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Discussion Starter · #1 ·
The CM450 has been giving me some starting trouble lately. I finally got it pulled in the garage and started running through the list of problems and causes. I have done carb mod with pod filters and I pulled the carb and cleaned and rejetted it to a better setup but then I started checking combustion and spark to see how good it is and noticed I only had 90 PSI in both cylinders. Looks like spec is 150-200 PSI. I know some people say older bikes have lower compression but others say a healthy engine should have at least 130 PSI.

At what point does it make sense to fix compression issues? Sounds like some people get away with riding on lower compression for sometime but considering i'm having start up issues, I think it might be time for a rebuild. I've never been in one of these engines before but I have worked on cars engines. It sounds like this motor can be rebuilt in place, on the frame and is relatively simple procedure. Does anyone have a link to a good 450 rebuild? What are the main points to look for and is anything tricky when rebuild these engines? How much am I likely to spend on a decent rebuild kit or replacement parts?
 

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I do not trust the automotive compression testers on motorcycle engines. They are designed to be used on cylinders with a lot more displacement then most motorcycle engines have. I am more concerned with any difference between cylinders. You do not want there to be any more then a 10* difference between cylinders. As for things to watch on rebuilding your engine. No#1 would be to make sure that the threaded holes in the case for the head/cylinder bolts do not strip out, and No#2. Do not let the cam chain fall down and come off the crank sprocket. If this happens it could get lodged between the sprocket and the balancer chain guide. If this happens, you will be splitting the case.
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I'd adjust the valve clearance if you haven't already. Can make a big difference if they need some attention.

If the engine runs strong when it is warmed up and doesn't burn oil or look like a 2cycle I would leave well enough alone.
 

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Compression test must be done with the engine at operating temp and with the throttle and choke wide open (after adjusting the valves as Franz said).

If the compression really is bad it's best to take the cylinders and pistons to a machine shop. They can measure them and tell you what needs to be done (hone and new rings or bore and new pistons and rings).
 

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Discussion Starter · #6 ·
Started taking the top end apart to see if I could check the valves. Went ahead and took out the cylinder head bolts and found some interesting rust and build up patterns. At first, the number 5 head bolt was not loosening right. Ended up snapping it off because it was rusted solid. That was the only snapper of the bunch though.

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I'm guessing a seal leak has occurred near the rusty head bolts. The internal set next to the rusty bolts have a blackened oily build up on them too. Probably burning oil near there. Both outer sets of head bolts are perfect with no build up or rust.

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I did have the throttle open when I did the compression test. I even checked the compression gauge by hooking it up to my air compressor with a known pressure to check the gauge was reading correct. It seems good. Now I'm diggin in and have reason to believe rust might be in the cylinders, I'll probably take it down to the case and rebuild cylinders. Luckily I work at a machine shop that can probably help out with getting it all back to spec.
 

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Discussion Starter · #7 · (Edited)
So the top end is completely apart and it doesn't look too bad. The valves and pistons are black as hell so they are currently taking a chemical bath, but over all pistons and cylinder walls look like they are in decent shape. Even the valves and rockers look decent, though I haven't measured anything yet. Nothing bent or noticeably broken.

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Started looking for replacement parts for reassembly. I have 4 head bolts that look nearly perfect. Does anybody ever reuse head bolts on these engines? They are expensive! Also, when buying new piston rings, should I buy enlarged size rings if I plan on honing the cylinders? Would anyone suggest honing or machining these?

I did a google search for rings and found honda OEM parts that seems good. I didn't find much for the head gaskets kits except on ebay. There is a $45 CM450 engine gasket and seals kit. Has anyone used these or would you suggest some other source for engine rebuild items?
 

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You can test if your low compression is a valve issue by pouring solvent (kerosene, acetone, gas etc) into the combustion chamber and watching for leaks. There are tons of youtube videos on this that explain better.

Everybody seems to re-use head bolts, I know it's a car thing to get hardened super magic head bolts but I think that is only necessary in cars that have cheap stock ones unable to handle the power increases people get from other mods. Just be sure the ones with seals (the little rubber tubes on two or so of them) go back where they should.

You should measure ring end gap asap too, if they're out of spec maybe that's your compression issue right there and you just need new rings. If not it's on to piston to cylinder clearance, cylinder roundness etc. which probably require a machinist's tools to measure.

