Bubba, Modquad, anyone?? This may be long so please bear with me. Ok first off, I have a Scrambler 400 4x4 with these mods. HPD airbox with Twin Air Slip on filter, 38 TMX carb, Rad Valve, Shaved head (12.5 :1 Compression), Wiseco .020 over piston, Trail port, BR9EV spark plug and clutching. Also run Polaris synthetic oil through oil pump and Klotz racing gas. Ok problem is... I have a hesitation on take off. I know It is the carb, not clutching because I have no slippage whatsoever. Ok right now I am running a 25 pilot, 58 needle in 2nd position from top, and a 430 main. I am jetting strickly from feel and a EGT gauge. According to my gauge, I am rich from idle to 1/4 throttle but I have a hard time believing i need to go down to a lower pilot?? Is this possible? I am fairly new to the jetting scene but It seems to me that anything less than 25 is low? Can anybody comment??? Another thing is when I turn my idle screw there is a point were it is idling high and a half a turn later it just drops off idle very low. Is this normal? I am at about 1800 ft. above sea level and the temp yesterday was around 55. Anybody, thanks.

 

Focus on plug color, not so much the EGT gauge. You didn't list an after market pipe. Start tuning from idle-1/4 throttle first. operate the machine for about 1/4 mile staying below 1/4. Cut the ignition with the handlebar switch, and take a plug reading. Turning the low speed screw out, leans (lets more air in), turning it in richens. Also at this point try to gain a compromise with a crisp throttle response. Do this for 1/4-1/2, then 1/2-3/4. For this range you adjust the clip position on the needle. Doing the same ignition chop after holding the throttle steady for the 1/4 mile. As you lower the clip on the needle, you richen those mid-range settings, and raise the clip to lean. Then go for the wot- wide open throttle...make sure you have ample shut down area. You want cocoa brown plug readings, or, darker, for all areas. You also don't list any clutch mods...do you have any?

 

Oops, forgot the pipe huh? Knew I would miss something. It is an Aaen top-end pipe with their silencer with no spark arrestor. Ok clutch mods are a top cog pro belt (new), s58 weights(new), 44-36 teflon helix(new), I think aaen blue secondary spring, an aaen red primary spring. I will have to try the spark plug reading again, but I am not too good at that. Used that method in the past and burnt up my engine, so I got the EGT gauge for a better reference. I guess I should check plug color too though, I will try that and see what happens, prob. be wed. before I get to though and the temp is supposed to be in the 30s, so... Anyway, thanks for the reply.

 

The pilot does seem to be on the low side.You didnt mention the air screw settings either. A hesitation is almost always a sign of too lean or an air leak. I can tell you that I messed around with needles on my wife's 250 Suzuki. It has a 34mm flatslide mikuni. Call it wierd but I had to go back to the original needle to get a good idle characteristic and response. Before I done anything else I would recheck the cylinder base bolts, check for air leaks, then if that is alright, put a larger pilot in it. Start with about a 35 and check it from there. Needle position with the air screw should be able to give you quite a bit of adjustment. Modquad will probably tend to be a little more picky on the jetting than me because he does more hard trail riding and I am just about flat out all the time. that's the difference in our bikes though.Get your idle and part throttle done first, then work on the full throttle last. Full throttle in my opinion is the very easiest to tune in. That main jet seems like a big one also. I dont think I have ever heard of anyone running that big of a main in a mikuni. I know guys with full race modded motors only running in the 300 range. Just one thing at a time and dont give up. The end results will be worth it[img]i/expressions/face-icon-small-smile.gif[/img] Hope this helps,Bubba

 

Take your time. Like Bubba said it will be worth it. Don't totally ignore the EGT gauge, use it in conjunction with the plug readings. Good point on cylinder retorque too. Anytime you do a rebuild on these engines, it's strongly recommended to retorque the cylinder after the motor has been warmed up good the first time.

 

