Carb Gurus, Ignition(?) problem redux...

When you punch the throttle, it will go WAAY lean if the accel pumps aren't working. Are they? Have you investigated the possibility of vacuum leaks? Pull the vacuum line off the manifold that goes to the brakes and plug the hole on the manifold to see if this helps. You might have a leaking vacuum line, although that would only effect cylinder #4.

Birdman

 

Popping through the carbs is pretty much either a timing issue or a lean running condition. If it seems to be happening across all four cylinders of the rear bank, the common thing is ignition (i.e. coil/rotor/points/condenser/main HT wire/ballast resistor). I mean, it's possible that both rear carbs have the same malady, but not likely, given that the fronts don't have it, right?

Birdman

 

When you say, "punch the throttle". At what rpm are we talking about here? Does it pop and hesitate at all rpms when this is done, or just low rpms? If it only happens at low rpms, IMHO it is undoubtedly not getting enough timing at the rpm in question. I set my timing so I get full timing at about 3500-4000rpm as opposed to the factory recommended 5000rpm. I personally like it much better this way because I am getting full timing right about when the main jets kick in. Your distributors might not be curving together and the two banks may also be fighting with one another with the differing timing at low rpms. I have always felt these cars are easy to set up over 3500rpm, but under that is very difficult due to how sensitive those distributors are.


The early carb cars are by far my favorite, but they can be a pain to set up.

 

I think you may be on to something here - some of your carbs may be incorrectly rebuilt.
Good catch.

 

Great catch. Let us know what happens.

 

Okay, this is long.....

If it ran great before, and it runs like crap now, there are only two simple possibilities. Either you broke something somewhere, or you assembled something wrong. Carbs dont suddenly change jetting, stop working, etc.. Distributors dont suddenly have faulty advance mechanisms. Points dont suddenly quit. These are mechanical wear items, not electronic things that suddenly stop working mysteriously.

Now dont feel bad. There isnt a mechanic worth his salt that hasnt screwed up and forgot things at sometime. I have a pair of pliers I found up inside the wing of a Cessna that had been up rattling around in there for years. Not only left there by a long since retired mechanic (they have his name engraved on them), but missed by the various other mechanics who annualed and 100 houred the plane over the ensuing years with blind probing fingers, mirrors and flashlights. The moral of that story, posited by all the old mechanics who recognised the name on them, was dont put your name on your tools, lol. Yes, people are only human and we make mistakes and miss things. Ive been in the same darned boat your in a few times, and it can be very frustrating.

After almost 40 some years working on engines, my advice is to go back to basics. Resist the urge to say you already checked something, and start from scratch and go through it system by system. Its a simple engine, you have to approach your problem that way, dont overlook anything. You checked the cam timing on both banks and verified flywheel marks? You do know you time cams on both banks on only number 1 cylinder TDC? You didnt time the front bank cams on number 5 TDC?

If your sure of cam timing, double check ignition timing. Now you did time the front bank on the front bank flywheel marks? Some would laugh, but you would be surprised how screwed up settings can be on an engine and it still runs. Birdman bought a Mondial with the front cams 180 out that ran "perfect". Just it was firing backward on the front bank, 8-6-5-7 instead of 5-7-8-6. Once you are proof positive, check resistance of all primary and secondary wiring with an ohm meter. This is important, you could have a coil to points lead broken internally. Bend and twist wires while you watch ohms. Did you mess with the points? Is the dwell correct? Did you check primary and secondary winding resistance in the coils? Are the engine ground leads intact and secure? Are all the power wires and ballast resitor wires checked? Is the carbon brush in the cap? Are the lead wires screwed down tight, are the extenders good, are all hoses secure and in good condition? Do not just say you checked this stuff, do it all again, and do it with your eyes fully open. Your missing something somewhere. Plug the line to the brakes vacuum, see if something is screwed there, loose, unhooked. Did you monkey with the fuse panel at all, maybe a loose fuse, bad contact, poor connection? Hotwire the ignition off the battery to make sure. Another nice thing you can only do with points.

Almost 20 years away from points ignitions reminded me how much I have forgot. I think I know more now of why so many hate points, and why so many see such great improvement installing anything electronic. IMHO its because thier ignition system was faulty to begin with, and the electronic systems mask the faults. This second car had MSD, and ran fine until it got hot because one box had timing faults. When I switched it to points it barely ran, popping and sputtering. Digging through it, I found two wires with internal breaks (infinite resistance) and an extender melted and burned through and resistor plugs (BPR6ES). Points ignition HAS to be maintained to a higher standard. Low resistance wire, black (not the brown) extenders, non resistor plugs (BP6ES). Points ignition FORCES you to be more thorough. I was looking at a web site recently that compared a dual point Mallory distributor with a Crane fired MSD ignition on a plus 700 HP Big Block Chev. Dyno runs back to back showed less than 10 HP difference, far within industry standards of 1% error in the dyno. So all the hype electronic makes more power than points is just that. Hype. These cars ran great when new, they should run great today. But they do require more maintenence.

Only after everything else is checked and put right, should you play with the carbs. None of us can witness your troubles, but if you monkeyed with the mixture screws or throttle adjustments, you are now going to need to follow Birdmans carb sync and tuning instructions. I had a very severe popping with mine, and ultimately I found the mix screws to be too far screwed in, and the throttle was opened to try to achieve a higher idle speed to compensate for the extreme leanness. When I backed out the mix screws, idle speed soared, and caused me to back off the idle speed screws many turns to get the idle speed down. On these aftermarket DCNF's I need + 3 turns out. But anyway you do it, back them out rich and then set the idle speed. It wants fuel. Now it will idle down to near 500 and is purring smooth and has great throttle response and no popping. Win win.

