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Discussion Starter #1 (Edited)
1974 Honda CB450 DOHC
ALRIGHT BACK TO BASICS...There are only so many things an engine needs to run, spark, compression, fuel, and timing.
Both carburetors have been rebuilt correctly and the correct amount of gas is flowing into the cylinders.
Both cylinders have about 150+ compression.
Both spark plugs are firing.
The cam and crank have been timed together correctly after rebuilding my cylinder head, timing for the ignition is timed correctly to the compression stroke for both cylinders.
The bike wont even fire up while I'm spraying starting fluid direct through the carbs (choked or not).
Is there something obvious I am missing?? WHY wont this bike start!?

thankyou all who respond i need as much insight as possible.
 

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Are you using the kick start, or electric? If electric, the Voltage may be sagging too low to get a strong enough spark, so try using a booster battery; if you jump to a car or truck do NOT start that vehicle's engine!
 

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Sensei
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What reference are you using to determine compression stroke for a cylinder?

Has the rotor been pulled and re-installed?

Please feel free to phone to complete this dialog.......
 

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My question... was it EVER running???
When?
Then... what was done to make it not run?
 

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Discussion Starter #7
Are you using the kick start, or electric? If electric, the Voltage may be sagging too low to get a strong enough spark, so try using a booster battery; if you jump to a car or truck do NOT start that vehicle's engine!

I've been using a booster with electric start.
 

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Discussion Starter #8
What reference are you using to determine compression stroke for a cylinder?

Has the rotor been pulled and re-installed?

Please feel free to phone to complete this dialog.......

I've pulled off the cam head cover to look at both closed valves on the compression stroke.
 

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Discussion Starter #9
My question... was it EVER running???
When?
Then... what was done to make it not run?
It ran great for about 10 minutes in June of 18, then a screw fell into the left cylinder and i got the cylinders bored and valves lapped and put everything back together but then it only ran on one cylinder (low compression) but would start (fall of 2018). Now summer of 19 I haven't touched it all winter, and the bike won't even start.
 

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You mentioned "The cam and crank have been timed together correctly after rebuilding my cylinder head" and "I've pulled off the cam head cover" - are we talking about a DOHC CB450, or the later SOHC CB450? big difference in ignition and cam timing between the two

I ask because when we don't have an Introduction, it takes longer for us to fully understand the new member and what bike he has...
 

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Discussion Starter #11
You mentioned "The cam and crank have been timed together correctly after rebuilding my cylinder head" and "I've pulled off the cam head cover" - are we talking about a DOHC CB450, or the later SOHC CB450? big difference in ignition and cam timing between the two

I ask because when we don't have an Introduction, it takes longer for us to fully understand the new member and what bike he has...
1974 Honda CB450 DOHC
 

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...then a screw fell into the left cylinder and i got the cylinders bored and valves lapped and put everything back together but then it only ran on one cylinder (low compression) but would start (fall of 2018)
So if I'm interpreting this correctly, the last time it ran it was the fall of '18 after the rebuild, but it only ran on one cylinder due to low compression... sounds like the top end condition would be the focus at this point, getting it started on one cylinder is pretty much unproductive
 

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Wait... What?
Did the screw fall into the cylinder while it was running?
Why did you bore out the cylinders? Typically only a slight hone would be needed as boring usually mean you're going larger in piston and ring sizes.

Even with lower compression, it should still fire if fuel and sparks are available and especially with the starting fluid.
I'm thinking your ignition timing is way off somehow.
 

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Discussion Starter #14
Wait... What?
Did the screw fall into the cylinder while it was running?
Why did you bore out the cylinders? Typically only a slight hone would be needed as boring usually mean you're going larger in piston and ring sizes.

Even with lower compression, it should still fire if fuel and sparks are available and especially with the starting fluid.
I'm thinking your ignition timing is way off somehow.
in June of 18 a screw from the carb butterfly dropped into the left cylinder and welded itself to the piston (while it was running of course) and scratched the sh*t out of the cylinder wall. Cylinders were bored and piston sizes were extended from rings, so compression isn’t an issue. I agree the only possible explanation is still timing.
 

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...and i got the cylinders bored and valves lapped and put everything back together but then it only ran on one cylinder (low compression) but would start (fall of 2018).
Okay - so this above makes the most recent statement "Cylinders were bored and piston sizes were extended from rings, so compression isn’t an issue" confusing. Just trying to get a clear picture
 

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Since you've run a bunch of raw gas and starting fluid through it, trying to start it, it might be time to try a fresh dry set of NEW spark plugs.
 

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Sensei
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Are you running stock pistons with over-sized rings, or did you replace both pistons and rings with oversize parts?

BTW, both intake and exhaust valves are closed for the compression stroke and most of the power stroke....
Intake valve closure (Just closing) indicates the compression stroke has started/is starting...
 

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Discussion Starter #18
Are you running stock pistons with over-sized rings, or did you replace both pistons and rings with oversize parts?

BTW, both intake and exhaust valves are closed for the compression stroke and most of the power stroke....
Intake valve closure (Just closing) indicates the compression stroke has started/is starting...
Oversized pistons and rings.
 

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And you've checked the ignition timing? Set to fire on LF and F?
 
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