Help with 73 BBC engine problem, looking for idea's ?
Thread Starter
Drifting
Joined: Nov 1999
Posts: 1,324
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From: St. Peters MO
Help with 73 BBC engine problem, looking for idea's ?
I'll be brief:
73 454 Chevy, basically stock, 13,000 miles since rebuild around 1995 (based on previous owners input)
Problem: Engine "miss" or "backfire into intake" under moderate load, or over about 2,300 rpm's. Idile is good, cruise is good till 2,300 rpm's (60 mph). Then, the harder you push the throttle, the louder the "rattling" type sound gets. Also, power is down noticeably, feels like it's running on 7 cylinders.
Checked timing, good, 12deg. initial, 38 deg. total mech advance with vacum line disconnected (Mallory Unilite distributer, vacum advance model) replaced plug wires, coil wire, dist. cap first, no improvement. Removed Valve covers, all springs and rocker arms appear fine (no broken springs), oil pressure is as normal as it was before.
I'll be hunting down a compression tester this week to see if that shows anything (Head gasket???)
These symptoms happened rather quickley, car not driven very often, seemed like from one outing it was fine, then next one was rattling around 3,000 rpm,s and getting worse quickley.
Any idea's would be appreciated! :confused:
73 454 Chevy, basically stock, 13,000 miles since rebuild around 1995 (based on previous owners input)
Problem: Engine "miss" or "backfire into intake" under moderate load, or over about 2,300 rpm's. Idile is good, cruise is good till 2,300 rpm's (60 mph). Then, the harder you push the throttle, the louder the "rattling" type sound gets. Also, power is down noticeably, feels like it's running on 7 cylinders.
Checked timing, good, 12deg. initial, 38 deg. total mech advance with vacum line disconnected (Mallory Unilite distributer, vacum advance model) replaced plug wires, coil wire, dist. cap first, no improvement. Removed Valve covers, all springs and rocker arms appear fine (no broken springs), oil pressure is as normal as it was before.
I'll be hunting down a compression tester this week to see if that shows anything (Head gasket???)
These symptoms happened rather quickley, car not driven very often, seemed like from one outing it was fine, then next one was rattling around 3,000 rpm,s and getting worse quickley.
Any idea's would be appreciated! :confused:
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Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
What do the plugs look like?
Any water/oil disappering?
Vacumn reading?
With a used engine, you just don't know what the condition is even though the owner may have told you ;)
Sounds like possible timing problem.....timing chain maybe........
Any water/oil disappering?
Vacumn reading?
With a used engine, you just don't know what the condition is even though the owner may have told you ;)
Sounds like possible timing problem.....timing chain maybe........
Burning Brakes
Joined: Nov 2001
Posts: 1,189
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From: Austin TX
Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
You might be having a lobe on the cam go flat, probably an exhaust lobe,
which will give you that popping back through the intake under load because
you're getting a lot more air into the cylinder then and it's not all getting out through the exhaust valve. A compression test will not confirm this either as
the static cranking pressure should still be good.
Just a thought.
Good luck,
Dan
which will give you that popping back through the intake under load because
you're getting a lot more air into the cylinder then and it's not all getting out through the exhaust valve. A compression test will not confirm this either as
the static cranking pressure should still be good.
Just a thought.
Good luck,
Dan
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Joined: Feb 2002
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From: Arlington Texas
Re: Help with 73 BBC engine problem, looking for idea's ? (danno85)
Here is a problem I had on a BBC somewhat similar to yours. The difference is I did not have the backfire problem.
The engine ran great after rebuilding for about three months. Then it suddenly developed a miss. I believe it was in the #6 cylinder. After scratching my head for a week (I checked everything imaginable) I accidentally noticed a vacuum cap missing where the direct intake manifold vacuum line attached to the vacuum canister on the A/C housing. It was an obvious vacuum leak. The vacuum fitting on the intake manifold was threaded into the #6 runner. Therefore it was causing a "lean" condition for the #6 cylinder causing it to miss. Replacing the vacuum cap solved the problem.
It's probally not what's causing your problem but it's a quick easy check. Hopefully it's something this simple.
Good Luck
The engine ran great after rebuilding for about three months. Then it suddenly developed a miss. I believe it was in the #6 cylinder. After scratching my head for a week (I checked everything imaginable) I accidentally noticed a vacuum cap missing where the direct intake manifold vacuum line attached to the vacuum canister on the A/C housing. It was an obvious vacuum leak. The vacuum fitting on the intake manifold was threaded into the #6 runner. Therefore it was causing a "lean" condition for the #6 cylinder causing it to miss. Replacing the vacuum cap solved the problem.
