[Cliff R]

Remove a small manifold vacuum hose, like the one going to the choke pull-off. The engine RPM should increase slightly. Turn the speed screw back down idle and look for both nozzle drip and more sensitivity with the mixture screws.

How much vacuum is it making at 750rpms?

While you are idle tuning I'd also loosen up the distributor and add some initial timing. If it gets really happy with that deal the VA can be employed via manifold vacuum at idle to offset all the negatives.

This will simply involve installing a VA with a lighter spring adding the amount of timing to get the initial where your engine likes it at idle speed. Personally I'm NOT a big fan of that deal when the real fix is about a full point more compression, 10 degrees less duration, or same cam moved out to a wider LSA, or combinations of all three.

In any case it appears the carb now has sufficient idle fuel, so just a few more tweaks and you'll be good to go........

[steve25]

I more then agree with everything in that second to last paragraph Cliff!

[TransAm 474]

Thanks! Should we try increasing the Idle Bypass Air first, before we try hooking the Vacuum Advance up to full mainfold vacuum? I am not sure what the Idle vacuum is currently, as I haven't yet checked it.

[steve25]

You need to do that since it will give you a real good clue as to if you needa different primary metering rod spring

[Cliff R]

Remove a small manifold vacuum hose first. Re-adjust the idle speed, mixture screws and see if it like s more bypass air. If so add some bypass air before messing with running the timing clear off the scale at idle speed......

[TransAm 474]

Cool, Thanks Cliff ... and if it does respond well to removing the small manifold vacuum hose, what would you recommend we increase the Idle Bypass too?

[TransAm 474]

Ok, this is gonna be lengthy, but I want to explain everything we did today. I think we have the carb getting a LOT closer now .... in the end, it was just mostly "user error" on our part �� .... I told you all that the Mixture Screws didn't have any response because of what I thought was the Primary Blades being cranked too far open. We were gonna take the carb back off and increase the Idle Bypass Air, because I thought that is what it was wanting. It turns out, that they weren't too far open at all. First thing I noticed today, was when I checked the ported vacuum source, it didn't have vacuum on it at Idle like I was expecting, so I grabbed a light and looked down the primary bores, no Nozzle Drip at all. The error was, a few days ago, when I was adjusting the Mixture Screws from their baseline setting of 5 Turns out, I didn't try adjusting them IN anymore than maybe 1 1/2 turns from that baseline setting(3 1/2 turns from seated), so it wasn't responding, same with adjusting them OUT, there was no response. Today, after I verified the blades were not cranked open, and had no Nozzle Drip, I decided to crank the Mixture Screws IN to try and seat them with it idling to see what happened, low and behold, they started responding, so it just wanted less Idle Fuel at the Screws than I was expecting. Anyway, we ended up with them at 2 3/4 to 3 Turns out from Seated for best Idle and Vacuum. Initial Advance is set at 12°. I did however still do the test pulling the small vacuum hose off the choke pull off to see what happened, and the Idle RPM still increases 75-100 RPM, so it might still be indicating that it could use a little more Idle Bypass Air. Idle Vacuum at 780-800 RPM is right at 10 to 10.5", so it seems maybe a bit low still, but I am not sure what we can do to increase it. We did try the Vacuum Advance hooked to Manifold vacuum as well, and it increased Idle RPM as well, so I lowered the curb idle screw back to 780-800 RPM, and it did seem to increase Idle Vacuum a little maybe, but not a huge amount, maybe .5" to 1" at best. We also did the APT Adjustment with the Tip-In Procedure in Cliffs book up at 2000 RPM. Baseline setting was 3 1/2 Turns up from seated, and I found that it actually lost a little RPM when tipping in the choke flap, so we adjusted back IN until RPM now increases 50 to 70 RPM when the choke Flap in tipped in, which ended up at only 1 1/4 Turns up from seated. Throttle Response from a stand still definitley Improved after adjusting the APT. It is Idling pretty well, and stays running nice when placed into gear now. I might still be able to make improvements somehow. I wish it had a little closer to 11.5 to 12" of Vacuum at Idle, but it may be as good as it gets. Anyway, I thought I would check in and update you all.

