Timing problem with rebuild - Chevelle Tech
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post #1 of 19 (permalink) Old Jun 28th, 20, 9:11 PM Thread Starter
 
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Timing problem with rebuild

Hoping someone can help us out! Last year after our all original 72 Chevelle was nearly obliterated by a landscaping truck and trailer we took on the mission to bring her back to life. My husband and son took on the rebuild over the winter, only thing original at this point is block and crank. (350 Bored 30 over, Edelbrock top end kit installed) They have been methodical about the work ( son is a mechanic who has done rebuilds before ) but one major problem remains. Timing is properly set, runs very well for about 10 minutes, starts running rough, they shut it down and the rotor is off 180. Does this every time. Reset it properly, repeat process and back to being off again.
Any suggestions would be so very much appreciated.
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post #2 of 19 (permalink) Old Jun 28th, 20, 10:19 PM
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leo
 
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Re: Timing problem with rebuild

Has the distributor gear been checked to see if the pin holding it on the shaft has been sheared??

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post #3 of 19 (permalink) Old Jun 28th, 20, 11:57 PM Thread Starter
 
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Quote:
Originally Posted by LeoP View Post
Has the distributor gear been checked to see if the pin holding it on the shaft has been sheared??
Thanks for getting back so quickly. Yes it has been checked and can’t find anything wrong with the distributor. Ordered a new one in the off chance it might work. Really appreciate your suggestion.
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post #4 of 19 (permalink) Old Jun 29th, 20, 7:03 AM
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Re: Timing problem with rebuild

Quote:
Originally Posted by Jimmary View Post
they shut it down and the rotor is off 180.
Are you sure it is off 180 and not just on #6 when they are checking it ? Almost impossible if the distributor gear isnt spinning on the shaft.

Something else is probably causing it to shut down.
.
.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md


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post #5 of 19 (permalink) Old Jun 29th, 20, 8:04 PM
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Re: Timing problem with rebuild

Impossible to shear the gear pin and have it end up 180° out....it will quit running long before it even got to 70° out.

Seems your confused to the fact you have TDC twice for every one revolution of the distributor. #1 cylinder and #6 cylinder are 180° apart in the firing order, and both come to TDC at the exact same time, just on different strokes of the 4 stroke cycle. One complete 4 stroke cycle in any cylinder requires two full crank rotations
When #1 is at TDC firing the plug getting ready to shove the piston down the bore on the power stroke...., #6 is at TDC overlap pushing exhaust out/exhaust valve just closing while intake valve is just starting to open and piston getting ready to head down the bore drawing in a fresh air/fuel charge.
By the same token, when #6 is at TDC firing, #1 is at TDC overlap.
Camshaft cycle decides which situation is happening, not just the timing line on the balancer.

The distributor timing is not what is causing an issue, you need to look elsewhere for the rough running problem. Valves adjusted too tight, fuel delivery issue, choke pull-off, vacuum leak, coil over-heating, etc.
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post #6 of 19 (permalink) Old Jun 30th, 20, 11:55 AM
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Re: Timing problem with rebuild

Just what distributor is in the engine and supposely doing the "180 free spin"?

As Eric said, the engine wouldn't stay running if the rotor was doing the 180 free spin.

One test, take the cap off the distributor, no matter what kind it is, and grab the rotor, try to turn it around with your hand. The rotor should only move a short distance, then, snap back when you release the hand from it. If the rotor still moves past that, and the gear pin is OK, pull the ritir off and take a look at the top of the distributor shaft where it interacts with the rotor mount. Some shafts are pressed into the timing center, some are brazed on place.

