timing chain question?


gotmudd

gotmudd

15+ Year Member

Joined
Mar 5, 2008
Messages
618
Points
3,101
City
PORT ANGELES, WA
Vehicle Year
1973
Transmission
Automatic
o.k, so i have '73 390c.i.d motor other day while in town started running like crap[checked all plug wires, they are in order and good shape and none are crossed]anyway it was spittin and sputtering like maybe it jumped a tooth or two on the timing chain, so today i decided to dig deeper by starting truck to see if it happened at an idle or just when the throttle was applied. truck started o.k ran so-so off idle for maybe 20 seconds then started to sputter. i thought that if i snapped the throttle a couple of times to clear it i would have time to get out under the hood to check things out. it ended up backfiring and dying so i tried to restart, it backfired REAL LOUD twice but would not start. am thinking it's my timing chain[ which i intend to replace], my question is if the chain broke what do i have to look out for?i was told it might bend a valve but not sure:temper:any clues before i put it back together would be great, am going to tear down now
 
If it was going to bend a valve, it already has.

Test for such damage with a leakdown test. Pressurize each cylinder to 100 PSI at TDC/compression and listen at the TB and exhaust pipe for hissing. Should be dead-nuts silent. Note that leakdown past piston rings is normal, and you'll need a calibrated leakdown tester to determine if that's excessive. But ANY leakdown into anything else is not normal.

It's more likely one of the sprockets wore down to nothing, than your chain broke. But you'll be able to tell on teardown. Some of those older sprockets are made of Nylon....

Do check IGNITION timing before a teardown. It's not unsual for the distributor to work loose and make just ridiculous timing. If it won't start, time it at cranking speed or at turn-the-crank-with-a-ratchet speed. It won't be right on, but you'll detect FAR off timing that way. Note that old HEI ignition may not fire slower than cranking speed; points or Hall Effect HEI ignition will fire at any speed if good.

And before you get too far down into the engine, remove the valve covers, and put each cylinder on TDC/exhaust. Close valve timing will have both valves slightly open. If valve timing makes it backfire, they would have to be WAY off. Sometimes cams advance or retard the valve timing by a few degrees, so the height of the opening may not be exactly equal.
 
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MAKG is right; I would have checked/replaced the points and even pulled the fuel pump to feel for timing chain slack before I tore the whole front dress apart.
 
MAKG said:
Note that old HEI ignition may not fire slower than cranking speed; points or Hall Effect HEI ignition will fire at any speed if good.
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HEI ignition is General Motors. The ignition on this truck would be Duraspark.

A compression test will be as good as leakdown testing for this purpose. Most don't have facilities for the leakdown test.

This could be an igniton problem. Bad coil, intermittent connection to ignition circuit. I went thru this last year with a 350 Chevy Rat Rod. shady
 
shadetree said:
HEI ignition is General Motors. The ignition on this truck would be Duraspark.

A compression test will be as good as leakdown testing for this purpose. Most don't have facilities for the leakdown test.

This could be an igniton problem. Bad coil, intermittent connection to ignition circuit. I went thru this last year with a 350 Chevy Rat Rod. shady
Click to expand...
yeah, i thought the HEI thing was only for chevy too, as for4 the dura spark, mine is breaker point which predates dura spark by a year or two. talked to a friend back home and told him what was going on and he said it was definately in the ignition/firing sequence, but that it might also be a fuel problem also. so i am going to replace the timing set cuz it looks a little wore, but not too bad and then do a tune-up and see if i can get the old girl running:3gears:
 
gotmudd said:
mine is breaker point which predates dura spark by a year or two.
Click to expand...
You are right. It was late 70's for the duraspark. The problem I had with the Chevy was occasionally it would start & run. Then it would start missing, backfiring like hell. Then sometimes when you tried to start it, it wouldn't start and would backfire all over the place. Cam and ignition timing were perfect. The coil was new, but I changed it anyway. The car fired and ran perfect. We thought the problem was fixed. I talked to the owner of the car later, and the problem was actually the wire from the coil to the dist. He had rewired the car, and the crimps on the connectors were sloppy and made intermittent connections. shady
 
Yeah, HEI is a GM trademark, but the idea of a high-voltage electronically controlled ignition based on a variable reluctance sensor was copied by everyone. The problem is the VR sensor; its signal is proportional to crank speed, so it gets really small if you turn the crank with a ratchet. By contrast, the later Hall Effect sensors have the same amplitude regardless of speed, much like points.

If this thing has points, it can be timed with a ratchet.
 
IT LIVES:icon_hornsup::icon_hornsup:stil not sure what the problem was :icon_confused:so i will chalk it up to gremlins:annoyed:. anyway i replaced the timing set cuz it measured right at the tolerance of 1/2inch deflection, and i also replaced the dist. cap and double checked the wiring. got her glued back together and we now have VROOM-VROOM:D
 
I've seen whipped points blow a muffler a good block.

HEI was good. I doubt the Frisbee-sized cap was really necessary. I mean, a spark will jump a long way if it has nothing better to do, but your supposed to provide a good landing for it. Some motors you had to convert to something else--like a point distributor with a Hy-Fire conversion--if you wanted to run an aftermarket intake.

I liked the GM points because you could set the dwell with the motor running. That was just smart.
 
Will said:
I liked the GM points because you could set the dwell with the motor running. That was just smart.
Click to expand...

For a small-block, yes.

Not on my I-6. The points are adjusted from inside the dist cap. There is no window.

One of these days, I'll put a 1973 HEI distributor in there.
 
MAKG said:
For a small-block, yes.

Not on my I-6. The points are adjusted from inside the dist cap. There is no window.

One of these days, I'll put a 1973 HEI distributor in there.
Click to expand...

i am probabily mistaken but HEI is chevrolet and in 1973 points were the only option unless you went after market with a electronic ignition and according to MY clymer shop manual your choices were breaker points[pre-73 and some 74]SSI[75-76] and different variations of dura spark on everything else.NOTE, this is for in-line 6 and V-8 engines UNLESS the guys who wrote my manual were smokin' somethin'
 
gotmudd said:
i am probabily mistaken but HEI is chevrolet and in 1973 points were the only option unless you went after market with a electronic ignition and according to MY clymer shop manual your choices were breaker points[pre-73 and some 74]SSI[75-76] and different variations of dura spark on everything else.NOTE, this is for in-line 6 and V-8 engines UNLESS the guys who wrote my manual were smokin' somethin'
Click to expand...

His '72 is a GM.
 
85_Ranger4x4 said:
His '72 is a GM.
Click to expand...
my bad, but MAKG still ain't going to find an HEI from 1973 to put in his ride because according to BOTH my firebird AND camaro books hei wasn't an option until 1975 UNLESS my books are mistaken, which i don't think so:icon_confused:
 
Yup, they are mistaken. I have a 1973 HEI 250 I-6 distributor sitting in my spare parts pile, along with a set of spark plug wires (they are different). I just need to find the time to run the wiring. Not a big deal, but I have to TRUST that neither of the other vehicles will break while it's down.

Were I-6s even an option for Firebirds and Camaros? I've only seen them in Novas and "sport vans." And Pontiacs may be GM cars, but they are NOT the same as Chevy engines. Also, don't trust Haynes for model years. They get it wrong often.
 
HEI was available in the 60's. You can now get the HEI for most any engine. I just put one in a 302 engine last year that went into a 56 Lincoln. shady
 
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