350 timing how much

[Billla]

The engine doesn't require heavy load for testing either mechanical or vacuum advance. Mechanical advance is strictly RPM, and the load for vacuum advance can be just cruise If you're not sure about what detonation sounds like, get someone to ride along...detonation will *destroy* an engine in short order.

I'm not sure the timing is coming in too early - it just seems a bit aggressive. I'll see if I can get to a mock-up today on the vacuum advance; I'll be out working with pmski_1 on his overhaul and my Impala SS suspension rebuild today.

As noted previously, the Edelbrock typically (not always) requires 1 spring lighter than stock calibration - refer back in the post for links, etc.

[dznucks]

I know what detonation sounds like and what causes it. but i am not hearing it with any spring combonation. it could be that i cant hear it over the exhaust.
i will keep listening for it.

I am guessin when you say too agressive you are talking about how it comes in too fast. It does seem weird that at idle i am at 16 deg and then at 1000rpm i am at 30deg. is this an issue that i need to address?

[Billla]

All good - sorry; just wanted to be sure It may be that with a low-compression engine you're just not hitting the limit - I wouldn't worry about it too much.

Yes, it seems like it's coming in fast, but if there's no detonation then....I guess I'd leave it. I am a bit surprised that it takes 16 initial...did you install the limit bushing that came with your timing kit?

[dznucks]

I didnt get any limit bushings in my kit. I bought the Crane kit and it just had the adjustable can, springs, allen wrench, and that limiting plate.
do i need some of the limit bushings?

i thought the 16 deg was too much as well. i set the max timing at 36 deg at 3000rpm and 16 was where it sits at idle speed.
Is this correct....to set max timing you adjust the dial on the back of the light to 36, raise the engine rpm until timing is all in, then adjust the dizzy until the timing mark is on zero. ??

i have never run the timing that advanced but across the driving range it runs better there. there is a mild cam in the engine but i didnt think that would matter that much on timing.

[dznucks]

i found the instructions for installing the limit plate on the Crane website.
I have it in but i havent limited the max advance yet.

it looks like the limit plate keeps the vacuum advance from resting to far advanced when no vacuum is applied to the can. i thought the vacuum advance increased the advance with an increase in pressure. it appears to work backwards. I have to be missing something here. ???

[matty&d67slb]

boog, would you send that to me as an email please I had a buddy that understands all that and try to explain to me,but after a few beers and with hot rods sitting there in front of us well,,, please send to dndrm4@aol.com thanx

[Billla]

Originally Posted by dznucks

I didnt get any limit bushings in my kit. I bought the Crane kit and it just had the adjustable can, springs, allen wrench, and that limiting plate.
do i need some of the limit bushings?

i thought the 16 deg was too much as well. i set the max timing at 36 deg at 3000rpm and 16 was where it sits at idle speed.
Is this correct....to set max timing you adjust the dial on the back of the light to 36, raise the engine rpm until timing is all in, then adjust the dizzy until the timing mark is on zero. ??

i have never run the timing that advanced but across the driving range it runs better there. there is a mild cam in the engine but i didnt think that would matter that much on timing.

Hmmm....yes, pretty sure there's a brass limit bushing in the kit - it's just a little collar that fits over the limit pin for the mechanical advance. It allows a limit to total timing; if it's not there then we end up with too much mechanical advance...which is what I'm thinking is happening here.

Yep, you're doing the advance check exactly right!

Virtually every SBC will run better with better fuel economy with this advance curve. An even more custom curve can make more power...but this will get you 90% of the way there.

[Billla]

Originally Posted by dznucks

i found the instructions for installing the limit plate on the Crane website.
I have it in but i havent limited the max advance yet.

it looks like the limit plate keeps the vacuum advance from resting to far advanced when no vacuum is applied to the can. i thought the vacuum advance increased the advance with an increase in pressure. it appears to work backwards. I have to be missing something here. ???

