DR 100 top end help

[Butthole-surfer]

I’m getting ready to put my childhood bike on a milk crate and get it running for my nephew. If I learned on it he can too. Problem is it lost compression a couple years back so I’m about to dig in to the top end and redo the brakes, bearings ( rear end is seized shut won’t roll). And hopefully make it run. My main question is would it be better to bore it a little rather than find a new top end? This is literally my first time working on a bike and I’m trying to learn so please be gentle. If it helps then yes, I used to live under a rock. 

[turboguzzi]

Are you able to start it? If yes, compression could come back just form running it around for an hour or two with new oil an filter.

if that doesnt work, then i'd dismantle and check what needs to be renewed. even valves can be reused after some lapping on the seats and maybe just a new set of rings will set the piston right.

a complete new top end will be worth more than the bike....

[Butthole-surfer]

No it won’t start. It used to be able to be drag started and would stay running but it sat for a while and that no longer works. I was gonna just pull the top end off and possibly have it resleeved or oversized slightly to save the jug. I’ll update once I get the damn thing apart. I figure minimum it needs new rings, piston? A chain, a carb rebuild, gaskets and bearings along with brake shoes and it should be mint. It was a tank before a previous owner mistreated it before I got it back. Thanks for your help!!!

Edited June 8, 2020 by Butthole-surfer

[S.O.A.N.Z]

i guess you wont know until you pull the jug off where to go next

[turboguzzi]

agree with soanz, it could be just rings a bit stick, rust on valve seats.

[tripicana]

I just did a top end on a dr100. Take a close look at the rocker arms. Mine were worn, and the cam was worn also. Got it back together and the damn thing still wouldn’t start. Ended up being a bad cdi, search TT for dr100 cdi conversion, $10 Amazon cdi got it running. Google dr100 service manual on reddit, that’s where I found a manual.

i would do a compression test first, anything close to 100 psi should run. If it doesn’t even cough with ether, and still has spark, suspect cdi, test per manual.

[Butthole-surfer]

Cdi tested okay almost no compression. Got it apart Everything looks great for what it is. valves are tight no play  rockers aren’t worn. Piston is in good shape rings looked alright and the jug was flawless. Got new rings today and gaskets on the way anyway gonna do those and new oil and filter and rebuild the carb and see where it’s at. Worst it could do is nothing. Rear end can move now. Needs sprockets and shoes since the old ones were rusted shut hence no rolling it around. Mice chewed the headlight wires but that’s a simple splice. Seems like I just need to restore compression at this point and the rest are simple fixes in comparison ?. 

Edited June 14, 2020 by Butthole-surfer

[turboguzzi]

how are the valve seats? is somewhat rusty, lap them with fine grinding paste