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Discussion Starter · #1 ·
My mito evo 96 has mikuni tm35, and standard pilot jet there is size 45, however it was running insanely rich - even with air screw fully out. Trouble starting, completely wet spark plug, juices coming out of the exhaust.
The cylinder I had at the time (now seized and destroyed, oops) was unmarked mitaka(i guess it is a0401, will try to check today), but from the factory the bike had 73037 cylinder(at least the manual says 130 transfer and 190 exhaust). Everything else afaik is standard from the factory.

So I got some jets, and re-jetted it with size 20 pilot and upped main jet from 310 to 350(standard mikuni jetting for tm35), to keep the total fuel flow unchanged. Aaand it seized after running wide open throttle for quite a bit and closing the throttle fully before a turn.
I suspect that this happened because I was running too lean pilot jet, causing detonation (there is a small molten spot on the piston). Contributing factor might be tight clearance between piston and cylinder (yes, rookie mistake to assume that cylinder kit comes with matched piston).

I get the idea behind rich pilot, it is exactly for extra cooling when going from full throttle to closed throttle, however now I dunno if it was designed to idle like crap.

Now I am thinking that port timings (124 degrees for a0401 and 130 for 73037) affect air velocities - same amount of air is to be delivered in a shorter time - increasing air flow through the carb.

I will get my old 73037 replated and new vertex piston, see how it goes.

So, question:
Why would cagiva use such a rich pilot jet, that can not (in my case) be leaned out with air screw? What might be the reason for this rich situation?

Pics here: https://photos.app.goo.gl/7rWuxXodtZir6F4y7
 

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Discussion Starter · #3 ·
As i saw photos the problem is from piston quality. Usually Mitaka cylinder is ok but pistons coming together are crap. If it's this type that's one of the reason .
Also small pilot can do this but it needs time and not at the time you are closing throttle next minute seize.
Always companies tune richer all high performance 2 strokes bikes, just for their engines to be accurate and have longability before failure.

Btw check carb specially air chocke, if when is close has small flow then it will be like you have rich mixture at low throttle opening or idle
Thanks mate! Getting my 73037 recoated and vertex piston to go with it :] hopefully no more destruction and pain.
Do you maybe know if (like on probably most automotive carbs) there should be a gap (and how big) between throttle slide and carb body? Should the bike even start with say fully closed slide?
 

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Discussion Starter · #5 ·
Summink seriously wrong with the Engine or/and setups..... the Evo1 Tm35 can run even #300 main jet. "Equimania" even made the landspeed record in 125 Stock Class with his '95 Evo1 at Saltlake hotness.

#310 in northern Scandinavia tends to be on Rich side. but Aftermarket TMX35 #350 size would be an overkill.... about same viceversa issue compering idle jet #20 into #45.

that Pilot #20 and closing from WOT killed it, but im seriously wondering an deeper issue like "AirLeak" on intake manifold/crank seals...No matter if it doesnt Hang Rev's/idle Rev's for a while as a 1st Symphtom

Throttle slide's "CutAway" depends on Carb specs (richer or leaner function.... more or less Vacume needed for idle jet side.....".
iirc "Dellorto basics principals" carb pdf book explains this and shows some photos etc...linked on this forum too....

Engine starting with or without fully closed throttle depends various matters, like is it cold engine, is it running temp, using or not using Choke circuit. etc

if the slide has a "Medium" or smaller "CutAway" , healthy engine should start with closed throttle with using Choke circuit, from cold stand still, sea level ~+20Celsius. with clean good plug and strong sparg etc Healthy specs.

some 2T carbs at least from dellorto moped carbs, iirc, do not have slide "CutAway"....
Thanks!
The bike I have is was abused a lot in the past, however I have fully rebuilt the engine, new crankcase gaskets should be tight, will check anyway. I will double check for leaks on the intake, and with new cylinder kit definetely will go first with factory spec jetting.
Carb slide I have now if I recall correctly is 6.0, and factory should be 5.5, I guess should be fine.

With air leaks isn't it supposed to run leaner?
 

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Discussion Starter · #7 ·
Yes, any CrankSeal or intake manifold airleak will cause running Lean. on Cagiva's the CrankCase pressure is known to sometimes PopOut crank oil seal at Magneto side.
Thanks! I believe there is a stopper/holder for the seals, but anyway, how do I check if the seal is in place? Preferably without splitting the engine :)
 

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Discussion Starter · #8 ·
Quick update:

Fitted 73037 cylinder, standard jetting/settings, 5.5 slide instead of 6.0 - idles almost perfectly. My guess is that mikuni + any other cylinder than 73037 will run rich :] Thanks everyone!

Did not ride it yet, the bike still has to be fully assembled, but I guess it will be fine.
 
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