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Username Post: 1975 Chevy Timing        (Topic#341803)
Thumpit 
Silver Supporting Member
Posts: 43
Thumpit
Reg: 11-26-16
01-09-17 09:19 PM - Post#2670377    

I have been all over this board and the net reading about setting initial and total timing. I have a 75 Chevy 350 with HEI and 4 speed manual with the granny gear. I have to replace the advance vacuum and want to check timing. I understand part of the process but the 'total' gets me a bit on how to set it.
I unplug the vac and plug it and make sure of the TDC at 0*. And then start engine and set to about 8* BTDC. Make sure the idle is right. And then plug up the vac.
That's what I usually do. But how do I make sure the total is right?
Do I leave the vac plugged to set the total?
What else will I need to do and will I need anything other than a regular timing light?
Thanks




 
LMC Truck
0utlaw 
Very Senior Member
Posts: 2711
0utlaw
Loc: US East Coast
Reg: 09-10-02
01-10-17 05:30 AM - Post#2670412    
    In response to Thumpit

Verify that your TDC timing mark is correct.
(Outlined in several previous posts)
BASICALLY
Run down to your local speed shop or order a timing tape from Summit, Jegs for YOUR DIAMETER DAMPER. Money well spent. Clean your damper with solvent WELL and apply tape. Now you can set the curve any way you want, any time you want with a regular light and be pretty accurate. Most 350 mild Chevies (old school) like 8-10 initial. Your combo of springs should get you a max timing (vacuum disconnected) between 32-36 degrees (depending on many variables. I'd go for 32 to start) You could start off with 2 medium springs from ex. a Mr Gasket Spring Kit, reset your initial and check it out. I would suggest an ADJUSTABLE vacuum can and adjust it per INSTRUCTIONS.... if you hear the engine ping at light throttle cruising back off the can and not your initial. If the ping continues, back off the initial. Be prepared to spend time on this...EVERY ENGINE IS DIFFERENT....NO SET RULES. It might run unbelievable one day and not quite so good the next...just the weather can change it. Remember, for max performance, you MAY need hi-test. You can try manifold vacuum or spark-ported vacuum on the vacuum can to find out which works best. Either vacuum will EVEN OUT at 1100 rpm or so. Having vacuum at idle aids in warm up and light throttle cruising, but may cause a slight skip out the pipes at idle. OUTLAW



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18514
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
01-10-17 06:26 PM - Post#2670509    
    In response to 0utlaw

Total timing equals initial (or base) + mechanical advance, so total = initial + mechanical.

There usually is some way to limit mechanical advance, but you might have to purchase a weight and spring kit to get it, or there may be a bendable limit stop depending on what distributor you are using.

Vacuum advance doesn't figure into this equation, because the vacuum advance goes away after you open the throttle all the way. It only operates when the vacuum is high, or in other words when the throttle blades are closed or close to closed (intake manifold vacuum, port vacuum works a little differently). Also, on most muscle cars the vacuum advance should normally be limited to 10 degrees, and there should be a limit strap/cam/mechanical add on inside any ADJUSTABLE vacuum advance kit to limit the overall vacuum advance.

Typical initial advance is 4,6,8,10,or 12 degrees, and usually depends on the type of combustion chamber design you have. On older heads, the upper limit is usually 12 degrees initial, on better (newer) heads it may be limited to 8-10 degrees, depending on how good the head design is.

It is all based on how well the gasoline explosion is burning inside the cylinder, and it can vary if the humidity is high, or there is a change in elevation, or a host of other factors.



 
0utlaw 
Very Senior Member
Posts: 2711
0utlaw
Loc: US East Coast
Reg: 09-10-02
01-11-17 04:34 AM - Post#2670544    
    In response to 0utlaw

Vaughn did a great job of further explaining.
I outlined how to buy a timing tape which is the key element to get everything right..
HOWEVER there is nothing stopping you from making one. You need a suitable piece of tape, a cloth tape, permanent marker or scribe and a good ruler.
I have done that before and it is not difficult.
I used chrome tape because I had some.
I was going to explain, but this article does it better. There is even a picture to show you the correct orientation of the tape.
http://www.crankshaftcoalition.com/wiki/How_to_mak...



 
Thumpit 
Silver Supporting Member
Posts: 43
Thumpit
Reg: 11-26-16
01-11-17 08:58 AM - Post#2670566    
    In response to 0utlaw

Thanks for the great replies. This is my work truck that I use on jobs occasionally. 1975 C30 dually with 9 foot dump bed.
The motor is still fairly sound. But I discovered in pricing a vacuum advance for the hei distributor that I could get a new distributor from Advance Auto parts for about the cost of 2 vacuum modules with my discounts. .. Less than $100. It's basically the carquest standard unit but I believe it would be ok for this old motor. I think I am going to brush up on the removal and install of the entire distributor and then work on the timing. It's been a lot of years since I have done it but there should be some posts on here that detail it.
If anyone has had bad luck using a distributor from Advsnce Auto please let me know. I don't want to put a real expensive one in this old motor but I don't want junk either.