OK, well when I had all this work done to my motor last summer I asked Olav Aaen what kind of jetting etc. I should use. He recommended starting with a 390 main up to a 410, but... that was before i had the clamp on air filter , I was using the factory box at the time, so knowing this I ordered bigger mains. 420,430,440. Ok, needle is what came with carb and he said this is prob. what I would need. Also I couldn't remember what pilot came with the carb so I called and asked and they said a 25. He said prob. run 30 in winter 25 summer. I had this thing running good with no hesitation a few weeks ago, but since I have changed gas (from Turbo Blue 115 mixed with 93 pump about 50 % to Klotz 113 straight) and I have also added the HPD box with the clamp on air filter. Before I had 30 pilot 1.5 turns out , 58 needle in center position, and 410 main. Ran great. I also thought maybe air leak, retorqued everything and couldn't find any evidence of any leaks. I am thinking maybe the main is a little big maybe put in 420 and try that. I had a 30 pilot in the other day when I was doing all this and seemed to rich, but didn't check the plug just by gauge. Had hesitation then too. Dropped clip one because the gauge showed not hot enough. Today I had about 15 minutes, done a run to 1/4 throttle pulled plug and It looked good to me. Light tan color. But when I get to the 1/4 or more point It sounds real tinny. Could this mean needle to lean need to go back to middle position of clip? Oh yeh I have retorqued the head and base about 4 times now so I am sure they are tight. One more thing, this thing hasn't really idled right (idles high) ever since I started adding performance parts to It. I thought since I had no leaks anywhere that I could find that maybe It was the airbox, but now with new one no change. I also just put in brand new reeds thinking maybe they were worn out, which I believe they were anyway, but still no change. Maybe I just have my jetting so out of wack It does that.? Well, I will be running It more this weekend getting plug readings and trying It again and I will get back to you two. I appreciate the help and recommendations, If you can think of anything else I should try I am all ears.Thanks!!

 

OK, first when you go from a mixed ratio to a full race gas, you have to lean it out some becaue it will go richer. The high idle really bothers me. When you said you retorqued the base bolts (dont take this wrong) are you using the torque wrench or just hand tightening them? I hate to ask this question that way, but just checking. Do this check before going any farther. Get it running, then take a new can of WD-40 with the nozzle on it and start spraying around the cylinder base, reed cage, and carb boot. If the idle comes down in your case then you have an air leak and that will be the entire problem. Also why are you running that kind of gas? Given the specs you have you should be able to run 92-93 octane without any problems. I also dont agree with the spark plug choice, it's too cold of a plug, especially when coupled to race gas on a mild compression machine. I would go back to a good ole BR8ES or just B8ES. If you want to run some race gas with your setup, you should only be running about 1 gallon of race fuel to a tankful on that one. If you get alot more octane than needed you slow down your burn rate and lose power as a result. Rule of thumb is to only use as much octane as needed without detonation. As far as the air leak, I had a very small piece of the base gasket come out on me one time and I played havoc with jetting on it and getting the idle right. That was a long lesson learned and I just wasn't wanting for something like that to give you alot of grief. Bubba

 

When I retorqued the base bolts I used a micrometer type torque wrench (about $175 wrench). I haven't tried the WD-40, but I will. Honestly I thought I needed that kind of gas because I have read around on the internet alot on race gas and from what I could tell with the 12.5:1 compression I should need It? I also read that mixing leaded racing gas with unleaded pump gas can cause alot of headaches when It comes to jetting because It just doesn't mix right together. I asked that question on here earlier about what gas should I use and no one answered, so I just ordered the Klotz and now I am using It. I went to Klotz's site and If you look at the gas they offer the Klotz 113 is for 12.5:1 or higher compression and 108 is 12:1 under, so that Is what I went by. Now If that is wrong tell me and I will switch, but I figured since they made the gas they new what they were talking about. Olav recommended the switch to the BR9EV plug, so that is why I went to it. I will check for a leak next chance I get. Thanks..

 

Klotz is rating their octane on a 4-stroke engine compression, which is different than a 2-stroke. For instance, when HPD and such sells their 12:1 cool head, it is for use with pump gas. Those things can really confuse a person. It doesnt hurt anything in mixing leaded and unleaded gases. Since you have the klotz, use it, but mix it lightly with premium. I have a buddy that is full trail modded with the pipe,head,reeds and such, and he mixes one gallon race gas with 4 gallons of premium and it does great. When I ran race gas on the 15:1 compression with a 9 spark plug, it actually tried to foul on 2 different occasions, once at the sand dunes riding hard. I went back to the 8 plug and bam! never happened again. These aftermarket carb can be trickey also when adjusting for full throttle. On some if you get full throttle your slide dont go all the way back down and vice-versa. Check that out.Bubba

 

Hum see I didn't know they were talking about 4 strokes, they need to specify that. I will go back to 25% which is what I was running before. [img]i/expressions/face-icon-small-smile.gif[/img] I know my slide is going all the way down, but I don't think It clears the throat at WOT. I asked Aaen about that and they said sometimes It does that, so. I am confused about the throttle adjustment though. I have messed with It alot and am still not sure If It is right. I read on here that the contacts don't touch, but that gave me alot more than the recommended play. It is really unclear In the owner's manual and the other manual I have (clymer I think) where the play is measured from. Measured from the throttle itself or the parts inside the housing??? I haven't had any problems with the spark plug fouling but I may try to go to an 8 if I do have that problem thanks!