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post #1 of (permalink) Old 05-21-2005, 10:18 AM
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vacuum advance question

Hi all, been a long time. My 1979 CJ-7 has been revived with a engine transplant, and now we're in the long slow road to recovery. Now I'm stumped, and I am in need of some sage advice. Here's my problem:

almost two years ago, my CJ threw a rod through the cylinder wall and so I had to get a new 258. My money situation being what it was (still is), I bought a used 258 from a Jeep junkyard (1975 cj-5) and put it in there. I put my MC2100 back on it, along with a new-to-me Chevy HEI upgrade with all new guts except the vacuum advance (which I had checked with my patented precision suck-on-the-hose test). After much drama with other systems, I got it to start and stay running. However, it's straining to go 40 mph.

So I bought a vacuum pump and ran a few tests:

- tested the vacuum with the engine at idle using my timing light. The timing appeared to advance way too fast when I would increase RPMs (as in it idled at about 3 BTDC, and jumps quickly past 21 BTDC and off the charts).

- tested the vacuum advance diaphragm with the dist. cap off using the vacuum pump. It started moving a little before 5 in-Hg, and reached maximum movement at about 18. I tested a new diaphragm, and it maxed about 15.

-Tested the vacuum off the MC2100 at varied RPMs. I didn't have a tach hooked up, but I can tell you it sucked past 18 in-Hg at light throttle (approx 1500 RPM, but it's just a rough guess).

I figured that there was too much vacuum for it to go directly to the distributor, and the vacuum diagrams I found when I searched this site and looked here looked like a road map to parts I didn't know existed. I tried porting out of the top of the carb, under the air cleaner, but that's just open air behind the air filter, so no vacuum. I also tried porting it through the valve cover emission stuff, but that also is no vacuum.

I need to reduce the amount of vacuum my mc2100 is providing. I am using the ported vacuum at the base of the carb on the engine side (right under the choke assembly), and that's the only vacuum port I can find. It varies (a lot) under load, so I know it's not manifold vacuum. Anyone got any advice? Thanks for lookin'.
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post #2 of (permalink) Old 05-21-2005, 10:58 AM
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Re: vacuum advance question

Nearly impossible to change the amount of vacuum from the carb. The 2100 is designed to be a strong vacuum, the stock HEI needs a weak vacuum.
But there is a simple and easy solution - match the distributor to the carb!

Get an adjustable advance diaphgram - parts stores etc. About $20. Preferably a Crane. Make sure it has a little cam like thing in the kit for limiting the total. Sometimes they get lost - some of the Chinese junk doesn't even have them.

If you don't adjust it properly your troubles will be WORSE!

Install it - the vacuum pump helps you get to the screws.

First step -- Install the limiter cam for the adjustable - it's a small piece that goes under a screw to limit total travel. Initially set it so it limits travel to about 1/3 of the total available travel. That's very important - without the limiter it can move it about 40 degrees -- waaaaaay too much.

Then through the vacuum nipple, adjust it to the LEAST SENSITIVE position -- if I remember right, that's counter clockwise - opposite of what you'd think - but look at the instruction sheet to make sure.

Do not use the mechanical weights and/or springs with it, unless you are building a street machine. You want the mechanical to come in sloooow too. Stock weights and spings work best for an off-road machine where you want low end torque. They are fine for the street too. The light springs and fast curved weights are good if you are launching at 3000 R's.

Install the distributor, time it with vac disconnected to about 8 degrees.
Connect vacuum - check to make sure the advance does not change with the line connected - vacuum should be at it's lowest point if it's the correct ported vacuum.

Check the full advance to make sure total advance is about 35 degrees at 3000. Use the limiter cam to get it in range.
Now, adjust through the nipple to get the vacuum advance so it doesn't start moving till about 1200, advances slowly and evenly to full at about 2500. You'll probably have to reset the limiter cam a couple of times.

Remember - vacuum advance readings are in addition to the mechanical, so it's best to get a "feel" for the mechanical's curve before adjusting the vacuum curve.

It takes a little trial and error, but you'll be ALL SMILES once it's dialed in!

And - a thought -- you did eliminate the resistor or resistor wire feeding the HEI - it needs full 12 volts through a heavy wire - preferably 10 Ga. It's power hungry!

Who says 258's can't turn 6000? (But don't with an unbalanced used engine.)

Enjoy!!!!

Let us know how it fares.
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post #3 of (permalink) Old 05-21-2005, 09:15 PM
 
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Re: vacuum advance question

I just bought the crane cam vacuum advance unit and it does not work on a 258 distributor. The mounting holes are wrong and the shaft length is too long. This was the "Ford Kit".
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post #4 of (permalink) Old 05-21-2005, 09:22 PM
 
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Re: vacuum advance question

I am going to adjust my last statement. I tried to put the crane cam kit on today and it is close but not a drop in replacement on the stock jeep distributor. Tomorrow I need to redrill the mounting holes slight on the distributor and I think it will work.
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post #5 of (permalink) Old 05-21-2005, 09:54 PM
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Re: vacuum advance question

Sorry - by your reply I ASSumed when you said HEI upgrade, you put a real HEI distributor in it. You must have just done the module & coil switch?
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post #6 of (permalink) Old 05-22-2005, 01:28 AM
 
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Re: vacuum advance question

I don't know JACK S**T about HEI but I think you are barking up the wrong tree getting all twisted up about vacume.

