School Me please - Tuning procedures

[JR74CJ5]

I have always tuned timing per spec, then turned a bit +/- to get the best response and smoothest running symptoms. Then I reset the carb mixture screws to zero, out to recommended spec (I think 3-1/2 out for my MC2100) then increase / decrease the idle speed and mixtures as needed to smooth out...

And called it good from there. I have been unhappy with my tuning by feel / trial and error though... My power top end is lacking when I really get on her, which I understand can be a non tune-able fix (could and probably is internal engine wear). She runs good for what I use her for mostly, but hoping for some improvements...

I've recently read though that the best method of tuning is to use a vacuum gauge and timing light to accomplish a greatly tuned engine...

Please school me on the procedures, etc. I believe it was 007 that had a setting marked on the distributor for elevation and sea level (or close to) locations, a on-board vacuum gauge, and had things spiced up a bit... This got me thinking about how ignorant I am on this subject... I know some, but for the sake of completeness and to ensure what I know is correct, please start from the beginning...

Links to great articles is fine too

FYI - 304v8 - should be stock (Cam and bore etc)
DUI HEI style distributor
MC2100 1.08 Carb with minimal vac hooked up.

I could even have my vacuum lines way out of whack and hooked up wrong...

I'll try to get a picture or diagram of my hook up to add here to start with too.

Want a clean running beast for Colorado trail ride appearance this year...

 

[007]

I sent you a PM.

Here's a vacuum diagram from the book...handy reference.




Sent from my iPhone using Tapatalk

 

[JR74CJ5]

That looks really close to how mine used to be set up. There are a few missing parts these days. I may have to go seaching

 

[007]

Check your PMs


Sent from my iPhone using Tapatalk

 

[Posi]

When was the last time you rebuilt your carb? (sorry if I missed it).

What I do is straight forward.
A fresh rebuild I set the adjustment screws 2 turns out.
A non-rebuilt carb, I turn the screws in and seat both screws easy then back out 2 turns.
Attach a vacuum gauge to manifold full vacuum.
Start a warm engine and note vacuum
Turn both screws a quarter turn in ( keep both screws in the same position, if one is at 12 o'clock so should the other).
Keep adjusting the screws in a quarter turn until you see the vac needle drop a hair.
Back off the screws a quarter turn, and your done.
Take your time between adjustments, and give the adjustment time to work.
If your out 3 turns chances are you have a vacuum leak or carb needs a rebuild.
If possible adjust carb with air filter installed.
This is what works for me.
If you need any assistance with your carb while in Ouray I'd be happy to help.

 

[elwood blues]

https://youtu.be/UPGLUsW5ZeE
idle mixture screws

https://youtu.be/yPe9OsnLzQ8
curb idle speed

https://youtu.be/UYGU7mTwsZc
setting ignition timing

So, the first 2 videos are for a Holley carb, the basics are the same with the MC2100 carburetor. The third one is pretty generic to any V8 and is from MSD, the guys seem to know what they're talking about.

As far as I've always been taught, and how I do it, always adjust the the timing first, then the carb. Do you have a timing light? Do you have a vacuum gauge? If you don't want to buy them, get them from Autozone, they lend tools for free now. Or you can rent them from O'Reilly's or Advance Auto.

 

[JR74CJ5]

Posi - Rebuilt probably a year or two ago - super simple rebuild, but I have bypassed a lot of Vac system. Going to get some pictures or draw up a diagram of mine to see if I'm doing something wrong easy.

Elwood - Thanks bud. I do have a timing light, don't think I have a vac gauge yet, but as much as people are talking about how useful they are, I'll probably buy one soon to have.

I'll check some things out this weekend and get back here to post findings.

 

[007]

As you mentioned earlier, I put my vacuum gauge on the dash. If you do this, make sure you have a long enough hose on it to move around to what you want to measure (then cap off where it normally stays), then you can put it back where you want to leave it for normal operation.

 

[elwood blues]

Elwood - Thanks bud. I do have a timing light, don't think I have a vac gauge yet, but as much as people are talking about how useful they are, I'll probably buy one soon to have.

Good to go JR

I actually installed one under my dash

https://www.summitracing.com/parts/atm-2337



It's handy to have just there in the dash all the time, it's a basic troubleshooting tool, and fun to watch bounce back and forth as you get on and off the throttle. I also bought one at Sears a few years ago for like $20 to use under the hood, I found the same one on Amazon;

https://www.amazon.com/Actron-CP7803-Vacuum-Pressure-Tester/dp/B0006V2BS2

 

[Posi]

A vacuum and hour meter gauge is a good edition, I have both. I could run my engine all day on a hard trail and not go 10 miles. Both gauges are good for maintenance.

 

[JR74CJ5]

Just bought a remanned 2100 and a few other parts to play with this weekend. My 2100 may be a bit suspect, and has been hard to find the right (flush ended) power valve in rebuild kits, and the choke system has been through the ringer... New mounting gaskets too etc.

I'll keep the old for a bit as a spare, or ship it back for the 75 dollar core.

Also bought the distributor advance CTO switch to re-hook that up. As mentioned earlier, in relation to 007's posted vac diagram above, I've minimized my vac lines. See attached - "now" is what I remember having now (not looking at jeep, but will correct it if I'm wrong...) and Planned is what I plan to try out first this weekend.

However, slight wrench in the plan - I was reading here: Pirate4x4.Com : 4x4 and Off-Road Forum - View Single Post - Tuning CARBURATED old school jeeps....

and according to this, the ported vacuum set up was basically a early emissions band-aid. This poster's original Poster of that info makes a good argument for why manifold vacuum is supposed to go to the distributor advance during normal operation instead of Ported...