It's probally not what's causing your problem but it's a quick easy check. Hopefully it's something this simple.
Good Luck
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Re: Help with 73 BBC engine problem, looking for idea's ? (danno85)
A couple of thoughts. First, danno85's suggestion of a flat cam is VERY possible. With it missing at idle, try wetting a finger or two and lightly test the temps of the various header tubes or exhaust manifold runners. If one in particular is cooler than the others, you have isolated the offending cylinder. An inferred(sp) heat gun is a good idea if you have access to one. Even new plug wires can have problems. Check the resistance on each as reassurance. Pull the valve covers and crank the engine, while watching the valve action. One or two that don't move as much as the others, will indicate flat lobes. Even a new cam, especially if it wasn't broken in, correctly will fail. Hell! Even big block cams that are broken in properly, are subject to early failure. Good luck, and...
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Thread Starter
Drifting
Joined: Nov 1999
Posts: 1,324
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From: St. Peters MO
Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
I did a static compression check and all seems fairley well.
highest was 88 p.s.i. and lowest was 79p.s.i. rest were 83 to 85.
Lowest was number 8 cylinder.
I have not found access to a leakdown tester as of yet.
The "popping" sound seems to be isolated to the #8 cyl. maybe #6.
Spark plug in #8 was slightly fouled, but not horrible, rest look great.
I'll try the watching the valve action with the motor running method to see if the exhaust valve on that cylinder is openning fully.
Can one cam lobe wear much quicker than the rest? :confused:
Almost sounds like an intermittent exhaust leak at high load or high rpm's at #8, can't really see underneath exhaust manifold, may go ahead and replace the exhaust manifold gasket on that side just in case it's leaking.
Thanks,
highest was 88 p.s.i. and lowest was 79p.s.i. rest were 83 to 85.
Lowest was number 8 cylinder.
I have not found access to a leakdown tester as of yet.
The "popping" sound seems to be isolated to the #8 cyl. maybe #6.
Spark plug in #8 was slightly fouled, but not horrible, rest look great.
I'll try the watching the valve action with the motor running method to see if the exhaust valve on that cylinder is openning fully.
Can one cam lobe wear much quicker than the rest? :confused:
Almost sounds like an intermittent exhaust leak at high load or high rpm's at #8, can't really see underneath exhaust manifold, may go ahead and replace the exhaust manifold gasket on that side just in case it's leaking.
Thanks,
Safety Car
Joined: Aug 2000
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From: Clymer PA
Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
A flat cam lobe is a possibility. I would also check every plug to ensure that none are fouled. I know you verified the timing of the motor with a timing light. However if it's an old balancer that has the rubber that holds the two pieces together, the other ring has been known to slip and throw the timing out of whack. You may want to try backing the timing down. If you are backfiring through the intake then you are too far advanced. Retard the timing little by little and see if that helps matters.
Keep us posted.
Keep us posted.
Melting Slicks
Joined: Jun 2001
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From: CA
Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
Your compression readings are way low. You should be getting at least 160 psi, depending on the cam & c/r. Make sure you get 5 good pumps w/all of the other plugs out and with a freshly charged battery. Dont forget to wire open the throttle.
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Thread Starter
Drifting
Joined: Nov 1999
Posts: 1,324
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From: St. Peters MO
Re: Help with 73 BBC engine problem, looking for idea's ? (danno85)
O.K., Looks like danno85 is correct!
Pulled valve covers, cranked motor, and cyl. #8 exhaust valve just bareley opens :eek: so it's leaning out in that cylinder.
I assume the cam lobe is flat, or possibly a bad lifter? But looks like I get to install a new cam and lifter set now :hurray: At least I'll know what I have in there.
Compression readings? I'm going to try another compression tester as the first one was an old beat up piece drug out from an oldtimer's tool box, probably from the 50's!
And I didn't open the throttle plates either, good idea. I'll also charge up the battery as I've been doing lots of cranking on it.
Does it matter if the motor is cold or warm when doing a static compresion test???
Thanks for all the idea's,
Tom
Pulled valve covers, cranked motor, and cyl. #8 exhaust valve just bareley opens :eek: so it's leaning out in that cylinder.
I assume the cam lobe is flat, or possibly a bad lifter? But looks like I get to install a new cam and lifter set now :hurray: At least I'll know what I have in there.
Compression readings? I'm going to try another compression tester as the first one was an old beat up piece drug out from an oldtimer's tool box, probably from the 50's!
And I didn't open the throttle plates either, good idea. I'll also charge up the battery as I've been doing lots of cranking on it.
Does it matter if the motor is cold or warm when doing a static compresion test???
