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Old 06-22-2013, 11:06 AM   #1
ragge_ OP
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DR 790 Big bore too tight

Hi! Im crossposting this from tt, will post updates soon as I get too the bottom of this. But anyways, this is it

I need some advice, as usual i tune the dr some for the annual bike vacation i take. This year, the mighty 790 kit is installed (and holy crap it pulls). Since i live in sweden i turned the cylinder in at a local firm for the drilling, which now seems too have been a misstake, the bore is too tight o.O
I cant get the starter too move the piston, all i get is a clicking sound from the starter relay. Too ensure that the issue is a too tight fit, piston - cylinder, I did a bumpstart (got pulled by a car) and the bike came too life.
From fully charged battery, the startmotor got power too move the piston just a bit, my estimation is aprox 1 revolution, then the starter relay clicking again.
Today I rode the bike 40km (64miles) from a bumpstart, hoping the piston would break in enough for being able too use the electric start, no dice, its still too tight/heavy too move.

Has anyone encountered this problem before, and if so, found a way too fix it?
What do you guys think, should i do a pulled by car start again and just take a freakish long ride and see if everything gets seated?
Any risks i can have overlooked?

And btw, once the bike was running, it was one hell of a beast! Pulled strong and felt solid, bit weak in the upper mid range, got some jetting too do i guess. The low end was just amazing!! 3.5k-4k rpm felt way stronger than the 644 @ 6k rpm

Any input on this will be very appriciated :)
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Old 06-22-2013, 11:18 AM   #2
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Did you mic the bore and gap the rings? Never put it together before that is done.
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Old 06-22-2013, 11:26 AM   #3
itsatdm
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http://advrider.com/forums/showthrea...=860980&page=5
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Old 06-22-2013, 12:20 PM   #4
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If he's already ridden it 64 miles, I doubt he screwed up the ring gap. They would have torn themselves up by now and he'd be able to hear it.
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Old 06-22-2013, 01:40 PM   #5
itsatdm
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You did not read it did you? I will be more exact. Read post 71. Sounds kind a similar to me, even melting down after doing 60 trouble free miles.
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Old 06-22-2013, 01:41 PM   #6
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Yeah, I kinda doubt it's too tight if it runs O.K. without seizing. I'd check cables & connections first. Battery may be too weak also.

I'd also pull the oil filter & look for shrapnel before I'd go too far tearing it apart.

Good Luck,
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Old 06-22-2013, 02:18 PM   #7
itsatdm
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I hope you 2 are right. The first thing I would do is contact the person who assembled the engine and see if he gapped the rings and if he did, to what spec. If you read Planeman's post, he did not do that and the rings do not come pre gapped. There is warning included to do that.

You mention a machine shop that did the drilling. The kit comes with a liner and the machine work is to install that. I guess they could fail to make the hole big enough and with enough heat and pressure force the liner in. Probably would cause some distortion, did you measure it? Did you measure piston fit without the rings?
It Doesn't seem likely IMO.


I have done a big bore. It is a slip fit once you get the rings constricted and I could do that with my fingers. One thing I did before trying to start, is crank the engine with the plug out, a half dozen times to get oil to the upper end. Doesn't sound like that was done.

The engine should start on the bike battery and this one won't start even with a jump. Something isn't right

Planeman got 60 some miles out of the engine before it blew up, so the fact that the OC has also proves nothing.

If my bike, I would not ride it until I pulled the head and checked the cylinder.
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itsatdm screwed with this post 06-22-2013 at 06:42 PM
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Old 08-10-2014, 11:20 PM   #8
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^ another expert


Though.....he's probably right. This grumpy bastard knows a bit.
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Old 08-10-2014, 11:29 PM   #9
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Pull a spark plug and see if the thing turns over easily with the starter motor. If so your problem is likely somewhere else.
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Old 08-11-2014, 01:03 AM   #10
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Quote:
Originally Posted by itsatdm View Post
You did not read it did you? I will be more exact. Read post 71. Sounds kind a similar to me, even melting down after doing 60 trouble free miles.
I read that build thread too, and this sounds like the same thing. The rings DO NOT come pre gapped.
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