Hey guys, I'm in the process of recurving my dizzy. I did this many years ago with a recurve kit from H-O Racing still, lol, but want to recheck everything. I found the info below with the kit I bought, but not an expert and don't know exactly what it means. Can use your help with this. There are 3 different kinds of springs with the kit: Gold, Black and Silver. What does the below mean??
That makes sense, the heavier the spring the more centrifugal force it's going to take to expand.
Well if you bought the kit from HO Racing it might be meant for a race engine with very high cylinder pressures. One may not want to get full mechanical until higher rpm unlike a street/highway car that would want it all in in the early 3000 range.
I have mine set at 28 total, I had to make a lot of adjustments to get 28 total at 3200 and still have the mechanical start to come in at higher than idle rpm and have a good initial for starting and keeping a smooth, cool idle. Lot of trial and error involved.
Most engines will run great with a close to stock timing and advance, but once it's modified it takes some screwing around to get optimum setting. Chassis dynos help for this but cost big $$$
You recurving due to problems with detonation or power?
Thanks Al, the car is kinda dogy from a dead stop, I'll recheck the timing and curve next WE and report back. HO did sell racing and street application aftermarket parts, I'm sure I got the street, need to verify that as well....
If your engine wasn't a dog on launch before and it is now it could be the advance, could be air fuel delivery as well. If it's not a case of slowly getting worse it may be the above or gearing. You'll want to check to see if the mechanical advance is even functioning, they can get gummed up and stop moving, springs can break, weights can jam. Same with the Vacuum advance, they can stick or the vacuum can can get a leak.
That curve kit you bought come with a selection of cams and weights as well as springs? Different shaped cams and different weight weights will give you a range of total mechanical advance as well as the speed it comes on. The springs will also vary when advance starts and stops in the rpm range. The idea is to find out what timing and advance curve is going to be best for your engine, An initial of 18 BTDC and a total of 30 at 2800 RPM may be great for one engine and crap for the next. Only way to tell is testing , by driving the car or on a chassis dyno. I set mine on an engine dyno, worked great with the engine on the dyno but I had to test and adjust the curve once the engine got in the car.
There are places one can send a distributor to along with info on the car, gearing, transmission, weight, engine specs, type of driving etc. and the company will set the advance curve using the supplied info. Even then I think further real life testing on the car on the road is the only way to get the timing and the advance correctly set.
Thanks for your input. Not slowly getting worse, its just not as responsive as it was. Its the original untouched engine so with that compression ratio I'm thinking 34-36 total advance should be good. The kit came with 3 sets of springs, one busing and 1 set of weights. I rebuilt the dizzy 3 years ago and know the vac advance works and springs are good and lubed the weights, so nothing stuck and I took out the excessive play. I'll start from baseline and check timing next week, record initial, mechanical advance and total and go from there. Rocky Rotella wrote an article on Pontiac Engine timing and suggested to leave the original weights in place. Only use bushings and springs to get the timing right. I know I have the initial set at 12 believe I was able to get to 36, but don't remember the RPM. I'll record everything and post current settings on here. I think you're right that my curve is just not there cause once the car moves it goes with no problem. I can also hear a slight miss fire at idle and know its ignition related. More to come....
12 initial and 36 total is a common number for our engines, when the vacuum is added we can get close to 50-52 which will be great for cruise and fuel economy. Maybe all you need is to do is use some lighter springs to get more advance sooner. That may give you some more acceleration while keeping the initial and total the same.
Took a break from this and am now back at it. Trying to find out when the distributor bushing comes in to change the mechanical advance. One tuner swears by setting the mechanical advance as low as possible like 14 Degrees, others say Pontiacs like 20 or 24 degrees at the distributor. Total 34-36 Degrees, but that would to make initial at the crank 22 Degrees to end up with 36 total? I know they say don't pay attention to the initial, but what do you think? Have a new set of springs and will go at again this WE. That just is new territory to me.
