Need ideas

My 260 started popping on the right side. Mostly light or no load. It seems to run fine accelerating. I put in a new set of plugs and points, no difference. Tonight we switched around plugs and plug wires but the popping stays on the same hole. Compression on that bank is about 140 except one hole 120 (not the misfiring hole). We also used a heat gun and the reading bounces around on that hole. Cap. rotor and wires have maybe 200 miles on them. Tomorrow I'm pulling the valve cover and look at the springs.
Any other ideas?

change the wire on that hole and put an old plug back in .

Sounds a valve isnt opening as it should. Could be the cam, a lifter, the valve, or the pushrod.

What does that plug look like?
Perhaps a sticky valve?

21willys said:

Sounds a valve isnt opening as it should. Could be the cam, a lifter, the valve, or the pushrod.

Click to expand...

X2 flat lobe maybe? one busted coil on the valve spring? or maybe a burnt seat or not sealing well.

Use your timing light on each plug wire and you'll know if you're getting a good constant pulse or crossfire, etc.

21willys said:

Sounds a valve isnt opening as it should. Could be the cam, a lifter, the valve, or the pushrod.

Click to expand...

X3, this is where I would start.

I'd say it's time for some Marvel Mystery Oil top end lube! Do they still even make it?

Zipper

VonBoneMan said:

change the wire on that hole and put an old plug back in .

Click to expand...

We switched the wires and plugs from hole and the miss stayed in the same hole. My best guess is a valve spring. I'm getting ready to take a look this morning. Also plan to run it with a compression gauge in the hole. I've seen sbf engines have good cranking compression and none with the engine running.

When you pull the valve cover, if the problem is not obvious, pull the valve springs and check the guides. I had a Ford in the shop years ago and it was the exhaust guide causing the problem. It was loose enough that the valve would not seat every time. Causing a random misfire.

Thanks RPM. I'll check that. I have the valve cover off and didn't see anything obvious. Cranked it over and no sign of a weak lobe or any broken valve springs. I still haven't checked the firing with a timing light but I suppose it isn't completely out of the question that I have a little cross fire going on in the cap even though it's virtually new. You never know with the quality of this Chinese made parts.

Flat cam

ronnysplymouth said:

Flat cam

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Don't think so, I cranked it over and all the rockers seem to be doing their thing. I threw the valve cover back on last night so I could take my daughter for a promised last ride in the cool evening air. We went out of town about 5 miles and back at 50-60 mph and it wasn't popping for about a minute after we got back to town, then it started again. I have a couple suspicions. One is some sort of lean misfire in my home made twin carb set up. I know that both the power valves are out because the carbs leak down when it sets. I checked for vacuum leaks and found none. I'm also going to try a new cap when I get back to it.

I think I have it narrowed down to a lean misfire. I screwed the front idle screw out a couple turns and the pop went away.

Excellent!

Yep....still make it...

von-zipper said:

I'd say it's time for some Marvel Mystery Oil top end lube! Do they still even make it?

Zipper

Click to expand...

Use it for air tool oil among other things....

I wanna pic of this homebrew carb set up or a link to the build thread.

A vacuum situation can find everything lined up for one hole and it's looking more like that might be.

Thread theft: My experience has been that Marvel Mystery Oil has too much detergent and shortened the life of my die grinders. ATF is the worst. This was in an industrial setting as a composite toolmaker and jig builder. The fact that you give them a drink of something is better than most. Find dedicated air tool oil and not the stuff for in-line oilers. You will enjoy increased life, torque, and RPM

Don