CB350 Engine rebuild: first timer, need some advice

SONICJK

Reminds me of...me No, I'm sure of it. I hate him
Alright guys,
First off this forum is awesome glad I found it..

I have a 73 cb350 that I picked up pretty cheap. It didnt run, so i did all the usual adjustments and still couldnt get it cranked (i didn't try that hard) I mostly got this bike as a toy and to learn about motorcycles and their engines (first bike).
So I began the process of tearing down the engine.
Ive got the whole top end off. I cant decide if i want to break into the bottom end or not...(opinions)
It turned over just fine, had (some) compression etc.

First question. I did a PB Blaster valve seat test.....
PA100195.jpg


and 5 minutes later...
PA100196.jpg


So it looks like I have a leaky exhaust valve.
So whats the consensus on what to do? I have seen some mention of a little lapping, and some say hell no. What do you guys suggest?
I have thoroughly cleaned the combustion chamber with a dremel and wire wheel and it looks good as new now (left the buildup on the valve faces for a little extra compression ;D .
So what do I do from here?


Second, I didn't do a full compression test, just did a thumb test and it felt pretty good, had a good strong kick to it.
I really don't want to have to bore it over, and the PO said it ran just fine before it died (electrical I think, had no spark when I got it) So should I replace the piston rings? How do I tell if they are still good?

Anything you guys recommend while i have this thing apart? Im going to paint the whole engine eventually. All new case bolts, all new gaskets, am I missing anything?

Thanks,
Justin
 
Well, first off...

Welcome to DTT.

Secondly, you need to clean ALL OF THE head and lap the valves. Check to see that theyre not bent also. Make sure the valves themselves are physically within spec, as is valve clearance. (If you dont own a manual, get one)

You'll need to make sure the cylinders are in good condition (no scratches, rings seat properly, etc...).

Reassemble the top end and do a proper compression test. The thumb over the spark plug hole means NOTHING. Thats the easiest way to "test" the state of your rings and cylinders. Make sure the top end is within factory spec FIRST though...

Only then can you start to diagnose the issues at hand. There are several possible problems you may find.

You didnt say what the mileage was. That will help to determine some of your next steps. The motor needs certain adjustments and services depending on the mileage. Again, the service manual will explain these in detail.

Ok... Now onto the POs claims. Every bike was running "fine" before it died. Ive bought bikes that were complete wastes that "ran great when I parked it". All POs say that. Dont believe it, even if theyre probobly right. Assume the bike is in need of the most serious work you can think of. It just may be...

Oh, and electrical issue = no running bike. Find the gremilins before you try to get it running. It'll save you from wanting to shoot yourself later.

Well, thats all I have for ya... Good luck, and...


BUY A MANUAL IF YOU HAVENT ALREADY!!!!


It will save you a ton of time and trouble.
 
Thanks!

I have a manual, (several actually)
Everything is really clean now (lots of dremel time and a lot of simple green)

Is there a write up somewhere about how to lap valves? I get the concept but have never done it before so a write up would be helpful.

Dunno what the mileage is (not stock speedometer/odo)
The cylinders look good, perfectly smooth and the rings look good as well, how do I check that they seat properly?
Sorry for the newb questions, I am a lot more used to suspension and fab work on jeeps than this motorcycle thing.
So far its awesome though!
Thinking about grabbing a cb550 to make a cafe racer. Oh its just as bad as the jeep sickness!
 
I would not lap any valve ever take the head to automotive machine shop and have a vac. Test done. They will tell you what the next step is 350 valves are big enuff that tooling will not be a problem (some mc valves are small enuff that auto machine shop dont have the proper size cutters) i dont see any point in rebuilding just to pull back apart go throu the head and re ring and hone the jugs and replace the cam chain & rollers while your in there. ask von what happens when they break, i would leave the bottom end alone for now good luck and get a manual. The only way to own you bike is to rebuild it then there is no one to blame but you lol
 
Not quite sure why you tore the engine down. That's not usually the first step one takes when an engine doesn't start. <G>

First you eliminate the usual causes of an engine not starting.
Is it getting fuel?
Is it getting spark?
Does it have good compression?

A shot of starting fluid can often answer those first two questions.

Once you resolve why an engine doesn't run, THEN you determine it's mechanical condition. A no start condition can be a simple as needing to clean the points, or a valve adjusted too tight. It is just plain silly to pull an engine apart just because it won't start.

