Let me set the stage, I am not a mechanic just a guy with a cool car but I have a 1932 Ford with a 1969 SBC and a 671 blower. The car started a backfire/popping (small pops) while at cruising speed and light on the throttle. However as I push on the throttle the popping goes away. I started with the easy things (plugs & wires) and found the wires to be in bad shape as well as a couple of the spark plugs had cracked ceramic and replaced. I also installed a new coil and "Hyfire" box along with a different distributer and it seemed the backfire got a little worse and now backfiring at idle. I verified compression (120 at each cylinder) and began to pull the plug wires one at a time to locate the backfire. When I pulled the number 2 plug wire the backfire goes away. I changed out the plug and the wire but still have the same issue. I have also checked the power valve and it seems to be functioning properly. I pulled the valve cover and checked the push rod and spring which appear to be ok but the valve seemed a little loose. I put a quarter turn on the rocker nut to remove the vertical slop but the issue still remains. What am I missing?
I see no mention of dist. cap, carbon tracking/ corrosion? I did notice you changed the dist. new / used?
He said he replaced the distributor. My guess is cam going flat. When you checked the lash did you check on how much lift you have? I’d start there.
Pull the #4 wire when idling and see if the popping stops. If so, you have a head gasket blown between 2-4.
"I put a quarter turn on the rocker nut to remove the vertical slop but the issue still remains. What am I missing?" Solid or hydraulic lifters? Adjust them either way. Don't randomly tighten the rocker nuts.
lot of unknown here, is there abnormal smoke coming from exhaust , it was running then you started cracking plugs , that screams a jump in timing for whatever reason , i don't think somebody built a blower motor with cheap cam gears , an if it ran this long cam is probably fine , i would think your dizzy springs got realy weak an you changed to another dizzy not knowing how the other was set up , how many plugs cracked because a cracked ceramic is a extreme timing event and or lean issue that suddenly appeared an that is less likely , how much was the rocker adjust an was it just that cracked ceramic or others , you have a lot of explaining to do Mister how you screwed up a blown Sbc lol
The compression test is only part of the battle. You need to put a vacuum gauge on the engine, which will likely shed some more light on it. You also need to do a leak down test, which will tell you if your cylinders are holding compression, or bleeding it off through a blown head gasket or elsewhere.
Sounds like a cam lobe going flat. Those engines had some,but not all, cams that were prone to do that. I've replaced a number of them. It seemed to be a quality control issue as one would go bad and thousands would not. With the cover off look at the number two valves with the engine idling. You can usually see if one is not moving as far as the others.
Did the backfire 'pops' come on suddenly, or was it gradual? If it was fine yesterday but not the next, it may not be a cam issue. Comparing the valve lift(s) as mentioned is a bit of a time investment but it also answers or eliminates questions on the cam. Cracked plug insulator tips are not a normal event. That points to excessive heat from the air/fuel ratio being off (probably lean). The blower may be sucking more air than the carb can meter at some point. I'm not an expert so check out what I post and pay attention to the guys here who have the right experience.
Yes, a lean fuel mixture can cause backfiring but usually not at idle. Which type of lifters do you have? Solid or hydraulic?
Unless I missed it he has not said and no one has asked where the back fire is, through the carb/intake or through the exhaust ?
LOL we used to pop the cap and mark the inside with a lead pencil to cause a little back fire when I was in high school. OK not we me and only to guys I did not like. LOL
Try adding some fuel to it, before tearing the cam out, blower manifolds don't evenly distribute fuel. What carbs are on it ?
Easy to pull valve cover and visually check the valve lift on #2 cyl. If a worn lobe is the issue it should be easy to see one rocker is not moving as much as the others while cranking the engine over.
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