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Old 08-30-2004, 12:27 AM
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454BBC backfires all over all the time. I Need Serious Help Fast.

Hello, i was just bought a "bad lemon" 454 Big Block. I bought it under the impression it was going to run "great" but instead, it back fires all over the place, UP threw the carb and DOWN out the headers. Its a stock 1974 454 2bolt main chevy big block. the timing mark on the dampner is at the top of the motor (55 degrees off) the first timing i tried to run it, so i figure the TIMING CHAIN was to blame, so sure enough the nylon teeth were busted off about 5 of the aluminum teeth on the cam sprocket and the chain was so streched, it could touch the side of the block/time cover. So i REPLACED IT. I try it again crossing my fingers and it still idles like crap. i can rev it up but its kinda shakey, theres no throttle reasponce, and it still back fires out the exhaust and up threw the carb. I was able to get the motor to run at top dead center after replacing the timing gears; however, it backfires at TDC on almost ever cylinder until it kills a few moments later after moving the timing to 0 degrees and it still wants to run the best (which is still crappy) at 55degrees.

I have a new cam and lifter kit, but i don't wanna put it in and have it not fix the problem; thus, destroying the cam.

When its running at about 1500rpm it just runs kinda rough but still backfires ever no and then and its somewhat even firing and not irratical. IF you flick the throttle, it'll backfire, and if you rev up and release the throttle fast it will backfire if not kill. I think the backfires broke my edelbrock carb (is that possible?) b/c i can't adjust the fuel mixture, even when i put the carb on a motor i know runs good. But it will still make the motor run... the carb just won't make the good motor idel below 1500 rpm.... this backfiring 454 is one peice of work.

I need answers, i'm far beyond all the time and money this motor is worth, i just wanna get this nightmare over.

Its not just "tuning and spark timing" it definatly has a componant failure somewhere (least thats what i think after using 2 different carbs and distributors)

Any hopful information or definate answers.. i'm THINKING it can only be VALVE TIMING b/c the lower rotating assembly doesn't blow/burn any oil when i run it. I know its my fualt for buying it but i'm in over my head and i gotta figure this out.

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Old 08-30-2004, 12:49 AM
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Did you realign the cam gear with the crank when replacing the timing chain? Have you checked it for leaks and replaced the spark plugs since? They can foul pretty easily.

Yes, a radical backfire can kill a carb.
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Old 08-30-2004, 12:52 AM
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your not off a tooth on your timing? maybe you need to re-adjust your rocker arms?
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Old 08-30-2004, 01:03 AM
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Eventhough the wires "look" new and i was told they were new, I'll replace them, and i'll throw in some new plugs and gap them around 40. I'll take the valve covers off and TRY to check to see variance in rocker travel (the rocker in big block chevys are staggared which might pose as a problem to visually check) re-adjust the rockers (haven't done that yet) and thriple check the carb gasket and vacuum ports. If this does not solve the problem what do you experts out there suggest? ALSO, if it comes down to having to remove the cam... you think i should measure the lobes to see if there is any variance/difference in lift? How much difference in lift would pose this type of a problem?


OK. so is there an easy way to fix my edelbrock 650cfm carb? I haven't really ever messed with carbs before.

I know i made good and sure the cam and crank gears were aligned.

I have yet to run a compression test on it. if the compression test shows 90-100psi in all cylinders with no dramatic pressure differences then it means its good. What else would it be telling me if the compression was all messed up? (as far as cam lobes and rockers etc...)

I KNOW I TYPE A LOT BUT I JUST WANNA GIVE ALL THE DETAILS

THANK YOU

Last edited by MuscleBlock81; 08-30-2004 at 01:03 AM.
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Old 08-30-2004, 01:28 AM
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it sounds like its way out of time

did you move you rotate the engine at all while you had it apart before putting it back together and maybe have either one of the gears or the dist. out of time.or did you have it set at TDC when you put it back together

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Old 08-30-2004, 01:37 AM
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he said the dampner is at 55 degrees when the engine is at tdc, and he has moved the timing around and it sounds best at 55 degrees but it's still crap, maybe he's 180 out????
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Old 08-30-2004, 04:04 AM
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If you only have 100psi of cranking pressure something is definately wrong. With the symptoms you are having I have to say it is that the cam timing is screwed up. If the valves are opening at the wrong time, it will bleed off al the cylinder pressure. The hard starting and constant backfiring also point to this. I think it is time to take the timing cover back off and then we can talk about the timing marks. You also need varify that your balancer timing mark is right or get a new one. If the cam timing is off, the balancer might be correct at 55 degrees.

