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Discussion Starter #1
Recently bought a CB450 with an intention of rebuilding it. Just did a cold engine compression test. My first time doing a compression test, just following instruction that came with the tester kit. With throttle open, kicked it (kick starter) 4 - 5 times and it reads about 75 psi on both sides. Is this normal? or am I doing the test right? I read in some other posts, it needs to be somewhere around 150 psi. Would appreciate any help. I'm not exactly mechanically inclined, just trying to learn as I go along. :|
 

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Just make sure that your throttle is wide open and the choke plates are fully open as well.

If you redo the test and it's still that low then it's long past due for a rebuild.. That's VERY LOW.


GB :mrgreen:
 

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Sensei
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It needs to be somewhere around 180 PSI.... 160 is MINIMUM spec....
Warm/hot test is preferable as cold it may read from 7 to 10 PSI low.... Throttle wide open, no choke... (plugs out helps)...
 

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Discussion Starter #4
Thanks. I'm pretty sure I did the tests correctly. However, I find strange that both cylinders are equally VERY LOW. Some other posts also talked about compression below 100 psi isn't worth rebuilding. Would it be because too much work is involved? Never took an engine apart, so I haven't an idea how complex the job will be.
 

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ninepeaks9 said:
Thanks. I'm pretty sure I did the tests correctly. However, I find strange that both cylinders are equally VERY LOW. Some other posts also talked about compression below 100 psi isn't worth rebuilding. Would it be because too much work is involved? Never took an engine apart, so I haven't an idea how complex the job will be.
Not strange at all, just worn evenly.
At the worst, new pistons and a valve job will have you back at 180 psi. Could be less than that even......
 

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As long as you have a good workspace like a garage and patience you should be fine. I'm on my first honda twin as well and it's very easy to work/learn on. I'm more comfortable under the hood of a fox chassis mustang than anything else; but these bikes are way simpler. The brains on this forum are invaluable for such work though, make sure you take lots of pics.
 

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Sounds like the problem I had when I first got my 450.

Although it looked in pretty good condition from the outside, it had been stood for quite some time. As a result of the inactivity, the top piston rings had frozen to the sides of the bores - and when the PO tried to fire it up, he used the starter without first putting any thin oil (to free up the rings) into the bores. Somewhat unsurprisingly, the starter power with a new battery overwhelmed the rings and they snapped.

But before going mad, remember to check all of the usual suspects like valve clearances before yanking the engine out. If you decide to pull, opening the engine up is pretty straightforward. Just take your time and work methodically.
 

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Discussion Starter #8
the-chauffeur said:
Sounds like the problem I had when I first got my 450.

Although it looked in pretty good condition from the outside, it had been stood for quite some time. As a result of the inactivity, the top piston rings had frozen to the sides of the bores - and when the PO tried to fire it up, he used the starter without first putting any thin oil (to free up the rings) into the bores. Somewhat unsurprisingly, the starter power with a new battery overwhelmed the rings and they snapped.

But before going mad, remember to check all of the usual suspects like valve clearances before yanking the engine out. If you decide to pull, opening the engine up is pretty straightforward. Just take your time and work methodically.
Thanks. The bike had been sitting for 20+ years before PO found it. PO did the minimum required to get the bike running when I bought it from him. Maybe, he went through same type of situation you describe.

I'm going to check valve clearance, but does valve clearance make that much difference in compression readings? If it's similar in shape to other things I've noticed like the points ignition gap (there's a wide valley there), it's probably out of whack too.
 

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ninepeaks9 said:
I'm going to check valve clearance, but does valve clearance make that much difference in compression readings? If it's similar in shape to other things I've noticed like the points ignition gap (there's a wide valley there), it's probably out of whack too.

If the valves were too tight, they wouldn't close all the way - you wouldn't be able to build up compression.

Loose is a little different - they'd have to be really loose to affect it much.

Don't get caught up in the point gap Game - read the article here in Tune-Ups and Tricks section.
 

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Discussion Starter #10
What do you mean by point gap game? I just got my feeler gauges delivered. Do I need to drain the oil when I remove the alternator cover?
 

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ninepeaks9 said:
What do you mean by point gap game? I just got my feeler gauges delivered. Do I need to drain the oil when I remove the alternator cover?
No, you don't have to drain the oil, though there may be some leakage.

Read the thread about timing these bikes - turns out the point gap is the least of your problems.
Setting the timing is a little goofy....

http://www.hondatwins.net/forum/viewtopic.php?f=18&t=883
 

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Discussion Starter #12
OK, I found sometime to.... remove fuel tank, spark plugs, alternator cover, breaker point cover, carburetors and then onto valve covers (or what I think are valve covers - screwed on cover on the same side the breaker point cover is on and the other one on the opposite side). Couple screws on these covers won't budge. I've managed to mess up couple screw tops. Plenty of WD-40 won't work. Any suggestions?
 

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First, If you aren't sure what parts are the rocker covers (valve covers) you should check your manual.....IF you don't have a manual, get one....The 450 engine is not all that complicated, but it has to be taken apart properly to avoid damaging parts....IF your only problem is reluctant bolts/screws, use an impact driver....
 

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ninepeaks9 said:
OK, I found sometime to.... remove fuel tank, spark plugs, alternator cover, breaker point cover, carburetors and then onto valve covers (or what I think are valve covers - screwed on cover on the same side the breaker point cover is on and the other one on the opposite side). Couple screws on these covers won't budge. I've managed to mess up couple screw tops. Plenty of WD-40 won't work. Any suggestions?

The tappet covers (to set the valves) should be held on by 6mm bolts, not screws.
Each one has 8 bolts......
 

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IF you are taking out Phillips head screws, you are attempting to remove the camshaft journals (bearings), and you DON't want to do that.....At least not yet....
 

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Discussion Starter #16
Thanks guys! That's actually a good news. I can deal with the bolts, I think. I do have a very old copy of Clymer manual downloaded from the net, but pictures didn't come out very good and instructions aren't very detailed.
 

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Discussion Starter #17
Removed the valve covers, reset the cam chain tension, checked and adjusted the valve clearance to .002" (this was the smallest gauge I have), checked breaker point gap, put covers back on and then checked the compression again. It's still 80 psi on both cylinders. Looked at the tops of pistons through spark plug holes and they looked to be burned both sides - caked on black soot on top. Was going to do timing adjustment, but I guess there's no need since I need to open the engine and see what's contributing to low compression and fix that first. I may be way in over my head. :roll:
 

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ninepeaks9 said:
.........I may be way in over my head. :roll:

You may be right. You nearly pulled a cam journal off, that wouldn't have been good.
And I still don't think you have the gist of the timing.
2-thousandths on the valves is ok though.

In light of your compression readings, you're probably looking at a valve job, rings at the best, pistons/bore job possibly, plus whatever other stuff you find "tore up" in there.
You could be facing substantial (to guys like us) cash outlay, definitely a lot of work.
 

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Discussion Starter #19
Can you give me an idea what sort of cost I'm looking at? A range from at best to the worst possible scenario? Depending on the cost, I may rethink this job. I don't want to spend a lot of money on this bike.
 

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ninepeaks9 said:
.......... I don't want to spend a lot of money on this bike.
You're well past that stage already, just on the engine.....

Valve job ~ $140 at a machine shop
Boring ~ $40 per hole at a machine shop
Piston kits ~ $100-$140
Rings ~ $30
Gasket kit ~$60
Seal kit ~ $40
Cam chain ~ $40

Bad parts you run in into (or break) - who knows $$$$$
 
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