Milage problems...as in, sucks fuel like a tank!

Duke · May 17, 2008

Morning all,

I am getting really BAD gas millage. Something like 10 or less to a gallon. I have a four barrel Holley (1850-2, 600 cfm) on a high rise manifold. This is on the highway too. I know that 4.22 rear end does not help but man.

The engine is a 260 bored .030 over with a "hotter" cam. I do not know what cam it has at this time.

My question is - you guys running a Holly four barrel on your 260, what is your primary main jet size. I have not cracked open my carb to see what I have but I would like to know what I should have. I do have the original Autolite two barrel, will the primary jets from it fit the Holley?

Thanks,
Duke

64beam · May 17, 2008

Hi Duke,

Are you sure that it is your carby that's causing the excess fuel useage? Is the ignition system working OK? Has it always used that much fuel since you have had it? The other option is that the 600 is too much carby even for the hotter 260 (just a thought).

Regards, Robin.

husky drvr · May 17, 2008

Duke,

I will guess that with the -2 designator you have an earlier production carb that if built prior to 1992 would not have the power valve blowout protection that is built into later carbs. You should check the power valve(s) prior to changing the jetting ( both for blown diaphragms and proper opening point ).

As Robin says, even on a warmed up 260 a 600 cfm carb is a bit large. Also since the 1850-2 is listed as a universal carb, then it was probably originally optimized for a mild 350 type usage.

You might try this link for more info:

http://www.holley.com/TechService/Library.asp

If you were to consider a new carb and wanted to stay with a Holley you might consider this link. This carb should have benefits for a sports car just as much as for its designed purpose.

http://store.summitracing.com/partdetail.asp?autofilter=1&part=HLY-0-90470&N=700+115&autoview=sku

Good luck,

V_Mad · May 17, 2008

Duke
I am running a 465 cfm carb (1848-1), which comes with a stock jet size of 57 primaries which I think needs to be reduced for my stock 260. Your 1850-2 (600 CFM) comes with 65 primaries, which is a lot bigger than I am running. That plus your axle ratio surely must be the reason your car is drinking heavily.

Duke · May 17, 2008

OK guys,

Great tips and info!

Did the power valve test and they are good.

I pulled the bowl to check the primary jets and they are 66's.

So I need some WAY smaller jets. What do you think about 55's?

I gotta get this fuel consumption issue under control before I drive the car to CA next month.

husky drvr · May 17, 2008

Duke said:
OK guys,

Great tips and info!

Did the power valve test and they are good.

I pulled the bowl to check the primary jets and they are 66's.

So I need some WAY smaller jets. What do you think about 55's?

I gotta get this fuel consumption issue under control before I drive the car to CA next month.

Duke,

Did you check your idle and cruise manifold vacuum readings by any chance?

Check the note under MAIN METERING SYSTEM at this link:

http://www.holley.com/data/TechService/Technical/Carburetor Tech Info.pdf

You should pick up a book on Holley carbs. The one I have has a good troubleshooting chart to work with on solving problems.

Duke · May 17, 2008

I just bought a 1848-1 563 C0AE-9510-N off of ebay. It has a manual choke and I believe will be the proper size for my current engine.

All of the carb calculators I have found tell me that 389.063 CFM (@ 83 % VE) is what my engine needs. So the 1848-1 at 465 cfm should me more than enough.

V_Mad · May 17, 2008

I didnt want to say it before, but I think the 600 cfm is a bit oversized for the 260 (unless its quite highly tuned); you should find the 465 much better. Make sure your fuel pump is not flooding the carb. Holleys dont like much more than 6 PSI and some of the Facet ones push out too much pressure.

64beam · May 17, 2008

Hi,

The Holley interactive carburettor selector on their site is a bit weird. When you punch in your details, it does not matter whether you choose a stock or highly modified engine and still comes up with the same carburettor selections

. Even then the 1848-1 (465cfm) does not even get a mention.

Regards, Robin.

alpine_64 · May 18, 2008

Duke,

Even if you change the carb down if you have a hot cam and that 4.22 I am thinking you are going to be lucky to see anything over 17mpg.. and if you put that right foot into it...

Duke · May 18, 2008

alpine_64 said:
Duke,

Even if you change the carb down if you have a hot cam and that 4.22 I am thinking you are going to be lucky to see anything over 17mpg.. and if you put that right foot into it...

I agree. I am pulling the rear axle next week to have a 3.07 gear installed.

agmason · May 18, 2008

How are you measuring your gas mileage? With the 4.11 rear gear ratio the odometer and speedometer will be off unless it was changed. Have you checked the linkage for the additional barrels of the carb, maybe they are stuck open or opening too soon. The Edelbrock opens via engine vacum and I think the Holley is by linkage.

64beam · May 19, 2008

dude234 said:
How are you measuring your gas mileage? With the 4.11 rear gear ratio the odometer and speedometer will be off unless it was changed. Have you checked the linkage for the additional barrels of the carb, maybe they are stuck open or opening too soon. The Edelbrock opens via engine vacum and I think the Holley is by linkage.

Hi,

Duke actually has the Holley 1850 fitted to his car, which by the part number has vacuum secondaries. I believe he has checked the carburettor operation and is working correctly.

Regards, Robin.

buckalpine3 · Nov 10, 2009

mpg

Duke,
Looking through these old threads, I wondered what your mpg is with the new carb and gear?