So I've been trying to figure out an engine miss from stone cold start-up for a long time now... it's an on and off project. Once the engine is warmed a bit and rev'ed to clear it out, its good for the rest of the day, even after sitting for hours it will then start and run fine.

So I start the engine, it starts right up but has a definite miss. When put into gear and rev'ed to move it, you can definitely feel it missing. However give it a minute, then rev it up to 1500rpm, it will run rough.. blow a bit of black smoke then smooth out and run fine for the rest of the day.

I have a cold intake manifold (cross-over blocked) so I thought it was fuel condensing on the runners. Also the pre-heat cowling from the exhaust manifold is long gone so it doesn't get pre-heated air. When I rev'ed it, the increased air flow picks up the condensed fuel and makes a rich mixture for a few seconds then its cleared out and all is good. I have Throttle body EFI (originally Howell now EBL with Rochester TB) so adjusting the startup mixture is easy. Maybe fuel-drop-out is normal with a cold manifold. When I start it and pull it out of the shop, there is a heavy gas smell, so I can likely lean the choke setting even more.

What drives me nuts is the miss on one cylinder (I believe its only one). Over the years I have changed plugs and wires (DickP's wires and his recommendation of two different spark plugs). New coil and pickup of the correct polarity from DickP. New ignition module. BTW it did this running on OEM HEI and also now on Spark Control from the EBL.

I've tried starting it and let it run for 30 seconds or so, then pulled the plugs to see if I could troubleshoot which plug the miss was on. I could not see much difference between them.

I tried wrapping little copper wire out of each of the sparkplug wires at the distributor. Using them I could short the spark for each cylinder and see if I could determine which one was missing. I could invoke a miss on every cylinder, even while it was missing. So this lead me to believe it must be a random miss and not one particular cylinder.

It never backfires. However, even warm when rev'ed up to 1500-2000 rpm it runs a bit rough... not smooth like I would expect. It was rebuilt by DickP back in 2009 and I've asked him about it a couple of times... he though it was fine. I didn't specifically ask to have it balanced during the rebuild... I didn't know of such a thing at the time.

Anyway, does anyone have any ideas on my "missing" problem? I've lived with it for years but it just bugs me. I was thinking of putting a pickup coil on the HV output of the coil so I could look at it on my oscilloscope, but with HEI, there is no way to get to that. Maybe scoping the Tach output would give me some indication if an Ignition system problem...?

What say ye?

Thanks

RF_Burns wrote on Sun, 31 May 2020 07:06

So I've been trying to figure out an engine miss from stone cold start-up for a long time now... it's an on and off project. Once the engine is warmed a bit and rev'ed to clear it out, its good for the rest of the day, even after sitting for hours it will then start and run fine.

So I start the engine, it starts right up but has a definite miss. When put into gear and rev'ed to move it, you can definitely feel it missing. However give it a minute, then rev it up to 1500rpm, it will run rough.. blow a bit of black smoke then smooth out and run fine for the rest of the day.

I have a cold intake manifold (cross-over blocked) so I thought it was fuel condensing on the runners. Also the pre-heat cowling from the exhaust manifold is long gone so it doesn't get pre-heated air. When I rev'ed it, the increased air flow picks up the condensed fuel and makes a rich mixture for a few seconds then its cleared out and all is good. I have Throttle body EFI (originally Howell now EBL with Rochester TB) so adjusting the startup mixture is easy. Maybe fuel-drop-out is normal with a cold manifold. When I start it and pull it out of the shop, there is a heavy gas smell, so I can likely lean the choke setting even more.

What drives me nuts is the miss on one cylinder (I believe its only one). Over the years I have changed plugs and wires (DickP's wires and his recommendation of two different spark plugs). New coil and pickup of the correct polarity from DickP. New ignition module. BTW it did this running on OEM HEI and also now on Spark Control from the EBL.

I've tried starting it and let it run for 30 seconds or so, then pulled the plugs to see if I could troubleshoot which plug the miss was on. I could not see much difference between them.

