I think you are one of the only guys on here running a similar setup on your trailblazer.I have the 300 that you gave me some jetting advice about a while ago. It has the 34mm scram carb with a UFO on it also.I am having a problem with it running lean at about 1/8 throttle but it is alright at idle and anywhere above 1/8 it pulls like a freight train.If I am cruising along when the bike is totally warmed up, if I let off the throttle to coast, the bike will run on sometimes like a 2 stroke does when its running out of gas.I know its leaning out but I cant figure out why.Also when it starts to run on , it will detonate too.If i give it more throttle it will come out of it.My EGT guage looks good while its doing it though.I would think it would go out of sight if it is leaning out enough to detonate, but it won't go above 1100.My carb setup is as follows:
22.5 pilot jet
needle in 2nd groove with .020 shim.
230 main jet
2.0 UFO cutaway.

This bike is so sensitive to the needle position that if I raise the clip to the 3rd groove the bike is too rich right across the range.If I take the shim out it is way too lean on the 2nd groove alone.

The strange thing is too , the bike does it a lot worse as it gets hotter and the air temp gets lower.When the bike is just warming up for the first few minutes it works great but the warmer the engine gets , the worse it does it. I was out yesterday afternoon and the air temp was about 32F and the bike worked excellent with almost no detonation. And then last evening the temp dropped a few degrees and the bike started to lean out and run on.I have replaced the cerb vent lines and a new high flow fuel filter too.

I think a richer cutaway would cure it but the 2.0 is the richest one that comes in the kit. I even talked to Lonn Peterson at Thunder Products and he was no help at all.He said that with my jetting the bike shoud be very rich.Well it isn't.I have also replaced the crank seal on the mag end and it didn't help either.I am going to check the pto one next.

If you have any ideas please feel free to give me them.

 

sparkys,

I know exactlly what you're talking about. Yours is running richer than mine is, but then you motor is bigger too. You would need a richer UFO, but they don't make them. Try adjusting the air screw and lowering the idle a little. Also try going to a bigger pilot jet. Maybe a 25 might be rich enough. The hot start problem occurs in mine too. It takes a while of cranking to get it going when warm, but starts up easily when cold. As far as the run on goes, mine does that too. It can be annoying at times, but if the EGT isn't getting above 1100 degrees you should be fine.

The upper jet needle diameter could be the problem too, not to mention the needle jet diameter. Still, if it were me, I would leave it as is except for the detonation. Switch to a higher octane fuel to see if will go away.

Hope this helps!

 

As far as starting when hot, mine starts very well when hot.It also starts excellent when cold too. Just give it half choke and roll the engine about 3 or 4 times and it starts right up.It idles at about 2800 when cold and drops to about 1800 when cold though. I am going to check the clutch end crankseal tomorrow just to verify its not leaking.The detonation is not that bad to alarm me its the run-on thats very annoying.I would say the heck with it and remove the UFO but that throttle response is very addictive.I know I wouldn't be happy without it now I've had it on the bike for a while.It got a lot worse after I put the pipe on the bike.I guess thats cause it leaned the jetting out somewhat.The thing thats strange is when I called Lonn Peterson at Thunder products, he told me that most engines are way too rich with even a 2.0 cutaway.He said that most engines run best with a 3.0 and up.I have to say that I wasn't that impressed with his technical knowledge though.I know that these carbs come with a 1.5 cutaway stock so the needle and needle jet are calibrated for that setup.I have tried a 25 pilot and the bike gets so rich it doesn't idle very well.It needs a richer cutaway or a different needle.When the weather warms up a bit it might get better because yesterday when the air was warmer it worked excellent.It didn't start running on until the temp dropped from about 32deg to 20 deg.I wouldn't mind changing the needle if I knew which way to go.I know I want a slimmer needle at the top.Actually I might try to get another stock needle and shave it down a bit myself and see what happens.

 

Kind of sounds like an air leak when you close the throttle. You might consider doing or having a leak down test done just to be sure you don't have a leak. The EGT will read low when the engine is detonating. I have done the retapering of the needle, mark the needle at the throttle opening where your lean spot is and thin the needle there a little at a time (.001) till you get what your after. Make sure your intake boot doesn't have a crack in it.

 

sparkys,

O.K. I think I see what's going on. Your idle is set too high. Can you turn it down a little? This did happen to mine, it seems like it wants to keep a high idle for a bit, then it settles down. I turned the idle screw down until it just lopes along at around 800 RPMs. The engine does take a bit of time to spool up, but nothing like it did when stock. Nothing shy of a set of reed valves is going to cure that.

