Low Load Miss...
 

Been experiencing a low-load, mid-rpm miss on my 408 lately. Well, really for a long-a$$ time, now. Trying to even remember when it started. Seemed to start when I reterminated my spark plug wires to get them the right length, but other plug wires don't make any difference. I'm starting to think fuel.

Miss happens when I'm very gradually accelerating in any gear except 5th. It's as if the engine is stopping and starting as opposed to losing 1 cylinder. I'm not sure if I'd be able to notice losing 1 cylinder at speed.

The car lightly bucks, lurches, whatever you want to call it. I notice the miss up to about 4K RPM as long as I'm being really nice to the motor. If I lay into it, no such thing as a miss.

Could this be a primary circuit fuel problem? Leaning out?

Possible solutions? This car sounds like such a beast at WOT but I wouldn't mind having to not be embarassed at more reasonable throttle applications!! :LOL:

What carb are you running? be specific

Barry Grant Speed Demon 750 DP.

Need more specifics? Don't remember what jetting we have in (it's on the dyno sheet somewhere) but it is up a step (maybe 2?) from stock. Also went up a power valve size.

Let me know if you need more info, and I will do my best to get it quickly. Thanks!!!

Maybe a vaccum leak? Also, the front accelerator pump may need more squirt. When you floor it, the back pump, which is usually bigger, squirts to cover the lean spot until your on the main circuit. This could be why on partial throttle, you get the stumble. Good luck, scott.

Tighten up?
 

I had a similar hesitation under part throttle, almost like the car was running out of fuel! And yet it worked great at w.o.t! Also a back fire out of both side pipes while down shifting or easing off the accelerator.
I noticed a spary pattern of dry fuel on the passenger side valve cover and on the fuel log, It turned out that the six phillip head screws that hold the base plate to the main body of the demon carb had loosened and caused a small vacum leak between that gasket. I hope yours is an easy fix! %/

My first thought was a plug wire that was not on properly...

I am thinking more accel pump squirt needed, or a longer duration of pump squirt (smaller nozzle size) on the primary side. If you can't get it out with the accel pump, try one primary jet size bigger.

Do you have vac advance on your distributor? Could be too much advance under the conditions you describe. Had exactly the same symptoms on another engine - nothing to do with carbs, as I found out eventually.

HTH

Could be a few things. A lean mix, try pulling on the choke and see if it goes away, that is if you have a manual choke if not does it miss when it is cold? if not it is lean.
To much spark advance will cause a condition like you describe. If you have a vacuum advance you may have a bad one that is causing the spark to bounce. Try disconnecting and plugging the hose. You can also back off the timing a bit to see if it helps.
You said you went up on the power valve? The valve should be a bit under half of your idle vacuum reading. Normal valve is 6.5. Idle vacuum is normally 14-18. So half of 14 would be 7 to be safe a step down gives you 6.5. So if your car has a reading of 12 you would want to go to a 5.5 or less. The idea is to richen up the mix when engine vacuum drops below the power valve setting as can be seen at a high cruise speed.
That may also be why you do not see it in 5th gear as the engine is turning slower so the valve is not needed.
The accelerator pump have nothing to do with a cruise miss. What does the tach show when the car starts to miss??

Miss happens when I'm very gradually accelerating in any gear except 5th.

(he didn't say a steady miss while cruising)

vacuum leak is good possibility

Ok fine. Still stands that under light acceleration the accelerator pump is doing very little. He did not say for how long he was holding it at the miss point and since I was looking at it as if I would be test driving it to look for the problem I put what I hoped would be another opinion and idea of a possible solution.
I have seen this same problem on 6-pack cars and most can be traced to a mix or spark problem. Just going on my 25+ years of working on cars nothing more nothng less.

Hey guys, little bit more info...

Miss is happening during the course of an acceleration (very light accel) in first 4 gears. Given my shifting pattern through the neighborhood (when it's most noticeable), I'm seeing the miss between 2.5-4K.

I could be wrong but I don't really remember noticing the miss that much during a TRUE steady cruise. Although come to think of it, maybe it is missing then. I DEFINITELY feel it, however, when I "breathe" on the gas.

There is no choke on the carb. It misses when very cold, yes, and it is somewhat difficult to keep running for the first 5-10 seconds.

I did notice the allen-head bolts running horizontally that seemed to "Keep the front and back of the carb on" were loose when I first inspected the motor this season. I snugged those up. Could it be over the winter that the gaskets (are there any?) deteriorated?

