Distributor caps and rotors

[m3a1]

I've been helping a fellow member with a M151 that never really ran well but DID run. Now, it is reluctant to run, it will only start with starting fluid, runs rough when it decides to run for a moment and backfires through the carb just as it dies (and burns the hair off my arm). After we did some tweaking, it now also backfires a little through the exhaust as it dies.

Carburetor has been rebuilt and he reports fuel flow is fine. Engine was rebuilt locally several years ago by a long-established and reputable shop. I've seen this truck run and drive and IMHO it was running tolerably well.

So, after looking everything over and finding nothing obviously out of place, we went back to the basics, beginning with valve adjustment. This would be the tweak that started the exhaust backfire which wasn't occurring beforehand.

Since we were working with a cold engine I was a little surprised to find the clearances tight and ranging anywhere from .007 to .012. I was expecting measurements just a little bigger than spec. So we marked the adjusters where they were (just in case we needed to go back) and brought everything to about .015, cold. I know it's supposed to be .015, hot but, ya gotta start somewhere.

So began the exhaust backfire.

We tried making minor adjustments to the distributor but it was behaving oddly. I would scootch it one way and there was resistance and it would try to scootch back.

That's odd and minor changes in the timing really made no difference.

Checked the spark plug wires. Several were damaged with cuts through the silicone shielding just about behind the little flat spring. Appears someone tugged on them a little too energetically and they were reluctant to come out of the plug. However, it was highly likely that those cuts would close completely once everything was put together. Didn't get a look at the working end of the plugs because he didn't have the socket needed to remove them.

I don't recall needing a socket with extra depth to remove my Mutt's spark plugs but I DID need one to remove the extra long plugs in the Doom Buggy because they were for something else entirely.

One plug looked odd in that it had a grey appearance on the outside, a bit like chalk, but not something that wiped right off so I'm not sure what that was all about. Anyhoo, there will be new plugs and wires as soon as they are shipped.

Here's where it gets really interesting. Inside the distributor I found points ignition that looked clean and new. The rotor was on the shaft extraordinarily tight (not sure that was the correct rotor and I was pretty sure it would break if I tried to pry it off.

The cap showed evidence that the rotor had actually been dragging across the contacts. Two consecutive posts were deeper than the other two.

Ah HAH!

So I removed the cap and when I got the last screw to a certain point, the cap kind of popped toward me off the distributor housing cover. So, it had been under a bit of stress. In examining the rubber seals I found one had kind of galled up in the center but they aren't double stacked. However it happened, I can only conclude that the cap wasn't flush and level with the distributor housing cover.

At this time, the spark plugs haven't been examined but from what I've seen so far, the question is, (assuming the valves are now reasonably close to being adjusted properly), how does an engine with such low mileage run as though its timing is both retarded and advanced at the same time?

My guess is, the rotor was really hanging up at two spots and then snapping forward with the assistance of the spark advance springs once it got past a certain point. It hanging up would explain the resistance I felt when trying to adjust the timing.

Why this is happening now when it wasn't happening before is the mystery. My long socket has been delivered to him and hopefully, we'll soon have a look at those spark plugs.

I would love to hear comments on this one. It's quite a conundrum.

Cheers,
TJ

[kmam]

I had problems with a Series 2A Land Rover that would start and when revved up would stall. Not the same symptoms but tracked that down to a problem with the springs that control the advance/retard. This can cause a range of symptoms, so it may be worth checking that out. Probably the easiest way is to put on another distributor.
Also had problems with the MUTT and it turned out to be fine crack in the distributor cap which was not obvious until inspected VERY closely.

[m3a1]

Yeah, I'm familiar with the gummed up spark advance. Don't think this is going to be it. This acts like either burnt valves (highly doubtful) or that the ignition timing is all over the place.

May very well be we are working with multiple problems at the moment. It will be easy enough to sort out the distributor and cap and wires and plugs. With that done, we'll see what's next.

