Technical SBC 350 Timing Mark Question

Hi All,

I have a 53 Chevy with a SBC 350 (1973 Chevelle motor) and I had taken the driver side head out and had a valve fixed earlier in the year. I don't think I have the timing set right and it runs ok but an exhaust shop had fixed an issue and pointed out I might be running a little rich. I have a couple questions:

I had taken pictures of the timing marks on the harmonic balancer and timing cover tab. I had measured 0.015 to 0.025 inches from the #1 piston top to the block height so I'm pretty sure the piston itself was at the top of its travel. Is it normal for the crankshaft to be 2 to 4 deg off while the piston is still at its top? If so, is it safe to assume zero is the TDC position in my photo and I just happened to be off 2 deg with the crankshaft at that exact moment?

Does anyone know if each mark is 1 deg, or 2deg?

I also find it weird that the timing tab has an R or B and the A in the reverse position. Is this likely some tab that belonged to a different motor? This motor was rebuilt by someone before I got the car.

Lastly, how would I time this with an analog timing light for 10 deg BTDC? It doesn't seem like the tab extends enough for 10 deg (unless each mark is 2deg).

Here are some photos from a while ago as I was taking the head out:

 

Marks are probably 2# increments. "R" is for retard "A" is for advance. There could be some slippage in the rubber isolater in the balancer but it is close enough. After you use the timing light hook up a vacuum gauge and adjust for maximum reading at idle.

 

I think the A (advance) and R (retard) are in the right place - since the engine rotates clockwise, the plug firing before the mark on the damper reaches the "0" on the tab would be an advance, while the line on the damper being past the "0" towards the R would indicate a late firing (retarded) condition.

 

Advance=A Retard =R From your pic of the tab,it is at 4* btdc each hash is 2* on what you have.
When you go to set it- Find TDC=compression stroke, line up the balancer roughly between the 3rd/4th notch up^ =a lil farther than the pic,and align your rotor pointing right at your # 1 terminal in the cap. You will be at 6-8* which will be VERY close to optimal. I use a timing light with a dial,so I just put my mark where I want it,and dial zero. Sometimes it's tough to see the marks,regardless of what light you use,but you'll get it. Just takes a lil bit.

 

Thanks everyone.
Is it ok to assume the zero is TDC? I imagine the engine was rebuilt so the TDC was at zero.

If so, should I remove the vacuum advance and plug the canister and the vacuum line and then set it at 10 deg BTDC? I had read that is a good setting for my setup. I have a mild cam (an RV cam supposedly). I don't have a vacuum gauge.

 

vacuum gauge is pretty cheap to buy and invaluable for setting up your carb and timing. Keep in mind that
the engine will tell you what is the best setting for a specific setup, what works for me may not be the best
for you even with identical specs. You can start with the settings you have listed, but try advancing or retarding a couple of degrees at a time till either you see a decrease in performance or get pinging. usually
the best (highest) vacumn reading will be good, but remember the carb effects it also.

 

Thanks so much! I'm going to set it near 10 deg BTDC and then drive it to a work related colleagues house to see if he can help me adjust everything including the carb. This person does throttle bodies for a living so I will hopefully get good guidance.

I will look for a vauum gauge so i have it in the future.

 

It's a nice thought (and would be helpful to rely) but I wouldn't trust that the marks on the tab are at any exact spot.

 

Yes^ I should've mentioned that. The vac gauge will help afterwards,but like mentioned,the engine will speak to you. Not always the highest vac tell's the tale. the carb want's to play too,and could be fussy for the happy spot...... Try no less than 6,but 10 might be a tad much..... Then there's total advance.......,but,we'll go slow and get it running without death ping first....

 

Thanks. I set it at 10 or just a tad bit past 10. It didn't ping when I let it idle and then rev'ed it up a bit. I'm not sure if I should back off to 8 deg BTDC? I was going to drive it at my current setting and see how it goes and then drive it some time this upcoming weekend to my colleague's place where I can get some help with setting the carb right etc.

 

It also doesn't seem like the vacuum advance is changing the timing at all when I plug the line back in. I set initial timing with the hose and canister plugged.

Should I get something like a new MSD distributor, cap and rotor? I'm not sure if vacuum advance is something easy to fix typically without replacing the entire distributor assembly?