So i assumed the "spark CTO switch" shown on the diagram sends either ported or manifold vacuum to the distributor advance port depending on coolant temp, I may need to play with it some to see what vac actually makes me run better with good operating and low RMP / idle temps. Idle and low RPM temps are important to me with my type of wheeling... The question I guess is does the CTO switch just change once based on low and high temps, or does it change from low to normal, and change again at too high as some people seem to suggest, which would seem to confirm some of what that above linked poster said...

What i thought was, manifold vac is high during idle and zeroed during high RPM and ported was opposite. It appears it may be slightly more in depth than that.

Manifold Vac is high during idle, zeroed (almost) during high acceleration, and doesn't really have anything to do with RPM truly...

Ported Vac is low during idle / steady throttle, and high when throttle plates open wide (acceleration).

DUI's FAQ covers this slightly too:

Q: What’s the difference between mechanical and vacuum advance?
A: Mechanical advance is the centrifugal weight and spring assembly located underneath the rotor that provides the largest part of the
timing curve. The vacuum advance canister, located on the side of the distributor, provides additional timing only when the engine is pulling
vacuum. The mechanical advance along with the initial advance (base timing set at idle) is what gives you total timing. An example of total
timing would be 12° initial plus 24° of mechanical which equals to 36° total. Vacuum advance is not figured into total timing because it will
only function when the engine is not under a load. Example: If your vehicle is cruising on a flat stretch of road or going downhill, vacuum
advance will come in and add as much as 15° to the total timing for increased fuel mileage and cleaner spark plugs. It is not uncommon to
see as much as 50° – 52° of timing with vacuum advance. But don’t be alarmed by this being too much timing as the vacuum advance will
decrease once you accelerate and put the engine under a load. Under hard acceleration the timing will go back to the original total with no
vacuum advance.

So if this is the case, Shouldn't I hook my distributor's vacuum advance direct to manifold vacuum only? Then what (since no smog stuff needed) would be the purpose of having ported vacuum hooked up anywhere really?

Getting in the weeds a bit here I suppose...

I have some testing and and more reading to do... I want to start with ensuring no vacuum leaks to start, then install the "new" carb, and go from there...

 

[007]

IIRC - my Jeep seems to like ported vacuum at home (400') elevation, but runs way better on manifold vacuum in the mountains. Could be lots of reasons for this, but it's consistently been that way no matter what else has been changed or the reason(s) why.

 

[Posi]

All the older, and newer emission systems were engineered to work together. If you start removing parts it will affect fuel burn. Same goes with the vacuum advance and timing.

That said, if you don't have a complete emissions unit it can be made to work, an adjustable vacuum advance and carb adjustments help. There is some advantages going full vacuum in the right circumstances. Ported advance is just an emission thing.

 

[elwood blues]

JR, I think you may have chased the rabbit in to the hole, don't go in too far or you'll confuse the **** out of yourself. If you don't speak engineer, don't read the stuff the engineer wrote and try to understand it...

I'm not great with the AMC V8 vacuum and emissions stuff, I'm much better with the AMC 258 i6 / 4.2l . That being said, don't put the vacuum gauge on the distributor vacuum line, that will change with the advance of the distributor, you need to find a place that is just manifold vacuum, I would suggest using the one at the base of the carburetor on the intake manifold.

Speaking of the intake manifold, check to make sure the mating surface is square, I've had an adapter that warped, it looked fine until I put a straight edge on both sides of it (steel ruler) and noticed that it was not in fact straight. Replaced that and it made a world of difference. It would be a little more difficult with the intake manifold, but it if's not straight and you're mounting the carb to it, it's going to leak vacuum, and even a little will make the engine run bad and make you think the carb is junk.

 

[JR74CJ5]

Just bought cheapo vac tester for tuning:

https://www.napaonline.com/en/p/BK_7001715?ref=MA==

Now I can install new carb and play with some stuff.

Maybe one day will dash install so I can see what's happening while driving, but for now this will be a good tuning tool.

007 - I may text / call you this weekend - will be playing with the jeep while drinking green beer and eating corned beef all day so we'll see... :booze: :wasted:

 

[JR74CJ5]

https://youtu.be/UPGLUsW5ZeE
idle mixture screws

https://youtu.be/yPe9OsnLzQ8
curb idle speed

https://youtu.be/UYGU7mTwsZc
setting ignition timing

Just went through those videos fully - good info in them all. Like the timing one for explaining a bit more about total timing and curve adjustment. I remember my DUI distributor having curve paperwork inside the box and not knowing what it was.

Also the mixture setting one was great for understanding process of using the vac gauge to set the mixtures.

Thanks buddy

 

[elwood blues]

 

[R717]

Ported vacuum is purely an emissions concoction which started back in the late 60s. All it does is prevent a vacuum signal at closed throttle (idle). Without vac advance the engine doesn't run efficiently at idle and generates a lot more heat in the cylinder and right at the exhaust port This helps burn extra hydrocarbons and gets​ the catalyst (if equipped)up to operating temp faster. If you don't need to pass emissions testing there is no reason to use ported vacuum, it will just contribute to sluggish off idle performance​ and contribute significantly to overheating problems at idle.

 

[JR74CJ5]

Replaced the carb among other things this weekend.

Tested Manifold vac before the swap at idle warmed up was about 16 - 17

After the swap it started about the same. I think I got it to about 19 and left it there for now. No big needle jumping.

Going to mess with it a bit more, but have to find a new manifold vac connector as I had pulled and plugged the manifold vac port before.

Did a full tune up and fluids change - checked timing (seems to like about 11-12 degrees advance for starts and smooth running).

At this point, she's running great, but I've noticed a gear noise coming from the drivetrain so going to start tracking that down next - hopefully something simple...