General tip in question.

[Pav8427]

When tuning APT using tip in procedure, would it be correct to say that when tipping in and you have little to no RPM change, you would raise APT? And on the opposite end, if you have more than the 50-100 rise in RPM, you would lower APT? Would that be the case as long as everything else was close and there are no obvious other issues?

Thanks. Doug

[Cliff Ruggles]

Raising the APT will cause less change, lowering more change in RPMs as it makes things leaner.

The APT test only tells you that you are close and have control, any/all final tuning should be done based on engine performance, fuel economy, etc......Cliff

[Pav8427]

If I undertand correctly. What you are saying is that when richening things up, a 1/4 turn will give you 'x' amount of RPM change, and leaning up, a 1/4 turn will give you 'x'+ RPM change. Thats good to know if that is the case. 
To clarify my question. Upon intitial start up, and to get to the 50-100 RPM rise for the base tip in tuning, if there is no RPM change during tip in, would you raise APT? And on the other end of that. If you had a lets say 200-300 RPM drop during tip in, would you lower it? As long as there are no other obvious issues rearing there ugly heads, would that apply?

[Cliff Ruggles]

The APT is set after the engine will fully heat soaked and idle mixture screws have been set.

Using the fast idle adjustment set the engine rpms around 2000rpms for the test.  This will insure the carburetor is well on the primary main system.

If you see a 200-300rpm's drop the carb is pretty lean, so raise the APT some.  Raising the APT lifts the rods, richens up the A/F ratio and you'll see less RPM drop with "tip-in".  I like to see a very slight change, about 50-100rpms and use that as the baseline setting.

Also make sure the vacuum advance is hooked up and applied during the test.

The big myth in all of this is that a leaner setting will improve fuel economy.  For most set-ups it will not.  All engine need adequate fuel in the mix for complete and most efficient combustion.  The more efficient the engine the less fuel it will need, same for timing.

What I see here is that engines with very tight squish, optimum compression ratios and very well chosen camshafts are the happiest everyplace when it comes to how much fuel and timing they will need. 

Engines with "low" compression ratios, a lot of squish area, poor flowing heads, crappy combustion chambers, and poorly chosen cams are the worst in terms of power production, and how much fuel and timing it takes to make them happy.

I'll also add this while on the subject, the camshaft choice alone can hurt engine efficiency.  Cam companies push very tight LSA's because the customer base demands a some "attitude" at idle speed.  I've watched this trend for many years.  They will also tell you that the "whiz-bang" modern lobe profiles will make more power, more area under the curve, improved throttle response, blah, blah, blah.

I've tested modern camshafts on tight LSA's and not one single example has made the grade.  I will say they certainly produce an aggressive idle quality, but on the dyno and at the track we've made more power and ran quicker with camshafts that have longer seat timing, less aggressive seating velocity, and wider LSA's.

When you get time wonder over to the Lobby and read my thread on camshaft selection........Cliff

[Pav8427]

Thank you Cliff. That explains alot. I have read many posts. Over and over. Lots of good info. My question actually applies to a recent build up of a 17058253 with one of you hp kits,44 rods and jets. Just didnt want to throw out a ' whats wrong with my build ' question. You learn by doing.
Thanks again for all the great help from you and others here. And good parts.

[Cliff Ruggles]

The 17058253 is an excellent unit.  Very little need to modify very much with one of those to set it up.  The airbleeds can all remain stock, it may need larger idle tubes/DCR's and a little idle bypass air for engines with "hefty" cams in them.

They main system really benefits from using our custom metering rods in lieu of the larger factory "K" series rods. 

The CV secondaries aren't too far off either having .052" tips.  We may drop down to our .044" secondary DA rods for some set-ups, others will be fine with the larger CV rods.

All in all a very good carburetor and big cfm too.......Cliff

[Pav8427]

Built it to recipe 2 w/ mods suggested from book. Air flap notches,.093 drilled idle screw holes ect. Did use CV rods with the tips nocked down to .048. First fire up was nice. As soon as it got gas went straight to smooth 800 RPM idle. NEVER had one do that. First initial tune and drive was NICE. Another session tuning, and would hope its all nailed down.

[Pav8427]

Another question I have. From the tip in description in the book, and various posts, the mention of pulling a small vacuum hose to varify settings is clear. What is not clear is, do you pull a hose while at 2000-2200 during the tip in procedure? Or when back down at idle speed?

[Cliff Ruggles]

Pulling a small manifold vacuum hose can be done for either idle or "tip-in" at 2000rpms.

If pulled for an idle test, idle speed will increase slightly and will allow you to lower the idle speed screw.  If nozzle drip was present, it may go away, telling us that the carb needs some additional idle bypass air to stay on the idle system.  This assumes that control with the mixture screws is still present during the test.

If you pull a small vacuum hose during the "tip-in" test it leans things up slightly.  Depending on whether or not the engine speeds up, slows down, etc, it can help determine if you need to raise/lower the metering rods a little more or go in and change parts to changes part throttle A/F.

One also has to consider that a "tip-in" test around 2000rpm's is a no-load test and just helps get you close.  A road test should follow and further adjustments made based on what the engine actually wants or needs.

We always tune for best results in all areas.  What many folks don't realize is that the carburetor is a load sensing device and will add fuel when engine load/throttle angle, pressure differential above/below the venturi and airflow past the boosters dictates.  This happens even when we don't change parts or adjustments to the APT system. 

Carburetors by design have been doing this quite well for decades, and why we still see them in service today despite the fact that fuel injection has been around and commonplace for 30 years or so at this point.......Cliff