I decided that it was time to install a new muffler on my 2004 Yamaha Grizzly, and with all the choices out there, HMF had a nice slip on assembly that seemed to be widely used, and suggested when surfing the web. Below is my review from start to finish, what it entailed, and an overall review of the installation procedure, tuning, and after thoughts.
First off, I was impressed with the packaging that HMF uses. The pipe and muffler were very well cushioned within the box. The hardware and instructions came in plastic packaging, along with a small aluminum or tin HMF marked box, which holds a few different size carburetor jets. Jetting is necessary for proper performance and warranty, as per their website. This was going to be the first time this quad was going to be jetted or have anything really done to it outside of oem.
The first thing I did was lay out my parts and grab the tools I needed. There was going to be 2 steps here, exhaust install and re-jetting the carb per HMF recommendations. So I decided to tackle the exhaust install first. I removed the plastic panels from both sides of the quad, removed the seat also, to expose the air box. The actual pipe and muffler looked to be an easy removal with a clamp on the engine pipe/header, and one frame bolt at the muffler. I proceeded to try and loosen the muffler bolt, and then it snapped! That set me back a little in time, but was an easy fix, as I tapped the remaining portion of the broken bolt out with a steel punch. I then removed the clamps from the header pipe, and slid the old exhaust off. The HMF pipe slides right into the muffler and all you need to do is attach the tension spring to keep them together as one unit. I slid the slip on unit in through the back and onto the engine pipe, then installed the clamps. I aligned it up and installed a new bolt and nut on the rear muffler frame bracket. That's it! Took less than 30 minutes (not counting the broken bolt) to install this slip on exhaust.
My next step was to jet the carb with the included 175 main jet as recommended here: HMF Specs . I loosened the carb clamps and a few bolts, then pulled the air box out of the way. I rotated the carb enough to expose the bowl without removing completely. (The proper way of course would be to remove the entire carb and take it to a bench..) I proceeded to take out the 4 carb bowl screws, and 2 out of 4 stripped out. I couldn't get them out. I finally was able to remove the stripped screws with the help of some needle nose vise grips. I ordered a new gasket and new screws. However, I took the advice of a fellow member (Dirt Demon), and bought some allen head screws from Home Depot to use instead. I removed the factory main jet and installed the one that came with the HMF kit. Worked out well.
I buttoned it all back together, and started it up. Wow, what a difference in sound. Much "throatier" and in my opinion sounds awesome. I don't have a dyno to test, but it definitely feels much more responsive and definitely has more torque. It sounds real nice, check out the before and after video I posted to see for yourself. I'm very impressed with the ease of installation and the actual look and sound of this muffler. I definitely recommend this exhaust. Maybe my next step will be a high flow air filter and new jet! HMF
Exhaust http://www.hmfracing.com You can check out my threads on this install in the forums.
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