The adjustable vacuum cans have a total advance of around 20 degrees - more than the 15 we expect. Sometimes if the engine runs better with more vacuum advance early, it's possible you end up with a vacuum advance adjustment that provides too much vacuum under heavy load. The limit plate allows you to adjust the vacuum so it works well for daily driving, but has a "stop" to prevent too much vacuum under heavy load.

The vacuum advance works off vacuum - not pressure. There is virtually always vacuum in the manifold, technically unless you're at WOT.

[dznucks]

i meant vacuum.

so when your under heavy accel is the only time the vacuum advance is sitting against the limit plate, ie. when there is no vacuum. That and at idle. right?

no, there was no bushing in the kit. i may go buy another advance curve kit to get the bushings, i saw them for less than $10.

let me make sure i am understanding this...or not. My mechanical advance is at a max of 36. your saying that i need the bushings to restrict the amount of mechanical advance???? Why is this? i am missing something.

i thought we needed to limit the total timing, which you stated the bushings will do. The amount of total timing, beyond the max mech. of 36 is controlled by the vacuum advance. I am not understanding why the bushing is going to help total timing.

sorry for all the ??'s but i am just trying to fully understand whats actually going on. regardless i appreciate your help billa.

[Billla]

No worries on the questions at all - it is not straight-forward how this stuff works and I've spent a lot of time scratching my head on this too. I think I noted in a previous post that I spend like a week just mucking about with timing curves and vacuum cans just to get my head wrapped around it.

Let me see if I can net out some of your questions; let me know where we need to dive further. And if I missed something in your threads, let me know - I've been running the last few days and probably haven't paid as close attention as I should.

There are two types of limit "devices"; one is the limit bushing, which is only used for mechanical advance. Typically the mechanical advance kits, since they get full timing in early, over-advance at higher RPM - the limit bushing prevents that. If you're at 36 mechanical, then you're PERFECT and you don't need to muck with it.

The other is the limit set for the vacuum advance that only comes with an adjustable can. That is intended to limit the total vacuum advance to 15 degrees or less...as with the mechanical, sometimes by having more vacuum advance at lower vacuum, we end up with too much a cruise and high load.

Does this make sense? Again, sorry for not spending more time to make sure I understood your comments/questions before answering - I'll do better moving forward

[LONGHAIR]

dznucks,
You are confusing a few things...and I hope I don't make it worse.

First, "heavy acceleration" and "heavy load" are not necessarily the same thing. Your "load" changes as you go up/down hills, because it takes more power to push up the hill...and less to coast down it. This is just the truck itself, it is more pronounced when towing/hauling.

The vacuum advance and the mechanical advance are totally independant of each other. The vac adv can add too or subtract from the mechanical (centrifugal) advance. Mechanical advance is "Locked" into RPM of the engine. Advance goes up and down at a specific rate in relation it.
Vacuum advance is more "dynamic", it moves with changes in power requirements to keep the truck moving, based somewhat on your throttle input.
Say you are rolling along on flat ground at an even speed. The mech adv is steady and in relation to RPM. The vac adv will probably be giving you something too. It depends on many factors though. The actual number that you would see, if you had a guage on it, will be different based on things like the engine size, compression ratio, cam, gear ratio, rolling resistance of the truck, etc. (The mech adv is not effected by any of these factors)
Ok, you are rolling along....at whatever actual vac reading, you stab the throttle to "wide open". The vacuum in the engine drops to near zero and any "vacuum advance" you had is gone. The vacuum "pulls" the advance in...no vac, no advance.
When you are idling you have the highest vac reading and the vac adv contributes to this idle. That is why you have to remove and plug the vac line when adjusting the "initial" timing with a light.
So, the vacuum advance is "all in" at idle and in reality, it is "releasing" this advance as the vacuum drops (accelerating). When the engine needs it because of a load increase (hill) the vac increases and the advance unit gives some back to compensate. You are in effect starting with all of the vacuum advance that you are ever going to get, and all of the above factors require the cannister to adjust. I hesitate to use the word "retard" because it is not going backward beyond "zero" (zero meaning as if it weren't there at all, not zero degrees). But all the cannister really can do is allow the timing to "retard" from the maximum that it started with at idle.
You rarely get "all" of both at the same time. Very low throttle opening at above 3000RPM would really be the only case. High RPM and "High enough" vacuum don't really go together.
All of this is why they make adjustable vac cansiters. Not only can you adjust (limit) how much advance you get, but you can adjust how much vacuum it takes to make this happen. "Wilder" engines generally produce less vacuum at ilde and low RPM cruise. So you may need to have a cannister that needs less vacuum to do it's job.

