Diagnosing a misfire

[Mrturner1]

Originally Posted by prodjay10

Anyone ever try using this calculator to figure out which size carb your vehicle needs? Last time I checked, it said my stock 350 cid needs about 340cfm

http://www.carburetion.com/calc.asp

Also depends what you're using the truck for. 750 is just right when paired with big valves, big intake runners, big intake and exhaust, and 6500 rpm

[1969c10joe]

yes displacement and rpm . It sounds like your truck runs pretty well with 750. with my gearing and sm465 I only got to enjoy it getting on on ramp never did find out when it stop pulling too chicken with those drum brakes

[Mrturner1]

Originally Posted by 1969c10joe

yes displacement and rpm . It sounds like your truck runs pretty well with 750. with my gearing and sm465 I only got to enjoy it getting on on ramp never did find out when it stop pulling too chicken with those drum brakes

I still have drum brakes to haha that's why Grumpy and a few others give me such a hard time. I have a 12 second capable motor in a truck that shouldn't be trusted over 65 mph

[1969c10joe]

It was set up like that from previous owner. He lost his license med. reason before he sold it to me wild bill our friend would take it out and really open it up greg always got out of that truck laughing

[Mrturner1]

Well I re-adjusted lash, most of them looked good except one exhaust valve was a little too tight, and a few other valves were a bit too loose. I used a .018" feeler gauge this time, and set them all consistently. It fired right back up and idled great, but I went to drive out of the driveway and as soon as it was under load it was misfiring again.

SO, we've ruled out lash being too tight causing valves to stay open. It's still VERY rich at idle, so much so that you can see a haze all around the back of the truck from fuel smoke. There's actually little tiny vague puffs of black smoke coming straight out of the tailpipe, so it's fairly rich. This is my first Holley so I don't exactly know where to start. Im gonna run to the parts shop and grab a new set o spark plugs, and recomendations on which plugs to use this time? I'm using Accel header plugs as of now, and if I can stick with a shorter plug I'd like to, these headers are a PITA. Also, if I'm going to start checking/fixing my carb for a rich condition, where do I start? The floats look fine, the sight window looks to be half full all the time. It's a brand new carb, but if there are any parts I need to pick up do tell because they have a whole wall of Holley crap at the shop I'm going to. Could it be the wrong Power Valve? Maybe it's feeding fuel when it shouldn't be? It's a 6.5 power valve, so what does a 4.5 or 8.5 do differently?

[davepl]

A 6.5 power valve cuts in and adds fuel when your vacuum gets down to 6.5", whereas a 4.5 wouldn't kick in until it got down to 4.5"

So if your idle vacuum were below the power valve kick-in point you'd be dumping raw fuel.

Also if you've had a carb backfire you can easily (on older Holleys anyway) damage the power valve and make it leak. I've heard it can still happen on newer models, but never has to me.

[Mrturner1]

Originally Posted by davepl

A 6.5 power valve cuts in and adds fuel when your vacuum gets down to 6.5", whereas a 4.5 wouldn't kick in until it got down to 4.5"

So if your idle vacuum were below the power valve kick-in point you'd be dumping raw fuel.

Also if you've had a carb backfire you can easily (on older Holleys anyway) damage the power valve and make it leak. I've heard it can still happen on newer models, but never has to me.

Well I got a vacuum gauge and it says that my idle vacuum is 6", which I kindof expected since I have a ton of overlap in the valve events. So that means the 6.5 power valve was feeding fuel all the time pretty much. I plugged the vacuum gauge into the full manifold port on the carb, will that work? There's a port in the actual manifold but I figured manifold vac on the carb was the same. It's weird though, when I revved the motor up, the vacuum went up....isn't it supposed to be the opposite?

[1969c10joe]

hate to flip flop if the black smoke only shows up when its misfiring that could be electrical.