I know your pulling your hair out, but just hang in there. Your just around the corner from finding the holy grail, and that car is going to make you forget all your troubles when you get it sorted. You'll be amazed.

 

It's definitely the gaskets.

 

I've been there. The previous owner of my 308 messed up the firing order on the front bank. It ran, just not well. After I got that sorted out it took me 3 or 4 tries to get the dwell set right on the points. I would get it "right" for about a week and then it would start missing again. I finally got it right (using a dwell meter) the week before Thanksgiving, and I haven't had to touch it since then.

Aaron

 

This is such great advice I just want to emphasize it. I've been "stuck" troubleshooting my car a few times, and I was "sure" some things were good when it turned out they weren't. The best thing to do when you're stuck is to pretend it's someone else's car. Do everything by the book: electrical first, then carbs. Assume anything can be bad, from spark plugs, wires, timing, etc. Don't say "I changed that last week so it's fine". Pretend you know nothing of the history. Not only will you solve your problem, you might also catch something that needed attention anyway.

Ken

 

Sheesh, who woulda thunk! You rebuilt the rear carbs with the wrong gaskets AND goofed up the ignition timing on the rear bank! LOL! I'm surprised it would run at all! I can't answer your timing question....I'm not a points guy! Paul...?

Birdman

 

We have all done stuff like this. What's important is that you figured it out!

 

Thanks, JR,

Your sharing some "oversights" make me feel better about some of the bonehead stuff i have done. Glad you got it sorted out.

best,
chris

 

Great post!!!!

 

Dont kick yourself man, anyone who hasnt done this stuff hasnt been wrenching very long. Things I have seen and heard, some ive done myself:

A seasoned aircraft mechanic with decades of experience doing an extensive annual and re-rigging on a Beechcraft Bonanza accidently crisscrossed the control cables to the ailerons. The aircraft didnt get off the airport property before it went into the ground inverted at high speed and killed himself and his pilot friend.

A guy that was working out where I was, had removed and replaced the magnetos on a Mooney. The mags are simular to a 308 having twin distributors. He didnt time them, just put them in and placed them "about right", and left the bolts loose. He parked it outside with the keys in it, no note, and buttoned up the cowling. The boss went out and was going to test fly it before returning it to the owner. On startup he noticed a drastic RPM drop when switching between mags and shut it down. Boy was that old man pi$$ed when he took the cowl off and found those mags loose. It probably wouldnt have killed him if he got airbourne, but it would probably have wiped out the engine.

I removed the generator and reinstalled a new alternator on an old Cessna, but I didnt mark the wires because I thought I knew which one was which. But when I was hooking it up, I was confused about two of the wires because it was a different hookup. So I guessed. Well duh, it didnt charge. That was the only time that old man ever yelled at me. Its a part your not allowed to repair yourself, has to be sent back to the manufacture to be gone through. $900 alternator, looks just like a Ford alternator only gear driven. Luckily with 7 heads all looking in books we all got it figured out. At least as well as 7 heads could decifer. But still fingers crossed that I didnt short it out. Luckily it worked. Yeah, I felt like an idiot.

And an airplane again, but the mechanic was the owner and also a licensed FAA inspector. Probably didnt help he was also a flight surgeon to top it off. He had a fast twin engine airplane. People had seen he put a new windshield in it. But as judge and jury, no one had to look over his work, he could sign his own papers. He flew it around the patch, but he splattered himself and his bird in a farm field when the windshield blew inside on him.

We are humans. We make mistakes and errors. Not to far away from where I live we have a 1900 foot bridge lying in the river. You probably seen it on TV. Some human or humans made a goof. Someone missed something. I doubt there is a real mechanic alive anywhere who doesnt see that bridge and feel bad, because once you start taking things apart, repairing things, or inspecting things, your assuming responsibility. For you it just a dumb car and an ignition thats never going to harm anyone. For another man its loose lug nuts on his girlfriends car he forgot to tighten, crisscrossed control cables on a friends airplane, a faulty O-ring on a rocket booster, a pair of forcepts in a persons chest seen on an x-ray, or a pair of pliers found lying inside the wing of an airplane for almost 10 years that no one spotted in 10 previous inspections. I found those pliers where other more capable hands and eyes had looked before. Will someone find some tool I left accidently somewhere in some place they dont belong? God, I pray not. And if they do, I pray they did no harm. But at least I know we all are capable of goofs. What seperates some of us from others, is our ability to admit our goofs. If people could not do that, we could never have gone to the moon after the fire in Apollo 1. Sometimes its good we goof. We needed Apollo 1 to change our course. It makes us much sharper and more commited to doing things better the next time. Some write thier name on thier tools to show off, or keep them from being stolen. Some dont put them on out of fear. Some put them on out of honour. Some of mine have other guys names on them, lol.

So lets re-word your quote. Lets say "Life is hard. But it's even harder when you're to stupid to admit your mistakes.."

Go have a beer, you deserve one. You learned a great deal through all of this, your no longer questioning how stuff works, you nailed it. Glad you like how it runs now.