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Discussion Starter #1 (Edited)
1997 Polaris scrambler 400

I have a loading up situation and I have bought two different types of rebuild kits and neither of them have fixed my issue, I even made sure that the float is adjusted right. The one consistent thing about the rebuild kits is the needle seat has half the amount of holes drilled in it then the factory one, I put the new seat in and it wouldn’t rev up it would get to maybe half throttle. I put the original seat in that has twice the amount of holes and it runs like it should, except for after a few days of sitting it won’t start and then detonates in the exhaust. What are those holes in the needle seat for. I’m getting really frustrated with this machine and I’m about to burn it. When it detonates in the pipe it’s like a .45 going off, it even makes my ears deaf for a second and I don’t want to be causing anymore damage to the already rebuild cylinder




 

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The holes in the needle seat are just assure flow of fuel into the bowl so the flow is not obstructed by the needle.

Are you shutting the fuel off when you park it? Usually backfire in the exhaust is caused by a late spark (fouled plug, excessive gap or other ignition system problem). You might try holding the throttle full open as you try to start it and then backing off the throttle while the engine is cranking, that might alleviate the backfire and promote easier starting. Also, the backfire will not hurt the engine - it is irritating and unnecessary, but I have never seen engine damage from backfiring except on reed valve two strokes where a backfire through the intake can break the reeds.
 

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Discussion Starter #3 (Edited)
I am not shutting the fuel off, I will start doing so but really shouldnt have to, right? So the aftermarket needle seats are restricting to flow, I am going to try and find a new oem seat, maybe the one in there is out of spec causing the carb to leak down, can’t be the needles I put two new ones in and it didn’t really change anything. Glad to hear the backfire isn’t hurting anything. Well except the donut gasket everytime it blows the exhaust off lol


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Discussion Starter #4
Also meant to mention it has a brand new br7es in it and once I get it started it will be fine starting back up, this thing is so confused sometimes. It’s the last demon I have to beat out of it and it doesn’t want me to


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Don't be concerned with the flow of fuel being blocked by the needle seat - the holes are just there to assure flow is not obstructed by the needle. 90% of float needle seats do not have those holes and fuel flows just fine.

I don't know if you should have to shut the fuel off, but why do the manufacturers install a fuel shut off? In most owners manuals there is an instruction to shut the fuel off when not in use. Most motorcycles and many ATV's that have the fuel tank above the engine have vacuum operated fuel shut offs. Before 1970 riders were expected to shut the fuel off. My 72 Triumph Bonneville will school you in that matter quickly. Leave the gas turned on, park it on the side stand and 30 minutes later all your gas in on the ground. My 72 Kawasaki had a vacuum petcock that shut the gas off for me. My 75 CB750 Honda did not have a vacuum petcock and if I didn't shut the gas off the left two cylinders would flood and it ran like crap till the left cylinders cleared out and started firing again. My Kawasaki KFX400 had a vacuum petcock that failed - I ran it in the "prime" position while waiting for the replacement petcock. It ran fine, but if it ran out of gas, it was OUT OF GAS and if you forgot to turn it from pri to on or res, it would fill the air box. The air box had a drain, but after draining out the fuel it would run like crap till the air box dried out and it didn't have all the excess vapor to deal with. Not turning off the fuel can also fill the crankcase with gas. I worked on a Polaris 500 that had about 4 quarts of gas mixed with the oil. Didn't hurt the engine, but it was hard to start and ran like crap. I just got in a habit of shutting off the fuel on all my bikes with the fuel tank above the engine.

Machines with the fuel tank under the seat, rear fender or anywhere level with or lower than the carb didn't have as much of a problem. Although the Polaris has a fuel pump, it it not a positive obstruction to fuel flow. Take the fuel line off the carb and you will find the fuel still flows at a slow rate. Faster with a full tank than a near empty tank, but still flows. Some electric pumps will allow fuel to flow when the pump is not running.

Each machine is different. Live and learn. Try it and if it solves the problem make it a routine.
 

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Discussion Starter #6 (Edited)
I will check those things, just never needed to shut the gas off on anything else before, just something to learn. Why would the newer needle seat change the performance? Unless it was just a fluke. No fuel pump on this or vacuum actuated petcock but the gas tank sits directly above the carb so I could see how it could affect the carb, will try turning the gas off and see if it helps


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OK - I just verified that you do not have a fuel pump, but I suspect that your floats might be heavy and not shutting the fuel off at the proper level in the float bowl.

You can adjust the float arm, but that does not guarantee a correct fuel level. In addition, the fuel can siphon from the carb into the intake flooding the engine if the petcock is not shut off. To verify proper fuel level, you have to modify a float bowl cap with a nipple - attach a clear hose to the nipple - let the hose loop down and back up along the side of the carb - turn the fuel on and watch the fuel rise in the hose as the float bowl fills - the fuel should stop no higher than where the float bowl seats against the upper part of the carb. Using this method, you can compensate for floats that have absorbed some fuel and do not float at the proper level, but the floats will continue to absorb fuel and eventually will not float at all. A new float is $29 and two are needed.

Three things you can do to attack the problem; 1) simply shut the petcock off 2) elevate the front of the quad - if fuel is siphoning out of the carb, with the front elevated the fuel will flow into the air box instead of the cylinder 3) reset the float level so siphoning does not occur

If you have no other performance problem, just get in the habit of shutting the fuel off.
 

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I don't think the amount of holes in the seat will make a difference. Are you changing the needle valve and seat together? Like somebody else mentioned check your float for leaks. You might be better off just replacing the float just to be safe. The problem you describe does not sound like a stuck or leaking float. Just my opinion.
 

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Discussion Starter #11
So I am planning on changing the floats at some point but since this post has happened I followed latebirds suggestions but with a twist, I shut the fuel off and let it idle till the bowl is empty, no more backfire and fires rights up, after letting the bowl fill


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