Manta Outing, Terrible Gas Mileage

[tomking]

While I purchased my 73 Manta about two years ago, I have never taken it out of town, except by trailering to a couple of shows. I had found a fraternity brother that I hadnt seen in 40 years was living just up the road in Springfield MO so I contacted him and asked to take him to lunch. I decided to drive my Manta and check fuel mileage. So I filled up and left. I had worked on the Solex carb many times to correct a vacuum leak and had it running excellently. The idle is the best it has ever been, idling smoothly at 800 RPM. Had adjusted it so the hesitation was gone when opening the throttle and so I thought we are ready for the open road.
Now this is a 73 with 140K miles, automatic with airconditioning and cruise control. The air kept it almost comfortable but not quite; any backseaters would still be hot. It was about all it could do to run 70 mph and the RPM was 4K. I expected it to lose a good deal of speed going up hills but it didnt. As the miles clicked by I could see the gas gauge dropping like a rock! This I didnt like. So upon arrival in Springfield I stopped to top if off and check the mileage. 80 miles and 7.9 gallons! This sucks; well it sucks gas to be exact.
So as I drove to the resturant suddenly it began to miss and cut out. The tach was going erratic as well. So I pulled off and shut down. Waited a couple of minutes and fired it up; things are OK so I continued. But within a short distance the same thing. I pulled into a Hardees lot and shut down. I called my buddy and he came to pick me up for lunch. Afterwards I had him take me to Pep Boys before back to the car. I thought the likely culprit was the coil going out so I bought a new coil. I installed it and fired it up. I took off for home and never had the cutting out again so I felt like the coil was bad. But the mileage sucked nearly as bad going home as coming up, but I didnt run the AC.
I am puzzled why the mileage is so bad when the car is running better than it has since I bought it. It is timed correctly. It has a Crane ignition and CD spark box. Plugs are new as well as wires, rotor and cap. I dont think there are any vacuum leaks as it idles nicely and the throttle response is good.
Where should I start to find the problem? Thanks guys.

[neuropel]

ANY chance there could be a fuel leak somewhere? That's a LOT of fuel. No fuel odors, etc.? My manta was getting horrific mileage for a while, until I realized that the vent hoses at the fuel tank were not set up right and I was losing fuel. I would have never caught it except that after Carlisle this year my wife drove the car home and Mach1Tom, who was following, spotted fuel leaking. If you're really only getting 10mpg, something's WAY wrong I'd think.

Did it have a LOT of power? Perhaps someone dropped in a big block!

Todd

[Bill Hoffmann]

Could the brakes be dragging? I would lift each corner and see how freely each wheel turns.

I don't think the A/C should hurt much on a long trip. I once drove my '74 Manta on a weekend trip from Philly to upstate NY (not flat land). The car was fully loaded - four people with luggage, and the A/C was on the whole time. My first fillup revealed that I was getting 33 mpg, which was higher than normal. I still haven't figured that one out.

Bill

[oldopelguy]

If that much fuel was running through the engine it would be fouling plugs and you'ld be able to smell it in the exhaust. That's almost 3 times as much fuel going through the engine as it needs, it's got to be going somewhere.

Are you sure you were in third? I know my Kadetts have a taller rear end, but 4k seems a bit high to me for 70mph in third.

Did you check your oil level at all? I have seen a failed seal in the fuel pump dump fuel into the crankcase, but that much fuel should be squirting out somewhere.

[opelnut10]

Originally Posted by oldopelguy If that much fuel was running through the engine it would be fouling plugs and you'ld be able to smell it in the exhaust. That's almost 3 times as much fuel going through the engine as it needs, it's got to be going somewhere. Are you sure you were in third? I know my Kadetts have a taller rear end, but 4k seems a bit high to me for 70mph in third. Did you check your oil level at all? I have seen a failed seal in the fuel pump dump fuel into the crankcase, but that much fuel should be squirting out somewhere.

With an automatic and a 3:44 gear should put you at around 2800-3000 at 70 MPH. If you find no fuel leaks, check the oil as a faulty fuel pump can allow gas to go into the oil pan (Not Good for bearings) also the transmission converter and transmission may be slipping causing the engine to rev at high RPM's. Take the car onto the interstate and check the speedometer by the mile markers and a watch (at 60 MPH it should take 1 minute between mile markers. A slam wore out Manta should have a top end of 90-100 MPH something is sure not right somewhere.

