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Another Honda XR600R starting problem

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Hi All,

I've been creeping around here for quite some time, but this is my first time posting. I know most of you reading this are groaning and about to type up an understandably frustrated rant about getting the bike to TDC, squeeze the decomp till just past TDC, no throttle,  half choke and a good deliberate kick. Unfortunately, this problem is not so easily solved. 

I bought a XR600 1991 a little over a year ago, from the second owner. I registered the bike and as used it as a commuter straight away. It was pretty hard to start cold, but that was immediately fixed by adjusting the float height to the correct 14.5 mm. After riding it daily for several months, I decided to rebuild the top end, as I think it's good practice to do so on a secondhand bike with unknown engine condition. At this point there was absolutely no problem with the bike running. When I opened it up, I found one badly worn rocker (left exhaust) and corresponding cam lobe. I replaced the following: camshaft, both exhaust rockers, cam chain, cam chain tensioner spring, valve seals, all gaskets, piston and rings. Evert hing else was checked and in serviceable condition.

Once the engine was rebuilt and back in the frame, I gave it a good kick and... nothing. After a good while of kicking and swearing, I decided to push start it. The bike fired to life, but died at idle. After repeating and keeping the throttle slightly open, I decided it may help (I don't know why) to take it for a bit of a break in ride. I took it to my local mountain and did about 40 km of piston ring break in riding. The bike ran superbly, but when it came to idle, it died and had to be push started again (luckily I was on a mountain with lots of declines to aid in this).  This post is getting pretty long, so I'll just tell you what I have done in an attempt to fix this still unresolved issue.

Done

  • Taken apart carb, soaked in carb cleaner, blown all holes and galleries out with an air compressor
  • Checked carb diaphragm for tears
  • reset float height t 14.5 mm (measured in line with the main jet)
  • replaced fuel mixture screw and O-ring
  • replaced float valve needle
  • replaced intake manifold seal
  • checked petcock and fuel lines
  • replaced spark plug - gapped correctly
  • set valve clearances to 0.10 mm for intake and 0.12 mm for exhaust
  • checked coil and spark plug cap - secondary resistance was fine, but the primary resistance was 1.3 Ω, supposed to be 0.3-0.4 Ω

Yet to do

  • replace coil - I have ordered a new coil, spark plug cap and CDI
  • check stator
  • check decompression plunger/spring thing in head - I was pretty careful to make sure it was working well when I reassembled it
  • check decompression cam - when the decompression mechanism was fit onto the new cam, there was a very slight amount of play on the decomp cam. Perhaps the spring under it is worn? It didn't seem to matter on the last cam, though.

All in all I'm stumped. I bloody hope it's just the coil, but as with most of the things listed, I'm not sure how the bike runs so well, but just not at idle or when trying to kick it over. Any ideas?

 

Thanks in advance,

Luke

Edited by LukeCB
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Although adjusting the idle does keep the engine from dying at idle, it doesn't help the hard start issue. I think the problem may be more complicated. 

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It might be, but I do hope not, haha. My technique was working fine for several months. Before the rebuild it would start first or second kick. There's no difference hot or cold. Equally won't start without a push start.

 

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If it runs fine but won't idle and is hard to start sounds to me like you have a carb issue.  You say you cleaned the carb, but did you make double sure the low speed circuit was clear?  You are talking very small passages, and sometimes soaking and blowing it out doesn't clear it.  Sometimes a thin piece of copper wire is what you need combined with canned carb cleaner and the straw.  I would start there before going any further.  If the bike ran like crap then the problem may be bigger, but you said it ran fine.

BTW, here's the proper way to set the float level, from a Honda service manual:

float_zpscyfpkzlh.jpg

Edited by cjjeepercreeper

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Thanks cjjeepercreeper, I'll give the copper wire a try. Although, when I blew all of the carb passages, I could feel air coming out the other side. I hope it's a simple as a carb issue and nothing to do with the decomp. 

