I have never posted to the Technical thread before, but this time I'm stumped. My son picked up a real nice 1973 Maverick with a 250, 50,000 miles and stock. We have been struggling with getting this engine running right and have tried about everything. The symptoms are a very rough idle and a timing mark that is crazy (the timing light is randomly at anywhere within 360 degrees and appears to be rotating around the balancer, I can send a video if that helps, but I don't seem to be able to upload videos). The engine also jumps around a lot. I know the plug wires are right (153624 with a clockwise dizzy). The timing light works fine on my other cars. The carb has been rebuilt twice. The fuel pump is new, the gas tank and lines are cleaned and fresh, the spark wires are new twice (and separated), the distributor has been changed twice (stock and Mallory Unilite, wired right), confirmed distributor rotor and balancer mark at zero with #1 at TDC with compression, all cylinders have good compression within 15 psi, vacuum is good at 19 etc. Removed valve cover and all valves are moving OK. Nothing has made much of a difference except maybe the carb a little when we changed it out. I know the harmonic balancer slipping might explain the timing marks movement, but we confirmed many times #1 is at TDC with the marks aligned, so slippage seems unlikely and that really would not explain the poor running. I also think the distributor gear teeth being loose would not explain the timing marks being so random (they would be off, but not random). The timing chain slippage also seems unlikely for the same reasons. Anyone ever experience this and have a solution? Thanks (I'm out thru Wednesday 7/20, so I may not reply right away)
Loose timing chain...Check your plugs too...Could have a bad one or a miss gapped one. My $$ is on a loose chain though. Not unusual for the balancer to go bad either. Especially after 40 yrs...
if your light is jumping around that much then you either have a crossfiring issue within the cap.. bad ignition ground.. or you hopped a tooth on the chain. Could be a bad lobe or two on the cam too.. but as you said.. that or the chain slop/hop would be unlikely to cause that much variation. Plus it would likely not be making 19 on the gauge either. If you hadn't changed the distributor already I would also point you towards a sheared roll pin/spun gear on the distributor shaft as well. Even though the rotor statically lines up.. they keep spinning out of phase during run time to make your life hell. I would also run the engine around the entire firing order while paying close attention to the rotor position. I would also pull all plugs and mock up the entire works to ground the plug out so you can physically see the sparks occuring. Flat bar stock works well if you have a big enough thread tap.. or keep the holes very tight to hold the jammed plugs. It's a PITA to do all at once but it can show you misfires once you get it all set up and grounded. With what you describe here.. my moneys on ignition related problems. Be sure the coils resistance is out of whack too.
Thanks for the additional ideas. We will try some of this before we pull the front off to check the chain. We have not tried a new coil yet. I'm still thinking maybe the carb was just un-rebuildable (Carter RBS, single barrel). I had the Carburetor Shop in St Paul do it and he said it was bench tested, so I'd think they verified proper misting thru the venturi. It sounds (and looks) like clumps of gas going in it until we get it up above 3000 rpm, then it seems to mist fine and run much smoother (but even though it seems fine at higher rpm, it still is a dog on the road at 40-50 mph).
I fabricated the setup you mentioned (see picture) and it appears to be an ignition problem (I had to use aluminum since my bit was dull, but I'm thinking that should be fine as they self threaded in). Missing spark often on some plugs and weak at other times. The order is right and sequences OK, but the spark is not right. The two coils we used were both older, so I got a new one. We will try this and a few other things this weekend to see if we can narrow in on the cause. You mentioned a bad ignition ground. What exactly are you referring to?
yep.. AL works fine too.. simple troubleshooting tool creates a WYSIWYG effect to eliminate the guesswork check engine ground to frame(required to ground the magnetic or hall effect ignition module within the distibutor.. usually they work or they don't but sometimes an intermittent ground situation can make you chase you tail too) check ignition switch ground make sure you don;t have the newer ignition/coil running through the old resistor wires shorted coil(bulk of my money is bet on this being your main cause of issue here) no time to go into details right now but found this on a quick search. http://www.aa1car.com/library/ignition_coils.htm
Huh?? AFAIK the ign sw only passes voltage to the different circuits, grounds are supplied by chassis, engine block etc... Points are grounded by a small flexible cable inside dist, can cause all sorts of erratic ign problems, but you'd subbed dist so I doubt that the cause... Have you checked voltage on battery and points side of coil when running??? Maybe resistance wire in harness has a issue...
Tom, it's been awhile since you have helped me out!. Thanks. Anyway, based on lessons learned from my first project, I have rewired the distributor and coil so it is driven by a relay directly off my battery and I'm using a ballast resistor (wired right in parallel with the aux starter wire). My old ignition resistance wire is just now driving the relay, so I'm confident I don't have any ground issues (as you said, the Unilite sub also proved that). The battery is old, so a new one is going in tomorrow along with a new coil (and new plugs again). If problems still exist with both distributors, new coil and new plugs, then.......... I quit and drive the working Maverick for awhile.
Installed a new battery and now I have good spark on all plugs!!!! The old battery turned the starter, but must have had such a low voltage that the coil could not deliver. This was probably enhanced by partially fouled plugs from such a horrible week of running. So, we are running a very smooth idle for the first time (and new plugs for the 3rd time). I am still chasing 2 problems: 1) The stupid timing light marks are still all over the place (360 degrees). But, every time I check the mark with #1 physically confirmed at TDC, it is right on, so I don't think the balancer is slipping. I can give up and ignor this for now and just set timing based on sound etc. 2) When it revs above idle, it begins to "dog out" at around 1800 rpm. When I look in the thrice rebuilt Carter RBS carb, this is exactly when the main jets kick in and start dumping fuel into the venturi. When I look in the carb, the main jet just starts dumping chunks of fuel in to the throttle plat, no mist at all. If we can get it past this point, I can get nice misting at very high revs. So.... I think the Carb is shot. Anyone have a lot of experience with the Carter RBS on the 250's? I am definitely interested in a swap to a newer carb is someone has a good plan. Thanks for any help.
Based on how your engine is performing, I'm of the belief that you have timing light connection issue or the timing light is malfunctioning. In other words your timing is at least operable. Second, you have a carburetor flow issue. Who rebuilt the carburetor and were all the passages blown out with compressed air?
Thanks for responding. As for the digital timing light, it works perfectly on my other old car, so I don't think it is the light. I have separated the wires (and replaced the wires), so I don't think the spark is jumping wires. Also, since I replaced the battery and plugs, I finally get RPM right (I could not get a good RPM when I was having the misfiring issue). But again, I think I can ignor this issue until after I can solve the issue related to getting it above 1800 rpm. (I should also mention that I installed a new coil today, but that again made no difference) As for the carb rebuild, 1st time was by me, second and third times were by a professional. I'm sure passages were confirmed open. This carb has an internal vacuum driven metering needle which I don't understand, but that might be part of the problem. It will cost me near $200 for a new different rebuilt Carter RBS, so I would like to consider alternatives before I make this investment.
I'm going to close this tread and open another on the 250 Carter RBS carb since I think that is my problem.
Just to summarize this: A new battery fixed my poor spark issue and rough idle issue. A different rebuilt carb fixed the stalling issue at 1800 rpm. My first rebuilt carb was probably so fouled up from an old rusty gas tank that it was never going to clear out (removed tank and did a POR-15 seal job and bypassed the old fuel line with rubber for now from a 5 gallon boat gas tank). All simple issues compared to what it could have been! I still have timing marks all over the place, but it runs great and I don't care right now. I'm sure it is something simple and I'll eventually find it and post the find. Time to drive it before winter comes around!!!!!