[tomking]

All good suggestions for sure.
There were no odors of fuel either inside or outside nor when had the hood up. I saw no leaks or drips. I have an electric fuel pump with fuel pump location blocked off on block. I dont think the tach works correctly since I installed it. Some may recall I had issues with that in connection with the Crane ignition. I purchased the tach from Gene and he suggested I go back to points so it would work. I dont want to do that. So the RPMs may have been wrong. But I dont think I could have gotten it to 90 MPH either as 70 was nearly full throttle. Oil level was full as I checked before departure. I am sure it was in third gear. I didnt check the tranny fluid level but had previously and it isnt leaking in the garage so I was confident it was still full.
"
Did it have a LOT of power? Perhaps someone dropped in a big block! "
It didnt have THAT much power! The one thing I havent checked is the dizzy for proper advance. I will have to buy a new timing light as my old one doesnt have that advance feature. I agree it was using nearly three times the fuel it should. No black smoke at any time! I havent looked at the plugs.
Either the fuel is being burned or I am losing it. I will have to look again for leaks but again didnt smell anything at any time nor see any when stopped.
Everything is simple once you know the right answer; I havent found the right answer yet.

[Paul]

Originally Posted by Bill Hoffmann Could the brakes be dragging? I would lift each corner and see how freely each wheel turns. I don't think the A/C should hurt much on a long trip. I once drove my '74 Manta on a weekend trip from Philly to upstate NY (not flat land). The car was fully loaded - four people with luggage, and the A/C was on the whole time. My first fillup revealed that I was getting 33 mpg, which was higher than normal. I still haven't figured that one out. Bill

I got the answer for that one Bill. Your 1st fill up was without anyone in the car, so you got the tank full. The 2nd fill up was with the back of the car lower since people were sitting in the back seat. Therefore, you couldn't fill the tank up all the way.... an easy way to get better milage for 1 tank

[tomking]

I pulled the plugs just now and they are black, black and sooty. I am suspicious of the charcoal cannister hook up; so I went to the FSM. Mine is not hooked up correctly and I wonder if it is possible to pull liquid gas thru the cannister? Surely the hose connection at the tank that connects to the cannister does not have a drop tube in it? Surely it just picks up vapors from the top of the tank? If so I dont see how it could pull liquid. But none the less it is not properly connected to the carb. Plus there is supposed to be an orifice in the vac line to carb. My line is connected to the INTAKE manifold! Not by me, but there it is. Danged PO. I hope this is the problem, but I have not seen it spewing black smoke either idling or under way. But I guess some small amount of black smoke I probably would not see under way. So back to the garage to see about proper charcoal cannister hook up. More later.....

[tomking]

After researching how the charcoal cannister should be hooked up vs. what I have I dont think that is THE problem. It is A problem, but minor. The third port was open and with that open the intake couldnt have been pulling but a slight, very slight bit of vacuum at the cannister. Therefore that isnt the reason it is running so rich. I will need a new hose which I dont have to restore it to stock configuration, but that isnt my problem. I am pretty sure my jetting is bone stock as I recall comparing it to stock listings a few months ago. But I believe now the stock Solex carb is the problem. I dont know why/how/what. But it is running rich by the look of the plugs. By goosing the throttle I can see some black smoke out back. But it wasnt enough to see while driving down the road. Perhaps if I had been behind the car with someone else driving it I would see the tell tale black smoke.
A couple of months ago I tried and tried to make a Weber work but I could not stop a vacuum leak between it and the intake manifold. I tried different gaskets, thin ones, then thick ones. Tried with and without the heat shield and I could not stop the leak. I cut the threads deeper on the studs and even installed shorter studs but that didnt cure it either. The Solex didnt leak there but the Weber would so I gave up for then trying to get a Weber to work. But I believe something is wrong with the Solex so perhaps I will go back to trying to make the Weber work. That is where I stand now. Thanks.

[scrapman]

Hi Tom. My 73 Manta has a gas sucking problem as well, not as bad, but like yours not what it should be. Mine does have the Webber on. For a long time I was getting only 18 mpg and knew there was a problem. Checked everything I could think of and could not find a problem either. I did have a gasoline smell when I went into the garage but could never find a leak. Finally a while back, I had the car backed out of the garage and it sat warming up (winter). I got out for some reason and walked up behind the car, I caught a glimpse of something dripping at the left rear. I watched longer and sure enough, a leak in the gas line where it went up over axle. The gas hit the driveway and almost immediately it evaporated. That's why I never spotted it sooner. Fixed that problem and mileage went up to almost 22mpg with the next tank. I have an electric fuel pump also and it was pumping it right through the leak as I drove it.