Perhaps it would be in my best interest to pull the carb off and replace the carb to intake gasket, snug all bolts on the carb itself, etc...

To check vacuum, just hook up a vacuum gauge to the PCV hole on the carb and simulate the driving conditions??

A little more history: When this first started, I had switched to Champion race plugs. I thought they could be the source (after I changed them, the car missed...but then again I did some other things, too), so I pulled them all back out. #7 was clean as a baby's butt. Wait, that's not clean, that's smooth. #7 was as clean as something that's really clean...sorry, at a loss of great metaphors or hilarious comparisons.

So, needless to say, I've been trying to pinpoint the problem to the seventh hole. But to me, the miss feels more like the whole engine is going back and forth between choking and breathing fire...rather than just having one dead-miss cylinder.

I'll check back later and see how everyone feels with this info.

Thanks
JP

Aww, man, I thought this was gonna be some kinky discussion...I used to date a "low load miss"...
Seriously, if it misses at a steady light-throttle acceleration, I would doubt it is the accelerator pump, as the throttle probably isn't moving, after the pedal is pushed down. That makes me think too much advance or too lean (but not on primary jets unless you did something to the carb)...had both happen. disconnect the vacuum pot on the distributor if it has one, see if the miss goes away. If not, maybe a vacuum leak on the #7 runner?Let us know what you find.

If it started after you changed the plugs put a standard set in and see if the problem goes away.
Repair shop motto. Start at the last thing the customer did before the problem.
I can not tell you how many sets of "Fancy" plugs I have replaced over the years. Bosh platinum seem to be the most at going away before they wear out. Just from what I see nothing more.
Still sounds like a lean miss to me. If changing the plugs does not help, be sure to gap them, you could try and old trick for carbs with out a choke. Plug the air filter to see if a richer mix fixes the problem. I have seen folks use an old t-shirt tied around the filter and it works to richen things up. Just an idea.
You may wish to spray a bit of carb clean around the carb base and edges of the intake to check for leaks also. If the engine speed changes you have found the problem are. BE CARFUL as some of the cleaners are VERY flamable if you go that way. Also be sure to stay away from the top of the carb as that will give a false rpm change.
Good luck and let us know what you find out.

Thanks, Bruce.

By the way, it's a mechanical advance distributor. Changed the plugs back to the Bosch Platinums. The spark plugs didn't seem to be the difference at all.

While not a pro, I consider myself to be a decent shade tree mechanic...I have spent some time doing computer programming, etc... and live by the "last known good configuration" policy. Didn't get me very far this time. Could've been coincidental that something I didn't touch happened to fail or change as I was doing the plugs and wires. I will try some of these fuel fixes and let you know. May be a week or two.

Thanks for all your help!
JP

J.P.

Please let us know what you find out.
Good luck on the hunt for the problem!

If it feels like a lean stumble: Pull carb, make sure baseplate is snug to main body. Replace carb to intake gasket, and go up one primary jet size.

If you think it's an ignition miss: Inspect cap and rotor, make sure all plug wires are snapped on good on both ends. You didn't accidently pull one of the sparkplug wire's brass terminals off the wire's core when changing plugs perhaps?

When you first roll into the throttle lightly, if it stumbles for a few hundred rpm and then clears up, like from 2500 to 2900 rpm, then from 3000 rpm up, its smooth, its a fuel problem or a tiny vaccum leak. Once the throttle is depressed slightly, if it stumbles from 2500 all the way to 4000 rpm, its electrical. Plugs, wires, timing, cap, rotor, plug wires arcing, etc. Really direct misses tend to be electrical. Good luck, Scott.

Are you one of the first people trying to integrated GM's DOD (Displacement On Demand)? That's what it sound like you got.:D :LOL: :D :LOL:

John

Engine is quite disassembled right now. Took the front of it off to replace the front crank seal (it was chewed up and spit out pretty bad) and took the carb off, too. Every screw in the baseplate was loose. I'm assuming those are what holds it to the main body? Hope those weren't metering screws of any kind, they sure didn't look like it.

I rebuilt a carb once from an exploded drawing, but I know very little about them. Know the principle, but couldn't build you one, you know?

Anyways, screwed those down tight. All screws are now tight except the 4 corner idle mix screws. And any other ones that I KNOW shouldn't be.... ;)

This weekend will be the test. Will be reassembling with a much *SAFER* carb (can't believe how everything was so loose). I will post results!!

Thanks again, everyone, for your help!

JP