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Old 08-02-2019, 07:15 PM
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Timing, timing, timing

Okay, this engine has been talked a lot in my other posts: 408 BBC 9.7 to 1 compression. Stock closed chamber 101cc large oval port heads (3964290). Weiand dual plane intake and 770 street avenger. Hooker super comp headers and 2.5 dual exhaust. The cam is a comp 270 magnum hydraulic flat tappet timed 6* advanced. Cranking compression is 185 +/-. I degreed the cam to where it is now and put on a new neutral balance balancer. The timing tab I'm almost positive lined up with the balancer 0* mark on the 0 on the tab. The distributor is a Pertronix HEI street and strip flame thrower w/ adjustable vacuum advance that I bought 15 years ago. It has less than a thousand miles on it. I timed this engine by ear last year due to not having a timing light. I know, I can hear your groans. Anyway, after growing pains with the 770 SA I was quite surprised at how much fun the engine was in my 1984 Olds Cutlass with 3.42 gears and a turbo 400 with a 2400 stall. So just today I decided to throw a timing light on it and see what's what. (finally bought one). The numbers floored me! At a 750 RPM idle with the vacuum advance unhooked and the hose plugged I had 40* of timing. I verified this number by using the dial back on my light. So I checked the mechanical advance and it only advanced 10* and moved no more all the way past 4000 RPMs; and it was all in by 1800 RPMs. Checked with vacuum can hooked up and it throws another 12* on top of all that. This is suspect of the balacer and timing tab not being in sync. I will re-verify TDC this weekend. Even more weird I reset the timing to 18* BTDC and adjusted the idle and it ran. (Couldn't drive it due to tranni issue). Could shut it off and hit the key and it would fire and idle- tho you could tell it wasn't optimal. Because there was only 10* mecanical I pulled the cap. Several things were discovered: When I moved the rotor back and forth without actuating the mecanical there was a fair amount of slop. I figured this explained my somewhat erradic timing at idle. But you could also pull the shaft assembly up and down quite a bit. The mechanical advance seemed like it took alot of effort to move so I looked at it and saw what looked like light weights and heavy springs. I can recurve it once I figure out TDC but is there a shim kit to take the up and down slop out of the shaft? The back and forth slop I haven't nailed down yet. I believe it's a hardened gear on those. Is this compatable with a flat tappet cam? Should I just buy a new distributor and go from there? If so can you recomend a distributor? BTW the timing was steady above idle. Just fluctuated at idle. Does anyone know where I should time this combo to start with? To me it's obvious why this engine never detonated, not enough mechanical advance. With that said I wonder how much power I left on the table? So I'm new to proper timing tecniques to get the best power. Please guys I need all the help I can get to do this right.

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Old 08-02-2019, 08:59 PM
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Mechanical advance is floppy, needs to be free. If the hole where the pin comes up, is egg shape replacement required, weights can or will get hung thru the RPM range.

Springs can be mixed or matched to obtain an optimum advance curve. Bring the timing in then mess with the curve (springs)

VAC & mech advance, for me it has always been one or the other, I fall on the Mechanical side of that line.

Pep
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Old 08-02-2019, 10:37 PM
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The factory Melonized gear will run on a cast flat tappet or factory roller cam which is also cast just not the same stuff as a flat tappet is cast from. It cannot be run against all aftermarket cams here there is a wide array of materials used so you need to contact the maker if you don't have the instructions for your specific cam.


At the dead centers the crankshaft moves through a lot of degrees with very little piston movement. The only way to nail this down if you don't trust the damper and pointer alignment is to do the TDC drill with a positive piston stop and degree wheel. This is fairly complex you'll find instructions on the web and in this site's tech help section "The Hotrodders Knowledge Base".


Bogie
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Old 08-03-2019, 06:18 AM
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Wide open quesion. If you want a max of say 36 degrees timing, then set the initial to 15-18 and bring the balance of it in before 2500. Unless you have access to a distributor machine, this is just a trial and error deal with the weights, springs and advance limiter. It's also helpful to drive the vehicle so sounds like you need to correct your trans issue first. AS to the factory pointer, find TDC yourself and either make a new one, buy one or modify the old one. Vacuum advance is mostly for part throttle driving and should be worked on after everything else is settled.
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Old 08-03-2019, 08:48 AM
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I know how to find TDC and recurve the dizzy. Worried about the back and forth and up and down slop in the dizzy. The weights don't appear to be sticking, just stiff springs. Once I get TDC sorted out, should I just buy a new dizzy and recurve from there? Going to the shop today to try and get more info on what's wrong with the distributor. Can you recommend a good distributor if I decide to go that route?
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Old 08-03-2019, 09:43 AM
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Quote:
Originally Posted by jjjaffo View Post
I know how to find TDC and recurve the dizzy. Worried about the back and forth and up and down slop in the dizzy. The weights don't appear to be sticking, just stiff springs. Once I get TDC sorted out, should I just buy a new dizzy and recurve from there? Going to the shop today to try and get more info on what's wrong with the distributor. Can you recommend a good distributor if I decide to go that route?

Your first thread leads one to think you didn't do an effort to find true TDC so the question left in my mind is how do you know where it is if you suspect it's off. This would be an essential element to get nailed down. A constant problem as more aftermarket parts are used.

