8.5 mpg help

surfsup · 02-27-2005, 07:47 AM

I’m getting 8.5 mpg , the motor is an 85 all stock with the exception of the exhaust, 2 ½ cat back, and a Mallory coil. Everything else is stock. 5 spd, 273 rear..This is what I have done to the car so far.
Replaced cap rotor wires plugs, cleaned egr plate, rebuilt carb, the only thing I changed on the carb from stock was the power valve, orig was a 2 stage, I put in a single stage 6.5.. Replaced all filters. All emissions are disconnected except for the egr valve. I have been over the motor several times to adjust the carb, reset floats and mixture, tried setting the timing at different degrees, nothing. If some one could maybe shed some light here on what’s causing this rotten mileage, would be great. I drive easy, no heavy foot.

Gearhead999 · 02-27-2005, 09:05 AM

How does the engine run?? Are the spark plugs reading good??

How about the float level?? Has the engine backfired, blowing the power valve??

I would guess that either the float level is way off or the powervalve is bad.

surfsup · 02-27-2005, 12:09 PM

At first the plugs were black, sooty, not oil, I found the egr plate clogged with crap, I cleaned all the carbon out, and now the plugs look normal. I also changed the plugs, first I had the bosh platinum, then I put the bosh copper core in. Float level was set dry when I rebuilt the carb, then I set the float level at idle with the side plug out. I replace the two stage power valve with a new single stage. The car has never back fired. The car runs great, no hesitation, no lag when accelerating, The carb kit i bought was new, but an older kit, if the power valve I put in went bad, Is there a way I can test the power valve?

Rev · 02-27-2005, 04:15 PM

If the power valve was leaking, I don't think the engine would idle well. Same with the needle and seats leaking (the float level wouldn't work). What sort of vacuum are you getting at idle? If the PV is actuated most of the time during normal driving (due to low vacuum), that could account for the bad mileage. Something is wrong somewhere to get 8.5 mpg without the lead foot.

Rev

grey88gt · 02-27-2005, 08:26 PM

This might be way off, but if the plugs look good and its ideling right and you got good manifold vacuum say 16-22+ at idle, id say the fuel is going some where else. Have you checked to see if the diaphram in the fuel pump is cracked. Over the years I have had three diffrent pumps, one on 460, a 400, and a 2.8 v6 crack, the engine still run great but have poor fuel mpg. All of them was squirting fuel out the weep hole on the side of the pump. Just a guess probally not your prob but something ive seen before. Good luck!!!!!!!

surfsup · 02-27-2005, 10:01 PM

I believe the Manifold vacuum is between 16- 22. I can take an actual reading tomorrow and post the number. If your talking about the pvc valve, it’s connected directly to the carb, and pulls a strong vacuum at idle, if I pull the pvc out of the valve cover, you can see the valve open and hear the suction, if I put my finger over the bottom valve completely, it will suck my finger in, and the motor will idle down a few rpms and still idle smooth. I don’t believe the fuel pump is leaking, I don’t see any wet areas, or smell gas.

grey88gt · 02-27-2005, 11:54 PM

Well I agree with Rev . If the power valve is opening to soon, it will cause low mpg. Factory power valves normally open when vacuum drops to around 6.5 ". I always run power valves that opened up at 8.5 " and run a little smaller main jet. My old 400 in a 75 Cougar got 14-16 with a 8.5 " power valve # 61 jet and the car is very heavy 4000 + lbs. Anyway enough of my BS IAm outa ideas for ya again good luck!

surfsup · 02-28-2005, 07:08 AM

Grey88gt, Thanks for the reply, your post didn’t make sense at first about agreeing with rev, so I read his post over, I thought he was talking about the pcv valve when i replied, I now see it’s the power valve. Like i said earlier, i changed mine to the single 6.5, maybe i should put back the two stage, which is 11.5 - 6.5. The primary jets are 622, and the secondary has a metering block with out jets. The carb i have is the 4180.

grey88gt · 02-28-2005, 09:18 PM

Maybe this will help. When ever you increase load or open the throttle, manifold vacuum will decrease until the load is mantained or goes away. When you take off or want to go faster you decrease manifold vacuum. If you have say a 6.5 " power valve in your carb, the vacuum has to drop to that point before the power valve will open thus enriching the fuel . So the bigger the number on the power valve the sooner it will open and the longer it will stay open thus using more gas. You might try droping a jet size or so. The jet you have is 62 and the extra 2 at the end I do belive is the grade or quality of the jet (someone correct me if Iam wrong ) . I think I saw a post on here , where someone explained that. I still dont a 62 jet is big enough for your mpg to be that low.

