Poor Gas Mileage, Plus Other Misc. Carb Issues

Joshua Stinnett

Daily Driver
Hey everybody, ever since I got my 1974 X I've been getting much lower gas mileage than I would expect to get. When I got the car it was around 10-15 mpg, and after a witch hunt that involved rebuilding the carb, replacing the fuel hoses and a stuck thermostat, adjusting the timing, replacing the points and pickup wire, and adjusting tire pressures, I've managed to squeak out about 20 mpg. As for my driving habits, I regularly get around 24-30 mpg in my Spider 2000. I'm really at a loss at this point. The few clues I have to work with are that I smell gas sometimes at open throttle under load and the car occasionally backfires and cuts out on cold starts. It is also difficult to start when hot and generates what I would consider an excessive amount of blow-by. The spark plugs are normal, it idles fine, and the car doesn't seem to be low on power. Can someone please help me?
 
I've looked absolutely everywhere and replaced all the hoses, so I really doubt it. I also don't smell gas besides except for an occasional smell when at open throttle.
 
The fuel leak could be into the crankcase... if the pumps diaphragm is perforated then fuel can leak directly into the crankcase which could explain the excessive blow-by. Check the oil...does it seem thin and smell of fuel?

Badly blocked exhaust system would cause excessive fuel consumption, but should also affect performance at the same time.

A badly worn engine will use a lot more fuel, and have lots of blow by too.

SteveC
 
A badly worn engine will use a lot more fuel, and have lots of blow by too.
As Steve states. You say it's a '74. It may be difficult to know but how many miles on the engine? Try a leak down test and/or compression test and vacuum test to help assess the condition.
Also you rebuilt the carb but how worn is it? The smell of raw fuel under certain conditions might indicate a carb leak or excessively rich mixture? Is it the stock carb and jetting? Are there other modifications done to the engine?
And keep in mind your Spider 2000 is a completely different vehicle with fuel injection and likely much less wear. So it's not a fair comparison. But I get your point.
Hopefully we are prompting new ideas to pursue in finding your answer.
 
I would look at the carburetor float height to verify that it is at the correct height to start.

I would gather the correct info for jets, emulsion tubes and so on for the carb and verify that they are correct in the carb itself. It would not be unusual for someone to change those components to get more performance and was pretty common practice. The wrong jets could be the culprit. You may want to find a magnifying glass to read the numbers on them. Additionally someone may have drilled out the jets to a larger size which could complicate verification, perhaps having some that are known to be the correct size would be good to have on hand.

I would verify that the advance mechanism of the distributor is actually advancing as that could also be part of the problem.
 
I don't remember if this suggestion is appropriate for a carbed 74 X ( I had one a million years ago) but I had a similar issue on my FI 83X and traced it down to a bad fuel pressure regulator. The diaphragm was blown and it was allowing gas to pour straight back into #1 cylinder, past the rings and into the sump. If that's the case, you might see a rise in your oil level. Do you have a fuel regulator?

Ed
 
^^ I dont recall them as having an FPR from factory, but I could be wrong. My '78 and '79 didnt, but maybe the '74/75s did. I think FPRs are for the FI crowd. The fuel pressure in our carb cars shouldnt be more then about 3-4psi. The issues described sounds to me, like its possibly the combination of several things.
 
My '78 and '79 didnt, but maybe the '74/75s did.
They did not. The mechanical fuel pump in the 1300 motors is pretty much self-regulating as long as you get the projection of the actuator rod right - the procedure is in the shop manual.
 
As Steve states. You say it's a '74. It may be difficult to know but how many miles on the engine? Try a leak down test and/or compression test and vacuum test to help assess the condition.
Also you rebuilt the carb but how worn is it? The smell of raw fuel under certain conditions might indicate a carb leak or excessively rich mixture? Is it the stock carb and jetting? Are there other modifications done to the engine?
And keep in mind your Spider 2000 is a completely different vehicle with fuel injection and likely much less wear. So it's not a fair comparison. But I get your point.
Hopefully we are prompting new ideas to pursue in finding your answer.

