RD400 first run after rebuild, very low power

[Striker1423]

retorque the heads. 18-20ft-lbs, you can also spray some permatex copper gasket sealer on them.

[ElFuegoBlanco]

retorque the heads. 18-20ft-lbs, you can also spray some permatex copper gasket sealer on them.

Are you saying to remove them, spray permatex on the gaskets, then reinstall and retorque?

[Striker1423]

I would remove them, clean them, spray with permatex copper (2-3 coats each side), reinstall and torque to 18-20ft-lbs.

[ElFuegoBlanco]

I would remove them, clean them, spray with permatex copper (2-3 coats each side), reinstall and torque to 18-20ft-lbs.

Thanks for the advice. Not how I was hoping to spend the weekend, but that's life.

Any idea what could have cause it?

[Striker1423]

Pulling the heads to look is easy. Are you positive the head gasket is leaking? The thing is, if you had that big of a leak, the head was either loose, or it's warped. Cause 120psi is good compression. If the head was leaking enough, you'd be seriously low on compression.

Before going too far, I'd check to make sure your exhaust gaskets are good, and also check to make sure the exhaust headers aren't leaking at the flange first. Those leaks will sometimes waft upwards and make it look like your heads to blame. Also, oil pouring down the cylinder from the exhaust flows all over when you ride. Could explain that too.

[SoCal250]

+1 Agree with what Striker said
Also make sure that after the engine has been run (heat cycled) that you re-torque the head bolts. 

[ElFuegoBlanco]

I'd say I'm sure. It's blap blap blapping out the side of the motor between the cylinder and head.

I wish my camera wasn't busted (and that my new phone arrived on time this morning instead of being shipped to Indianapolis (I'm in WV lol)) or I'd take a video of it. I'll go ahead and pull the heads to take a look. I know it's not hard, more like an inconvenience. It wasn't leaking before this last bit of work. All I did was change the throttle cable, change the timing from 2.2 to 2.0mm, and chang the float valves.

+1 Agree with what Striker said
Also make sure that after the engine has been run (heat cycled) that you re-torque the head bolts. 

I haven't retorqued the heads since installing them. I'll try that first just to be sure.

[Striker1423]

You can retorque the heads sure, but I would recommend you check and clean the gaskets before just applying torque. The leak will build up crap on the gasket surface.

[ElFuegoBlanco]

You can retorque the heads sure, but I would recommend you check and clean the gaskets before just applying torque. The leak will build up crap on the gasket surface.

Will do. Thanks for the advice. Tank is off, I'm off to get some permatex copper.

[SoCal250]

You can also anneal the copper head gaskets if they're being reused.

[Clem710]

Anneal and clean gaskets, make sure heads are flat, check to see if cylinder is flat, be sure head studs are clean and that the bolts thread easy, make sure you use the washers.  Reinstall and tighten slowly and evenly in an alternating crisscross manner, use a torque wrench and just keep going back and forth.  Theres probably a pic in a manual for which order to do them in but I doubt most people follow it.

Assuming its sealed, heat cycle, then check em all again. I try em at the low end of the spec kinda thinkin that its less likely to warp the head and I can always go up but thats probably all meaningless if you do it evenly. 

Sometimes I'll use a very thin film of goop on ones that dont like to stay in place.  Permatex copper certainly doesnt hurt anything.  In theory, neither should be necessary.

Doing them slowly and evenly is the trick imho.

Done probably less than a 50 so not a pro, there are certainly plenty of them here though, you can always check the manual, it should be in there:)

If you are up in the panhandle or at least above Wheeling, I'm not far at all and could help, but again, not a pro, just patient and persistent.  Been done quickly in the pits by racers 1000s of times so you should get it.

Lol, I type too slow but here it is in one place:)

[ElFuegoBlanco]

Annealed and sprayed with permatex copper spray. I wasn't fast enough to read the reply saying to just anneal.

So, now that permatex has been sprayed, how long should I wait before installing? The can just says to let the solvent to evaporate before installing.

[ElFuegoBlanco]

If you are up in the panhandle or at least above Wheeling, I'm not far at all and could help, but again, not a pro, just patient and persistent.  Been done quickly in the pits by racers 1000s of times so you should get it.

Thanks for the offer. I'm farther down, south of Morgantown.

[m in sc]

retorque and check again

[ElFuegoBlanco]

retorque and check again

Okay, so I'm back to where I was at the very beginning of this thread. Heads are torqued down and the leak is fixed. But it just won't rev under load. In fact, my driveway is sloped and I can't even get it up to the road. It bogs down to the point where it dies (this is new, it didn't die before, but I wasn't trying to get it up the driveway).

I can't think of what I'm missing here. Here's where I am:

Moved from 310 mains to 250
Moved needle clip from second notch from the top to second to the bottom, then the middle, then back to second from the top.
Cleaned the carbs again
Swapped the throttle cable (the carb ends where pretty bent up)
Swapped the float valves
Moved from 2.2mm btdc to 2mm
Checked and rechecked the oil pump gap

I haven't changed the reeds. They looked to be in good shape when I got the bike,though they are probably original.