Comparing Performance of mixed Evo and Pre-Evo Rotax Max Parts


(Dom Callan) #41

Just a thought… and maybe I am naive since I just do club stuff… maybe you can talk to the guy and ask?

(Marin Vujcich) #42

Chris, Your torque down low looks real similar. So if this is the case you make have to drive the corner different or free the rear of the kart on exit a bit.
Just make sure you are not comparing your data to a “freak” engine, that you may never get near in regards to power.

If it is any consolation, mine is similar to the other blokes, but not quite as peaky but it is an evo. If you are confident you have explored all the fuel tuning tricks outlined, the one thing I would try is raising the barrel by 0.2mm and see what that does.

Hope this helps

(Christopher Ramnauth) #43

Marin, I’m gonna run the same gearing as them and then go as lean on the main jet that I can until it pops, then go back up one. I’ll see what the stop watch says as well as the data.

I am not going to adjust my float height to match the other guys though…3.5mm leaner than where I’m at is just insane in my books. Where mine is set is very close to what John Savage out of the UK recommended for this pre-evo carbs. Again, no clue how what he has is working for him.

One last thing Marin, can you recall how many clicks out from fully in you usually ran the power valve?


(Marin Vujcich) #44

13 clicks I ran, which is pretty much the base setting.
Your power graph looks really good through the transition, so I would say you have it spot on.

(Christopher Ramnauth) #45

Gotcha, will let you know how it goes…thank you for all your help.

(Christopher Ramnauth) #46

Guys I thought I’d put some closure to this thread after figuring the problem out with my engine. Lots of threads have really good info but never follow up with any fixes that solve the problem.

So to recap, the problem was that when I compared data from myself vs another driver on same track using the engine rpm analysis feature which basically calculates hp and tq on track, my engine seemed to be missing a heap of up top power compared to the other guy. Also my jetting was quite a couple jets richer than him and I could not go leaner else my engine would pop.

So now on to the fix. I’ve been heavily involved in rotax over the last 4 years and I’ve done tons and tons of research learning about these engines but no on ever told me to check what I’m about to tell you guys. Basically the problem with the engine was the PV was not opening 100%. This has nothing to do with the red adjuster or improper PV operation, everything was spot on but I never physically checked to see if the PV was opening and closing 100%. To check this you need to remove the exhaust and look in through the exhaust port and operate the PV in both positions by hand. In the closed position you should hear it hit the metal stops and be right up against the piston skirt and when released and fully open the valve should not be seen at all protruding into the exhaust port. It should be flush against the top of the cylinder head giving the exhaust port full opening. Mine was not, when in the open position piece of the PV was still partially in the exhaust port. Essentially I was running with a restrictor on my engine.

The error that was made was following instructions from people blindly. I was always told to set the PV length to 75mm for a pre-evo senior, but this length is what governs if the valve is fully closed or opened. The confusion I think comes from rotax having 3 different length PV studs over the years, this essentially means to get your particular valve to work the way it is supposed to you need to experiment and find that optimal length by physical inspection as I outlined above.
I reckon the PV not opening 100% was the reason the engine would not take leaner jetting up top as well.

One other important thing to note is that the engine rpm analysis feature is a very accurate and powerful tool if used properly. If not for this I’d have never known my engine want making maximum power while on track.

(Lee Swindell) #47

Interesting saga Chris.

  1. How did you fare running the taller gears? Until you diagnosed this Id been wondering whether you were burning up your acceleration advantage in wheelspin.

  2. The engine analysis tool you refer to - is that a function in Race Studio?

(Christopher Ramnauth) #48

Lee, running the shorter gearing I wasn’t too badly off, I think if I’d have been getting maximum power up top I’d have been really close to them. Initially I thought the reason they were quicker on the last sector of the straight away was due to the gearing solely but now I’m not so sure. There was no excessive wheel spin.

Yes the engine rpm analysis is a feature on race studio, it is dependent on driver a bit so if you put a really fast driver and a slow driver to drive the same kart it’s gonna calculate the hp/tq a bit differently. But once you’re pretty much close to the competition it can be a useful tool to see if they have an edge on you in terms of all out motor performance…that’s if you can convince anyone else to share their data lol

(Christopher Ramnauth) #49

I should also add, adjusting g the lengtth of the blade assembly did not fix the problem. I wrote the post above almost certain that it would but when I actually got down to doing I realized that even shortening the assembly did not stop the valve from going fully recessed into the PV slot on the cylinder head. I had two engines side by side , one the valve assembly was functioning 100% correctly and then my engine was the other. I swapped the valve assemblies from one engine from the next and it did not change the issue on my engine. It seems that there is something off with the actual geometry of my cylinder head…it’s a junk head essentially that was palmed off unfortunately on me. I guess this is why people always say on older rotax engines there is a big amount of disparity between the engines

(Liam Sergeant) #50

What year is your barrel?

(Christopher Ramnauth) #51

It’s either an 09 or 1999 I am unsure.

(Liam Sergeant) #52

Oh interesting. The 09 is supposed to be one of the very good Pre-Evo years. I don’t know what the differences were, but I’ve been told they were highly desirable before they started using CNC technology/producing the Evo barrels.

(Christopher Ramnauth) #53

Yeah so I’ve heard myself but I can’t see what else could be making mine not work properly aside from a flaw itself in the cylinder head. As I said the valve assembly that was working 100% on the other engine still didn’t with mine. The only way I could get it to work properly was to leave the o-ring out and allow the valve more movement. Other than that it would require some machining on the PV metal backing to remove some material which would then render the motor illegal.