I tried wrapping little copper wire out of each of the sparkplug wires at the distributor. Using them I could short the spark for each cylinder and see if I could determine which one was missing. I could invoke a miss on every cylinder, even while it was missing. So this lead me to believe it must be a random miss and not one particular cylinder.

It never backfires. However, even warm when rev'ed up to 1500-2000 rpm it runs a bit rough... not smooth like I would expect. It was rebuilt by DickP back in 2009 and I've asked him about it a couple of times... he though it was fine. I didn't specifically ask to have it balanced during the rebuild... I didn't know of such a thing at the time.

Anyway, does anyone have any ideas on my "missing" problem? I've lived with it for years but it just bugs me. I was thinking of putting a pickup coil on the HV output of the coil so I could look at it on my oscilloscope, but with HEI, there is no way to get to that. Maybe scoping the Tach output would give me some indication if an Ignition system problem...?

What say ye?

Thanks

Bruce,

I just had a new GMC owner on his maiden voyage home to CA from MI stop by here a couple of weeks ago with a similar "miss". I chased ignition replacing coils, modules, spark plugs to no avail. I was certain it was ignition as that is the way it behaved. This was with just a Howell system. I put my EBL on thinking that might clear up the issue but still persisted.

For some reason, I got the bright idea to watch the injector spray pattern while the miss was occurring. The right side was easiest to see so started there but everything looked good. I then watched the other one. Every time we felt a miss, that injector would not spray for one or two cycles. Felt just like an ignition miss. Going back and studying the log, I could see a momentary lean condition followed by rich as the EBL compensated for the lean cycles. I happened to have an extra set of injectors of the same type that we exchanged and the coach ran great after that.

For what it is worth...

Randy, are you using a timing light to “see” the spray? Or can you just look at the injector in use and see when it fails?

Hi Randy.
I'll take a look at the injectors, but I had this problem with the Howell Holley before I installed the Rochester TB. Still could be the issue since the Rochester had one bad injector when I got it. I replaced them with two new injectors before installing it. New doesn't always mean works perfectly thought.

On my choke settings, the Choke Decay Multiplier %... do you know how that parameter works? I'm assuming that every so many seconds after start-up, it decays the choke by this percentage. Would you know what that timer period is? or am I wrong?

Hi John. I have fuel injection, however there are "choke" settings to adjust the AFR dependent on coolant temperature and a decay function to lean off the choke.

OK,
First time I heard of using the strobe light to freeze the action of spray.
One can learn a lot and also refresh the mind by reading these discussions.
Thank you all for asking and responding.😷

Sir, so possibly you have an injector slowly leaking pressure down when you shut it off causing to rich on next morning start up? Maybe after a hard run shut it off and pull air cleaner lid and look for smoke like a Q-jet does when the Welch plugs leak. But with the intake blocked it may not get hot enough to smoke. Maybe put a small piece of cardboard under each injector and look for possible leak - Wet spot? Do you have a schrader valve in your fuel line? Maybe install a pressure gauge and see how long it holds pressure.




[pquote title=RF_Burns wrote on Sun, 31 May 2020 15:55]On the choke decay loop, from its description, "Choke Initial Delay", I took that as meaning it first delayed for 5 seconds (cold) or 2 seconds hot before starting to decay out. Maybe calling it "Choke decay loop delay" or such might make it clearer.

Rich, This only happens at stone cold first startup after sitting overnight etc. It happens within seconds of starting... after giving it maybe a minute to warm up, I need to rev it up to 1500rpm or so to clear the condensed fuel in the intake.. it will blow a bit of black smoke out the exhaust.. then its alright for the rest of the day... no more missing. I can shut it off and let it sit for hours and it start right up with no issues. Never any hot-start issues.

The fact it does it only when cold, and I can clear it before it gets too warm makes me think its not an intermittent connection.

When I rev it to clear the excess fuel there is sometimes a bit of blue smoke too, but not always. I figured that is just from the PCV as I never need to add oil between oil changes (once a year in the Fall)

I haven't tried a different ECM, but I do have the original Howell unit for a spare. I should plug it in to test it.

Battery is fairly new (2 years). It sits in the shop with the smart converter/charger plugged in so the battery is charged up to snuff. I have an isolator with a combiner connected across it.