View Full Version : SBC Popping

05-27-2012, 06:48 PM
Was out with our SBC running at WOT. Ran low a little low on gas and heard a loud pop. Now having a "popping" noise and a mis-fire. Replaced all plugs and checked wires. Still same issue. Any ideas????

05-27-2012, 11:53 PM
Popping out the carb? Broken valve spring?

05-28-2012, 01:12 AM
Usually, popping is a sign of a lean condition which makes sense when you were low on fuel but not quite sure why it would continue like that. How low on fuel did you get? Did you run out of gas shortly after? Hopefully its not damage from having run WFO with little gas such as a burnt piston crown.

05-28-2012, 01:05 PM
Or a burn't exhaust valve.

06-02-2012, 10:43 AM
Check your fuel filter for water.
Often times when running low on fuel, the pickup will deliver all the crap from the bottom of the tank. And these days with the hydroscopic ethanol they insist on putting in our fuel, the crap is usually water.
I use a pickle jar to pour the fuel from the filter into so I can see water separation.
Good luck.

06-03-2012, 08:17 AM
Is the popping intermittant or steady? If it is steady then the valve or piston issue rings true, if intermittant it should be a fuel or carb issue. Mechanical is rythmic happening with each rotation.

Ralph Savarese
06-03-2012, 01:09 PM
Defenitly valve spring or rocker arm or valve

Dax McDavid
06-05-2012, 09:26 AM
agreed....valve spring or rocker arm. has happened to me twice...happens on rotation.

06-05-2012, 12:23 PM
I had a popping through the carb at higher rpms, this was the cause. A cranking compression test and leak down test will tell you a whole lot.

06-10-2012, 09:24 AM
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All are great suggestions, the compression and cylinder leak down test will tell you what cylinder is low (compression test), and what is the cause (leak down test) whether it is valve, or piston related. If you don't have access to a cylinder leak down tester, if you hook shop air to the compression tester hose, or use the adapter that NAPA and Discount sells for holding the valves up to replace the valve stem seals, you just listen for where the air leaks. Leaking from the Intake = Intake Valve, leaking from the exhaust = exhaust valve, leaking from the valve covers or dipstick tube = piston.
Also if you're running a Holley carb be sure to check the Power Valve, when the engine pops back through the carb it will normally "blow" the power valve which will cause it to run exceedingly rich at Idle.
Be sure to check the valve springs closely before you tear down the engine. Remember, if it's a broken valve spring, you can change it with the head on the engine.

joseph m. hahnl
06-10-2012, 05:33 PM
It's probably just ethenol related. :wrench: A clump of gloop fouled the jets.:spongebob:

06-10-2012, 05:48 PM
DTM has never answered the most important question! Rhythmic or intermittent, the answer will lead to the right fix.

06-10-2012, 07:55 PM
Thanks for everyone's guidance.... The root cause was mis-adjusted valves with the flat tappet hydraulic cam/lifters.

I had someone else help my son (17) adjust the rockers while I was away, as my son couldn't wait to get the boat out (don't know where he gets that from). This person was suppose to know what they were doing. Well they didn't and we had to re-do a lot of valve train. I had not re-adjusted the valves after initial rebuild as the risers have to be removed to remove the valve covers to get to the rockers. Lazy - stupid move on my part.....

When we did the compression check we found one cylinder at ZERO, one at 90 and all the others at ~ 150. So we removed the riser and valve covers and found the over tightened rockers on the zero cylinder. I checked the studs and found the groves at that time. Opened up the other side and found the broken rocker (bottom broken out), the sheered oil splash guard and a sightly bent stud.

So, we had to: replace 3 rocker studs and reseat 3 others (pulled partially out). Note: We checked the height of each stud to find this issue (I was going to but in screw in studs when I had the heads redone and now I kind of glad as they took the abuse not the springs etc --- I hope..). Purchased a valve spring compressor designed for use while head is on engine (worked great) and replaced the sheered splash guard. Then installed all new roller tip rockers with new rockers balls and polylocks...

Cut a access holes (a long rectangle) in a set of old valve covers and installed them so we could readjusted rockers while engine was running (after doing a basic adjustment when replaced the rockers). Backed nuts of till "clatter" then 1/2 turn and lock down with the polylock. Recheck timing (set at 9 BTDC)...

Took the boat out Friday evening and it ran better then ever (what a surprise). We did not go WOT (had to hold my son's hand back) as I wanted to rockers to get a little broken-in / seated before WOT. Boat idles smoother and not "bogging" (thought we had a carb issue before - must of been the valve train "binding").

Thanks again everyone and remember: You always have time to do it right the SECOND time....

Note: Planning on pulling heads at end of season and maybe going for a set of aluminum ones and a roller cam as I am not sure about the stress that was but on the springs and the rest of the valve train.....

06-11-2012, 07:20 AM
On the zero cylinder I would suspect a valve stating to tulip, as a valve starts to tulip the stem rises hence no clearance. It by your description ran fine until the what I believe was a fuel issue, it takes a very short period of a lean condition to tulip valves. I think the heads need to come off sooner than next season, if a valve head comes off you will have much greater engine damage.