Manifold Vs Timed Port Vacuum

My68Vette said:

I know this topic has been brought up a handful of times and its not my intention to start another debate about it but I do have one question..
If my motor was tuned on a dyno using ported vacuum for the distributor vacuum, what would happen if I changed the hose to a manifold vacuum and without making any other changes or adjustments?

I'm asking because I cut some hoses to length, including my distributor vacuum hose and believe I may have installed the hose on the wrong port because my motor is running like crap now on cold starts and at idle.. It will be a few days before I can get a longer hose to put it back where I believe it was initially hooked up so meanwhile I was hoping someone would tell me the switch could cause allot of issues with cold starts and idling.. My timing light just took a crap and I don't have a vacuum gauge so I cant check either of those right now either..

Right now I'm experiencing backfires during a cold start and severely fouled out plugs and I haven't made any other changes since the motor came off the dyno.. It also seems like my lopy idle smoothed out some..

Thanks

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A manifold port would apply full manifold vacuum to the distributor as soon as the engine started and idled. A timed port would withhold exposure of vacuum from the distributor when the engine was idling, assuming the carb's curb idle is correctly set up.

The way in which these two ports affect engine operation is with idle RPMs:

1) When the distributor is exposed to manifold vacuum at idle, it moves the vacuum advance to fully applied, this speeds the idle speed up to which when tuning the enigne, you would respond to by reducing the throttle opening to slow the idle speed and making any idle fuel adjustments needed to correct the mixture.

2) If the vacuum advance is connected to the timed port, and the engine idles with the throttle positioned to keep the blades below the port, there is no vacuum advance being applied. When you tune the engine you would have to open the throttle's curb idle setting to increase RPMs to make up for the lack of advance, or you would add more base advance to speed the idle up. As soon as the throttle blades opened far enough to expose the timed port to manifold vacuum, the vacuum advance would function normally.

These two distinctions are the only difference in operation of not timed or timed vacuum advance. There are subtleties in tuning between them that are greatly, but not, exclusively tied to compression ratio, cam timing, and idle emissions.

Certainly the problems you're having could be the result of the vacuum advance sourced from the wrong port for the way the engine was tuned. If the engine was set up for full manifold vacuum at idle there would be full vacuum advance applied which provides more burn time for the lower density idle mixture, a good thing. If in that state of tune the vacuum advance was changed to a timed port, the idle would have insufficient advance causing the idle RPMs to be slower, combustion temperatures to be lower resulting in fouled plugs as a result of both the lower temps being unable to burn deposts off the plugs and more deposits being put on them from unburnt fuel resulting from the lack of sufficient burn time, as well as missfires that occur because the low density mixture often won't catch fire from the spark at the plug.

Bogie

cobalt327 said:

You added timing at idle speed by hooking the vacuum advance up to full-time (manifold) vacuum, so you would now have more advance at idle, compared to where you were before.

This could manifest itself as having an idle speed higher than before, it may exhibit "run-on" when shut off. I wouldn't expect it to suddenly start fouling plugs, or backfiring- but I could see it smoothing the lope some.

My feelings are, there's something else being overlooked here. You can check the timing chain to be sure it hasn't jumped w/o tools. Align the timing tab/damper mark to "0". Pull the cap and see where the rotor's pointing. It should be very near either terminal #1 or #6. For this check, it doesn't matter which. But it will be one or the other.

You sure the choke's not stuck closed or something simple like that?

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Ok, I brought it up to TDC and the rotor was very close to the #1 terminal.. Just so you know, I had the engine builder install a gear drive..

A buddy just dropped off some hose for the Vacuum on the Dist so I hooked it up and now it wont fire up at all.. Yesterday when I got it up to temperature the heater core started to leak so all I did was bypass it by plugging off the water pump and intake.. I waited until today to give the permatex time to cure and now I cant even get a cough out of it.. I pulled a plug, rolled it over and I do get a spark but cant really tell if its weak or not.. My plugs are pretty black and and its only been ran once (as mentioned in the first post) so I dont know if they are to fouled to fire it or not.. As mentioned, I do get a spark, just dont know if its strong enough or not.. I dont have spare plugs here.. I checked for 12v at the HEI dist and its there.. I then checked for gas at the carb and the squirters are working when I pump the throttle. One problem after another ain:

My68Vette said:

Ok, I brought it up to TDC and the rotor was very close to the #1 terminal * snip* now I cant even get a cough out of it

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The plugs could be too fouled by this point.

Don't worry- you will get it running again.

To start with, if you have a battery charger, top up the battery. All the cranking will deplete the voltage during cranking to the ignition, you don't need this.

What you described in your first post sounds like you were on manifold vacuum, but then switched it to ported. You sure this isn't the case?

