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stinkwheel |
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![](images/spacer.gif) stinkwheel Bovine Proctologist
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 05:17 - 13 Mar 2024 Post subject: |
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From memory, the C70 head IS the "hot head" for these engines, to the point C90 owners will fit one to their bike as a tuning aid.
Clean it properly, thoroughly inspect the valve and seat for damage like cracks and nibbles and seat it properly with a spit-stick and grinding paste. If it's pitted and nibbled on the seat/valve, you might need the valve to be reground or the seat to be recut and then lap it.
A quick and dirty test is to fit the plug and with the valves in place (pushed in, not necessarily with the springs fitted) sit it upside down over a bit of cardboard and put some petrol in the head. See if it leaks out. If it does, you aren't finished.
While you have the head off, you may as well check the bore too. Visual inspection for damage/wear. Look for signs of blowpast on the piston and carefully remove a piston ring and check the end-gap in the bore at a couple of points on the working area. ____________________ “Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles. |
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jeffyjeff |
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![](images/spacer.gif) stinkwheel Bovine Proctologist
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Cub650GS |
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 17:32 - 13 Mar 2024 Post subject: |
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Lots of good response; even the snarky ones
"How poor is compression?" 90psi, since I last took it out and tinkered with it, it won't start but I don't have a tester to see if the compression dropped. I think I got the timing off.
"don't use a drill to lap valves. hand tool with a suction cup, back and forth 1/2 turns clockwise and then counter clockwise." I never lapped a valve before and the mechanic friend who told me how just said to spin it with a drill. I actually bought a drill suction cup one that didn't suck to the valve, It was as hard as a rock even after applying heat. I was gonna buy one that had a mechanical gearing to step up the drill speed but the suction cups were too big for my little valves. I'm taking it these aren't the right tools?
"If indeed compression is leaking past a valve, you should be able to ascertain which valve it is by listening carefully to the intake and exhaust ports on the cylinder head as the piston comes up on compression stroke." A local mechanic told me it was low compression off one of the valves and that it needed a head job and it does sound like it is coming from the exhaust valve. He was going to rebuild the head for me, gave me a quote and everything, but didn't get back to me when I tried to book it (multiple times). I must assume he is either too busy or didn't want to. The other mechanic just wanted to throw amazon parts at it and I was less enthusiastic about that.
I don't want to get down into the piston and cylinder; I don't have the tools to deal with it, or hone it, or the personal constitution. If it needs all that, it will have to go to a shop.
"From memory, the C70 head IS the "hot head" for these engines, to the point C90 owners will fit one to their bike as a tuning aid." That's what I figured, I thought there might be a better head for it that was worth upgrading to.
Aside from a really dirty exhaust valve it looks good. The casting is a bit rough but there are no gouges, cracks, or anything that looks out of the norm. I did look at the bore and it looks smooth, my fingernail didn't catch anything and I don't want to get into it. I know there are people who feel strongly about going all the way all the time, I just don't have the confidence to.
"I was suspecting half a job." Kinda like your half a comment? Haha! You know I never done this before; it's not like I went "Huh, must be good enough", it was more like, "Hummm, if I keep using the sand paper on this am I going to ruin the valve? Should I use a wire brush? My honing stone?...".
After all this I am thinking maybe it's best to send the head to a head shop? There is one local to me and I am working with probably not the right tools. ____________________ Bork Bork |
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jeffyjeff |
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 18:57 - 13 Mar 2024 Post subject: |
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Cub650GS wrote: | I don't want to get down into the piston and cylinder; I don't have the tools to deal with it, or hone it, or the personal constitution. If it needs all that, it will have to go to a shop.
Aside from a really dirty exhaust valve it looks good. |
Dude. I read your post and all of the responses, and I would like to comment:
1: Good on you for having the cajónes to tackle this project yourself. Trust your skills and don't be afraid to try something you have never done before.
2: Think about the exhaust valve for a moment. The exhaust valve is the hottest running part in any internal combustion engine. Superheated exhaust gas blows past that exhaust valve every time it opens. The operating temperature causes the exhaust valve head to glow red when in operation. That is normal. What is not normal is carbon build up on the valve. Sure, it happens; what you have is nothing unusual. But it would be unusual for a well-tuned, finely running engine to have deposits on either valve. Even a 42-year-old 6 hp Honda Cub engine.
Something abnormal caused the incomplete combustion that resulted in carbon build up on your exhaust valve. Could be low compression from a leaking valve. Could be low compression from piston rings not sealing. Could be a restricted exhaust or a restricted air filter. My point: Don't get myopic and focus just on the head. The exhaust valve could be the symptom, not the problem.