Gaskets I'm not sure, the PO used one of the cheap sets on my cb400t engine and it seeps a bit after coming out of storage (head to cylinder gasket). I can't say if it's the gasket's fault because it could also be due to poor surface preparation or even just my use of synthetic oil.
 

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As long as it starts and idles well, and doesn't use oil, I would be inclined to run it. Dynamic ring seal is different on a running engine, combustion pressure,finding its way behind the ring, is a whole lot more effective at creating ring seal than just relying on the ring tension to do it at cranking speed. There could be a lot more miles left in it before it needs to come apart.
 

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Sorry to see another broken bolt. Have done the same.

In answer to your question though, "acceptable" is relative. I had one that was down to 125 in each jug and I only took it off the road because of very serious oil consumption and smoke. At 125psi it had no drivability issues and just felt a little tired. Would still pull a 6.6 second 0-60 and just reach the ton.
 

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Discussion Starter · #11 · (Edited)
I checked the piston rings last night and they seem pretty close to .013" gap or so. It was kind of hard to check them without feeler gauges though. I used calipers. I brought the cylinders into the machine shop today so we can measure and check the wall condition.

So the reason I'm breaking this engine down is because I want a more reliable bike to take longer trips to the mountains. I don't really want to have to worry about if it will break down or have enough power to do the job. I know others just ride it on 110 PSI around town and it does fine. But considering my goal, I want to make it like new again. My goal is 180 PSI or better. I'll keep checking and I'm sure I'll find the 90 PSI issue somewhere. I did pour cleaner on the valves upside down and let it sit for like 2 hours and the fluid was still there with no apparent leaks. Perhaps valve cam or timing...

If valves are really dirty and not well greased, can they get sticky and not open and close properly?
 

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A 5$ set of feeler gauges would be a good investment, you need them to adjust valves anyway after you put the top end back together.

If valves aren't getting oil to the valve guides they will tell you about it and something will break, possibly other things first. If there is enough carbon build up it can get stuck between the seat and valve and cause sealing issues. I think your valves are probably okay since they aren't leaking.
 

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What did the machinist say about the bores? They look pretty glassie to me. How do the rings fit in the piston ring grove? This is critical for a good seal. If it was my bike I would find a set of oversize pistons and get it rebored and new valves and get the head done.
 

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Head bolt: If there is no signs of corrosion on them reuse them. Any signs of corrosion they should be tossed.
Compression: Spec is 185 +/-14 psi. Somewhat acceptable power down to 135psi.
Bore & pistons: those need to be measured by a machinist and be within spec.
Valves: the seat face is hardened to a depth of .0002" so refacing them cuts right thru that. Replacement only. Seats can be ground, refer to the FSM for seat width. A good valve job does not require any lapping to get a seal.
Parts: I prefer to use OEM Honda gaskets even though they are more expensive. Haven't been impressed with Athena gasket, Vesrah and DK seem to be ok. The lower cylinder O-rings all seem to be too thick except the Honda ones so if you have trouble installing those that's the reason.
Broken bolt: I haven't tried this yet but you might consider it https://hackaday.com/2015/08/03/dissolve-steel-drill-bits-in-alum-from-the-grocery/
 

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I used one of those Pittsburgh compression testers on my CM400 and got 90 psi as well. I don't know if it's just a coincidence or if there's something off with those gauges. Can't attest to performance because I'm having other issues too, but it's something to think about.
 

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Discussion Starter · #16 ·
Machinist said we could probably run scotch brite through the cylinders just to resurface the walls, but they don't look too bad. I got a feeler gauge and measured the ring gaps at .010" and .012" of an inch and toward the upper limit of tolerance. I measured the pistons outer diameter at 2.9265" and the book says spec is 2.9512" - 2.9524" with the wear limit 2.949". So my piston is out of spec by .025" and past the wear limit. So now I'm looking at pistons... they seem to come with matching ring sets. The ring grooves had a little slop but I don't know what specs are on these and I'll probably replace pistons at this point. The head does look like it could use resurfaced in the middle. Probably .005 - .008 out.
 

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Scotchbrite?, never heard of that one before. A few passes with a 400 grit cylinder,to take the glaze off, is what I use. It's a good idea to replace the pistons when replacing the rings anyway, these things turn enough revs to stretch the ring grooves, might as well have everything a proper fit if you are doing the job.
 

