XT engine working, but impossible to start

[Troxx]

My 4PT (1998) engine runs fine, but if it stops it is near impossible to start again. It is also near impossible to start any time. But if I manage to start it runs fine.

I've cleaned carb twice, plug seems fine and is 1 year old.

Problem started instantly. Started it up easy, rode to shop, but when trying to start again, totally dead.

Back home I changed to fresh gas.. All good with carb as far as i can see.

I'm out of ideas. Please help :-)

[scottieh]

How old is the battery? Weak spark can manifest itself in funny ways. Even a charged battery can still deliver weak spark. I’d start looking at electrical parts, checking specs for the coil, reg/rect, stator, even plug cap.

[motorang]

Easiest way to rule out a dead battery is to connect XT battery to car battery using jumpstart cables. If the problem ist gone the battery is the culprit :-)

Startup problems may well be carbish though ... idle jet or some air leak, maybe intake rubbers loose or delaminating. Choke may be stuck slightly so mixture too rich.

My first go there would be to check the rubbers (also check if any hose has come loose or hardenened), then remove the carbs and give them a clean.

Tutorial available :-)
viewtopic.php?p=34435&hilit=carbie+tutorial#p34435

Andy

PS: Choke can be checked without unmounting the carbs

[Troxx]

Thanks for trying to help :-)
I had an incident with my back, and was in great pain for a week, sorry for responding so late.

Battery is good, and I recharge while and after trying to start.

Cleaned carb twice. New o-rings. Bike runs fine if I'm lucky enough to get it to start, but if engine dies it is near impossible (as first time starting) to get it to start again.

Could "runs fine" indicate coil and CDI is ok?

Measured 4 Ohm on coil, as I remember.. What should it be?

[donnybach]

It’s probably fair to say nothing is out of adjustment on the carb if it was working fine up to that point so like Andy and Scottie were saying it’s probably electrical or rubber damage. Doesn’t mean it’s not carb though, one bit of shite in the pilot for example)Hopefully as simple as one connector corroded or wire pulled out slightly. Use the workshop book and measure all you can, dry, and running if you can. Might find it’s obvious once you see or test it.

[scottieh]

Thanks for trying to help :-)
I had an incident with my back, and was in great pain for a week, sorry for responding so late.

Battery is good, and I recharge while and after trying to start.

Cleaned carb twice. New o-rings. Bike runs fine if I'm lucky enough to get it to start, but if engine dies it is near impossible (as first time starting) to get it to start again.

Could "runs fine" indicate coil and CDI is ok?

Measured 4 Ohm on coil, as I remember.. What should it be?

Some more tests:

Have you checked your compression?

Have you tried bump starting your bike down a long hill?

Pursuing the carb theory, next time you get the bike running grab a can of carb cleaner and spray around the intake boots. Try both sides of the carb, where it connects to the airbox and where it connects to the engine head. Listen to rev changes or if it outright dies. If there are any changes then we can assume air leaks are causing your hard start issue.

You can also try this: install a brand new plug and then start the bike and let it sit at idle for 10 minutes then pull the plug and look at its color. If grey and ashy then you have a lean condition at idle, and if blackish and sooty then too rich and either of those may also be the culprit.

[Troxx]

Did some tests today. I managed to get it to start, let it run at 2000rpm for a while, and it responds nice to speedhandle input... full range. It also ran at 1200rpm idle. When dieing I managed to get it to start again by holding full speed and then let speed go while running starter. Several times I did this.

Coil tested by multimeter: primary = 4 Ohm, Secondary = 12,75k Ohm, Plugcap =10k Ohm all within specs according to workshop manual.
Adjusted exhaust valve, Intake was within specs < 0,10mm. Located CDI, and brushed connectors. Bike charging ok. Pick up measured withing specs.

Got a (almost mc mecanic) friend come by while doing it all. We think it is carb that gives me this problem. Intake manifoils is not leaking. He thinks this is strange, all good while running, just near impossible to start. Plug checked a little rich, but not bad.

We will take apart carb at his workplace later this week, replace fuelhoses, insert fuel filter and more.

[motorang]

Carburettor idle system, the idle jet has a very small opening which is easily obstructed.
Therefore with any carb problems I fit a non-paper non-cloth inline fuel filter just before the carb. I also omit the fuel pump should there be one, for testing and run tests with a full tank of fresh fuel.
Sinter filters work fine and do not shed any fibers themselfes just because they do not have any ...
Sinter filter example

Furthermore idle and mid range performance relies heavily on air flow in the air channel from intake to the main jet where it helps creating the mixture.
This is often overlooked - carb gets thrown into a chemical cleaner or ultrasonic cleaner bath, without removing the dirt/crust afterwards.

After every soak the carb channels need to get flushed with pressurized air (all jets and adjustment screws removed!), ideally try poking the channels with some soft wire (copper wire) to remove any sediments which may sit there, maybe even CAUSED by the cleaner bath.

If you blow into the holes where the jets did sit, you should get clear airflow out of some of the channels.
Also check the choke. Due to the silly construction water runs along the choke wire and messes things up inside the carb (corrosion).

Andy

[scottieh]

Did some tests today. I managed to get it to start, let it run at 2000rpm for a while, and it responds nice to speedhandle input... full range. It also ran at 1200rpm idle. When dieing I managed to get it to start again by holding full speed and then let speed go while running starter. Several times I did this.

Coil tested by multimeter: primary = 4 Ohm, Secondary = 12,75k Ohm, Plugcap =10k Ohm all within specs according to workshop manual.
Adjusted exhaust valve, Intake was within specs < 0,10mm. Located CDI, and brushed connectors. Bike charging ok. Pick up measured withing specs.

Got a (almost mc mecanic) friend come by while doing it all. We think it is carb that gives me this problem. Intake manifoils is not leaking. He thinks this is strange, all good while running, just near impossible to start. Plug checked a little rich, but not bad.

We will take apart carb at his workplace later this week, replace fuelhoses, insert fuel filter and more.

While the carb is apart check the rubber o rings all around. The idle mixture screw has an o ring that is giving me grief right now. Bike is hard to start but runs well once warm. I’ll be diving into my own issues this week and will chime in with info if anything is obvious. Good luck and keep eliminating issues. You’ll find it

[Troxx]

I did this: AFAIK all parts of carb was in its smallest pieces and I found nothing... Used guitar strings and put them in all holes, and used pessure air to blow all holes... Found nothing .. put it all tougether and then bike was fine. No problems...

So ... bike is now fine, and I don't know why, but now I know all part of carb, and that all holes should be picked with guitar strings and blowed clean.

I also did correct the valves but this could not be reason.. anyway it now runs clean and chill at 900 rpm.