Topic: Rebuild # 2, should I replace rings? Tips on cleaning carbon from pistons/head.

[hevykevy420]

Hi everyone, I've torn down my engine due to oil leaks.  I only put about 1k or less miles on 836cc rebuild #1.  I realized after tear down I missed the o-rings that sit under the cam towers (not the 6 puck gaskets).  This time around, I"m hoping to get all the o-rings right, and I'm also using HD cylinder studs and replacing the cam tensioner assembly.  My questions are:

Can I get away with reusing my Wiseco rings?  I really don't want to replace them at $28/per piston.  I don't see any visible damage whatsoever to the rings, pistons, or cylinder.  All appear to be OK (see pics), but I"m going to have a machine shop double check the bore and rehone as recommended by Wiseco. 

I had a lot of carbon build up for so few miles, after speaking with Dynoman, he thinks my 4-1 exhaust with the packing that I added and also the stock airbox is too restrictive, and causing the carbon.  I'm going to try pods and at least unpack the muffler.

any tips on cleaning off carbon?  Other thoughts/suggestions??

[hevykevy420]

Here's the other pic...

[hevykevy420]

I did notice some very light scratches on the cylinder, not sure if this is just part of the hone:

[hevykevy420]

Here's the head:

[Bodi]

I don't see any reason to replace rings after 1000 miles. I wouldn't even hone it either, but that won't hurt if you use a really fine stone.
The carbon is from the fuel/air mixture being too rich. This is a carb tuning problem, not an exhaust or intake problem. If the bike looks and sounds nice to you with the airbox and pipe you have, then tune the carbs properly. Just fiddling with the packing won't do much, if it's a glasspak muffler the packing affects the sound a LOT more than it affects restriction. Putting pods on will mean rejetting anyway, and will give you a much louder engine sound when riding. Some folks have assorted troubles with pods too - stumbling in crosswinds and rain for example.
So I would recommend cleaning up the head and pistons and using the old rings, and then deciding what intake and exhaust you really want before doing a careful carb tuning. What took us many days to accomplish with plug chops can now be done in a few hours using a dyno and exhaust gas anylyzer.

[hevykevy420]

Thanks Bodi-

I do prefer the airbox because I don't like crosswinds and leg positions affecting the engine, and I also prefer a quiet exhaust.  I'm surprised I was too rich as I am only running the stock 120 mains.  I think what I will try is to use the pod filters to lean out the mixture before rejetting.  If I don't like the pods, I'll go back to the airbox but try a 115 main jet.  It is just counterintuitive to me increase the displacement, but decrease the main jet size, but I know some people think a larger displacement draws more fuel, so maybe this is the case!

I just wish I had access to a dyno and an exhaust analyzer!!!

[vintage_racer]

Large displacement draws more fual only if you can get more mixture in. i.e. you had done porting and installed larger valves, longer duration camshaft, etc.. and use high RPM.

If you just incresed your bore, you don't need more fual, you use a bit more mixture at lower RPM (that's why you have more torque at lower RPM) but the air to fual ratio remains the same.

It actually makes sence to try and go down to 117.5 or even 115 jets on larger bore since you are probably using now less revs and hence the carbon buildup.

[hevykevy420]

vintage racer-  that makes sense, I didn't do port and polish but i did use a longer duration cam.  Also, i started with a 130 main which was way too rich, and while breaking the bike in I did not rev real high.  I'll start with the pod filters to lean out a bit...

[Jerry Rxman Griffin aka MuthaF'er]

"stock 120's" Are you sure about that? I thought stock was 105's. Perhaps not.

[Heironymous Josh]

stock is 105

[DammitDan]

Wow....   So THAT's why my carbs used to have 105 mains in them (and why my bike ran like crap...  stock for a CB650 is 120  )  The PO must have misread somewhere that a CB750 and CB650 have the same main jets.

Anyhoo, if you need 105s I can send em to you.  I've got extras at the apartment (if I can find em! )

[martin99]

I've just been involved in a very similar thread and the consensus there was that by adding pods you would definitely be creating a leaner mixture, and would need to increase the jet size. My F2 is running pods with 120 jets, mixture screws out 1 1/2 turns and it's fine - better than it was with stock 105s. I would suggest you try the pods with your 120s first as it might save some fiddling about. Carb balance and timing need to be spot on too

[TwoTired]

When you increase the displacement of an engine, assuming no other changes, you increase the volume of air being drawn though the carb throats.  This increases the vacuum (decreases the air pressure) seen on the outgoing side of the fuel jets relative to the atmospheric pressure, and causes more fuel to flow through the same jet orifice.

Stick a straw in a drink don't you get more drink if you suck harder?

Pod filters usually reduce some of the resistance to draw at the mouth of the carb and bring the carb throat pressures closer to outside atmospheric, thereby causing less flow throught the fuel orfices due to decreased pressure differential across the jet orifice boundary.

If pods have the same pressure differential resistance curve over the entire RPM/ airflow range of the motor relative to the stock filter, you can adjust overall mixtures with the pods.  If however, the resistance to airflow is not a direct relation ship I.E. a differential pressure at different speeds relative to the stock filter, then some aspects of carburetor jetting will have to be changed.   Maybe just main jet, maybe just slide needle position, maybe just slide needle taper, maybe all of the above, or any combination of the above.   I view the pods as a Hail Mary attempt at adjustment.  Might work.  But, you may just have to rejet and readjust carb settings either way.

I also don't understand why you are re-honing the cylinders after just 1000 miles.  Yes, if new rings.  But, if what you have hasn't been damaged, just put it back.

Cheers,

[hevykevy420]

Gracias for the explanation/advice TwoTired, you are a wealth of knowledge!  I decided not to rehone, I was thinking the same thing (why rehone?).  I was only going to do this because a tech person at Wiseco recommended it along with new rings.  After inspecting everything, I can't see any damage to the pistons or rings at all so I am just going to clean up reassembe and reuse the rings after having the bore checked at a machine shop with a micrometer (only costs $17).  I figured checking the bore is a wise thing since I didn't do it the first time around...

Also- I realized the stock jetting for the k8 is 110 according to the specs page on this site, I could of sworn somewhere I saw that it was 120.  I thought I was reasonably jetted all along when in fact I was not!