Working on a friends Pilot and could use some advice. The problem was that the Engine would not fully rev out, ran ok at low revs but would not respond to full throttle. Would not bog but just wouldn't rev. Went through the carb a good 4 times cleaning and checking jets with no sucess. Fuel pump tested and is good, lines are good etc. Finally tried running it with the airbox lid off and presto, runs extremely strong all the way through. Filter looked OK but I installed a New OEM filter anyway. No obstructions in the upper snorkel and air scoop, but even with a New filter, the Engine will not rev with the airbox lid on, but runs fantastic with the lid removed. Scratching my head on this one, anyone been through this before? Thanks for any replies...

Check the compression..

Good spark?

Smoking more than normal?

Mouse nest in exhaust?

Fairly recent fresh rebuild on the Engine and was previously running fine, spark is strong, no excessive smoke. Have not checked exhaust internals. I don't understand how that might be a factor as it runs Great with the airbox lid off. Could the airbox really be that restrictive?

Possible that the choke cable is being pulled on a bit or binding internally not releasing the choke. Someone put a fatter jet in after the rebuild to help prevent a lean seizure on a new Engine or did someone put a fatter jet to compensate for leaking crank seals on the previous Engine and forget to put the correct one back in? What is the main jet that is in it now?

Appreciate the replies. Choke cable is free and functional. Running 140 main and 52 slow jet... Stock jets I think.

a plugged up fuel filter is possible

Fuel fump is outputing over 50cc at idle in 10 seconds, which is per service manual spec. Fuel filter is prior to fuel pump, so an obstruction at the filter should be reflected in fuel pump output.

Maybe the airbox is restrictive enough to make the mixture way rich?

You could pull the choke with the airbox lid on... to see if the revving is any different. If not you know you are too rich.

A couple plug chops with and without the lid might give some info.

Anything in fuel delivery (before the carb), exhaust, compression, or ignition should not be affected by running with or without a lid.

Have you run with and without the lid multiple times so you know it was not a coincidence that it ran better without the lid?... and it is not related to something completely different like the Engine being cold or warmed up?

Compression is?

Reeds ok?

Have yet to run an actual compression check, might have a gauge here might not. And yes, New Honda reeds were installed in the midst of the 4 carb R&Rs, mainly because they were on-hand and wanted to eliminate the possibility.

During the last 2 test rides, I hopped out in the middle, while the Engine was running and subsequently re-installed and removed the lid and got underway. The difference was immediate. Putting the lid back the other way would then again reproduce the other result. I expected to find a tennis ball or something stuck in the air intake, but it is free and clean. Should be something obvious but I've double-checked all the basics a couple times now.

Mine would start and idle fine even go up until 1/4 throttle but blubber around over that point.
I took the filter off and back washed it into a paper towel. It was then clear the filter was half full of rust dust from the tank.

Please share more info on parts installed. stock pipe and silencer, plug gap and #. If you went one day with lid on same riding location ran fine to next day wont run unless lid is off. Do compression check before any thing else then start on carb. This is why it's important to get base line #'s pre and post ride. save you some time on chasing problem. If you remove the intake boot after running measure the blow back strenght in intake boot and shine light in reed cage or just remove it and check reeds. It is interesting aint it.

Interesting it is. This is a Bone stock 89 Pilot. We have a 90 Pilot also, no issues with it other than it occasionally leaks fuel out the overflow when running. Both are stock jets, stock pipes, no Engine mods whatsoever. I even grabbed a spare carb and installed it on the problematic 89. Exactly the same situation. Runs good with the airbox lid off. We are up at a Thanksgiving get-together at a second home. The Pilots stay here so don't have constant access to the machine, and I don't have a compression gauge available here. Before I tried running the Pilot with the lid off, I was suspecting CDI (Capacitor Discharge Ignition), as all other aspects had been addressed. But with the 89 Pilot ripping along *strong* with the lid off, the CDI (Capacitor Discharge Ignition) concerns are not applicable. Plug reads lean if anything, not rich. Raised the needle to 3rd groove, runs fine but did not fix the problem.

Change the plug?

http://www.pilotodyssey.com/PO/viewtopic.php?t=4910

Did your change of location( second home) have an elevation change as well.He said "We are up at a Thanksgiving get-together at a second home". The 89 and 90 carbs are diff. The 89 is more supseptical to elevation change than the 90. A air screw, neddle andjustment and mian jet change is more sensitive than the 90. As well as outside air temp. If you change location often get a 90 carb less adjustments. The other thing would be to change to the 90 intake ( reed assembly and reeds)as well and this should solve issue if you don't like doing the work which can be done at pre ride. I bet "H" has a pic of carb setting to show this If you do not have book showing adjustments.