xs400 starting problem

[David Houston]

Hi there i have a 1979 xs400 , I have overhauled the carbs , Fitted new HT coils. Fitted new condensor and points getting a great spark . Timing is spot on. the problem is the bike wont start from cold either with or without the choke . I squirted a small amount of petrol in each chamber through the spark plug holes and the bike will start and run. It will start and run until i leave it overnight then it want start again until i squirt some petrol back in the spark plug holes. the compression is 120 psi each cylinder..Has anyone any idea why fuel is not getting to the cylinders when the engine is cold. Any help would be greatly appreciated.
The only problem when overhauling the carbs was that the pilot screw on both carbs had snapped and seized the tip of the pilot screw in the carb body , i managed to remove both tips and fitted new pilot screws.By the way i have standard petrol tap on the petrol tank , (Not a Petcock Valve)
I look forward to any advice to help me.
Regards Dave

 

[xschris]

120 psi is very low for these bikes. Do a wet test of the cylinders and see if they change. If not your valves could be bad or out of adjustment.

 

[David Houston]

hi chris
how do you carry out a good wet test for the compression.
cheers dave

 

[xschris]

Remove the plugs put a little oil down the holes and turn the motor over with the throttle wide open. I like to use stable fog spray for this but bike oil will do. 2-stroke oil is good also

 

[David Houston]

thats mate i will try that tonight.
cheers dave

 

[David Houston]

hi again chris
i have carried out the wet test with 2 stroke oil and the psi went upto about 140psi , does that point to piston rings being badly worn. should i strip the the engine ,fit new piston ring sets and regrind all the inlet and outlet valves and fit new gaskets.
cheers again dave

Remove the plugs put a little oil down the holes and turn the motor over with the throttle wide open. I like to use stable fog spray for this but bike oil will do. 2-stroke oil is good also

 

[Tyler Shiells]

Not sure about the fuel issue.

Compression wise though...Are the valves set proper?

You can make a simple leak down tester and see where its loosing pressure.

If it's coming through the crankcase then you have to take the jugs off and get an accurate measurement at varying depths of the cylinders. If your lucky they are in spec and new rings may work for you. If they are over spec then a rebore is necessary. Oversized pistons for these motors are harder to find then gold. Someone in Germany makes over sized rings. Very pricey. I just went through the whole process and it was over $500 in parts and shipping.

 

[amc49]

Unless the mileage is high commonly it's exhaust valves that are the problem, Japanese ones have never been know to last a long time in great numbers and the totally unleaded fuel simply burns them up.

 

[David Houston]

i have trouble with low compression mine is 120psi should be about 165psi i tried a wet test with small amount of 2 stroke oil in each cylinder, refitted the spark plugs (did not fit the HT leads) the wet test brought the pressure up to 135 psi on each cylinder. stripped head off to make sure the valves and seat were ok, all valves and seats were good no leaks. measured cylinder bores 69.97mm on one clinder and 69.90mm on the other , should be 69mm.do i have to get a rebore then fit oversized rings or would fitting a new set of standard rings help.
i appeciate any recomendations.
cheers dave

 

[Tyler Shiells]

What tool are you measuring with? And at varying depths that was the average? I would double, triple, quadruple check those measurements. If those are accurate your jugs are 4 sizes over already. if you have standard pistons I dont think you'd have anywhere near those psi numbers with the jugs being that far from factory spec.

 

[amc49]

Like said in the last post, you are talking about almost 1 entire mm. in wear there and it wouldn't be running without rattling like skeletons like that. You cannot use dial calipers for measuring that, it must be micrometer work and an inside one to measure the bore IDs.

They are worn out at .010" (.25 mm.) of clearance and you are saying it's almost .040". It's pretty unusual to even get to .020" much less the bigger number.

 

[David Houston]

What tool are you measuring with? And at varying depths that was the average? I would double, triple, quadruple check those measurements. If those are accurate your jugs are 4 sizes over already. if you have standard pistons I dont think you'd have anywhere near those psi numbers with the jugs being that far from factory spec.

I assume my micrometer is out of calibration and the engine has been rebored to 70mm , which is max on the xs400 i believe. The bores are in great condition no scratches or scores ( reading the same just under 70mm through out the chambers). Should i try fitting new 1mm o/s rings to pistons . The engine was part of a project bike . No history but it was very cosmeticaly clean all the gaskets look like they have been replaced lately.

 

[Tyler Shiells]

Take a picture of the pistons for us. Do they have +1mm stamped on them? Measure the ring gap and see if the rings are still in spec. The procedure is in the manual.

 

[amc49]

If the cylinder wall to piston is too loose then it doesn't really matter what the bores look like and new rings may not help much. When pistons rock too much from being loose you lose lots of ring seal.

Need to tune up your mikes and measure for that clearance.

If you haven't solvent tested the valves you have no idea how much they may be leaking.

As to the OP, you need to make sure the choke is working right to start cold. You don't tear down simply for that and you never assume the single compression numbers are right either, the gauge type or technique used can commonly tilt compression numbers and it usually does. Car gauges can easily drop motorcycle numbers when they use hoses that are too long, and the gauges can vary wildly by themselves. Cheaper gauge is always suspect there. Even the little release valve can affect the numbers bigtime. Why you never mess around with budget gauges on that, get a good one, hundreds of $$$ rely on it.

 

[David Houston]

What tool are you measuring with? And at varying depths that was the average? I would double, triple, quadruple check those measurements. If those are accurate your jugs are 4 sizes over already. if you have standard pistons I dont think you'd have anywhere near those psi numbers with the jugs being that far from factory spec.

cheers mate
i will get my mates micrometer which is calibrated constantly due to work he does and will do a load more test all over the cylinders
dave