I guess this is opening a whole can of worms!!Let's see if you can convert the "no"'s amongst us!!
fierofetish

If the teeth on either the cam gear or crank gear (or both) are extremely worn along with a stretched chain it is posibble for the chain to jump a tooth . I personaly would think the chain would break first but I've seen stranger things :watch out:

[This message has been edited by FIEROPHREK (edited 09-15-2004).]

Hi FIEROPHREK
If I accept your theory,can you explain why it should only jump one tooth, and then stop?Surely, if it can jump once, it would not stop, but go on jumping..This is what baffles me.I am sure strange things happen, but not just once...
fierofetish

I have had a chain jump on a Chev 250 six and a Dodge 360, but not a Fiero. The timing was thrown off enough that investigation lead to the discovery before the chain broke. I have also had a chain break, there were many, many miles on the car. It became hard to start and rather than work on it I just left it alone till it died.

I guess you can believe whatever you want in a case like this. It really doesn't matter.

OK, so the chain jumped a cog..did it advance or retard the camshaft timing?
fierofetish

Posted it in the tech forum too:

What can happen is this:
If the chain stretches the length of one or 2 links, there will be slop in the chain. As the gear spins, if the chain "bunches up" and compresses itself a link, the extra link will come out once it spins around the top of the cam gear. Once this happens, the chain has effectivly "jumped" a tooth.

Its the same idea that some IVTs use. They have a very loose chain and an arm that "whips" the chain. This causes it to jump teeth on the other gear and change the drive ratio of the transmission.

Its very rare for the chain to "whip", but things like oil hitting it at just the right time in just the right place, or revving the engine at just the right time can cause a stretched chain to do this.

[This message has been edited by PontiacMan (edited 09-15-2004).]

My brother 's 69 Cougar (351W) jumped. Sure ran like poo poo

I'll explain why I find it so difficult to accept this phenomenum, and then you can tell me where I am wrong.
The cam sprocket or timing wheel is twice as big as the crank gear.The engine runs clockwise, and the crank gear pulls the cam gear round, and so there is constant tension on the right hand side of the gears because the valve springs try to stop the camshaft turning.Therefore because there is always tension on that side, there is no possiblity for the chain to flex enough for the crank gear to skip every tooth once.It can't skip only one cog at a time, because of the tension that exists.So, go to the other side of the gears, where the chain will always be slack.That is obviously why the tensioner, if fitted, is on that side, to take up the slack caused by wear and the dynamics that exist when an engine is running.In order for the chain to skip a tooth on the cam gear, the chain has to flex enough outwards for the links to form an outward "V", meaning that the camgear will engage with the next but one link, instead of the next one.As the cam gear is pulling the chain from that side, it will not allow the chain to flex outwards enough to form a "v", and anyway, if you were to take a chain, and make a v with it, you can see that the two rollersections of the links when doubled up, are a lot wider than one tooth of the gear.Therefore, it would be impossible to jump one link.That is the best I can do by way of justifying my beliefs without doing technical drawings or simulations.
fierofetish
Pontiacman...you posted whilst I was typing ...the "kink" in the righthandside of your drawing would appear to me to be impossible, by way of my explanation of my concept above.I would be more inclined to accept it jumping three links on the left side, but one is impossible
fierofetish

[This message has been edited by fierofetish (edited 09-15-2004).]

It doesnt have to skip over the cam gear. It can skip over the crank gear too. Itd actually be much easier for it to happen on the looser side of the chain as you said. Plus Ive never seen a timing chain setup with a tensioner on it...

You'd be suprized at how much a piece of metal can stretch after being used at 5000 rpms for 200,000 miles. The chain link bushing things usually wear out, which adds a bunch of slop.

Like I said, look up a chain driven IVT (inf. variable transmission), they use this exact principle.

[This message has been edited by PontiacMan (edited 09-15-2004).]

