My 75 won't start.

[Tone]

The engine has been rebuilt, rebored, new pistons crank rebuilt valves reground, New advance unit new points and coil new condenser good spark. Cam timing is correct with the line on the sprocket level with the top of the head/rocker join and I have checked the valve opening degrees all good. valve gap is set at .003/006. I have points ignition which are gapped .014" advance unit is new and working good. Plugs are new gapped .032" . Hard to tell if engine is getting gas as the plugs are new so I can't tell if they are wet or dry. I assume it is getting gas as the carbs are new VMs been cleaned and blown out multiple times with compressed air jets are all clear holes are also all clear choke is working Float height is 24mm . Engine has been timed statically on that F mark on the alternator. Battery is charged and showing 13 volts, engine turns over very fast. Standard exhaust and silencers, air filters are new pods. Won't start with them or if I take them off.

The bike was going, but now it just refuses to start, even with a cap of gas thrown in. I get the occasional bang from the exhaust, but nothing else. What am I missing.

 

[Jan_P]

Maybe Ignition wires Flipped from Points sending spark at the Wrong Top Dead Center ---On the Exhaust TDC
Try the Kick starter but be careful it kan Kick Back

 

[Tone]

i did try swapping spark plug cables same problem won't start

 

[Jan_P]

I would take out one spark plug Ground it against the cylinder .. Try to start / crank See if there are any spark And how it looks
Petcock on Pri ?

 

[JOHNDADDY]

A little more on how it was running and when it quit.
Did it die or just sat and wouldn't restart?
Last mods if any before it wouldn't start?
Might give a clue.

 

[Tone]

Everything was done at the same time the motor was rebuilt new plugs points condensers coils pods carbs floats jets valves and so on after putting it all together I started it and and tuned it. it ran great rode it around for a few weeks then one morning it just refused to start. Checked. points coils good spark from plugs valves timing cleaned the carbs checked float height blew out the jets and holes must have done that half a dozen times. It just won't start. Battery is showing 13 volts and motor spins. Over quite fast all I get a a pop from carbs or exhaust everyone and again.

 

[Jan_P]

I believe 13 V on a charged battery is on the low side Even with kick start attempts
E start you get Voltage drop and then you have some voltage drop to ignition parts somewhere low 11 s
If you now have fouled plugs and keep on trying the e - start the battery gets weaker and are less likely to start

Please do # 4 so there is a spark that looks OK
Charge the battery
A cleaning of the plugs would not hurt
Check the fuses
Give it one ( Maybe 2 ) priming kicks ignition off Petcock on pri.
Then ignition on 3/4 or Full Choke
Kick start
Please report back what happens

 

[Tone]

13 volts from 12 battery is not low. Each cell of a good fully charged 12 battery is 2.1 volt there are 6 cells that is 12.6 volts a good healthy battery at rest after disconnect from charging after an hour will settle around12.6 to 12.8 volts
After winding the engine over dozens of times trying to start it registers around 12.8 volts.. It is a new battery
I've already stated twice I tested the spark from the plugs it is a good spark and the plugs are clean and new
If a fuse was blown the plug wouldn't spark and the electric starter wouldnt work but the fuses are al good anyway
Please don't comment any further you are not helpful.

 

[Paul Sutton]

If you are satisfied with the timing and electrics then try putting a fan heater in front of the cylinders and warm them up - need to be just warm to the touch, then try starting. Maybe just a short term issue with richness/leanness/choke circuit. I had to do this with my lawn mower a couple of years back.

 

[JOHNDADDY]

Any help is helping, that's one of the reasons were here.
Something stupid simple gets overlooked and you're rippin out your hair.
How's your fuel flow out of the tank to the carbs?
If you've played with the points check that your timing's not out 180°
very common easy mistake, and it will pop but not fire.

 

[Its been a long time]

If the OP has a fully rebuilt motor that was running fine. But then suddenly stopped doing so, then if the ignition and carburettors are 100% I would be tempted to do a compression check. I did that myself a couple of months ago to rule out a sticking valve or similar. The compression test showed it was fine, but I had to do that to prove to myself it was the case.

It wouldn't be the first time on a newly rebuilt top end that a valve or two picked up in their guide and reduced compression to near zero.

 

[JOHNDADDY]

If the OP has a fully rebuilt motor that was running fine. But then suddenly stopped doing so, then if the ignition and carburettors are 100% I would be tempted to do a compression check. I did that myself a couple of months ago to rule out a sticking valve or similar. The compression test showed it was fine, but I had to do that to prove to myself it was the case.

It wouldn't be the first time on a newly rebuilt top end that a valve or two picked up in their guide and reduced compression to near zero.

Good point. Might explain the fast cranking.

