Valve timing
02-13-2013, 03:11 PM
#1
Le Mans Master
Thread Starter
Valve timing
I have what I think is a valve timing issue. I have a 70 SS 396/402 big block Chevelle. Just before I put the nose on the car, I broke in the engine at the usual 2000 rpms for 20 minutes. The car ran rich, but I kept it going for the break in period. When trying to set up the car, I dropped the dist. in at 0 on the mark on the compression stroke at #1. The car fires up quickly. When I try to set the timing at 10 degrees initial with the vacuum advance off, the engine vacuum is at 3"
. I can get the vacuum at 11" only at 40 plus degrees of initial advance. The rpms at these settings are 950. I got the engine as a part of a package with a frame and a Turbo 400. Long story, but the engine was sold as a fresh rebuild that sat for 5 or more years. The cam is a mild hydraulic flat tappet with no lope. The carb is a q jet also stock with a fresh rebuild. I am thinking that the cam was not ligned up correctly with the crank, or the balancer has slipped. I plan to use a stopper on the #1 spark plug hole to determine if everything lines up. The vacuum guage is fluttering by about an inch or so. Is it also possible that the distributor was rebuilt incorrectly? I am looking for some opinions before I remove the left side head and measure TDC with a dial indicator and pull the timing cover off also to check the marks on the timing chain. Even if the balancer has slipped, I should get more than 11" of vacuum out of this motor. Lastly, the gauge is good and I do not have any vacuum leaks. What do you guys think?? Jerry
. I can get the vacuum at 11" only at 40 plus degrees of initial advance. The rpms at these settings are 950. I got the engine as a part of a package with a frame and a Turbo 400. Long story, but the engine was sold as a fresh rebuild that sat for 5 or more years. The cam is a mild hydraulic flat tappet with no lope. The carb is a q jet also stock with a fresh rebuild. I am thinking that the cam was not ligned up correctly with the crank, or the balancer has slipped. I plan to use a stopper on the #1 spark plug hole to determine if everything lines up. The vacuum guage is fluttering by about an inch or so. Is it also possible that the distributor was rebuilt incorrectly? I am looking for some opinions before I remove the left side head and measure TDC with a dial indicator and pull the timing cover off also to check the marks on the timing chain. Even if the balancer has slipped, I should get more than 11" of vacuum out of this motor. Lastly, the gauge is good and I do not have any vacuum leaks. What do you guys think?? Jerry
02-13-2013, 03:46 PM
#2
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02-13-2013, 03:48 PM
#3
Burning Brakes
Still running rich or no?
How are you so sure you don't have any vacuum leaks?
I don't think it's the distributor at all. What I would worry about with a motor that was rebuilt and sat for 5 years is whether you wiped out the cam or not. How long will break-in lube sit on the lobes before it just drops to the pan?
How are you so sure you don't have any vacuum leaks?
I don't think it's the distributor at all. What I would worry about with a motor that was rebuilt and sat for 5 years is whether you wiped out the cam or not. How long will break-in lube sit on the lobes before it just drops to the pan?
02-13-2013, 03:56 PM
#4
Melting Slicks
It sounds like the cam is retarded, but also could be that TDC is not where you think.
To do a rough check...
1. Set #1 to TDC on your balancer - you can get it within about 3 degrees of the piston's real TDC if you feel the top of the piston with a small screwdriver. Compare to the balancer's mark. If ok, proceed to the next step...
2. With the valve covers removed, look at the #3 intake valve - if it is open, you are at #1 firing... so now look at #6 which is also at TDC, but is in overlap. Look to see if the valves of the overlapped cylinder are equally open (both should be nearly closed. If the exhaust valve is more open than the intake, the cam is retarded; if the intake is more open than the exhaust, the cam is advanced.
3. If you find that the exhaust valve is open about 1/16" more than the intake, the timing chain was improperly installed and you'll have to redo it.
To do a rough check...
1. Set #1 to TDC on your balancer - you can get it within about 3 degrees of the piston's real TDC if you feel the top of the piston with a small screwdriver. Compare to the balancer's mark. If ok, proceed to the next step...
2. With the valve covers removed, look at the #3 intake valve - if it is open, you are at #1 firing... so now look at #6 which is also at TDC, but is in overlap. Look to see if the valves of the overlapped cylinder are equally open (both should be nearly closed. If the exhaust valve is more open than the intake, the cam is retarded; if the intake is more open than the exhaust, the cam is advanced.
3. If you find that the exhaust valve is open about 1/16" more than the intake, the timing chain was improperly installed and you'll have to redo it.
02-13-2013, 04:02 PM
#5
Team Owner
It sounds like the cam is retarded, but also could be that TDC is not where you think.
To do a rough check...
1. Set #1 to TDC on your balancer - you can get it within about 3 degrees of the piston's real TDC if you feel the top of the piston with a small screwdriver. Compare to the balancer's mark. If ok, proceed to the next step...
2. With the valve covers removed, look at the #3 intake valve - if it is open, you are at #1 firing... so now look at #6 which is also at TDC, but is in overlap. Look to see if the valves of the overlapped cylinder are equally open (both should be nearly closed. If the exhaust valve is more open than the intake, the cam is retarded; if the intake is more open than the exhaust, the cam is advanced.
3. If you find that the exhaust valve is open about 1/16" more than the intake, the timing chain was improperly installed and you'll have to redo it.
To do a rough check...
1. Set #1 to TDC on your balancer - you can get it within about 3 degrees of the piston's real TDC if you feel the top of the piston with a small screwdriver. Compare to the balancer's mark. If ok, proceed to the next step...
2. With the valve covers removed, look at the #3 intake valve - if it is open, you are at #1 firing... so now look at #6 which is also at TDC, but is in overlap. Look to see if the valves of the overlapped cylinder are equally open (both should be nearly closed. If the exhaust valve is more open than the intake, the cam is retarded; if the intake is more open than the exhaust, the cam is advanced.
3. If you find that the exhaust valve is open about 1/16" more than the intake, the timing chain was improperly installed and you'll have to redo it.
Except I prefer a small wooden dowel instead of a screwdriver in the #1 cylinder (or holding a finger over the plug hole to feel compression if you have a helper to crank the motor).
02-13-2013, 04:03 PM
#6
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Before you pull a head, use a piston stop tool to find true TDC, see where that aligns with the timing tab, and go from there.
02-13-2013, 04:50 PM
#7
Le Mans Master
Thread Starter
Timing
Great advice. I know there are no vacuum leaks, because I probed all the usual spots for leaks with starter fluid. I also disconnected all vacuum accessories like the P/B and A/C and transmission and plugged the manifold ports. I will tear into it this weekend. Thanks for your help. Jerry
02-13-2013, 05:14 PM
#8
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I don't think a timing chain installed wrong will make the vacuum gauge "flutter". BWTFDIK.
02-13-2013, 07:50 PM
#10
Le Mans Master
...only real way to find exact tdc....