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95 sp 587 backfire

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It's a core exchange engine from sbt so I really have no idea on the internals. Here is a pic at tdc of the rotary valve with degree wheel... one thing I noticed is all the bolts for the valve cover are the same size and there was a sticker on the core which said long bolts in bottom...
 

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Sorry for quick short posts but in the shop wanted to note that I never understood why the ground was attached to the starter...seems like it could short out if there is a gasket in there especially.
 
Ryan356 said:
The negative yes.
Click to expand...

The negative from the battery goes to the ear on the starter and the positive in on the rear part of the starter. Every negative I have seen on cars (I dont generally work on these in case you couldn't tell.) Has been directly to the block.
 
Well, the starter is the big current hog so that's maybe not all that bad. The starter bolts connected through the starter frame into the casings should be good enough ground for the electronics and general purpose.

The spot facing where the cable ring-eye is connected should be clean of paint before the cable is attached.
 
Ryan356 said:
It's a core exchange engine from sbt so I really have no idea on the internals. Here is a pic at tdc of the rotary valve with degree wheel... one thing I noticed is all the bolts for the valve cover are the same size and there was a sticker on the core which said long bolts in bottom...
Click to expand...

So this motor never ran correctly since it was installed?
 
Sportster-2001-951C-Stock said:
This is what I'm thinking as well, just I have no way to prove it. Cylinder casting number?

And I'm still wondering if you have a big air leak. I realize this doesn't help much.
Click to expand...

It would be an air leak between the carb and engine wouldnt it? This is why I mentioned the bolts that go in the rotary valve cover. I had suspected that possibly a couple of the bolts may be too long and not allowing the ring seal to seat but I have no way to confirm that.
 
Ryan356 said:
It would be an air leak between the carb and engine wouldnt it? This is why I mentioned the bolts that go in the rotary valve cover. I had suspected that possibly a couple of the bolts may be too long and not allowing the ring seal to seat but I have no way to confirm that.
Click to expand...

Yeah, an air leak will not run well depending on how large, others have reported air leaks around the cover that gave them lots of problems. Call SBT and see what they say?
 
I started with sbt. Their tech support recommended the new valve and cover due to the tolerance being out. I have the new one installed. I should mention that the new one with the new rotary valve is out of tolerance as well a bit but wasnt sure this would cause this massive backfire. The guy also thought it was the mpem after that...that's why I got the used ebox... after that he basically said he didn't know.
 
Ryan356 said:
I'm leaning towards not four stroking. This thing backfires violently... I am also wondering if there was something changed in this engine core from sbt where it may require a different valve setup or something...?

Also is there a certain position that the flywheel should be in at tdc? I wondered if it was possible to spin the center part of the flywheel if it got overtightened...I suppose that's a possibility as well. Still puzzled.
Click to expand...
Had a 720 that was backfiring through the carb and couldn't for the life of me figure it out. Checked and double checked everything. Swapped parts from it's good running twin ski, and still nothing. Finally noticed that the center of the flywheel had a small crack from one of the bolt holes to the edge of the center hub. Upon further inspection, I found that the flywheel had rotated on the center hub. Replaced with a used flywheel and it ran perfect. I've never seen this happen on any other ski, but may be worth taking a look at at this point as you've gone through everything else
 
Ryan356 said:
Is there a way to see if it has spun? For instance should there me a marker in a certain position at tdc?
Click to expand...
There are no alignment marks. I referenced the keyway in the center hub to holes on the flywheel and compared with known good flywheels to determine it had spun
 
Yeah I had found a replacement one on Ebay and compared pics to what I had and they matched. However, I heard there was a flywheel change around 95...I wonder if this could come into play...?
 
Another thing I just remembered which is making me wonder is that when I first got the new engine I compared everything before I sent the core back. One thing I noticed was that the pulse line was on the opposite side of the carburetor. I'm just wondering if it's possible that this is a different block or something? I had asked the sbt tech at one point and he said it was fine. All else looks the same. The sbt listing says 92 to 96 for the white motor as well and I noticed that there is a different flywheel for the 92's...also stators appear to change in 93 so not sure if this means anything...
 
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