 

[gggGary]

Easy to tell good compression or not with the kick starter. Kicking slowly you should be able to feel two solid resitance to kicking through, one per kick though.
your ocasional backfire sure sounds like advance rod is installed 180 degrees off, VERY easy to do, can happen at the pin on either end.
"Swapped plug wires"... Not sure what you actually did, but the quick spark on the wrong side test is to swap the points wires left/right at the coil/condenser junction.
Easiest way to divide problem in two is to spray a bit of carb cleaner in both carbs then try to start.

 

[Tone]

The fast cranking can be explained by a 13 volt fully charged new battery, overcharged if anything. Rebored engine with new pistons rings valves reground and 150 psi is not not low.

 

[gggGary]

The fast cranking can be explained by a 13 volt fully charged new battery, overcharged if anything. Rebored engine with new pistons rings valves reground and 150 psi is not not low.

We can feel your frustration.
We're trying to help you asked.
Looked back up the thread, and see it WAS running then quit suddenly.

150 psi is not not low.

Checked again after it quit running?

 

[gggGary]

All the ignition timing checks done AFTER it quit running?
Cuz my first guess is a pin went missing in the advance rod.
That happened to me once.

 

[Tone]

The pin can't go missing as long. As the points cam is in place and the advance unit is not removed. Just not possible unless the points cam and advance unit suddenly disappear. But I have checked that anyway, all these checks done after. It stopped. unning.
Points cleaned and gapped .014
No play in rod
Advance working no binding
plugs cleaned and gapped ..032
timing at 15 BTDC,
Valves at .oo3/006
Float at 24mm
Jets clear
Carb body holes clear
Compression 150 both cylinders
Both coils working
Tried swapping the plug leads
Very good spark both cylinders
Condensers ok
Noloose wires
Fuses all good
Battery now at 12.8 still above fully charged
Engine still turning over quickly
Cap of petrol thrown in carbs
No leaks around carb mounts
No leaks from head gasket
Cam chain tension checked
Haven't checked the cam again except for the making sure the pin in the rod was upright. But its a new chain so couldn't possibly have jumped.
Used a degree wheel to check valve opening times all good. besides it started and ran fine before.
Everything Checked when engine rebuilt and after it stopped occasional pop from carbs and exhaust when trying to start

I rode it around for a few weeks after the engine rebuilt it went great no problems at all then one morning it just refused to start
I've rebuilt 3 XS650 engines 3 honda fours BSAs velocette peewee honda50 lots of stationary engines and car engines. Never had a problem before this has me absolutely stumped

 

[gggGary]

More than once site sleuths have found a problem when they can look at good clear pics of engine, ignition, carbs.

It didn't just quit while running, it wouldn't start one morning and never ran again?
Have you tried kickstarting it? (Chassis/engine ground issues, weak spark with starter motor engaged.)
Have you had the carb slides out?
I won't make you wait, it's possible to swap them left to right carb and they go in (backwards) it won't run when that happens.

On the advance rod pins you are right, Early pamco E-advance used a copper advance rod aligning cap that would let the pins fall out on occasion, that's what allowed mine to fall out during a ride. Maybe one other time, a PO had used "something" as a pin, it was soft and sheared.

 

[JOHNDADDY]

That sounds like a perfect set up.
So, what's the problem?
Air. fuel, compression, spark, right? - (at the right time.)
And in the right amounts, that's doubtful per your description but first things first.
Aughta run just fine.
Pull the spark advance unit and look for alignment!
What's the harm? 3 min job.
1 thing off the list. If not move on.
That pin can come out.
And, if you over rev it. it will tear the back of the unit and set your timings off.
I've done it three times!
It's worth a look.
We can go from there. 1 by 1
Tryin to help just like everyone else here.

 

[Tone]

'''The pin can't go missing as long. As the points cam is in place and the advance unit is not removed. Just not possible unless the points cam and advance unit suddenly disappear. But I have checked that anyway, all these checks done after. It stopped. unning.'''

Today I pulled the rod out, pins still in place, greased and reinserted it. Advance locked in place, springs OK spring tabs OK circlips in place points cam locked in place not damaged points contact clean not pitted rod springs back when advanced. Stripped the carbs again, cleaned em blew em out with compressed air, checked ffuel height checked the advance unit again points again, timing again, valves again. Battery charged back up to 13 again plugs again still a good spark. Still won't go. I've done all these checks I've mentioned at least half a dozen times probably more no point in going over the same thing over and over again.

Vm carbs aren't truly left and right. Only the idle screw has been moved to the left side on the left hand carb other than that they are identical even the choke levers face the same way. But they are on the right sides but that would make no difference other than making the idle screw impossible to get to

On my last ride it went fine on about a 25mile trip, rode home, turned it off, turned off the taps. Next morning nothing. Pressed the starter and kicked it until I fell over then resorted to the starter again nothing. Battery was still up around 12.4 when I gave up and tank over half full. Battery put on the charger then pulled out the toolbox and started with all the checks above.
There has to be something I havent though of something simple.