Playing with a wideband

[madkaw]

That's probably close to my engine as far as compression. I have a stage 2 cam with no overlap and had no problem running 20 initial with premium gas. I am running a 81 dizzy which has 17 degrees mechanical.

I will say that I have no proof that my timing is best for power, but I get decent MPG. I also have installed a wideband, but have yet to use it---can't wait to play like you.

[beermanpete]

A few things I did not see mentioned:

The suction springs can be changed to adjust the mixture. Stiffer springs will richen the mixture. Shims can be used to add preload as well.

The mixture adjustment nut is an idle mixture adjustment. It has far more effect to the idle than at large throttle openings.

Make sure the jet nozzle and all the other parts at the bottom of the carbs are clean and operate smoothly. If they get gummy or corroded and don't move freely the mixture will change every time you use the choke or make an adjustment.

As mentioned by Cygnusx1 the Zs idle rich. The CO spec is 6% or less. This is very rich compared to typical American cars of the same period, which are typically about 1 to 1.5% CO.

[LeonV]

The mixture adjustment nut is an idle mixture adjustment. It has far more effect to the idle than at large throttle openings.

Are you speaking of the big nut on the bottom? The big nut on the bottom affects the mixture across the entire band since you're changing the position of the jet with respect to the needle. It is not just an idle mixture adjustment even though the FSM does call it the "idle adjusting nut." You will see definite changes across the spectrum as you vary the mixture nut position.

Testing shows this, using Steve's data:

I leaned it out 0.5 turns on each carb. The idle went to 12.2 and the low speed cruise went to mid 14's. However 75MPH cruise went to 16ish. This seems a little high to me and I am getting popping from the exhaust on the down shift. So I backed it down a quarter turn. Now high speed cruise is about 15.2, idle 11.5.

It seems that I can't get the idle up without going lean on the highway.

[Zed Head]

I'm not clear on why people run without vacuum advance. Even with a modern engine management system, MAP values are still used to control timing and fuel. MAP control is essentially like vacuum advance control. What is the reasoning behind getting rid of it, or is it more of a carb tuning thing?

steve911t, I thought that your initial 1 - 5 list might have more value with engine RPM added. Can't really tell what the engine is seeing otherwise.

Anyway, interesting discussion.

[LeonV]

I'm not clear on why people run without vacuum advance. Even with a modern engine management system, MAP values are still used to control timing and fuel. MAP control is essentially like vacuum advance control. What is the reasoning behind getting rid of it, or is it more of a carb tuning thing?

Many people with triples don't use the vacuum advance for mainly two reasons. One reason is that individual runner manifolds will provide a noisier signal to the diaphragm especially with a high rpm cam. This can be somewhat alleviated with a balance tube or vacuum canister/damper. The other reason is that some manifolds don't provide a port for the vacuum advance thus the diaphragm has nowhere to get its signal from. Both reasons are not good enough to justify eliminating it. It has more to do with laziness and broken vac advance diaphragms.

Vacuum advance is a good thing, though many don't take advantage of it.

[madkaw]

For some, the vacuum advance pulls in too much advance for their modified motor. If you have a 81 dizzy on your 71 , it will pull in an additional 17 degrees on top of your mechanical which might total out around 40+ to 50 degrees(no laziness here, it just doesn't work for my engine). I agree that would be ideal to keep it for mpg and cleaner running engine.

As far as the mixture nut effecting what. It really does only effect 0-1/4 throttle opening, but that would effect cruise speeds, so I guess it is how you want to look at it.

[steve91tt]

Vacuum advance is a good thing, though many don't take advantage of it.

The advance curve I am using has all of the mechanical advance in at 2500RPM. The vacuum advance on the Mallory distributor will not allow advance beyond the maximum mechanical advance. Therefore, in my case, the vacuum advance only serves to increase the timing when cruising below 2500 RPM.

