Mark Maras

Sounds like we're on the right track. My gut feeling judging by your float setting pics, the number of turns down and the color of the plug is the float level is a bit too low. Are you going to double check the float level with the sight tubes you have? Remember, with the chambers and pistons removed and the nozzles 2 1/2 turns down the fuel should be 1/16" below the top of the nozzles. That is what your goal should be when setting the floats.

I'm surprised we've heard nothing from @Redwing about Atlanta yet.

That works too. I've always preferred using up and down to describe the direction of travel. Left is loose (rich) and right is tight (lean) would work too.

I Agree. The 260's were-are the red headed stepchild for various reasons but being single year model they will be a rare commodity in a few years. They really aren't much different than the late 73 240's. Most people wouldn't know the difference if a 260 had 240 emblems.

Thanks for bringing that up Cliff. There has been a bit of a perspective problem in the past when it comes to adjusting mixture screws. Some, when looking down on the carbs, picture the mixture screws from that perspective, which reverses the direction in ones mind. Just for the record, clockwise brings the nozzle up and leans the mixture. Anti clockwise lowers the nozzle and richens the mixture. Should anyone still be confused, pull the piston and needle out and watch the nozzle as it is adjusted.

They look a little lean to me. How is the engine running? Have you installed the sight tubes on the float chambers to check the float level? In old post by @Bruce Palmer(Ztherapy), he says "Pull the chamber and piston. Look down at the top of the nozzle. The fuel level should be about 1/16" below the top of the nozzle for proper operation. Congratulations! It looks like progress to me.

Middle names are only worth 1/2 point.

How far off were they, and were they both the same?

OK, Sounds good. Let us know what you find.

Did you change, adjust, or do ANYTHING else (screw with the carbs, change the timing, adjust valves, etc.) at the same time you upgraded the exhaust? What do you mean by race the engine before checking the plugs? You're looking for a tan color plug in normal driving.

Add Eastern Oregon and Eastern Washington to the list.

Thanks for the confirmation. @jalexquijano Start leaning the carbs out. If you think the floats are close to the right setting, set the nozzles at 3 turns down, drive it for a while and check the plug color. You'll need clean plugs to get a good reading.

You seem to believe that richening the mixture will make it stop popping. Where did that idea come from? Did the seller with the 72 and the same exhaust tell you if he had a hot cam in his engine? The cam may be contributing to the popping.

What color are the plugs at 4 turns down?

Which direction are the Gros Jet needle valves installed? The rounded end (not the ball) is the contact point to the float tab (tang).They were supposed to be a better design (better mouse trap) but they too leaked at times undoubtedly from foreign matter in the valve. Could some more crud be in them? I've not installed the lids without gaskets before. I don't see why it couldn't be done temporarily. The float will be a little lower (the thickness of the gasket) but I doubt that will affect anything. I doubt that most floats are set exact.

Google "spark plug color chart". Almost every spark plug manufacturer has one on their website. The shade of tan you're shooting for is anywhere from an Ivory color to the color of your spare tire well, to the left of the tank, in the pic above. As i stated before, the important thing is to get the mixture right first and then assess the exhaust noise. Some of the backfire may go away with the right mixture. It's also possible that the new exhaust is causing most or all of the problem with noise but you won't know until the mixture is right. To any and all Hitachi SU freaks out there, does anyone out there have experience running SM needles? They seem to be a common replacement but not really satisfactory, at least from what I've read.

I realize the exhaust popping is driving you nuts but achieving a light tan spark plug color should be your goal. When the plug color is right then worry about what the exhaust sounds like. Whatever is causing the problem, be it, rich mixture or back pressure you want the mixture to be right.

It's possible he has Gros Jet needle valves if there's a steel ball bearing in one end of the needle. The standard needles have a rounded end that contacts the float tab and a cone shaped end that is the fuel sealing surface. The cone shaped end is either rubber? or brass.

The richer the mixture, the more it will backfire in the exhaust.

When the gaskets are removed, put them in a plastic bag or wrap them in foil and put them in a book to keep them flat.

SCORE! I slipped one by.

It's just cockamamie. I can Google George Carlin's list of seven or Lenny Bruce's list of nine but i can't use them.

I love it. It lacks duck tape but there's a bit of Red Green in that set-up

You can use the old plugs IF you can get the porcelain around the center electrode white. Good luck with that.

www.ngksparkplugs.com has a good chart. You'll want to install a new set of BP6ES's to get an accurate reading.