Terrible Gas Mileage....

[LordFly]

I just took my car on a 190Km (120 mile) trip, and burned 3/4 of a tank of gas! Previously I had been getting about 25-ish MPG on the highway. The only thing I have changed recently is my vacuum lines. The PO has done something wonky with them, so I put them back to how they SHOULD have been, based on the hose diagram on the engine cover. I did not take note of exactly what he had done, usually because whatever he had done was a stupid idea. Possibly he was bypassing something? I remember one (maybe 2?) of the lines running to the Thermal Valve Control were not hooked up, but I figured that was because it was a HUGE pain to connect them with the intake in place.

On what I'm almost certain is a related note, the car seems to be down on power, especially in the lower RPM ranges. VERY noticeable in 1st gear. It's very sluggish until it hits about 3000 rpm, then it has a very distinct jump in power. I assume the thing is just dumping way too much fuel in, especially at low RPM. I'm usually very good with cars, but the K-jet is still black magic to me, so I'm not sure what the problem might be or how to check it. Suggestions would be great! Thanks guys!

[DMCMW Dave]

I just took my car on a 190Km (120 mile) trip, and burned 3/4 of a tank of gas! Previously I had been getting about 25-ish MPG on the highway. The only thing I have changed recently is my vacuum lines. The PO has done something wonky with them, so I put them back to how they SHOULD have been, based on the hose diagram on the engine cover. I did not take note of exactly what he had done, usually because whatever he had done was a stupid idea. Possibly he was bypassing something? I remember one (maybe 2?) of the lines running to the Thermal Valve Control were not hooked up, but I figured that was because it was a HUGE pain to connect them with the intake in place.

On what I'm almost certain is a related note, the car seems to be down on power, especially in the lower RPM ranges. VERY noticeable in 1st gear. It's very sluggish until it hits about 3000 rpm, then it has a very distinct jump in power. I assume the thing is just dumping way too much fuel in, especially at low RPM. I'm usually very good with cars, but the K-jet is still black magic to me, so I'm not sure what the problem might be or how to check it. Suggestions would be great! Thanks guys!

Check your ignition timing, this is indicative of very retarded timing. Maybe it was set with the vacuum advance hard-plumbed to the intake, and now it goes through the correct switches.

OR

Set to spec with the vacuum line off, then hook it up, crack it off idle and see if the advance goes very high (this would be normal). If it does not jump, another possibility is that it was plumbed incorrectly on purpose to bypass a failed vacuum switch or solenoid that you have now put back into the circuit.

[David T]

Check your ignition timing, this is indicative of very retarded timing. Maybe it was set with the vacuum advance hard-plumbed to the intake, and now it goes through the correct switches.

OR

Set to spec with the vacuum line off, then hook it up, crack it off idle and see if the advance goes very high (this would be normal). If it does not jump, another possibility is that it was plumbed incorrectly on purpose to bypass a failed vacuum switch or solenoid that you have now put back into the circuit.

To elaborate on what Dave S said, recheck your vacuum hoses. It is VERY important to get the right connections on the electric solenoid valve on the front of the motor. If they are reversed the vacuum advance will not function correctly. Make sure the electrical plug is connected and finally, as Dave says, check the timing. I would also check the mechanical advance as long as you have the timing light hooked up.
David Teitelbaum

[LordFly]

So as best I can tell, all vacuum things and solenoid related to timing advance work properly. Although I didn't let the car warm up, so I can't say for sure the ones related to coolant temp.

I figured I might as well set timing anyways, so I loosened off the nut on the distributor, but I can't move the thing! I even tried giving it a few taps with a hammer, won't budge. there is only 1 nut holding it, right? suggestions?

[DMC5180]

So as best I can tell, all vacuum things and solenoid related to timing advance work properly. Although I didn't let the car warm up, so I can't say for sure the ones related to coolant temp.

I figured I might as well set timing anyways, so I loosened off the nut on the distributor, but I can't move the thing! I even tried giving it a few taps with a hammer, won't budge. there is only 1 nut holding it, right? suggestions?

1 NUT. It is likely just corrosion between the Head and the distributor housing (shaft). there is a sealing O-ring on the distributor. What is your fixed timing set at now? 13˚ BTDC +/- 2 is standard spec. Check it with the vacuum advance hose removed. Plug the end of the hose just incase too.

[David T]

It is 13 degrees at 775 RPM. You should see it move as you increase the RPM's and/or add vacuum. The charts for advance are in the Workshop Manual. If it is OK you do not have to move the distributor.
David Teitelbaum

[RammJaeger]

I have the same problem with my car - terrible gas mileage and serious lack of power at low RPM's. I've been curious what the problem might be.

[LordFly]

I haven't had a chance to check this yet (business trip). One thing I'm a little lost on is how you make sure the timing plate is in the right spot. I naturally assume the PO screwed up every possible party of the car. So before I check timing, I want to make sure the plate is in the right spot.

I thought the manual said that with cylinder one at TDC, the timing mark should be at 13°. But but that seems really wrong. I would think it should be 0°, hence the "top dead center" name. But I didn't think the timing plate even went to zero (I'm going by memory as I'm on the other side of the country). What am I missing?

