The bad news is that you'll be out some cash and that the new diaphragm will fail one day and eventually need replacing. Doing so reduces the afterburn 'popping' you get with a lean mixture and allows bikes to be jetted leaner from the factory for whatever purpose. Okay, if that hanging idle is still there it's time to tear the carby down. Make sure you get rid of any traces of polish residue. What does cause problems are valve clearances that are too tight. The key to fault-finding and tuning is isolating issues and controlling variables. That said, some people advocate a couple of more invasive solutions that you might like to know about.
And while we're talking Allen heads, if the cross-heads on your twistgrip throttle body body have corroded themselves into place as steel bolts tend to do in aluminium housing M6x15mm is the replacement size. If the diaphragm has any kind of leak, the good news is that you've found the problem and dropping in a replacement is a simple fix that should cure things. He even pushed the idea of a four-stroke motocross bike in the face of an advancing two-stroke tide. The chances of drilling into a sheared M3 screw and keeping the threads intact are pretty slim, but if you're very careful you can drill the old screw out and tap the hole for an M4 which will work just fine. Honda's engineers bored the crankcases to accommodate a 3mm-larger bore, while a new crank yielded 5mm more stroke for dimensions of 97.
Okay, fire her up, let her warm up, and set the bike upright and level. Pilot screw: that's the little knurled brass slot-head screw at the front engine side of the carb just ahead of the float bowl. Besides checking for smooth operation and any binding caused by routing or fraying, it's worth pulling apart the twistgrip to make sure it's not gummed up and that free-play is as per spec. Unless you're a race mechanic the only thing you'll play with is the position of the needle clip on the needle. Honda specifies an idle speed of 1300rpm plus or minus 100rpm but that's probably a little on the high side. With the diaphragm breached fuel can be sucked up directly into the carb body at low speeds and idle making you run rich. Bolt it all back up together and the job is done.
Some people call this jet the pilot jet see where the confusion comes in but we're sticking with slow jet. If that called for racing an already dated four-stroke against a two-stroke that was lighter and faster, then so be it. The downside is that you'll get more pops from the exhaust on the over-run hey, some people like that! My bike is a so called B-version bike. You need to remove the throttle cables, pop the top off the carb, unscrew the linkage arm, withdraw the throttle spindle, pull the slide and linkage out of the carb, and reach deep down to undo the linkage plate that holds the needle and clip in the slide. Don't just slap in the biggest jets you can find though — optimum jetting varies according to weather, altitude, engine condition and modifications. The 'nipple' on the carb side of the diaphragm acts on a spring-loaded button valve behind the little brass ring so check that this isn't sticking and give it a bit of a flush out.
In a nutshell moving the clip down raises the needle and makes the jetting richer, moving it up closer to the top lowers the needle and makes the jetting leaner. When he came upon lapped riders, they knew who was coming and would get out of the way. The convention is to measure pilot screw position in the number of turns out from fully-seated in , so note that. The edges on the carb body are pretty sharp, but Honda puts a slight bevel on the base of the slide so this shouldn't really be an issue. The engine is fed by a 39 mm piston-valve. If you're in the the 1. The 'standard' setting is 2.
The emphasis should be to keep that jetting diary too — besides being invaluable while you're actually juggling jets, it'll get you back on track after a break and help eliminate guesswork in the future. A few light taps will reveal the spring and allow you to install the shaft so it meshes with the tang inside the spring while the outside tang sits in the recess in the carb body see photo. Hanging idle vs high hot idle: chances are you'll experience both, so let's differentiate between a high or fast hot idle and a hanging idle. Chances are that the screws will come straight out without a problem, but if you do shear the heads off don't panic. After decades of adjustment the O-ring may well have lodged itself in the carb body and be flattened, scuffed or torn. Turning the knob clockwise raises the slide and airflow to increase the idle speed, anticlockwise backs it down. The slide itself is hard chromed so it shouldn't theoretically be worn much, but you'll probably see some dirty deposits on it and, if you look closely, some vague outlines of where it's been spending most of its time.
Honda calls it the air cut-off valve, but some people like to call it the air control valve. G ently turn the screw in until the idle starts to drop or miss a little, and note that position. Make sure your fuel is good fresh, with no water or dirt , open the petcock and float bowl drain screw to make sure you're getting decent fuel flow clogged petcock, filter or even a blocked tank vent cap , and check for blockages in the overflow and breather tubes. Turning it out anti-clockwise makes the mixture richer. The purpose then, as now, was to get rid of a potential failure point and make race-tuning a bit easier. He was a four-stroke man and made no apologies. Some people refer to this as the slow speed screw or air screw that's just wrong , but Honda calls it the pilot screw.
The motor still had two carbs and it was still heavy, but the bike was taking shape. A can of electronics cleaner works well here too. He was one of very few four-stroke riders in the woods and he always said that worked to his advantage. If you're too lean you need bigger jets more fuel , too rich and you need smaller jets less fuel. A hanging idle is when your bike idles high when you back off the throttle or come to a stop but returns back to 'normal' idle speed after you blip the throttle. That creates a 'vacuum' that pulls the slide against the barrel of the carb body. This model also brought back the 18inch rear wheel.
Honda took the racing effort in Baja in house with the likes of Bruce Ogilvie at the helm. If you're checking float levels with the carb on the bike and you don't have a float gauge cutting a piece of stiff card into a 14. Replacing the stock unit with a 'billet' aftermarket unit is a pretty common modification. They close up over time, so make sure the clearances particularly on the exhaust valves are right. You should end up with a carby barrel that's smooth to the touch and a slide that.