mojoffdawal · 03-01-2005, 11:06 PM

Man, I was just reading this yesterday and i noticed today that my mustang ate up around the same amount of gas. Another symptom that I don't believe anyone mentioned here is that (occasionally) when I am stopped and neutral, t hear the engine rev up and down without my foot on the pedal. Any diagnosis would be helpful as I'm not mechanically inclined yet, so I wouldn't want to take it to the shop and get ripped off.
Oh yeah, the mustang is a pretty much stock 86 gt

surfsup · 03-02-2005, 10:36 AM

So what you are saying is a two stage should consume more gas then a single stage. But I wonder if that holds true for this carb. I’m not familiar with the 4180 carb except for what has been written, this carb is suppose to be an economy, low emission and performance carb. And in one of the web sites I found, say to change the two stage to a single for performance. So I wonder if the two stage pv is what makes this an econo carb, which would use less gas, and because of the design changes that have been made, the power valve would not operate in the manner that you have described. I changed the single stage back to the two stage; I’ll run a tank of gas and see if my mileage is any better.

Capri306 · 03-02-2005, 04:00 PM

Sounds like a classic case of a vacuum leak. Whether it's in the carburetor itself (i.e. the power valve, gaskets, etc), a manifold leak, or a popped-off line will take further examination. I'm willing to bet it's something simple, though. I'd change the power valve back to what it was before or just *one* click up. A faulty spring or dirty seat on the power valve could cause problems as well. I'm agreeing with Rev that an unseated power valve would cause a rich idle, but it's also possible that the spring is shot or there's a gasket not sealing off somewhere.

I believe the notion of using a 2-stage power valve as a means to have both power and economy is just hyberbole for a smoother power transition between loads (vacuum levels) upon the engine. Fuel injection made this happen with simple programming, but in this case dealing with a caburetor, it's all a matter of pressure differentials.

Good luck!

grey88gt · 03-02-2005, 11:40 PM

I agree with you Capri. But if he takes a vacuum reading with no load and its at least 15+ inchs and a steady needle that almost elimanates a bad intake gasket or a line popped off somewhere. But as you state it very well could have an internal leak in the carb.

surfsup · 03-03-2005, 08:58 AM

The carb was rebuilt, . I did change the power valve back to the two stage, one thing I noticed on the one I took out ( the new single stage ) the diaphragm looked distorted, wavy, I know it wasn’t like that when I installed it, so I wonder if the kit I bought was to old and the diaphragm was dried out, as far as vacuum lines go for a possible leak, I have very few under the hood, the vacuum advance and pcv valve to the carb, the egr valve, the brake booster, and I think that’s it. I also replaced the intake gaskets before i installed the motor along with some other things i did. At idle the vacuum gauges reads 17 and it’s steady, no movement at all, at 1500 rpm the gauges reads 19.5 and steady. The car idles at 700 rpm, and seems smooth. Maybe fuel injection is the way to go.

grey88gt · 03-03-2005, 09:19 PM

Sounds like you got a good healthy engine. Have you tried dropping a jet size or maybe two? I put 56s or 58s in my dads 351w when we built it for a 66 Ford F100. But its only a 2-barrell carb (2100 Motorcraft) the engine has a mild rv cam. It runs great. I tend to think its all in your carb.

02-27-2005, 07:47 AM	#1
surfsup Registered Member Join Date: Sep 2004 Location: jersey Posts: 147	8.5 mpg help I’m getting 8.5 mpg , the motor is an 85 all stock with the exception of the exhaust, 2 ½ cat back, and a Mallory coil. Everything else is stock. 5 spd, 273 rear..This is what I have done to the car so far. Replaced cap rotor wires plugs, cleaned egr plate, rebuilt carb, the only thing I changed on the carb from stock was the power valve, orig was a 2 stage, I put in a single stage 6.5.. Replaced all filters. All emissions are disconnected except for the egr valve. I have been over the motor several times to adjust the carb, reset floats and mixture, tried setting the timing at different degrees, nothing. If some one could maybe shed some light here on what’s causing this rotten mileage, would be great. I drive easy, no heavy foot.

