phillybuilt Posted January 27, 2017 Report Share Posted January 27, 2017 I've got a really strange popping and backfiring at full throttle, and I'm kind of puzzled thus far. When I got my banshee originally, it had FMF's, a boost bottle, jetted WAY too lean, air leaks everywhere, TORS was on it, leaky head gasket, it was a big mess. Did the same thing, popped at full throttle trying to do a donut or make a really hard pull. Stock bore/port/stroke too nothing crazy. Ditched the boost bottle, eliminated TORS, installed a brand new cool head, dialed in the jetting pretty well, switched to T5's, fixed EVERY air leak (including the crank seals), pulled the airbox lid off, running 30/320's/1 turn currently. Starts first kick, idles like a dream. Has 130 PSI compression both sides, holds 6 PSI for 6 minutes. Timing is set at +4. I'm running BR8ES plugs. Both carb floats are adjusted to 21mm. As far as I can see without disassembling anything, the reeds are intact (though I think stock). I'm about 700 feet elevation. Carbs are sync'd, idle is set same both sides, fuel is 93 octane with Klotz premix at 32:1. Originally, I thought this was happening because it had 260 main jets and 25 pilots, but it doesn't seem to make any difference going between 280-340 mains (I've tried them all with no difference) and going from 25's to 27.5's and 30's as well. Air screw tuning does nothing to help. Throwing the airbox lid back on didn't help either, made no difference at all. I can't even make full pulls to chop the plugs. Happens with pod filters, or with the airbox (lid on or off) Starting to become a little neurotic, I tried chasing down the smaller details. Removed the fuel tank and petcock, cleaned out all the dirt and debris inside. Cleaned the petcock screen as well. Removed the ignition coil, and ground down the mating surface on the frame to bare metal, and cleaned up the mounting points on the coil as well. Every time it's apart, I double check the carbs to be safe. They come apart, I recheck the float height, and also clean them every time as well, along with testing the fuel inlets and float needle function. There's no issues there either. I took my spark tester and ran it on each wire between the plug and wire, both look to have equally intense spark. I also pulled the stator cover, and the flywheel is turning true, no rattle, and the pickup lugs are clean. The boots from carburetor to air box are okay, the intake boots going from reed cage to carbs are good too. When I had the engine apart for crankshaft seals, the crankshaft appeared to still be in phase. I considered adding a ground wire in between the coil and engine cases, and also between the engine and frame. My next step is going to be checking the voltage at the coil and the CDI, and also checking for resistance between the coil and CDI... I'm just really not sure where to go from here. For what it's worth, it's a stock Yamaha coil, stator, and CDI. Thanks guys. Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 27, 2017 Report Share Posted January 27, 2017 What size domes are you running? Sent from my VS990 using Tapatalk Quote Link to comment Share on other sites More sharing options...
phillybuilt Posted January 27, 2017 Author Report Share Posted January 27, 2017 Knew I'd forget a detail somewhere... 21cc's Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 28, 2017 Report Share Posted January 28, 2017 What compression tester are you using? 130 psi with 21 domes seems low if you have a good tester Sent from my VS990 using Tapatalk Quote Link to comment Share on other sites More sharing options...
phillybuilt Posted January 28, 2017 Author Report Share Posted January 28, 2017 Just a Craftsman compression tester. Doesn't leak or anything. When I had installed the cool head, the pistons looked great, and the cylinder walls had a very clean cross hatching in them. What should I see for compression with 21cc domes and good pistons? Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 28, 2017 Report Share Posted January 28, 2017 I'm at about the same sea level and with 20cc domes I was right around 160 psi. With 21cc domes you should be closer to 145-150 psi. Sent from my VS990 using Tapatalk Quote Link to comment Share on other sites More sharing options...
bigweav81 Posted January 28, 2017 Report Share Posted January 28, 2017 (edited) Have you tried swapping out cdi, stator, coil with known working ones? Does it break up, and pop in the upper rpm? Sent from my SM-S820L using Tapatalk Edited January 28, 2017 by bigweav81 Quote Link to comment Share on other sites More sharing options...
