High throttle miss

[Texasdoc]

New carb not better. Ran a bit hotter but still missing. There is a local guy/shop with a five gas analyzer, but not sure if they have the scopes or oscilloscope. Will put everything back together and take it to them.

[Gaz64]

Originally Posted by Texasdoc

Question: Would a cam that is incorrectly degree'd cause a high speed miss? I degree'd it when I installed it, but if I made a mistake then, would it cause a high RPM miss? Just grasping at straws here. I don't think this is the problem as it ran great with the EFI (whenever the O2 sensor decided it wanted to work that day). I don't remember what I ended up setting it on but I think it was advanced.

I also had to use a shorter timing chain due to excess slack from having the block bored/honed. I doubt a shorter timing chain would affect the cam to crank position enough to cause problems.

Sounds like you have a chain set for main saddle line bored block.

The sets I have used for this have slightly larger gears, same chain.

Your misfire is ignition or engine related (valvespring etc), single 4 barrels don't cause misfiring, they can cause surging or low power, flat spot etc.

Misfiring is a dead cut on one or more cylinders.

Gary

[Texasdoc]

Just for documentation:

Note - this is on a cold engine. I know the leak down is more accurate on a warm engine as the rings will seal better. Plus, I forgot to hold open the throttle when doing the compression test - but I kept cranking until the pressure was stable.

Compression / Leak down % :
Cyl 1 - 180/5
Cyl 2 - 180/3
Cyl 3 - 175/3
Cyl 4 - 183/3
Cyl 5 - 185/2
Cyl 6 - 184/4
Cyl 7 - 187/6.5
Cyl 8 - 185/3

[Gaz64]

Maybe you can run the car on a dyno with an ignition oscilloscope and wideband O2 sensors per bank.

The scope may show which cylinder is misfiring and the O2s will show which bank has high oxygen, (no combustion).

[Texasdoc]

I finally got the WBO2 sensor installed. I previously set the idle screws at best vacuum at idle.

The youtube video (click here) shows some interesting results. The video has four snippets.

First section is cold start - cold idle - i have to hold the pedal for a few seconds at 2000 rpm or so until it warms up enough to run.

Second section is about 20 seconds at steady cruise. Third section is hot idle waiting at a light. Fourth is the two WOT pulls in second gear. There was a bit of traffic this morning, so I couldn't stay in it very long. The last few seconds of the clip you can see it starts to break up at around 4500 rpm.

Some observations:

Cold and hot idle is very rich. 11:1 or so. This surprised me since I had set the idle screws to best vacuum on a hot motor.

Cruise is about 11.5:1 - this is because I am basically running on the idle circuit, maybe a little of the transition circuit. Since the idle circuit is so rich, cruising around 1500 rpms would be too.

It is hard to read the meter at WOT with all the shaking and the video frames fighting, but it looks like WOT is around 12.5-13. This seems fine. It doesn't seem to go dead lean as would be expected with it mis-firing.

When I got home, I adjusted the idle mixture screws. It was very sensitive to even slight changes. Screwing in the screws even 1/16th of a turn resulted in about 0.5 to 1.0 change on the gauge. I didn't know the idle mixture screws were so sensitive - plus, it dropped my intake vacuum from 14 to 13 inHg going from 11:1 to 13.5:1. I also had to increase my idle screw a touch to reset the idle back to 1000 rpm as it this change dropped my rpms about 75-100 rpms.

[olddog]

Although I never adjusted a carb with a wide band, that seems way too big of a change for a 1/16th of a turn.

If the O2 senses no change when it is breaking up, that would tend to indicate that the fuel is getting burned.

If there was a cross fire and a cylinder fired at the wrong time, the fuel would get burned, but the engine would miss.

If there was a valve issue such as float, the fuel would burn, but the engine may seem to miss if the float was only on one cylinder.

Thought to consider: Are you experiencing pre-ignition. A high compression engine with a big cam has a low dynamic compression at idle. At high rpm the dynamic compression goes much higher. Is your dynamic compression going high enough that the fuel is lighting before the spark. Fuel lighting too soon kills power because it is preventing the piston from coming up. It may seem like a miss-fire.

I hate running a cold engine hard, but would it break up on a cold morning with a cold engine?

PS
Do you have an O2 on each bank? Or is both banks going into one pipe? If the O2 is on one bank, it may mean that the cylinder that is breaking up is on the other bank.

[Texasdoc]

My O2 sensor is only on one side - the passenger side. So, if it is not firing, it would be on the driver's side.

