Adjusting timing on mechanical advance distributor.

[o_rod]

Hey fellas,

 

Now that I have been enjoying cruising in a somewhat trouble free monte I'd like to start checking the tuning aspect of it. I'm going to start with the timing then move onto the carb.

 

For ignition I have a mallory distributor that doesn't seem to have a vacuum advance. I'm assuming that means it's mechanical instead. Do I check for base timing at idle without touching anything? Also how do I go about adjusting base and full advance on a mechanical distributor? Do I still rotate the distro as I would a vacuum? I'm not worried about the advance rate just yet, just base and max. Tried searching but not much information came from it.

 

I'm experienced with a timing light and understand timing advance, just no experience with SBC timing adjustments. I'm coming from the turbo import world!

[wallaby]

You would be much happier with a distributor that has a vacuum advance. The vacuum can adjusts your timing depending on the load the engine is seeing. Right now, without the vacuum can, your distributor only adjusts the timing dependent on the engine speed. 

With a mechanical advance it should be "all in" or fully advanced by 2500-3000 rpm. For timing, it adjusts just like any other distributor; check the timing at idle then bring up the engine speed to 3000 and check for the total advance.

 

The mechanical advance distributor is really a race item, and doesn't work well on the street. With mechanical advance on a race car the main concern is idle and full throttle. There really isn't any need for anything in between. A street car spends most of its time in that in-between range, where the vacuum advance can bring in more timing at cruise and part throttle than the mechanical-only dist can. It makes for a cooler running engine and better mileage, and generally makes the engine much happier, and throttle response is much better. The vacuum advance can also back off the timing as engine load increases preventing pinging or detonation.

Imagine you have a cruise control and it's set at 60mph. Everything is working fine, and you start to climb a hill. With a vacuum advance, the vacuum will start to dial back the timing as the engine load increases. The mechanical advance doesn't do anything because the engine is still going 60mph and it doesn't even know you are trying to go up a hill. We all know that to keep a steady speed when you encounter a hill it requires more throttle... and that tells the carburetor to put out a richer fuel mixture. The vacuum distributor is on top of all these changes and adjusts accordingly, while the mechanical distributor does nothing because the speed in our example has remained constant. Because of the non-response from the mechanical distributor, it often takes even more throttle to overcome the sluggish performance to get over the hill.

 

Set your timing and it will be as good as it gets, but think about changing to a distributor that has vacuum advance.

[LS65Speed]

You would be much happier with a distributor that has a vacuum advance. The vacuum can adjusts your timing depending on the load the engine is seeing. Right now, without the vacuum can, your distributor only adjusts the timing dependent on the engine speed. 

With a mechanical advance it should be "all in" or fully advanced by 2500-3000 rpm. For timing, it adjusts just like any other distributor; check the timing at idle then bring up the engine speed to 3000 and check for the total advance.

 

The mechanical advance distributor is really a race item, and doesn't work well on the street. With mechanical advance on a race car the main concern is idle and full throttle. There really isn't any need for anything in between. A street car spends most of its time in that in-between range, where the vacuum advance can bring in more timing at cruise and part throttle than the mechanical-only dist can. It makes for a cooler running engine and better mileage, and generally makes the engine much happier, and throttle response is much better. The vacuum advance can also back off the timing as engine load increases preventing pinging or detonation.

Imagine you have a cruise control and it's set at 60mph. Everything is working fine, and you start to climb a hill. With a vacuum advance, the vacuum will start to dial back the timing as the engine load increases. The mechanical advance doesn't do anything because the engine is still going 60mph and it doesn't even know you are trying to go up a hill. We all know that to keep a steady speed when you encounter a hill it requires more throttle... and that tells the carburetor to put out a richer fuel mixture. The vacuum distributor is on top of all these changes and adjusts accordingly, while the mechanical distributor does nothing because the speed in our example has remained constant. Because of the non-response from the mechanical distributor, it often takes even more throttle to overcome the sluggish performance to get over the hill.

 

 

Set your timing and it will be as good as it gets, but think about changing to a distributor that has vacuum advance.

 

Just my $.02 here.................read on

 

Agreed....distribs. w/o vac advance on the street are just racer wanna be stuff.

 

Nothing but trouble tuning then to get livable  street performance  and "manners' out of engines running those things. I have friend with a 375 HP 396 Chevelle that is all rigged up for racing,  it was very successful on the track, the car is just a HUGE PIA whenever he drives it on the street. Runs like crazy at full throttle but try crusing at steady speed, even at highway speeds with his racing gears in the rear, the thing just makes you want to scream it is so temperamental.

[o_rod]

That is some fantastic info, Thanks very much...

 

I did notice that while cruising the car seems rather sluggish and heavy, throttle response especially. At full throttle it's incredibly powerful but it seems there's too much of a gap between performance and driveability, its not linear at all. I'll go with a vacuum unit.

[o_rod]

so I tried timing my engine and not sure if I made it better or worse.

 

I went by the timing mark the previous owner put on the crank pulley and it was really advanced,liked 25 degrees. I set it at 14 degrees and noticed no difference in its running characteristics. But when I Rev it up to 3k rpm the timing doesn't advance at all, it stays at 14 degrees, it may even retard a few. What's going on? The engine runs well and cruises well but at WOT it's weak. what would cause no timing advance?

