CK5
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Timing with a .... timing light?
- Thread starter 78 GMC Jimmy
- Start date
While I am by no means an expert, I can speak to the expertise of the folks answering your questions. I just went through this with a different setup. It took a bit for me to realize what was happening...there is a lot more to timing than set it and forget it. There's always going to be a trade-off, it cant run at it's best all the time, even with variable timing controlled by a computer. You have to either tune for all-out WOT or find a happy medium where it's 'close enough' to meet your needs. I do have a bit easier way to find TDC, maybe not as accurate, but close enough for me. Try a plastic drinking straw in the #1 cylinder. It will come up, pause, then start to drop as you turn the balancer. When it stops the upward motion, you're about TDC. Then figure out if you're on the compression stroke with the finger on the hole trick and you're good.
I used to use a chopstick, but then I lost one in the cylinder. A straw won't hurt anything. Apparently, the chopstick didn't either, but i was a little worried the first time I started it.
I used to use a chopstick, but then I lost one in the cylinder. A straw won't hurt anything. Apparently, the chopstick didn't either, but i was a little worried the first time I started it.
78 GMC Jimmy
1/2 ton status
Thanks for the info. Ya, I pretty much gave up on trying to time it. I just advanced the distributor until it would kick back on the starter or ping under acceleration, and then retarded it slowly until those symptoms went away. I'm pretty sure it has an aftermarket cam in it, and maybe even pistons as well, and I just don't think that the traditional timing settings will work for this engine. That combined with the overheating of the cylinders (without any indication of this on the temp gauge) resulting in detonation made it very difficult to track down the problem because it would ping and kick back at ANY timing setting when the engine was warm, but not when cold. But the issues are solved, and its good enough for my needs.
You should try timing it with a vacuum gauge. With a vacuum gauge connected to manifold vacuum source advance timing until you reach maximum vacuum, and then back it off 1-inch of vacuum. Then tighten down the distributor. When it comes to aftermarket cams that have not been degreed in with a degree wheel plus aftermarket cams having manufacture tolerances not being perfect using a vacuum gauge can be the only way to time the vehicle at least somewhat close to correct.
78 GMC Jimmy
1/2 ton status
Thanks for the info. I tried a vacuum gauge, but the needle bounces a bit and its hard to calculate what max vac is, because the needle is not steady. How do I stabilize the needle?
Hopefully it's just the sign of a big cam. Usually the gauge bouncing at idle means you have an intake valve not closing properly. It's possible your original issues are not just the timing.
You can try putting some kind of restriction in the line, which will slow the response of the gauge.
You can try putting some kind of restriction in the line, which will slow the response of the gauge.
78 GMC Jimmy
1/2 ton status
Pretty sure its a big cam. Who ever had this rig before me built it from the ground up. They swapped out the motor to a 400, switch from auto to manual trans, 3/4 ton axles, lifted, tires, etc and then threw tons of upgrades and chrome at it. Intake, exhaust, the works.
You might have a vacuum leak that is causing the vacuum to not be steady. If the vacuum leak is bad enough no amount of timing adjustment will make the engine run right. A wild cam will make the vacuum a little unsteady with a low reading, but not so much fluctuation that you have no idea of how much vacuum your engine is creating.
78 GMC Jimmy
1/2 ton status
OK, I will block off unnecessary vacuum hoses, such as the brake booster and then check the rest of the motor with starting fluid. If there is a vacuum leak, I will find it. I will also check the torque on the intake manifold bolts.
You might also use that vacuum chart posted by Blue85 to see if anything other than a vacuum leak is happening.
78 GMC Jimmy
1/2 ton status
Cant see the pic, my account wont allow it. Link to a google pic?
