Just got my son’s Mini Rok motor back on after he ran 206 last season. We last ran it in December of 2023, where it was running really well. It has the older Dellorto carb.
Before getting it mounted, I cleaned the carb pretty well with air (ran out of carb cleaner), and added a new plug. Since mounting it, I’ve been chasing issues with it for the past few weekends at the track.
It runs pretty well down low and in the mid-range, but it falls flat up top. It sounds really quiet and clearly off and it’s really low on top speed (hitting 48-49mph on our track instead of 54-55+mph).
I’ve played with the idle adjustment and fuel mixture screws, setting them both at baseline and since adjusting them. I’ve pulled the head and inspected the piston and cylinder. Both look pretty good - no crazy scoring on the cylinder.
CHT is barely hitting 200 degrees, which seems really low, so we may be running rich. I leaned the fuel mixture screw pretty dramatically and was only seeing a ~10 degree improvement in heat. I’ve also adjusted the throttle needle to different lean/rich settings from baseline, with no improvement.
At this point, I’m wondering if it’s a carburetion issue or a top end issue. I haven’t replaced the top end since owning the engine, which probably has ~12-15hrs on it. That could be a part of the issue, but it was running so well before it sat, and now there’s such a steep drop-off.
I’m looking at a couple of options -
Clean carb out thoroughly, with carb cleaner
Do a compression and/or leak down test (any recommendations on a decent compression tester?)
Replace piston and ring
Ditch the old carb and spring for the new Tillotson carb
I’m willing to do each of these, but I’m hoping that someone in the KP community has experienced this and can point me in the right direction so that I can shortcut the fix and get my son running sooner rather than later.
Thanks Simone. The interesting thing is that I’ve ran this same jet in the past and it ran well. Perhaps it points to a carburetion issue in general, that a better cleaning or a fresh carb could fix.
The first time we ran this engine after it sat, I made the mistake of putting washers in between the exhaust and the cylinder. This created exhaust leak and my latest hunch is that the heat/dissipation of the exhaust/loss of compression compromised the base gasket.
I’ve got a few different sizes of gaskets coming in tomorrow.
I’ll get it assembled and hopefully out to the track this weekend.
It does not look comprimised to me, except that it broke When you tolk off the cylinder.
I would look at the fuel pump. Clean the carb, install a new sparkplug and try out different jetting.
Remeber when you are changing gakkets, you also changing port timing and squish, both things needs to be within spec according to the rules.
If you are In doubt if it needs a new piston, then get the cylinder honed and a new piston installed, at the sametime get the shop to check the engine over.
Thanks @Bai7200 . Yes, I’m aware that it split when I took the cylinder off. What I’m referring to is the seepage from the back of the cylinder/exhaust area that resulted in all of the fuel/oil residue.
I measured the old gasket and bought a few in that size, as well as a few of a different size in case I needed to adjust squish.
Carb has been cleaned/rebuilt multiple times over (and I just cleaned & rebuilt it again since the last track day), the fuel pump has less than an hour on it from late in 2023 when we last ran the motor (it was the cause of the last issue). With the current jetting, the motor was running strong the last time we ran it.
Piston/ring/cylinder honing will be next up if the new gasket doesn’t fix it.
New base gasket, carb rebuild, and fresh plug and still having the same issue. Motor is strong down low and in the mid range, but falls off up top.
I played with air/fuel mixture quite a bit in both directions, adjusted the throttle needle in both directions, and adjusted the idle screw a bit. All in this same track day. No real changes. Can’t tell if it’s a carburetion issue (carb is shot) or if bad compression is the culprit.
At this point I’m going to buy a new carburetor and probably freshen the top end.
Let me know if any of you have experienced this and can help pinpoint the issue.
It looks like its smoking, are you running the Dellorto carb? If you are you really should look into changing the main j’et Down a bit. And check that the neddelvalve (inlet valve) is working.
Yes on the Dellorto. I’ll look into main jet options. But again, I’ve run that same carb/jet and it ran strong before. Do you think the main jet got fouled/compromised?
We had this setup when the engine was on another chassis. But with this new chassis we opted against it since the tank didn’t have another line to route to.
Right now the fuel tank has the main line out and the other line goes to a catch can. If we reinstall the y-line, do we need to tap another line to the tank? Or where would we run it?
Needle inlet valve was seated properly, but what @Ed_H mentioned about the y-splitter got me thinking.
We ran a y-splitter when this motor was on another kart. That kart’s fuel tank had 3 fittings - one for the fuel line, one for the catch can, and one for the return. When we mounted this motor on the new Tony chassis, the tank only had fittings for the fuel line and the catch line. So we opted to run without the y-connector & fuel return line. My latest hunch is that the lack of the return line is leaving it fuel-starved at high-throttle.
I just tried to jerry rig it by using the catch fitting on the tank for the return line from the y-connector. This bypassed the catch line. The kart started up easier than it has recently and ran really well on the stand. Only issue was that the catch line fitting is designed more as a breather and doesn’t have metal barbs, so it was leaking fuel. I wanted to test and see if this fuel return line fixed the issue before tapping another fitting onto the tank.
This brings me to my next question for all those who have ran Mini Rok recently - Does the new Tillotson carb require a fuel return line? If so, are they all tapping another line into the fuel tank? If it doesn’t, I’d rather buy the new carburetor (which I’ll need to race anyway) and save tapping another fitting into the tank.
Had the same issue on my son’s cadet. I added the third line into the tank with a barbed pipe fitting. I recall with the right size drill the fitting self taps fairly easy into either the top of the tank or the fuel cap.
