[Thoms69]

My engine has been running a little rough lately and smell rich at idle. I decided to pull the plugs and see several that show rich mixture and a few that look pretty good. (see pic)

Before I pulled the plugs, I checked cylinder temps at each header tube close to the head and noticed temps varied from 160 F to 240 F.

Plugs are Autolite 3924

Any ideas on what may be causing this?

 

[SoCal805]

plug wire (s) to the fouled plugs (1,3,7??) could be bad. Is header heat killing those plug boots?

what do the corresponding terminals of the inside of distributor cap look like?

rotor look ok??

 

[Thoms69]

Fairly new MSD distributor and coil all still look okay. I replaced the wires with MSD at the same time. I have insulators over the plug boots to protect against the headers.

 

[SoCal805]

you pulled the cap & looked inside?

Carbureted motors specific cyl foul plug "typically" is bad plug (cracked), plug wire or C&R specific to those offending cyl.


IDK if intake manifold leak, like on computer controlled cars, causes the same rich condition as you have although on those cars it would be for the whole L or R banks depending on what side the plugs show rich

You could try one of those inline spark check tools (fairly cheap at auto part stores) to see if the corresponding cyl show weaker spark. Your temp gun though pretty much shows that though

IDK if there is a way to test distributor per cyl terminal..."maybe" something is off internally

any weird noise coming from under valve cover? (broken spring for those cyl??) might be worth a valve cover pull if you can't find that this is a ignition issue....

 

[DOUG G]

Plug gap ?

Plug number/brand ? Autolite 3924

Fuel type ?

Carb size ?


How old are plugs ?


Thinking these have a lot of idle time and/or cold starts....a hotter plug "may" help also.

 

[Thoms69]

Plug gap ?

Plug number/brand ?

Fuel type ?

Carb size ?

Autolite 3924 gapped to .045. These plugs probably have about 600 miles or so.

91 Octane

QF 750 Q series

 

[Thoms69]

you pulled the cap & looked inside?

Carbureted motors specific cyl foul plug "typically" is bad plug (cracked), plug wire or C&R specific to those offending cyl.


IDK if intake manifold leak, like on computer controlled cars, causes the same rich condition as you have although on those cars it would be for the whole L or R banks depending on what side the plugs show rich

You could try one of those inline spark check tools (fairly cheap at auto part stores) to see if the corresponding cyl show weaker spark. Your temp gun though pretty much shows that though

IDK if there is a way to test distributor per cyl terminal..."maybe" something is off internally

any weird noise coming from under valve cover? (broken spring for those cyl??) might be worth a valve cover pull if you can't find that this is a ignition issue....

I have checked for vacuum leaks around the intake using carb spray.

I will take a closer look at the distributor cap again.

No unusual valve noises but I think I will pull the valve covers and visually inspect everything.

 

[Thoms69]

I inspected the distributor cap and rotor and didn't see anything unusual. I posted a few pictures in case some of you can see something I might be missing.

Thanks.

 

[SoCal805]

I can't zoom in on pics but if the inside terminals look good, especially the ones that correspond to the affected plug/cyl, than cap should be fine

I assume you made sure plug wires were pushed in all the way on cap & plug end

Given your look/testing already I just suggested looking at valve spring/rocker as if a rocker came off or a spring broke or a PR popped out those cyl would not be firing right but in either case you should be able to hear a problem like that....of course looking under VC would confirm. Just pull the one side (DS) that seems to have the affected fouled plugs if you run out of anything else to check

Not sure if a carburetor can FU cyl specific on a motor so my thought is this is either ignition or valve train with the later being unlikely but possible

 

[Thoms69]

I was just checking all my ignition voltage, etc and noticed I had grounded the MSD distributor to my aluminum intake manifold. I wonder if I should ground to the chassis of battery.

 

[SoCal805]

I was just checking all my ignition voltage, etc and noticed I had grounded the MSD distributor to my aluminum intake manifold. I wonder if I should ground to the chassis of battery.

wait, what?

do the instructions that come with the MSD distributor say to ground it to "chassis"....or - terminal on coil?

and are you using the "stock" white wire that goes to the + coil terminal for your "HEI" distributor? (the answer needs to be no)

 

[Thoms69]

No resistor wire. Its getting a full 12 volts.

This is a MSD ready to run. MSD instructions says to go to engine ground. I went to the aluminum intake because it was close. I am wondering if this is a sufficient ground.

 

[SoCal805]

I would have gone to the ground stud on firewall where the braided ground straps from VC bolts go.

With that said the aluminum is a conductive metal and should ground to the iron block....but you could move your ground to engine or other non aluminum base and see if your rough motor runs smoother if you are ? the aluminum ground point

 

[Thoms69]

I would have gone to the ground stud on firewall where the braided ground straps from VC bolts go.

I would have but it is buried and difficult to get to.

With that said the aluminum is a conductive metal and should ground to the iron block....but you could move your ground to engine or other non aluminum base and see if your rough motor runs smoother if you are ? the aluminum ground point

I just talked to MSD and they recommend the battery or a battery junction to ground. I am going to try that.

 

[bob15]

You could swap plugs around, drive it and see if it follows the plugs or stays with the same cylinders. And/or try doing the same thing with the plug wires.

 

[ilikeike]

If everything else looks ok,you could play with plug heat range a little,
Just for a reference,I run an Autolite AR3923 with AFR heads.

 

[COPO]

Try 1 heat range higher and if that doesn't help then lean out the jetting on the carb.

 

[Thoms69]

Try 1 heat range higher and if that doesn't help then lean out the jetting on the carb.

What would be 1 range higher than the 3924?

For reference my carb jets are 71 primary and 81 secondary. The idle air bleeds are .070. PV is 6.5.

 

[COPO]

What would be 1 range higher than the 3924?

For reference my carb jets are 71 primary and 81 secondary. The idle air bleeds are .070. PV is 6.5.

1 number higher would be 1 heat range hotter. So a 3925 would be hotter.

FYI, A 1970 LT1 Z28 with the 780 Holley with sec vac uses stock 70 prim and 76 sec jets.
You are definitely fat on the jets especially the rear.

 

[DOUG G]

How much idle time or how many cold starts ?

Lots of low rum rides around the block ?

I'm leaning heavy on carb needs tuned...jetting won't help below cruise RPM.