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sbc timing issue

3K views 47 replies 6 participants last post by  gonzo08383 
#1 ·
hey guys new here on the site hoping to find some answers to my problem I have a mild 350 with vortec heads and an xe268h comp cam and it seems like i,m not getting enough mechanical timing out of it. I'm only getting 12 degrees and its all in my around 1700 rpms. i'd like for it to be at 12 initial it seems to idle good there and have around 30-32 full mechanical timing. I also have an msd distributor and i even tried with the stock distributor with same results. any feed back would be appreciated.
 
#2 ·
typical sbc's like no more than 36 total. 12 mechanical seems low. The real question is does your motor like the time all in that early? I found with my sbc it likes 38 total, all in at 2200. Sounds a bit much, I know, but if the motor likes it then the motor likes it. No knocking or pinging on yours, right? What's the exhaust temp looking like at idle? Assuming your AFR is right a high exhaust temp at idle indicates too little timing. If that's the case you'll see the manifold tubes glowing a bit like I did. My cam requires 22 initial to make a happy motor. What springs are you running in the dist.? Vacuum advance hooked up or no? Throw more info out.
 
#4 ·
The problem with hooking it to manifold vacuum is that as soon as you hit the gas pedal manifold vacuum drops off thereby decreasing your timing which you don't want. The only way that works is if the mechanical advance comes in slightly faster than the vacuum advance drops out which won't happen with your cam. Manifold vacuum for the vacuum advance works with small cams I believe. Use ported vacuum source because as airflow increases through the carb venturis the amount of vacuum on the ported source increases which in turn pulls the advance diaphragm to increase your timing. The plus side of this is (in my case with my sbc) at cruise the unit pulls extra timing in to help burn the lean AFR experienced while cruising (assuming the carb is tuned right). Hooking to manifold vacuum will do the same thing at cruise since you have manifold vacuum available at part throttle and it helps while at idle because, again, you are leaner at idle. You want your timing curve, if graphed out, to look like a smooth curve upward that matches your power band. If the mechanical advance come in fast enough to counter act the vacuum advance pulling time out and the timing curve is nice and smooth then no problems. I suspect your engine stumbles a bit at first while putting the pedal down? Just out of curiosity, you did set the total time with the vacuum advance disconnected, right?
 
#5 ·
yes i set my total timing with it disconnected I've made that mistake once before haha. but the car doesn't seem to stumble at all. I just can't figure out why i'm only seeing 12 degrees of mechanical timing id like to see 20. to set me in the ball park in the low 30s where the vortecs seem to like it.
 
#6 ·
having the stock springs in the dist. surprises me. There's no way you should be all in that early with the heavy stock springs. I'd use the medium(silver) springs found in the common curve kits with that cam you have. If you lighten the springs you may be able to stay on the manifold vacuum source with no problems. It's been my experience that fine tuning the ignition curve is much easier using ported vacuum though. I hooked up a vacuum gauge to my ported source and went for a cruise to get a good idea of how much "suck" was present. Then I used my vacuum brake bleeder to adjust the adjustable vacuum canister to get the right amount of advance out of it without exceeding my engine's preferences. Basically I manually pulled the 5" of vacuum with the bleeder(it has a gauge on it) and measured how much time that equated to with the timing light and adjusted the canister accordingly. It's common for a mild sbc to see mid 40's timing with the vacuum advance at cruise. Remember, more timing is needed for the lean cruise mixtures assuming the carb is tuned properly. That's the difficult part of setting up the ignition curve, everything the engine is doing matters. Your exhaust temperatures will tell you a lot of where your timing needs to be up or down.
 
#8 ·
Well if it isn't stumbling I wouldn't worry too much about the manifold source. Can I assume you've already checked the weights to see if they move freely? Put in the next step lighter springs. The stock springs may be stopping the weights from going all in if they're too stiff. I'd bet a spring change will blow your socks off. When I did mine it was like finding an extra 40 horses.
 
#11 ·
I recommend the lightest springs your engine will handle. It's trial and error. Start with the two medium silver springs and go from there. If you get stuttering under acceleration go a step heavier. I used the Mr Gasket set that had gold silver and black in order of stiffest to weakest. the two blacks worked for me but the timing is slightly scattered. I think a higher quality kit would be best. It works for me and my Camaro tho.
 
#14 ·
I have a setup similar to yours but I run a voodoo 219/227/ @50 515/530 lift with dart shp 180cc aluminum heads with a 650 custom made vacuum secondary holley. If your stock springs are the stock really stiff ones they only offer about 10 degrees of timing or a little more and won't give much more until it would be revved past 6500 rpm plus and still not give total mechanical advance. I have had similar issues like you but not as bad.

I have my initial idle setting about 16 at the least or a little more but my balancer is really hard to read and I mark it with a slim paint line at the 18 degree mark to get very close within a few degrees. I have tried the lightest springs to make the timing come in as fast as possible by a little over 2000rpm and it did not like it as it was to fast.

I then went up to the two medium springs and it works like a charm and it starts to offer timing right after 1200 rpm and steadily goes up and is all in around 3000 to 3500rpm and it runs nice and strong and my carb smoothly climbs the rpm's nice and quick without any issues.

As far as the whole ported vs full manifold for vacuum advance that will be a battle till the end of time but each engine is different and mine will work either way so I use full manifold as it gives me better off idle response vs ported. Best thing to do is trial and error on that.
 
#19 · (Edited)
If you have a MSD distributor what advance stop bushing is installed. They are colored so you know the degree it is. This is the kit you should have used it has everything you need. http://www.summitracing.com/parts/msd-8464 .here is a set of instruction for my distrbutor but all are basically the same. http://static.summitracing.com/global/images/instructions/msd-85561_frm28755.pdf I had to use 1 heavy silver spring and 1 light silver spring. I tried two blue springs first but was too fast.
 
#24 ·
If all else fails throw a can of octane boost in it. Let's review, what springs are in it now? Who makes the kit? Stock advance weights? Is the vacuum advance hooked up? Is the pinging at WOT under load/on the street or just in neutral? What does the AFR look like at WOT? You may be leaning out horribly. Give me some info on the carb, ie jet size cfm, brand, etc. Vacuum or mechanical secondary? Or is it fuel injected? That pinging could be lean burning and it does make the most sense since it's only at WOT now. For the record, the octane boost will only mask the issue. It's a band aid fix. I figured that out when my motor wouldn't take any time and my dynamic CR is low enough to run 87 octane. Problem is my cylinder pressure builds in the 5500-6000 rpm range so at WOT I would detonate. 1 can of Octane boost later no more ping. 93 octane from now on and she purrs like a kitten. Just something to think about. Remember the more info you give us the better we can help without doing a lot of un necessary stuff.
 
#26 ·
Manifold vacuum source and "ported" vacuum source explained. Read this thoroughly and make sure you understand what he's saying and you'll never have another timing problem ever again. That guy is one of the last great mechanics. Now a days all we have are "certified technicians" that can't figure out a collapsing lifter. Let us know what you find
 
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