Rebuilt engine won't start

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namtip
Posts: 5
Joined: Sun May 15, 2016 2:14 pm

Rebuilt engine won't start

Postby namtip » Thu Oct 27, 2016 2:00 pm

Ok guys - I wonder if you could help me out here.

Full strip-down and rebuild. New gaskets, seals, rings, plugs. Valves lapped, leak tests passed. Timing checked, double-checked, and triple-checked (at 1.4T the notch in the camshaft is exactly horizontal to the mating surface and pointing to the front of the bike).

All four plugs spark - visible in bright daylight. Left-hand coil (feeding plugs 1 and 4) is switched by CDI unit via blue wire. This CDI gets its trigger from the pick-up with the blue wires positioned to the rear of the crankshaft. The right-hand coil (feeding plugs 2 and 3) is switched by the second CDI unit via yellow wire. This CDI gets its trigger from the pick-up with the yellow wires positioned to the front of the crankshaft. The pulser cam lobe passes the 'blue' pick-up when 1 and 4 are more or less TDC, and same for the yellow pick-up when 2 and 3 are TDC.

Valve-clearances set and checked (inlets 0.05mm, exhaust 0.08mm)

Carbs disassembled, ultrasonically cleaned and reassembled. Fuel gets to all bowls and doesn't overflow.

When hitting the start button the engine just spins over and over and... nothing. I have a barely-visible light-white smoke/mist/vapour that curls out of the exhaust while doing this. Newish battery - engine spins over really well.

Played with choke settings and pilot/air screw settings (1.5 turns out)

Sprayed Easy-start into the intake and had a couple of quiet "sighs" from the exhaust and once had a fizzle shoot back out of the 4th inlet!

Tested compression 1=61 psi, 2=57 psi, 3=60 psi, 4=57 psi - seems low? Maybe expected with new rings and honed cylinders?

On removing each of the plugs for the compression test I had the same barely-visible light-white smoke/mist/vapour coming out of the hole. It smelt a bit like gas, but not entirely.

I'm not entirely sure what to check/do next - it's hard not to feel down about it. Any tips/pointers anyone?

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tario
Posts: 166
Joined: Sat Mar 07, 2015 2:26 pm

Re: Rebuilt engine won't start

Postby tario » Fri Oct 28, 2016 12:11 pm

Compression seems really low. I've heard people say they get readings of 80psi to 130psi. A splash of oil in a cylinder is supposed to increase the compression a bit. You did run the engine a bit to get oil circulating, right?

Clymers says you should read 142psi to 198psi...
1979 cb650
1980 cb650

namtip
Posts: 5
Joined: Sun May 15, 2016 2:14 pm

Re: Rebuilt engine won't start

Postby namtip » Fri Oct 28, 2016 1:41 pm

tario wrote:You did run the engine a bit to get oil circulating, right?

The engine was turned over a hell of a lot all the time I was trying to get it working. I imagine that would be enough to get the oil flowing to all the right places. I could certainly see it pooled in the camshaft housing. But maybe it's not yet lining the cylinder walls...

tario wrote:A splash of oil in a cylinder is supposed to increase the compression a bit.

I didn't try this yet. If I can get out there this weekend this will definitely be my next try! I'll report back...

namtip
Posts: 5
Joined: Sun May 15, 2016 2:14 pm

Re: Rebuilt engine won't start

Postby namtip » Sun Oct 30, 2016 2:06 pm

namtip wrote:If I can get out there this weekend this will definitely be my next try! I'll report back...

Ok, so I squirted some engine oil into the cylinders and now I'm between 75 and 80 psi.

namtip wrote:Timing checked, double-checked, and triple-checked (at 1.4T the notch in the camshaft is exactly horizontal to the mating surface and pointing to the front of the bike).

Whilst this is certainly true, I totally forgot about setting the position of the pick-ups. It wasn't far out, but a couple of millimetres may make all the difference. I had taken the carbs off to examine them, so with fading daylight I resorted once more to Easy-start straight in the inlet. This time I'm getting some better-sounding thumps out of the exhaust. I'll finish examining the carbs to see if anything there is amiss.

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Folsoml
Posts: 1634
Joined: Sat Apr 07, 2012 9:02 pm
Location: North Florida

Re: Rebuilt engine won't start

Postby Folsoml » Mon Oct 31, 2016 5:11 am

I'm feeling like you may be leaking compression through one or more of your valves. Your compression is really low, and engine oil in the cylinder did not help that much. You may need to lap your valve seats and put in new stem seals.
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