I sure hope I didn't make this worse

[dznucks]

we're trying to talk about something very technical over a forum. its not the best of educational settings.

Longhair... all that makes sense. but in order for all that to be reality, that would mean that the ported vacuum is not where i should have the vacuum hooked too. i would need full manifold vacuum to get what you are talking about.
after reading that GM eng. article i have questioned what port to hook into. The article said full manifold but the article was also talking about more aggresive engines than i have.
but everything would make sense if i switch to the full manifold vacuum.

can anyone confirm that i am supposed to be on full manifold vacuum, this may be what i have been missing this whole time.

[70rs/ss]

Yeah, full manifold vacuum. I just got that part too. All along I have used ported and never had an issue, but seem to wonder how much I may have been leaving on the table? Good luck, I have followed this thread as I recently (got it" but wanted to let those that knew speak as I am not 100%, so I learned along with you and the OP.

[dznucks]

ok. so i am not the only one.

I have searched the forum for pics of edelbrock carbs to see which port everyone else is using. all the pics i saw had the vacuum at ported. thats why i thought i was correct. then i read that article and questioned it.
everything make sense now and i need to start at square one with the tuning billa explained to me.

Thanks for everyones patience in explaining this to me.
i'll try to get it right this weekend.

Longhair if you hadn't submitted that post i would still be

[Billla]

Originally Posted by LONGHAIR

dznucks,
You are confusing a few things...and I hope I don't make it worse.

This was an awesome post - I was answering the mechanism, but you addressed the how and why. Major props, dude Time for a FAQ

[LONGHAIR]

Sorry man, that's my fault. I should have stated that from the beginning.
If you take an engine that is timed and the distributor calibrated to run with full manifold vacuum and simply switch the vacuum line to the "ported" fitting, you will see the difference. Most aftermarket distributors have a fairly "average" curve that is designed to run with full vacuum. With a used stocker of unknown origin....you just never know. If it comes from the mid 70s it probably has the curve set-up for ported. Those will probably act "funny" in comparison. To make one set-up like that idle better, many guys put more initial timing to it...but then when the vacuum advance comes in, you have too much and get "pinging".

The bottom line (IMO) is: To get the most out of your engine, you need to know where you are and adjust accordingly. A timing light is really a must, along with a full timing tape. A vacuum guage will be a huge help too. Together with some willingness to fiddle with this, you can gain a lot. Fuel mileage, performance and over-all driveablilty can all be improved.

[LONGHAIR]

Thanks bill, that means a lot.

I find that everyone learns better when they "get it", which I feel is more related to why and how. Simple facts, however accurate, are to easy to forget. An over-all concept seems to work better for me.

[Mertz]

I guess I really started something but I think that it is all clear to me now. Just to recap then if I got it. There are two different curves dependant on if you are using ported vacuum or manifold vacuum and that it does not give the optimum performance by just switching from ported to manifold without making a change in the curve.

[LONGHAIR]

Just to recap then if I got it. There are two different curves dependant on if you are using ported vacuum or manifold vacuum and that it does not give the optimum performance by just switching from ported to manifold without making a change in the curve.