To satisfy your open minded curiosity take out your #1 spark plug and rotate the engine until the piston is at top dead center on the compression stroke at this point your rotor should be pointing dead on to the #1 plug wire,if the distributor was not removed before this then obviously the chain has jumped one or more teeth you should also have a hard start condition.If you need ony more convincing than that replace the motor

It cannot, in my opinion, jump the cogs on the crankshaft gear, because there is always tension there on the lead-in side of the chain.
fierofetish

In the Fiero timing cover there isn't enough room for the chain to jump without knocking a hole in the timing cover. Were that to happen there would be symptoms. Really loud and oily symptoms.

The older Chevy chain was known to jump because the cam gear was aluminum coated with thick plastic for noise abatement. The plastic wore off leaving enough slop for the chain to jump. Fiero gears are all metal.

JazzMan

Well sadly the only way to get solid proof would be to put a highspeed camera in the timing cover of an engine and catch it skipping a tooth.

If you want to believe it or not is completely your choice, but this is how it happens.

EDIT: Im talking general V8s btw, I dont know enough about the 60* V6s to comment on clearence issues etc.

[This message has been edited by PontiacMan (edited 09-15-2004).]

i have seen it many times on small block fords and once on a small block chevy. it does happen on some engines.

i've never heard of it on a 60' V6

I get that part of it, Akimoto Master Tuner, but that is not an explanation as to how the chain jumped, just an assumption that the chain has jumped, and your assumption that the chain has jumped one cog is your explanation for the discrepancy in the valve to distributor timing.I admit I have mostly worked on British cars all my 42 years of mechanical involvement, and most early Brtish cars had a timing chain tensioner. The only times I ever came across an engine where the timing chain had allowed one gear to turn one or more cogs is when the tensioner had broken, releasing the chain into a situation where the chain could disengage from either of the gears.OR, when somebody had fitted a new component involving the removal of the timing chain,and the des-synchronisation of the relative gear positions, and they had put the chain back one or more cogs out of time.I have never had one of my American cars suffer this problem, so maybe the system is different.But, when I took off the timing chain cover on my v6 Fiero, I do not believe there is enough room for this to happen within the confines of the timing gear area.
fierofetish
Edit: Firstly, I must point out, which I failed to do earlier, that I was speaking about the Fiero V6 engine.I apologise for this oversight.I have no experience of American v8 engines, as far as timing gear set-ups, and therefore cannot apply my theories to that type of engine.I have also revisited the thread that started me on this thread, and I see now the engine was not a standard v6 Fiero, but a bigger one from another type of Pontiac of which again I have no experience.

[This message has been edited by fierofetish (edited 09-15-2004).]

I had a Pontiac 301 jump a tooth on the timing chain. I don't know why, but one morning I went out to the car, started her up, and she ran awful. I thought I had somehow fouled 6 of the 8 plugs, as that's how it felt, but nope......no power, next to nothing for acceleration, and she really didn't want to stay running. After playing with it for a while, including retiming the car, I knew it was something bigger than plugs. A friend suggested the timing chain had skipped a tooth, so we pulled of the front cover and sure enough, it was off. The chain had stretched enough so when I had started the engine, it must have fired unevenly and caused the skip. New chain and gears and she was back to normal.

It does happen.

Mark

Hi Jazzman.
I tried to locate the previous thread some time ago, when you tried to explain how this can't happen on a Fierov6.You posted a brilliant(as usual!!) photo of the open timing area on a v6...would you be kind enough to post it again in this thread please?
fierofetish

quote Originally posted by JazzMan: In the Fiero timing cover there isn't enough room for the chain to jump without knocking a hole in the timing cover. Were that to happen there would be symptoms. Really loud and oily symptoms. The older Chevy chain was known to jump because the cam gear was aluminum coated with thick plastic for noise abatement. The plastic wore off leaving enough slop for the chain to jump. Fiero gears are all metal. JazzMan

quote Originally posted by Firefox: no power, next to nothing for acceleration...