By rotating timing 55 degrees you may actually be causing the distributor to arc to the previous plug wire. Since there is an ignition event every 90 crank shaft degrees, the rotor is actually closer the previous pole than the desired one when you rotate the distributor this far. This would cause irratic firing and arcing in the cap.

Chris
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Old 08-30-2004, 06:38 AM
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sounds like when the timing chain jumped you might have bent some valves. Do a leakdown test.
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Old 08-30-2004, 11:15 AM
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I dont believe that the 454's were interference engines.
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Old 08-30-2004, 01:05 PM
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I had one that did the exact same thing...drove me nuts. Turned out to be a bad ignition coil. I saw it when it was dark out, arcing all over the place.

I always wait unti dark now and take a look under the hood with the engine running before I tear anything apart. You can see if the ignition system is arcing quite easily.
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Old 08-30-2004, 01:57 PM
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Ok, i have yet to do the compression test.

when i originally replaced the OLD timing chain and gears, the gears were in the right spot (MAYBE one tooth off) but the chain was so streach that it probably ran like it skipped one tooth. The only thing that changing that did was make it possible to turn the distributor from 55 degrees off to 0 or (tdc) with out killing the motor; however, it backfires so bad at tdc that it kills itself from backfiring.

I don't think THAT could have been far enough to bend the valves. And at 1500rpm if i slowly increase the RPM to like 4000rpm it doesn't backfire (usually) so if it had a bent valve, wouldn't it backfire ALL the time?

Is it possible that a valve could have gotten "burned" like the edge got taken off, or would that make if backfire all the time?

Also.. when i said runs "crappy" i meant it shakes the car a lot but doesn't backfire irratically.

Finally... the dampner is not 55degrees off at TDC.... the dampner isn't spun. the motor just runs the best at 55degrees...

I've used 2 distributors, i know at least one of them works correctly.


TYPES OF BACK FIRES.

1-flames shoot out the headers
2-one REALLY loud bang out the headers
3-fuel squirts up out the top of carb and starts on fire
4-flame ball shoots up out the carb
5-really wierd "hollow sounding" poping like the noise you get when you hit a hollow log almost.

Something still seems physically screwed up b/c the distributor and the carb can't be adjusted to make it run any better the way it is now... Somthing HAS to be physically screwed up still. I'm going to try another distributor, new wires and plugs and adjusting the rockers and a compression test. after all that... it has to be the cam, but if it wa that cam, wouldn't it backfire ALL the time NO MATTER what rpm or fuel mixture and amounts. or am i wrong?

Any ideas? THANK YOU

Last edited by MuscleBlock81; 08-30-2004 at 02:02 PM.
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Old 08-30-2004, 02:02 PM
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probably not it but are the plug wires on cylinders 5 and 7 reversed?
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Old 08-30-2004, 02:05 PM
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I use HEI style distributors.

I have triple checked the firing order.

whats a "leakdown" test?
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Old 08-30-2004, 02:38 PM
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Timing

Doc here

If you are absolutes sure the valve and distributor timing is correct, Then It sounds like you may have some valve issues...

Timing chain is Dot to dot...#6 TDC Compression and Both dots at 12 O'clock #1 TDC Compression correct?

Rotor pointing to #1 Plug at the distributor on TDC Compression ,both intake and exhaust valves closed?

If you have another Distributor , try that, the old one may be defective, from backfires, (broken teeth, slipping gear, ETC)

If all that is good, Then Do the Compression test, 125 to 150 a hole is good, less, you may have some bent valves, or timing chain is off.

Do you have an EGR Valve? It may have some serious leaks...

Doc
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Old 12-04-2004, 09:31 PM
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hey, i was looking for a for something else when i ran across you problem. if you haven't solved it by now it might be a vacuum leak caused by a hole in the intake manifold under the carb. if memery serves me correctly i had to drill, tap and plug a small factory hole under the base of the carb in the manifold . i can't remember the backfiring but do remember that to make the engine run i had to advance thr timing about 50*or so . i hope this helps
studebakerclyde
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