Odd "miss and hit" during acceleration
 

Car has been working great, but today a new problem started. During fairly hard acceleration, the engine suddenly seems to "miss and hit" for a second. Almost like opening secondaries, but quicker. It's a hiccup of an accelerating engine, not a pop out the exhaust or carbs.

Very difficult to describe, but almost like a shot rod bearing allowed the piston to hit the head for a second. So it could be a mechanical problem with just rebuilt engine, but there are no signs such as smoke or metal in the oil.

I suspect a carb issue. I have twin vacumm Holley's with the light springs installed. I checked the vacumm hose connecting the two secondaries and found it accidently bent down under the ignition wires, but it seemed "crack-free" other than that.

Not sure what symptoms to expect if a power valve failed.

Anyone got any ideas for this "hard to describe" problem?

Problem gone, but new one now. Seems the vacuum secondary line may have been kinked a bit, and on this particularly hot and humid day, the vacuum may have collapsed the balance hose so the secondaries opened at different times. Anyway, my best guess.

So it seemed to work well. Went and got gas, and on the way home, it seemed to miss immediately when I tried to accelerate and with a lot of popping out exhaust. And that seems like excess fuel getting to the sidepipes due to weak igntion. And it's humid....HV leak no doubt. Will check at night for arcing......

Remember the FE's need to have the #7 & #8 plug wires separated a bit.

An overly rich condition would be the result. Make sure the vacuum opening rating on your PVs is roughly 1/2hg below your engine vac reading at WOT. Having the PV open too soon can cause a bog condition.

-Dean

Thanks Guys. I've been thinking some more (which can be dangerous....LOL) and all things point to the problems being ignition related. Going back over the facts, all problems started once engine was run quite a while....i.e. "hot". I notice the coil is also quite hot. Somewhere my ignition is breaking down.

In the first case, it was happening at 1/2 way through an acceleration run (1/2 way between gear shifts), but that also could have been a point where the Hp, spark advance, rpm, etc all determined the point a weak spark would misfire. In the last case, the bogging and popping out the exhaust are almost certainly due to ignition break-down.

I am using 9mm Accel wires, new plugs, Mallory dist with Pertronix unit and an Echlin internal ballast coil. This old coil, of dubious origin, I had laying around in a box.....never used, but the box looks like it's 50 years old. NOS with a stress on teh "O". It's been successful now since 96 when I first got the car on the road. Perhaps it's time to upgrade.

Darned thing......coils hardly ever go bad. Guess I need to do some more testing this evening. Look for arcs and check the max spark travel from the coil to ground. Darned hard to nail down when it seems to be causing trouble only after it gets heated up.

Check for a fouled plug. Just one fouled plug can really upset the engine.

Bob

I didn't do that yet, as when the problem happens, it's the same both sides, so i figured it highly unlikely all 8 plugs fouled at the same time. Plus, they are new...less than 50 miles on them. But anyhting is possible. I intend to pull a few anyway to see what's going on. Thank-you.

How long have you had the car? Is there any chance you could get an audio file of the miss? And before anybody laughs at that idea all he has to do is use the recording function on his BlackBerry and email me the file and I'll convert it and post it for him.:p I'd really like to hear what you're talking about.

A friend had this problem almost exactly. The distributor was the cause. It wasn't lined up right and the vacuum wasn't hooked up right, and the distributor just wasn't working right.. He got a new distributor, and had it re-curved and installed. The only way we found the problem was using a dyno.

Funny you should say that as I have just gone through a lot of distributor tuning, however evrything has been working great. So it's not a "design error", merely a small hiccup, the first symptom of which, I've never seen before.

Sorry don't own a Blackberry. Darned weird...the first time I heard it, I thought a rock had flown loose from the tire tread and hit the inner fender. Then it happened 3 or 4 more times....all with the engine accelerating in the middle of it's power band.

Anyway, that symptom went away after I let the car sit for a bit, plus move the vacuum hose around. Then it was fine. Until it warmed up. Then the symptom went to a major bog and backfire out both sidepipes immediately upon giving it gas vs the earlier symptom when it happened after the car had already started to accelerate.

I seem less confused now that the symptom has changed. Whatever I find, I'll make sure to post the answer in case anyone else ever has a similiar problem.

Oh....I finished building the car about 1996. Just finished my 3rd engine build. First time to get it ready for the car, second time: block cracked.....third time: balancer slipped, timing marks were way out, and I set the timing to the marks....broke a piston. I know a fair bit about engines, but obviously not as much as people in the field, or those more nuts than I am about internal combustion....LOL. But I wish I could record those earlier symptoms today.....I've never heard/felt anything like it before. I can imagine a complete ignition failure, but only for a moment (so when it comes back on, the engine hits hard), might be it.

To elaborate a little on what Dean said about the power valve .... if it fails , the engine will run really rich . Where it will be the most noticeable is at idle with the engine running really rich and rough . If you take the metering block off and find fuel in the well behind the PV , it`s bad .
Another thing I`ve had happen is the gasket under the pv is a two piece gasket that is cemented ( ? ) together . If you tighten the pv too much , the gasket will separate and give the fuel a sneak path to screw you up . When you take the pv off , the gasket will actually be in two pieces .

Well....it does seem to be a bit rich at idle and lower speeds and when accelerating seems to clear itself. But not rich enough to cause black smoke. Good tips on the PV though. Thank-you.

Last night, in the dark, I started it up. No arcing or anything. I then hooked by spark tester (just an adjustable gap device with a sparkplug tip on one end and an alligator clip on the other). I set the gap for just over 1/2 inch and connected the HV wire form the coil to it. Cranking the engine over I had a nice blue spark. It was hard to tell, but it seemed that every so often I'd miss a spark during cranking. This seems to match up when I used a timing light and every so often it misses a flash. Previously I blamed it on the light. Now I'm not so sure.