3: You have already done the hard part in disassembly down to remove the cylinder head. Removing the cylinder is easy from this point. Why not go just one more step? There must be some instructional video on YouTube to check the bore dimension, piston, ring gap, and hone. On assembly, you can compress the piston rings with a hose clamp (probably with just your fingers).
Good luck to you, wish you all the best. ____________________ History shows again and again how nature points out the folly of men - BOC |
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stinkwheel |
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![](images/spacer.gif) stinkwheel Bovine Proctologist
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 22:16 - 13 Mar 2024 Post subject: |
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The valves are hard steel, you can use a wire brush/wire wool to remove the carbon, especially on the face, just keep off the valve stem which is a ground diameter. Your spit-stick won't stick to carbon deposits, needs a clean, shiny metal surface. The valve seats are hardened too but the alloy of the head is soft so you can be fairly rough but in a more targetted manner with them.
Quote: | 2: Think about the exhaust valve for a moment. The exhaust valve is the hottest running part in any internal combustion engine. Superheated exhaust gas blows past that exhaust valve every time it opens. The operating temperature causes the exhaust valve head to glow red when in operation. That is normal. What is not normal is carbon build up on the valve. Sure, it happens; what you have is nothing unusual. But it would be unusual for a well-tuned, finely running engine to have deposits on either valve. Even a 42-year-old 6 hp Honda Cub engine. |
Valve stem oil seal? Won't be causing compression drop but could cause oil ingress. You probably have new ones with the gasket kit.
Are the valves a good sliding fit in the valve guides, no lateral play? ____________________ “Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles. |
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 16:43 - 15 Mar 2024 Post subject: |
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doing the vales with a stick is like 5 minutes max. very quick. I'd suspect using a drill isnt that great an idea. ____________________ Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R |
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 19:56 - 15 Mar 2024 Post subject: |
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Get OEM parts if you can unless they are stupidly expensive. They lasted this long... There are loads of aftermarket parts out there for these motors if price is an issue but in my experience, honda OEM stuff is top quality.
Normally when asessing a bore, you check for a lip (which you have done). Look for any damage, scores and smears with a good light. Have a look at the piston, see if gas has been blowing past the rings.
The proper way to check the bore is with a bore guage but they are fiddly and tricky to use, there's a knack to it.
The good enough way is to get a new standard size piston ring and push it down into the working part of the bore with the piston so it goes in totally square, then measure the end-gap on the ring with a feeler guage. There will be a specified tolerance for this in your workshop manual. If the gap is too wide, you may need a rebore. Or it may be cheaper to buy a complete barrel and piston kit for these bikes.
Here's a picture of me checking the ring end-gap in a different engine. This is a 2-stroke so it has ports cut in the barrel that you don't have.
https://lh3.googleusercontent.com/pw/AP1GczPRA7uH6Bfsss1xwYHrj_z9-839Dgfel35v6I-MLeTBnQ4uCdevQP9a9xi4Igo2_3QI-wE5Jhw-nBhUENwmmDLie51Z5orKPuyYPJqVnnCqpa5jtvhbjCqz_oXszXNaeW6fVPi6GnXxGTI3381dcRhW=w1547-h870-s-no
If it was me, I'd start by carefully (they can snap) popping one of the existing rings off and checking the end gap. If it's within spec, and there's no significant blowpast, I'd reassemble. If it's within spec and there is some blowpast, I'd hone the bore and reassemble.
If the gap is outside spec but there's no visible wear on the bore (that step you checked for) I'd order a new set of standard rings and check the end gap on those. If it's within spec, hone the bore and assemble with new rings. If it's outside spec, you just wasted the cost of new rings (that's just the nature of the job) and either need to have it rebored for the next size up piston or (may be cheaper for one of these) buy a barrel and piston kit and just swap in a new barrel.
If you're getting it rebored, you can usually buy a "piston kit" which has the piston, rings, gudgeon pin and circlips in it. You'll also need a top-end gasket set which should have head gasket, rocker cover gasket, valve stem oil seals, carb flange seal, exhaust gasket and base gasket. ____________________ “Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles. |
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![](images/spacer.gif) stinkwheel Bovine Proctologist
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![Post Post](templates/standard/images/icon_minipost.gif) Posted: 13:38 - 21 Mar 2024 Post subject: |
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Throttle valve could be sticking, most commonly due to an old/badly routed cable. You should be able to see it in operation if you take the air filter off.
Also a fast/variable/hanging idle can commonly be due to an air leak round the inlet manifold (causing air to get in and a lea running condition) so check the rubbers for cracking, make sure any gaskets or o-rings are in good condition. ____________________ “Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles. |
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