I know they're a bit of a magical item, but can anyone think of a way in which the decompression cam having a small amount of play might do this? Perhaps the it's not sitting below the exhaust cam lobe at low rpm?

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Backwoods-Bomber, not yet. I am assuming that I reassembled everything correctly and the new valve seals and fresh hone/rings should have everything in good order. It certainly feels like it has more compression than before the rebuild. However, I'll do a compression test over the weekend. 

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Backwoods-Bomber, not yet. I am assuming that I reassembled everything correctly and the new valve seals and fresh hone/rings should have everything in good order. It certainly feels like it has more compression than before the rebuild. However, I'll do a compression test over the weekend. 

I only ask because typically a bike that requires a bump start has either low compression or a fuel issue in my experience. As said above, the tiny sideways holes in the jets/emulsion tube need to be clear and many times a soak won't clear them. I use a torch tip cleaner, some use guitar string and others use a bristle from a wire brush. No matter what method, those holes need to be perfectly clear and that would be my first focus point. Your concerns about the decomp mechanism can be addressed with a compression check. Even with the decomp active you should read in the 90 psi range. If it is much lower than that, dump a tablespoon of oil down the spark plug hole and check again. If still way low, you can assume it's a valve issue. If comp improves with oil, you have a ring issue.

Anyone who hass done a compression test on an xr650l with decomp active can feel free to correct my 90 psi number. That is an average I look for on various bikes to get a starting point.

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On 4/13/2017 at 11:34 AM, Backwoods-Bomber said:


I only ask because typically a bike that requires a bump start has either low compression or a fuel issue in my experience. As said above, the tiny sideways holes in the jets/emulsion tube need to be clear and many times a soak won't clear them. I use a torch tip cleaner, some use guitar string and others use a bristle from a wire brush. No matter what method, those holes need to be perfectly clear and that would be my first focus point. Your concerns about the decomp mechanism can be addressed with a compression check. Even with the decomp active you should read in the 90 psi range. If it is much lower than that, dump a tablespoon of oil down the spark plug hole and check again. If still way low, you can assume it's a valve issue. If comp improves with oil, you have a ring issue.

Anyone who hass done a compression test on an xr650l with decomp active can feel free to correct my 90 psi number. That is an average I look for on various bikes to get a starting point.

After going through the carb passages and jets with car cleaner and copper wire, and checking the float height again, I can confidently say it's not the carb. 

I tested the stator and pulse generator; both fine.

Then I tested the compression. I started the bike up and ran it for a while. I couldn't get the compression tester in there without burning myself, given how awkward the spark plug position is. I waited about 25 mins for it to cool down enough. It was still warm, but not quite running temperature. With the throttle open, I was kicking a measly 42 psi (according to clymer, it should be 58-87 psi with the decomp for XR600). Given the fresh hone and rings, I dare say they're not the problem. Could testing the bike at less than running temp cause that large of a drop?

I don't think it has much to do with it, but the bike is running a +1.00 mm wiseco piston.

 

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After going through the carb passages and jets with car cleaner and copper wire, and checking the float height again, I can confidently say it's not the carb. 
I tested the stator and pulse generator; both fine.
Then I tested the compression. I started the bike up and ran it for a while. I couldn't get the compression tester in there without burning myself, given how awkward the spark plug position is. I waited about 25 mins for it to cool down enough. It was still warm, but not quite running temperature. With the throttle open, I was kicking a measly 42 psi (according to clymer, it should be 58-87 psi with the decomp for XR600). Given the fresh hone and rings, I dare say they're not the problem. Could testing the bike at less than running temp cause that large of a drop?
I don't think it has much to do with it, but the bike is running a +1.00 mm wiseco piston.
 

I always comp test cold. You have low compression. Dump a tablespoon of oil down the spark plug hole and check compression again. If it increases, you will know it's the rings. If it stays low, it is a valve or head gasket issue.

