A bit puzzled with my timing, mechanical only on my 340, who can help me?

[idrivemopar]

So, long story short, my builder stated I should set my timing at 17 degrees BTDC, and the distributor is only using the mechanical advance with the 272 cam installed on my 340. So I have been running like that for a while, and it runs good, but since I started tuning the carb, I noticed when you stomp on the throttle, it really bogs, easing into it, no problems.

So today, for giggles, I hooked up the vacuum advance which added another 10 degrees at idle, however, the throttle response was 100 times better, and just putting the car in gear and giving a goose, the tires broke loose, so it would appear to be exhibiting more power then ever has before. When I checked with my timing light however, the total advance went off the chart, above 50 degrees when I rev it hard, but no back firing through the exhaust or carb, but you could hear combustion through the carb it that makes any sense, really throaty sounding! Did he make a mistake in telling me not to use the vacuum advance?

Just curious now what my timing really should be, and I want to make sure that using much advance is not gonna be an issue. Can someone provide some guidance here?

 

[Photon440]

Your 'full throttle' advance should be any different, since your manifold vacuum should be gone then anyway. You might start to see double digit gas mileage now with the vacuum advance hooked up.

 

[Tempest 455]

Vacuum will add a lot of advance, hence why you set initial and total without vacuum connected.

Somewhere (and I say somewhere) between 30-40 deg total (without vac adv) is where most motors like to run. My street car is set at 36 deg. My 13.5:1 race motor is locked at 30 deg. Much depends on cam timing, static comp ratio, stroke and a host of other factors.

Most times, you will need to play w/ your total and vac advance curves to get it dialed it.

 

[idrivemopar]

Vacuum will add a lot of advance, hence why you set initial and total without vacuum connected.

Somewhere (and I say somewhere) between 30-40 deg total (without vac adv) is where most motors like to run. My street car is set at 36 deg. My 13.5:1 race motor is locked at 30 deg. Much depends on cam timing, static comp ratio, stroke and a host of other factors.

Most times, you will need to play w/ your total and vac advance curves to get it dialed it.

Ok, maybe that is what I misunderstood, was I thought the total advance between 30-40 was including vacuum advance, but you are saying with out, so then what I am seeing with the vacuum advance connected up would be above that on occasion.? Correct? So that would make more sense then if that's the case.

- - - Updated - - -

Your 'full throttle' advance should be any different, since your manifold vacuum should be gone then anyway. You might start to see double digit gas mileage now with the vacuum advance hooked up.

Don't know about gas mileage yet, car has not seen the street yet, but with vacuum hooked up, smells a bit leaner burning now.

 

[lewtot184]

i always use a vacuum advance in my street driven cars. it has no adverse effect on performance and keeps the combustion process cleaner. needing 17 degrees initial with that cam seems high. i suspect that the distributor has heavy springs in it and short travel advance plate.

 

[70bigblockdodge]

We need more info to help
Total timing 3000+ rpm no vacuum connected
Do you have a mopar dist. Made during last dozen or so years?
Cam specs

 

[idrivemopar]

The cam is 272/284 duration with 454/480 lift. The distributor is an aftermarket OEM replacement one newly purchased. I can check timing tomorrow to get specific, but I know its in the 30-40 total without vac advance. Just want to make sure I am ok here. Will do some more fiddling tomorrow but a buddy of mine seems to think its ok, and if I don't get any pinging under load should be good, but if I do, needs to be backed off a bit.

 

[snakeoil24]

1. if your not getting pinging that is a good sign. Does your vacuum adnce diaphram have a allen screw adjustment in the hose connector? if so you can adjust the vacuum advance. You could benefit from a good timing light with digital advance.

 

[idrivemopar]

No, the distributor is a stock oe replacement type. As far as timing light, I still use my 30 year old craftsman timing light that thing is bullet proof! I did add timing tape to the balancer.

 

[Meep-Meep]

I like to run the vac advance off the ported port on the carb. This is the vacuum port that sees vacuum just as you get off idle. The vac advance will add about 10 degrees of timing but that gain will go away at WOT when the vacuum is nearly zero. It's not uncommon to run at 50 degrees of advance during cruise under light load, but when you roll the throttle on the vacuum drops and all that extra advance goes away. Your total mechanical advance (vacuum disconnected and hose plugged) should be in the 35 deg range and all in by about 3 KRPM. FYI: A hot cam will tend to want a lot of initial advance.

 

[idrivemopar]

I like to run the vac advance off the ported port on the carb. This is the vacuum port that sees vacuum just as you get off idle. The vac advance will add about 10 degrees of timing but that gain will go away at WOT when the vacuum is nearly zero. It's not uncommon to run at 50 degrees of advance during cruise under light load, but when you roll the throttle on the vacuum drops and all that extra advance goes away. Your total mechanical advance (vacuum disconnected and hose plugged) should be in the 35 deg range and all in by about 3 KRPM. FYI: A hot cam will tend to want a lot of initial advance.

Ok, stupid me, RTFM! Its been a while since I had played with the thermoquad, like 30 years ago when I still one on the car so I have some re-acclimation to do and after double checking the service manual, I had the vacuum advance on the wrong port on the carb, all is better now, and it responds correctly and isn't going off the timing chart now!. Thanks Meep-Meep for your comments which made me double check what I was doing!

 

[70bigblockdodge]

The 17 btdc initial seems high for that cam. Did you ever verify your timing marks, thses are notoriously inaccurate from factory it is worth checking so you know you have a good place to start tuning.

 

[idrivemopar]

Are you referring to the marks on the cam and crank gear or the balancer and engine block marking? If its the balancer and engine block marking then yes, I did check that when I installed the motor, i.e., hand cranked it to TDC and checked the distributor location on number one cylinder.

 

[Cranky]

But to make sure your timing marks are correct you need a degree wheel. This is usually done when you degree in the cam. First thing done when degreeing a cam is to check for true TDC. I've seen new balancers that were off nearly 10 degrees before and old ones way worse.

 

[PoppaSmurf]

Cranky...100 % absolutely correct Thank YOU ,THANK YOU :icon_bigsmurf:... what's the history of the Harmonic Balancer, OEM old/New? better yet did you in any way stretch the timing tape while installing it?