Idle Misfire (1 Viewer)

[nachohouse]

Looking for some advice on a persistent idle misfire I’ve got on my 2/70 1F. I purchased the cruiser a year ago and I’ve been a frequent visitor here to learn about everything FJ40. I don’t know much about the history of the engine, if it was ever rebuilt, etc. I do know that the entire block + rocker cover is painted orange . Maybe I’ll just try to convince myself that the miss is actually just a cam.

I think I’ve read most of the top threads on mud about all things related to vacuum leaks, carb tuning and ignition. Rundown of everything I know so far/looked into/completed.

• Capped my vac retard dizzy and capped all open ports on the carb
• Replaced the PCV valve, grommet, and hose clamps
• Sprayed carb cleaner all around the intake manifold (top + bottom), carb base, etc…no change to idle. All other vac hoses to the vac transfer + brake booster are good to go.
• Carb was rebuilt a couple years ago (not sure who did it, but PO told me it was sent off right before I purchased)
• I don’t have a tach, but the engine feels like it’s idling in the 600-700 ballpark. I confirmed by looking into the carb that there is no fuel coming out of the main jet at idle. The engine also dies when the idle solenoid is disconnected, so I assume the idle circuit is functioning like it should?
• Vacuum steady at 16-17 when I hook my gauge into the brake booster hose plumbed into the intake manifold
• Timing set at 13 deg. advanced with my harbor freight timing light
• Replaced rotor, cap, coil, OEM plugs, OEM wires, condenser and OEM points. Points gapped to 0.0018, plugs to 0.032.
• Adjusted valves, exhaust 0.0014, intake 0.008. I don’t know if feel totally confident in the adjustment here, so I plan to go back and take another look. A few of the valves seemed a little tight, so I did make sure those were closer to spec.

Running out of ideas, any help appreciated. I feel like the last thing to do is bite the bullet and get the Pertronix? Thanks for the help!

 

[45Dougal]

Tried pulling the plug leads one at a time to see if you can isolate it to one cylinder?

How does the spark actually look?

Any other known problem areas? Ignition switch? Battery? Grounding straps? Fuel tank & filter...?

 

[charliemeyer007]

Look at the contacts inside the distributor cap - if that are aluminum, they get an oxide layer very quickly which acts like an insulator. I used the small blade on my swiss army knife to scrape it off. NAPA Echlin caps had brass which stays clean longer. Make sure your vacuum booster isn't leaking - it will burn the #5 cylinder.

 

[mattressking]

Stop chasing other fixes, your problem is your vacuum. Fix your vacuum leak. If it's a points truck check/adjust dwell also.

16-17inHg is too low, so cap off all vacuum operated accessories, if it's still low, you need to replace your manifold gasket/seal it correctly. Also do an inspection of spark plugs, make sure they are firing evenly on all 6. Note color and condition for each cylinder....i.e., oily, black, gray, white....carbon build up.

In order to have a smooth idling truck and smooth fuel transition in carburetor....you need a bare minimum of 18inHg. Ideally 21-22 at sea level.

 

[nachohouse]

Tried pulling the plug leads one at a time to see if you can isolate it to one cylinder?

How does the spark actually look?

Any other known problem areas? Ignition switch? Battery? Grounding straps? Fuel tank & filter...?

I haven’t tried isolating it to a particular cylinder. I can do that and report back. Battery is a new optima red top. No ground straps, I suppose I could add some. Block seems to have good ground according to the multimeter.

The spark seems weak when I grounded the #1 plug to the block. What should it look like? I’m new to all this stuff and might be comparing to YouTube videos of msd ignition muscle cars in my mind.

 

[nachohouse]

Look at the contacts inside the distributor cap - if that are aluminum, they get an oxide layer very quickly which acts like an insulator. I used the small blade on my swiss army knife to scrape it off. NAPA Echlin caps had brass which stays clean longer. Make sure your vacuum booster isn't leaking - it will burn the #5 cylinder.

I pulled the new cap after running it for maybe 20 miles and it already had some oxide buildup. Do I need a brass studded cap?

How do I test the brake booster for leaks?

 

[nachohouse]

Stop chasing other fixes, your problem is your vacuum. Fix your vacuum leak. If it's a points truck check/adjust dwell also.

16-17inHg is too low, so cap off all vacuum operated accessories, if it's still low, you need to replace your manifold gasket/seal it correctly. Also do an inspection of spark plugs, make sure they are firing evenly on all 6. Note color and condition for each cylinder....i.e., oily, black, gray, white....carbon build up.

In order to have a smooth idling truck and smooth fuel transition in carburetor....you need a bare minimum of 18inHg. Ideally 21-22 at sea level.

I don’t disagree with you, I would like to have more vacuum but I’m not convinced there’s a big vacuum leak yet. I think you’d also be hard pressed to find original F engines out there routinely pulling 21-22 based on what I’ve read. Correct me if I’m wrong.

I fogged the manifold gaskets with carb cleaner and the idle didn’t change. The FSM says 16.5+ is in spec. I know I’m right at that threshold.

