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Bike rebuild and engine won't start

BlueGnue

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Hi all,

New to the forum and a bit of a newbie question here...

I've just rebuilt and un-seized a CB750 Kz 1979 DOHC engine - replaced items: new main cam chain, valves, piston rings, solenoid, spark units, coils, battery and plugs etc.

The starter motor is good, there's spark in each of the plugs, compression appears ok (will test once the kit arrives), I thoroughly cleaned out the carbs, but cannot get any of the cylinders firing!

Following the Service Manual when rebuilding, I aligned the no. 1 cam lobes towards the sparks and the alignment dots horizontal and the spark advancer to centre F I T

Coils are ordered - Left 1 & 4, Right 2 & 3

The coils have a yellow, blue and black wire - blue to left coil, yellow to right and black joining both b/w coil wires?

When I fill the carbs with Cold Start spray, the spray is igniting and flames shoot out the exhausts (no pipes yet), or if the choke is open, flying out the carbs!?!

Before I dismantle the entire cylinder head and rebuild it again - any advice on where to start checking would be greatly appreciated.

Thanks in advance for your help.


Sam
 
So it cranks, but wont start correct? Does it show any signs of ignition while cranking? Sort of sounds like the valves are not doing what they are supposed to be doing. Maybe trapping the un burnt fuel and then its released by a flame. Almost like the power stroke is not executing. Sorry
 
So it cranks, but wont start correct? Does it show any signs of ignition while cranking? Sort of sounds like the valves are not doing what they are supposed to be doing. Maybe trapping the un burnt fuel and then its released by a flame. Almost like the power stroke is not executing. Sorry

Thanks Wez!

Yeh, she's cranking but not igniting at all.

However, I've just done a compression test and it's not even registering on the scale :((

So the head's going to have to come apart and replace the gaskets I think...

Wish me luck
 
Hi Pidjones - is the easiest way to check this with the no.1 (left hand side) cam lobes pointing towards the plugs, cam sprocket dots parallel and timing at F | T?

I'm guessing the timing marks should be central to the viewing window in the plate and the small cover pointer?

I'd prefer to check these than take the thing apart again.

It also doesn't appear to be leaking from the sides or blowing.

many thanks
 
Thanks Wez!

Yeh, she's cranking but not igniting at all.

However, I've just done a compression test and it's not even registering on the scale :((

So the head's going to have to come apart and replace the gaskets I think...

Wish me luck

It's usually the piston rings that are the problem.

Also. make sure you have a full battery, warm conditions and test each cylinder one at a time. Also, you must be WOT (wide open throttle) while cranking. Just keep the ignition switch set to off, not run. Check all cylinders. Once cylinder might mean burn valve. Leak down test will help you pinpoint the motors condition.
 
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