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Post by Hs1973 on Sun Jun 28, 2020 7:59 am

hi guys. 
Me and wanda need your help again. Recently i wrote in here that i thought about changing my rear end gears because i wanted more umpf from my new engine. I belive it Might have been Joe that mentioned timing could be an issue. 

Anyway i thought about it and decided to advance my timing a bit, and it made a noticeble difference in performance. But it also made me Think about my timing advance vacum canister and vonder if it is working proberly. Its a pretty new hei dissy, but i know things can stop working right without me noticing. 

So is there a Way to check if i works right without a timing light? I have plans for buying one and learning to use it, but for now i dont have one. Can i test it out anyway? Or do i need to buy a timing light? 

Thank you in advance. Henrik.
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Post by Joe73 on Sun Jun 28, 2020 9:25 am

Absolutely. Timing needs to be a TOTAL of 32-36 degrees. Theres a couple steps involved in getting there, its pretty easy, just have to take your time. It will make a world of difference.

Heres a good, but long read, about how to do it. https://www.chevelles.com/forums/374-ignition/189195-ignition-101-a.html

In a nutshell, heres what you have to do. More precise details in the read above.

First find out what you currently have. This is assuming your TDC (top dead center) AND your "0" mark on the timing tab match each other. Theres a process to verify that.

Pull the vac hose off the VACUUM CANISTER (and yes you should be running vacuum advance) and plug the line with a screw or something so you dont have a vacuum leak.

Its preferable and a bit easier if you have a timing light with the advance knob on the back end. Start it up, double check your idle rpm (should probably be around 900 since you have an aftermarket cam and looser convertor). Now take a timing reading, write the rpm and the timing reading down on paper.

Now, while shining the light on the timing mark, raise the rpm slowly til the timing mark stops advancing. Write down the rpm and timing number that they max out at. A timing tape on the harmonic makes things easier especially if your timing mark is off from TDC on the piston. Subtract your idle timing from your advanced timing, thats the amount of MECHANICAL advance the weights are giving you. You want this number to be between 32-36 degrees. If your low, increase your initial timing at idle.

Also, you will also be looking at what rpm your timing comes in at while you increase rpm. You want this to be close to your CAM rpm band. If you need the timing to come in sooner, you just light ONE or BOTH of the weights under your rotor.

Now, since you know what your engine timing is at idle with NO VACUUM advance. Now check it with your vacuum advance hooked up at idle. You dont want this number too high because it will make the car hard to start since the timing will be advanced too high and the starter might not be able to handle it.

Three things going on here. Initial timing, how much does the vac advance add and how much (and at what rpm) are the weights giving you.

Read the above link to set your timing for max power and it will be much more fun to drive and doesnt cost anything but some time.
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Post by Hs1973 on Sun Jun 28, 2020 10:41 am

I was hoping to hear from you joe73. Since im a bodyman i have not the greatest off experience with engine work. However i have Found that you dont always get things done right when you pay someone to do it. So im slowly starting to learn how to do things myself, of course i can do avarage mechanical work like brakes and putting a motor in or changing a trans. But i have not had the curage to mess with timing or carbs. But im going to buy a timing light and learn. So Thank you for the writeup im sure it will help me alot. I noticed that the guy i used for the initial startup did not tighten the bolt for the dissy enough so it has slowly turned making it run worse. Advancing it a little helped alot, and it feels more powerfull and peppy now. But im sure there is more in it, so i have gotten a vacum gauge and are going to attempt adjusting the carb too. That however i know how to do. So im hoping i one day dont have to Call for help anymore. Thanks Joe.
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Post by Hs1973 on Sun Jun 28, 2020 10:58 am

So i just read some off the link, and he States that i need to change to full manifold vacum. I have been running portet vacum. But i guess if he says all streetdriven cars should run manifold vacum i guess he knows what he is talking about.
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Post by Joe73 on Sun Jun 28, 2020 11:25 am

Definitely FULL manifold vacuum!! That would be any port at the base of the carb (throttle plate) or below. Should be a constant suction.

Once you get the carb adjusted correctly and the timing squared away, you will have a nice "crispy" throttle response. No lag in revving up.

Good luck with it.
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Post by Hs1973 on Sun Jun 28, 2020 11:48 am

I just read the whole thing in the link. Now it makes sense, i thought the vacum canister gave more advance and that was it. I did not know there was a cetrifugal advance too. I will tjek that its working right cause i can see how performance is Lost if it is stuck. 