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Discussion Starter · #18 ·
Head bolt: If there is no signs of corrosion on them reuse them. Any signs of corrosion they should be tossed.
Compression: Spec is 185 +/-14 psi. Somewhat acceptable power down to 135psi.
Bore & pistons: those need to be measured by a machinist and be within spec.
Valves: the seat face is hardened to a depth of .0002" so refacing them cuts right thru that. Replacement only. Seats can be ground, refer to the FSM for seat width. A good valve job does not require any lapping to get a seal.
Parts: I prefer to use OEM Honda gaskets even though they are more expensive. Haven't been impressed with Athena gasket, Vesrah and DK seem to be ok. The lower cylinder O-rings all seem to be too thick except the Honda ones so if you have trouble installing those that's the reason.
Broken bolt: I haven't tried this yet but you might consider it https://hackaday.com/2015/08/03/dissolve-steel-drill-bits-in-alum-from-the-grocery/
I have 4 head bolts that are reusable and the inner 4 are not. I definitely think my compression was low and with the filter pods I believe it becomes even more important to have good compression to fully combust that more rigorous air flow and increased jet sizing. I suspect the valves do need some work or replacement but I have not gotten there yet. Still focused on cylinder piston area right now. I was think vesrah gasket set for this one. I saw a guy use gasket sealant with the gasket on a CB450 and he claimed it added a little extra sealing insurance so it doesn't seep oil. I was looking at Honda OEM parts and will probably buy some parts from them as well.

Dissolving broken bolts! now that's an idea. I've never heard of this before. My method is heat to extract this one. The one bolt is rusted to hell, but with some heat the aluminum will expanded enough to break loose I think. Been hitting it with PB blaster for a few days too.
 

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Discussion Starter · #19 ·
I used one of those Pittsburgh compression testers on my CM400 and got 90 psi as well. I don't know if it's just a coincidence or if there's something off with those gauges. Can't attest to performance because I'm having other issues too, but it's something to think about.
They are cheep gauges but I tested mine with the air compressor gauge and it perfectly matched compressor gauges so I would say it's fairly correct. If your bike is like mine, it's 30 plus years old and without good compression everything else just doesn't work as good, especially the carb and spark troubles.
 

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They are cheep gauges but I tested mine with the air compressor gauge and it perfectly matched compressor gauges so I would say it's fairly correct. If your bike is like mine, it's 30 plus years old and without good compression everything else just doesn't work as good, especially the carb and spark troubles.
Yes, the gauge will read correctly with constant pressure and an unlimited supply of more pressure behind it..... The problem is, these engines have a small compression dome (volume-wise) and can only compress the amount of air they can take in at a "gulp" when the intake valve is open...... Since pressure is directly and inversely related to volume, adding the volume of the hose to the equation automatically reduces the pressure reading..... here's how you can figure out how much effect it has: (using round numbers).....
Your 450 has approximately 225cc per cylinder when the piston is at BDC.... (to make the calculations easy , I'm using a 10/1 compression ratio......So the dome's actual volume is 1/10th of 225, or 22.5cc at piston TDC....and the pressure would be 10 times atmospheric, or about 147 PSI (Actual is more like 170ish as air rushes in faster to fill the vacuum left by the exiting exhaust, so it "overfills").... This is now where the hose volume messes-up the readings.....Lets just say the hose has an inner diameter of 6mm (almost 1/4") and the hose is 14" long (352mm).....Volume of that hose is circular area X length..... Area =Pi X R squared or Pi X R X R... R=3 R squared =9 and Pi = 3.14159 so Area = 28.274 square mm......

28.274 Sq mm X 352 mm long equals 9952.49 cubic mm or 9.9525 cc.......Added to the actual dome volume, you are now compressing the "gulp" not into 22.5cc, but into 32.4525cc

The inverse ratio is 1.57566, so multiply that times the 90 PSI reading and you get 141.81 PSI.....(Close to the presumed 147 PSI after all the assumptions and rounding off of the numbers).......

Your gauge isn't as inaccurate as you thought it was....It is reading pressure in the total volume, not just in the actual compression dome..... and 93.3PSI would give exactly 147 PSI, a mere 3% error in reading
 
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