[Spittybug]

I haven't had a chance to check this yet (business trip). One thing I'm a little lost on is how you make sure the timing plate is in the right spot. I naturally assume the PO screwed up every possible party of the car. So before I check timing, I want to make sure the plate is in the right spot.

I thought the manual said that with cylinder one at TDC, the timing mark should be at 13°. But but that seems really wrong. I would think it should be 0°, hence the "top dead center" name. But I didn't think the timing plate even went to zero (I'm going by memory as I'm on the other side of the country). What am I missing?

In order for the spark to actually detonate the cylinder charge, it must be sent to the plug well before the piston is at a maximum compression (TDC). If you sent it with 0o of advance, it would arrive late and the cylinder would already be depressurizing as the piston withdrew. That is why you get reduced power. Conversely, if you fire too early you get the mixture combusting while the piston is still compressing, thus fighting the situation, not helping. This is detected as knock or ping. The higher the RPM, the sooner the spark must be sent to reach the goal of completing the combustion at the same point in the downstroke.

Ideally our cars would like more than 13o at idle, heck it jumps way up as soon as you come off idle, but too much more and you get a harder time starting the car. Understanding advance is key to developing good power and optimizing performance. It is the biggest challenge to get just right for those of us running EFI/spark control setups, but it is really rewarding to feel the benefit when you get it right. On a stock setup your only adjustment is to rotate the whole distributor so that you get ~13o advance at idle. The strength of the springs in the bottom of the distributor control the rate of advance based on RPM. Using 2 springs lets the curve be non-linear; at lower RPM the advance goes up quicker, as you get closer to ~3500 RPM the rate of timing advance tapers off. Vacuum advance is done by using the vacuum of the engine (greatest when your throttle plates are closed, nearly non existent when open) to rotate the insides of the distributor and thus advance the timing. This is done for non-rpm related reasons. In a non "full" cylinder (one where the relatively closed throttle plates have allowed less air to fill the cylinders - like rapid deceleration or just cruising as opposed to accelerating) the molecules are much further apart. A flame front therefore has a harder time moving through it and therefore the spark must be sent sooner. A "fully packed" cylinder of air and gas has the molecules tighter packed and doesn't require that additional time. This means that the amount of vacuum advance is being reduced as you accelerate (more load on engine). This is why some people refer to this as vacuum retarding since you are decreasing the total amount of advance compared to if the engine were unloaded.

So if your vacuum is working right, your static is set right and your springs aren't shot, problems lie outside of timing.

Easy, right? :shyshy:

***I just reread your post and think that you're talking about the timing marks down by the crank pulley...... sorry, my bad. I spent so damn long typing that explanation out thinking you were talking about the rotation of the distributor for static advance....that I'm not deleting it! Sorry if I've told you what you already know. Maybe it will help someone else. There are 2 notches in the pulley, for #1 and #6 cylinders. I too believe that the notch for #1 should be at 0o on that scalewhen you can confirm that #1 is at TDC.

[David T]

To check the timing plate you find TDC (Top Dead Center) of cylinder #1 and the timing mark on the crank pulley should line up with the TDC mark on the plate. Pull the spark plug out of cyl #1 and manually rotate the crank till you get the piston in cyl # 1 to TDC Once you are certain you have TDC you can now run the motor and check the timing of the spark to cyl #1 which should happen at 13 degrees BTDC (Before Top Dead Center) at 775 RPM's with the vacuum hose plugged. If it does not you rotate the distributor till you do. Once you have it set you can watch the mark move as you rev up the motor. That tells you the mechanical advance works. You can also check the advance against the chart to see how accurate it is working (how much advance and at what RPM). Now you reconnect the vacuum and do the same thing subtracting the mechanical advance to see what the vacuum advance is. Most of the time if the advance works at all it is probably accurate enough. Most Deloreans (automatic AND stick) only have 1 spring and they all have the SAME advance. Very unusual for automatics and manuals to have the same advance. Typically a manual will have a much faster advance curve.
David Teitelbaum

[Rich]

One thing I'm a little lost on is how you make sure the timing plate is in the right spot. I naturally assume the PO screwed up every possible party of the car. So before I check timing, I want to make sure the plate is in the right spot.

While it's true that the timing plate position is slightly adjustable there's next to no reason for a anybody to fool with it.

Seems like a safe bet to check the timing with the timing plate as-is and then go from there.

Yes, it's true that PO's can manage to screw up almost anything so I do understand the concern.

[LordFly]

***I just reread your post and think that you're talking about the timing marks down by the crank pulley...... sorry, my bad.

Haha, I was wondering what you were going on about... ;) I was indeed asking about the timing marks :)

[LordFly]

So I just worked out my mileage for the trip I was just on.... I tried to squeeze every MPG that I could out of the car, and after going through a tank in under 170 miles, it worked out to just a hair under 18 MPG :jawdrop:

Still lacking power south of 3000 rpm. I've messed with the vacuum lines a little to see if I could figure out what was bypassed previously. I've tried vacuum advance plumbed straight to intake, vacuum advance disconnected, and WUR disconnected from vacuum (haven't had a chance to try any other combinations yet). So far, oddly enough, I have noticed no difference in the cars performance under any of these conditions, still the same lack of power below 3000 :(