heres a problem I hope you guys can help me solve. I just recently rebuilt my wifes SBC in her 69 Camaro. The car ran great for the first 100 miles. I broke it in correctly and cehcked everything twice. I ahve no metal deposits and have relilgiously changed the oil. Now, all of a sudden, we were driving on the freeway and the engine started backfiring through the carb. It only happens on acceleration above 1500. That same night we were out I yanked the vavle covers becuase I could hear a distinct clanking sound from the rocker arms. I pull the covers and one rocker arm on each bank of cylinders is completely loose. The valve was not being opened at all by the pushrod and cam. So I tighted them down and we were on our way home on the freeway and it started up again after 15 minutes at about 2250 rpm. I pull the covers the next morning and completely reset valve lash (they are hydraulic lifters) thinking that the rocker arms were still a little loose and that the nuts themselfs were coming undone. I used new adjusting nuts WITH loctite this time, and it STILL happens! WHY? I checked the timing, it is dead on at 8 deg initial 32 total. I built the engine myself and the timing mark were dead on! I used the same HEI dist from the previous motor that was almost brand new, and had the carb reabuilt about 2 years ago (q JET). The thing that really dumbfounds me is that the engine does NOT miss on idle or acceleration. It only backfires through the carb, It sounds like a damn glasspack when it does. What is the problem? I had no probs firing this thing up initially, so I dont think its a wiped out lifter?? maybe? THe only problem I had is that the carb caught on fire for 3 sec when cranking it over. Too much starting fluid. could that have hurt the carb to cause this???? please help.. my wife is not happy about her Camaro, that is 99% finished. thanks ahead of time.

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1970 Chevelle 400hp 350ci, th350, locking 10bolt. Go Earnhardt Jr.! "The importance of material things is invaluable if you loose the things essential to the spirit of life."

Any chance you've got a broken valve spring? That usually affects the idle, but the engine will run pretty well at higher RPM. It could explain a backfire, and the valve lash. A broken spring can also be tough to see.

Look closely at the amount of thread that is above each rocker's adjusting nut. Are the ones that were loose the first time showing more thread than the others? Are the same ones loose again?
I'm guessing that the loose ones were (are?) exhaust valve rockers based on your description of the backfire symptoms. If your rocker studs aren't pulling out (pressed-in studs) or unscrewing themselves (threaded studs) then, I hate to say it but, I think your cam is going flat. Check the lift on the end of the pushrod with the rocker out of the way on the suspect positions versus another position. The lift should be at least close to the cam's specs. Be careful to make sure that you're checking an intake lobe against the intake spec and an exhaust vs. exhaust spec, etc.
Good luck,

Louie

Its funny you mention the thread amount on the studs becuase the ones that were loose have more threads than the others. I have one that seems to be really screw down. I am running stock 1.6 steel rockers on this engine since the cam is basically stock with lift numbers at .442" and .473" with the 1.6 rockers. I looked at all the springs, they are also brand new, and installed by a machine shop. Perhaps the cam has gone flat? or a lifter bad? I didnt think so at first becuase this engine has 200 miles on it?! sheesh, I just got it in the car and the cams gone? I had plenty of lube on the cam using good redline ass. lube and ran it at 2200 for at least 20min than I ran it up and down from 2500-3000 for the last 10 min of the 30 min break in. I even primed the engine to 50 psi before initial start up. And the studs are screw in studs that I torqued to World's specs. Where did I go wrong? I've been messing with thing this too much and coming up empty. any other ideas? I ready to take it to my local mechanic and let him check it out.

---> by the way, the loose rockers were intake valves, which makes sense to me why fuel and air would be coming back up through the carb.
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1970 Chevelle 400hp 350ci, th350, locking 10bolt. *Soon to have 450hp 383ci* "The importance of material things is invaluable if you loose the things essential to the spirit of life."

[This message has been edited by green70 (edited 02-08-2003).]

Green, the thing that doesn't sound right to me is that a flat cam , or a pulled out stud, would increase the lash, and cause the valve NOT to lift as much as it should. This will affect the performance, but I don't see why it would cause a backfire. It sounds more like the intake valve isn't closing, which is why I suspected the valve spring. The spring height should be the same on all cylinders, and the stud heights should be close. Check those springs again. Remove the rocker and check the spring tension. Check Louie's suggestions. It's not time to call the mechanic yet! I hope you're not dealing with 2 different problems, like a bad distributor cap AND a valve problem. Good luck.

By the sounds of it, your cam may have wiped a lobe or lobes. If you have more threads on the stud compared to the others, keep that valve cover off, crank the engine over and watch the valves open and close on each cyl. If the valves that are suspect barely move, and are on the lobe, there is your problem.

Pull the intake, and yank those lifters out and inspect them along with the lobe. I bet you find something amiss. I know this probably means little right now, but the cam in my 427 went after just 30 min of running, I had to do it all over. Please make sure, if this is the problem, change the oil and filter before running the engine again. Metal particles have gone through the engine and that filter hopefully picked up most of them. And pick up a bottle of GM EOS available at the parts counter and douse the new cam/lifters with it before buttoning it all up before starting for the first time, again.

Let us know what you find.

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Craig Sanden

Distant thunder, cold as stone, a big block screams down from it's throne, one by one each car succumbs, something WICKED this way comes.

the only reason Im tempted to take it to a mechanic is that I have to work. I dont have lots of time to diagnose the problem thats all. WHen I was younger I had plenty of time to fix almost anything. Ill check the lift and the springs again. Perhaps I missed something. Im going to probably pull the dist also. If there are alright then I pull the intake and check the lifters. Then I pull out the cam and lifters and THIS TIME, Im going to put in a ROLLER cam. I know they arent cheap, but they dont require the break in a flat tappet cam does. Its going to take me a few days to do all of these things. I will report back to you by Thurs if not Friday evening. talk to you guys later.

Greg

okay, heres the update. I pulled out the lifters and cam today after checking everything else that I could like timing, studs pulling out, compression, etc. Three of the lifters have DEEP grooves in them. And on the cam two of those three bad lobes are COMPLETELY wiped out- there isnt any lobe left. Now that I discovered what the problem is... WHAT CAUSED THIS TO OCCUR? I know I broke in the cam properly with the correct rpm and priming the motor before startup. Is if possible that my stock oil pump isnt up to par and no providing enough oil to the cam lobes? I have 25lbs at idle and 50lbs by 3000rpm. I know the old rule of oil pressure- 10lbs for every 1000 rpm. SO what happened? could it be too stiff of valve springs? I used the ones that came with the S/R heads which are rated to .560 lift and 104 lbs. Perhaps the spring is too stiff for a stock cam- IE .442 lift 80 lbs stock? Please let me know what oyu guys think before I go and put this thing back together.

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1970 Chevelle 400hp 350ci, th350, locking 10bolt. *Soon to have 450hp 383ci* "The importance of material things is invaluable if you loose the things essential to the spirit of life."