crankshaft

Is it possible for the crankshaft to twist causing engine to be out of time enough to make engine pop thru carbs and not start?

possible but not probable,

what you are describing sounds like a high mileage bike that needs shimming,the clearances have closed up and is keeping the intake valve/valves off the seat ever so slightly,this gives the combustion fire a chance to spit back thru the carbs and lowers compression to the point of not starting with the starter.

if you pulled it with a four wheeler it would prally start but not idle right.

just my .02B)

popping thru the carbs can be a few things

low battery
plug wires on wrong cylinders
ign. coil signal wires mixed up
tight valves
burnt / arcing points
bad gas from storage

if you drag race the crank can be damaged but that is very difficult to do

heck if it hasn't run for some time it will do this for a while as the build up is burned off the combustion chamber inner surfaces

if you use ether to get it started be careful not to catch your bike on fire
the back fire is common for bikes coming out of storage

Misfiring spark plugs can also cause popping back through the carbs.

I run a GEREX CDI unit on my '73 Z1-900. One day it started to miss and spit back through the carbs. It turns out that I had not checked the spark plugs for 7 years! Found that all the plug gaps were at least 0.110". I normally run 0.065" spark plug gaps. Replaced the spark plugs with new ones which ended the problem.

Just bought bike few weeks ago and don't know its history, but the odometer is showing only 9,300 mi., valve clearance is good and bike has 130-150 psi. of compression. plugs also were replaced, and it has good spark and all timing marks line up. tried two different sets of carbs and still no start. just pops and spits!

It is easy to check for a twisted crank in the motor.

Get your degree wheel and TDC tool, and find TDC on cylinder #1.
Then repeat the process on cylinder #4. If they are not both TDC at the same place on the wheel. the crank is out of index.

Jay

man, did I ever miss on this one, could the spark plug wires have been crossed when you changed plugs?

are the carbs jetted for pods or are you using your stock airbox,spitting back thru the carbs usually means a lean condition or my first guess (shiming/valve off the seat)

If you have BS34 carbs and the airbox isn't in place, the stock jetting won't allow the bike to start in all liklihood. The BS34s are kind of sensitive in this way. Try putting the airbox on if you have the stock jets.

If I had a "new to me" bike, I would FIRST check compression and valve clearances. NORMALLY, previous owners weren't keen on adjusting valves if they are selling a bike. If the valves are stuck open due to lack of clearance, the bike won't start:

www.wgcarbs.com/269767.html

If the valves are OK, check for spark as you turn the engine over.

Three things are necessary for the bike to start:
1. compression
2. spark (at the right time)
3. correct air/fuel mix

Last, put a clear hose on the drain nipple on each carb and ensure they have gas flowing into the bowls. You can do this by opening the drain screw and holding the hose up next to the outside carb with the bike on centerstand. After tapping out the air bubbles and NOT moving the hose, the gas should be about 1/8" below the gasket mating line on the carb body.

Post edited by: wiredgeorge, at: 2006/07/24 10:02

airbox was missing when I bought bike, could be my problem!

Choke it, crank it, then take off the choke and crank it without touching the throttle. Let it start on its own. And make sure the throttle is completely closed by backing off the idle screw until it isnt contacting, then turn it in until it engages the stop. Give it a quarter turn. Go for it.

You have good compression. Sounds like it may have a vac. leak or your just not starting it right. If you end up taking the carbs off again, hold your hand over the intake boots one at a time and feel for suction. If you feel air pushing back at any time while cranking, the intake valves are not sealing or the cam timing is not correct. If the engines normal range is 150psi, cam timing is not likely an issue. But do not rule it out. compression figures can look as yours do either way. keep tinkering and asking questions and we will all help u to get this thing running. Make certain that the wires are on the right cylinders. Only other thing i can think of that would cause spitting through the intake is a faulty pickup or one that was wired in reverse, and also possible the coils input wires reversed. There are ways to test for this if it comes to that.

Post edited by: nads.com, at: 2006/07/25 00:46

Post edited by: nads.com, at: 2006/07/25 00:49

try quick start spray (ether)this will fire it up

if it does NOT you have ign/timing problems

if it DOES fire it up you have fuel delivery problems

only 3 things needed for combustion in an engine

fuel
compression
timed ignition

simply complicated??