The distributor shaft should have and does use shims that control the shaft thrust clearance to the distributor base, this actually is fairly loose from about .050 to .100 or more. Some guys like to tighten this to .005 or so. This does carry a risk of "stacking" the pump drive elements.

The mechanical advance system is loose till moving, the rotational forces tighten it up pretty quickly.

Wear points with a distributor are certainly gear and the includes the cam's gear as well as that of the distributor. Bearings and bushings of the distributor shaft, lubrication being more hopeful than not for these elements is often a problem especially where attempts have been made to dry out rocker oiling as drain back through the valley is where the distributor gets what little oil it does.

Loosness of the cam drive is seen on the runnng engine. Some can't be avoided but this can also indicate poor manufacture of the drive, use of a standard drive on a block that's been align bored or honed, wear of the drive elements, cam walk along the logitudinal axis the Chevy using lifter to lobe interferance to control this instead of a physical barrier is somewhat fluid this forces the gear mesh position to move which changes the timng.

I've run distributors from Pro Comp to MSD with satisfactory results. There are some things to look out for with the ProComp and other import distributors using a Chevy .491 inch shaft so Chevy gears fit, but these use a larger diameter retention pin which doesn't fit the gear, if you use the Chevy pin that fits the gear it is now loose within the shaft and will introduce movement in the timing till the shaft breaks and things go quiet. When you read about broken shafts in web reviews you can believe the author didn't check this out.

Bogie
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Old 08-03-2019, 09:50 AM
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I installed a new PerTronix Stock Look distributor in my 350:CI Chevrolet engine but before I did that, I checked the end play clearance of the driven gear and the housing. It was within the recommended thrust clearance of .015” - .020”.

If the distributor shaft thrust clearance is more than .020”, the ignition will have spark scatter. I have seen a new stock GM point type distributor thrust clearances of .025”-.035”.

There are GM and aftermarket shim kits that consists of hardened washers you install between the gear and housing that will reduce the distributor end play clearance to an acceptable limits.
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Old 08-03-2019, 10:10 AM
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That's about how much up & down play is in my dizzy. You say this is normal? I was worried it was too much. I will be locating true TDC today. I just assumed it was off due to there being 40* of initial on the balancer. I have a complete re-curve kit including weights and springs.
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Old 08-03-2019, 10:17 AM
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Dizzy up/down is fixable with AN shims from your local hardware store.
Side to side requires bearing/bushing and hand honing. It’s likely less expensive and faster to buy new at that
point.
Verify the gear mesh too. Intake heights and basic machining move things around so a gear mesh and pattern too high/low causes problems you don’t want.
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Old 08-03-2019, 10:18 AM
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Quote:
Originally Posted by MouseFink View Post
I installed a new PerTronix Stock Look distributor in my 350:CI Chevrolet engine but before I did that, I checked the end play clearance of the driven gear and the housing. It was within the recommended thrust clearance of .015” - .020”.

If the distributor shaft thrust clearance is more than .020”, the ignition will have spark scatter. I have seen a new stock GM point type distributor thrust clearances of .025”-.035”.

There are GM and aftermarket shim kits that consists of hardened washers you install between the gear and housing that will reduce the distributor end play clearance to an acceptable limits.
That sounds more realistic to me. Think maybe it's time for a new dizzy.
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Old 08-03-2019, 10:21 AM
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Quote:
Originally Posted by johnsongrass1 View Post
Dizzy up/down is fixable with AN shims from your local hardware store.
Side to side requires bearing/bushing and hand honing. It’s likely less expensive and faster to buy new at that
point.
Verify the gear mesh too. Intake heights and basic machining move things around so a gear mesh and pattern too high/low causes problems you don’t want.
I close mine up to .005-.010
.020 is safer
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Old 08-03-2019, 10:23 AM
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Quote:
Originally Posted by johnsongrass1 View Post
Dizzy up/down is fixable with AN shims from your local hardware store.
Side to side requires bearing/bushing and hand honing. It’s likely less expensive and faster to buy new at that
point.
Verify the gear mesh too. Intake heights and basic machining move things around so a gear mesh and pattern too high/low causes problems you don’t want.
The side to side is good, it's the back and forth that bother's me. I actually am worried about gear mesh as it seems like the most likely culprit. That's why I asked about the distributor gear being compatible with the FT cam. No real abnormal wear on dizzy gear but cam gear I don't know yet.
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Old 08-03-2019, 10:32 AM
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Gear marking compound works super well here.
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Old 08-03-2019, 04:21 PM
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Do not reduce the thrust clearance between the gear and the distributor housing to less than .010”. If less than that you may seize the gear due to thermal expansion. You need no less than .010” thrust clearance in order to oil the gear and thrust washers.

You may not get the exact shaft end play you want because the hardened thrust washers only come in .005”, .010”, and .020” thickness. The factory put a .030” thrust washer on the distributor shaft to achieve .025” to .030” clearance. You need to add a combination of thrust washers to the factory washer in order to reduce the end play to a more acceptable clearance.

When I checked the thrust clearance of my PerTronix Stock Look distributor it was acceptable right out of the box. The thrust clearance on those distributors is critical so the factory pays close attention to it when the distributor is made.
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Old 08-03-2019, 09:07 PM
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This is more than what you need but you might get a couple of pointers out the information.


Build a Crank Trigger Distributor – Rutherford Motorsports
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