The Combine Forum banner
1 - 20 of 25 Posts

· Registered
Joined
·
31 Posts
Discussion Starter · #1 ·
I just rebuilt the top end of a 2011 ranger500 efi, and I cant get it started. the timing is good, the spark and fuel is good
It cranks real hard when I try starting it. But when I jump the starter solenoid with the key off it spins real good
so I thought to myself the only difference is we are adding fuel and a spark, so first I unplugged the fuel pump
and tried to start it and it still cranks hard, and did the same with the spark plug and still cranks hard.
The funny thing is I've had it running for about 15 min after the rebuilt, but just cant get it to go again.
 

· Registered
Joined
·
6,647 Posts
A single cylinder four stroke is a tricky thing to get started. I presume it has an automatic decompressor that has a centrifugal actuator in one cam lobe that holds one of the intake or exhaust valves in the head a few thousands off of its seat as the piston goes through the compression stroke. If the valve lash is worn or set to great on the particular valve that the decompressor works on it might not lift the valve off the seat far enough and the compression stroke pressure will be too high for the crankshaft to maintain enough rotational inertia as the piston comes to top dead centre.

That’s one portion of the voodoo. The other part is that a small amount of compression stroke leak down past the piston rings is normal in an engine in good running regular use. Too much assembly oil on the cylinder, piston or rings can seal the rings so perfectly on a brand new rebuild that it can be impossible to start because of excessive pumping pressure.

I’ve fixed a few rebuilds the young guys have done on 450 MX bikes by putting small amounts of gasoline in the spark plug hole and blasting it out with an air hose. The gas will also thin out the assembly oil.
 

· Registered
Joined
·
31 Posts
Discussion Starter · #5 ·
I presume it has an automatic decompressor that has a centrifugal actuator in one cam lobe that holds one of the intake or exhaust valves in the head a few thousands off of its seat as the piston goes through the compression stroke. If the valve lash is worn or set to great on the particular valve that the decompressor works on it might not lift the valve off the seat far enough and the compression stroke pressure will be too high for the crankshaft to maintain enough rotational inertia as the piston comes to top dead centre.
I got new camshaft and rocker arms, the only thing that I didn't replace is the valves and the springs
 

· Premium Member
Joined
·
1,790 Posts
Huh, recheck your valve lash. when hot sometimes the clearance is gone and will not have enough compression for ignition. Been through this exactly with a Suzuki quad for 3 years, hard to start warm, went through everything even a new carb. just for a giggle, I grabbed the rockers, and they were tight and holding the valves open. I just loosened until I felt play and the thing started up like new and never skipped a beat.
 

· Registered
Joined
·
3,674 Posts
Does it crank over easy with the spark plug out. Also are you 100% sure that you have the cam timing correct ( and valve clearances ) as well as nothing changed with the spark timing. If everything seems to check out in that way, do a compression test if you have a tester and see what sort of reading your getting. The ring end gap comment was to see if you had fit the compression and the oil control rings on their own into the bore and measured the end gap of each ring to ensure they were correct and that you have not caused cylinder scoring by having installed and run the engine with rings that expanded with heat and ruined the cylinder surface causing major friction. Something seems very odd that it ran and now does not, the only other thing I can think of is that for some reason the injector flooded out the cylinder with fuel and is causing the high compression issue and flooding out the spark plug.
 

· Registered
Joined
·
3,674 Posts
The piston, is it shaped on the top so that it fits the profile of the valves in either 180 degree turned rotation relationship to the cylinder head. Also is it the stock piston and not some aftermarket unit. The next thing is to look closely at the top of the piston to make sure there are no "witness marks" that would be a tell tale that for some reason ... valve timing or piston orientation that any of the valves hit the top of the piston. Also check the head to make sure the valves are seating and you haven't created a bent valve head. The cylinder base gasket, is it the same thickness as the original, the same applies to the head gasket as those two items if different ( thinner ) can sure change the compression ratio as well as valve clearances. Now hopefully the cylinder itself is not off as per having the same height from base gasket to head gasket surfaces as again that would change clearances as well as piston clearance. Double check you received the correct parts from your dealer. Just some items I would go through in a mystery situation like this. And the spark plug, make sure if its a new plug that it is the same and has the same reach, its not the first time someone installed an incorrect plug and had the piston hit it !
 

· Registered
Joined
·
31 Posts
Discussion Starter · #14 ·
Thanks Norther Farmer for your reply
So I took the valve head off again and the piston is lining up with the valves, But the
old gasket between the piston head and the valve head was more then twice the thickness
then the new one so ill put the old one in and try it
 

· Registered
Joined
·
3,674 Posts
Can you hold up the two cylinders lower and upper mating surfaces side by side or better yet if you have a vernier caliper to measure top to bottom of each cylinder to rule that out as being different. Also how did the cylinder base gaskets compare for thickness ? . Oh, it just donned on me that for now you left the cylinder on so can't really measure that nor the base gasket or not easily. Before you dive into too many directions, the parts you bought although aftermarket, did you buy them from a dealer or was this a ... I'll call it a Chinese purchase ? . Another words if bought from a local shop that you can have a discussion as well as if they have used these kits and if there is something quirky that you should be aware of.
 

· Registered
Joined
·
3,674 Posts
In my opinion you answered you own question . The only difference between turning freely without spark and turning hard with is ignition timing .
I thought of that right away as well but then he stated he had the spark plug disconnected and cranked it over hard then as well but only cranked over better when he had the key off and jumped the starter solenoid it turned over easier. Somewhere we are more than likely missing some key item, and yet it initially ran for 15 minutes.

Wouldn't it be funny if something was fiddled with after the initial start ( if its possible for the starter system to do this ) and the starter is turning the engine over backwards.
 
1 - 20 of 25 Posts
Top