Thanks, that's kinda what this guy swears by...I'll try that, don't want to race just good overall street performance. Is a good way to check mechanical (distributor advance) to look at the difference between, vacuum hose plugged and unplugged?
Mechanical advance is totally separate from the vacuum advance. Setting the timing i described above has nothing to do with racing, every car i set this way and they run great. the more initial timing you have will help the engine idle and run much better and pull more vacuum. the factory setting of 10 degrees is not enough. All vacuum advance does is pull in more advance when the engine is at the least amount of load when the engine produces the most vacuum. It is primarily intended for gas mileage. I don’t even hook up the vacuum advance on these cars, the mechanical advance does everything you need providing the distributor is not worn out.
Yes, the vacuum advance is entirely separate from the vacuum advance and yes, to check your mechanical advance disconnect the vacuum advance. Pull the vacuum hose off the vacuum advance can and plug it. Then you will need a timing light with a dial back feature or have the harmonic balancer marked with degrees of rotation before top dead center. Easier to use a dial back light. Start and warm up engine. Set your idle rpm somewhere around 750-850 RPM where it runs smoothly. You want the idle low enough so the distributor's mechanical advance has not started to move. Point the light at the timing mark on the front cover and note what the initial timing is. Let's say it's 14 degrees. Rev the engine until the timing stops advancing, Turn the dial back feature of the light until the gun shows the timing mark at the same 14 degrees you had when at idle. Whatever the dial on the light is set to when the mark is at 14 is your amount of advance. If you get a harmonic balance tape to show the degrees on your balancer or mark it yourself. You start the same way as above, pointing the light at the timing mark and noticing the degrees of advance at idle. Let's say 14 degrees. Then you rev the engine until it stops advancing and note the amount of degrees the timing mark lines up on. Let's say it stopped advancing at 38 degrees, 38 degrees less the initial 14 idle degrees will be 24 degrees of mechanical advance.
Forgot, sorry. Your idea of plugging and unplugging the vacuum advance hose is going to tell you what the vacuum advance is not the mechanical advance. Say you have the car at idle, the vacuum advance hose connected to manifold vacuum and your timing light shows you have 32 degrees of advance. Then you disconnect the vacuum hose from the vacuum advance can and the timing light shows you are now down to 14 degrees of advance. 32 degrees with the vacuum connected minus the 14 degrees with vacuum not connected is 18 degrees of vacuum advance.
Now if you include the theoretical mechanical advance of my last post, we now have: 14 degrees initial; 24 degrees mechanical; 18 degrees vacuum. Now at cruise speed, if you are cruising at an RPM above the point where the mechanical advance is all in you will have an advance of 56 degrees. Good for mileage and a cool running engine. If you decide you want to 'floor it' to pass someone the carburetor's throttle plate opens all the way, causing vacuum to drop, and you are left with a total advance of 38 degrees. Total is initial plus mechanical.
I'm not saying these are ideal settings, just theoretical numbers to try to explain a little about mechanical and vacuum advance. Some cars run well with 12 degrees of mechanical some run well with 20. Depends on the set up of each engine.
Well explained,- thanks. I go it at 35 total and am trying the different springs now. Once I get it "all in" by, say 2800rpm, I'll test drive it and listen for knock or ping. I guess I got confused reading too many articles and was unsure what the best way is and what our Pontiacs like. Right now with the 35 total, I have 2BTDC at the crank (initial) and I guess 33 mech advance (in dizzy). That is an area I still like to confirm as I read Pontiacs like 24 mech advance, but also read to limit the mech advance to 14-15 degrees (as Chief has and one other article indicated) to give you ideal overall performace, cooling and gas mileage. So for my application that would mean if I add a larger bushing in the dizzy to bring down the mech advance by 18 degrees to 15, that would give me 20 initial/crank timing (roughly). Seems my dizzy is set up wrong?
