Shee_screams Posted March 21, 2016 Report Share Posted March 21, 2016 To start out,back in February I acquired my first Banshee,1994 421 cheetah (about time). Got it from a dude that seemed pretty mechanically inclined, said he bought as a trinity big bore and with the help of FAST made it in to a cheetah 421...I asked him the compression of the bike and he showed me that it was 180PSI (holy sh!t), and he said he ran premium pump gas. I was a little surprised at this, as research on this forum has concluded anything over 165PSI on premium pump is a bad thing, anyways knowing there are more things than just compression that will dictate octane needed I took his word on it (it ran great!). Fast forward to March 6th, took it out for its first real rip at the farm. There was still some snow on the ground so it was hard to tell the true power of the machine, but Holy sh!t, it ripped! and ran MINT! Anyways now for the interesting part... We get out riding today, or well Sunday, let the banshees warm up and go out for some track time. I ran the banshee for probably 30 min of messing a round on a ghetto MX-ish track, she ran mint and the power was great. So now we are hitting the open fields after with 5" of snow on them (riding rural Alberta). Anyways she had balls and killer power, but once i hit 5th gear power band it seemed to fall flat on its face (almost like a 4stroke rev limiter) and stutter(think it might be pre detonation), Anyways after some figuring and listing me and my buddy discovered that the spark plugs were loose, like wobbly loose, WTF... We tighten the buggers back down and started her up again, the sound of the engine falling apart was gone. Anyways I take off and the same thing keeps happening(in every gear), it keeps falling flat and making weird sounds when the power hits. Get her back to the shop and pull the plugs and it turns the ground electrode one one of the plugs is almost gone and the other plug fully intact, but the center electrode looks pitted. Throw the compression tester on back at home and attempted to get a psi reading. One cylinder was 120PSI and the other was aprox 110PSI. Pretty close together I guess ( the tester may be off), probably not a hole in the piston. Anyways buddy is getting a scope from work to check out the cylinders without taking off the heads to see if there is damage. So my plan is to get some octane booster or a bit of VP race gas and mix it with pump gas and try it out Easter weekend and see what happens... Any input on whats wrong, what to correct or experiences would be extremely appreciated! Would like to have a running banshee to take out during semester break to St. Anthony lol. BTW on a semi limited budget, in collage and working at the hmedepot atm. 1994 Banshee: Cheetah cylinders (stock from FAST) Hot rods 4mil crank Pro desisngs cool heads (no clue what domes) Trinity 2 into 1 Carb Trinity Pipes V force reeds (number 2 i think) Thanks! Seth (Don't know how to post photos of the spark plugs... will use the search in the morning) Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted March 21, 2016 Report Share Posted March 21, 2016 Honestly it sounds like you cooked the top end. If it had 180 psi you are well past pump gas. What brand compression gauge are you using? If it's a cheap one it might be giving you a bad reading. The plugs backing out is a sure sign of detonation. Before you tear the motor down at all, do a leak down test also. It will tell you if you have any air leaks Sent from my VS990 using Tapatalk Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted March 21, 2016 Report Share Posted March 21, 2016 All of your fears about detonation issues are real... Sorry but octane booster isn't gonna fix anything. Just run the proper fuel a motor needs and avoid these costly mistakes. Tear it down and get it rebuilt and order a different set of domes if you wanna stay on pump fuel. Quote Link to comment Share on other sites More sharing options...
Surfrjag Posted March 21, 2016 Report Share Posted March 21, 2016 yep she sounds toast... unless you did compression test wrong Quote Link to comment Share on other sites More sharing options...
guns4children Posted March 21, 2016 Report Share Posted March 21, 2016 Shes a gonner. You could in theory take the head off and have a look inside to see what the extent of the damage is. If there are any loose chips in the top end just rebuild now. If you don't your expensive 4 mill crank is next to go. If you don't see "much" damage to the tops of the pistons you need to order new or used domes (yours are probably toast now toonif you had part of a plug doing the cha cha slide inside of your running cylinder) that will allow you to run on pump gas. Quote Link to comment Share on other sites More sharing options...
Shee_screams Posted March 21, 2016 Author Report Share Posted March 21, 2016 Honestly it sounds like you cooked the top end. If it had 180 psi you are well past pump gas. What brand compression gauge are you using? If it's a cheap one it might be giving you a bad reading. The plugs backing out is a sure sign of detonation. Before you tear the motor down at all, do a leak down test also. It will tell you if you have any air leaks Sent from my VS990 using Tapatalk Not sure on the brand of the compression tester that we used or how well the threads sealed up. Anyways buddy rebuilt his banshee this summer with a fresh bore, and in theory he figured that he should have around 160PSI and that gauge read 140PSI, so i don't believe its accurate (I wish I had a primary standard to calibrate it with lol). Anyways for the leak down test that would tell me if the plug was mangled from an air leak or detonation? I guess it's possible the guy I bought it from bullsh!tted me and the bike didn't have near 180PSI? All of your fears about detonation issues are real... Sorry but octane booster isn't gonna fix anything. Just run the proper fuel a motor needs and avoid these costly mistakes. Tear it down and get it rebuilt and order a different set of domes if you wanna stay on pump fuel. Given I don't know what what domes are in it or if they're still good. what dome size should i be running on that set up? My elevation changes occasionally. Right now my home riding area is about 2100 feet mark, however my "Home Home" is in the mountains and the base elevation of 1700 feet ranging up to probably 7000 feet. Also hit up St.Anthony once a year or so, just not sure how much the atmospheric pressure at different elevations effects the engines compression. Shes a gonner. You could in theory take the head off and have a look inside to see what the extent of the damage is. If there are any loose chips in the top end just rebuild now. If you don't your expensive 4 mill crank is next to go. If you don't see "much" damage to the tops of the pistons you need to order new or used domes (yours are probably toast now toonif you had part of a plug doing the cha cha slide inside of your running cylinder) that will allow you to run on pump gas. What do you mean by the crank is next to go? Is it taking a pretty big chance if the compression is that low even though the the top end doesn't look horrible? I don't want to wreck anything more but I also would like to get away with out a huge parts bill lol. Thanks all! Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted March 21, 2016 Report Share Posted March 21, 2016 With the amount of detonation that thing was forced to live through, you should be doing a full tear down to atleast check and inspect that crank. Your cheetah, is it an actual Cheetah with powervalves, or just a Cheetah Cub? And it's 4 mil crank in 68mm bore? If so, 24cc domes would make it pretty happy on pump fuel. Quote Link to comment Share on other sites More sharing options...
Shee_screams Posted March 22, 2016 Author Report Share Posted March 22, 2016 (edited) The guy I bought it off of didn't say it was a cub, but I think it's a cub, don't recall seeing any power valves. Approximately how much compression would those domes give me? Yes I believe it's the 68mm with a 4mil crank Edited March 22, 2016 by Shee_screams Quote Link to comment Share on other sites More sharing options...
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