The car has 57k miles that I am certain are genuine, as I'm the 2nd owner and the condition of the car would suggest it. I'll be sure to run a compression test when I get home today. The car is bone stock aside from the period removal of the smog pump, but I've capped off the vacuum line that came from the carb. I certainly appreciate any suggestions and guidance, as I really don't know where to go from here.
 
The fuel leak could be into the crankcase... if the pumps diaphragm is perforated then fuel can leak directly into the crankcase which could explain the excessive blow-by. Check the oil...does it seem thin and smell of fuel?

Badly blocked exhaust system would cause excessive fuel consumption, but should also affect performance at the same time.

A badly worn engine will use a lot more fuel, and have lots of blow by too.

SteveC
No fuel smell from the oil. A fellow X1/9 owner suggested that I clean out the pcv vapor separator, as it can tend to cause blow-by. It was pretty filthy, but I couldn't do a full clean without emptying the oil pan, so I decided to wait until my oil change that's coming up soon. I didn't think that would cause bad gas mileage though.
 
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I don't remember if this suggestion is appropriate for a carbed 74 X ( I had one a million years ago) but I had a similar issue on my FI 83X and traced it down to a bad fuel pressure regulator. The diaphragm was blown and it was allowing gas to pour straight back into #1 cylinder, past the rings and into the sump. If that's the case, you might see a rise in your oil level. Do you have a fuel regulator?

Ed
No fpr, the 74s are strange and simple beasts indeed. I had a similar issue with my 79 spider so I was sure to check. Thanks for the input.
 
I would look at the carburetor float height to verify that it is at the correct height to start.

I would gather the correct info for jets, emulsion tubes and so on for the carb and verify that they are correct in the carb itself. It would not be unusual for someone to change those components to get more performance and was pretty common practice. The wrong jets could be the culprit. You may want to find a magnifying glass to read the numbers on them. Additionally someone may have drilled out the jets to a larger size which could complicate verification, perhaps having some that are known to be the correct size would be good to have on hand.

I would verify that the advance mechanism of the distributor is actually advancing as that could also be part of the problem.
When I rebuilt the carb about 1500 miles ago the listed sizes checked out, but I plan on replacing all the jets pretty soon to be safe. I also replaced the needle valve and set the float when I did so.
 
The car is bone stock aside from the period removal of the smog pump, but I've capped off the vacuum line that came from the carb.

OK this could have something to do with it... The smog pump pumps air into the exhaust ports, and changes the metered CO reading for emission testing... but the vacuum line at the carb doesn't connect to (or at least I'm pretty sure it doesn't... Australian spec car never had a smog pump per se) the smog pump... it's for the vacuum advance module on the distributor, and incorrect advance (mechanical and plus/minus vacuum advance / retard) will definitely affect engine consumption and performance...

SteveC
 
OK this could have something to do with it... The smog pump pumps air into the exhaust ports, and changes the metered CO reading for emission testing... but the vacuum line at the carb doesn't connect to (or at least I'm pretty sure it doesn't... Australian spec car never had a smog pump per se) the smog pump... it's for the vacuum advance module on the distributor, and incorrect advance (mechanical and plus/minus vacuum advance / retard) will definitely affect engine consumption and performance...

SteveC
My carb is connected to the vacuum advance from a different port. I guess it could be that the line for the advance was connected to the wrong vacuum port by the PO, but would it matter? I'm pretty sure that the shop manual said the port was for the smog pump, but I'll double check to be sure.
 
I just discovered something. My Haynes manual and the Fiat service manual have the marks on the timing belt cover listed backwards; the Haynes manual (which I've been using) has it listed as 10, 5, 0 and the Service manual reads 0, 5, 10. Maybe I'm running 10 degrees of timing out?
 
Sounds like you've covered most of the suggestions so far. Even though the mileage is low I would still recommend checking the compression because of the blow-by you are experiencing.

the marks on the timing belt cover listed backwards
Maybe I'm running 10 degrees of timing out?
Easy to tell by connecting your timing light and watch which way it moves as you change the position of the distributor. You can also do a basic check of the distributor advance this way, by watching for timing advance as you increase RPM's (not real accurate but should let you know if it is advancing at all).

