Grenaded Engine

[bmw633]

Hi,

I bought a 2001 GZ250 with 10,000 miles on it for my 25 year old daughter to learn to ride on the road. I removed and cleaned out the carburetor, put a new spark plug in and changed the oil and filter, and 500 miles later, she was on the interstate going around 75mph when the engine made a noise and stopped running.

The head was pulled off and what happened was one of the exhaust valves broke off and embedded itself in the piston, causing catastrophic damage to the engine. I found a junkyard engine with 4500 miles on EBay, so my question is, what caused the exhaust valve to break off from the stem? Is it a manufacturer defect? I have read about other GZ250's losing their exhaust valves, so is this common with this bike?

The carb was clean as a whistle. She was running 87 Octane gas, not the 93 octane high test stuff. Could a lean situation caused the valve to get overheated?

Seth Anderson
Moon, VA

[jonathan180iq]

First though I had was that it simply got too hot for one reason or another. A very lean condition could do this, yes. But I think you would have noticed if it was really bad.

The fact that you already have a junkyard motor means you are a step ahead of the game. I would just do what you have to do and get back on the road. She'll have to learn to trust the bike again, which shouldn't take too long. But I'll bet she'll be skittish about it for a little while.

[Water Warrior 2]

A couple factors might have come into play. Did the bike ever have a valve check? If it was overdue that would affect the valve operation somewhat.
75 on the interstate is a little tough on a GZ. The GZ is not and never will be an interstate bike. It doesn't have the power or size to compete with fast traffic.
And a safety thought here concerning a new rider of any age. A new rider on an interstate at 75 mph is just plain nuts. Slow down and stay off the slab. Did your daughter take a professional rider's training course. She should. And no you can not teach her yourself for 2 reasons. #1, you are personally involved in your heart, #2 you will teach her your bad habits.
Next, get a copy of Proficient Motorcycling by David Hough. Rider training and the book will give a new rider a geat chance of surviving the roads and inattentive cagers. As an off again, on again rider for most of my life with various bikes I finally took the course in my 60's and read the book. It made a tremendous difference in my riding ability and enjoyment for the last 6 years. Your daughter needs the training and your need to keep her as safe as possible on 2 wheels.

[rynr]

It' 90% wrong valve clearance (must be checked every 5000 km (3000 miles)). Perfect valve clearance is when valves click when engine is cold, quiet when warm and click delicate when hot.

[bmw633]

She took a 3 day motorcycle rider safety course, but, like most of the fairer sex, she is hardheaded. I specifically told her to take the back roads, but she was in a hurry, so she says.

Should I rejet the carburetor a couple of sizes richer to help it run less hot. The day the engine blew, it was in the low eighties, very nice day.

Seth Anderson

[Water Warrior 2]

like most of the fairer sex, she is hardheaded.

Seth Anderson
Sounds like an ex wife of mine. I said "don't drink and ride", she did anyway. She eventually recovered from her injuries but never rode again.

[raul10141964]

this engine is design to run lean
I more incline to ting she read line the engine

[Water Warrior 2]

Red lining the engine on an interstate would certainly find the weakest link in the mechanical chain of assorted parts. Stuff does happen no matter how much we don't want it to happen. The maximum for the GZ to run happily is 60-65 mph. The GZ is a small workhorse and not a small racehorse.

[bmw633]

Would putting on a 16 tooth front sprocket help with anything?

[rynr]

It depend on actual engine condition. If it's in mint condition 16T sprocket lets riding almost top speeds on lower rpm value, and you can get slightly higher top speed.

Some guys tell that there isn't much difference between engine oil producers or types.

I can't disagree more. Last year did 2500 km (1600 miles ) trip on GZ125, with passenger, side and top luggage. Bike was almost overloaded. I handled full throttle almost all the time riding 80-90 km/h (around 55 mph) - it couldn't go faster and GZ's top speed is 100 km/h (almost 65 mph) when solo riding. Couple times i get over 100 km/h when riding downwards. And nothing bad happen.

And this GZ125 wasn't new, had almost 20 000 mileage.

Anyway, if you're using top-end oil, you propably won't have any major problems with engine.

[jonathan180iq]

Would putting on a 16 tooth front sprocket help with anything?

If she is stubborn enough to want to pull 75MPH on the highway again, the 16T sprocket won't let her do it. She'll be maxing out around 70MPH or so and with lower revs. So it would possibly help. You could slap one in there and force her to slow down because of it... :) But then she might not like the bike afterwards, and you don't really want to deter a new rider from experiencing the ride and enjoying their machine.

A single upjet would help the thing run a touch cooler. I would only go one size though, if you're looking at keeping everything else stock.
This is a procedure I've used on the last 3 bikes I've had. Small engines have to work harder than bigger ones to do the same thing. That's just a fact. Anything that we can do as users to limit the stress the engines have to endure are little check marks in the list towards engine longevity.
My cool-down favorites include:
-Slightly larger gearing (Weights in scooters)
-Slightly oversized jets (very slight - you still want it to run right.)
-High end oil

Keeping up with all the other maintenance is necessary before those things work, obviously. A leaking carb inlet, for example, or a leak anywhere else will throw your little tweaks out the window. Causing a leaner condition on top of your upjet. But you already know that.

[raul10141964]

my cruse a 75+ mi
my settings are:
main jet 140
pilot 3 turns
needle mod
16T
filter mode

http://s17.postimage.org/qcfafu4yz/IMG_20120830_110644.jpg (http://postimage.org/image/qcfafu4yz/)

[bmw633]

I placed an ad for an engine for the GZ, and a guy is trying to sell me his GZ250 with a bad exhaust valve for $250. It has a new battery and new tires, and has a title. I am thinking about it, but am hesitant to buy another Zuki with an engine that is prone to having exhaust valves break off.

Is this bike a 10,000 mile engined motorcycle? How long can I expect these to last with some 60mph cruising. I use Mobil 1 synthetic oil and never skimp on maintenance.

Seth Anderson
Moon, VA

[JWR]

Read this account.

The GZ only had 35,000 miles on it when he left for Alaska.

https://lh4.googleusercontent.com/-ChwF561gddk/Rsn8aeBljvI/AAAAAAAAAC8/jFhJG5DCQvU/s640/DS2_0057.JPG


http://home.comcast.net/~pat1776/Alaska05/

[Water Warrior 2]

I placed an ad for an engine for the GZ, and a guy is trying to sell me his GZ250 with a bad exhaust valve for $250. It has a new battery and new tires, and has a title. I am thinking about it, but am hesitant to buy another Zuki with an engine that is prone to having exhaust valves break off.

Is this bike a 10,000 mile engined motorcycle? How long can I expect these to last with some 60mph cruising. I use Mobil 1 synthetic oil and never skimp on maintenance.

Seth Anderson
Moon, VA
Cruising at 60 is well within the GZ's ability. But be forewarned, hills and headwinds will be a bit tough on occasion. Ride the GZ within it's capabilities and it will likely never leave you stranded.