[muttguru]

uote=m3a1 post_id=115877 time=1640027254 user_id=30391]

..................Here's where it gets really interesting. Inside the distributor I found points ignition that looked clean and new. The rotor was on the shaft extraordinarily tight (not sure that was the correct rotor and I was pretty sure it would break if I tried to pry it off.

The cap showed evidence that the rotor had actually been dragging across the contacts. Two consecutive posts were deeper than the other two.
Cheers,
TJ[/quote]

TJ, that looks like your main problem. If the rotor arm has been forced onto the rotor and/or it has not been seated properly, it could be out-of-synch with the vehicle's timing, or it could be mis-firing under the distributor cap. Usually the rotor arm fits reasonably tight (there's a flat spring in the arm to locate and secure it in the correct position) but if the arm has just been forced in place, the arm is either the wrong one or the flat spring is missing or damaged. Whatever it is, that rotor arm has to be removed and replaced with one that fits correctly or you'll never get the ignition system working properly. And when you've done that, check the points mounting plate to ensure that the plate is also located correctly in the distributor and that the big circlip is fully engaged in the housing slots.
Also, did you use the "rule of nine" when you adjusted the valve clearances?

Ken

[rickf]

Rule of nine, It is just so much easier to just turn the engine over one half turn for each cylinder adjustment.

[m3a1]

The rule of 9 for a four cylinder engine is clever but if you think about it, you're actually just identifying the particular valves that need to be in compression before you adjust its opposite number and the only thing you need to accomplish that is to realize that you're just working from both ends toward the center to accomplish it, then crossing over and working outward again until they're all adjusted. Thus, the continuation of my diagram (#1 is the front end of the engine) would be to simply swap the word 'in compression' for 'adjust' on each one.

IMG_8071 (1).jpg

HOWEVER! Using the Rule of 9 does have one undeniable drawback. Customers get kinda nervous when a mechanic is working on their car and holding up his (or her) hand in front of their face and counting their greasy little porkies over and over again. The casual observer doesn't know that and quite frankly, it doesn't build trust... Just having some fun with you, Ken!

On another topic, I found the following chart online the other day. Its validity is unverified but it looks 'official'... Thought I'd toss it in here as a JFYI.

s-l1600-1.jpg

Cheers,
TJ

[rickf]

je ne parle pas français monsieur.

[Cliffaliff]

I’m about ready to throw in the towel!! Problem is I have no spark from the distributor. I’ve changed out the capacitor, coil, electronic Swiss control , the inner distributor cap,and put them back in when still no spark! I have 24 volts going into the distributor. Help me! M151a2, 1977.

[m3a1]

Just spitballing but it sounds as if your 24v going in is grounding out if everything else is 100%. I guarantee you that the power is going SOMEWHERE. So, you're going to have to work the problem backwards. Where does it stop being hot?

And, I believe Swiss controls are also available for 6 cylinders as well (our more scholarly members may want to comment on that as a possible mix-up). I have no idea if they look the same but that could be another less-than-obvious thing to check.

[rickf]

Have you checked inside the distributor to make sure the power is actually AT the coil? The input capacitor which is located behind the four screws where the hot wire screws onto the distributor is known for going bad, Hence power goes in at the connector but does not reach the coil.

https://shop.midwestmilitary.com/produc ... capacitor/

[Cliffaliff]

Thank you gentlemen, I’ll dive into it tomorrow, great suggestions, thanks!!

[Hambone]

you can delete the capacitor if needed.

[Cliffaliff]

Gentlemen, I opened up the dist. today. I was never happy with the ground connection.So I improved the ground connection by retapping the hole and getting a longer screw, both sides. No change. I checked the coil with the switch on, 24 volts. Still won’t start.

[Hambone]

Have you tried this test? If you have 24v to coil, it can be done in vehicle, process of elimination.
https://www.google.com/search?q=m151+di ... FM-Gs,st:0

[rickf]

What ground connection are you referring to? And did you check to see if you had 24 volts at the coil positive?