The Stovebolt Site Forums The Shop Area Engine Shop 305 V6 update... doesnt look good.

I have tried everything to get this thing running right. I think the timing chain has slipped. I first got this idea when I was trying to adjust the ignition timing. I used a jumper light and made sure the #1 cylinder was at TDC. What does UDC mean? it has that marked on the damper pully. Anyway, I used the jumper light, and loosened the distributor, and turned it until the light went out. I then turned it little by little until it came back on, and finished that. THis is the way to do it, according to my book. I started it up, and it ran horribly. Since I know I did everything by the book, I can only assume the timing chain has stretched and slipped. What do you guys think?

To check for slop in the chain, rock the crank back and forth by hand. You can feel the added resistance when the cam starts turning (or you can watch the distributor rotor for movement.) Take it in one direction until you feel the cam start to move, then back it up until again you feel the cam start to move. More than a few degrees (5-10?) and the chain is loose and should be replaced.
Have you tried "power timing" it? Advance the dist a bit at a time, until it pings on hard acceleration. Back it off a hair and you're done. Primative but effective.

HI GMCFAN,

The "UDC" I believe stand for Upper Dead Center it is designed for the #1 piston to be at the top of the block in the max compression stage. You need to get the timing mark on the correct line as shown in the shop manual, If you need a copy let me know and I'll send you a copy of mine. Right now if you take a look at my short block pictures you will see the piston in that position When I mount my head the wek I will carefully re-read all the section in my shop manual on getting the engine in the starting position, I still get a bit overwelmed (read Scared) because I have never done this type of stuff before EVER. I feel if I can read I can follow direction, when in doubt get on the board and beeeeeggggg for help.

JAke

g m c fan dose this motor have a governor on it, if it has look for a leak in the vacuum lines.my book says running timing should be 5deg. before tdc. i have owned two of these motors,one in a pickup the other in a 4000 ,good motors, they do like gas.dave h53 gmc 55gmc

Just a guess, but you may want to try loosing the rockers one at a time while someone else turns the engine over. Your lifters may not be pumped up?

Hey 42, this motor has solid lifters no hydraulics. With an engine over 100k, it is very possible that the timing chain has slipped, especially the V6, which has a very large distance between the camshaft and the crankshaft.I would line up the timing pointer on 0 TDC, and check the postion of the rotor under the distributor cap.Since it is a 4 cycle motor, the is a power stroke every 2 revolutions.Every second revolution the timing pointer and the distributor rotor should line up with #1 cylinder. If it does not line up, the timing chain has slipped a couple or more tooths.

My 305 seems to run best when set between 5 & 7 btdc on the timing light, idled down low w/vac advance disconnected and port plugged while setting timing. Have had some trouble with the stromberg carbs wanting to flood when idled down-needle and seat leaking. If you can't get consistent timing, you might pull the distributor and check the pin through the distributor gear and shaft bushings for slop, and the vacuum and mech advance components before tearing down to the timing gears. Other problems I've had with these engines include gummed up pvc valves and breathers and previous owners over-tightening the valve lash to quiet valve noise (makes solid lifter engines run like crap after they warm up).

Joe

allstock, I have tried the distributor thing, but I had the pointer on 7 1/2 UDC instead of 0. I am going to try that tomorrow. This engine has run almost 270,000 miles and isnt rebuilt. Man, GMC sure could build a good motor, when it runs right.

Just another guess, did you check the upper distributer bushing?