YZ250F Only starts with Hot Start engaged?

A little background - 100 hours on the bike, checked valves again and they were tight. Intake were .8, .9 ,.10 (spec is .10-.15) Shimmed them all to .13. Exhaust were both .15 (spec is .18 -.22) Shimmed them to .20. Put in new OEM piston and rings, new OEM timing chain.

 

Go to start it after top end, kicking with choke on, a couple short fires but wont start (has always started well with choke on when cold) After a lot of kicking tried a couple with hot start on and it starts? Went for a ride, seems to run fine. Again yesterday went to start it, will only go with hot start on. Once warmed up it starts normal, no hot start, no choke.

 

I have the carb off now and am going to blow it all out.

 

Does it sound like something related to the valve clearance or when I had the carb off did I get dirt in it?

 

Thanks in advance for opinions.

I posted over in the YZF section but thought it may be more applicable here:

 

A little background - 100 hours on the bike, checked valves again and they were tight. Intake were .8, .9 ,.10 (spec is .10-.15) Shimmed them all to .13. Exhaust were both .15 (spec is .18 -.22) Shimmed them to .20. Put in new OEMpiston and rings, new OEM timing chain.

 

Go to start it after top end, kicking with choke on, a couple short fires but wont start (has always started well with choke on when cold) After a lot of kicking tried a couple with hot start on and it starts? Went for a ride, seems to run fine. Again yesterday went to start it, will only go with hot start on. Once warmed up it starts normal, no hot start, no choke.

 

I have the carb off now and am going to blow it all out.

 

Does it sound like something related to the valve clearance or when I had the carb off did I get dirt in it?

 

Thanks in advance for opinions.

 

Hot start adds air. Lot's of air.

So, that would lead to the assumption that you are excessively rich on the piliot circuit or fuel screw.

Hot start adds air. Lot's of air.

So, that would lead to the assumption that you are excessively rich on the piliot circuit or fuel screw.

 

Hmm, started fine before the new top end. 

 

When I was first trying to get it to start and it would not, I pulled the plug a couple times, after 20-30 kicks the plug was coming out dry? Then when hot start was engaged it fires. Make any sense?

If it needs hot start lever when cold then its rich

Possible fuel leak past float valve needle or pilot jet fell out or tick over set too low

Edited by GuyGraham

Hmm, started fine before the new top end. 

 

When I was first trying to get it to start and it would not, I pulled the plug a couple times, after 20-30 kicks the plug was coming out dry? Then when hot start was engaged it fires. Make any sense?

 

No it doesn't .

 

The information you are providing is kind of spotty and incomplete.

If it needs hot start lever when cold then its rich

Possible fuel leak past float valve needle or pilot jet fell out or tick over set too low

Thanks for the reply!

 

Have the carb off now, will clean it up good. Piliot was in tact. What is "tick over set too low"

No it doesn't .

 

The information you are providing is kind of spotty and incomplete.

Not sure what other info to provide:

 

I think my first post describes in pretty good detail what is going on?

When the bike is cold it will not start unless I engage the hot start. Once warmed up it starts normal. I am in the process of cleaning the carb. Wondering if there is anything specific I should be looking for. 

Idle speed

Should 1900 to 2100 rpm wh8ch is quite fast

Edited by GuyGraham

Actually jetting, incudling the pilot air jet and main air jet (which if clogged, would give you your symptoms..)

Hours on motor

Hours on carb

Last time the carb was serviced for wear components (slide plate, plate seal, needle jet, apump diaphram)

Had the carb apart last night, cleaned everything, blew out all circuits and jets. 

 

The only obvious problem I found was the oring on the fuel screw was ripped in half, wouldn't think it would cause my problem but not sure?

 

In around the slide area it was somewhat dirty as well. Gave it a thorough clean.

 

Finished late last night, too late to start it, will be trying it out tonight.

 

Thanks for the suggestions on what to look for. Will offer feedback later.

That little oring on the pilot screw is very important

You need a new one just to rule it out as a culprit.

Replaced the oring on the fuel screw, cleaned everything blew it all out. It fired up second kick with the choke on. All seems good. Does still seem to want the hot start when starting hot which I guess makes sense, just never seemed to need the hot start prior to the rebuild?

 

Thanks again to those that took tie to offer advice.

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