[1969c10joe]

and i was just gonna add low vacuum can do this at idle on an edelbroc its wrong stepup springs dave sounds like he knows what hes talking about

[1969c10joe]

looks like its time for a vacuum gauge

[1969c10joe]

I have one plug it will break if i put a socket on it. I ground down an open end /box wrench no sweat now im learning about holley carbs cool place this forum

[1969c10joe]

only under load. not in neutral. when in gear and you hit the loud pedal vacuum will drop you need more fuel and your vacuum advance will drop. I use full manifold vacuum some dont. I wonder how much vacuum advance you have at idle

[1969c10joe]

as i recall you said it ran fine with that carb before you did valve train work. Is it possible your idle was set higher before work? your vacuum would be higher. Try turning up idle and see what happens.

[1969c10joe]

Try hooking up vacuum advance to carb so it always has vacuum and raise idle get your vacuum up above powervalve values and see what happens

[RichardJ]

>>when I revved the motor up, the vacuum went up....isn't it supposed to be the opposite?<<

You got something right, maybe? When you blip the throttle, manifold vacuum should drop for a split second, then rise as rpm increases if you are holding the throttle.

The port on the side of the metering block is ported vacuum. Vacuum will rise at this port when you blip the throttle. 6" at that port means you don't have the mixture screws adjusted properly, you don't have the idle speed screw adjusted properly and you probably don't have the timing set properly.

The Power Valve is selected by dividing the manifold vacuum by 2. The 6.5 or 65 is "Hg that the valve opens. You would use that if your manifold vacuum is 13"Hg.

The problem you've got is that everything is too far out of adjustment to know what your actual manifold vacuum is.

A lot of guys are going to disagree, but I'd remove the Vacuum Advance from the manifold vacuum and leave it off and that port plugged until you get your other problems solved.

If this helps.



What is your initial timing?

What is the idle rpm?


If Comp cams say .020" then set the damn lash at '020" and stop experimenting.

Just to be clear .020" is a light drag on the feeler gauge. .021" is a heavy drag and .019" feeler will drop through.

[Mrturner1]

Originally Posted by RichardJ

>>when I revved the motor up, the vacuum went up....isn't it supposed to be the opposite?<<

You got something right, maybe? When you blip the throttle, manifold vacuum should drop for a split second, then rise as rpm increases if you are holding the throttle.

The port on the side of the metering block is ported vacuum. Vacuum will rise at this port when you blip the throttle. 6" at that port means you don't have the mixture screws adjusted properly, you don't have the idle speed screw adjusted properly and you probably don't have the timing set properly.

The Power Valve is selected by dividing the manifold vacuum by 2. The 6.5 or 65 is "Hg that the valve opens. You would use that if your manifold vacuum is 13"Hg.

The problem you've got is that everything is too far out of adjustment to know what your actual manifold vacuum is.

A lot of guys are going to disagree, but I'd remove the Vacuum Advance from the manifold vacuum and leave it off and that port plugged until you get your other problems solved.

If this helps.



What is your initial timing?

What is the idle rpm?


If Comp cams say .020" then set the damn lash at '020" and stop experimenting.

Just to be clear .020" is a light drag on the feeler gauge. .021" is a heavy drag and .019" feeler will drop through.

I think the low vacuum is directly related to the big cam with lots of overlap, not all the settings wrong. Timing is set at 18* initial, 36 total all in at 3000 rpm. And the mixture screws are 2 turns out from bottomed. Idle rpm is 800

[1969c10joe]

Big cam low vacuum try what i suggested if it works then replace power valve your right and throttle response will improve not that you need it mr turner

[1969c10joe]

my suggestions are merely to help diagnose rich mixture after you can return to correct settings

[Mrturner1]

I'm gonna do a compression test tomarrow, I got a new gauge today. I have two gut feelings, one, my rings are giving up and letting a ton of oil and blowby through, and two, I think my new solid cam may be going flat. I set the lash too tight, and if the lash is too tight then there's no buffer between the lifter and rocker arm and that's how you get a flat cam correct? I think that's why my rocker arms keep coming loose. Just a couple of hunches is all, I'll have facts tomarrow.

