Mods on a 7041262 for a RAIV 455 engine. Are they OK?

[MattK2]

Did the tip-in test. Too lean now. APT screw is at max rich. Increase of RPM went from 2020 to 2280. So an increase of 260. Not the preferred 50-100.  My understanding of the mods I made yesterday increased air slightly (tubes) and reduced jetted fuel 77 to 73 jets.  Guessing a 75 jet may get it closer. The bucking is clearly a symptom of lean air fuel mix.

I’m not sure what down channel restriction does. Can’t find a description on Cliffs book or where it’s located. I’m sure it there somewhere.

[Kenth]

The bucking may also be an indicator of a weak ignition system.

The downchannels delivers idle/lowspeed A/F mixture to the lower idle airbleeds, off-idle slots and finally to the idle needle screws.
Note: the lower idle airbleeds changes to deliver A/F mixture when throttle blades are opened past the holes.
Too small downchannels may in some cases restricts A/F mixture to the off-idle circuit to the point of hesitation.

[MattK2]

The ignition coil is new, has been working fine.  Let’s assume for now the ignition is working as it should. I don’t want to be chasing 2 things at once.

The tip in test done to analyze A/F mix is quite telling. At 2000 RPM, the APT screw was in all the way, so at max rich. It leaned out further when turned out with measurable drop in RPM and even rougher idle. As it should. I was surprised that it actually affected the mix to that degree.  So in theory if my initial A/F mix is close, this screw can fine tune it from there.

[Kenth]

Does your new coil get proper current/voltage for the ignition system used?

[Cliff Ruggles]

Just a note on tuning with jets.  Changing jet size tunes both heavy and light part throttle. 

Tuning with primary metering rods only effects light part throttle or the "normal" driving range. 

Any heavy or full throttle rods are up and you are on the jets plus the .026" end of the rods. 

It is best to tune for jet size first by doing some heavy part throttle driving without the secondaries.   Once you have found the ideal jet size then tune light load/light throttle driving with the metering rods.  Idle system mods have an impact there as well since some of the fuel comes from the transfer slots.

The "tip in" test is used to tell you when you are close but final testing is always done by looking for the best results.  Timing is a player for part throttle tuning as well as the vacuum advance is applied when engine load is light and engine vacuum is high. 

It's really not all that complicated but for tuning in the normal driving range you need to do both timing and fuel to get the best results.  What makes it not so complicated is that all N/A engines run pretty much in the same range for A/F.  I also know what needs to be done for any particular engine combo having spent nearly half a Century figuring all that out.

What I've seen so many times over the years is that folks want to yank the top off these carbs and start throwing bigger jets in them, when more times than not the jet size was adequate and they just needed to change to slightly smaller primary metering rods.......

[MattK2]

Coil gets proper voltage.

Jets went down in size from 77 to 73. Rods remained at 43.

APT at max rich.

The way I read your tip-in procedure, anything over 100 rev increase was out of range. You used 50-100 revs. I had nearly 260+ rev increase at tip in. Used the choke valve. Went from 2000 to 2260. That’s not “close” from what I read. Conclusion, it needs a little more fuel. Already increased the air by bumping the tubes from 36 to 38.

I’m willing to try other parts. I don’t have much to choose from so I’ll source what I think I need and buy it. Just looking for a suggestion on what a possible part change could be. Smaller rod? Perhaps a 75 jet. I know what you mean about the jets controlling higher rev and how the rods control lower revs when party raised.

[Cliff Ruggles]

In any and all cases tune for best results actually driving the vehicle. 

Tune for jet size first.  If the 73 jets are fine leave them alone.  If it needs more fuel at light engine load go to smaller primary metering rods.

It may be fine as-is, do some testing before making any more changes.....

[MattK2]

I have a nice variety of jets and rods to work with now. Will block off the secondaries to assess primary jet performance. Once that’s good the rods will be dialed in.