900 MONSTER CARB : ANOTHER ONE OF BRAD’S BIKES

Due to my inability to control the urge to buy anything presented to me at a good price, and the fact that I can ride again, I now have a 900 Monster as well. The bike is an original 93 model, one of the very first that Moto Italiano, as we were then, sold new. It now has 40,000 km on the clock, as well as some evidence of normal wear and tear.

As purchased, the bike was fitted with a pair of STAINTUNE mufflers, DYNOJET jet kit, K&N filter and a cut open airbox lid. It was very, very loud, but didn’t go as well as I would have expected. The low speed rideability was also fairly atrocious. I found some old crash damaged standard mufflers up stairs to reduce the noise, but the low speed jerkiness drove me nuts so I put the whole thing back to standard. I wanted to check its power output anyway, given the kilometres.

The graph below shows the as purchased state of tune and the standard tune power. With 72 Hp in standard trim, the 40,000 km hadn’t taken away any of the performance. The red line is as purchased with STAINTUNE mufflers and the DYNOJET kit, the blue line is std mufflers and DYNOJET kit and the green line std all round. Fitting the std mufflers didn’t take very much power away, but they certainly blunted the response.

While in std trim, I took the chance to do a bit of a muffler comparison. The STAINTUNEs fitted are an early pair, with a baffle construction instead of the later ( and universially more common ) absorption core. We had some new MADAZ mufflers to try as well so on they went. The different styles show up with different curve shapes, which is also quite interesting. The graph below shows the MADAZ mufflers in red, the STAINTUNE blue and std green.

I rode the bike around for a few days in totally standard trim. At first it was nice and quiet and went well, but I got bored pretty quick. However, I went against my best advice here by fitting a second hand FACTORY jet kit I found at work. I normally use the FACTORY jet kits and have quite a bit of experience with them. The needles were quite marked, but I figured a polish would see them right. I did fit new needle jets, which I had the pleasure of replacing less than 150 km later due to my tight arsing. I also screwed around with lots of different main jets, eventually going back to the jets marked 160 that were in the bike when I bought it. On a positive note, I did dial the cams into the specs I like to use. The graph below shows the final spec of cams dialed, K&N filter, no airbox lid, s/h jet kit and MADAZ mufflers in red, MADAZ mufflers but otherwise std in blue and totally std in green. The power above 60 mph is less than I would expect, but this may have been main jet related.

Next I went a slightly different direction to that I would usually travel. I found a crash damaged exhaust that originally came from a 600 I think. It certainly wasn’t built for a 900. This is an ARROW full system with a 2 in, 2 out single muffler that sits on the RH side. ARROW TWIN TUBE is how I shall refer to it in the future, for want of a better term. To get it to fit the 900 I had slip joints put into the two header tubes to extend them as well as a 20 mm extension welded into the front header at the exhaust port. The slip joints will let it go back on a 600 if needed. The muffler had taken a bit of damage during the crash. Fixing this required filling up holes in the stainless end caps, rubbing down scratches in the carbon sleeve ( as well as rotating it so the inside is now out ) and making new end rivet bands and mount clamp. It probably absorbed more than 25 hours all up, but I spend my time far easier than my money. The clearcoat I used to make the carbon sleeve look nice again also seems to have a problem with the heat it has been subject to.

This system has the same size tube as the std exhaust headers, but no real crossover. There is a small balance pipe between the two header pipes, but this wouldn’t allow any real flow between them. This would be the most likely cause for the resultant curve below I feel. The graph below shows the same curve as the previous graph with the MADAZ mufflers in red. The ARROW TWIN TUBE system, fitted with no other mods to the bike, is green.

At this point I admitted defeat with my s/h jet kit. It had destroyed brand new needle jets in less than a tankfull of fuel and I couldn’t really get the main jets I wanted. So I did what I should have done a couple of weeks prior and bought a new jet kit. Although WOT is basically run on the main jets, the needles have some influence on the under 6,000 RPM range ( that I don’t really understand ). So I now had known good needles and most importantly of all, some 155 main jets. The graph below shows the result. The new jet kit curve is red, the old jet kit curve is green. The difference goes beyond dyno power and smooth delivery, and is very noticable in response. Back to very nice to ride.

An observation at this point though. I haven’t ridden one of these for any length of time for a long time. The last 2 valve I spent lots of time on was the 900 SS IE that is featured on this site, which is quite different in engine character. You really notice the low RPM of the power peak on this bike, mainly because it drops the front wheel down on a power wheelie before you expect it to. The SS would just wheelie into the rev limiter, with the 900M it’s all over by around 7,500 RPM or so.

The next curve compares my current spec of the ARROW TWIN TUBE system with the best of the previous MADAZ spec and the as purchased STAINTUNE/ DYNOJET kit spec. The ARROW TWIN TUBE is red, the MADAZ green and the STAINTUNE blue. Getting back the top end will be the aim of the next mods, which will be set out in another page ( this one is now a little full up ). The effect of our cam dialing can be seen in the drop off of the green and blue curves. With the cams dialed as we do ( green ), the power does drop off quicker. The increase in power and response below the peak power RPM more than make up for it, however, in my opinion.

Finally, a comparison of how far we have come to now. Compared to the std spec, the current ARROW spec looks pretty good in the graph below. ARROW TWIN TUBE is red, std green. We just need to pick up some higher end power.

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