[1969c10joe]

black smoke is fuel and blue smoke is oil.Yes you should select correct power valve.Also if oil is fouling plugs they look different. you might have more than one thing wrong withcarb

[midniteblues]

Dude bro...your lash is fine .15-.18 is ok but ya gotta remember a good warm engine things will expand some and loosen up.

Its a shame somebody cant masterlock up your internals and keep you out of the innards!

VERY BIG QUESTIONS HERE.

Did you replace that oe resistor wire properly? Please tell me you did. Gotta feed it the proper 12v juice for that coil and plugs too fire properly and burn all of the cly. Fuel/air charge

Do you have more than enough clean and proper engine to frame to body grounds? If ya feed the plugs the good 12v its gotta have a good clean path to travel out of the block and back too the charge system. Believe it or not bad motor grounding can lead too weak spark and fouled plugs.

If your bowls are "half" way up the sight plug the float level is too high.

You should be able too remove the plug and have no fuel come out of the hole while its running.
Only if you gently rock the truck should it trickle out.
That is a starting point for float level.Start there and adjust the float up during test runs. After you get the plugs too stay clean.

A 750 is big but can be made too work ok.
At this point i would not be concerned with the power valve forget about it for now unless you had a serious back fire.

You need to find what your fuel pressure is. Holleys can take alot but you can still over power the needle and seat.

Can you turn mixure screws all the way in "lightly" seated will it still run without dying out?

Your getting TOO much fuel and also you could not be burning it completely.
Lets be sure you dont have dirt in the needle and seat and good fuel flow.


There is lots of little bits of very good advice in this thread its up too you on what you do with it.

Btw i would give g.o.m the key😁

[Mrturner1]

Originally Posted by midniteblues

Dude bro...your lash is fine .15-.18 is ok but ya gotta remember a good warm engine things will expand some and loosen up.

Its a shame somebody cant masterlock up your internals and keep you out of the innards!

VERY BIG QUESTIONS HERE.

Did you replace that oe resistor wire properly? Please tell me you did. Gotta feed it the proper 12v juice for that coil and plugs too fire properly and burn all of the cly. Fuel/air charge

Do you have more than enough clean and proper engine to frame to body grounds? If ya feed the plugs the good 12v its gotta have a good clean path to travel out of the block and back too the charge system. Believe it or not bad motor grounding can lead too weak spark and fouled plugs.

If your bowls are "half" way up the sight plug the float level is too high.

You should be able too remove the plug and have no fuel come out of the hole while its running.
Only if you gently rock the truck should it trickle out.
That is a starting point for float level.Start there and adjust the float up during test runs. After you get the plugs too stay clean.

A 750 is big but can be made too work ok.
At this point i would not be concerned with the power valve forget about it for now unless you had a serious back fire.

You need to find what your fuel pressure is. Holleys can take alot but you can still over power the needle and seat.

Can you turn mixure screws all the way in "lightly" seated will it still run without dying out?

Your getting TOO much fuel and also you could not be burning it completely.
Lets be sure you dont have dirt in the needle and seat and good fuel flow.


There is lots of little bits of very good advice in this thread its up too you on what you do with it.

Btw i would give g.o.m the key😁

I don't know what wire you're talking about. I was supposed to replace a resistor wire? I had some wiring trouble a while back but that was one of the old wires from the points system that was on it. I didn't think that needed to be replaced because I have HEI now.

[davepl]

Originally Posted by Mrturner1

I don't know what wire you're talking about. I was supposed to replace a resistor wire? I had some wiring trouble a while back but that was one of the old wires from the points system that was on it. I didn't think that needed to be replaced because I have HEI now.

You should search the forums on this one (resistor wire distributor) as it's fairly important.

