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Discussion Starter · #1 · (Edited)
Hey everyone, sorry for the long winded post; I want to give all the information I have on this problem. I recently purchased a used 2019 Grom with 2,700 miles on it. It was 100% stock except for a Ransoto full exhaust that still had the baffle in it. I bought this with the intention of stunt riding and riding to work so I knew I would immediately tear into it. I changed the oil and rode it for a few days to make sure everything was in working order. I also checked the spark plug and it looked to be close to "optimal" based on some pictures online. The bike would idle as smooth as a single cylinder would at 1800 rpm. Nothing out of the ordinary.

Here are the following mods performed:
  • DHM Stage 1 tune
  • Chimera Intake w/K&N filter
  • Remove baffle from exhaust (per DHM instructions)
  • Install external idle adjuster (had to remove throttle body from manifold but o-ring appeared to be in great condition and I properly torqued on reinstall)
  • Reset ECU (per DHM instructions)
As far as installation goes, if there's a torque spec in the manual, I use it. I'm pretty OCD about that stuff.

Per DHM instructions, I reset the ECU. I have ridden about 250 miles on it and I am having some odd idling issues. Sometimes the bike will bog down at idle and you will hear popping from the intake and the bike will stall. Other times the idle surges back and forth from 2.5k to 3.5k. Occasionally I'll get the intake pop when getting back on the gas after stopping. Sometimes the bike seems to run fine. While on the move, the bike runs fine.

For the idle adjuster, I started with making sure it protruded past the throttle body housing the same as the OEM screw. Like is said, sometimes the bike runs fine but then it can bog and die. For now, I just have the idle set to around 3k which seems to keep the bike running pretty well aside from occasionally surging.

Last night I decided to pull the plastics off, double check vaccum lines, intake couplers, exhaust header flange, etc. I reset the ECU again but this time also reset the TPS (which I didn't do originally). I also ordered a new spark plug since the original, although the original looked "optimal", the outside was looking like it had some miles on it. I haven't tried the bike again yet. I'm waiting to pick up the spark plug from the local auto store around 1030 this morning.

The only other things that I know to try if there is no improvement is replace the throttle body to manifold gasket just for peace of mind and check/adjust valves.
 

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What is the external idle speed adjuster?

I'm guessing one of the aftermarket parts is throwing off your mixture.

3k rpm idle is very high. Is that for stunt riding?

Have you adjusted your throttle body air screw with stock idle speed settings?
 

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Discussion Starter · #3 · (Edited)
The idle adjuster is basically a idle screw that doesn't have a lock nut. It's held in place with spring tension so it doesn't back out. I measured how far the OEM adjuster protruded past the throttle body housing and matched that with the new adjuster. So at the moment I'm not running a high idle for stunt riding. For the sake of diagnosing the idle issue, I would bet it's unrelated except for the fact that I had to remove the throttle body to install.

I haven't touched the air mixture screw yet. Trying to rule out the things I'm familiar with first.
 
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