 
0utlaw 
Very Senior Member
Posts: 2711
0utlaw
Loc: US East Coast
Reg: 09-10-02
01-11-17 06:33 PM - Post#2670644    
    In response to Thumpit

  • Thumpit Said:
Thanks for the great replies. This is my work truck that I use on jobs occasionally. 1975 C30 dually with 9 foot dump bed.
The motor is still fairly sound. But I discovered in pricing a vacuum advance for the hei distributor that I could get a new distributor from Advance Auto parts for about the cost of 2 vacuum modules with my discounts. .. Less than $100. It's basically the carquest standard unit but I believe it would be ok for this old motor. I think I am going to brush up on the removal and install of the entire distributor and then work on the timing. It's been a lot of years since I have done it but there should be some posts on here that detail it.
If anyone has had bad luck using a distributor from Advsnce Auto please let me know. I don't want to put a real expensive one in this old motor but I don't want junk either.




I've seen adj. name brand cans for around $15. I'll point out that I'm using a 1974 distributor with a new module and a Hypertech coil, cap combo. With me...The old distributor must have a tight shaft, advance mechanism and drive gear in good shape. These babies run a long time..even through light racing. It is always good to have a spare module and tools to change in the glove box.
This is my 74 distributor in action.
http://s1226.photobucket.com/user/edopholeteus/med ...
One turn starts hot or cold.



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18514
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
01-11-17 09:39 PM - Post#2670673    
    In response to 0utlaw

If I were you, I would use a distributor from at 1974-78 Camaro, they have a better stock timing advance curve (different advance weights and springs). The dually distributor has a far less aggressive curve, mostly to keep the engine from over advancing in a high heat/heavy towing load situation. If you aren't going to be towing anything, there isn't much need for a restrictive timing advance curve. The motor will run a lot better with better fuel economy with the above distributor.

Before you pull the distributor out, make certain that the old distributor is at TDC on the #1 cylinder by pulling the distributor cap and making sure that the rotor is pointing at the #1 spark tower and that the balancer as at the TDC mark. Getting this right before pulling the distributor will make it a lot easier to put the new distributor in. If you don't check the rotor's position, you will have no idea where the motor is from top dead center on #1, and this will make it a lot harder for you to get the timing right.



 
0utlaw 
Very Senior Member
Posts: 2711
0utlaw
Loc: US East Coast
Reg: 09-10-02
01-12-17 04:06 AM - Post#2670690    
    In response to 0utlaw

What I usually do is do what Vaughn says, but I mark the distributor with a little white out on the BASE under the cap for number 1 and a little mark on the right valve cover where the advance is pointing. You can transfer the mark to the new dist. When this all lines up perfect and your dist. pops in and is FULLY SEATED , your timing should be as it was. Don't forget the distributor gasket. OUTLAW



 
Thumpit 
Silver Supporting Member
Posts: 43
Thumpit
Reg: 11-26-16
01-12-17 09:29 PM - Post#2670868    
    In response to 0utlaw

Thanks for the replies. A couple questions on this total timing to be sure I am on the same page.
All the timing is done with the advanced unhooked and Out of the equation. It's just hooked backed up when you are finished with all the timing adjustments and are ready to drive?
2. On the total I understand the engine is supposed to run pretty much wide open or until the advanced mark on the balancer stops moving up and then note what the number is. Or do I need to just idle it up to a certain rpm and hold it there and note the number. And hope its between 32 and 36.Note that I don't run this truck hard and very seldom would the rpms go over 3000-3500 and that's just shifting through the gears under a load.
And lets say that at 3500 rpms or full throttle that the number only comes up to 28 and with an 8* initial. Would that be in a good range to let it ride or should I change the springs?
This is the first I have heard about total timing. We always pulled the vac and set the mark to 6-10 and ran with it as long as it didn't ping or have a hard start. I guess I will have to stop shade treeing it from now on....lol



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18514
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
01-12-17 09:52 PM - Post#2670874    
    In response to Thumpit

Yes, you unhook the vacuum advance while setting base or initial timing, then reconnect it once you are done.

Mechanical advance should be all in by about 3000-3500, depending on what weights and springs you have and the limit on mechanical advance.

You do not have to have a total timing of 36 degrees (initial plus mechanical), it is perfectly acceptable to have an initial of 8 and a total of 28 if your truck runs best with this. IF you go too crazy with the initial and mechanical, you could go into pre-detonation if you are using regular unleaded gasoline and maximum total advance. More timing may require higher octane to be able to run that timing. So, just make sure your engine is running well with whatever gasoline you are likely to use, and base your timing advance on that.



 
75gmck25 
Contributor
Posts: 367

Loc: Alexandria, VA
Reg: 12-25-07
02-28-17 03:00 PM - Post#2679002    
    In response to Vaughn

I tried installing a timing tape on my balancer but it didn't stay on for long, so I started using a setback timing light instead. It was relatively cheap, and seems to work well.

Most stock truck HEIs I have looked at will have about 18-20 degrees mechanical advance. This means that if you set it for 8 initial you will only get about 28 (8+20) total. However, I also know of several guys that set their initial to 12-16 degrees (32-26 total) and did not have any issues. I am currently running 12 initial on my 350.

The vacuum advance is there only for fuel economy at cruise, so all your other advance measurements and settings are with it disconnected and the vacuum line plugged. Some aftermarket vacuum advances are adjustable, but most stock units are preset to 18-20 degrees.

Once you get initial and mechanical set, you check vacuum advance by coming up to highway cruising speed and then accelerating slowly. If you start to get anything more than very light pinging (since your advance is now 8+20+20 = 48) then you may have to dial back the vacuum advance. Some stock low compression SBCs will be able to handle up to 56 degrees total, but YMMV.

Bruce



 
LMC Truck
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