Vacuum advance timing question

[thundley57]

Hi all,

Just recently joined this site and am enjoying reading through the forum posts learning more and more about my CJ (1st time owner).

I am about to complete the Howell TBI upgrade and am curious whether those of you that have done this upgrade used manifold or ported vacuum for your distributor advance? Also, is the recommended 6 to 8 degrees of initial timing sufficient or is there another setting that will give better performance?

BTW, I have a working Carter BBD 2 barrel carb (with stepper motor) if anybody is interested in it. I will ship it to the first responder for free. Can't see just throwing it in the trash.

Tom
1983 CJ7 AMC 258 i6 / 4.2l

 

[Lumpy Grits]

That Carter is the only carb that's 'smog legal' for our late model Jeep, CJ's 6cyl in PRK.
LG

 

[Torxhead]

I use the t.b.i. vacuum port closest to the distributor which is manifold vacuum. In the Howell instructions they do say that "Some earlier, non emission models, and non-CA legal kits may perform better with manifold vacuum for distributor vacuum advance". I can't figure that out.
I set my initial timing to 6 degrees, even then it would ping with regular gas. So, I made some adjustment to the rate of vacuum advance by using a 1/8" allen wrench into the vacuum canister until it quit: http://www.jeep-cj.com/forums/f2/vacuum-advance-adjustment-distributor-23788/
When you go see the smog man, make sure the yellow sticker that came with the kit is in plain view as that has the info to check your emissions. Along with a check engine light that you will have to purchase mounted inside the vehicle.

 

[Captainlance]

ported vacuum is always used for the distributor, also, if you use a timing light and set the max advance at 2500 rpm to 35-37 degrees, the engine will run better more efficiently and stronger. Initial advance is only a starting point and not meant to be a final adjustment. this holds true for all gas engines.

 

[thundley57]

Thanks guys. Very interesting and informative article by TeamRush in the link you sent me Torxhead, appreciate it! Had no idea that there was so much involved with proper ignition timing.

Offer is still valid for the free Carter carb. About the only thing I can offer at this point to give back to the forum.

T

 

[Lumpy Grits]

ported vacuum is always used for the distributor, also, if you use a timing light and set the max advance at 2500 rpm to 35-37 degrees, the engine will run better more efficiently and stronger. Initial advance is only a starting point and not meant to be a final adjustment. this holds true for all gas engines.

He has to pass our dang SMOG test. So he will have to set it to a given spec. because the timing is checked. You will fail, if your timing is more the 2* off the factory spec and your idle is below 550 or above 950 rpm.
I have to retard my timing every 2 years for this SMOG test, and then reset it to where the Jeep runs 'rite'.
LG

 

[gyvrr]

Just a suggested note... I was tagged because I used the shift lite for my Ck Eng lite. Ref made be install a sticker with the lite stating "Ck Eng lite". I put it on a refer magnet and keep it in the glove box just for inspections. The local smog shop would not pass me until the CA smog ref approved the upgrade....
The Howell setup was the best investment to get by Ca's smog rules....https://www.jeep-cj.com/forums/images/smilies/smile.gif

 

[Torxhead]

Just a suggested note... I was tagged because I used the shift lite for my Ck Eng lite. Ref made be install a sticker with the lite stating "Ck Eng lite". I put it on a refer magnet and keep it in the glove box just for inspections. The local smog shop would not pass me until the CA smog ref approved the upgrade....
The Howell setup was the best investment to get by Ca's smog rules....https://www.jeep-cj.com/forums/images/smilies/smile.gif

Yeah, that's too bad. Since the Howell t.b.i. kit does already have a California Air Resources Board approval and number for the smog man to check with. I just went to a regular smog shop and they checked it off.

 

[thundley57]

ported vacuum is always used for the distributor, also, if you use a timing light and set the max advance at 2500 rpm to 35-37 degrees, the engine will run better more efficiently and stronger. Initial advance is only a starting point and not meant to be a final adjustment. this holds true for all gas engines.

Captainlance, when you say set the max advance to 35-37 degrees, does that include vacuum advance or only centrifugal?

Thanks!

 

[mstar]

looks like the captain is busy.I will tell you would I consider total advance.
initial (static) +centrifugal = total .so the answer to the question is Yes.
As a general rule when we refer to total advance it is used in dyno testing and of course vacuum adv. wouldn't have any bearing on wide open throttle. I have seen numbers from 32-40 * for different applications(not talking jeep engines) . What I have done with my own engines is have the distributer setup on a dist. machine with the specs and curve you want.The addition of a adjustable vacuum advance is good so you can set the amount of advance for that separately. You may end up with say 35* + 15* vacuum for 50* advance at cruise speed or very light throttle.
There are some very good articles on the net about this subject and a LOT of misunderstanding also. mike

 

[thundley57]

Appreciate it Mike!

 

[thundley57]

I know there are numerous articles (and opinions) out there on proper timing tuning and trust me, I've been combing through all of them. I'm hoping that somebody has the answers to the questions I still have below.

Did a timing advance curve on my AMC 258 i6 / 4.2l liter engine today and found that my maximum centrifugal advance is 26* (total was 34* minus 8* initial) and that the advance was not all in until 2600 rpm's. Was wondering if anybody else has a similar curve since what I've been reading is that mech advance should be mostly all in at around 2000 rpm's? I have the Mr. Gasket 925D springs installed. I'm still trying to tune out some pinging so any help would be appreciated.

While I had the Duraspark distributor out I noticed that my advance slots are 16R and 21R with it currently set on the 16R slots. I've seen a lot of talk and pics of the plates with 8R and 13R slots. Should I consider buying a distributor with these lower value slots or is there an easy way to decrease the slot size without buying a new distributor or plate? Some pictures I've seen show a small clip on the limiter pin to reduce the travel. Anybody know if these can be had anywhere?

Another thing I did while the distributor was off was to perform a vacuum advance profile on the vacuum canister to determine at what vacuum the actuator started and finished applying advance based on the adjustment screw setting inside the diaphragm housing. This was done using a Miti-Vac. Mine allows for 13 turns of the screw from all the way out to all the way in, the results are as follows:

Adj screw all the way out - starts @ 8" & is all in @ 14"
Adj screw 1 turn in - same results as above
Adj screw 2 turns in - starts @ 7.5" is all in @ 13"
Adj screw 3 turns in - starts @ 7" is all in @ 13"
Adj screw 4 turns in - starts @ 7" is all in @ 12.5"
Adj screw 5 turns in - starts @ 6.5" is all in @ 12"
Adj screw 6 turns in - starts @ 6" is all in @ 11.5"
Adj screw 7 turns in - starts @ 5.5" is all in @ 11"
Adj screw 8 turns in - starts @ 5.5" is all in @ 10.5"
Adj screw 9 turns in - starts @ 5" is all in @ 9.5"
Adj screw 10 turns in - starts @ same results as above
Adj screw 11 turns in - starts @ 4.5" is all in @ 9"
Adj screw 12 turns in - starts @ 4" is all in @ 9"
Adj screw 13 turns in - same results as above

I'm wondering if anybody else has performed this test and gotten similar results?

T