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· Registered
237 Posts
Discussion Starter · #1 ·
Uh Oh, not liking the sound of this! :unhappy:

The '68 suddenly started running poorly last night, acted like it was running out of gas.

Could not be, the tank is full.

So today I changed out fuel lines, filter, replaced the missing filter in the Quadrajet, and ran some spray cleaner down the venturies.

OK good, runs much better............... except!

I had noticed when I first got it that there was some weakness in the upper RPM range.
No surprise, the car sat a long time, so I put some Lucas fuel system cleaner in at the first gas fill.

After doing what I could to clean up the fuel system today I took it out for a little test drive.

Engine is a 400SB+30/400 trans with shift kit.
So far as I know the engine is basically stock.

Good idle and pulls strong now, BUT!

At higher rpm I hear a sound I dread, light popping at the carb!

It could be I'm floating the valves, no tach so I'm unsure of the RPM.
What is the max RPM for a stock 400 anyway?
Doesn't seem like it's really being spun that high.

Lean carb?
I get gas smell, so a high fuel level and rich seems more likely, and not expected to cause popping.

My cam is going flat isn't it! :banghead:
I did not have this engine built, but the first thing I did when I got the car home was an oil change, "racing oil" WITH ZDDP to protect the cam.
Seems I was already too late, maybe why the car sat?

Hydraulic lifters, but I suppose I should still be able to measure or at least compare the lift if I pull a valve cover?

I looked at cams on-line, then I looked at roller cam$, going to have to be another flat tappet cam due to cost.

Car is a daily driver, I like power but still need decent mileage and do not race.
Will be doing occasional light towing.

I'm thinking Edelbrock Performer cam, put on my Performer manifold at the same time, keep the quadrajet until I can get the Edelbrock FI system.

So, is my thinking sound?
Something simple I missed?
Better cam for a 400 engine daily driver?
400 Red-line?

Serious suggestions?

· Registered
237 Posts
Discussion Starter · #3 ·
Appears to be, have to chalk the timing line.

Appears to be, have to chalk the timing line so I can see it.
Now that I have identified the engine year I should be able to find specs.

Engine pulls strong at the low to mid-range end, no rattle on acceleration, pops ONLY at high RPM, steady popping if RPM is kept up.
It IS popping, not pre-ignition. :frown2:

Last time I had a flat cam it was just one lobe, you would not believe how loud the popping could be.

This time, popping is steadier, and not so bad.
Car sounds fine as long as RPM is kept fairly low, but once they go, they get worse.

· Banned
5,787 Posts
i was thinking a sticking valve-you could try some marvel mystery oil but,,,,
first i would isolate which cylinder it is by pulling plug wires one at a time,,,, find the offending hole and then do a visual inspection,, if that passes the mustard its time for dial indicaters and see if you have a intake lobe gone away,,,, hope this helps a lil bit any ??? others will ring in also im sure:beer:

· Registered
1,348 Posts
The popping would likely be a valve hanging up. This can be difficult to find, although the trick of pulling off one plug at a time might help. Be sure the timing is right. Find out if it's just one cylinder. Pull off the valve cover to see if you see an obvious problem. You might have to do a very close inspection; if you have the typical double springs, the inside spring might be broken, but it's almost impossible to tell without taking it off.

· Registered
237 Posts
Discussion Starter · #9 ·
Long list of minor issues, this is one.

Car basically drives "OK", but not well enough to make me happy.

Took it for a longer than usual drive today, cooling appears marginal.
Heats up while idling, 16" flex fan is clearly too small, changing to 18".
Not sure the radiator is up to cooling a SB-400.
Suppose I should have a BB radiator in there.

Drum brakes, weak.
To be changed to disc, after the gas gauge is fixed.

I will do a compression check, examine the plugs.
I think my float level is high, smell gas after tight corners and sudden starts. (That's ALL starts! :wink2:)
I have a pro-built carb on order.

I'm pretty certain this is a cam going flat, or really weak valve springs.

Engine is supposed to be a fairly low mile +.030 over rebuild.

Car sat for a few years, so I expected some issues, but a flat cam is more than I bargained for, about $500.00 more.

Busy week coming, probably next weekend before I can get into it more.

· Premium Member
2,536 Posts
Car sat for a few years, so I expected some issues, but a flat cam is more than I bargained for, about $500.00 more.
The Edelbrock cam and lifters is About $130 on ebay. Add timing cover, intake, and valve cover gaskets and you're only another $25 - $35.

So, $150 - $175 if you do it yourself. It's pretty easy.

Hardest part is pulling and replacing everything it takes to get the cam in and out.


· Registered
237 Posts
Discussion Starter · #11 ·
Edelbrock 2103 - Edelbrock Performer-Plus Cam and Lifter Kits $159.95
Edelbrock 7800 - Edelbrock Performer-Link True Roller Timing Chain Sets $41.95
Mr. Gasket 1099 - Mr. Gasket Quick-Change Timing Covers $54.95 (In case this happens again)
Mr. Gasket 1183 - Mr. Gasket Cam Button Spacer $6.95
Mr. Gasket 4403 - Mr. Gasket Cam Change Gasket Sets $24.95
Mr. Gasket 948G - Mr. Gasket Cam Locking Plate and Bolt Kits $10.95

Does not include oil and filter, probably upgrade the intake gasket, antifreeze, and whatever I have forgotten.
If it's apart, it gets new parts.
So total close to $400.00.

Basically stock 400 SB, so no point in a wild cam.

Will change out the carb and manifold at the same time.
Of course, if it IS junk valve springs, this would all be for nothing!

· Premium Member
12,128 Posts
If you aren't looking for a bunch more power, checking out the possibilities of the cause for the problem would sure be less expensive. You can change valve springs with the right tools, and replace the valve stem seals too while doing the springs. If you have a flat lobe, it should be apparent when watching rocker arm movement with the covers off.
And it may be an ignition problem also.......
Some aftermarket ignition suppliers claim the the HEI starts losing it after 4500 RPM, which is mostly BS, since 5800 is a more realistic number, from my experience.
Without a tach, you may not really know how high it's running. 7000 is not a good number. I broke a crank 7 cracked the balancer doing that.....
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