66 427/390hp, Which spark plugs??

[William B.]

I have a 66 427/390 in which the previous owner installed R44XLS ac delco plugs. The book calls for R43N's. The car is hard to start, I have already changed points, cap, rotar and coil, STILL HARD TO START?? Which plug is best for my car or what else could be making my car hard to start?

[mike cobine]

Re: 66 427/390hp, Which spark plugs??

The 43N were the proper plug, but the 43N was a bit cool, so a 44N would be a better choice around town and most driving.

Now the hard part. 43N and 44N (and the R43N and R44N resistor versions) are not available in the stores anymore, thus the R44XLS that was in your car.

If you want the 43N, get your wallet out. They sell on eBay from $30 on a low side to $65 on a high side. The 44N are high also, but never go that high on the high side.

The XLS means it has a full-threaded extended-tip plug unlike the N. It should be a good replacement, as long as the extended tip doesn't hit the tops of the pistons.

Hard starting is probably another matter. Is timing set right? Carb adjustment?

[Stephen L.]

Re: 66 427/390hp, Which spark plugs??

Need some more information.......

ELECTRICAL
Are the plugs clean?
Is the battery fully charged?
Are the battery connections clean?
Are the battery cable connections at the starter and block ground point clean?
Is the starter dragging (running slow)?
Is 12vdc present at the coil + terminal during the "start" phase?

FUEL
Is it hard starting only when sitting for an extended period?
After running and its shut off, do you notice gas leaking into the carb throats? Possibly a little smoke coming from the carb?
After its running and shutoff will it restart immediately?

[Jon #40768]

Re: 66 427/390hp, Which spark plugs??

If the extended tip does not hit the piston or cause detination, it will run cleaner than the same heat range of a standard plug. I have used them for years in my 427.
Jon

[Clem Z.]

you can use a hotter plug with extended tips

because the fuel wash cools the plug tip and i would use R45XLS on any BBC used for street driving

[Dick W.]

Re: you can use a hotter plug with extended tips

Every big block that I put 44's in eventually fouled them. I have had real good results with R45XLS

[mike cobine]

Re: you can use a hotter plug with extended tips

You didn't drive them hard enough.

[Dick W.]

Re: you can use a hotter plug with extended tips

I use Autolite plugs in my driver. Only thing that will last! Never had any luck with AC plugs for daily drivers, pickups included.