I just put my freshly rebuilt 350 back in my 69 and I'm really excited given that it has taken me over a year to get it back in the car. I've got the motor running pretty well, however, the engine wants A LOT of initial timing to start and idle well. Like, 27 degrees initial... (!!). Anything less and it doesn't idle well and/or start consistent. I measured this with the vac advance disconnected.

I do have a comp solid lifter cam in the car. I've posted a link to the cam card below. I installed it straight up, degreed it and I think it came up right at 1 degree retarded. I left it alone.

Any thoughts on why this thing wants so much advance? Could this be a carb tuning issue? I have a brand new Holley street HP 4150 750 cfm on the car and overall, it's running great. I drove it down the street and back and it ran really nice, especially from 1500 RPM onward. With the 27 degrees advance it even starts off well and has no bog down low. I'm just concerned with getting into detonation once the mechanical advance comes in.

Here are the specs.

Comp XS268S http://www.compcams.com/Company/CC/c...?csid=225&sb=2

268/274 duration
.488/.501 lift
230/236 @ .05
110 lobe sep
106 intake centerline
.016 lash (I lashed .012 cold per Brodix recommendations for my aluminum heads)

9.75:1 compression
Brodix IK 180 heads, 64 cc chambers
Holley Street HP 4150 750 cfm vac secondary
Petronix Ignitor III ignition
Stock distributor with vac advance

 

Heres a video of it running and idling. Hope you enjoy the open headers as much as I did.

http://youtu.be/soRsSynYvAE

 

Give it what it wants, NOT what you THINK it wants or what the internet tells you...

I would just limit the amount of total advance you have...You will have to curve the distributor to get the extra mechanical advance you need without exceeding the total amount needed..

Example:


Say your target is total timing at WOT is 36*

1) Set 27* initial
2) Play with your weights/springs/bushings to add another 9* mechanical advance.

Now your total is 36*.

As far as vacuum advance, you will have to play with it. Most vacuum advances will add 16-18* additional advance which will likely be WAY too much for your combo... I would make a limiter plate for the vacuum advance to yield around another 10* of advance.

So when cruising, you will have your 27* initial, your 9* of mechanical (now youre at 36*) and add the 10* of vacuum advance.... That gets you a total timing of 46*... It still may be too much and you will have to limit the vac advance some more..you just have to play with it and see what it wants and make sure it doesnt ping and low loads.

FWIW I run 31* LOCKED OUT...(all initial timing, meaning no mechanical advance) and I add 4.5* of vacuum advance... That gets me around 36* of total timing at cruise and 31* at WOT which is all my engine needs and will tolerate.

No problem starting it. I use a mini starter off of a 97' 350 p/u (vortec 350) and it starts it fine....This is a 10.0-1 compression 496.

Hot rod motors love a lot of initial timing... More power, lower coolant temps and better idling.. I would never run one with LESS then 20* initial...

Give it what it wants..

 

Thanks for the thoughts ajrothm. That eases my mind somewhat. I was concerned that I had done something wrong in the build process, i.e. setting up the cam timing... this is my first motor build myself.

I'll have to look into the curve kit.

So I guess my question now is WHY would it want that much timing??

 

wow, good post! thanks

 

Many factors can determine how much initial and total timing an engine wants..

Compression ratio, camshaft events, head/chamber design/efficiency etc etc...

In my opinion the main factor is the camshaft itself...its specs/characteristics and where its degreed in at.

A 230/236* cam is a pretty decent sized cam, ofcourse the valve events (opening/over lap etc etc) will dictate the behavior of the engine and affect the vacuum it produces(which in turn can mess with timing settings)...

Yes usually most engines with aftermarket cams will want more initial timing....

Your best bet may be to see if you can find a local shop that sets up distributor curves on a "Sun" Machine... Then they can dial in the ignition curve for you based on the initial timing you tell them you are running,... They can spin it to 7-8k rpms and make sure the whole assembly doesnt "over advance" at high rpms..

I recently had this done on a 66' TI distributor for my dads L79 Vette... I set his initial at 20*, I had about 16-18* put in the mechanical advance and that got use at 36* or so when its idling... Then I hooked the vac advance back up and that put us at about 52* total at 3000 rpms.... Thats about like a stock total advance but with more initial and less mechanical.... Its really a lot of timing for light cruise but this is only a 9.5-1 motor so there has been no issues with it.



With timing, every engine is different, you just have to play with it and see exactly what it wants for the best idle/cruise timing. WOT timing advance is a crap shoot...general consensus says around 36* total but....every engine is different. Unless you get it on a dyno or at the drag strip tuning it and seeing exactly what picks up power, its hard to know what it wants. Without dyno or track tuning, I normally never run them over 36* total for WOT... Just to be safe.

 

If you have not tried it yet, set your total advance to 34-36 degrees and use manifold vacuum rather than the carb port for the vacuum advance. The carb port provides no vacuum at idle.