In brief, however, in order to avoid burning the points the factory used a wire with resistance built into it. It's the white wire, normally. When you change or upgrade to an ignition that wants a full 12V, you need to get rid of the resistor wire and power your ignition with a full 12V from the fusebox.

If you keep the resistor wire you're basically trying to power a 12V unit with 6V, and the results can be unpredictable.

In your case, however, you've got a fuel issue to fix before you start chasing zebras. Fix the horses first.

[RichardJ]

>> I set the lash too tight, and if the lash is too tight then there's no buffer between the lifter and rocker arm and that's how you get a flat cam correct?<<

Buffer? What the hell do you think you're talking about?
If the valves are too tight, you run the risk of holding the valves off the seat or not letting the springs hold the valves tight against the seat for good heat transfer. That can lead to burnt valves.
If the valves are too loose, the valve train is noisy and really loose, hammering causing excessive wear.
The most important aspect of valve lash is to allow for heat expansion and not reach either of the extremes mentioned above. Hydraulic lifters self adjust to allow for expansion, never need readjusting and are much quieter.

Stop trying to use valve lash as a tuning aid.

The cam is not causing the rich mixture. The rings are not causing the rich mixture. A flat cam lobe is not causing a rich mixture.
You don't wipe out 16 cam lobes at the same time. If all the adjuster nuts are at approximately the same height, you don't have a cam problem. You did just adjust them. I have to assume you adjusted them correctly.

Stop flitting from one wild guess to the next.

Get rid of the vacuum advance. adjust idle down to 600 and see if the idle mixture screws are responsive to adjustments in or out from the 2 turns you set them at.
I'm making the assumption that your 2 turns out is a setting and not an actual adjustment. Use your vacuum gauge and a tach.

>>36 total all in at 3000 rpm.<<
Who cares. At this point it is more important to know at what rpm mech advance begins.

[Mrturner1]

Originally Posted by RichardJ

>> I set the lash too tight, and if the lash is too tight then there's no buffer between the lifter and rocker arm and that's how you get a flat cam correct?<<

Buffer? What the hell do you think you're talking about?
If the valves are too tight, you run the risk of holding the valves off the seat or not letting the springs hold the valves tight against the seat for good heat transfer. That can lead to burnt valves.
If the valves are too loose, the valve train is noisy and really loose, hammering causing excessive wear.
The most important aspect of valve lash is to allow for heat expansion and not reach either of the extremes mentioned above. Hydraulic lifters self adjust to allow for expansion, never need readjusting and are much quieter.

Stop trying to use valve lash as a tuning aid.

The cam is not causing the rich mixture. The rings are not causing the rich mixture. A flat cam lobe is not causing a rich mixture.
You don't wipe out 16 cam lobes at the same time. If all the adjuster nuts are at approximately the same height, you don't have a cam problem. You did just adjust them. I have to assume you adjusted them correctly.

Stop flitting from one wild guess to the next.

Get rid of the vacuum advance. adjust idle down to 600 and see if the idle mixture screws are responsive to adjustments in or out from the 2 turns you set them at.
I'm making the assumption that your 2 turns out is a setting and not an actual adjustment. Use your vacuum gauge and a tach.

>>36 total all in at 3000 rpm.<<
Who cares. At this point it is more important to know at what rpm mech advance begins.

Give me a break dude, were you born with all your knowledge about motors or did you learn by trying it out and making mistakes?

Solid cam lash is actually a tuning aid for those who know what they're doing, not there yet but it's not like I'm blowin it by giving it a shot to see how things change.

There's no flitting from one wild guess to another, I'm here with the truck, and know the thing inside and out so there's a good chance my hunch is correct, and that's all I was saying, it was a hunch.

You asked about all my timing and carb settings, so I told you what they were and you say it's not an adjustment it's a setting, and who cares about my timing. You asked, I assumed you wanted to know. So does my timing and mixture screws really seem that off to you??