Sounds like about right for a 301 to me

I've seen them jump-several times. Never while driving down the road tho, at highway speeds. It has always happened upon sudden decelleration/stopping, or upon turning the engine off. It's especially easy to happen if the eng rotates backwards on shutdown, or if it diesels a little bit.

I drove out to a drilling rig location one day in a company owned Ford F-150, with a 302 cu in V-8, turned into the rig's dirt road, and the engine died. Had spark-had fuel. Engine rotated , spit once in a while but wouldn't start. Got pulled onto the location, did my diesel work, and checked the physical timing. It was way off. I pulled the timing chain cover, and sure enough, the chain had jumped. Been 15-20 yrs ago, so I can't remember which direction it was off, but there is no doubt it had jumped a tooth or two. Yess, the chain was stretched/worn, but I pulled the gears, put the chain back on correctly, made a new cover gasket, and headed for home, 200 miles away. Made sure I kept the rpms up, by putting it in neutral when slowing down, and at a stop light.

quote Originally posted by maryjane:

you need to change your user name.
everytime i see it i think that you are female.

------------------
_{Buddy Craigg - there are two "G"s in my last name
Ling = 84SE-Modified
Julia C = 85GT stock (kinda)
Ivy = 67 Pontiac Catalina
KCFOG}

Nick,
i have seen it first hand on 3 or 4 small block fords and once on a small block chevy, i've never seen it on a V8 dodge.

there are a lot of 60' V6s out there...
GM has been making it for the last 18 or so years.

if it could happen, i would expect that we would have seen more instances of it by now.

------------------
_{Buddy Craigg - there are two "G"s in my last name
Ling = 84SE-Modified
Julia C = 85GT stock (kinda)
Ivy = 67 Pontiac Catalina
KCFOG}

quote Originally posted by fierofetish: Hi FIEROPHREK If I accept your theory,can you explain why it should only jump one tooth, and then stop?Surely, if it can jump once, it would not stop, but go on jumping..This is what baffles me.I am sure strange things happen, but not just once... fierofetish

Because after It skips once, you engine (provided you still have straight valves) is running like total ass and doesn't have the power to do it again. A worn sprocket and a streatched chain do not engage the way you are thinking. The chain obviously only wears the side with load on it. As the tooth tries to pull the link along, the link climbs up to the top of the tooth. I don't want to step on any toes, but the kink theory earlier in this thread is completely impossible. There is NO slack in a bad timing set while the engine is running. The chain is climbing the teeth on both gears. Any slack is taken up by the effective increase in sprocket diameter. The chain continues to streatch under this tension that is more than it was ever designed to withstand. This make it ride even further out on the teeth, which by the way, get thinner as you go out. Eventually, The resistance of the minimal engagement will be less than the torque your crank is exerting on the chain and it will jump. If you are lucky it's only one or two teeth. any more any your valves are destroyed.

If you want to know if your timing chain is bad, just put a breaker bar on the motor like you were going to turn it over by hand. If there's play, it's streatched and needs to be replaced wehter or not it has jumped time.

[This message has been edited by Azriel (edited 09-16-2004).]

Well, at last my eyes have been opened to the possibility of cog-jumping in certain engines!! I think it is a long shot, but the possibility is there, I guess...sharp deceleration transfers the slack in the chain to the lefthandside of the gears momentarily, causing a lash, which, if the chain is slack enough, might be carried over one of the gears.I would like to point out one slight flaw in this argument,though...if the chain was that slack, say 2 cogs worth, or 1 inch or more,then the timing of valvetrain to crankshaft would be at least 5% retarded at the time of the jumping...this is a hell of a lot, and the motor would have been running like the proverbial POS for a long time prior?
BUT..I still think this is impossible in the Fiero, which was the subject of this discussion, as per Jazzman's contribution above.
I want to thank you all for your contributions to this theme, and your explanations as to how you think it could happen.Enormous patience and restraint displayed, in most cases, for this stubborn ol' limey's tunnel vision!!!
fierofetish