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Bought a 4mil stroker and it doesnt run properly


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Guest Brapppitron

I bought a 4mil j arm bamshee stroker has fmf pipes stage4 trinity head with 20cc 9mm domes charity intake and uni pod filters and bigger carbs not 100% on the mm has stock cylinders and no porting it had stock pistons in it and hasnt ran properly yet bottom of one of the stock pistons snapped off so I ordered new wiseco pistons still waiting on one more to show up but the bike has been a slug oh and it has a spacer plate I just ordered a timing plate and reeds and carb sync tool amd jets with the new pistons my questions are 

1. Was it running horrible cuz the pistons were wrong? 

2 what should my timing be set to when installing the plate 

3. I'm 1100 above sea  level where should I start with jetting 

4. Do I need to get it ported or different cylinders 

Thanks guys for any info this is my second banshee but last one i had went 2 seasons without having to even change a plug before I sold it and regretted it for a year before finally saving up for this one I trusted the guy and paid 5g all to be very disappointed in his work now I have had it a couple months and finally got motivation to get the beast unleashed  so pretty new to working on them I'm pretty mechanically inclined but not a small engine mechanic in any means 

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You said it is a 4mil, which having a spacer plate makes sense. If it has stock cylinders that is fine, but if it has stock pistons then that is your problem. The stock replacement pistons are 513s. If it is a 4mil, then you need to be running 795s. Running the wrong piston is probably why one of them failed. 

Next, you say you are running 20cc domes, that means this bike is setup to run on either 105 or 110 octane gas. If you are trying to run pump gas, then that is a problem as well. You need to run 23cc/24cc domes if you want to run 93/91 octane out of a pump. If you are going to change domes, get some cut domes so you dont need the spacer plate.

To answer your questions:

1: It could be what I mentioned above, wrong pistons or wrong domes for the fuel you are using. You didnt mention your fuel ratio, you did say you just bought it so you should know that you need to be running 2stroke mix in your gas. Personally I use Amsoil Dominator at a 40:1 ration, and thats what I would recommend as well.  It could also be your jetting as I will talk about on 3rd question.

2: with your pipes I would run +3/+4 for timing. 

3: Jetting varries motor to motor. You said big carbs but didnt mention their exact size or brand. I have a similiar trinity 4mil that is a 397 with its 66mm bore. It runs Mikuni 34s with main jet at 280 for 1800 elevation. I would start there as your build isnt to far off from what I have. Put in fresh spark plugs, ride it hard for a bit with WOT(wide open throttle), then pull the plugs. If they are black/wet then you need to go down a size and start over with fresh plugs. If they are white/ light brown then go up a size and try again. Ideally, you want them to be a chocolate brown.

 

I will check back for any questions you might have or help you need.

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Thanks a bunch grant lots of good information for me to start trying to figure this out I'm guessing pistons and gas would more then likely be my issue as it started and ran just fine but just had no balls what so ever oh and  my carbs are also mikuni 34's would you suggest me trying to find some one that does porting and get the cylinders I have done or try and find some cylinders for the 4mm

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Race gas definitely means more power, but it is expensive to run. When I go out to the dunes, I run through roughly 10gallons a day, which comes out to $150 daily to run 110. If you don't ride much, or your wallet is fat,  then go for it.

 

You should be fine with the cylinders you have, if you want more power send them out to one of the sponsors of BHQ for a dune/trail port. You said you ordered new pistons, you should read the number on the top of the old pistons to see what size they were, that should have told you the bore size. If they were the 795s for a 4mil and it truly was standard bore, they should have read as 795M06400. The last 4 denote the bore size, 64.00mm is standard. If you are putting in new pistons, I'd recommend just getting a bore done when you send the cylinders out for porting. This way you can get the pistons matched for your setup. Go to like a 64.50.

 

When I bought my trail bike it was already at 65.5. this means it had a bore done several times, which was expected from a 30+chest old bike. You should make sure of your bore size by checking the old pistons out, as people do this usually when they purchase the bike and most banshees are now old enough to buy alcohol.

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Your domes will determine the compression, but you need to have the right fuel for the compression. If you're running high compression domes, running the wrong fuel will damage the motor and obviously cause loss of power.  He is right that fuel does not determine power, but running better fuel will make you able to run higher compression domes.

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  • 1 month later...

how much horse power should i be pushing with my set up?

Fmf fatty gold series with core 2 

Vforce 4 

Pwk 35 

+4 timing 

Stock bore no port job 

Trinity stage 4 cool head 20cc domes

Uni pods no air box 

Charity intake

4 mil long rod 

Am i missing out on much power by not having a port job done or by having stock bore ? Should i bore it and how much can i bore on stock jugs safely they are brand new should i try and sell them or trade for differnt instead ? 

 

 

 

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  • 1 month later...

So.another question as I have the bike back together after a full rebuild now I have low compression could this be due to squish and my domes ? As I havent actually checked squish as i want to ditch the spacer plate and get my head cut I'm running charity stage IV 20cc domes  ( the domes have pitting on them  so figured it would also be fixed with cutting )but only have about 80 psi I'm wondering what I need to tell the shop to shave from it for my 4mm

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