Two weeks ago, I was cleaning it up for a car show. There was grime on the Webber so I cleaned it off and had a screwdriver handy so I touched a couple of the bolts just to check them. Loose. Checked every bolt and screw on it. Every one was loose. Vibration worked them loose. The thick filter practically oozed gas. Wonder it never caught fire. Lesson learned, Check the bolts mounting the carb regularly. Since tightening them, the gas smell is greatly reduced but have not checked the mileage yet. It seems like it is not going down as fast so this may be part of it's problem too. I also think the Webber is not ported correctly but not sure yet how that is done. Hope you get to the bottom of it. We ought to meet in Neosho for lunch one day. Maybe we'll both figure out all our problems with the Manta's!

Rick

[tomking]

Hello Rick. I will crawl under and check for small leaks with the fuel pump on. But I solved all the leak problems back a few months; but it never hurts to check. Rick and I have practically the same car for those who dont know. Thanks.

[timemachine]

On two of the four mantas I have owned I have had a gas leak in the short rubber section of the fuel line behind the left rear wheel. It's right at the top of where the fuel line follows the arch over the axle, there's a piece of rubber hose a couple of inches long held by two clamps. Since it's at a high point in the line, it quickly stops leaking when the car is turned off and is hard to detect, but it was pretty obvious when the car was running. If your brakes were sticking you'd know by the hot hubs, pulling, deceleration and performance.

My all-stock manta w/solex gets what they all have in my experience, about 28mpg at legal or faster highway speeds. I've been lucky with this solex so far, in the past I had my share of black plug syndrome and fuel dumping in hot engines.

Jim

[Gary]

Did you check the filler hose for cracks or leaks? (2 1/4 inch, NAPA No. NBH 1055) I had a crack just above the lower clamp on my wagon and the mileage improved once I replaced it. I noticed it when I had some spillage when topping off the tank.

[tekenaar]

Tom,
Since yours is an A/T, I assume the Solex is a vacuum secondary type, right?

The most common problem with that type Solex is a ruptured secondary vacuum diaphragm, but that would cause an intermittent/steady vacuum leak and loss of higher RPM power because of lean mixture when on the primary alone (most of the time normally, all the time with leak) and the secondary never opening when "your foot is in it!" . . . neither condition the likely source of your problem.

However, thinking outside the box for a moment and assuming that your secondary vacuum diaphragm is indeed intact, if moisture/condensation has ever entered that chamber, the vacuum diaphragm return spring inside may have become corroded and, hence, may have broken. That means greatly reduced spring pressure to keep the secondary closed until designed vacuum opening level is reached, i.e. the secondary opens much more easily and frequently at a lower vacuum level!

Also, are you sure the secondary throttle fully closes and/or operates freely? I don't believe you have a fuel feed leak of some sort as most of those would have been noticed before. Either way, I'd take a serious look at your carb's secondary functionality as a more likely source of your "fuel dumping" problem.

[thetourist]

You mentioned an electric fuel pump. Is it pressure regulated. If you discontinued the the return line and are pumping fuel without a pressure regulator it could be dumping huge fuel into the engine.

Check the plugs. That much fuel would be black and gunky plugs.

[tomking]

I did check the plugs and they were indeed black and sooty. The number two was least sooty but others completely covered in black soot. So it is burning rich.
Otto I have had the Solex apart several times in the past months working to correct problems and the carb is clean throughout and the secondary diaphram did not have a hole or tear. The spring is not corroded and all parts work freely. Now perhaps the secondary has subsequently torn the diaphram but it was good when I last had the carb apart. I had to sand the throttle body down twice to stop the leak at its base. And of course rebuilt the carb then.
My fuel pump is NOT pressure regulated; it is the carter pump. I did have leaks in the lines, but all have been fixed. I had that same leak at the rear drivers wheel but fixed it. I replaced all the vent hoses. I have not replaced the main fuel line or return line from the rear wheel forward. But all between there and the tank as well as in the engine bay have been replaced. I havent energized the pump and crawled underneath to check leaks since the trip but will do that today. Again I cant smell any gas while driving or working around the car, so I dont think I have leaks. I think there is something wrong with the Solex. I have ordered a piece of phenolic in order to make a spacer which will allow me to install a Weber and I may go that way if I cant find anything with the Solex. One thing I did find yesterday that with the amount my tach is off, I was turning 3100 RPM at 70 MPH; it read 4K, but I knew it was off some. Plus I checked my pedal and it was misadjusted enough to keep me from opening the throttle100%. I have adjusted on it but need another person for me to check it will fully open. I will catch my wife out there sometime and have her hold the accel down while I check at the carb. But not allowing me to open it fully wasnt causing this problem either. Something must be amiss with the carb, so I guess I tear into it again.