You positive it couldn’t be ignition? Do you have an F? What manifold gasket are you running? Remflex?

Also, when you say to check dwell are you saying I need to run a tighter points gap? What’s the dwell spec to shoot for?

 

[45Dougal]

I haven’t tried isolating it to a particular cylinder. I can do that and report back. Battery is a new optima red top. No ground straps, I suppose I could add some. Block seems to have good ground according to the multimeter.

The spark seems weak when I grounded the #1 plug to the block. What should it look like? I’m new to all this stuff and might be comparing to YouTube videos of msd ignition muscle cars in my mind.

There must be at least one ground strap from the battery to the engine. Worth checking its in good shape.

A consistent blue spark is what you want. If its yellow then there's something wrong.

A lot of these misfires are a combination of several minor issues coming together in combination.
Improving one issue might fix the symptom.

 

[45Dougal]

Re. Testing the booster:

Suck and listen?

 

[nachohouse]

There must be at least one ground strap from the battery to the engine. Worth checking its in good shape.

A consistent blue spark is what you want. If its yellow then there's something wrong.

A lot of these misfires are a combination of several minor issues coming together in combination.
Improving one issue might fix the symptom.

Gotcha, spark definitely looked consistent and blue. Just wasn’t the lightning storm I was expecting I guess.

I’ll see about adding a ground strap. Ideal location for this? Looks like I have a bolt hole below the front engine hook that might work.

 

[nachohouse]

Re. Testing the booster:

Suck and listen?

Ok, booster seems solid then. I have a manifold vacuum port that’s normally capped at the base of my carb also (I think it went to a now-removed VSV) and the vac level is the same with the booster hooked up.

 

[charliemeyer007]

You can put a big hemostat on the booster hose, see if your misfire goes away. I you drive it that way you will not have power assist brakes - it will still stop you just need to press down harder. Or disconnect the hose and plug the port on the intake manifold.

I like the brass contacts and that is all I would buy.

The really good spark plug wires are worth the investment.

At the hanger we had a spark plug breakdown tester. Even new plugs could have the spark blown out at 60psi. Proper gap and a square edge on the center electrode that same plug would still dance fire at 175psi

 

[nachohouse]

Another detail I forgot to mention, the rpm’s and firing seem smooth off-idle. When I give it a little gas things feel and sound pretty steady.

 

[45Dougal]

Another detail I forgot to mention, the rpm’s and firing seem smooth off-idle. When I give it a little gas things feel and sound pretty steady.

Check your battery voltage - if its too low without the alternator, then can cause a weakened spark. (Another contributor)

 

[pb4ugo]

What's your compression #'s? Does your harbor freight timing light have a tach? You need a tach so you can watch the rpms drop when you disable one cylinder at a time. The cylinder that effects the rpms the least is probably the problem.

 

[pb4ugo]

Does your vacuum gauge needle shake?

 

[nachohouse]

Check your battery voltage - if its too low without the alternator, then can cause a weakened spark. (Another contributor)

Voltage is good, 12.6. Alternator works good also, ~14 when the engine is running.

 

[nachohouse]

Does your vacuum gauge needle shake?

Maybe a tiny bit, but I’d say the vacuum is pretty steady. I haven’t tested compression yet. My next step is probably going to be that and getting a tach.

I noticed that my multimeter has a tach actually but the numbers were way off. I hooked the neg lead to battery negative and the positive lead to the neg side of the coil and it gave me 2000+ rpm which is definitely not accurate. Then I tried wrapping a wire around #1 plug wire and hooking the meter’s positive lead to that and got ~160 rpm. Meter is not from harbor freight but it definitely acts like it. How do you guys hook-in a tach?

 

[charliemeyer007]

I have a tack/dwell meter I seldom use. I like my old craftsman inductive timing light. I run a vacuum on my rig - really helps on mpg - I watch the needle and set my gas pedal by it. A lot of time the engine noise changes but the speed doesn't

Tuning by ear is becoming a lost art.

 

[pb4ugo]

Maybe a tiny bit, but I’d say the vacuum is pretty steady. I haven’t tested compression yet. My next step is probably going to be that and getting a tach.

I noticed that my multimeter has a tach actually but the numbers were way off. I hooked the neg lead to battery negative and the positive lead to the neg side of the coil and it gave me 2000+ rpm which is definitely not accurate. Then I tried wrapping a wire around #1 plug wire and hooking the meter’s positive lead to that and got ~160 rpm. Meter is not from harbor freight but it definitely acts like it. How do you guys hook-in a tach?

Usually a tach needs 12volts, a wire to the neg side of the coil and a ground. I don't know how you would hook up a multimeter. Do you have the instruction for the meter? You might try hooking up the positive probe on a 12 volt source and the neg probe on negative side of the coil. I like my timing light, it has many functions, tach, volt, dwell, rpms, and adjustable timing advance strobe. You asked about dwell, Points can be set with a feeler gauge or set by using a dwell meter. Imo, dwell is more accurate. Maybe somebody can chime on what that setting is.