02-27-2005, 09:05 AM	#2
Gearhead999 Registered Member Join Date: Sep 1998 Location: Oviedo, FL Posts: 992	Re: 8.5 mpg help How does the engine run?? Are the spark plugs reading good?? How about the float level?? Has the engine backfired, blowing the power valve?? I would guess that either the float level is way off or the powervalve is bad. __________________ "Support our Troops" Dave 1968 Cougar 2004 Thunderbird 2007 F150 Harley-Davidson, SuperCrew 1986 LTD 1997 Ranger 1992 Honda Civic

02-27-2005, 12:09 PM	#3
surfsup Registered Member Join Date: Sep 2004 Location: jersey Posts: 147	Re: 8.5 mpg help At first the plugs were black, sooty, not oil, I found the egr plate clogged with crap, I cleaned all the carbon out, and now the plugs look normal. I also changed the plugs, first I had the bosh platinum, then I put the bosh copper core in. Float level was set dry when I rebuilt the carb, then I set the float level at idle with the side plug out. I replace the two stage power valve with a new single stage. The car has never back fired. The car runs great, no hesitation, no lag when accelerating, The carb kit i bought was new, but an older kit, if the power valve I put in went bad, Is there a way I can test the power valve?

02-27-2005, 04:15 PM	#4
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	Re: 8.5 mpg help If the power valve was leaking, I don't think the engine would idle well. Same with the needle and seats leaking (the float level wouldn't work). What sort of vacuum are you getting at idle? If the PV is actuated most of the time during normal driving (due to low vacuum), that could account for the bad mileage. Something is wrong somewhere to get 8.5 mpg without the lead foot. Rev __________________ '66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi. O.B.C. #2 '66 coupe

02-27-2005, 08:26 PM	#5
grey88gt Registered Member Join Date: Feb 2004 Location: Skiatook, Oklahoma Posts: 193	Re: 8.5 mpg help This might be way off, but if the plugs look good and its ideling right and you got good manifold vacuum say 16-22+ at idle, id say the fuel is going some where else. Have you checked to see if the diaphram in the fuel pump is cracked. Over the years I have had three diffrent pumps, one on 460, a 400, and a 2.8 v6 crack, the engine still run great but have poor fuel mpg. All of them was squirting fuel out the weep hole on the side of the pump. Just a guess probally not your prob but something ive seen before. Good luck!!!!!!!

02-27-2005, 10:01 PM	#6
surfsup Registered Member Join Date: Sep 2004 Location: jersey Posts: 147	Re: 8.5 mpg help I believe the Manifold vacuum is between 16- 22. I can take an actual reading tomorrow and post the number. If your talking about the pvc valve, it’s connected directly to the carb, and pulls a strong vacuum at idle, if I pull the pvc out of the valve cover, you can see the valve open and hear the suction, if I put my finger over the bottom valve completely, it will suck my finger in, and the motor will idle down a few rpms and still idle smooth. I don’t believe the fuel pump is leaking, I don’t see any wet areas, or smell gas.

02-27-2005, 11:54 PM	#7
grey88gt Registered Member Join Date: Feb 2004 Location: Skiatook, Oklahoma Posts: 193	Re: 8.5 mpg help Well I agree with Rev . If the power valve is opening to soon, it will cause low mpg. Factory power valves normally open when vacuum drops to around 6.5 ". I always run power valves that opened up at 8.5 " and run a little smaller main jet. My old 400 in a 75 Cougar got 14-16 with a 8.5 " power valve # 61 jet and the car is very heavy 4000 + lbs. Anyway enough of my BS IAm outa ideas for ya again good luck!

02-28-2005, 07:08 AM	#8
surfsup Registered Member Join Date: Sep 2004 Location: jersey Posts: 147	Re: 8.5 mpg help Grey88gt, Thanks for the reply, your post didn’t make sense at first about agreeing with rev, so I read his post over, I thought he was talking about the pcv valve when i replied, I now see it’s the power valve. Like i said earlier, i changed mine to the single 6.5, maybe i should put back the two stage, which is 11.5 - 6.5. The primary jets are 622, and the secondary has a metering block with out jets. The carb i have is the 4180.

02-28-2005, 09:18 PM	#9
grey88gt Registered Member Join Date: Feb 2004 Location: Skiatook, Oklahoma Posts: 193	Re: 8.5 mpg help Maybe this will help. When ever you increase load or open the throttle, manifold vacuum will decrease until the load is mantained or goes away. When you take off or want to go faster you decrease manifold vacuum. If you have say a 6.5 " power valve in your carb, the vacuum has to drop to that point before the power valve will open thus enriching the fuel . So the bigger the number on the power valve the sooner it will open and the longer it will stay open thus using more gas. You might try droping a jet size or so. The jet you have is 62 and the extra 2 at the end I do belive is the grade or quality of the jet (someone correct me if Iam wrong ) . I think I saw a post on here , where someone explained that. I still dont a 62 jet is big enough for your mpg to be that low.

