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Unread 04-02-2014, 11:05 PM   #1
Longpond
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Timing?

I checked my timing at idle. I don't have a tach. on my jeep so I went by ear. My tag says 5 deg. BTDC at 700rpm. That is where I put it, before I went through all the work on my carb. My question is... with a 4bbl, headers, glass packs, new plugs and coil, will my timing still need to be 5 deg. BTDC? I only ask this because I get some backfire coming off a high rev, or high down shift. It doesn't really bother me but my wife doesn't like it. I checked all my header bolts some were a little loose, didn't seem to make a difference.

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Unread 04-02-2014, 11:42 PM   #2
BagusJeep
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Wives are like that, mine complains about the wheels falling off.

When you close the throttle at high revs you are leaning out the mixture. Usually the vacuum advance would advance the ignition such that it fires a lot earlier for that slower burning mix. I have run the dizzie with no vacuum advance and it has backfired.

You may have an issue with the vacuum advance. Is it connected? Does it give you around 20-25 degrees of advance under that condition? Yep, it really is that much when you are pulling down 18" of mercury!!

Th way to test is to run the engine but connect the vacuum advance to a pump and gauge. For given vacuums the timing should move by a set amount. It is not a terribly exact amount as it is there for emissions, fuel efficiency and backfiring etc.

Here is a curve for the 258, you will have to find the one for the 304. To adjust when it comes in there is probably a screw inside the vacuum canister that you can reach with an Allen key through the port and wind in and out. The total possible advance is governed by the canister.

It is not unknown that the vacuum canister is defunct or mechanism jammed. At idle it should move if you connect / disconnect the vacuum pipe to a manifold vacuum source.
dizzie-curve-very-small.png

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Unread 04-02-2014, 11:49 PM   #3
Longpond
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Yeah its connected, I was considering getting a new distributor from summit but I'm getting pretty tapped out on the budget. If I hook my Mittyvac up to it would it work? It has a pressure gauge on it that just measures pressure in the line right?
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Unread 04-02-2014, 11:56 PM   #4
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Always better IMHO to find the fault BEFORE buying parts. How do I know that?

I have no idea what a Mittyvac is but does it give you measurable vacuum in the range 0 to 20 in Hg? If so you can use it to measure the amount of advance. You will also need a timing light and I suggest an expanded scale to stick on to the timing marks so you can read up to 30 degrees of advance. You can use a dialback type of timing light but they are not always accurate.

I would start off at idle with it hooked to manifod vacuum and just see what difference it makes pulling the hose off and putting it back on. If you have it on manifold vacuum you should see the arm moving back and forth at least 1/2". If you have it on ported vacuum you should see little or no movement.

This is a diagram of the differences in ported v manifold vacuum. It also include the impact of a NLVR valve if you have one (unlikely)
nlvr-valve.png

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Unread 04-03-2014, 12:05 AM   #5
Longpond
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Yeah Mittyvac is a vacuum pump that I try to use to bleed my brakes. I have a timing light, so I just need to search for the curve for a 304.
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Unread 04-03-2014, 02:33 AM   #6
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I would search on this Forum for anyone who has set up the vacuum on a V8.
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Unread 04-03-2014, 04:45 AM   #7
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Quote:
Originally Posted by Longpond View Post
I checked my timing at idle. I don't have a tach. on my jeep so I went by ear. My tag says 5 deg. BTDC at 700rpm. That is where I put it, before I went through all the work on my carb. My question is... with a 4bbl, headers, glass packs, new plugs and coil, will my timing still need to be 5 deg. BTDC? I only ask this because I get some backfire coming off a high rev, or high down shift. It doesn't really bother me but my wife doesn't like it. I checked all my header bolts some were a little loose, didn't seem to make a difference.
Bad header/collector flange gaskets or header/exhaust combo's that are too short can cause an exhaust backfire on de-eacceleration. If your bolts were loose, then you may have to replace them.
Just something else to think about.

Most folks bump their timing up into the 10 degree range.
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Unread 04-03-2014, 06:19 AM   #8
colojeepguy
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Not sure what the elevation is in Idaho, but you also need extra timing to compensate for higher elevation.
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Unread 04-04-2014, 09:15 AM   #9
Longpond
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I found a link on another site, the guy said to T into the vac lines with a gauge, and leave them all hooked up, turn your timing up until it is highest on the gauge, then back it off a tiny bit. That's what I ended up doing, and ended up around 13 or 14 deg. Seems to run really good, still back firing coming off the high rev so I think it must need new gaskets. I just thought the trick by setting your timing with a vacuum gauge was pretty cool. Especially if you don't have an rpm gauge.
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Unread 04-04-2014, 09:23 AM   #10
Longpond
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By the way elevation where I live is 4200 on the flat to 8000 in the mountains. I like trailing it in the 6000' range, nice and cool in the summer time.
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Unread 04-04-2014, 10:17 AM   #11
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I bump timing up until it starts to detonate under a load and then back it off two degress at a time until there's no more detonation.

Timing isn't an exact science where a certain mark applies to every engine. Altitude has an effect as does wear over the lifetime of the engine.
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Unread 04-04-2014, 01:51 PM   #12
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For 35 bucks, these are well worth the investment. I bought mine new in the 80's (for my VW's) and it still works great for the Jeeps. It will save you time from guessing!

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Unread 04-04-2014, 05:35 PM   #13
LumpyGrits
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First-BUY a tach and stop guess'n.
Then set the timing as per the FSM.
LG
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Unread 04-05-2014, 06:34 AM   #14
colojeepguy
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Quote:
Originally Posted by LumpyGrits View Post
First-BUY a tach and stop guess'n.
Then set the timing as per the FSM.
LG
I agree about getting a tachometer, but as I posted above, the factory timing specs don't apply at high altitude.
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Unread 04-05-2014, 06:55 AM   #15
Matt1981CJ7
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Quote:
Originally Posted by colojeepguy View Post
I agree about getting a tachometer, but as I posted above, the factory timing specs don't apply at high altitude.

The factory timing specs include high altitude settings, at least for 80's models. Not sure about earlier Jeeps.

And I agree with LG. Using a vacuum gauge to set timing is a very inaccurate method, and often results in way too much advance.

Matt
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