I also use the slow idle as a way of making sure the pilot jet is correct. If you can adjust the idle with the air screw (not the idle screw) up and down, then you have the right pilot jet. It took me a long time to realize this with the old 38mm TMX carb on my Sport. For the longest time I couldn't get it to idle worth a darn, then I started messing with the pilot jets (going to leaner ones) and the air screw. Lo and behold, when the right jet was in there, I could actually adjust the idle speed with the air screw.

Yeah, the cold could also cause the lean condition at 1/8 throttle. As far as the Thunder Products people, I wouldn't knock them. Just try to understand where there coming from (snowmobiles). They probably don't have a clue that we are using them on piston port ATV motors which are run in warmer weather. That could tend to mess them up slightly on their advise. Reed valve engines tend to run slightly richer than their piston port counterparts. They probably haven't seen a piston port motor on a snowmobile in years.

 

Mike, I know what you mean about too high an idle but I don't think its whats happening to my bike.If I drive along and hold the throttle at 1/8 the bike has the burbly 4 stroke sound of being slightly rich but then the engine will start to smooth right out like its leaning out when they run out of gas.And if I continue to hold the throttle in this 1/8 position it will detonate sometimes but not that badly.Then if I let off the throttle completely the bike will continue to run on like I didn't even let off the throtle. I know the cable isn't sticking either because the ETC switch doesn't shut the bike down.I know buy the sound it is a lean condition. I just still can't figure out why.Also its an intermittent problem and it doesn't do it all the time.Sometimes it might do it and then it won't do it for a few more minutes.If it was because of the idle set too high it should do it all the time. The strangest thing about it is that it does it worse the hotter the engine gets.If its a lean jetting problem it should be worse when cold and get a little better whan the engine is warmed up.It is the total opposite.This is really starting to frustrate me too because I am a licensed mechanic and I do this kind of thing for a living at my own shop.

Another strange thing is that when it does it my EGT guage doesnt get any higher.Lonn Peterson at Thunder Products aggrees with me that if the engine is in fact leaning out the EGT guage should skyrocket at the same time.

Powerslider, when you shaved the needle do you mark it at the base of the carb bore when the throttle is in the 1/8 position?And what did you use to shave the needle down?Also the needle is quite straight for the first inch or so, so do I shave it straight also?

Today I took the UFO out of the slide and put a shim under just the front part where the slimmest part of the cutaway is.This actually tips the UFU slightly in the bottom of the slide so its at approx. the same angle as a 1.5 cutaway.I am going to test it tomorrow to see if it richens it up any.

If not I am going to look for any air leaks and if I don't find any I am going to order another stock needle and do some shaving until it gets better.It has a 6CEY-6 needle.

 

Yes Sparky mark the needle at the bottom where it enters the needle jet, use 400 grit wet/dry paper with the needle in a dremel tool. Measure the diameter before you start with a micrometer and reduce the diameter of the needle by .001 at a time till you get enough fuel at that throttle position. It's not always possible to find an aftermarket needle that will fit your needs. It's not that difficult to do but it is time consuming so be patient. I would still make sure the engine is air tight before you go to this needle mod. Also when you say "head shaved" was it just milled off or was it rechambered? Your squish angle may be too tight causing the detonation. Then there's the dial a jet, I'm wondering why you need this on this carb, could this be part of the problem? Maybe you should disable it and try the needle in the third groove again. I remember this carb on my Scrambler when new, it did not carburate cleanly at all at lower throttle openings. I changed to the TMX when I did the motor mods and had to do some serious internal mods on that carb but it works very well now.

 

No the head was just shaved. When I first had it done they took off .035 and it was detonating bad.I had to double up the head gasket to take some of this away.What do I do now? I can't put it back. I wish I had never shaved it at all now.How do I get the head rechambered? What angles do I look for?

 

The squish angle is the first bevel you see on the head chamber starting at the edge of the cylinder and going toward the spark plug, if this area is to tight the velocity in this area while being compressed will be so high that the fuel/air will ignite before the spark plug fires (detonation) There are various numbers here, I don't know the optimum figure. I think .035 would be the minimum with .050 being better. HPD or RCR would be able to guide you on this better than I. You can have the head rechambered to get the right setting.