I have a TY175 that has a delay before returning to a slow idle.  When cutting the throttle, it will stay at a high idle for 3 to 5 seconds before returning to a slow idle.  Just guessing... but if a slow idle is 800 rpm, the fast is maybe 1300.  Besides that it runs fine.  Is that normal or to be expected?  I have cleaned the carb multiple times, checked the reeds and gone from a 30 to 32 and then 35 pilot jet but it is much the same.  I tried spraying WD40 all around the intake system to see if there was a leak.  It is mostly stock with dual stage reeds, fmf muffler and a little weight removed from the flywheel.  Is there something else to look for?  IIRC 4 strokes can do this when rich and 2 strokes when lean.  Is that correct?      

Could indicate air ingress and if so, most likely the magneto side crank seal.

Could also indicate a sticky throttle cable or slide.

In neutral the revs should go straight to idle RPM when the twistgrip is released.

First thing is to replace the O rings in the carburettor float bowl, you will see them when you remove the float bowl. 

Can the magneto side seal be replaced form the outside?  Are there any tricks or advice?  I did replace all of the seals in the carb.  Someone sells a kit.  

We have exactly the same problem, crank seals replaced cable free and same fault with two different carbs one sonically cleaned with new seals the other off a working bike. New reeds & as yourself tested for leaks.

I read that a lean needle can cause this so I looked at the carb again.  The needle did have a very slight amount of up and down play in the slide, maybe 0.010 inches.  I have not seen this on other carbs but don't know that I would notice.   I raised the needle one position to full rich and added a small spacer on top of the clip to get rid of the play.  The hanging idle seems gone or at least greatly reduced.      

The raised needle masked the problem for just one ride.  I pulled the point plate and it is quite dirty near the seal but not really wet.  

Will a leaky mag side crank seal / air leak cause a bog in throttle response right at the bottom? My 1975 TY175B has a nasty bog right at the bottom as in an off camber up hill first gear low RPM situation. New carb, re jetted, pilot, needle and main just to cover every aspect. DT175 for a little more compression (problem?) new piston and rings, modified air box, indexed spark plug, modified header, mid box and muffler. Crank seals are my last option! Well probably not but...

Thanks in advance.

9 hours ago, HOT101ROD said:

Will a leaky mag side crank seal / air leak cause a bog in throttle response right at the bottom? My 1975 TY175B has a nasty bog right at the bottom as in an off camber up hill first gear low RPM situation. New carb, re jetted, pilot, needle and main just to cover every aspect. DT175 for a little more compression (problem?) new piston and rings, modified air box, indexed spark plug, modified header, mid box and muffler. Crank seals are my last option! Well probably not but...

Thanks in advance.

Left crank seal leaking is one thing of many things that could cause that.

First thing to check is if the idle air screw has been adjusted correctly. Other possible causes include pilot jet restricted or pilot circuit restricted.