Hi…
I want to setup my Carb… but because I am new in this sport i couldnt take risk and do it right away
Does any body use the application of “Jetting Rok”?
With this setup am I going to risk my engine?
I did my setup on that application but i didnt check it on the grid… !
Engine is Vortex Rok Gp
Carb is Dellorto vhsh30
Weather : 30 degreas celcius
Humidity : %60
Grid has some medium straights
1 tour of grid is 1200 meters
So I thought I would resurrect this topic as it seemed relative to my current situation. I bought a gently used Rok GP about a year ago coming from an Iame Leopard MY09.
On the Iame with the Tillotson Pumper carb, I could adjust the jets on the fly which made it easy to make changes as needed. However the GP uses the Dellorto Slide carb that you have to change the physical jet screw at the bottom of the float bowl. It seems by the rules you are only allowed to change the main jet and not any of the secondary jets.
I picked up an EGT sensor when I bought this engine knowing I would need some data points to know if I was moving in the right direction with jet changes. The carb came with a 130 main jet installed and I decided to order four more to have some range (two lower and two higher) to play with. Having no idea what I actually needed, I went out to practice before the race and saw my EGT’s were just under 900F peaks. It was relatively cold out with a strong steady breeze. My Relative Air Density Meter was reading around 94% for most of the day. Having run the same jet last summer and hitting readings above 900F, I decided to go up a jet to the 132. Went out again and no noticeable change in EGT temps. If anything, they went down slightly. So, I went the other way. Again, no noticeable change to EGT temps.
At this point I was getting frustrated. I downloaded the Jetting Rok app listed above and input my specs for engine model, fuel type and oil mix. It then pulled local atmospheric data from the web and return a suggested jet value. To my amazement it called for a 145 main jet. At my last race, the local team mechanic suggested I would only need 2 or 3 jets. This app was now telling me to go up 6 jet sizes from what I originally stated with. I also did not have that. The best I could do was throw in the largest jet I had and raise the needle position to its highest setting and hope I didn’t run it too lean to cause damage.
I ran that setup for the prefinal and the final. The engine did end up revving higher than before and felt like it was making good power, but the EGT temps never really went up by more than 10 or 20*F. Was I just so far off the mark that they could never get close to target? Being new to using and EGT sensor, do the values increase or decrease in a linear function or is it more exponential? I am in North Texas and with the seasons changing, it is not uncommon to be in the upper 30’s one day and in the 80’s (F) the next day. What range of Jets should I be keeping in the tool box to cover both the colder days and the extreme summer temps over 100F ?
In my experience the jetting apps are always very rich. For the cold 30F days you might want to swap out to a BR9 or similar heat range as it’s a hotter plug. For anything over 50F and over a BR10 should be good. It’s been a few years since I’ve run the GP, but I recall the jets being much leaner 130 and below (for my area anyhow). The inner pilot you can change to a B45 and that’s it, definitely recommend doing so if you have a CD1.
Thanks Ricky, I will check which pilot jet I have in there. I can understand the app running a little on the rich side to error on the side of caution. I thought the 145 was way too big, but with the 135 and needle all the way up, it was making better power and I was able to get closer to the rev limiter. So, maybe it was closer to the 145 jet than the 130 jet.
Another thing to consider, some run a bypass on the carburetor and some don’t. It can change the jet you’re using quite a bit. Pump rebuilds are something to keep up on also, if you start losing top end randomly you may have a fuel pump getting weak.
On the Tillotson I would change the diaphragms on carb every six months of running. On the Dellorto, all I have done is clean it, spray it out with compressed air and put it back together. How often would you suggest I tear the pump down and replace the diaphragms? Same as I did on the Tillotson? I will order a set when I order some more jets.
I don’t run a bypass (assuming you mean a “pump-around” system). I have only really seen them used on shifters. Not sure what the advantage would be on the GP. In what way does it affect the jetting? Is the same amount of fuel not entering the float bowl that the floats allow in either configuration?
You should be able to run about the same frequency of rebuilds. It’s the diaphragms and little check valves. If you run without a bypass (which I recommend) then you have to run smaller main jets. If I recall correctly 1050-1100 are rough numbers for good egt. You can run lean on a gp without too much trouble, if you’re too lean usually it will fall off on the top end.