03-01-2005, 11:06 PM	#10
mojoffdawal Registered Member Join Date: Feb 2005 Location: Los Angeles Posts: 10	Re: 8.5 mpg help Man, I was just reading this yesterday and i noticed today that my mustang ate up around the same amount of gas. Another symptom that I don't believe anyone mentioned here is that (occasionally) when I am stopped and neutral, t hear the engine rev up and down without my foot on the pedal. Any diagnosis would be helpful as I'm not mechanically inclined yet, so I wouldn't want to take it to the shop and get ripped off. Oh yeah, the mustang is a pretty much stock 86 gt Last edited by mojoffdawal; 03-01-2005 at 11:07 PM.. Reason: spelling

03-02-2005, 10:36 AM	#11
surfsup Registered Member Join Date: Sep 2004 Location: jersey Posts: 147	Re: 8.5 mpg help So what you are saying is a two stage should consume more gas then a single stage. But I wonder if that holds true for this carb. I’m not familiar with the 4180 carb except for what has been written, this carb is suppose to be an economy, low emission and performance carb. And in one of the web sites I found, say to change the two stage to a single for performance. So I wonder if the two stage pv is what makes this an econo carb, which would use less gas, and because of the design changes that have been made, the power valve would not operate in the manner that you have described. I changed the single stage back to the two stage; I’ll run a tank of gas and see if my mileage is any better.

03-02-2005, 04:00 PM	#12
Capri306 Moderator Join Date: Sep 1998 Location: Grand Rapids, Michigan, USA Posts: 1,001	Re: 8.5 mpg help Sounds like a classic case of a vacuum leak. Whether it's in the carburetor itself (i.e. the power valve, gaskets, etc), a manifold leak, or a popped-off line will take further examination. I'm willing to bet it's something simple, though. I'd change the power valve back to what it was before or just one click up. A faulty spring or dirty seat on the power valve could cause problems as well. I'm agreeing with Rev that an unseated power valve would cause a rich idle, but it's also possible that the spring is shot or there's a gasket not sealing off somewhere. I believe the notion of using a 2-stage power valve as a means to have both power and economy is just hyberbole for a smoother power transition between loads (vacuum levels) upon the engine. Fuel injection made this happen with simple programming, but in this case dealing with a caburetor, it's all a matter of pressure differentials. Good luck! __________________ Capri306, Moderator The Mustang Works Online 1979 Mercury Capri 1987 5.0L Mustang LX Notchback 1993 5.8L Eddie Bauer Bronco

03-02-2005, 11:40 PM	#13
grey88gt Registered Member Join Date: Feb 2004 Location: Skiatook, Oklahoma Posts: 193	Re: 8.5 mpg help I agree with you Capri. But if he takes a vacuum reading with no load and its at least 15+ inchs and a steady needle that almost elimanates a bad intake gasket or a line popped off somewhere. But as you state it very well could have an internal leak in the carb.

03-03-2005, 08:58 AM	#14
surfsup Registered Member Join Date: Sep 2004 Location: jersey Posts: 147	Re: 8.5 mpg help The carb was rebuilt, . I did change the power valve back to the two stage, one thing I noticed on the one I took out ( the new single stage ) the diaphragm looked distorted, wavy, I know it wasn’t like that when I installed it, so I wonder if the kit I bought was to old and the diaphragm was dried out, as far as vacuum lines go for a possible leak, I have very few under the hood, the vacuum advance and pcv valve to the carb, the egr valve, the brake booster, and I think that’s it. I also replaced the intake gaskets before i installed the motor along with some other things i did. At idle the vacuum gauges reads 17 and it’s steady, no movement at all, at 1500 rpm the gauges reads 19.5 and steady. The car idles at 700 rpm, and seems smooth. Maybe fuel injection is the way to go.

03-03-2005, 09:19 PM	#15
grey88gt Registered Member Join Date: Feb 2004 Location: Skiatook, Oklahoma Posts: 193	Re: 8.5 mpg help Sounds like you got a good healthy engine. Have you tried dropping a jet size or maybe two? I put 56s or 58s in my dads 351w when we built it for a 66 Ford F100. But its only a 2-barrell carb (2100 Motorcraft) the engine has a mild rv cam. It runs great. I tend to think its all in your carb.