I tried vacuum advance with my setup when I first got the distributor and the car tended to hesitate and buck more at at low RPM so I disconnected it. I now understand that this hesitation may have been partially caused by a lean condition due to low viscosity dash pot oil (discussed above). I have gotten rid of 80% of the lean condition by going to 20wt oil so I may go back to the vacuum advance. I may try 30wt oil as I am still a little lean on transitions at times. It may make things worse but it's a quick, easy experiment to run and I can't help myself. I must tinker!

[steve91tt]

As far as the mixture nut effecting what. It really does only effect 0-1/4 throttle opening, but that would effect cruise speeds, so I guess it is how you want to look at it.

I agree. I would love to turn the mixture nuts in a turn or two to lean things out at idle and WOT but when I do the car goes quite lean on the highway.

[madkaw]

Steve,

curious of what you are using for a fuel pump? The reason I ask is that I have a suspection that my fuel pump causes a lean condition at top end. Plus the fact that your running more cfm with that motor than with a stock 73 since you have a rebello bottom end.

Again, I can't prove this because I haven't run yet with my wideband.

As far as timing, around 2500 is where I had my detonation issues---because all your vacuum advance is in at a slow to moderate acceleration, and also all of your mechanical is chiming in at 2500 also, so you're looking at 40+ degrees for me. So if your set-up won't allow past mechanical, I'm suprised the motor won't support more timing.

[steve91tt]

I am running an RX7 electric fuel pump at the tank to boost the stock mechanical pump. The RX7 delivers about 4psi to the mechanical, it's nice and quiet, easy install and easy to find new or used.

I will try hooking back up the vacuum advance this evening and see if things have changed now that I no longer go excessively lean on low RPM tip in.

[LeonV]

For some, the vacuum advance pulls in too much advance for their modified motor. If you have a 81 dizzy on your 71 , it will pull in an additional 17 degrees on top of your mechanical which might total out around 40+ to 50 degrees(no laziness here, it just doesn't work for my engine). I agree that would be ideal to keep it for mpg and cleaner running engine.

Sure, that is a valid point. Some people's distributor vacuum advance curve doesn't match their engine. Three options here: switch the module for one with less advance, modify the current one for less advance, or go to a programmable advance which of course requires further modifications.

As far as the mixture nut effecting what. It really does only effect 0-1/4 throttle opening, but that would effect cruise speeds, so I guess it is how you want to look at it.

It affects everything, although it does affect the mixture less as the butterfly goes wide-open. However you want to say it, the mixture nut does more than change the mixture at idle as the round-top carb only has that one circuit. You can't change the idle mixture without changing everything else.

The advance curve I am using has all of the mechanical advance in at 2500RPM. The vacuum advance on the Mallory distributor will not allow advance beyond the maximum mechanical advance. Therefore, in my case, the vacuum advance only serves to increase the timing when cruising below 2500 RPM.

I tried vacuum advance with my setup when I first got the distributor and the car tended to hesitate and buck more at at low RPM so I disconnected it. I now understand that this hesitation may have been partially caused by a lean condition due to low viscosity dash pot oil (discussed above). I have gotten rid of 80% of the lean condition by going to 20wt oil so I may go back to the vacuum advance. I may try 30wt oil as I am still a little lean on transitions at times. It may make things worse but it's a quick, easy experiment to run and I can't help myself. I must tinker!

Try it out and get measurements of fuel economy before and after. Driveability in transitions and light throttle should also improve.

I'm not trying to chastise anyone here, my L24 with triples has its vac advance currently disconnected as well. However, I don't plan to keep it that way. What's stopping me is getting the correct fitting that matches my vac advance hose and a broken advance module. I'm still rebuilding the suspension so it's not quite at the top of the list right now!

[steve91tt]

I hooked the vacuum advance back up and went for a test drive. All seems as it did without the vacuum advance but I guess that's a good thing. I also noticed that I had to force myself to drive below 2500 RPM. I find that I don't drive at that low an RPM very often unless I am accelerating or idling. As I understand it the Mallory vacuum advance has no effect while accelerating or idling or any condition above 2500 RPM so I don't see any advantage to it. But I also don't see a disadvantage so I 'll keep it hooked up for now.