[Leghome]

It could have a bad diaphragm in the adjuster on the distributor . I have seen them go bad before kind of rare but can happen.

[o_rod]

Gonna take it out and have a look...

 

is it possible that the timing has been locked out? like I said it was set very high at idle and doesn't budge when revved to 3k. It does start very easily though...

[Sam (Bones)]

it is a good possibility that it's locked

[MCfan]

Gonna take it out and have a look...

 

is it possible that the timing has been locked out? like I said it was set very high at idle and doesn't budge when revved to 3k. It does start very easily though...

 

Bryan, 

 

If your mechanical advance has been "locked out" either on purpose (I've personally never heard of that) or due to mechanical failure of the spring/weight advance linkage (more likely, IMO), you probably didn't do yourself any favors by reducing your static (base) timing down to 14 degrees. In addition to weak power output at WOT, I'll bet your engine is running hotter and your fuel economy has gone into the toilet.  If your engine has a mechanical (only) advance distributor that is not functioning properly, it's not going to treat you very well wherever you set the base timing.  If you confirm that it is not advancing between idle and 3K, you would do well to either repair it or ditch it for a unit that also offers vacuum advance, as Mark already suggested.

 

I assume your Mallory distributor is a pointless unit, but either way it has a rotor under the cap.  If you remove the distributor cap you should be able to grasp the rotor and turn it approximately  20 degrees (only 1/18 of a full circle) without anything else in your engine moving at all.  That is the normal amount of mechanical advance that the springs and weights would provide with increasing rotational speed (rpm) if it was functioning correctly.  If the rotor does not rotate at all, then it is locked either on purpose or due to failure.  If it rotates a bit but requires some force then you either have way too strong of springs installed or there is some mechanical binding of the advance weights.  

 

I am not personally familiar with your distributor, but I know that worn bushings between the weights and their pivot pins on a stock HEI distributor can allow the weights to overlap and bind instead of nesting together correctly so you might check for loose fitting or misaligned weights ( you'll probably need to remove the rotor to check this).  If the weights are locked in place by misalignment or overlap, you'll want to realign them and see what is allowing them to become misaligned (i.e. worn bushings, a thrown spring, etc.).  

 

It is also possible that someone installed a super strong spring on one or both weights so the weights don't move until after 3K rpms. Normally your mechanical advance would be "all in" at or before 3K.  Let us know what you find.  Good luck.

 

[o_rod]

So I removed the cap off the distributor and snapped a pic. Obviously it has no Springs or weights and it can only move about 3 degrees as it is. It's definitely locked, not sure what's holding it in one spot though or which weights Springs to install... it's a Mallory 29215 if that helps...

 

[o_rod]

So the part # for the distributor is 8548201 which comes in a race prepped version with the vacuum and mechanical advance locked. This is obviously the version I have. Guess I'll have to convert it back to mechanical and vacuum.

[MCfan]

So the part # for the distributor is 8548201 which comes in a race prepped version with the vacuum and mechanical advance locked. This is obviously the version I have. Guess I'll have to convert it back to mechanical and vacuum.

 

Well, there you have it.  You might be able to acquire the right parts to add mechanical advance and even vacuum advance to your current gutless wonder, but you also might be time and money ahead to just buy a new Series 85 with all of those parts included and then swap in the high performance coil from your current distributor.  Unless you plan to run your Monte only at the track, you need a good street distributor with both mechanical and vacuum advance.  In the meantime, you may want to return to the base timing you started with.  Good luck.

[Sam (Bones)]

if you remove the 2 allen head screws, it will unlock the mech advance, then buy a kit and a vacuum can 

[o_rod]

Thanks guys, that's a start. It's just a weekend warrior and it's in winter storage so I have time to mess with it with no real consequence. I emailed MSD/mallory to see if the parts are available but the parts aren't listed anywhere individually and the website says if you want to modify the units you have to send them in. Might be stuck buying a new unit. I didn't like the way it drove locked out at anything but full throttle.

[monte70car]

Bryan if you want to go old school I have a Mallory dual point dist that I couldn't use due to my motor choice.

[o_rod]

Thanks for the offer but I think I have it solved. Looks like i wont need a new distributor, yay! Called a few shops today and any HEI advance curve kit will work with mallory 75 and 85 series distributors. Remove the 2 hex bolts, install weights and springs, seems simple enough. I was told just mechanical advance would work fine for my application but will need to play with the spring combination to see what I like best. I'll update this when my kit comes in and I play around with it!

[o_rod]

Finally got my advance curve kit in and put it all together. Very straightforward, going to start with the lightest springs and see how the curve looks, hoping for all in around 3K.

 

Only issue I'm having is my distributor doesn't have the little e clips that seem to be a safety net for the middle weight. Finding these are becoming shockingly difficult and my local parts places are clueless. Anyone know the size of them or have people left them off? I wouldn't risk it but just curious...

[o_rod]

Here's a look at the progress so far. Met a very nice mechanic at work who happened to build rat rods and gave me a spare distributor she had laying around. Gotta love the car community, best people in the world!

 

I was able to salvage the vacuum advance off of it and added a centrifugal weight kit to get both types of advancements going! Been -30c up here so haven't had a chance to fire it up but I'm hoping it will help my throttle response and power band.