[oldopelguy]

With no pressure regulator it's entirely possible that the pump is delivering fuel at a high enough pressure to overcome the needle and seat inside the carb and it's overflowing the fuel bowl right into the engine. I'd start there for a fix.

[scrapman]

Question about the fuel pump. Mine is also electric, not sure what kind it is. I am not familiar with pressure regulating for the fuel pump. Can you explain further for fuel pump novices, how to tell if it is regulated, if not what is required? Different pump or something different? Thanks, Rick

[opelnut10]

Originally Posted by scrapman Question about the fuel pump. Mine is also electric, not sure what kind it is. I am not familiar with pressure regulating for the fuel pump. Can you explain further for fuel pump novices, how to tell if it is regulated, if not what is required? Different pump or something different? Thanks, Rick

When using an electric fuel pump it is important that the fuel pressure does not overcome the needle and seat. A good rule of thumb is 3-3 1/2 PSI with the Solex and 4 to 5 PSI with the weber 32/36. you need a fuel pressure gauge and adjustable regulator between the pump and the carb. and set your pressure accordingly.

[tekenaar]

Originally Posted by tomking I did check the plugs and they were indeed black and sooty. . . . And of course rebuilt the carb then. My fuel pump is NOT pressure regulated; it is the carter pump. I did have leaks in the lines, but all have been fixed. . . . Something must be amiss with the carb, so I guess I tear into it again.

. . . yeah, but WHICH Carter E-pump?!! . . . makes a big difference as most are designed for American engines - surprise! surprise! - and typically put out 5-8 PSI fuel pressure. As stated in the post before this one, Solex and Weber carbs both prefer ~3.5 PSI MAX!!

[tomking]

Well thanks to all of you for your help. I did check for fuel leaks and found none. I just let the pump run and looked all over and no leaks. Otto its the same Carter pump you are running, whatever the number. Not finding any leaks I pulled the carb. While I thought I had rebuilt it, when I got it out I realized that all I had done to it was to sand the bottom of the throttle body flat to stop the vacuum leak. So following suggestions I looked into the secondary diaphram. It was not cracked, torn or broken so I cleaned it and reinstalled. But I did find the secondary throttle plate not closing all the way. It was gummed and dirty so I cleaned it and got it working freely. Then I noticed that if I manually pushed the secondary vacuum plunger open, it stayed that way instead of closing. I traced it down to a plugged vacuum port that goes through the throttle body. I could not get it open by using carb cleaner and air pressure from either end. So whatever is plugging it is pretty hard. So I exchanged all the jets etc. from that throttle body to one I had cleaned and sanded flat which was on the shelf. Buoyed by new enthusiasm I installed it and fired it up. I did a quick tune and have it close but the noon day heat had gotten to me and I buttoned it up for another time to finish up, I hope. It doesnt yet have a really good idle but hopefully more tweaking will get it just right. But no more for today.
I dont understand something and maybe the experts can explain this. The FSM says to check the port that advances the dizzy and if the vacuum is between 1 and 8 inches leave it alone. If it isnt between 1 and 8, then adjust idle screw on throttle to get 6 inches. Well if between 1 and 8 is OK, then why does it need to be set at 6? I am sure this has been discussed, but I dont understand it as it seem confusing. Is 1 inch better than say 4 or 6 or 8 inches? Which is better and why? Thanks for your help as your suggestions led me in the right direction.