[Cliff R]

The engine will always increase speed slightly if you pull a small manifold vacuum hose. If you don't have nozzle drip and control with the mixture screws it doesn't need more bypass air.

The only time I'll add more bypass air if no nozzle drip is when there is too much transfer slot exposed and I don't have control with the mixture screws.

10" vacuum sounds about right for that engine combo as it's got a pretty "hefty" cam in it on a tight LSA.....

[Kenth]

I have found the APT screw starts moving the power piston at 2 turns up from bottomed.

FWIW

[TransAm 474]

Originally Posted by Cliff R

The engine will always increase speed slightly if you pull a small manifold vacuum hose. If you don't have nozzle drip and control with the mixture screws it doesn't need more bypass air.

The only time I'll add more bypass air if no nozzle drip is when there is too much transfer slot exposed and I don't have control with the mixture screws.

10" vacuum sounds about right for that engine combo as it's got a pretty "hefty" cam in it on a tight LSA.....

Thanks Cliff, just for testing, we are thinking we might try increasing the Initial Timing a bit, maybe up to 16° or even 18°, from the current 12°, to see if Idle Vacuum and Throttle Response improve, no wide open throttle of course, because they way you have the distributor set up, it would probably have near 38-40° Total, instead of 32-34° with it set down at 12° Initial. This may be one of those engines that likes a LOT of timing?

We were also thinking that maybe a Vacuum Advance can such as the Standard Motor VC302 would work better with it, and could possibly improve throttle response a little, since we seem to have pretty low Idle Vacuum. That particular can starts pulling at 4" of Vacuum, with a max of 5°(10° at the Crank) all in around 8" of Vacuum.

What do you think, and what would you do if Idle Vacuum and Throttle Response improves by bumping up the Initial Timing?

Again, Thank You for all the help along this journey

[TransAm 474]

Originally Posted by Kenth

I have found the APT screw starts moving the power piston at 2 turns up from bottomed.

FWIW

Kenth, so basically what that means, is I am running it as lean as possible with the APT then, to get the desired slight RPM increase up at 2000 RPM, during the Tip In procedure? I forgot to mention that I am using Cliffs custom 50c rods, which are full smooth taper, instead of stepped. I am also using the Light Blue spring out of Cliffs kit under the PP. I did check and verify that at Idle, even with its pretty low Vacuum, it is still sucking the PP down completley. All Vacuum tests are being done with my brand new Mighty Vac.

[TransAm 474]

What do you think, and what would you do if Idle Vacuum and Throttle Response improves by bumping up the Initial Timing?

Any input Cliff?

[Cliff R]

If the engine responds well to more timing at idle and you end up that high I'd bring it in with Vacuum Advance vs base timing and shortening up the timing curve as too much initial can cause difficult hot restarts.

I wouldn't worry much about APT or lean tip-in testing at the moment. Nail down the timing and fuel settings to get the engine happy at idle speed where you have no nozzle drip, and good sensitivity from the idle mixture screws.

APT adjustments are best made by driving the vehicle and making very small changes to nail down the best power, efficiency and fuel economy in the "normal" driving range......

[TransAm 474]

Originally Posted by Cliff R

If the engine responds well to more timing at idle and you end up that high I'd bring it in with Vacuum Advance vs base timing and shortening up the timing curve as too much initial can cause difficult hot restarts.

I wouldn't worry much about APT or lean tip-in testing at the moment. Nail down the timing and fuel settings to get the engine happy at idle speed where you have no nozzle drip, and good sensitivity from the idle mixture screws.

APT adjustments are best made by driving the vehicle and making very small changes to nail down the best power, efficiency and fuel economy in the "normal" driving range......

Much Appreciated! I will be back with Results

[TransAm 474]

Originally Posted by Cliff R

If the engine responds well to more timing at idle and you end up that high I'd bring it in with Vacuum Advance vs base timing and shortening up the timing curve as too much initial can cause difficult hot restarts.