bigweav81 Posted January 28, 2017 Report Share Posted January 28, 2017 (edited) I noticed not on your list of things already done....are your carb slides in correct? Cut out faces air box. Never know. Sent from my SM-S820L using Tapatalk Edited January 28, 2017 by bigweav81 Quote Link to comment Share on other sites More sharing options...
phillybuilt Posted January 28, 2017 Author Report Share Posted January 28, 2017 Have you tried swapping out cdi, stator, coil with known working ones? Does it break up, and pop in the upper rpm? Sent from my SM-S820L using Tapatalk Unfortunately, I don't have any of those for spares. Yes, it does pop and break up in the upper RPM's. And I hear you with the cut outs on the slides, keep it simple and don't overlook anything! They are in correct though. And while we're on the stupid things subject, the choke tube is good to go. I re-visited a few tests and theories this evening. Didn't have batteries for the meter so couldn't do any electrical testing. Got a different compression tester, without the gas tank in place so the line wasn't kinked. 138 on left, 140 on right. Kicked it until it stopped going up, engine was cold. Also air leak tested again. Holds 6 PSI for 6 minutes. Sprayed EVERYTHING with soapy water, no signs of air leakage anywhere. Used a starter cable from another ATV as a ground, between case and frame, and ground all contact surfaces. Tried swapping coil wires around for the hell of it. No change. I also noticed that the right side pipe has less smoke coming from it at idle, just hitting the throttle a little bit, or even at wide open. Thinking maybe the carburetor wasn't getting fuel properly, I tried another right side carb. I took it apart, everything was cleaned then blown out with compressed air, adjusted float to 21mm with no gasket, sync'd carbs after, etc. No change at all. Also noticed the right side will pop at high RPM's, the left side never does. The crank seals were OEM Yamaha as well, no aftermarket junk. When I had the exhaust off the engine, there was an equal amount of residue on each of the exhaust outlets, leading me to believe the crank seals are fine. It runs beautiful at idle, and for the most part runs great all around otherwise. Just the full throttle high RPM part sucks. Quote Link to comment Share on other sites More sharing options...
special06shee Posted January 28, 2017 Report Share Posted January 28, 2017 Pull spark plug boots off wires and cut 1/4" of wire off and get new boots Quote Link to comment Share on other sites More sharing options...
phillybuilt Posted January 28, 2017 Author Report Share Posted January 28, 2017 Thanks for the tip special06shee. I've been considering a Dynatek coil and wires anyways, perhaps even their CDI as well if it's worth it. Maybe now is the time to do it? I forgot to mention, also re-checked the flywheel lugs for any dirt or debris and very lightly polished them with a scotch brite pad... Though they were clean to begin with. The pickup gap was way off though. I reset that to .020 per factory spec but unfortunately no change. It had to be in the .030's with how much I could wiggle the .020 feeler. The pickup itself was clean as well, I didn't touch it. There's two brand new plugs in there as well, to rule out the chance of even a slightly fouled plug. Still weak smoke on the right side. Pretty stumped right now, and unfortunately I don't have anyone local with a banshee to swap parts with. Going to get some batteries for the meter and do some testing later. Thanks for the input guys Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 28, 2017 Report Share Posted January 28, 2017 Where are you located? Sent from my VS990 using Tapatalk Quote Link to comment Share on other sites More sharing options...
phillybuilt Posted January 28, 2017 Author Report Share Posted January 28, 2017 Northwest corner of CT, near Torrington Quote Link to comment Share on other sites More sharing options...
hoppedupandcutdown Posted January 28, 2017 Report Share Posted January 28, 2017 I'm with Special06. Trim wires - new caps Quote Link to comment Share on other sites More sharing options...
sheerider11 Posted January 28, 2017 Report Share Posted January 28, 2017 Get a pic of the stator and flywheel Quote Link to comment Share on other sites More sharing options...
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