I started by cleaning up the lean spots using mods frequently discussed in this forum.  I uncovered the idle mixture screw so it could be adjusted, I enlarged the stock #52 pilot jet (.019”) by drilling out with a #75 drill (.021”), and I raised the slide needle by installing a .060” washer in place of the stock nylon washer (.113”).

Here is a picture of the pilot jet drill work:

Best idle was achieved at 1-3/8 turns out.  As expected, it was a quantum leap from the as-delivered condition. Easy starting, good smooth idle, no more surge, almost no afterfire.  Everything’s better….except the twist grip still felt disconnected from the carb.  Turn, wait, go.

I figured a softer slide spring would result in quicker response.  I tied off several of the coils with thin nylon chord to shorten the spring & reduce preload.  Stock spring free length is 6-15/16”.  With the coils tied off the free length was reduced to 5-3/4”.  I installed the spring with the tied off section down in the slide.

Here is a picture of the shortened spring:

Results: Not good.  If anything, the response was a little worse than the stock spring.  

After giving it some thought, I concluded that softening the spring is the wrong approach to better response.  The variable slide adjusts the size of the venture to the optimum position necessary to maintain velocity.  The vacuum signal ported to the area above the diaphragm is proportional to the velocity.  The spring opposes the lift force applied by the diaphragm.  If you soften the spring the slide assumes a higher position at a given air flow, and in turn results in lower velocity, which in turn results in a less responsive system.

Let’s try the vacuum port concept.  The stock slide has two vacuum ports in the bottom.  A #40 drill bit just fits into each port so the ports are .096” diameter.  A new slide costs around $125 so I was reluctant to just hack away.  I checked the screw holes for the slide needle retainer plate.  A #38 drill just fits.  I left one of the screws out to act as an additional vacuum port and tried again.

Note:  Make sure both screws are tight.  Then remove one while leaving the other tight.  The single screw is more than adequate to hold the retainer in place for a quick test.

here is a shot of the screws that hold the plate in place:

Here is a shot of the top of the modified slide:

Glad you gave this a try and the results are promising.

Drilling jets doesn't scare me - the only issue is that most folks don't have jet drills or reamers or gauges.  Your mixture screw ended up very near the 1.5 - 2 turns that is specified as the range that indicates the pilot jet is the correct size.

Since you are qualified to experiment - can you figure out a way to make the TEV valve provide a bit more fuel?  That will help to eliminate the popping/banging when folks close the throttle between shifts or when compression braking.

I will take on that TEV valve (I call it the coasting enricher system).  

My understanding of that assembly is that under normal steady state and acceleration the spool valve is parked and air to the pilot and transition circuits is provided via the #230 pilot air bleed and #45 air bleed jet, located in the dome below the diaphragm.  When manifold vacuum spikes during deceleration, the high vacuum acts on the TEV diaphragm to overcome the spring and shuttle the spool valve over the port that connects the #230 pilot valve to the pilot & transition circuits.  Under the high vacuum condition, the pilot and transition circuit receives air only via the #45 air bleed jet.  When I had the carburetor apart, I measured that #45 air bleed.  It was greater than .016" and less than .018".  That's pretty small.  I didn't measure the threads on the jet.  Does anyone know if these jets are available?  Installing a smaller one will make the mixture richer on hard deceleration.

I think Batman's spring mod probably has merit, but also imagine it would be easy to go a bit too far and end up with that #230 air bleed covered up under normal driving conditions.

I will test the spring and determine what the preload is with the TEV valve in the parked position.  Then can compare to diaphragm area and figure out how much vacuum is necessary to shuttle the valve.  I will also do an operational test with a vacuum gage & damper to determine average driving vacuum and deceleration vacuum.  We can get a better idea of where the spring should be set.

As I mentioned earlier, deceleration mixture can be made richer by installing a smaller air bleed jet (maybe a #40 or #35) if they are available.  I have noted that on earlier models (86-88 & 95) the illustrated parts breakdown only shows one air bleed in the dome (a #70) and the TEV valve is incorporated.  Those older models may have used a system where the TEV closed off ALL the air bleed.  Maybe I will try plugging the #45 entirely just to test it and see how it works.

Wish me luck.

Hey Batman, in your opinion, how did the additional vacuum port in the bottom of the slide work?  Do you feel that it improved the way the throttle responds to the twist grip?  Does it feel more connected?

BTW, I checked out the YouTube vids on Gadgetman's groove.  I am absolutely reluctant to turning a die grinder loose on that $525 carb.  Do you happen to have any drawings or sketches that show exactly how that groove is configured?  You know, depth, width, angles, etc.  Have you incorporated it into your carb?  If so, was it hard to do, did it work good?

Yes,  opening the vacuum ports made the throttle response seem much faster, but I caution anyone to go very slow when enlarging them ,you can easily make them to large , at the same time I trimmed the spring on the TEV valve to compensate for the faster slide movement, again going slowly 1/3 of a turn at a time . If you measure the TEV spring before cutting and go to far you could add a small washer to restore spring length.I have also done a mod to the bottom front edge of the slide facing the air filter slightly easing it's rather blunt edge ,hoping to increase air flow speed at part throttle. One might also ask themselves why the leading and following edges of the throttle plate and the shaft (and screws that seem to long)and the plate it's mounted to have such square edges ? and if eased might help produce a more laminar flow.I have also reduced the throttle return spring by one coil to lower wrist strain at steady highway speeds