Just got my raider back together (701 motor) and it seems to have a couple of issues with the carb tuning. Firstly it bogs off the mark when you gas it, not all the time but when it does it feels like it overfuels and dies. When you back off the gas and let it idle then come on slowely its fine again. Secondly it has fuck all top end, feels like the motor still has plenty more in it (my 701 single carb fx1 absolutely dusts it). Just wondering if anyone has any suggestions or is in the waikato and would like to take a look at it
I'm not "great" at carb tuning but I can get by, if you get in a pinch I’m in Taupo. (though I don’t have any tools or spare carb parts right now since my shipment hasn’t come in yet) Don't know how well versed you are in carb tuning, so you might know some of this already, ignore it if so. Also might help to post up some of the mods your boat has. If you haven't changed anything you should be able to tune just by changing the screws (assuming that everything is in good condition), if not you may need to change pop off pressure and jets. 1. Make sure your motor is sealed up well, do a leakdown check. A MOTOR THAT LEAKS AIR IS IMPOSSIBLE TO TUNE. 2. Check synchronization: pull off intake and look at the throttles to be sure that they move together, open and close fully when the throttle is pulled. If not refer to the engine manual or mikuni manual for instructions. 3. Use the low speed adjuster and idle set screw to set idle speed to about 1100RPM (out of water, in water can be lower, the SXR manual actually says 250 and 700rpm). If you don't have a tach most good automotive multimeters have an inductance tachometer that you wrap around the plug wire or you can do this by feel. The low speed and idle screw work together and only really set idle speed and mixture. Idle the boat around in the water a bit, shut it off, restart it. It should restart easily, if not, give it a bit more fuel. Work in 1/8 to 1/4 turns. 4. Here is where things get complicated above idle to about 3/8 throttle, since the pop off pressure and low speed jet adjustments overlap. If you went to aftermarket flame arresters, you will likely need to change this slightly as it messes around with your intake pressure. Keep the engine at about ¼ throttle, then launch it to full, there should be no hesitation, if it hesitates it is lean, if it is loaded up it is rich. Less pop off =more fuel. Larger jet =more fuel. As long as the motor is close to stock and pop off is where the manual recommends you should be able to get away with just minor jet changes (in reality you should adjust pop off first then low speed jets but if you haven’t changed much you should be close on this already and jets are easier to change, order one smaller and one larger than stock). It’s much easier to find a lean hesitation by the way, so move down, then back off till it disappears. 5. The high speed jet test works the same as the low but from about ½ to ¾ throttles. Ride around holding the throttle at about ½ for a minute then quickly open it. Again hesitates=lean, takes a second or two to clear out then accelerates = rich. 6. The high speed adjuster works from 3/4throttle till full. Pretty much adjust it till you get max speed running in a straight line or max RPM. You can also do what’s called a plug chop, where you ride around at full throttle, cut then engine then look at the colour of the plugs (brown is good), I find this less reliable in 2strokes since your oil colours the plugs. RPM is best to tune to, then max speed (assumes you are propped correctly) then colour. Records where you are to start, go 1/2in in one direction, ride it, and then go ½ in the other from start. See which is better, and then move to 1/4 turns from that position then 1/8th. Too lean or two rich will both loose top end power, there is a sweet spot where it is just right. For a starting point though, a 701 SJ motor should be Low speed 7/8 turns out. The Front high speed screw should be about 1 1/8turns and the rear 1 1/2 turns. The raider manual should have stock settings in the preventative maintenance chapter. More info in the SBN manual: http://www.mikuni.com/pdf/sbn_manual.pdf
Ahh just from reading that I think i might have worked out what ive done, the motor is a stock twin carb 62t and was in another hull before this, I rode it in the other hull and it went fine (I never did a top end run though) but it would pull well off the makr and not bog. When I fitted the motor to my current hull and started it up it felt like the idle was way to low and lumpy so I wound the idle screw in a touch which may have made it too rich down low hence why it bogs there
Great, hopefully that was of some help. I find ski's (especially twin carbed pole skis) the hardest of any motorized vehicle to tune since it's all seat of the pants, no gauges to look at. Been thinking about putting a Tiny-Tach on my ski to help with this stuff.
Yeah this one (sitdown yamaha raider) has gauges but none of them work anymore. Whole ski turned into a nightmare really, i thought it would be a simple engine swap (hull had an 1100 in it) but turns out nothing else fits so had to swap everything and make up custom bits etc
