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Discussion Starter · #1 ·
Hey Guys,

So the bike I bought last year for cheap ($300) with the intention of fixing it up and cruising around a bit, is looking like a bit too much work/money for its value.

1978 CB400tii

Low compression, so I'm thinking it needs a top end rebuild. Leaking near the head gasket pretty good, what are the chances it only needs a new head gasket to solve its problem? I would be okay doing that myself, but if I change the head gasket and it doesn't fix anything but the oil leak, I'll be out more time and money. Local shop has quoted me around $1k to do a top-end rebuild, but at that price, I might as well invest in something newer, I think. Assuming I could complete the top end rebuild myself (not sure), wouldn't I have to send out parts for machining and possibly buy some specialty tools?

So far: Rebuilt the carbs, new brake MC, new brake pads, new chain and sprockets, air filter, oil change, bought new rear shocks but havent installed yet. I'm probably $600 into the bike, plus whatever registration ended up costing (maybe $200, tags good for another year)

I've rode it only about a half mile at a time, and it sputters out once it warms up, making it unrideable. I'm assuming this is due to the low compression/ head gasket leak. Feeling pretty dumb at the moment, any advice welcomed.
 

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Rebuild the top end yourself. It's not that bad. Do you have a local machine shop? Do you have the FSM?
 

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Discussion Starter · #3 ·
Rebuild the top end yourself. It's not that bad. Do you have a local machine shop? Do you have the FSM?
I do have the FSM. Not sure on local machine shops, I assume so. I'm in Portland, OR. I spoke on the phone with Vicious Cycle here and they said of the $1k quoted, that some of that is for sending off parts to get machined, so I guess I could ask them who they use. What do you reckon costs on getting parts machined will come out to?

Thanks for the encouragement, I'm definitely up for giving it a shot at rebuilding the top end, it seems like that's my best bet at the moment.
 

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For me it would depend on the overall condition of the bike. If the bike is complete and cosmetically OK
I would rebuild the top end. I have done a couple of them and it is not that hard.
 

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How many miles are on the bike? And what was the compression on each cylinder?
 

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A compression test and/or a leakdown test could give you a better basis to make a decision.

If it is starting reasonably well and sputters and dies after it warms up the problem is something other than compression, most likely fuel. Are you using the choke to start it? Have you adjusted the valves?
 

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Time to start wrenching. Pull the engine out and pop the jugs off. Only then will you know what it truly needs. And, if you decide to bag it, you have already stated to part it out. There are a lot of people on the site who will help you out.
 

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Loss of compression, when warmed up, is a classic example of too little valve clearance.
 

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No need to beat yourself up. You have the FSM and this forum for help, plus I'm just across the river from you. I don't know the machine shops locally since I haven't needed one to date. If I did I would ask Beaverton Honda, Cycle Heap and Vicious for recommendations, Pro Caliber so far has been pretty much useless. Doc Brown moved to the Canby area and is basically finished with motorcycles.
Since these engines are not known for blowing head gaskets I doubt that's the low compression or running issue. Low compression is likely the rings and possibly needing a valve job.
So with the amount of time and money you've spent I would go forward and fix it.
 

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Discussion Starter · #10 ·
No need to beat yourself up. You have the FSM and this forum for help, plus I'm just across the river from you. I don't know the machine shops locally since I haven't needed one to date. If I did I would ask Beaverton Honda, Cycle Heap and Vicious for recommendations, Pro Caliber so far has been pretty much useless. Doc Brown moved to the Canby area and is basically finished with motorcycles.
Since these engines are not known for blowing head gaskets I doubt that's the low compression or running issue. Low compression is likely the rings and possibly needing a valve job.
So with the amount of time and money you've spent I would go forward and fix it.
Thanks for the words of encouragement, I think I just got bummed out that summer is almost officially over and never got more than a half mile out of her. Haha.

15k on the odometer and last time I (probably incorrectly) checked the compression it was around 95psi / 110psi. I will check the compression again to get a good reading, and go from there.

I checked the valve clearances and they seemed okay, what does a "valve job" entail?

Gas, I checked for the "whoosh" backpressure build-up, but it didn't seem to have a build up when it sputters out. Seems to come out of the petcock okay, but should I change that out just in-case? It has some new premium fuel in it.

Rebuilding the carb seemed pretty straightforward, so I am hoping I did that correctly. Although, I never had the carbs sync'd, maybe that could be part of it.

Cheers, thanks everyone for the answers
 

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Compression test is done with the throttle held wide open, both plugs removed and engine cranked until the gauge stops rising, usually @10 revolutions. 95-110 psi is so far out of spec, 185 +/-14 psi.
 
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