TL;DR at the end

1987 Kawasaki KLR 250 (9.3k miles showing)

Symptoms:

• Cruising during a ride, the engine speed dropped instantly and without warning as if someone cut the throttle cable. It was able to start right back up after coming to a stop, but would die if any throttle was applied.

Observations:

• Will start again consistently, but will not rev up.
• Any movement of the throttle's butterfly valve kills the engine.
• Modulating the choke with the throttle closed will increase engine speed from ~900 RPM - 2100 RPM. Idle is smooth and consistent.
• No knocking, pinging, or overheating issues have been noted.

Analysis:

1. Fuel
   1. Carburetor cleaned with liquid injector cleaner and sprayed out with aerosol carb cleaner.
   2. Serviceable jets (main and pilot) replaced.
   3. Diaphragm slide moves freely (with spring return) in its slot. Movement during normal operation is unconfirmed.
   4. Fuel-to-air ratio adjusted, but it has been noted to start easily at all ranges of available adjustment.
   5. Fresh fuel and fuel filter installed.
   6. Fuel lines are open and the carburetor's float is operational. Confirmed by opening the drain on the float bowl.
   7. Spraying starter fluid into the air-box will increase engine speed slightly (+1k-1.5k RPM), but only after the butterfly valve has been opened. The amplitude of the opening does not seem to matter.
   8. No backfiring has been heard except after liberal amounts of starter fluid have been put in the air-box.
2. Air
   1. Subjectively, it sounds like air is being pulled into the air-box side of the carburetor when kicking the engine over.
   2. Spraying starter fluid near the junction of the engine and carburetor does not affect engine speed.
   3. The air-box is open and clear.
   4. The air filter is clear and clean. No excess of oil was noted.
3. Compression
   1. Testing with the automatic compression release engaged reached 60 psi (with flickers on the analog gauge up to near 80 psi). Service range is 60 - 114 psi.
   2. Testing with the automatic compression release disabled reached 120 psi.
   3. No leaks were audible anywhere during either test.
   4. No blue smoke or blow-by is noted in the idle range noted above.
4. Spark
   1. The spark plug grounded outside the engine during compression testing consistently made sparks.
   2. Resistance of primary and secondary coils of the ignition cord were checked. The secondary was 5kOhm above the service limit of 9kOhm, and the assembly was replaced.
   3. Resistance values between pins on the CDI box were checked with an analog multimeter, and the source coil negative pin appeared disconnected (>100MOhm), so a new CDI box was swapped in. The new box had ~40kOhm resistance on the same pin, well above the recommended range, but was at least measurable.
   4. Resistance of the trigger/exciter circuit was measured and found to be perfectly within the acceptable range. The gap between the magneto and pickup point was also within tolerance.
   5. No deformities, lost magnets, or any other such physical abnormalities were noted on the magneto or stator.
   6. All wire connections are continuous and have not been found to be shorted to ground.
5. Timing
   1. Timing of the cam lobes was confirmed in relation to the camshaft.
   2. All joints are sufficiently torqued, lubricated, and not worn beyond service limits.
6. Valves
   1. Diameter service limits of valves and valve stems have not been checked due to results of compression tests.
   2. Intake springs were removed for inspection. No physical abnormalities noted. One set felt like it was weaker than the other while in the machine, but it felt just the same as the other as soon as it was out. Nothing about the seat in either spring's location seemed different than the other.
7. General
   1. Kick-starting does not feel restricted or as if the engine is binding at any point.
   2. All other electronic components operate as expected.
   3. Very little vibration is noted while the engine idles.
   4. Temperature of the coolant system rises within an acceptable amount of time (~4 minutes while held between 1500-2000 RPM).

TL;DR

I can get the bike to start just fine, but it will die as soon as the throttle is opened even a little bit. Moving the choke will increase engine speed. The carb has had a rebuild and a good clean, the spark is good, compression is fine, and air is no problem. Valve springs and timing have also been checked. Where should I look or recheck next?

 

Have you been able to pass compressed air through all the passages of the carburetor while you were cleaning it?

If the engine dies when the butterfly opens (when adding air and changing the air-gas ratio), it seems to be that not enough fuel is getting in to make up for the air that is being added. Needle sticking down in the emulsion tube?

 

Good thoughts! I have been able to pass compressed air in both directions through each passage and orifice of the original carburetor after soaking in cleaner.

The diaphragm slide has been confirmed to operate as expected with a shop vac being hooked up to the engine-side and the butterfly valve actuated. It is entirely possible that the needle has become coated with a layer of residue, so I can soak that in cleaner as well and then brush it off with a rough cloth.

A new carburetor assembly has been fitted to the bike, but no improvement has been noted. The engine will still only rev as high as the choke can push it.

 

An internal dowel of the intake cam's sprocket was broken and left the cam 180 degrees out of phase.

The good thing about old, small engines is that there are only so many parts that can be replaced before you've replaced the entire engine. The issue (so far) appears to be a mechanical break: the internal dowel between the intake cam sprocket and the intake cam valve had sheared. Over the course of this months-long repair, I tried replacing the CDI box, pulsing coil, spark plug, pickup coil, timing chain, intake valves, all valve stem gaskets, carburetor (multiple times), intake boot, piston rings, piston, fuel filter, and fuel petcock. The only items still new alongside the cam sprocket and dowel are the cam chain, intake valves, and valve stem gaskets.

A new cam chain (92057-1170) was left installed since the original was close (~0.5 mm/20 links) to its service limit and replacing it later would be an unnecessary pain. The cam sprocket with dowel (12046-1063) was purchased used from a 2001 machine. I probably would not have found this issue if I had not used a depth gauge and angle of the crankshaft to determine that the intake valve was wide open during the compression stroke. The bolt holding the sprocket on to the cam valve (12044-1351) was still tight, but it had allowed the two to slip just enough under load to wreak havoc. As a word to the wise, do not try to replace the dowel in these sprockets. I have access to an 8-ton press at work, and all I succeeded at when trying to decouple the parts was bending the part doing the pressing. The pins are also hardened, so drilling them out is not an option.

Cylinder compression was last seen at 100 psi. I attribute this entirely to the new valves and proper timing. I have my reservations about how well the piston rings are seated since my installation methods are far from professional, but time will tell. The next things I deem likely to fail are the piston rings or the store brand spark plug. The intake valve springs are also very old and also within 0.5mm of their service limit, so that is another way that the bike may show its age next.

Time will also reveal tuning needs of the carburetor. I cleaned it and sealed it before the last installation since it had been out of the machine for at least three months. I'm not certain that the idle screw was left in the same spot as when it was running well before, because initial startup left it idling at 4000 RPM. It has since been reduced down to 2200 RPM, but I have not let it cool down much this afternoon. If it proves difficult to start and holding the throttle open a little bit helps, I'll move the idle up as needed. I'm reasonably confident that the carburetor is not leaking a vacuum anywhere yet as I had sprayed small spurts of starter fluid around its seams and no effect was heard from the engine.



Overall, this experience has been long. I'm happy to have a running machine now. It may not run super well or ever win any races, but it is running. For now.