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Discussion Starter · #1 ·
a few questions:
i recently put together a sb 400 and everything seems fine and it runs smooth but when i "get on it", it starts a popping at higher rpm's. it doesn't do it other than that. when driving it normal, it runs fine.
yea, i know, keep my foot out of it, right....lol
i've been told it's a broken valve spring.
1. how do it test to see if that's the problem?
2. can i replace them while the heads are on the engine?
3. how do i know which springs to buy.(these are stock heads)
any insight is greatly appreciated.
 

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At what rpm does it start to pop and does it always pop at the same rpm in each gear?

I also feel ign is the 1st place to look too like dirty or bad plugs,plug wires arcing to headers or the inner winding in wires has broken down due to heat causing issues,coil ,etc.

Then after ign issues comes valve springs and or fuel delivery issue going lean on top end under load.

But if its valve springs it should pop at hi rpm not matter what gear your in but if its ign it may or may not pop all the time at hi rpm. It may pop worse in higher gears at hi rpm when under higher load if its an ign issues but not always,just depends on what type ign issue it is esp if its an issue with spark plugs .

Did you check your rpm limiter chip if running one to esnure its the proper one for the rpm range of your motor or that it's working properly by trying another chip?

These are some good places to start when having hi rpm issues.

Scott
 

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Discussion Starter · #4 ·
i'd say it starts popping around the 3500-4000 rpm mark. i don't have a tach so that's my best guess. it pops 8 out of ten times when in that rpm range. i'll try the plug boots first. #3 plug sits pretty close to the headers. thanks for the help.
 

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The coil is always my first item to look at.
 

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My car dead that when when I was using an HEI distributor. Problem was that the HEI module went bad. Got a new module from NAPA and the problem was gone.
 

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1968 chevelle SS, 540ci., th400
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i broke a valve spring with a 396,it will idle very roughly,run like crap,compression test will tell you which cylinder... i think you have a electrical problem... jeff
 

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I had that problem also in my car when I switched over to a hei dist. after changing plugs, wires, etc. I had to run a wire from dist straight to batt. with a fuseable link. so I would have a full 12 volt source. cleared right up and ran good.
 

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Popping could be backfiring as we like to call it here.

Where's your timing, speciffically?

I would put a timing light on there, and verify where it is. Next, put the timing light on EACH wire and rev it up to make sure your getting consistent spark out of all wires. Are the points OK if eqipped?

If your getting good spark, pull the valve covers a check all the springs. Start her up and let here idle to make sure all the valves move equally (look for a flat cam lobe).
 

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Further to the last post, can you see a worn cam lobe by watching the valves move up and down? How long will I have to run the engine at idle to spot the worn lobe? I am chasing down a stumble on acceleration and poor performance at highway speed and I am thinking worn cam lobe.
 

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Further to the last post, can you see a worn cam lobe by watching the valves move up and down? How long will I have to run the engine at idle to spot the worn lobe? I am chasing down a stumble on acceleration and poor performance at highway speed and I am thinking worn cam lobe.

You'll know right away - a valve will open a alot less, maybe not at all, compared to the rest. I had a 396 that cruised and idles real nice with several flat lobes. Would just pop and fart when floored.
 
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