Vacume is a direct result of throttle plate opening. You are trying to suck a given volume ( displacement of engine ) through a variable opening (carb throat)
When the throttle plate is closed as in ideling the opening is small meaning ( hopefully ) high vacume
when it is WOT Wide Open Throttle the opening is large meaning low vacume.
Ported vacume does not come into play till around 1000 rpm or so, after that manifold and ported vacume run In-Synch (And I don't mean the group) no matter how fast or slow you go untill you take your foot off the peddle.
Like I said I do not know Jack about HEI but most older engines I have tuned tell you to disconnect and plug the vacume line, so the timing is not affected by idle speed.
Are you sure you are not out by 180*
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post #7 of (permalink) Old 05-22-2005, 02:57 AM
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Re: vacuum advance question

""""Ported vacume does not come into play till around 1000 rpm or so,""""
True - usually.

"""""after that manifold and ported vacume run In-Synch (And I don't mean the group) no matter how fast or slow you go untill you take your foot off the peddle.""""

Maybe "in sync," but opposite.

-- Ported gets stronger the faster the R's - to about mid throttle, then drops down somewhat. At WOT it's near 0 again.
-- Manifold vacuum is strong at idle, strong at cruise, then drops under heavy load.
Totally different curves - almost opposite.

And -- there are many different types/curves of ported vacuum - depending on WHERE and HOW it's picked off the venturi bore, at the butterfly edge, above it slightly, above it even more etc, hole size, slotted or not etc.
Each slight change in position creates a different curve.

Several uses that require similar, yet different ported curves: Timing advance, EGR, Cannister purge, bowl venting, etc.

Look at a Thermo Quad - about 6-10 different vacuum nipples, each with a different function and curve.

Since his complaint is too much too soon it has to be slowed down.

Yes, if run without any vacuum, ping should go away - IF it was caused by too much advance too soon. But -- the reason vacuum is used is to advance it during light load for more efficiency - better mileage - kinda important at $2.50 a gallon.
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post #8 of (permalink) Old 05-24-2005, 10:22 PM
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Re: vacuum advance question

Thanks for the info on the adjustable vacuum, RRich. I ordered one from Advance Auto, took them a day to get it. When it came, it had no instructions and no Allen wrench. [img]/ubbthreads/images/graemlins/smile.gif[/img] But I had the wrench and I got the instructions online, so I'm good for the moment.

I installed the advance on my lunch break, so it wasn't the most meticulous of jobs, and I didn't have a tach handy. I adjusted the advance to the least sensitive, installed it, set the cam, and timed it to 8 BTDC. I haven't had a chance to do the fine-tuning yet (had a prior engagement tonight), but I'll try to get it done in the next few days.

I was a little disappointed on the drive home... It sputtered a lot on acceleration and just ran far worse then it had recently. When I got home and opened the hood I realized that I had forgotten to reconnect the #1 spark plug wire (I have the old style spring-and-clip-on-the-spark plug style timing light, rather than the new-fangled sensor kind). Felt pretty stupid. I'll see how it runs tomorrow... [img]/ubbthreads/images/graemlins/smile.gif[/img]
post #9 of (permalink) Old 05-26-2005, 11:58 AM
 
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Re: vacuum advance question

[ QUOTE ]
Thanks for the info on the adjustable vacuum, RRich. I ordered one from Advance Auto, took them a day to get it. When it came, it had no instructions and no Allen wrench. [img]/ubbthreads/images/graemlins/smile.gif[/img] But I had the wrench and I got the instructions online, so I'm good for the moment.

I installed the advance on my lunch break, so it wasn't the most meticulous of jobs, and I didn't have a tach handy. I adjusted the advance to the least sensitive, installed it, set the cam, and timed it to 8 BTDC. I haven't had a chance to do the fine-tuning yet (had a prior engagement tonight), but I'll try to get it done in the next few days.

I was a little disappointed on the drive home... It sputtered a lot on acceleration and just ran far worse then it had recently. When I got home and opened the hood I realized that I had forgotten to reconnect the #1 spark plug wire (I have the old style spring-and-clip-on-the-spark plug style timing light, rather than the new-fangled sensor kind). Felt pretty stupid. I'll see how it runs tomorrow... [img]/ubbthreads/images/graemlins/smile.gif[/img]

[/ QUOTE ]

When you get to adjust the unit you purchased, can you post the part number? The unit I ordered from Summit Racing will not fit on the 86 distributor, I have had to autoparts stores try and locate a true adjustable replacement with no luck. If you got an adjustable, I am very interested in the info.

Last night I tried a new trick with my vacuum advance and the mc 2100 install. My jeep will ping only at cruise when I crack the throttle, no ping at WOT. I installed the vacuum delay valve and vecuum reservior original installed on my Carter Carb and it seems to have really help releave the ping. It basically dampens the vacuum response. There is not much info on the net about these valves but I keep studing the shop manual and figured it would not hurt to try. I only tested it for about 20 minutes last night but I made a run with it and without it on the same section of road under the same conditions and it reduced the ping a bunch. With an adjustable advance unit.....I feel I will be right on the money.
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post #10 of (permalink) Old 05-26-2005, 01:04 PM
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Re: vacuum advance question

Those valves delay the vacuum from getting through, they don't lower it, just slow it down.
The color on the end of it determines the seconds it delays it.
As I remember, they come in red, green, blue and yellow, black. They range from 5 seconds to 20 or 30? seconds. But not necesarily in that color/seconds order.
They delay the vacuum from going through - like to the advance, but they don't hold it in - so it's one way only like a check valve. Handy to have sometimes.

Yes, it can help with the "tip in". Especially the longer delay ones. But if you are holding the throttle in that spot for longer than the delay, it'll ping again.
Better than nothing though.

Interesting -- 70's Fords used them on their vacuum advance - caused an advance delay - made it stumble on acceleration sometimes.
GM same vintage had a similar stumble - caused by too fast an EGR.
Easy way to cure 2 cars - take the delay valve off the Ford and put it on the GM! Must have done hundreds. Yes, not smog legal, He He - but what smog tech would ever notice it?
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