XR75 wont start

[Vince H]

brought it not going yes has points and they are clean was one of the first things i checked when wont produce spark after it was before. all that been down is a pull down and clean up

[sleepy]

What do you mean by pull down? Is that stripping the motor or just taking the tank and seat off to give it a clean?
Assuming the motor is sound, check that there are no insect nests in the carb vents or in the fuel tank vent hose(if it has one) and give the carb a good clean out, making sure nothing is bloked. Most faults with points igntions are condensors , resistor in plug cap , coroded points(clean them with emery tape not a file) sometimes top coil and bottom coil occansionally also check kill switches. Check correct stator wire is going to the coil and points, on a most hondas I have seen it is the black one. I'll say it again, the spark plugs with unleaded fuel can seem to go off after a period of sitting and go black. Make sure you have a non-resistor spark plug(that "R" on the side doesn't mean racing) It is quite common that you can see a spark at the plug but when it is inside the motor under pressure it will track down the side and not fire. The same can happen if the spark is week. If you put a timing strobe light on the lead when you kick it over you should see it flash.

[Momus]

Remove (unscrew) the plug cap and cut the last 15 mm of the high tension lead off.

This last bit seems to develop high resistance.

[Vince H]

The motor was stripped down cleaned and put back together car by is clean no blockages it had spark the other day pulled motor out to take off head to check valves and now back in frame and has no spark at plug. Has spark coming from stator wire when grounded to motor( orange in colour) but not at plug once all hooked back up.

[sleepy]

Are you sure the igntion wire isn't the black one? The plug coming out of the motor normaly has the black wire with a short extra wire connected to nothing. I use it as a test wire for setting the timing.
Make sure that the points wire isn't shorting on anything and disconnect the kill switch and check again for spark.
I would connect it again it no fault before starting.
Have you any resistance readings on the top coil, primary and secondary?
Try replacing the condensor, just cut it of the coil and wire in a anything you can get to fit. VW bug or early holden will do the job.

[Vince H]

the spark is orange is what i meant the wire is black like u were saying sleepy might be a short like u said will look tomorrow

[Vince H]

So have spark up to where coil plugs in but not at plug or plug lead. It's a 77 model so condensor is attached to coil can it be taken off and replace or is it easier to replace whole coil if q does anyone have a coil they are selling??

[sleepy]

Just cut the wire to the condensor and do as I suggested in the previous post. You can buy aftermarket coils with condensor but my money is on just the condensor.
You can test the coil with an OHM meter if you have one. Primary is about 3.5 ohm secondary is about 180k ohm.

If you are near the southside of Bris I could lend you a coil to test. but change the condensor first.

[Vince H]

will try that sleepy cheers

[sleepy]

Did you get it going?

[Vince H]

no not yet have  given up fpr the moment will be trying again at the weekend

[Vince H]

well think i have finally found the problem its compression got my old boy to show me how to properly use a pair or vernier calipers today and turns outs the barrel i have with the standard piston is at 48mm. does anyone know of where i can get my hands on a reasonably priced over sized piston kit or a barrel and piston and rings that will let me get the old girl running again that wont cost me and arm and a leg cheers

[Vince H]

what is the standard bore size service manual says 47 mm but piston kits on net i found say 48 is standard for a 77 model

[alexbrown64]

Hi Vince, looks like your bore and stroke are 48mm x 41.4mm
Below are the specs taken from all 3 mini bikes from 1977 including yours....



Cheers,
Alex

[bazza]

any bike wont go=fuel   compression   or   spark   so simple,  yet we get cases that do our heads in