I will Change to manifold vacum and tjek the timing with the ligt. If i understand correctly the advance should rise with the Rpm, if the advance mechanisme works right of course. Initial timing without vacum and at idle should be 14. then with advance it will be 34 correct ? there was something in the instruktions that Came with the engine about how much timing it likes. I need to look it up.
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Post by Hs1973 on Sun Jun 28, 2020 11:51 am

Does all dissys give 20 degreas more in advance or is it different from dissy to dissy. Mine is an aftermarket hei. But when i Think about it its actually 4 years old.
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Post by Joe73 on Sun Jun 28, 2020 11:58 am

Theres mechanical advance which are the weights and springs in the dizzy. They are different and comes in different weights as does the spring tension. The weights work centrifugally when the rpm increases. The springs that hold the weights closed also come in different tensions and changing them allows you to dial in when the advance curve starts at what rpm. You could go lighter springs to make the advance come in earlier, or heavier to come in later. And you can also have one heavy and one light or medium light spring to dial in when your advance starts coming in.

Now dont forget, the vacuum canister ONLY WORKS at idle and drops out when you open the throttle. Dont get that confused.

You will be dialing the dizzy advance with the weights. And you will be dialing in when the advance curve comes in with the springs.
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Post by Hs1973 on Sun Jun 28, 2020 12:21 pm

I know and i Think i understand how it works now. But i went over and Found the instruktion sheet that Came with the engine. And it says to give it 10 degrees initial and 32 at 3000 Rpm. I get that, however it says to unplug the vacum canister and run it plunger as the engine is designed to run without vacum advance. 

Is this ok? I guess if gm performance says to do it this Way it is ok. And shoulder it be 12 degrees initial to achive 32 all in if the cetrifugal ads 20? Sorry for all the questions, but im taking advantage off the fact that im talking to someone who actually knows.
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Post by Joe73 on Sun Jun 28, 2020 12:43 pm

You need to run the vacuum advance, you can go without but I think you should. I ran without it for years but the engine runs way better with it connected.

You have to check that your actually getting 32! And at what rpm. You need the timing light with the back dial and confirm the zero timing mark with actual TDC.

Changing weights and/or the vacuum advance will give you a different number of TOTAL.
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Post by Joe73 on Sun Jun 28, 2020 12:43 pm

Read this. Good stuff here too. Easier to understand.
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Post by Hs1973 on Sun Jun 28, 2020 12:59 pm

Thanks Joe. Ill get a timing light and start learning. But i Think i understand better and know more about what im doing and what it takes. As i Said i had a mechanic that initially helped me start this thing up. And im finding more and more things he did not do wery Well. So Might as well take the time and learn. That is how i did it with my rearend, so im sure i can learn this too. Thank you for your time and im happy you are willing to share your knowledge.
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Post by Joe73 on Sun Jun 28, 2020 2:06 pm

No problem at all. Lots of great people on this board always willing to help out.
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Post by bigredlaguna on Sun Jun 28, 2020 3:41 pm

If you don't have a dial back light, you should use a timing tape referenced to the true TDC. This may be more reliable because there is some debate as to whether the dial back feature is accurate with an HEI.

Vacuum advance is necessary for fuel economy on a street car. Ported vs manifold is a hotly contested subject, but there is no harm in trying both to see what your engine likes best.

Most distributors have weights that give 20 degrees advance. Just check them to make sure they are installed properly and move freely. I would also suggest to install the lightest springs to check total timing first so you don't have to rev the engine so high. Some factory springs are so strong they never allow full advance in the first place.

Then check initial after setting total by installing the strongest springs because the light springs can allow some advance even at idle in gear. After all that you can then tune the vacuum advance.

You can go down a rabbit hole about timing and thoroughly confuse yourself, but I think you will be just fine with just getting the total mechanical where it needs to be.

Once the timing is squared away, then do the carburetor. Good luck!
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Post by bigredlaguna on Sun Jun 28, 2020 3:59 pm

This guy has a lot of good tips that apply to any brand of vehicle.

https://www.youtube.com/watch?v=Q1B07CMRljA
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Post by Hs1973 on Mon Jun 29, 2020 10:45 am

Thank you again for info. I know its a topic that can be hard to get into. But it seems that the more i read about it, the more i understand. I will get a recive kit and take the dissy apart to see if advance mechanism works right. I will put lighter springs in and get a prober timing light. Im sure that when i first get into it, it is really not that hard. I was just a little confused as to why gm performance wrote on the instruktions to plug off vacum advance. But i will leave it on. Thanks again and im sure ill be back.
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