Topic: So what keeps killin my bike?

[dusterdude]

call me stupid,but in the automotive world,if you run high compression or spend a lot of time at higher speeds,you need to run a colder plug.

[HITMAN]

+1 on the colder plugs, in addition to higher compression and over-bore we had to increase the cooling system about a half over stock, but we also added headers to our cars. 

My guess would be with all these mods (bore, compression and larger carbs) the stock cans are a big contributor to higher engine temps.    In my experience with Resistor plugs up to NR plugs was the least and last effort to fine-tuning, not major problem solving.

[CBGBs]

I'll try the colder plugs but the piston blew with the engine stock, and I'll consider mikuni's but the piston blew with cb500t carbs too. I've noticed that the racers are using the stock carbs so I figure that they should work for me. I still think it has to do with the fuel supply. Do you  think an aftermarket petcock may help?

I'm almost ready to put the bike back together.

In the original ad for the 350, Honda claimed it would hit 100.

[Jay B]

How do the combustion chambers look when you tear it down? Carbon deposits can cause hot spots that will pre-ignite the mixture and raise all sorts of hell with things.

[CBGBs]

I don't recall that they were in bad shape but they likely had some carbon, but not a flakey buildup. I would hate to tear it back down just to check, but that is good thinking.hmmm

[jsaab2748]

Does the mechanical spark advance work like it should? Did a previous owner ever tamper with it, i.e. stretch the springs,or modify it to advance farthur than it's standard limits? Is it rusted/frozen in a full advance position? ........a few stabs in the dark

[CBGBs]

Both times the piston blew it was with points and the advance was working properly and was unaltered. The bike is in good shape and the bike had been running fairly well.

[CBGBs]

I mean running "fairly" well. I'm pretty sure this is a lean problem.
There had been times when the bike was completely stock that I would be cruising along a rural road at like 65 with just a big smile on my face and I would be thinking about how I loved my little honda and then I would start hearing that ping grow louder and I would start loosing speed and I would flog the bike to maintain 35.
I believe I was on one cyclinder and just abusing the crap out of the pistons.
This happened a few time, usually on hot days. Sometimes when I would come to a stop after a stretch the bike would not stay running.
I believe there was no fuel in the bowl.

[jsaab2748]

I'd agree that you have a lean condition. Another wild guess might be that the gas cap isn't venting, and not allowing atmospheric pressure to enter the tank as the fuel level drops. Pretty soon, fuel flow slows down, preventing proper fuel levels in the bowls. Another guess is that the balance tube that connects the two tank halves together is stopped up and not allowing fuel levels in each half of the tank to equalize. This condition can cause problems when the tank is only approx. half full if memory serves me....These are only guesses and if these are indeed problems, I'd think they'd cause less drastic effects on the engine than what you are actually experiencing. You could check 'em tho. If the problem was present with BOTH a stock AND modified engine, I'd think the problem lies in an area that was never changed, and both engine configurations had in common, such as the ones mentioned. Were the plugs of the correct heat range? any vacuum leaks?
Scrutinize each component of each system.

[Pinhead]

If you want to continue using your little 350 as a highway ride, you may want to consider upgrading certain components to help the engine resist detonation.

Everything that kghost said is true, no matter the engine you're working on. One thing that is especially important is getting the air/fuel mixture that is in the chamber as homogeneous as possible. There are a few "tricks" that have been developed since these engines were made, many of which aren't very well known.

Two things I recommend: MetricMechanic's Surface Turbulence valves and Mike Holler's PowreLynz.

Surface Turbulence Valves:


http://www.metricmechanic.com/pdfs/M3_Engine.pdf

Before you say something like "That may work with car engines but it won't work with our old motorcycle engines," do a little research. I know that this particular picture is of a completely different combustion chamber (utilizes squish/quench and Singh Grooves) but the effect is the same regardless of the chamber shape. The ST Valves help mix the air/fuel before it gets into the chamber.

The back side of the valves look the same. They will increase low-lift flow by about 10% and full lift flow by about 4%. They also aid in vaporising and mixing the fuel into the air charge.

PowreLynz:


http://powrehaus.com/2006/10/30/powre-lynz/

In fluidynamics, as an aerosol travels through a conduit, the gasseous elements will hug the center while the liquid elements will drop to the boundary layer.  Restated, as an air fuel charge travels through the cylinder head, the air and vaporized gasoline will hug the center of the port while the liquids will collect and puddle against the port walls.

By adding “screw threads” to the intake port walls, we are able to better vaporize the fuel through a couple of means:

- Increase boundary layer turbulence to excite the liquids and discourage puddling and condensing.

- Increase the surface area of the hot port wall allowing increased exposure to the liquid fuel.

- Create a “wick effect” where the screw threads will temporarily hold the liquid fuel and allow it to vaporize off into the higher velocity air stream.

- As a large fuel droplet bounces off the serraded walls, it will break into a dozen smaller, more readily vaporizable droplets.

- Promotes laminar flow outwardly, increasing the surface area of the fuel allowing it to vaporize more readily.

Realize that these mods won't cover up the problem you're having with your carbs. They're not meant as "band aids" to cover up more serious fuel problems. That being said, they greatly reduce detonation in an "overworked" engine such as yours.