I don't think it is cross fire. I've re-routed the wires multiple times. I've ran the motor in the dark and don't see any sparking.

Pre-ignition - maybe. The cam isn't that big and I only have 9.5 to 1 static compression so I don't think my dynamic compression is very high. I'll have to study more about this and calculate my dynamic compression.

Valve float: I just got a valve spring compressor. I'm going to check the springs. There aren't any obviously broken springs but I may have a weak or damaged spring.

Edit: Dynamic compression 8.76:1

[blykins]

Was that Wallace's calculator?

Download Pat Kelley's calculator and let us know what you have there. Also, make sure you're using advertised duration instead of .050".

If you are under 8.3-8.4 on Pat's calculator, you're golden.

[Texasdoc]

My Cam card doesn't list advertised timing events, only .050. Using the second tab of the program, it calculates the timing events based on .050 numbers.

So, my dynamic compression on Pat Kelley's program is 7.427:1.

[blykins]

Originally Posted by Texasdoc

My Cam card doesn't list advertised timing events, only .050.

I've got Gross valve lift, valve duration @ .006, valve duration at .050, valve timing at .050, lobe lift, and lobe separation.

Edit: Hang on, the second part of the program looks like it will calculate the advertised closing based on the advertised numbers and centerline/separation numbers....

Duration at .006" is advertised duration for a hydraulic cam. You always use advertised duration when calculating DCR.

[blykins]

You should have no problem whatsoever with compression ratios....

[Texasdoc]

Originally Posted by blykins

Duration at .006" is advertised duration for a hydraulic cam. You always use advertised duration when calculating DCR.

Yes, but the program was asking for advertised timing, which is not listed on the card. However, on the second tab of the program, you can put in the advertised duration, intake centerline, and lobe separation, and it will give you advertised timing.

[blykins]

The advertised duration *is* on the card. It's the duration at .006".

If you're using pat Kelleys program, you would input the durations at .006", the LSA, and the ICL.

[Gaz64]

After watching your video many times, I can't see the misfire being anything else other than an intermittent ignition misfire.

It occurs a various rpms and loads.

I had a set of Autolites in mine a few years ago, went hard for 300 km, then had intermittent miss, one spark plug faulty.

Generally the mixtures are too rich, have you got any photos of the spark plugs when they were out last?

Gary

[Texasdoc]

I just put in brand new plugs. The photos in this thread clicky are of the last plugs. It is going to the shop today.

[lippy]

Keep in mind that the wideband meter will point you in the right direction but it's not gospel. There are a number of factors that can affect the output of the meter, including: (1) sensor accuracy, (2) placement and variation from cylinder to cylinder and bank to bank, and (3) if the cam is big, readings at low RPM will be useless. Ask me how I know.

Long story short, I kept these factors in mind and used the meter as *one* tool, but relied more on the way the car felt and drove, and reading the plugs, to get the final tune right.

[blykins]

Lippy is spot on here. A wide-band won't do you much good with a camshaft with a decent amount of overlap. Your camshaft may not be that big as far as .050" duration goes, but most slower ramped hydraulic rollers will be fairly large as far as advertised duration goes, which increases overlap. Could very well be why the A/F ratio was showing oinky rich at idle.

Plug reading is your best friend. You can get a lot of info from how the tip of the porcelain looks, how the base ring looks, and how the strap looks, but you also want to look *down* into the porcelain and see what color it is down inside. If you're a doc, then you should have access to some pretty good tools for close inspection.

[jhv48]

I'm still betting on ignition breakdown.

[Texasdoc]

We will see. I'll report back here when I get a report back from them.

It seems like ignition break up to me to but for the life of me I can't figure out why. New 6AL box, tested with a different distributor, different plug wires, 3 different coils, new plugs. 6AL box has the MSD capacitor to provide clean power. Verified 12V at box on both big and small red wires. Verified rotor phasing. Verified ground at box, motor grounded to frame at two places. No vacuum advance. Verified balancer at TDC, verified total advance at 36 (tried 34-38).

[Jac Mac]

The cranking compression numbers (~ 185lb ) seem quite high for the cam & static compression you have mentioned and comparing your Autolite 3924 to Champion numbers ( RC12YC ) they seem too hot compared to similar stuff I have built ( RC9YC ). Can you verify following?
1. Bore & Stroke
2. Head Combustion Chamber Volume ( 58cc or 72cc ).
3. Assembled Piston to Head clearance.
4. Piston part # or dish volume.