Hotter Plugs?

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spikhead

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My Car
1973 Mach 1 351C
Hey everyone, I have just OEM recommended copper plugs for my 351, but need 2 steps hotter the engine guy told me. Anyone have recommended tips for plugs? We did mild stuff like cam, intake, carb, but changed some other stuff and guess they’re not hot enough but part stores don’t seem to know about older engines.

 
Hey everyone, I have just OEM recommended copper plugs for my 351, but need 2 steps hotter the engine guy told me. Anyone have recommended tips for plugs? We did mild stuff like cam, intake, carb, but changed some other stuff and guess they’re not hot enough but part stores don’t seem to know about older engines.
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Autolite 25's As used in the Boss 351

 
A. Your engine guy isn't one.

B. Verify ignition system integrity, timing, and advance curve.

C. Verify fuel pressure, float levels, power valve condition, and re-calibrate carb. (find a real engine guy to do this)

D. The B351 plug or Autolite Racing plug equivalent (or NGK equivalent) should be the right heat range for what you have described as the build.

Suggested reading: Tuning Carburetors, Reading Spark Plugs, Detonation and Pre-Ignition.

 You don't cure an overly rich condition by going two heat ranges hotter on the plugs, you find the cause of the rich condition and fix it. Chuck

 
I'd need to know you're more about your fuel system (carb or injected), compression ratio, type of fuel you're burning, and ignition type (points or electronic) to make a call on the right plugs.  All these factors contribute to performance efficiency of your motor.

Stu
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Carb, compression no idea, 91, electronic. Think it’s like 32 bore if that’s a right term, intake, 4 barrel, headers, bigger cam

 
Are there specific symptoms you are trying to deal with?
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Burning rich. We’ve adjusted the carb, but was told with the electronic switch from points needs to have a hotter plug? We also changed I think it’s the ignition when getting rid of points.

 
What do you mean by "burning rich"? Running rich or you think the car is rich because the exhaust is burning your eyes? It might help us to determine next steps if we had more details.

Other questions -

How does the car behave when driving, does it load up (act like its getting flooded) or does it surge at cruise?

If the exhaust is eye-burning, it might actually be too lean, so again plug color can be helpful. Have you checked plug color, and if so, what color are they?

If running rich have you actually rejetted the carb vs adjusting mixture screws?

Its very hard to give advice and really be accurate without being in front of the car,but maybe we can give some things to check out if you can fill in more of the blanks around symptoms you are experiencing.

Knowing you have an aftermarket cam, but arnt sure of the specs, I would also want to know initial advance on your ignition timing. Cam specs would be useful,but not critical.

 
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What do you mean by "burning rich"?  Running rich or you think the car is rich because the exhaust is burning your eyes? It might help us to determine next steps if we had more details.

Other questions -

How does the car behave when driving, does it load up (act like its getting flooded) or does it surge at cruise?

If the exhaust is eye-burning, it might actually be too lean, so again plug color can be helpful. Have you checked plug color, and if so, what color are they?

If running rich have you actually rejetted  the carb vs adjusting mixture screws?

Its very hard to give advice and really be accurate without being in front of the car,but maybe we can give some things to check out if you can fill in more of the blanks around symptoms you are experiencing.

Knowing you have an aftermarket cam, but arnt sure of the specs, I would also want to know initial advance on your ignition timing.  Cam specs would be useful,but not critical.
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It’s a 3/4 race cam. Because the exhaust tips are black someone said it’s running rich. At start up it ideals rough, which maybe is just the choke setting. Once it gets going after 5 mins or so it runs perfect, ideals really nice at a stop light.

 
If it runs good after warming up, I would bet your close and probably just need minor tuning. It is easy to overthink these things. Not sure I buy into the need for hotter plugs just yet - It can work for some situations, but isn't the first place I would go.

I've heard others talk about "3/4 race cams" but that doesn't mean much if we don't know specs, I can only assume its of longer duration than stock. Not trying to be rude, just being honest. I would still be interested in knowing more details, any way you can get the cam part # and possibly the timing specs (initial and total timing, as well as what RPM you reach total timing).

You should be aware that longer duration cams (i.e., race cams) can take a little longer to warm up, and sometimes like a rich idle. They also tend to like some extra initial timing to make up for the long duration at low RPMs. That is why I'm asking for those specs.

 
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Short trips will cause black exhaust tips, the exhaust system just doesn't get hot enough to burn the combination of hydrocarbons (from cold start enrichment) and condensation (water) out of the exhaust.

I would rely more on how it's running than on what people have to say about it.

What kind of electronic ignition? Did you also change the ignition coil?

I don't remember any electronic ignition conversions instructions recommending changing to hotter plugs. Changing to hotter plugs when not needed is a recipe for introducing pre-ignition in your engine, which can quickly cause damage to the pistons.

If you are really concerned about it install an air/fuel ratio gauge so you can monitor it under all driving conditions.

 
Good points Don.

An AFR will require at least 1 O2 socket (bung some call them) placed in the correct location in the exhaust about 18" from the manifold. The one I used had instructions on that location.

 
Some of this I have no idea what it means. It has the petronix system, and since it’s running well once warmed up a bit I guess maybe just leave it alone and if the exhaust normally might get black just from use I won’t worry :)

 
Some of this I have no idea what it means. It has the petronix system, and since it’s running well once warmed up a bit I guess maybe just leave it alone and if the exhaust normally might get black just from use I won’t worry :)
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 Do you know what Pertronix you have. Is the module black, red or dose it curve around the shaft, i.e. Pertronix I, II, or III. Is the distributor a Pertronix or factory Autolite or some other brand. Below is a pic of my factory Atuolite with a Pertronix II (red) I also have the matching Flamethrower coil.

I realize you may not be sure of what you inherited so to speak, but all this is useful when answering your questions. 

I'm not getting into timing and distributor curve right now as that is complicated, but helpful to other members help you.

Without going back, can you refresh me on the carb you have on the engine. Please add whatever you can in detail.

As you say it runs fine once warmed up, it may just be fine adjustments needed, but without knowing exactly what's in or on the motor, hard to diagnose.



 

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