Pontiac GTO Forum banner

21 - 40 of 55 Posts

·
Registered
Joined
·
1,028 Posts
Also know that many vac cans pull 20 and 25 degrees of timing at various load levels ...way to much causes poor running and detonation.

You have to put in the right vac can..that is what I am describing above......
 

·
Super Moderator
Joined
·
5,865 Posts
There is no "adjusting" the vacuum can, that is not unless you have an adjustable one - and most of them aren't.

The ones I'm aware of that are are adjusted by removing the hose and inserting a hex key through the fitting and turning a screw. On those, you're adjusting how much advance the can is capable of adding under light load/part throttle conditions where manifold vacuum is high.

The vacuum canister's purpose is to help a little with fuel economy and engine cooling. During conditions of light load/high vacuum, the fuel mixture will tend to be (notice I said tend to be, not will always be) a little leaner than during other conditions. Lean mixtures are a little harder to "light", take a little longer to burn completely, and tend to burn hotter than richer mixtures - so adding some ignition lead under those conditions helps to achieve closer to complete combustion (economy). For light load/part throttle you want 'enough' additional advance to compensate but not "so much" that you get detonation/pinging or surging. If you don't have 'enough' additional advance at part throttle/light load, then that air/fuel mixture is going to still be burning vigorously when the exhaust valve opens. That's going to heat up the valve, the exhaust ports in the heads, and the rest of the exhaust system more than if the mixture were to be completely combusted in the cylinder before opening the valve. It also means you're not getting the benefit of that combustion - you're wasting energy by sending 'fire' out the exhaust instead of using it to make cylinder pressure to move the car.

So you set 'total' (total mechanical) timing for best power, then rely on the vacuum can to compensate and optimize light load/part throttle operation. If you have an adjustable can, the sweet spot is to set it so that you're "not quite" into the realm where you start to get surging/pinging/detonation during light load/part throttle operation - AFTER you've optimized total (mechanical) timing for best power.

Bear
 

·
Registered
Joined
·
468 Posts
Probably best just to switch the can out. Lemans guy helped me out with a camaro I have . That can was pulling 20 so I purchased the can he recommended from Napa and that brought it down to 10 degrees. I switched it while the distributor was still on the car...took me all of five minutes.
 

·
Registered
Joined
·
1,028 Posts
Yeah RM it is worth the money, $15 and the effort 5 minutes to get that right....

most of the vac cans I see are pulling 20 to 25 degrees advance....just won’t make your engine run at peak efficiency
 

·
Registered
Joined
·
1,028 Posts
Excellent description bear, and I have a stack of adjustable vac cans in the cabinet under my two sun distributor machines. I always take them off. Now some do like them, that is OK.
I think maybe Big D uses one, but he is a racer and very knowledgeable about the nuances of his car, like you. You guys could optimize them.

But what I don’t like about them is that the Allen key in the stem adjusts the RATE of vacumn that the can pulls in....in other words at lower vac or higher vac. It does not change the total timing that the can pulls in. And most of them can pull like 25 degrees of timing.

Now Crane makes one with a stop limiter or you fabricate one and that will limit the total.

But, 10 degrees is optimal in addition to your 36 Total .......for 46..and that extra 10 is added to your base timing at idle that puts you in the perfect spot. Also since I recommend full manifold vac and most of the hot rods have somewhat lumpy cams there is almost always reduced idle vacumn and therefore can coming in low is better.

you must pull below your idle vac, and it’d that is already lowThat stock vac can will fluctuate in and out causing an erratic idle and drive one to drink.....

which maybe you should be doing anyway, being as we allneed to relax more
 

·
Registered
Joined
·
38 Posts
Discussion Starter #27
Are you running an original points distributor or an HEI distributor?
I’m running an HEI distributor which I just put a new MSD coil, ignition, rotor, and cap in. Would you suggest getting a new vac can to just add a fixed 10? I’m not sure if the one on there is adjustable or not but I personally don’t like a bunch of adjustments since it can lead to operators errors haha

could you possibly supply a pet number for it?

thanks for all the info guys, it’s been very informative!
 

·
Registered
Joined
·
1,028 Posts
Here is the one to get...All are made by the same manufacturer Standard Motor Parts, but different parts house put different numbers on them...

Standard Motor Parts ....SMP VC 302

Rock Auto calls it ..VC 302
O’Reilly sells it as ...BWD V482
NAPA sells it as....VC 1703

All will be stamped 626 10.........626 is an inventory manufacture number...The 10 denotes the number of timing degrees the can will advance at the crank.....10 degrees BTDC.

Specs it begins to move at 3 to 6 Hg......it will pull 5 degrees dist advance (which is 10 at the crank) at 7 to 9 hg.....many of our cars will run idle vac around 11 to 18 HG...with this can you can be assured that all 10 degrees of timing are in at those low vacs.....it drops out completely as you go to WOT.

I have tested a bunch of those cans on my distributor machines. They almost always pull 10, but a few pull 8 and a few pull 12 degrees. Sometimes I can loosen the screws on it and pull the can real tight against the front of the dist and get those2 degrees back. Or even a slight bend in the arm. But I am being way too exact....8 to 12 is perfect, and you will not need to adjust anything on it.

about 15 to $20 dollars.

Good luck and let us know how you do.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #29
Thanks for this! So once this is installed the initial timing I believe should be around 15 or so and total timing at increased rpm, with vac can off and line plugged, should be around 36? I think I read that in a earlier post.
 

