Jumped timing chain??

I recently started my rebuilt 355 last week for the first time. I noticed a rough idle, but it ran. After resetting the distributor a few times and working out some wiring issues with the coil & ballast, I now find some excessive backlash. If I turn the crank CLOCKWISE with a ratchet to TDC and then COUNTER CLOCKWISE, the distributor rotor won't move until the crank moves 8-10 degrees. Did my chain jump a tooth or is my distributor in need of replacement. I'm using a distributor with 38K on it. The engine it came from ran fine. How can I tell if the distributor is the problem before tearing the timing cover off? Everytime I crank the engine over, the rotor lines up with #1 wire on the cap, while the balancer is on TDC.

The timing chain & sprocket were new in 1999 and never used. My vacuum is about 17, but the needle was moving back and forth when engine ran on Saturday for a very short time. The dwell is about 29-30 degrees. The springs don't seem loose, they snap back as I twist the ass'y open. The mechanism underneath the springs allows me to lift the center shaft about 1/8". Should that be possible?

This might sound dumb but I'm speaking from experience...I was young so be easy on me!!!! I had a ford 302, put the plugwires on in the right order, just going the wrong direction.... caused me weeks of grief! Back to the basics and check everything over and make sure it's correct...could be something really simple that keeps getting overlooked...

The engine is a 1980 350 Targetmaster that was bored .030" over this summer. It has new pistons, rings,bearings, crank, hydraulic lifters, cam, fuel pump & oil pump. The build is aimed at stock & reliable - no high performance stuff. I have checked the direction and location of wires many times. The timing chain I used was given to me in 1999 when it was new. The distributor I am using is the same one from this very engine that has only 38K on it. The engine ran fine before disassembly except for a low reading of compression and a broken harmonic balancer. I replaced the balancer with a new one. My carb is a 2-barrel that is rebuilt and sitting on top of a cast iron intake. Saturday it started, but sounded rough at idle. So I checked for spark at each wire. There is spark. I don't have a leaddown tester. I did a compression check recently and the readings were good for 8.5:1 compression engine. I'm concerned as to where the 8 degrees of backlash is coming from?

were the rockerarms adjusted while it was running? Were the lifters primed during installation? This really sounds like a valve problem to me.

Backlash would most probably be in the cam/ chaiin assembly, loose on the key or possibly end play in the cam, I suppose the drive on the crank could be to , but would seem less likely to me. Is this a double roller chain drive on the cam ? Maybe double check the drive gear on the distributor for pin wear. Just tossin it out there, it would drive me nuts to.

The lifters were primed. The rockers were adjusted with engine off (zero lash - then 1/4 turn). Both times it has started there has been no noise of any slop in the rockers to make me think they are loose. The chain is a double roller. I'm going to swap the distributor with another one and see if the backlash changes. I'm going to pull the valve covers and check rocker arms and pushrods. Yesterday was too humid to work in garage. Thanks for your suggestions.

I swapped out the distributor for another and the backlash didn't go away. I then took off valve covers and looked for loose or overtight rockers or possibly bent pushrods. Didn't find anything wrong there. I took the front of the engine apart and found the chain to be tight and the dots line up. However I did notice there is some slop on the crank gear. I suspect that might be the source of 8 degrees backlash. I ordered a new double roller timing chain set and should have it in a day or two. My timing chain I installed is the only part I considered new that I didn't purchase myself, it was given to me. Hopefully the engine will run better by this weekend.

Hey KC... I noticed a significant reduction in timing chain deflection (slack) when I put a new set on my Falcon Six. Hopefully your new timing set will cure it. I'd probably look at the valve train to see what position they are in when the crank shows #1 @ TDC (geez, now which ones should be on intake and exhaust strokes?)

1980 TargetMaster 350... I had a 94 in my 85 Chev G20... I know the earlier blocks had crankshaft issues... probably used 2 bolt mains. Were the crank mains machined too? Just a thought that if not, there might be excessive play at the crankshaft.

Good luck buddy!

Sasktrini - My 1980 block is a 4-bolt and the crank has been replaced with a new one. My main bearing tolerance is about .002". If I remember correctly, my engine originally had a cam sprocket gear with plastic teeth? Does that sound possible? It was replaced in 1985 with a cast iron version. I heard stories from older guys that GM was cutting corners to save money in the late 70's early 80's. I ordered a timing chain set last night from Summit and it was sitting by my front door this afternoon. I thought of Wile E. Coyote.

Did you ever get those van parts from Doc thru vanner express? I think it was last December or January they left my garage. Take it easy!

I think the reason for the Teflon gears was just for noise reduction. All the replacement parts are metal. Dumb idea from GM!

Yup... I got the boxes from Doc just like you got the box from Summit! none of my boxes said "ACME" on them... LOL

Plastic teeth? No expert, but I doubt it... these engines are used in towing applications... can't imagine how bad they'd wear! They may have had nylon tensioner parts to keep the chain tight... they get chewed up real good.

It sounds like your rebuild was good and solid! Good choices! Don't know that I would have any other suggestions on any slackness in your setup.

sasktrini wrote:

Plastic teeth? No expert, but I doubt it... these engines are used in towing applications... can't imagine how bad they'd wear! They may have had nylon tensioner parts to keep the chain tight... they get chewed up real good.

Yes, GM factory upper cam/timing gear had Nylon/Teflon teeth... good for about 100,000 miles. Don't know all the years that GM did this but it was always the first thing I changed when I got a car or truck with a V8 or V6. Never seen them broken off but could always see the marks from the chain worn into the gears.

Mo_1040 thanks for confirming the Teflon gears. I never noticed a noisey sprocket & chain before, except for a gear drive which I think is a nice sound. It's just not in my budget.

I think you'd have to have the hearing of superman to hear the difference between steel gears and teflon gears. I know alot of stock car racers liked them for some reason??? I have thought about going with gear drive but have heard of more problems with them than your regular chain and gears....I think unless your after some serious horsepower the gear drive setup is a waste of money.
http://books.google.com/books?id=cEvdJ6pcRKYC&pg=PA32&lpg=PA32&dq=chevy+nylon+timing+chain+gears&source=bl&ots=nCh0cMWo0F&sig=0MAbLfbIrM177aU9oPrD_dVkMBg&hl=en&ei=HIaDSvrILIqcMLy-sc0E&sa=X&oi=book_result&ct=result&resnum=6#v=onepage&q=&f=false