trevor s

I am at a loss. My 350 was rebuilt maybe 1000 miles ago. 1973 block, 7a heads, Edelbrock performer cam and intake, q jet. It ran well but was starving for fuel at wot so I had a jet Performance q jet installed. Motor ran very well. I drove it for about 30 min and got on the gas a few times. The last time the motor made almost a backfire sound and then lost major power. It's making a popping almost a knocking sound. I read it may be detonation? The oil is clean with no metal. The timing was checked and it was at 10 degrees bdc with vacuum advance off. I let the engine cool down for a few hours and the popping sound is still there. I don't know if this matters but the distributor is an e bay hei unit.
i'm looking for advice on how to trouble shoot and what repairs may be required.
Thanks!

don71

trevor, If the carb is the last thing you changed, I'd look there first. Re-cover all those things that you touched or moved.

Don't forget the basic stuff as well. Look for a loose spark plug wire, might have fell off just like a vacuum hose.

young olds

Does it seem ok just driving normal but backfire when you get on it some, or a lot? I had a low miles rebuilt engine that did what I described, it was a worn exhaust love on the cam. Let just enough exhaust out to be fine while cruising but getting on the gas made it backfire out the car, actually melted my stupid foam air cleaner.

trevor s

the car ran well for a while. it idled nicely. it accelerated strong. then made almost a backfire and lost all power and started knocking. Now as soon as the car starts it makes a slapping or popping sound. I checked all the plugs. They look normal. The oil is clean with no shavings. I am going to pull the valve covers now to see if something is wrong with the valve train. The carb was installed by a mechanic. It was in for a transmission leak and ii figured get it all done while I was working. The engine was built August 14 by a reputable builder

trevor s

I took off the valve cover. I rocker is very very loose. I am guessing a collapsed lifter. Next question is why would it have collapsed?

olds 307 and 403

Was the cam broken in properly? That is a mild cam and mine broke in fine. I had the stock rockers not torque down properly and cause a tick. This sounds like a toasted cam. Pull the intake and inspect the lobes and lifters for wear. I have seen a lifter come apart on a Dodge 318.

trevor s

yes the motor was broke in by the builder. I guess I will pull the intake and see how what I can see. What's the best way to see if the cam went bad? The motor has 6-700 miles on it

trevor s

the pushrod on that cylinder is gouged up in the end that was in the lifter as well

oldcutlass

To look at the cam pull out the lifter and look down into the bore.

trevor s

cam looks fine as far as I can tell. the bottom of the lifter that rides on the cam is shinny. the insides of the lifter are totaled. ok So I know what the noise was, now what caused it?

oldcutlass

Are your rockers adjustable or stock?

trevor s

they are adjustable roller tip comp cam rockers. There was no ticking before the motor went kablooey. the insides of the lifter is gone. I turned it upside down and everything came out. So something really pushed back on that push rod hard. Could detonation have caused it? A misfire and blew back on that cylinder and squashed the lifter?

tru-blue 442

Get hold of your builder. My .02, good luck with it.

oldcutlass

X2 call the builder before you go any further and give him the excuse that you screwed with it.

70cutty

few other members including myself had the same problem with comp cam and their pushrods that come in the magnum kit. my 350 started ticking not even 200 miles after rebuild. Took it back to the builder and found 4 pushrods mushroomed. Replaced them with chromemoly and had no issues after that. I am never using anything comp cam again.

trevor s

so what I know so far:
Initial timing was 10 degrees BTD without vacuum advance.
sparkplugs are all fine
plug wires are good and in correct order
Cam looks fine
Ran well and accelerated hard the first 4 or 5 times
Car bogged then started clacking
Pushrod is mushroomed and a lifter is destroyed.
I'm thinking incorrect valve adjustment. Am I close???

trevor s

I had issues with the builder. He Is a seasoned builder (70 yrs old) who knows his small /big block Chevys. He had the motor for 6 months. His friend who did the body had to really get on him to put my motor together. He did not notice the push rods were to short (it says that cam may require longer push rods in the instructions that came with the cam) To cure the clacking another mechanic installed the comp cam 1411 Magnum roller tip rocker kit. This mechanic feels detonation collapsed the lifter. I don't know personally. It's been so many years since I worked on motors that I thought it was safer to work a lot of overtime and pay the professionals. I'm fixing it myself with the help from you guys!

coppercutlass

I would start talking to other people. Detonation would not collapse a lifter. Checking for proper pushrod length , spring heights , proper spring rates , installed valve stem heights ,all play a big part into proper working valve train. Personally i would take this time to inspect all the bores , i would not chance it and get new lifters, perhaps you had a bad set sent with the cam. 80 bucks for new lifters and re breaking in the cam is cheap insurance. i would also check for proper pushrod length , have your valve springs checked to make sure they are for the correct cam or meet the specs that the cam calls for , and check the installed spring heights , along with the installed valve spring heights. I trusted a reputable olds builder to set up my top end and the parts i needed and i ended up with pushrods that where too long and rockers just like the ones you have which could bearly handle the spring rate of the springs on the heads they built. Not bitter and im glad it did not cause a massive failure but i have corrected the pushrod length issue and hopefully i dont break another rocker. You also need more initial. Most of my builds even from a mild near stock engine ran well with 12 to 14 initial. with an hei that had 22 degrees mech adv. so 34 to 36 total. Oddly enough im running the magnum pushrods with no issues and i have been running them for some time now. There might be a diffrence between what you buy individually and what comes in the kit. I really like the trick flow hardened pushrods. For the price they are very nice one piece design for around 95 bucks or so.

70cutty

The pushrods in the Magnum roller tip rocker kit are High Energy and made from hardened steel.
Magnum pushrods are chromemoly but they are not in the kit, unfortunately I found out the hard way.

coppercutlass

I mentioned the trick flows where hardened they are chromeoly .

trevor s

What is the correct valve spring height installed on the cylinder head without the rocker installed on a stock 350. Is there a "range".

olds 307 and 403

While those Comp pushrods are known to be junk and too long, I bet the valve was adjusted too tight. Probably was OK cold but as temp climbed, no clearance eat the lifter and pushrod. I personally hate adjustable rocker arms but are necessary with big lift and full roller rockers actually gain power.

coppercutlass

As far as installed heights go it will affect a few things. The main reason to keep them at the stock height is when using non adjustable rockers . With adjustable rockers you can make up slight diffrences . It will affect installed spring height . But according to most sources .020 in diffrence of installed spring height is ok +\- . The other area it can affect is lift clearance ., That's usually in a good way though if you re grind stock valves but the added clearance won't be eneough to make a big diffrence. How big is your cam did you check for lift clearance ? You can bottom out the retainer @ .490 lift. I recently cut the guides down on a pair of heads and they didn't have enough to clear .490.

trevor s

ok so I backtracked to where the engine builder messed up. I replaced the ruined lifter, the cam is fine, went with the pushrod length Edelbrock recommended, put back the original non adjustable OEM rockers, put it all back together and now it runs just fine! Go figure following the instructions worked.

oldcutlass

Glad is was nothing major... Why was that one lifter so different, was it wore down?

trevor s

this is just a guess. the mechanic that had installed the new lifters and adjustable rockers did not pre soak the lifters and when the lifters pumped up it was too much pressure and it squashed the insides of the lifter. But I really have no idea.

oldcutlass

So your saying that one lifter wore down and did not damage the cam? It did not look like the other in the picture.

trevor s

the motor has under 500 miles on it, it never ran right since it was built last year. I think you mixed up my knocking lifter with the lifter in that 72 wagon

oldcutlass

You are right my bad...