 

"Give the engine what it wants", is spot on. I have a fresh 355 sbc with 60103 luanti cam, 180cc pro top line 64 chamber heads 9.48 to 1 comp. Initial timing is 24 deg. Mechanical advance set at 14 deg. Vac advance set at 12 deg on ported outlet. Vac at manifold is barely 14 inch at 1000rpm idle. The vac can drops out at aprox 12 inchs of vacuum so when auto trans is in gear idle is 750 rpm the vac advance starts hunting causing a very eratic idle. Went to ported vac advance set up and have a better idle in gear and the car runs good. Understatement

 

27* sure sounds like an awful lot of initial timing. If your distributor is providing 20* of mechanical advance, that puts your timing at 47*, which almost everybody would agree is too much. If your vacuum advance is providing another 20*, that puts you at 67*, also too much. What rpm does your mechanical advance come in at? How many degrees of advance is the vacuum advance providing?

I think that, in the end, you are going to find that your timing mark is not accurate, or some other mistake is being made concerning the measurement of timing. Your first step should be to verify that you are accurately measuring timing. Lars Grimsrud has a great paper on finding TDC on the #1 cylinder and verifying the timing mark on the dampener. Send him an e-mail and ask him for it. You might as well ask for his papers on setting timing and vacuum advance, too.

v8fastcars@msn.com

If you stood on a scale, and it said 500 lbs., would you go on a diet, or try another scale? I guess if you actually did weigh 500 lbs., that wouldn't be a very good example, but I think you can still see my point.


Keep the shiny side up!
Scott

 

You could use a B28 or AR31 vacuum advance can, they don't start to drop timing until around 7-8*, however, they will provide around 16 degrees of advance, which you would have to adjust your timing to compensate for, but they might allow you to run manifold vacuum to the vacuum advance. I don't have much experience with cars with really big cams, so take it for what it's worth, just throwing it out there. If you are bored one day, it might be something you could play around with.


Keep the shiny side up!
Scott

 

Way how much vac are you getting at a idle? your build is almost exactly like one I am putting together.

 

Run initial timing of about 18 with total limited to 36. Then, use a 12-degree vacuum advance control unit hooked up to manifold vacuum so you end up with about 30 degrees of timing at idle. The stock GM performance engines in the '60s ran idle timing in the 30 to 34 degree range with vacuum advance hooked up to straight manifold vacuum. This will produce good idle, low engine temps, and very nice throttle response.

On my solid roller 407 small block, I run 20 initial, 34 total, and 16 degrees of vacuum advance, which gives me 36 degrees of timing at idle. Runs perfectly.

Lars

 

Lars hooked me up with a 12 degree vacuum can for a rough idle (a bit too much timing with the old can at idle) and now it works great.

 

Your combination is like many others - nothing too radical, but the timing is a clue that something is amiss. I would consider it essential that you verify TDC.

If TDC is correct, verify the cam timing. If the cam had been installed one tooth off, you are looking at a 16 degree shift at the crank. Considering that the cam may be ground 4 degrees advanced with a stock timing set, you could be either 20 degrees advanced (not likely without detonation), or 12 degrees retarded which would require more spark advance due to the very late intake valve closing.

To check the cam timing without having to remove the water pump, you can do a quick and dirty check, or the more accurate cam degreeing process.

The quick & dirty process only requires removing one valve cover...
1. Set TDC-overlap; if the #3 intake valve is open (rocker pressing the #3 valve open) you're at TDC-firing, rotate crank another turn to get to TDC-overlap.
2. Look at the #1 cylinder intake and exhaust valves - the intake valve should be open slightly more than the exhaust valve (~.015"). If you find the exhaust valve is open about .050" more than the intake, you are off one tooth on the timing gear set.

Let us know what you find.

 

I recently read a similar comment by Lars and retimed mine with no regard for initial. After setting it up corrrectly for 36 all in, I'm ended up at about 30 initial and the old bastaard has never run better.

 

Thanks for the comments everyone. I degreed the cam when it went in so I am 99.9% sure that's not the problem. However, it was my first build so I could have messed something up. It just runs too well though especially cruising so I dont think that is the issue.

I'm going to follow Lars' advice here and try to tune with vacuum advance. Looking over his paper I think the VC1810 "B28" can is what I need. However, I see they are not available anymore from Napa. I found an auto zone part number DV1810 for a Duralast brand. Anyone tried that one? Anyone have thoughts on the adjustable vacuum cans from Crane or Accel?

I am out of town this week and won't get to tune on the car till this weekend. I'll be ordering parts tomorrow though so all feedback is appreciate. Thanks to all that have shared their input here. Very helpful.

 

nice cam!
using wrong carb base gasket! i made the same mistake.see article in sig
with my xe262 218/224, base timing is now 12, was 17 before.

 

nice cam! wrong carb base gasket will cause excessive timing needs, due to exhaust being sucked in during overlap.
When the engine injests exhaust, this pollution causes the mixture to burn slower. The more badly polluted, the more additional timing is needed. And bad air reduces power, and can cause more frequent downshifting in traffic, when vacuum is high, the problem is worst. This reduces MPG! I hope this helps.
sorry for dbl post, new browser works different

 

This is another reason I love my Holley HP EFI. While the car is idling I can change the timing with the laptop and enjoy the fruits of my labor immediately. It also allows you to adjust the A/F ration real time as well while it's idling. I started with 13.1 A/R and am at 15.1 A/R at idle and it's smooth and doesn't smell at all at idle.

 

if it requires more than 15 degrees base to run right, that is a clue. even EFI can't compensate for exhaust dilution. Look up Beejay, his c3 could not climb a hill without unlocking the converter, until he filled in the cut down divider that was allowing the contamination. How much is your base timing 68thumper?