Anyway, I took it for a drive after my testing and except for some light bogging just off idle, it seemed fine. Seems the problem gets worse when the engine is very hot and complimented by driving during very hot/humid weather.

More testing today........

Remember that occasionally a horizontally mounted oil-filled coil will have an internal intermittent fault that can be very hard to diagnose. A vertically mounted coil can also go bad as well -- before you moved on to any serious disassembly/exploration I might just go ahead and change out the coil if you have any suspicions at all about it.

That's my next step. Being cheap, I'm looking around at what I have on hand, and if I can get improvement, then I can select and purchase a good new brand name coil.

Notes on coils:

I have a brand new Jacob's variable permeability coil (which I never understood as I always thought permeability was constant for a specific material). I am not going to use that because the engine would barely run with it when I tried it years ago. Plus it got very hot, very quickly. Jacob's help line was of "no help". I am not impressed with Jacob's stuff based on that experience.....$60 wasted.

I do have a Mercruiser coil which worked fine when I swapped it out. It had a bit of arcing at the tower, but that was bad wires. I changed both wires and coil as a failure in open ocean is not an enchanting experience. I just need to check to see if a ballast resistor is required for it before I hook it up.

You know, when you look up coils on Summit's web site, they often check the box that says the ballast resistor is "not included", however I suspect many of them don't actually use one, so it should say "not applicable". Some of the manufacturer's web sites are not clear on this point either. Some things still need a phone call to sort things out properly.

You may want to consider going with a MSD High Vibration coil. I use one of these because our cars love to shake and shimmy. Never had a coil problem running these units.

-Dean

Well....I hate going in circles vs logical troublshooting, but that's what I'm doing.....

Bought a cheapie coil (available locally with no wait) just to try. Problem seemed gone, but just as I was heading home from a test drive, things started popping and snapping out the exhaust as soon as I started to accelerate. Sure seems heat related.

I have removed both valve covers looking to find a broken outer valve spring. Everything was OK.

I checked the plugs. Front 4 were 1/2 gray and 1/2 white on the porcelin. Back 4 were all gray on porcelin. Looked pretty normal, although I was using the 11Y vs the 9Y plugs.

I have twin Holley's and am going to check the vacuum secondaries next in case one is sticking or eratically operating.

Only other mechanical check is to drop oil pan and check for stretched rod bolts (as the cause for the high rpm "tchunk"), but I kind of doubt that's it.

I'll see about new 9Y sparkplugs. Better heat range and the new ones in it look a little like high speed glazing.....only because they are turing gray vs tan.

.......running in a circle like a chicken with it's head cut off......not fun at all !!!

Sounds like a fuel delivery/carb issue to me, possibly in the transition to the power circuit. Plugs are not the problem (based on how you described them)...going a few steps toward a colder plug is not going to resolve this issue. Just my $0.02.

-Dean

I guess there is no way to make a long story short, but I'll try. I just re-built this engine due to a broken piston....detonation. This occured because I set the timing incorrectly due to a slipped balancer outer ring. As the slippage can not be guarenteed to be "fixed", I have no idea where my timing has been set over the years, so no baase-line. So I have been playing with it a bit and finally got it where I'm happy with it. Although the best idle is at 38Deg BTDC, obviously it needs to be set lower with a mech-only dist and I've wound up with 18 Deg & 850 rpm with a max just under 38 Deg.

However, because I probably do not have it set as advanced as before, my engine heats up more quickly, my idle mixture screws had to be richened a bit (about 1/4 turn) and the idle stop screw had to be turned it maybe 1/2 to 3/4 turn more than before. This could be causeing a transition problem. I'll have to measure how much of the transition slots are being used up at idle.

I seem to have 2 issues...the "bang/clunck at higher rpms and sometimes a bog/misfire at lower rpms. All intermittent and all more likely when the engine is throughly warmed up on hot days.

As a hot engine requires a leaner mixture, I have given up on the igntion for now and am looking at the carbs. But I think I am looking for something not working vs not set-up right, but perhaps I am wrong about that.

Basic checks show the secondary float levels correct at base of inspection hole. Both Primary levels are a tiny bit below the inspection holes, but after sitting all night (primary bowls are vented)plus losing a bit during by tinkering (pump shots) it seems normal. Checked out both vacuum secondary's for diaphram leaks. None there although I noticed one flowed air easier. Due to different depths of the slots under the checkball. But that's from factory and they worked fine until now.

By the way, there's a miserable job....putting back together a secondary vacuum chamber...that rubber diaphram has a mind of its own!

Oh well.....back to it.....sooner or later I'll stumble onto the answer. Just wish it was due to good troubleshooting and not good luck.

Where is your ignition modual located?
Is it getting hot?--Maybe going bad?
Try by-passing it, can't hurt.
I think you are presentley going in the wrong direction.

JMO

Craig

Well....covered 20+ miles and problem is gone. To find out what the problem was, please send $1 in a self-addressed stamped envelope to.....

LOL. Actually, it's almost gone. Down to a few light misfires at various rpms and engine load. What is different form when the problem was really bad?

new coil
moved slightly chafed ignition wire
added alcohol to gas tank in case of water from condensation
day was not as hot and far less humid than before

So it may come back. I'm thinking my nice 9.5 mm Accel plug wires and other HV components may be the problem, and/or as above, the ignition module. Best guess on the "bang" is crossfire vs misfire.....a cylinder firing at the absolute wrong time from crossfire. So it looks like a ignition component replacement for a while.

Oh.....I have had a bog for a while which didn't help diagnosis.....trying to seperate that from some of the other issues. Fixed that. Bumped timing to 20 intial and 38 total and swapped accel pump cams for a bit bigger fuel shot.