Howdy. I have a 292 Y block in my '58 F100. Its been running pretty solid since I threw it in a few months back. I put a new Edelbrock 1406 on, ran it solid for a week and a half. Yesterday I noticed a bit of a big issue: at idle, I have rather noticeable pop through the carb/intake. it isn't a random pop, it happens exactly between 1 second intervals. Under any load or throttle, the popping turns in into straight up backfiring threw the carb. The backfiring is in sequence with RPM. My timing has not changed, and the problem doesn't go away if I mess with the timing at all. I pulled off the carb, went over the whole thing, everything seemed fine. My brain immediately went to truly shitty conditions, like a bent/failed valve or something. I did a compression test, because I figured if a valve wasn't shutting then that cylinder wouldn't have any compression. Please correct me if I'm wrong. All cylinders tested well, around 125-135 psi. The plugs did have some oil fouling, but nothing too serious. Here's some points that may or may not mean anything: -my ignition coil is a brand new MSD blaster. I have a pertronix flamethrower electronic ignition as well, no points. -the day before the popping started, I filled up from half a tank. Instead of using regular grade chevron, I used midgrade shell. Could the higher grade gas cause problems on a low compression motor? -the intake has been tightened. -my vacuum gauge in my dash jumps around quite a bit. Not exactly a goos sign I'd guess. Driving the truck isn't too great. I feel like I lose power on the popping, and it's pretty damn loud with the backfiring. Can anyone point me in the right direction before I start pulling heads off? Posted using the Full Custom H.A.M.B. App!
Back fire through exhaust is a rich fuel condition, firing through carb is a lean condition, or timing (but you have that set and probably not the issue). Spark is looking for fuel. How's the plugs look? Makes some sense with the lower octane.
The plugs are a little fouled. But nothing that wouldn't fire. Posted using the Full Custom H.A.M.B. App!
You may have developed a cross fire condition due to plug wires shorting one to another, or carbon arcing in the distributor cap, even condensation in the cap could account for this.......if you don't have a valve hanging open, then ignition would be the place I would spend some time before tearing anything apart. An exhaust lobe on the cam wearing flat could also cause popping back through the intake/carb but not so likely to also show up as exhaust backfire. Ignition issue is my bet. Ray
I took a look under the cap and didn't see any signs of damage or anything. How would I be able to verify this? I'm probably due for new plugs and wires. I'm going to take a look under the valve covers tomorrow. See if I can locate a busted valve spring or anything. Posted using the Full Custom H.A.M.B. App!
Get insulated pliers and pull plug wires one at a time to see if it goes away. If ignition issue it will, if mechanical issue it will not. Could be an exhaust valve issue(not opening) and that is why compression still checks good, but cylinder pressure is bled out (pop) when intake valve opens. Hopefully it is ignition...let us know what you find.
Exhaust valve is not opening. When the intake valve opens it lets compression to pop back through the carb. Bent push rod or bad lifter on one of the exhaust valves.
I pulled plug wires while the motor was running just now. When I pulled the wire for cylinder 7, the popping and backfiring went away entirely. So, new plugs/wires/caps, or is it possible that its more than electrical? Posted using the Full Custom H.A.M.B. App!
Many Y blocks came with solid push rods, and they can bend. The good news is, they can be bent back with a hammer on any flat surface. Adjust them to .016 to .019 and you are good to go. You mentioned its time for plug wires. On a Y block, its best to buy the grommets and run them like the factory did. They will last for years that way unless you pour oil on them.
There you go! If pulling the plug wire on that one cylinder makes the problem go away, then the cap, wires and plugs seem to be working OK. At least on that one cylinder. By not igniting the air/fuel mixture in that cylinder it can't ignite the mixture in the intake manifold (backfire) when it manages to find it's way back to the intake because the exhaust valve isn't opening sufficiently. An intake valve that's not sealing properly can cause a similar problem, but you would have probably seen a low cylinder when you did your compression test.
Unfortunately, I'm not going to be home until later on tonight. If I were to be looking for a bad exhaust valve, would it be on cylinder 7 or should I be looking further down the firing order as well? Posted using the Full Custom H.A.M.B. App!
just on the cylinder that you pulled the wire on . you can run it with the wire off and put a load on the motor it will shake ( a quick trip around the block should do but no more ) , but if it starts to pop again you have another cylinder acting up also .
Look at cylinder #7 first, since that is where the problem apparently resides. While I am in general agreement with the valve/pushrod theory, it is STILL possible you had a spark plug wire cross firing #7 wire and causing it to fire at incorrect timing. I AM NOT insisting that is what is happening, only saying it remains a possibility, especially if no mechanical problem is found. Hopefully, you will find a mechanical cause after pulling that valve cover. Ray
Hnstay is on to something, I used to run yblocks a lot and remember? that several wires cant even cross over others without bleed thru firing. Sorry I don't remember which ones.
As it turns out, the valve lash on the exhaust valve was so out of whack that the pushrod just came unseated from the lifter and fell through. Pulled the rocker arm assembly, inspected everything, verified that the pushrod was straight, and reassembled. Tightened up the lash, fired up and we are good to go. Gotta say, these Y blocks are amazingly easy to work on, especially with access to a great collective base of knowledge such as this forum. Thanks for your help everyone. Posted using the Full Custom H.A.M.B. App!
I used to have a 312 that would do that if I didn't keep a quart of ATF in every oil change, guides too tight. It's easy to try.
I thought I had fixed the problem, but on my way to the shop I guess the rocker came loose again. It seems to back out quite a bit after very little drivetime, letting the pushrod fall back down again. Anyone ever have any experience with this? Posted using the Full Custom H.A.M.B. App!
Well a flat cam lobe could be the issue but if you adj it and it loosened up right away it seems more like the adj screw doesn't have enough of a interference fit. How easy does it turn when your adjusting it?
Cam, just to keep in mind, any time you have a vacuum gauge that's bouncing around its a good sign of a valve issue . Also really helps set up the carb when all things are operating as they should. Posted using the Full Custom H.A.M.B. App!
I'm pretty sure the cam lobe is fine. The adjustment screw on the rocker can be tightened by hand. I make it just snug enough with a wrench, enough to not throw me out of whack, but I dunnno. I have a rocker shaft for my old 223 I'm rebuilding, all of those screws are tight and need to be wrestled with to even be loosened. I'm going to assume that this is a situation in which locktite isn't going to work, huh? Posted using the Full Custom H.A.M.B. App!
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