Take all the plugs out and clean them. Start by spraying them down w/carb cleaner, dry, then if you have a wire wheel, or a small wire brush, give them a brushing. Check that the gap's OK.

Crank the engine over a few times to expel any fuel that might be left in the cylinders, take your time while the plugs are out, you want the gas to be out of there, use compressed air if available to blow into the spark lug holes to speed up evaporation.

All the gas in the cylinder deal is likely not necessary, but sure as I say that, you'll have a cup of gas in there from a stuck float or some such!

Check all the vacuum lines to be sure they're all hooked up or plugged. A vacuum gage would be good to have- you might see that you have an intake gasket leaking.

From here, (still no timing light, right?) you should check the timing, lacking a light, you'll have to settle for it being close enough to run. The distributor hasn't turned? It's not loose- as in , you can't easily reach over and twist it, right?

Check the float level. If a float has stuck, or the needle valve/seat is clogged open- it can flood.

After all that- when the battery's fully charged, put the plugs back in, wire them correctly (1-8-4-3-6-5-7-2; CW rotation), cylinder 1 is drivers side, front, #2 is passenger front, alternating back and forth till #8, passenger side, rear.

Hopefully, it'll start.

cobalt327 said:

The plugs could be too fouled by this point.

Don't worry- you will get it running again.

To start with, if you have a battery charger, top up the battery. All the cranking will deplete the voltage during cranking to the ignition, you don't need this.

What you described in your first post sounds like you were on manifold vacuum, but then switched it to ported. You sure this isn't the case?

Take all the plugs out and clean them. Start by spraying them down w/carb cleaner, dry, then if you have a wire wheel, or a small wire brush, give them a brushing. Check that the gap's OK.

Crank the engine over a few times to expel any fuel that might be left in the cylinders, take your time while the plugs are out, you want the gas to be out of there, use compressed air if available to blow into the spark lug holes to speed up evaporation.

All the gas in the cylinder deal is likely not necessary, but sure as I say that, you'll have a cup of gas in there from a stuck float or some such!

Check all the vacuum lines to be sure they're all hooked up or plugged. A vacuum gage would be good to have- you might see that you have an intake gasket leaking.

From here, (still no timing light, right?) you should check the timing, lacking a light, you'll have to settle for it being close enough to run. The distributor hasn't turned? It's not loose- as in , you can't easily reach over and twist it, right?

Check the float level. If a float has stuck, or the needle valve/seat is clogged open- it can flood.

After all that- when the battery's fully charged, put the plugs back in, wire them correctly (1-8-4-3-6-5-7-2; CW rotation), cylinder 1 is drivers side, front, #2 is passenger front, alternating back and forth till #8, passenger side, rear.

Hopefully, it'll start.

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First of all, thanks for thr help!!
I just went to the aut parts store and picked up some new plugs and vacuum gauge.. I will pull all the existing plugs and crank over the motor to blow out any excess fuel.. Its impossible to mix up the wires because it has a Mallory HEI with top hat, which locks the wires in place and I havent touched them since the motor was delivered.. The plug wires are also in a loom so it would also be impossible to mix up the plug end so I know thats all good.. I also have heat shields over the boots so they dont get burned on the headers. The engine builder marked the position of the Dist and the marks are still lined up so it hasnt moved and its locked down pretty tight (wont turn unless you losin the bolt).. The carb has see-through float sights and you can see the level is right in the middle of them. Keep in mind everything on this motor is brand spankin new and it ran fine when it was on the dyno.. It pulled 451hp and 440 ft lbs of tq..

Ok, I'm off to change the plugs.. Be back in a little while to give you an update.

By the way, I'm possitive I had it hoooked up to manifold vacuum and not ported vacuum.. I have the paper on the new Summit 750cfm carb and it shows and lables the ports but I just dont remember which port the engine builder used.

cobalt327 said:

Good luck!

By what you've said, I really doubt it's anything serious.

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So much for that!
Pulled all the plugs, rolled the motor over and made sure the choke was open so it wasnt drawing fuel. Checked gap, installed plugs, doubled checked firing order (just to be sure) and then turned it over and it didnt even sputter at all. I then pumped the gas once and still nothing.. I then pulled the #1 plug to verify spark and it was sparking but again I dont know if it was weak or not.. Heck it started up with fouled out plugs the other day, lol..

I am starting to think its either the dist or valve timing. A friend just dropped of his timing light but that doesnt do me much good until its running.

It just doesnt make sense.. Although it didnt seem to run great, it at least ran yesterday and long enough for me to shoot the below video. Then I noticed the heater core leaking so I shut it off and bypassed the hoses and today it wont even fire at all.

Here is the video I shot yesterday and then below that is a video from when it was on the dyno..

Home Video

Dyno Video

carsavvycook said:

Are you running a MSD Box, along with the distributor?

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No, just a new Mallory HEI distributor with high output coil.