·
Registered
Joined
·
1,028 Posts
The first number that you want to get once you put on the new curve kit weights is what is the centrifugal advance. That determines what your base timing will be. It is not an arbitrary number like 15 or 10. It is 36 -your centrifugal....so if your centrifugal is 24 then set your base at 12......if it is 22 then set your base at 14....always shooting for 36....

once the weights are on put on the two lightest springs for test, just so you don’t have to rev the engine too high.....then set base somewhere where it will start, 10 is good....now with your timing light or timing tape on the balancer rev up the engine Until the timing mark STOPS advancing. Note that number, then subtract your base timing in this example 10, and you will know your centrifugal total....It cannot go mechanically higher that that regardless of springs.

now set you base timingwith your new number vacum disconnected and plugged.

You can then try springs that bring that total in when you want. Try one lightand one medium spring. 3,000 is good even 3600. Then add on your vac can and you are set.

don’t stress about trying to get every last degree of timing in by 3000. Often 2 or 3 degrees linger out to 34 or 3600. It is a result of the springs. With that vac can attached do one last look at your idle timing, if your base was 10 you should have 20. That will run cool and strong.

Good luck and let us know how you do!
 

·
Registered
Joined
·
1,028 Posts
PS also google this article “ How to optimize timing in Pontiac engines”,.where you can do it on the bench with a protractor. I have 2 Sun distributors machines which is really easy to set me up. But you can do it without that.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #32
Just took it by a buddy’s shop and he suggested the same thing to change the springs in the distributor to green and also to change out the plugs.

Changing plugs was on my “to do” list. He suggested the Accel shorty plugs since the hooker headers make it impossible to get #8 out.

What plugs do y’all recommend since it does have a cam upgrade and other performance stuff.
 

·
Registered
Joined
·
1,028 Posts
Those plugs are all good, sounds like you need the short one. Make sure your heat range on the plug is in the right zone. Too hot of a plug means the plug cannot dissipate heat into the head as well, too much heat causes pinging and detonation. So make sure you get it right for your engine. I run NGK plugs, ....R5670-6

Get Timing correct, right heat range plug, and fuel mixture right...(too lean runs hot causes detonation also and backfires..)

I like .35 gap...but you can go a little higher 40 even 45.....plug gaps are always working to get wider, not narrowing unless you have some major problems, normal wear widens the gap. If you start too wide and don’t keep an eye on em you with get hard starts and misfires.

I am not a fan of headers, because they often have these obstruction Problems. And put heat where you don’t want it. I run those big ram air exhaust manifolds, great flow and no maintenance issues.

But some guys love them and make them work....good luck!
 

·
Registered
Joined
·
38 Posts
Discussion Starter #37
Little bit of an update today:

Ordered the vac can along with a set of springs. Should be here the middle of the week. Decided to pull the carb off and get some info on the nozzle and jet sizes.
Found out the primary nozzle is a 28 and secondary is a 31. Primary jet is a 71 and secondary is an 80. Also found out that in the primary acc pump cam it was installed in the bracket hole 1 but pump cam slot 2. So I moved it to cam slot one. Changed fuel filter, adjusted float level and acc pump springs. Took it for a test drive and could tell it made an improvement.

It seems to me that the sizes of the nozzle or jets could be the issue with the stumble and backfire at wot but wanted to see what y’all thought.

Also got a video of it happening. First acceleration is rolling into the throttle. Seemed a bit sluggish but not bad. Second is from cruise idle to wot and that when it’s spits and sputters with a small backfire then takes off good.

Let me know if y’all are able to see and open the video.
 

·
Registered
Joined
·
4,396 Posts
I can't help get the Holley adjusted as I am not a Holley guy, but yes, if the carb is too lean when you hit the accelerator, the engine just gulps in air and it can also backfire up and out the carb - stumbles.

I would keep working on the squirters, maybe the 50CC pump instead of the 30CC accelerator pump to fatten it up when you nail the gas. I think there is also a "cam" that can be adjusted?
 

·
Super Moderator
Joined
·
5,865 Posts
Let me know if y’all are able to see and open the video.
I had to ' request access '.

Holley pump cams are color coded, with the color indicating how much volume they deliver, up to a point. What color plastic cam is on it now? There's also an adjustment for when the pump starts delivering fuel. You want it to start squirting immediately as soon as the throttle moves. If it's not adjusted right, then either there'll be a delay in actuating the pump (and you'll get a bog) or you won't get the full pump shot (too tight or too loose) or might even rupture the pump diaphragm by trying to push it too far (way too tight). Whenever the idle rpm is adjusted, also check the pump actuation. The change in the throttle linkage position due to changing idle also affects pump timing.

Try switching the pump nozzles, put the larger ones on the primaries. The larger nozzles allow the pump shot to be delivered sooner, smaller nozzles spread it out over a longer time. The pump cam combined with pump size (30 cc or 50 cc) control the total volume of the shot.

You can either make small incremental increases in delivery rate ( nozzle size ) and/or volume (cam and/or pump size) and "sneak up on it", or make a big, drastic change and work 'backwards', coming at it from the other direction.

If your problem is accelerator pump ( and it sounds like it is ) then main jet sizes in either primaries or secondaries won't affect it any.

Bear
 
21 - 40 of 55 Posts
Top