Regarding the SMOG pump, like Steve said I don't recall it having any vacuum connections (been awhile since I've played with one). Perhaps you are referring to the EGR port (if that was also removed)? Maybe retrace ALL of your vacuum lines and connections to verify everything is correct (compare with the factory manual...don't always trust the aftermarket books, as you found they can be wrong). Or you may find a sticker under the hood that shows all vacuum routes. Also try the simple vacuum-leak 'check' by spraying something like starting fluid around any possible leak areas. If the RPM rises then the fluid is being sucked in due to a vacuum leak.
 
Sounds like you've covered most of the suggestions so far. Even though the mileage is low I would still recommend checking the compression because of the blow-by you are experiencing.



Easy to tell by connecting your timing light and watch which way it moves as you change the position of the distributor. You can also do a basic check of the distributor advance this way, by watching for timing advance as you increase RPM's (not real accurate but should let you know if it is advancing at all).

Regarding the SMOG pump, like Steve said I don't recall it having any vacuum connections (been awhile since I've played with one). Perhaps you are referring to the EGR port (if that was also removed)? Maybe retrace ALL of your vacuum lines and connections to verify everything is correct (compare with the factory manual...don't always trust the aftermarket books, as you found they can be wrong). Or you may find a sticker under the hood that shows all vacuum routes. Also try the simple vacuum-leak 'check' by spraying something like starting fluid around any possible leak areas. If the RPM rises then the fluid is being sucked in due to a vacuum leak.
I've pored over the smog and carb diagrams, and I've yet to actually figure out what that vacuum port does. I've been trying to upload photos all day with no luck. It comes out of the base of the intake through a hole on the heat shield. I believe it's a part of some sort of air injection system that went with the smog pump. I don't believe 74s had an egr valve from what I've read. Anyway, I checked the vacuum advance, and it's working correctly. I set the timing to 0 by the factory service manual and it already seems to be running much better. I topped my gas up and I'm going to see what it does for now.
 
Honestly I don't know all the nuances of the '74. However keep in mind the manifold (or any other component for that matter) may no longer be a '74 item. These are cars that get a LOT of "tinkering", so it could be a manifold from another year that did have EGR (or something else) attached there. Can you get a picture of the port in question? I'm sure someone here will know exactly what it is.

Regarding the distributor, it may have a centrifugal (mechanical) advance mechanism internally in addition to the vacuum advance. It is not uncommon for the weights on centrifugal advances to get stuck. So just because you've checked that the vacuum advance is working does not necessarily mean the distributor is properly advancing. You can check the total amount of advance with a timing light.

Glad to hear it is already running better. Let us know what happens.
 
Because we're talking about a '74 here....
It comes out of the base of the intake through a hole on the heat shield.
when the car left the factory, that port operates the diverter valve for the air injection system to shut off the air injection when you lift off the throttle. If you no longer have a smog pump you should just cap it off (although it is also by far the most convenient place to hook up a vacuum gauge when you want to do that).
I don't believe 74s had an egr valve from what I've read.
You are right, they did not. Started with the '79 carbed 1500.
Anyway, I checked the vacuum advance, and it's working correctly. I set the timing to 0 by the factory service manual and it already seems to be running much better.
How exactly did you check the vacuum advance? Look carefully to see whether applying vacuum is advancing or retarding the timing. This is because in stock trim the '74 has a vacuum retard, not advance. The shop manual procedure is to disconnect the vacuum line and set the timing to ten degrees advanced, connect the vacuum line and verify that timing goes to TDC as the vacuum retard kicks in.
If you have everything working and you like how it runs, you're cool - you have the car working the way it did when it came from the factory. However, I if you have no smog pump I find that it's easier to just cap off the vacuum line to the distributor, then set the timing by ear - you want as much static advance as possible without pinging at full throttle and peak revs uphill.

The logic behind the vacuum retard is that under the '74 regulations emissions were measured at idle, mainly because back then it wasn't practical to do anything else. Retarding the timing under conditions of high manifold vacuum, which is to say at idle, lowers combustion chamber temperatures and cuts back on NOx; back before catalytic converters there was no way of getting rid of NOx except to not produce it in the first place. This cooler and less complete combustion also produces more HC and CO, but the air injection pump takes care of these by burning them to CO2 and H20 in the exhaust manifold.
 
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