one direction until you see the distributor rotor begin to turn, then see how far you can manually rotate it the other direction before the distributor rotor starts turning. If there is any perceptible lag, the T-chain is slack and needs replacement.

Are your EGR and OSAC systems still operational on this '73, or have you disabled/bypassed them?

Is your thermostatic air cleaner hooked up and working correctly?

Is your manifold heat control "heat riser" valve working correctly?

A 612 (#61, middle bin) jet is a bit large; if you are at or near sea level you ought to be able to jet down a little to save on fuel. A #58 jet might be worth trying.

I'm also curious how old the float is in this carburetor, and if the float counterspring is properly in place, and if you're getting any fuel leakage from the bowl gasket.
Author:  emsvitil [ Tue Oct 16, 2007 9:27 pm ]
Post subject: 
Specs for the distributor are in distributor degrees and distributor rpm.

Both are half of crank degrees and crank rpm.
Author:  newport77 [ Wed Oct 17, 2007 6:54 am ]
Post subject: 
OK guys, thanks.

It looks like I may have more mechanical advance than spec, also (which is not necessarily bad).

I'll check for timing chain slack the way you suggested, Dan.

And yes, you are right- I have bypassed OSAC and the EGR. I did this because the car could not get out of it's own way with them connected. I did this right after getting the car. Even with them bypassed, my mileage was still OK early on.

I'll double check the heat riser. The thermostatic air cleaner I'm sure is OK.

I'm pretty sure the float is original. I'm sure that's a possibility- though the carb doesn't seem like it's flooding as far as I can tell. The little counterspring I checked to make sure it went through the little hole in the float arm.

The bowl gasket was leaking at one point, so I replaced it. I am not sure how long ago that was now, though, so I might need to re-check the mileage to make sure.

I'll have to see if I can locate a smaller jet...

And thanks for all of the great ideas!!!
Author:  Aggressive Ted [ Wed Oct 17, 2007 7:41 am ]
Post subject: 
I run extra loose on the valve lash per Doc's suggestion (.012 and .022). It is worth the extra mileage, approximately 2 miles per gallon.

Like Dan says, more heat, especially since it is getting colder out. That is worth another mile or two. I got the best mileage when we were having 90 degree weather.

I tried a 61.5 once. That was extremely rich and kind of a waste of fuel and power in the colder climate (40 to 50 degrees). Might be fun at the track though, drag racing. For a commuter, a 57 is a little lean, a 58 is nice and a 59 is fun for passing quickly, the power is right there. I have tried all three sizes again recently.

I run allot more timing, 43 to 45 degrees overall at 2500 rpm. Initial is set at 20 degrees.
Author:  SlantSixDan [ Wed Oct 17, 2007 8:46 am ]
Post subject: 
Quote:
I run extra loose on the valve lash per Doc's suggestion (.012 and .022). It is worth the extra mileage, approximately 2 miles per gallon.
Mmmm...no, the difference between 0.010/0.020 and 0.012/0.022 is not going to give or take 2mpg.
Quote:
I tried a 61.5 once.
The 3-digit jet numbers have a 1, a 2, or a 3 as the final digit. The first two digits indicate the jet number (61) and the final digit indicates whether it's in the middle of the flow spec for that jet size (2), or on the low end (1) or on the high end (3). They are OE jets only; service replacement jets only have 2-digit numbers. There was no 615 or 61.5 jet; what'd you find and where'd you find it?
Author:  newport77 [ Wed Oct 17, 2007 9:54 am ]
Post subject: 
What's interesting to me is that the #612 jet is factory spec, but it sounds like a lot of people use smaller ones.

I wonder if they put in a big jet in '73 to compensate for the first year of OSAC and EGR.

Can I buy a new jet from someplace, or is it a situation where I have to pick up a bunch of junkyard carbs and hope I come across some smaller jets?

I will say this- the way it is, the car has pretty decent pickup and good high speed response. Not that it has to stay exactly that way. The mileage is a bit more important to me right now.
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