One day decades ago, my next door neighbor had a 454 Suburban, he went to "the best shop in Whittier, California", where they proceeded to do work on his wallet, and then, had to have the thing smogged. Didn't pass. so I got to fiddle with it. One fouled spark plug, bad plug wires and the large coil n cap HEI had the brazing holding the shaft to the plate was broken. Plate, reluctor and rotor were all over the place but still ran. I simply took the distributor apart, realigned the cracks in the brazing, redid the braze, put it back together, timed it, set the carb, and the smog guy couldn't figure out how this one passed, "they never do".
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post #7 of 19 (permalink) Old Jun 30th, 20, 3:47 PM Thread Starter
 
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Thank you everyone for the suggestions you guys are amazing.
We are out of town today but will be back to the head scratching and cussing over the chevelle tomorrow. You’ve given us plenty to think about until then and we will no doubt be in touch with more questions if you are willing. Have a wonderful evening!
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post #8 of 19 (permalink) Old Jun 30th, 20, 4:13 PM Thread Starter
 
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Update from my son... 🤔
Verified TDC on compression stroke after setting timing gear, assembled and ran engine until it stalled, reverified TDC on compression and rotor went from cylinder 1 to cylinder 6. Cam and distributor gears are intact with no odd wear and distributor gear roll pin is intact. Unable to grab and move rotor by hand more than a few degrees as normal. Will pull distributor apart and check for cracks or signs of breakage where distributor shaft meets rotor mounting plate.
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post #9 of 19 (permalink) Old Jun 30th, 20, 11:17 PM
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Re: Timing problem with rebuild

There is about only one way this could be happening, if it is....

The pin in the gear is sheared, but the jagged sheared parts still grip the edge of the pin hole in the shaft enough to drive if briefly....then it loses hold, spins 180° and catches up at the edge of the hole half way around the shaft. Same situation would apply with a broken braze or swedge fit on the advance plate at the rotor top end.

So if you examine the gear, you need to drive out the pin and take it off the shaft, not just look at it externally because the visible ends of the pin could look completely normal while the midsection of the pin is sheared off inside the shaft, out of view.

This would be a 1 in a million thing if it is happening.
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post #10 of 19 (permalink) Old Jul 1st, 20, 8:50 AM
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Re: Timing problem with rebuild

what does "setting timing gear" refer to? is this a gear drive cam setup? perhaps more information about the engine setup will help with understanding the issue because it makes no sense currently
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post #11 of 19 (permalink) Old Jul 1st, 20, 9:13 AM
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Re: Timing problem with rebuild

Where are you located ? Maybe there is someone local that can help you. A lot of times a second set of eyes will see the problem.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md


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post #12 of 19 (permalink) Old Jul 1st, 20, 8:33 PM Thread Starter
 
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Again thank you, the distributor was completely taken apart and examined and even though it looked to be fine it was replaced today and the new one hasn’t remedied the situation.
There was a question about the timing being set and perhaps my confusing explanation, my sons response was:
Double roller timing chain set
Timing marks are Crank at 12, cam at 6.
I’m going to do us all a favor and pass this thread over to my husband who has crawled out of the garage. He can fill you all in with the nitty gritty facts.
Btw, we are in Northern Michigan (Charlevoix) and a second set of eyes would always be welcome. 🙂
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post #13 of 19 (permalink) Old Jul 1st, 20, 8:54 PM
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Re: Timing problem with rebuild

Quote:
Originally Posted by Jimmary View Post
There was a question about the timing being set and perhaps my confusing explanation, my sons response was:
Double roller timing chain set
Timing marks are Crank at 12, cam at 6.

Btw, we are in Northern Michigan (Charlevoix) and a second set of eyes would always be welcome. 🙂

None of this matters if it starts and runs at first. The timing chain cant move. I am betting is is stopping due to some other reason such as running out of fuel. Or an electrical problem. Will it start and run at this point ?

Bill Koustenis
Advanced Automotive Machine
Waldorf Md


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post #14 of 19 (permalink) Old Jul 1st, 20, 9:25 PM Thread Starter
 
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Re: Timing problem with rebuild

Installed a new distributor today, double checked everything. #1 TDC compression stroke, rotor @ #1, all plug wires correct order. It popped fuel back out through carb once. Tried again & it backfired but wouldn't start. We are missing something, need to start over just not sure how far back I want to start.
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post #15 of 19 (permalink) Old Jul 1st, 20, 10:23 PM
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Eric
 
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Re: Timing problem with rebuild

As stated above, your confusing TDC on the compression stroke. Willing to bet the rotor is 180* out currently and firing on #6 when it should be on #1. Timing gear dots at 12/12 its firing on #1, 12/6 its firing on #6.

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