I wouldn't worry much about APT or lean tip-in testing at the moment. Nail down the timing and fuel settings to get the engine happy at idle speed where you have no nozzle drip, and good sensitivity from the idle mixture screws.

APT adjustments are best made by driving the vehicle and making very small changes to nail down the best power, efficiency and fuel economy in the "normal" driving range......

Will more Idle Timing normally want MORE Idle Fuel at the Mixture Screws(turned out richer)?

[steve25]

Your thinking is reversed .

A motor will want more timing at idle as the mixture is made leaner.
Basically as leaner mixtures are used it takes longer to get the fuel to burn completely and especially at low cycling rates

[Cliff R]

There is a fine balance between initial timing and idle fuel requirements.

Most folks get it backwards when doing this sort of tuning. It's long and complicated, plus a topic of much contraversy, but IF the carb doesn't have enough idle fuel to the mixture screws you will find that it will LOVE a lot of timing. This often has folks making huge increases in initial timing before they get the carbs idle system up to par.

For the most part IF you have adequate idle fuel to the mixture screws for the vacuum the engine is supplying you aren't going to have to run the timing clear off the scale to make this work.

In any and all cases this sort of tuning requires that we give the engine the timing and fuel it needs, not one vs the other.

Coincidentally I spent some time yesterday trying to help out a customer who just had his engine "built" by a well respected local builder. He said it runs well, but idle qualtiy is terrible and it's not very good right off idle and at light throttle openings/light engine load either. I had to ask but already knew that it was one of those "lower the compression ratio for pump gas builds" plus they installed a cam on a tight LSA. In the case here compression was lowered to 9.5 to 1 and cam is on a 109LSA. Well DUH?? How is that supposed to make the engine friendly at idle speed, produce good vacuum, strong power right off idle, fuel efficient, and not stink up your wife's hair when she rides in the car!.........

[MUSLCAH]

I need to save this thread

[TransAm 474]

Ok, just wanted to follow up on this thread .... we have been playing with the timing today, and adjusting the mixture screws on the Q-jet. We installed the VC-302 Advance can on full Manifold Vacuum so that it would add an adiitional 10°(verified with Timing Light, along with the Initial set at 12°, we had a total of 22° of Timing at 800 RPM Idle. We recored Idle Vacuum with our new Mighty Vac, and it produced 9.5 to 10" of Vacuum, Idled good and throttle response was good. For testing, we moved the VC-302 advance can back to "Ported", I readjusted the Mixture Screws for best Idle and Vacuum and readjusted the Idle back to 800 RPM. Now we only have 12° of Idle Timing(verified with Timing Light). To our surprise, Idle Vacuum increased to 10.5 to 11". The Idle is excellent and dead stable, In and Out of gear, and Throttle Response is instant, even better than it was with more Idle Timing. I guess this particular engine combination just doesn't seem to want more Idle Timing for some reason. It was surprising to us. Looks like we will stick with the 12° Initial and the VC-302 advance can on the Ported Source.

I appreciate all of the help

[Cliff R]

Good news, glad to see some more progress.

I'm not surprised by the results at all. There are many ways to tune these engines when we have a combination that's just not happy with "normal" timing and idle fuel delivery. What makes things even worse is that you can Google up a BUTT-LOAD of information on the subject in about .1 milliseconds and spend countless hours reading thru it, and much of it is old, outdated, frequently regurgitated with more crap added to it, plus has flaws and inaccuracies in it BEFORE they got it twisted up even more. The debates used to rage on here years ago, even though most of the contributors had about my actual timing doing it as I spent on the crapper this morning. Just muddies the water and causes folks to get their panties all wadded up, instead of helping folks get a good end result.

In any case you did it the way I do it here, get the carb up to par then find out how much timing it likes the best at idle speed. Less timing also puts the throttle plates a little more open so more exposed transfer slot, which no one seems to factor into these things. Timing will often be more stable that way in and out of gear, and we NEVER have to worry about loosing any timing from the VA unit if it's on manifold vacuum and the vacuum falls pretty low.