Carbs and Dynojet question

An update. We pulled my old 900 carbs apart and transplanted the relevant bits in to the newer 600 set. Here's couple of oddities.

The 900 carbs were found yet again to be containing what looks like real find sand in the float bowls. We put it down to rust/oxidisation coming fom somewhere within the carbs. Not surprising considering they bike was stood around for several years. We've rebuilt those carbs now as a fully functioning set of 600 complete with Dynojet kit, but I'm reluctant to sell them on as the corrosion problem means I'll probably be causing problems for the next owner. They spent several hours in a sonic bath as part of the rebuild last year but there's something not quite right.

Here's the real strange one though. Last year when I started this project a new stage 2 Dynojet kit was purchased and fitted. That kit is now in my "new" set of 600 carbs, along with all the other relevant parts needed - thanks for the chart above Al, that helped identify all the parts needed. However on close examination of the parts we found that the kit supplied had some parts from a 600 kit and some from a 900. That kit was purchased new last year. Spookily strange was that the kit fitted in the 600 carbs also had the same mix of parts. Both kits were purchased separately new, 2 years apart. I know this to be correct as Louigi Moto fitted the kit to the 600 2 years before I bought the 900. There is no way that they have managed to mix the 2 kits up. The two carbs were stripped next to each other on a bench and that was the first time that they had been apart next to each other. There was no way that they had "cross-contaminated" each other. It was actually quite fortunate that both kits were incorrect as it meant we could make a complete 600 and 900 out of the two mismatched sets. Somebody at DJ is picking the wrong parts when putting the kits together. The mismatched part was the needle valve nozzle.

The carbs have been fitted by me and fired up briefly with the original carbon fully open race cans. Without adjusting mixtures I managed to get a reasonalbe tickover. I didn't spend too much time on it as the goal is to run the bike with cored original cans, not quite as open as the race cans. It didn't run as well with the cored cans on, but it really needs a proper tune up now and some dyno time to get it right. That'll come soon I hope.

 

So my next question. Jet sizes and kits.

The dynojet US and UK web sites list 3 stage 2 kits for the 900 motor

7201 for the 91-98 900SS
7206 for the 94-97 900 Monster
7209 for the 98-99 900 Monster

The only difference between the two Monster kits is a small variance on the fuel needle size , 729 vs 730, presumably due to the smaller valves fitted to the later models.

However, there is a huge difference between the jets provided for the SS and Monster.

The 7201 SS kit contains 140, 144, 150 jets
The 7206 and 7209 Monster kits contain 165, 170, 175 jets

So why the difference between the two models? It's exactly the same engine isn't it (other than the tacho drive on the horizontal cylinder)? We have used a 94 Monster motor with the 91 SS barrels, pistons, and heads.

Which set of jets should I be using as a starting point?

Can anyone enlighten me as to what kit, and jet sizes, they've used in their 900 set up.

Thanks

 

I've got that link thanks, that's where I got my size info from. So, yes 150 is the top end of what comes in the SS kit, so why does the Monster kit run from 165-175?

I've got 165 in there at the mo. Rich has got all the jets so we can mess around on a dyno. I'm just interested in why the different sizes, and wondering what other folks found works best for them.

 

Ok, final update. We started off with the 165 jets which were the smallest jets from a Monster kit. Way too big. 14% over fuelled.

So we dropped the jets down to 140 which is the smallest set from the 900SS kit, set the eclip at position 4 and got good results. The bike is crisp and snappy and sounds great. That's running open air box, cored stock cans, blueprinted stock engine.

Here's a chart with some lines and numbers for the geeks.

 

That was the 140 jets but the needle on position 3, which is the recommended position with the 165 jets on the monster setup. Ive done a bit of digging and can find no evidence that the Monster engine is any different than the SS. I can only assume that the Monster exhaust system makes a difference, but I really cant see it needing that much fuel. I found with my 600SS DD bike that the smallest jets supplied were actually the best option when put on a dyno. I wonder how many people just stick the biggest jets in believing that it will give more power. The best way to get set up is certainly to grt the thing on a dyno.

 

All sizes are for dj jets. I reckon 150 is too big for a stock motor with race cans. As you can see, with 140 fitted we gained an extra hp and ft/lb by dropping a needle position, even though the needle isnt supposed to affect top end. I reckon a dyno set up is the only way anyone will ever know what its truly doing. Anything else is just guesswork and gut feel. Sometimes people can misinterpret a change as an improvement when its really not, they just notice that its different and assume thats a good thing.

I suspect we could have got away with 144s and position 3 as there was a slight lean spot on half throttle at 6500, but I dont ride half throttle at 6500 so we chose to live with it. Ill get it out over the weekend if I can get it motd. the real test will be how it actually drives on the road.