Thanks for all the idea's,
Tom
Burning Brakes
Joined: Nov 2001
Posts: 1,189
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From: Austin TX
Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
Sounds like you are on to something.
If you want to continue the diagnosis, pull the intake and remove that
one lifter and look at the bottom. If it's concave and/or galled, which I
suspect it is, then the cam is bad too. You can also watch the lifter
travel up and down in the lifter bore compared to the other to see if it's
moving the same amount. This will eliminate the possibility that it's just
the lifter that has collapsed.
Good luck.
Dan
If you want to continue the diagnosis, pull the intake and remove that
one lifter and look at the bottom. If it's concave and/or galled, which I
suspect it is, then the cam is bad too. You can also watch the lifter
travel up and down in the lifter bore compared to the other to see if it's
moving the same amount. This will eliminate the possibility that it's just
the lifter that has collapsed.
Good luck.
Dan
Thread Starter
Drifting
Joined: Nov 1999
Posts: 1,324
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From: St. Peters MO
Re: Help with 73 BBC engine problem, looking for idea's ? (danno85)
First thing: Compression test are fine with new tester borrowed from a dealer where my wife works, they test all there used vehicles with it.
See: http://www.motoexotica.com/
My correct compression is 150 -155 p.s.i. in all cylinders, I guess gauges can go bad over time. Makes me wonder about my old torque wrench!
Now I plan to remove the intake and see whats up.
Just hypothetically speaking, if a lifter were collapsed, could you just replace one lifter? Or, must you replace cam and lifters as a set? I know thats better, but I was thinking of getting myshelf thru the summer with one lifter then doing cam and lifters over the winter months.
Of course if it's concave and galled up, I'll do it all right now.
Note: I have adjusted this exact lifter twice since last October due to valve noise, I tightened it down each time to 1/2 turn past zero lash so the rocker arm nut is now noticeably lower than the other nuts on there respective studs.
I'm thinking this lifter may have :U it's guts out.
See: http://www.motoexotica.com/
My correct compression is 150 -155 p.s.i. in all cylinders, I guess gauges can go bad over time. Makes me wonder about my old torque wrench!
Now I plan to remove the intake and see whats up.
Just hypothetically speaking, if a lifter were collapsed, could you just replace one lifter? Or, must you replace cam and lifters as a set? I know thats better, but I was thinking of getting myshelf thru the summer with one lifter then doing cam and lifters over the winter months.
Of course if it's concave and galled up, I'll do it all right now.
Note: I have adjusted this exact lifter twice since last October due to valve noise, I tightened it down each time to 1/2 turn past zero lash so the rocker arm nut is now noticeably lower than the other nuts on there respective studs.
I'm thinking this lifter may have :U it's guts out.
Burning Brakes
Joined: Nov 2001
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From: Austin TX
Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
Note: I have adjusted this exact lifter twice since last October due to valve noise, I tightened it down each time to 1/2 turn past zero lash so the rocker arm nut is now noticeably lower than the other nuts on there respective studs.
indication of this yet.
Dan
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Re: Help with 73 BBC engine problem, looking for idea's ? (NASCAR314)
Note: I have adjusted this exact lifter twice since last October due to valve noise, I tightened it down each time to 1/2 turn past zero lash so the rocker arm nut is now noticeably lower than the other nuts on there respective studs.
Hyd lifter = set 1 or 2 times . If you have already done this and the threads showing are noticable more than the others......Exhaust or intake?
You can replace a lifter, they have been known to go bad, but......
Thread Starter
Drifting
Joined: Nov 1999
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From: St. Peters MO
Re: Help with 73 BBC engine problem, looking for idea's ? (ALLTHROTTLE&NOBOTTLE)
Removed lifter today, it's concaved so bad, I can't belive it even ran!
It is the exhaust on cyl. #8, last one in the back of the block.
Cam lobe is galled up and worn down somewhat, rest of lifters and cam lobes look good to the naked eye.
"Oiling problem in back?"
Researching cam specs now, will install new cam and lifters at a minimum.
Thanks for all your help :thumbs:
Now the fun begins, if assembly goes as smooth as dis-assembly, I've got her whipped! This being my first cam replacement job :eek:
It is the exhaust on cyl. #8, last one in the back of the block.
Cam lobe is galled up and worn down somewhat, rest of lifters and cam lobes look good to the naked eye.
"Oiling problem in back?"
Researching cam specs now, will install new cam and lifters at a minimum.
Thanks for all your help :thumbs:
Now the fun begins, if assembly goes as smooth as dis-assembly, I've got her whipped! This being my first cam replacement job :eek:
