125S Engine Rebuild

[davej]

I have recently rebuilt engine for above which is giving me starting problems.
Top end only needed new rings (no excessive wear in bore)
New 12v electronic kit fitted
Have tried 3 carb set ups
Standard 125S
Standard125
DL125
I can start from cold however after a few revs stop and leave for 10 mins I can not re-start, Engine firing but not catching on.
This happens on each carb set up I try
Fuel flow is ok.
Any Suggestions to help please?

[coaster]

Simple things first, are you using the correct starting procedure? You need to hold the throttle closed when starting, do not be tempted to twist the throttle as you kick it over.

You mention '3 carb settings' but what does that mean exactly? it is probably a bit rich at idle. New air filter fitted?

[CHRIS in MARGATE]

As above or you will flood it.

Is it the correct 20mm carb as was fitted to the 125S or the 18mm carb as fitted to the LI 125?

[davej]

Hi
Thanks for the replies
What I mean from the carb set ups is that I used the correct carb, jets and manifold for each 125 model (20mm carb for 125S and DL125 with correct jets/manifold)(18mm carb etc for 125) It just seems strange that I have the same symptoms for all 3 set ups I have tried.I have even tried 2 different cylinder heads. Oh and yes its a new filter.

[HxPaul]

Is it an Indian 12v system?
If the answer is yes,did you strobe it?If you set the 12v system up manually without strobing it,it could be miles out.

[coaster]

Hi
Thanks for the replies
What I mean from the carb set ups is that I used the correct carb, jets and manifold for each 125 model (20mm carb for 125S and DL125 with correct jets/manifold)(18mm carb etc for 125) It just seems strange that I have the same symptoms for all 3 set ups I have tried.I have even tried 2 different cylinder heads. Oh and yes its a new filter.

The most important part of my earlier response was about keeping the throttle shut whilst kicking it over, have you tried that yet?

[davej]

Hi
The 12v system is the one supplied with ducati/bgm items as advertised by one of our northern suppliers, I set up TDC by using a DTI and strobed the firing point, The spark I am getting is one of the best Ive seen (Ive seen many worse)
Regarding starting yes I am keeping the throttle closed, At cold she will start, switch off leave for 5 - 10 mins no start, leave for an hour and she will start again.

[CHRIS in MARGATE]

Are all the carbs fitted with red tipped nfloat needle valves?
Can cause flooding.
If you are satisfied with carb and ignition then you now have to look at possible blocked exhaust or excessive ring gap.

[davej]

Thanks Chris
Have fitted red tip needle in all 3 carbs, Fitted new rings and gap is ok, I am thinking that if it was the Exhaust I would not be able to get it running at all.

[coaster]

The symptoms you describe are similar to those caused by excessive spit back where fuel is blown back and collects in the bottom of the air hose. Blow back occurs when the exhaust pulse bounces back off the rear of the silencer and pushes unburnt charge back out through the carb. Overlapping port timings and expansion chambers exaggerate this effect. Not saying that is what's happening on your standard machine but a known 'work around' is to turn the fuel off 10 or 20 seconds before turning the engine off. Restart with a warm engine with fuel turned off.

Then idle screw adjustment could also affect your warm starting if it is set too rich as could the grade of plug.

[grandpa]

Hi Davej
Any difference In compression between hot and cold?
Possible air leak........ could spray engine start around carb manifold to see if revs increase could try inner tube leak test
Air leak would weaken mixture ,hand over Venturi or piece of cardboard to act as choke would richen mixture when trying to start engine. Leave petrol cap off


Good luck
Grandpa

[davej]

Thanks Lads,
More food for thought shall try things tomorrow, Cant say I notice a diifference in the compression.

[davej]

Update on latest: Today I changed the Cylinder, Head, Piston, Manifold and Carb from a known standard 125 runner and hey presto everything running and starting fine.The cylinders looked identical however I now need to measure both to compare, Anyone have any more suggestions that I should be looking out for? Cheers