Scooterotica

I suspect that the compression is so poor as to prevent ignition but here the story briefly.....

Engineer friend has rebored Indian 200 barrel to 70mm using a used 70mm Asso piston for tolerances. On fitting discovered that ring gap was over 1.3mm but being a know-all continued to fit barrel, head and piston.

In trying to restart engine there is virtually no sign of any detonation just the occassional 'cough' with throttle fully open and fuel turned off. Will do nothing with choke on/open. Ignition is Indian electronic and reamed out Jetex working before change of top end.
Ign...19 deg BTDC checked with strobe, Carb 125 main and 45 idle. Plug is wet after turning over and spark is strong and regular. Carb has been cleaned out, plugs changed and cleaned

Am I correct in thinking that the mixture is blowing past the piston before ignition therefore not enough mixture remaining for proper ignition??????

Any thoughts anyone?

i,m confused here...............
if you can,t start it.............how did you manage to strobe it.............?
and why would you try and start a motor with the fuel turned off............?
am i reading this post wrong or missing something..............?
ring gap is way out of course...............

sounds like its too slack on the bore. you can strobe it with a windy gun on the flywheel nut , with the plug out, on the bench.

ahhh yes...........good point,...............that explains the strobe thing maybe............

Thanks for replies

Engine strobed before top end change and has not been altered.
Fuel turned off as unable to start with choke/fuel on seems to be flooding....so fuel /choke off and try to start with throttel wide open to clear 'flooding'. This is when I some get a 'cough' from the engine.

Any clearer??

Was running with SIL 200 top end before this smart- alec decide to fiddle

Do you know what tolerances he used when reboring the cylinder?
I'd want 4 thou on a cast 225 set-up - even then running it in carefully. Also see Edens post re. old school 240 set-up and note how important it is to get the jetting correct. These big bore cast motors really can get hot.

my monies on the ring gap- seems excessive to say the least.

Chris

Just a randome guess.... but it sounds like its possible for it to have been bored to 0.4mm clearance.... instead of 0.004 thou of an inch clearance... Maybe?? as the ring gap you have stated would probably be about right at that, its mathamaticaly about three times bigger (ring gap) on circumfrence than bore... therefore... 1.3mm would equal approx 0.4mm...

Thanks again. Will do some checking and report back. Mate says tolerences for barrel to piston (no rings fitted) was 0.035mm.He says that's what it says in the Home Workshop Manual for assembly tolerence. Pretty sure arrow on piston pointed to exhaust but will check again.Seems that what JIMS reckons might be in the right direction.

4 thou is approx 0.1 mm give or take a gnats chuff..... and is the clearance being measured at the correct place?
does he mean 3 and a half thou ? (which is fine providing you run it in carefully)