 

Where is the OTHER end of the vacuum advance line attached?

 

it is not R it is a B for before TDC A is for after TDC B= advance A=retard

 

Your motor spins clockwise and the R stands for retard. The zero is top dead center and the A stands for advance. The hash marks are in 2 degree increments.

it is common for timing tabs to be off from the factory but that is because if you are a GM certified mechanic in a dealership close is cool. Some times the balancer slips as well. No hill for a stepper.

Time it, listen to it and if it doesn't sound right twist the distributer some more.

Sorry for the last statement, I seldom use a timing light.

 

Check the location you have the vaccuum line attached.

 

When ya set the timing ya pull the vacuum hose from the advance canister and plug it. When you pull it from the canister feel the end and see if it's sucking. (it should be, if the other end is plugged into the carb below the throttle plates. (manifold vacuum) For further info do a vacuum advance search and read all the good info.

 

You need to use the positive stop method. 2 degrees at TDC will give little if any piston movement.

 

once you determine true top dead center, beg borrow or steal a dial back timing light, Sears has craftsman ones pretty cheap and they are one of the better ones.

plug the vacuum and rev your engine to get the dizzy to advance, maybe 3-4000 RPM once fully advanced set your total timing to 34-36, then check your initial, the difference between the total and initial is the amount in the distributor. These can vary, what matters is the total timing 34-36, then you can add your vacuum on top of that.

Most SBC dizzys have 20-24 degrees in them so 10-12 initial is gonna get you close.

If you use a vacuum gauge for timing advance to highest reading and then back off 2 inches, advancing to the highest reading will result in too much advance.
Good Luck

 

Thanks. The other end from the canister is going directly into the edelbrock carb port. I will check to see if there is vacuum on that line. I plugged the canister and the hose when setting the timing to 10 deg btdc. The motor sounded good but when I connected the vacuum line back I didn't see any change in the timing. The canister looks old and rusty and ugly. This is where I wondered if I should get a new msd distributor assembly with everything.

I will definitely borrow or buy a timing light that I can dial so I can check total timing.

 

Here is a photo from earlier in the year showing the canister and line going to the carb.

 

That's connected to ported vacuum, I believe. You'll want to have it connected to manifold vacuum on the other side of the front. That port will be a bit lower on the plate. But changing it will require you to understand why.
Read this a couple times till ya understand it--->.vacuum tips (it took me a bit)
Good luck!

 

Thanks that is a good read! I will put it on manifold vac. I'm not sure why it was on ported (that's how I found it when I got the car).

 

some cars like manifold and some ported, neither have anything to do with setting your timing getting your initial and total timing down, they just add more advance on top of the 34-36 total timing

 

That makes sense. Also if I move to te manifold port, I don't know about the vacuum advance control unit needing to pull 2 in HG below the manifold vacuum at idle. I don't know if the vacuum advance control unit is the canister, etc that is in my setup and if it is, was it chosen approriately, hmmm? I saw mention of this in that article shared by pat59.

 

I always get a tickle when a young guy is worried about his timing marks . Hell it don't matter. What matters is what your ear and your butt tell you. You guys are being great today explaining. I think 53CH is learning a lot of valuable information and I think he will make a fair mechanic after all.

 

I am learning a lot that's for sure!

 

Should I buy this "Fuel Pump Vacuum Tester Kit" ?

http://www.ebay.com/itm/Fuel-Pump-V...hash=item2ee0fadb2e&item=201343032110&vxp=mtr

 

Yup. I use the timing mark for initial start up.

After that it's wrench, ass, and ears.

 

Nothing wrong with tuning by the "seat of your pants". But when you find a combination you like it's not a bad idea to know what you've got for initial timing, mechanical advance and vacuum advance. 'Cuz if you decide to change one of them later on you'll likely also want to change one or both of the other two.

Definitely get yourself a vacuum gauge and consider hooking it up inside the car, at least temporarily, to get a feel for all the things that can effect manifold vacuum. And how vacuum effects things like vacuum advance and when your power valves operate.

 

I understand the wrench and ears part... the ass.... is that for feeling the vibrations on the seat as you floor it on the street haha?