Well, too late for that advice. Now that you have already blown the money for gaskets, have the valves reconditioned by a competent machine shop, lightly hone the cylinders, and re-ring. Rings are cheap, and it would be silly to put it back together without changing the rings.
 
Don't forget there are two parts to valve sealing as well. The valves themselves are only half the equation. You also need to check the mating surfaces on the head. Even a very small nick will screw you over and it's possible you may be looking at regrind. I would avoid trying to so this kind of work at home. While it's certainly possible to get a good seal using an "at home" lapping kit, you almost always end up with too wide of a contact patch between the valves and the head.
 
Yup,
Pulled it apart for fun mostly. And to clean it up and get a nice coat of paint on it.

Anyone have an idea on how much it should cost to have it bored .25 over?

And the valves....conflicting views already haha. If it leaks liquid it obviously leaks air, no need for a vac test I don't think.

Whats the cost (estimate) of getting the valves set up correctly at a machine shop?

Again, new to this and just graduated from college waiting to hear from med schools so also broke.
 
It will be fine there pretty simple and parts arnt bad money wise but i would replace the cam chain rollers and remember this will make you better and the history books are writen by the victors.
 
SONICJK said:
Yup,
Pulled it apart for fun mostly. And to clean it up and get a nice coat of paint on it.

Anyone have an idea on how much it should cost to have it bored .25 over?

And the valves....conflicting views already haha. If it leaks liquid it obviously leaks air, no need for a vac test I don't think.

Whats the cost (estimate) of getting the valves set up correctly at a machine shop?

Again, new to this and just graduated from college waiting to hear from med schools so also broke.

I've never had valves done before (No problems? Leave it alone!) but when I went +2mm over on my 360 last year it was $20 for every .25mm.
 
That isn't how you test for leakage.
Stand the head up then fill the port, much easier to see whats going on.(even though it takes a little longer)
Should be able to get a 3 angle valve seat for about $20.00~ $25.00 a seat ($80~$100.00 total)
Probably get all of it done properly for $125.00
Boring will probably run from $35~$50 each hole
If you want higher compression, skim head 0.020"
 
Price varies around the country.
It's getting more difficult to find shops that can do 'small' cylinders
Now I'm part of the 'invisible' unemployed, I can't get boring done
 
Ya, last place I called up here wanted $90 a cyl. Another guy said "dont know if my machine will do it, but hell Ill try for $50 a side". Haha... No thanks man.
 
You could always find a 'normal' machine shop and get a quote
You need about 3~400 rpm with 0.004"feed/rev and 0.030" tool radius, zero rake.
Make sure they don't take more than 0.010" a pass as cylinder can heat up and liner can 'spin'
It can be done on a Bridgeport mill with a boring head as cylinder is less than 5" long (most have 5" quill travel)
I used a Bridgeport for first few bores I did (big bore CB160, fitted 175 pistons ;D )
Really need a shop with rigid hone to finish it though
 
Ive done jugs on a rod resizer wet hone but it sucked. my old job got a power hone a month after i left it will go to a real small bore, fuckers know better than to get it while i worked there i would not have done any customer work lol
 
<<Pulled it apart for fun mostly. And to clean it up and get a nice coat of paint on it.>>

I have to admit that I would pull an engine apart just to bead blast it and paint it. <G>

<<Anyone have an idea on how much it should cost to have it bored .25 over?>>

Why? You already said the cylinders look fine.

<<And the valves....conflicting views already haha. If it leaks liquid it obviously leaks air, no need for a vac test I don't think.>>

You could have had a valve with a little crud on it from the bike sitting with a valve in the open position. After running for a few minutes, it might have been sealing up just fine.

You really should have focused first on getting it running, then evaluate the engine's condition. I understand the urge to take apart just for entertainment value, but it probably wasn't the best strategy for a starving student budget.
 
I'll run a motor if I have to but I 'do' motors so often I'll build it the way I want (just because I can ;D )
As long as I have at least 1 bike running, I'm good 8)
 
What done is done the head gasket seal is broke the guy that owns the bike is willing. what is the problem with a just to see how it works and make it better rebuild... MAN Sweet pea, you really feel the need to talk down to everybody dont ya
AlphaDogChoppers said:
You really should have focused first on getting it running, then evaluate the engine's condition. I understand the urge to take apart just for entertainment value, but it probably wasn't the best strategy for a starving student budget.
 
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