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I've freshly rebuilt the engine with new gaskets and valve seals. I had all of the components inspected by my machinist and put new piston and set of rings in. Unless I've kooked it somewhere, which I'm fairly confident I haven't, I think it may be something concerning the decompression mechanism on the cam. 

scan0001.thumb.jpg.d3c30fb2340745577db253098dbf5a4a.jpg

Given the above diagram, even though mine is a '91, the slight play in the decomp cam doesn't concern me anymore. Perhaps the small plunger that holds the mechanism in place is malfunctioning?

Edited by LukeCB

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I've freshly rebuilt the engine with new gaskets and valve seals. I had all of the components inspected by my machinist and put new piston and set of rings in. Unless I've kooked it somewhere, which I'm fairly confident I haven't, I think it may be something concerning the decompression mechanism on the cam. 
scan0001.thumb.jpg.d3c30fb2340745577db253098dbf5a4a.jpg
Given the above diagram, even though mine is a '91, the slight play in the decomp cam doesn't concern me anymore. Perhaps the small plunger that holds the mechanism in place is malfunctioning?

That is very possible, however, the oil down the bore/comp test is a free, easy way to ensure it is a head issue without disassembly.

As far as the new motor goes, my grandpa had a saying; There are only two kinds of parts that fail, really old ones, and really new ones.
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Recheck the valve adjustments and loosen the right exhaust valve a couple of turns to remove the decompressor from the equation , then redo the compression check.

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Any update?

 

Not yet, I'm afraid. I'm out of town for work. I'll be straight back into it this weekend.

 

My plan is to try a comp test at operating temperature as the manual suggests (I bought heat resistant gloves), one with oil over the rings and another with the RH exhaust rocker fully slack. I'll also chuck the new ICM, coil and spark plug cap in.

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Any update?


So, I did the compression test at operating temperature, and boy what a difference! 80 PSI - well within the recommended 58-87 PSI with the decomp active.

After resetting the float height and idle screw/knob, as previously mentioned, the bike does now start with the kick start, but it is no easy task. Compared to how easy it was before the rebuild, it's as different as day and night. And, the bike does occasionally cut out at idleI

I've noticed now that there is also fuel overflowing the carb if the petcock is left on when the bike is off. This brings me back to thinking this is a carb issue. I have cleared all the passages, set the float height correctly, polished the float valve seat and replaced the float needle. No idea what else to do. Again, it runs well, just the idle and hard start.

PS. The new coil, ICM, lead and spark plug cap made no difference.

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Did you look at the size of your main jet and the position of the needle? Or try adjusting the fuel/air screw (that is accesible from the outside, while motor is running). I have the same kind of problem, my Honda XR250R does not want to start while kicking it, but it does start when doing a running start. And when it runs it is doing fine (no halts on low RPM).

The engine has a bigger cylinder then 250 (270 I think) and the PO changed the main jet (from 125 to 130) en changed the position of the clip on the needle. But the fuel/air ratio was way to rich. It did start with kickstarter however. Something it refuses to do now. Last weekend I changed it back to factory (main jet 125 and clip on needle in the third position (middle). My guess is that is running too lean now.

I am looking at playing with the fuel screw, and changing the position of the clip on the needle. Maybe these diagrams will help understand what does what when on a carb. It did certainly help me.

 

 

Carburator Concept.JPG

Carburetor Jetting Chart.jpg

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So, I did the compression test at operating temperature, and boy what a difference! 80 PSI - well within the recommended 58-87 PSI with the decomp active.

After resetting the float height and idle screw/knob, as previously mentioned, the bike does now start with the kick start, but it is no easy task. Compared to how easy it was before the rebuild, it's as different as day and night. And, the bike does occasionally cut out at idleI

I've noticed now that there is also fuel overflowing the carb if the petcock is left on when the bike is off. This brings me back to thinking this is a carb issue. I have cleared all the passages, set the float height correctly, polished the float valve seat and replaced the float needle. No idea what else to do. Again, it runs well, just the idle and hard start.

PS. The new coil, ICM, lead and spark plug cap made no difference.

Good news about the compression!

How many turns out is the fuel screw set at?
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