72 250 is running rich

[rgnprof]

I have done a lot of tinkering on this car the last several months and I have it running well. I have the zenith carbs, which I have really worked on and made some major improvements and I have all of the original emissions stuff connected correctly - timing is set 4 degrees ATDC (per the emission plate on the car) and it idles fine and starts with a touch of the key.

Trouble is it's running rich - tailpipe is black, and I pulled all of the plugs this weekend to test compression and they are all black and it it just smells like it's running rich. I am getting about 12-13 mpg in town (which I don't think is too bad for this car). Per some previous posts, I think I have valve stems seals that need replaced (A little oil on cylinder #5 and it blows out a puff of smoke on start-up). Compression was fine - ranging from 175 psi to 155 psi.

Question: Is it normal for these cars to run rich, or is there something I am overlooking? One recent problem that I have noticed is a slight surging when the car is accelerating under load - when I reached cruising speed of 70 mph on the highway - I don't notice it. Only when I am accelerating under load. What could I be overlooking, or am I being too picky with a car that's 30+ years old?

I am thinking of putting in a Crane (they don't make a pertronix for my car, although the company said they could custom build one for $250 - $300). Thanks for any suggestions.

Ryan

[psfred]

Poor ignition will make it feel and run like it's running rich, so get the ignition up to snuff before doing anything else.

The only other way it can run rich at speed is for the jets in the carbs to be too large (you didn't use a wire to "clean" them did you?) or for the air filter to be clogged.

Idle mixture won't have much effect once you get off the idle circuit.

Peter

[rgnprof]

By "up to snuff", do you suggest installing the Crane? I have been doing some reading and I haven't decided yet for sure what to do. Thanks.

ryan

[cth350]

The ignition system from the factory is a transistorized system that switches low current through the points. It doesn't get much better than that. (emphasis on the word "much"). Leave the ignitition system alone if it's doing what it is supposed to do and concentrate of looking for vacuum problems & correctly operating carbs.

There is sthe question of "is the ignition system operating properly?", and that is best answered with an ignition oscilloscope. -CTH

[ctaylor738]

Agree with Charlie - sounds like your ignition is OK. The mileage is pretty much what you would expect as well. Why don't you get someone to do a CO test at idle and 2500 RPM and see what that shows?

[rgnprof]

Appreciate the replies. I have tried to find someone to test CO with exhaust gas analyzer, but I can't find anyone. Equipment is expensive, and my understanding is that it's not used on newer cars.

As far as the ignition functioning properly, I guess that's my question??? I don't have access to an igniton oscilloscope.

How do you know when to replace points? Are there driveability symptoms? Mine look kind of blue grayish and the dwell reads 32, give or take.

If the valves need adjusting could that cause the car to run rich?

As far as vacuum leaks, I don't think I have any and the carbs have been adjusted and tweaked, synched and adjusted, etc.

Ryan

[psfred]

Ryan:

Use a digital VOM and check the resistance of the points closed. If higher than 0.1 ohm, they are shot. Sound burned to me, they are platinum coated and should be shiny with some grayness on the contact surface only.

Symptoms are "rich" running from random missfire, stuttering on heavy accleration, and poor milage. Cold starts can be troublesome as well.

On futher reflection, you also need to check the float height and fuel return lines for obstruction -- a high float will cause fuel to overflow the emulsion tube and spill out the bowl vent into the carb opening. Will run VERY rich no matter what you adjust!

If this is the case, your oil pressure will drop soon from fuel dilution.

Peter

[rgnprof]

Peter,

Thanks - can you humor me and describe exactly how to test the resistance?

I just checked the floats and the level and replaced the needle valves and the fuel lines are not obstructed. I don't think I have a problem there and I don't smell any gas in the oil, nor do I have any oil dilution problems or oil pressure problems.

Thanks.

Ryan

[cth350]

First, disconnect the battery negative lead. Not strictly necessary, the "worse case", is that you will blow up the meter and hate yourself.

Disconnect the lead from the points to the rest of the ignition. Then, with the points closed (crank engine a bit by hand is easiest), measure the resistance from the now dangling points lead to the body of the distributor (or any other ground point).

For completeness, check several other ground points. Notice the little braided bit of wire in the distributor that's soldered to the plate. I had a devil of a time with a car because that wire got damaged. Took a kid with no college degree getting paid minimum wage at a cut rate autoshop to notice it after some well paid mechanics, me and a few others overlooked it.
The symptom we never checked for was higher resistance between the points and the chassis. We only did points to the plate.

-CTH

[250 Coupe]

If you can't find anyone that can do a Co2 test for you, you can get this one for $170.
http://www.aep.bigstep.com/digital.htm

I have the Eezibleed and the Digital Gastester and both work as advertised.
My '76 280C ran so rich that it almost set off the evac alarm at the test station till I tweaked it.
I will not bleed brakes again without at least trying the Eezibleed (it won't fit older American cars without some fussing around).

I did put the Crane XR700 on the 280C and it made a difference in cold starting. I currently have the XR3000 on my 1971 250C mostly because my long since sold 250/8 died due to the points being burnt. That and I thought that the surging at speed was ignition. It turned out to be a plugged fuel filter.
My 250C runs strong, chirps the tires on the 1-2 shift, starts as well as a carb car can and gets 13-14MPG.

Michael

[rgnprof]

OK, I'm confused!

I checked the resistance of the points and it WAS greater than .1, so I changed out points. Old ones looked a little 'burnt'. Set the gap to .3 to .4 mm and measured dwell at 40 degrees. My manual says 30-36 degrees so I adjusted the gap. Manual clearly says "when timing angle is too small reduce contact gap, when timng angle is too large increase contact gap". So, following those instructions, I increased contact gap and dwell went >60 degrees???

I got the dwell down to 34 degrees by going the opposite direction, making the contact gap smaller. I am using a cheap Equus 3010 dwell/tach meter and I connect it to the ignition coil - green meter connection to + on coil and black meter connection to -, on either coil or battery, it doesn't matter as I get the same reading. I have the transistorized ignition (blue coil), and the connections described in the manual are really confusing, so I don't know if I am connecting it right or not.

Car runs fine and starts right up, so I don't know???

Can anyone tell me if I have the dwell meter hooked up right? The tach seems to be reading right when connected this way, but I am cnfused about the dwell.

Ryan

[cth350]

You can't have a dwell of greater than 60 degrees on a 6 cylinder car. Any chance you had the meter set to "4 cylinder" ? -CTH

[rgnprof]

No, I double-checked! My meter has 3 gauges - one for 4, 6 and 8 cylinders and I was reading the right one. It may not have been quite to 60, but it was close. I just saw that it was moving the wrong way (Dwell got higher when I increased contact gap) and I turned the car off and started over.

Ryan

[Tomguy]

Ryan: I notice the same as well. I go by the meter. I think the manual is a typo.

[psfred]

Ryan:

Does the (-) terminal on the coil go to ground or to the switchbox?

Let me know and I can give you the correct connections for the dwell meter. From (+) coil wire to ground is not correct for any MB transistorized ignition and will give you goofy results.

Peter