Old bike, engine advice

Oh wise one's, please fill me with your years of knowledge. This will be my first old bike, 1976 Honda CB400F (Four), shed find, very rough, apparently runs (although no carbs with bike; PO used his 350F carbs to test). No knowledge if the gears are ok as the PO didn't spin the motor because the rear wheel is damaged.

Used to high performance dirt bikes where parts are ususally replaced at the slightest hint of use, just rebuilt my KTM 2t enduro because it sucked water into the crank case.

My philosophy with motors has been to replace when in doubt, however I am not sure how robust these things are, my previous dirt bike was a WR and at 7,000 k you would probably be looking to replace the whole valve train. I would rather not be doing un-necessary replacements if its overboard.

So...

1. Should I look at getting some carbs and running the bike prior to splitting it open?
2. What should I look at replacing in the motor when I split it apart? I am probably going to bore the pistons out to 466, does that change the answer?

Was currently thinking of replacing gaskets, crank seals, crank bearings, cam chain and slider.

Thanks in advance.

I'd get some carbs, then do a compression, and leakdown test. That'll give you a general idea of how your engine is doing before tearing it down.

No problemo!

What are the wear parts in these motors? And how many klm's do you get out of them before they require a rebuild (assuming no damage)?

Reason I ask is that I see people talking about motors with 46,000 klm's on them and are not talking about a total overhaul in the top and bottom. That sounds like a bullet proof motor to me and I think I may be waaaaaaaaaaay too cautious in my planning at this stage.

Would rather not be replacing bits if its not required, the costs soon add up.

Buy a rack of carbs. Simple clean, throw 'em on, hook up an iv bottle and fire it off. If it'll idle, and doesn't sound like it's full of spare change, you're probably good to go.
Make sure and check the oil before testing, and I'd even recommed an oil change (with filter) before hand depending on how long it's been sitting.

Carbs are a must, obviously. Then give it a good clean and complete service. Check/replace points and plugs, air filter, petcock filter and do a compression test.

If it's all OK so far, go ahead and run it. They are very under stressed motors compared to that WR and with care will probably last longer than you want to ride it. OIf a prior owner let it rust or ran it without oil or did other damage, all bets are off.

Keen to tear into the motor anyway to check things out. What should I replace as a minimum:

Gaskets
Valve chain, gear and spring mechanism
Primary chains

Anything else?

neevo said:

Keen to tear into the motor anyway to check things out.

Click to expand...

What is it about all the newbies who can't wait to tear apart an engine that in all likelihood is perfectly serviceable?

Start by getting the engine running. Don't waste time/money tearing the engine apart just to evaluate its condition. Evaluate its condition by running it!

Build your bike, get it running, THEN you can do a valid compression test and listen to the engine with a stethoscope. It will tell you if it needs attention. Unless somebody fucked it up, that engine is most likely perfect.

BTW, you don't need a set of carbs to test compression or to do a leakdown test or to check oil, points, valve clearabces etc. Give it a complete service and test what you can while you search for carbs.

teazer said:

BTW, you don't need a set of carbs to test compression or to do a leakdown test or to check oil, points, valve clearabces etc. Give it a complete service and test what you can while you search for carbs.

Click to expand...

Ah, that's good to know. Was going to compression test today as I have the bike stripped down with the motor still in the frame.

AlphaDogChoppers said:

What is it about all the newbies who can't wait to tear apart an engine that in all likelihood is perfectly serviceable?

Click to expand...

Totally understand, must get frustrating. But I wouldn't say I'm a newbie when it comes to motorbike engines. Rebuilt a few hi-po enduro motors which is how I came across this project, I enjoy the wrenching and wanted something a bit more involved.

I want a brand new bike too, something I don't have to worry about. Cam and primary chains, seals etc is pretty cheap insurance, especially with the extra grunt of a 466 kit

If it has less than 30,000 on the clock, just ride it.

neevo said:

Totally understand, must get frustrating.

Click to expand...

Naw. I don't get frustrated with what others decide to do with their own bike. It does seem like a frequent pattern that guys get their first motorcycle, have very little experience wrenching and very few tools, and the first thing they do is tackle tearing down an engine. They jump in with both feet on the most critical and difficult jobs without even really having a reason to do so. They don't even know the basics about engines like how to properly do a compression test, and are poorly qualified to do engine work.

I'm not saying that's you, just saying that engine teardown should be done later, AFTER the engine has been run. A compression test on an engine that has been sitting for a long time will always read poorly. It doesn't mean the engine needs work, it just means it needs to be run!

70's Honda's had a 100,000 mile design life, IF used within design parameters and serviced at recommended intervals.
One bike I know about with a 12 hr design life was the Suzuki RMZ450 when first launched.
The servicing has now been extended to 30 RACING hours on crank.
KTM is similar, although a lot more expensive (I did a 525 head about 2 yrs ago, titanium valves, $170.00 EACH)

crazypj said:

KTM is similar, although a lot more expensive (I did a 525 head about 2 yrs ago, titanium valves, $170.00 EACH)

Click to expand...

And that's why I have a KTM 2 stroke.