[tekenaar]

Originally Posted by tomking Well thanks to all of you for your help. . . . So following suggestions I looked into the secondary diaphram. It was not cracked, torn or broken so I cleaned it and reinstalled. But I did find the secondary throttle plate not closing all the way. It was gummed and dirty so I cleaned it and got it working freely. Then I noticed that if I manually pushed the secondary vacuum plunger open, it stayed that way instead of closing. I traced it down to a plugged vacuum port that goes through the throttle body. I could not get it open by using carb cleaner and air pressure from either end. So what ever is plugging it is pretty hard. So I exchanged all the jets etc. from that throttle body to one I had cleaned and sanded flat which was on the shelf. Buoyed by new enthusiasm I installed it and fired it up. I did a quick tune and have it close but the noon day heat had gotten to me and I buttoned it up for another time to finish up, I hope. It doesnt yet have a really good idle but hopefully more tweaking will get it just right. But no more for today. I dont understand something and . . . Thanks for your help as your suggestions led me in the right direction.

Tom,
Since the existing carb now has an identified problem, let me suggest you fix that rather than switching everything over to another carb, which has to be retuned and may have problems of its own. KISS principle very much applies here, IMO. Cleaned vacuum orifice would fix your problem, now that secondary operates freely! Clip one of the steel, single strands from a wire brush and use it to clean that reducer/metered orifice for the secondary vacuum chamber.

I actually made a tool (below) to clean the Solex air corrector jets inside the throats, as they cause all sort of "rich running", mixture related problems when they get dirty/clogged . . . very common and, typically, never checked!! . . . could be used for clogged vacuum orifice as well!

[Bill Hoffmann]

Originally Posted by tomking I dont understand something and maybe the experts can explain this. The FSM says to check the port that advances the dizzy and if the vacuum is between 1 and 8 inches leave it alone. If it isnt between 1 and 8, then adjust idle screw on throttle to get 6 inches. Well if between 1 and 8 is OK, then why does it need to be set at 6? I am sure this has been discussed, but I dont understand it as it seem confusing. Is 1 inch better than say 4 or 6 or 8 inches? Which is better and why? Thanks for your help as your suggestions led me in the right direction.

That measurement is an indication of the amount the throttle is cracked open. Unlike the Weber DGxV, the throttle stop screw on the Solex is not intended to be the idle adjustment, and in most cases should not be adjusted. For idle you should adjust the idle mixture and air speed screws. In other words, the FSM is giving an acceptable range and I think discouraging the technician from touching the idle stop screw. Exceptions might be for high mileage carbs with wear in the throttle shafts, or adjustment after the carb has been apart for rebuilding. In that case, you should shoot for 6 inches of water in a closed tube manometer. If you don't have such an instrument, you can check it with a vacuum gauge, but don't expect a reading as such, because most vacuum gauges are not that sensitive. I usually connect the gauge and look for a small twitch of the needle. Any reading above an inch or so on a gauge and your throttle stop screw is probably open too far. This will make tuning very difficult.

Bill

[tomking]

Originally Posted by tekenaar Tom, Since the existing carb now has an identified problem, let me suggest you fix that rather than switching everything over to another carb, which has to be retuned and may have problems of its own. KISS principle very much applies here, IMO. Cleaned vacuum orifice would fix your problem, now that secondary operates freely! Clip one of the steel, single strands from a wire brush and use it to clean that reducer/metered orifice for the secondary vacuum chamber. I actually made a tool (below) to clean the Solex air corrector jets inside the throats, as they cause all sort of "rich running", mixture related problems when they get dirty/clogged . . . very common and, typically, never checked!! . . . could be used for clogged vacuum orifice as well!

Otto I tried for 20 minutes or more to get the plugged passage clear and could not. Whatever is in there is hard and fast. That is why I switched the throttle body out. I agree I didnt want to start over again, but when the obstruction would not come out, I switched. What I will do is now put it in the carb cleaner solution for a couple of days. If it comes clean then I may reinstall that throttle body.
Bill I think you answered me a year or so ago on the same question; and I have been guilty again of mistaking inches of mercury and inches of water. And as an engineer I should know better. I did set the throttle open to just get the vacuum gauge to register. After reading your post, I recall you saying the same thing a year or more earlier. Thanks Otto and Bill.

[Aardvaark]

Tom, you don't mention if it's using oil too or not. Maybe you haven't driven it far enough to notice. Does a cylinder compression test show a good bottom end to the engine? Could it be using so much gas and be low on power due to being worn out? 140K is allot of miles and could indicate a need
for a rebuild. Not necassarily though. Just wondering.