Mertz, that is a bit of an "over-simplification".
There are definately different advance curves and the point is that you need to know where you are starting from.
You are not going to get "optimum" by such a simple change anyway.
IMO, you will get the best over-all performance by using full vacuum and adjusting to that. This is something that takes some tweeking/tinkering to get optimum results.
There are just so many variables......

[dznucks]

Ok so i swaped over to the full manifold vacuum and I started to retune the timing and carb. I noticed a few things that i have questions about.

when i set the max mechanical advance i have it all in around 2600 rpm and set to 36 deg. the initial timing of the motor at idle is beyond the timing plate (which only reads to 16 deg) and i would guess its somewhere about 18-20 deg. if you dial back the dial on the timing light to roughly 22 the mark will line up with the 0 deg mark on the timing plate. this is way to advanced but it gives me my max of 36. why does my timing want to be so advanced.

Also when i hook up the vacuum advance, how much advance should that give me at idle. the idle timing jumps considerably when the advance is plugged in. i though that with a high vacuum at idle that should pull all that out.

Is there any possibility that there is something else wrong with the dizzy. its a typical store bought reman'd thats about 6 years old now. should i upgrade?

[Mertz]

Here is an update on my tuning efforts on my 350 power 72 Chevy pickup. I got the timing set and changed the rods and springs in the carb. It runs great but it used 1/2 tank of gas on a 80 mile trip. Most of it highway. Although I was looking for performance it was supposed to be based on economy as well. I haven't pulled a trailer with it yet but I'm sure it won't ping. Everything sounded and ran really good. Spark plugs were tan.

I will be changing back to the stock rods and back to the silver springs and see if the timing changes helped the pinging and the gas mileage.

I found a 17057204 quadrajet and factory aluminum intake manifold here in town for $25. I put the qjet on my 66 Catalina with a 428 and it ran great. The carb was really clean so I didn't do anything to it but put on the adaptor and some sealant on the old gaskets. The idle screws were opened about 6 turns so the idle was at 1400 and off the idle speed screw. I set the idle mixture screws to 1 1/2 turns and it ran fine. I set the idle speed to 800 rpm. Only 15 hg of vacuum but that is what I had with the AFB. The choke opened on its own but I didn't have it connected to anything. I plugged the connection with a hose. How do these chokes work?

I am going to install the qjet on the 350 and see if I can get better gas mileage with the same performance.

[Mertz]

I changed the rods back to stock and it appears to have increased my mpg but does not sound as good nor does it have as much power. I stayed with the stock springs. I tried to get some leaner rods. The chart shows the only way to go leaner is change the jets to .095. My test drive was only 15 miles so I will give it another long trip test.

BTW I will be leaving the qjet on the Catalina since it runs so well and will be running the Edelbrock until I find another good 72 350 qjet. So the tuning goes on.

Thanks everyone for the help so far. I think I am really close. If it wasn't for the price of gas I would stick with the richer rods and springs. It really did run good.

[70rs/ss]

There always seems to be give and take, perf and economy aren't close bedmates. A well tuned engine will get you more econ and perf than a poorly tuned one, but once you get into perf tuning you'll lose a little on the econ side, smaller jets and leaner rods will help mpg and may help the perf only if it was too fat, otherwise the perf will go down a little (less fuel) and the mpg may come up a tick. Either way sounds as though quite a few of us "got it" from this thread, so thanks to the OP and those that helped he and I out.

[dznucks]

yes thanks to all those that provided knowledge and insight to those of us who were confused.

My issues are not completely fixed, but i did determine that my dizzy needs to be replaced. there are so many worn parts in it that the timing fluctuates at idle. it move +/- 3 deg from where you have it set too.
However i do feel that i have it timed better and i have the appropriate (closer too at least) advance curve. My power has improved, my milage has improved and my idle has smoothed out.

I am going to wait until i get a new dizzy before i try and get it perfect. i figure its wasted effort to try and get the worn distributor timed perfect.