No, that's a bit off. Maybe it's just the definition of the terms we're using, but if you have 28 initial and 35 total there can only be 7 degrees of advance not 33. If you have 28 degrees initial and add 33 degrees of mechanical you'd end up with 61 degrees total. Putting in a bushing to limit the mechanical advance will not change the initial timing. the only way you are going to change the initial is by turning the distributor.
Probably just confused with definitions of terms. Initial is the amount of timing advance (or retard) gained by the position of the distributor. When the initial is set the engine is at idle speed, vacuum advance is disconnected with vacuum source plugged and the distributor is turned to get the desired amount of advance. I like to start with 16-18 degrees. Some like more some like less.
Total is the amount of initial plus all the mechanical the distributor will gain. Again no vacuum advance is included in total. I know it doesn't make sense but that's how it's defined
Vacuum advance is the amount of advance the ignition will gain when vacuum is applied to the distributor's vacuum advance can. Vacuum will change with the position of the throttle plates in the carburetor. High vacuum with the throttles closed, low vacuum when the throttle is open.
The timing is changed as we drive in order to get the spark plugs to fire when the pistons are in the optimum position in the cylinder in order to make best use out of the cylinder pressure as the fuel air charge expands. As the engine rotates faster the ignition has to fire sooner to keep the maximum pressure and piston position constant. Pontiac did a pretty good job of determining the best firing points. Timing events only started to be set less than optimum when they started to change timing in an attempt to satisfy the smog rules.
I'd say a way to get your distributor to operate well is to put it back to the original configuration, if you still have all the original springs, weights etc. then adjusting basic timing. Once you get the basic timing done the mechanical advance can be messed with to change to rpm at which the advance peaks as well as the rate.
Try to start with a stock distributor. Disconnect the vacuum advance and plug the source. Set your initial (no mechanical advance, no vacuum advance) so the engine starts easily and idles smoothly. Try for a number between 10 and 18. Change it up and down by rotating the distributor until you get the idle and ease of starting you want. Then determine what your total advance is and at what rpm it is all in at. Rev it up until the distributor quits advancing and note the timing and rpm. it should be all in by 3000-3200 rpm. Don't worry if it's a bit up or down or what the actual amount of total is yet. Drive your car out to a remote quiet road and accelerate in second gear, if there is no sound of pinging adjust the initial up two degrees and try again. Keep going until you hear a ping then back the initial down two degrees. Check what the total is with this setting. If this total gives you too much or not enough initial, then you can play with the distributors mechanical advance springs weights etc to get you the total advance you found plus the initial you need.
Say you found the most total you want is 35 degrees and the best initial you found when you first started was 16 degrees, that's 19 degrees mechanical, but when you finished road testing the initial ended up at 18 degrees, that's only 17 degrees of advance. To keep the initial at 16 you would have to adjust the mechanical to get an extra 2 degrees of advance. Yours will most likely not be anywhere near these numbers I'm just picking them out of air for examples.
After all that is done you hook up the vacuum advance, I use manifold source, and re adjust the idle. that way you have the best initial for idle, the best total for power, and the vacuum advance for cruise economy and cooling.
Of course I'm not an expert, this is just my opinion.
Thanks, I'll do just that. The car was running great the way I had it at 12 initial, but it was sluggish from a dead stop. I am leaving the Orignial "37" stamped weights in place. The springs,although they were from HO Racing are 30 years old and are stiff and a little bend. I will replace with slightly lighter and check the all in. The bushing I have installed in the dizzy is .265 in dia. and "should" give me around 21 of mech advance. I did have the original 2 port vacuum advance can installed, it still hold vacuum, but replaced with newer 1 port can. More testing and trial and error this week. Thanks again for your input!
Well, after re-evaluating my set-up and pulling my hair out over things that didn't make sense I forgot to look at the obvious. My timing light. That piece of $100 crap I just bought turns out to be off by 10 degrees! So using my older timing light, which I know works and is accurate I now have the timing tweaked at 35 total and 18 initial which would leave 17 degrees of mechanical. Still need to check vacuum advance and total everything up. She drives good, still need more testing before locking everything down.