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Spring 2016 Ask the Experts

Answers to your tech questions about idling issues, carburetors, backfiring and WOT
1998 Yamaha Mountain Max 700
Idling Issues
Q: I have a ’98 Yamaha Mountain Max 700. I just went through the carbs, completely cleared both jets, checked floats and checked reeds, which looked fine. I put it all back together and made sure the boots are perfect. Everything is put back just how I took it apart. It starts up great, but slowly revs up to 4000 rpms and does not come down when warmed up. When I pull out the choke slightly the idle goes back down to normal. Please help! – bigangel4

A: The symptom sounds  like you have a plugged pilot jet (or plugged pilot jet circuit) or the fuel screw turned in all the way. The engine is not getting enough fuel on its idle circuit, and that is what is causing the idle to climb. It may not be plugged on all of the carbs; one or two would do it. The fuel screw should be set on each carb the same. Back in the day on my Mountain Max with triple pipes, it was set at 2¼ turns off of the seat. That position will depend on the engine combination (pipes, head mod, porting etc.) that is being used and the elevation it is to be used at. Another cause could be an air leak at the base gasket of the cylinders, intake boots, case half parting line and/or crank seals. To check for an air leak, start the engine and let it warm up. Pull the idle back down by popping the choke on temporarily. Once the idle has normalized, spray starting fluid around each of the areas mentioned above. If a leak is detected, the idle will immediately rise. – Jerry Mathews, Starting Line Products


Twin to Triple Carbs
Q: Can you take a couple sets of 40mm rack-style carbs off a twin, pull them apart and make a set for a triple? I know that this would entail popping plugs and perhaps some drilling and maybe resizing of the spacer sleeves. Then the triple throttle shaft and choke shaft would be used. Other than that, is there a reason I haven’t thought of that this will not work? If it did work, I can have a set of 40mm flat slides on my XCR for about $200, probably worth 2-3 hp and I get the black slides all at the same time. – cool storm

A: When it comes to mods like this, our standard answer is that everything can be done for the right price. It’s definitely possible to take two sets and build a triple set. It will require some machining of parts to get it done, but all in all, it’s not a project that can’t be done. Good luck! – Jason Houle, Straightline Performance
Polaris RMK 600
Backfire problems
Q: My RMK 600 keeps backfiring on takeoff. First I thought it was because the throttle cable was bad. I replaced it as well as the plugs, had it re-jetted for higher elevation. I rode it for two days and it screamed, but after sitting for two weeks, it started acting up again. It’s been in the shop twice, and $300 later, it still has the same issue. – polarisdavermk600

A: It sounds like it is a carb model where you re-jetted it for elevation. Usually when we see a sled backfire it is because it has raw fuel in the pipe or silencer. This can happen when the sled is run hard and shut off without allowing the engine to come down to an idle. When that happens, the spark is shut off, but the engine continues to pump its air/fuel mixture through it, causing the air/fuel mixture to end up in the pipe and silencer. When the engine is fired up again, the unburned air/fuel mixture in the pipe ignites and backfires.

If the sled backfires when you take off (after it’s been running), it is most likely due to a misadjusted throttle cable/lever. The throttle override safety switches cut the spark for a second or less, pushing the unburned air/fuel mixture into the pipe and silencer. When the spark returns, the fuel ignites and backfires. A properly adjusted throttle lever should only be able to move away from the throttle block approximately the width of a credit card prior to pulling on the slides of the carbs. Too much slack in the cable will cause the throttle override safety switches to cut spark. I’ve also seen a dual throttle cause this problem when the throttle lever pivot point is tightened too tight, not allowing the lever to pivot smoothly and engage the switch properly. A faulty throttle override switch could also be the cause.

If the engine is allowed to come to an idle before being shut off, and it backfires when it is restarted, then I’d have to say the idle air/fuel mixture is too rich on fuel. The unburned fuel gets pushed into the pipe and silencer, where it ignites the next time it is started. Other common things that can cause this are a choke cable that is out of adjustment, a choke plunger that is stuck or leaking, a fuel pump diaphragm that is leaking slightly, a needle and seat within the carb that is leaking or a misadjusted float level. I’ve also seen it happen with a bad spark plug cap or wire that gives intermittent spark. Usually these items show up as more than just a backfire, though. They typically will result in a rich condition or misfire. If the sled has a low idle, or difficulty idling, then one or more of these items may be the culprit. – Jerry Mathews, Starting Line Products


Where to set WOT
Q: When setting WOT on your carbs, do you set the slide flush with the roof of the carb on the intake side (flush with cutaway) or flush from the engine side of the carb? Basically, how far should a carb slide be up for it to be at the optimum WOT position? – FrankieJames7

A: You need to set the slide flush with the backside, otherwise the bore will be slightly closed and the millimeter opening will not be correct. – Jason Houle, Straightline Performance


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