CB175 K4 - hesitation and sputtering on acceleration

The bike starts and idles just fine, however as soon as my fat ass sits on it, slaps it into 1st, and hits the gas it starts bucking, hesitating, and surging. In the garage it will not rev smoothly >2000RPM, and won’t go over 5000RPM anymore.

This is a brand new rebuild from the ground up. New Pistons, rings, hone, valves & seats lapped. It ran fine for a week. Then one day on the way home it started coughing, sputtering, and surging.

I’ve checked the following three times now:

1. Points: checked and adjusted
2. Timing: Checked and adjusted
3. Tappets: Checked and adjusted
4. Spark advance: Checked, not binding, its operational
5. Carburettors: Cleaned, checked and adjusted
a. Went all the way up and all the way down on the needle one increment at a time
b. Tried all three main jet sizes (90, 100, 105’s) whilst keeping the needle in the centre position
6. Battery: its good.
7. Compression: will check again but measured 155-160psi across both cylinders after I first fired it up.
8. Running a tad rich, but no cause for concern. Will run the same thing on fresh plugs.
9. Checked for an air leak - sprayed start ya bastard around both carbs, no change in RPM

What’s next on my list……

1. Check the torque on the head bolts
3. Put a scope in the cylinders to check visually for damage
4. Try the old condenser again, acquire a new one if available. The accel 140403s coil I installed was throwing a pretty fat spark, Perhaps it cooked the condenser? I’ve noticed an orange/yellow spark on the points recently….I recall it being a nice fat blue before.
5. Perform compression tests again.
a. If poor, I guess I’ll check for leaks and adjust the tappets one more time. After that, I’m going to tear the engine down.
b. If the compression is decent…..well I don’t know.

Can anyone shed some light on this situation. I'm at a loss and really really don't want to have to tear the engine down. It dosent smoke or backfire. Just a whaaaa......whaaaa....um....whaaaa when I try and ride it.

what do the plugs look like when you try riding it?

have you ever ridden the bike with those !@#$%^ing pods on it with sucksess or is this the first kick at the can?

spark cam not bouncing and dwell fluctuating as the bike runs?

Have you verified the timing with a strobe light or just done the static timing?

The plugs indicate its burning a tad rich. But no cause for concern. The bike runs the same when i take the pods off and on fresh plugs. its not the pods, I rode this bike with no troubles for a week before it started acting up. I have only done a static timing.

The cam on the advancer is a bit worn. Is that what you mean by spark cam?

yes it is you need to find both a dwell meter and a strobe timing light and rule those thing out before moving forward

Tunnelvision111 said:

.................., I rode this bike with no troubles for a week before it started acting up. I have only done a static timing.

Click to expand...

What changed between those two dates? Was sit cold or high humidity? Did you adjust anything else? If you did nothing it is either a flat battery or it's stale gas or water in the gas.

It takes only a tiny amount of water to block a main jet.

Not sure....I'm at a loss. An old wise mechanic is 100% confidant its the spark advancer.

That's really not a difficult thing to test.

Sonreir said:

Have you verified the timing with a strobe light or just done the static timing?

Click to expand...

Just replace it then I guess. Worth a try for $40 over tearing the engine down.

I give up on this thread.

Sorry. I misread your comment. How do I check it? Its free and appears to work. Perhaps it seizes when the engine warms up? Do you have any idea how to test it, a procedure of sorts.

A timing light is the traditional method.

Static timing is usually good enough to get the bike to run, but to run well, you want the timing light.

http://www.harborfreight.com/timing-light-with-advance-40963.html

Your description does not sound at all like the advance mechanism. However, it is indeed very easy to check. Just do a quick visual to see nothing obvious has happened like a spring came off or some other physical issue. It should rotate against the springs smoothly with no binding or sticking. If it passes this test move on. Fat blue spark at the points sounds not good. Should not be such an obvious spark. Try a new condenser. The point gap is important! replace the points or dress the ones you have and have a care for putting them in properly. Many times a bit of tweaking (gently bending) is needed to get the point faces nice and centered and parallel to each other. The rubbing block needs to be nice and square against the cam. The cam needs to be flawlessly smooth. Get it all right and reset the dwell (point gap). Points have good performance and longevity but only if installed carefully. If the gap gets too small all sorts of crazy irregular running can be expected, so make sure the gap is correct and the old crusty cam is not prematurely filing down the rubbing block. Set the timing statically to start it, but get a strobe light to check it while it is running. A lot of engines are well served by checking the timing marks with a degree wheel when you assemble your engine, but they will be close enough as is, and since you had the thing running ok already you can trust them assuming the rotor is not loose etc. Set the timing to spec, and you should see the advance do its thing with the strobe proving it is indeed ok.

If that does not help, try another condenser. I have had bad new ones, and not that rarely. Next I would focus on the coils, and while usually they start to fail after they get hot and work ok again after cooling off, they can have issues of any description.

I doubt there is anything wrong inside the motor, but check the compression and check the valve train. Verify that the cam is still correctly timed to the crank and nothing has happened to the cam chain adjustment/tensioner. Then check the valve lash.

If you still do not have a good running motorcycle, take the carbs back apart and clean them. Maybe a pilot jet fell out or some other wacky thing happened, but my money is on the ignition.

In any event, stop thinking you need to take your motor apart. Odds are it is a very small problem. Be methodical and thorough as you work through your troubleshooting so you can eliminate each thing is the problem. And don't discount a part as a problem because it is new:

Some time ago, I built a new motor for my girls boat. A boat old enough to still use points in the ignition. Well after a couple of good shake down runs we thought to set off to a nearby island to spend the night. All was well for about an hours cruise and the engine dies. But it starts right back up and dies moments later. Just like it is flooding, and there is a lot of fuel in the intake (this is an inboard 4 stroke motor a lot like a car engine). Make a quick look at the ignition, and it looks fine. Not that it is suspect, it starts right up and makes great spark when cranking. So I figure the carb is flooding by the evidence. Just rebuilt the entire engine including very thorough carb and ignition attention, so I figured it was some crap in the needle and seat. So I take it apart and it looks fine. Fire it back up, and runs perfectly for a minute, then dies as before with a lot of excess fuel in the carb. I can see it with a flashlight (we set out long after dark). Take it back apart to find out why. Well, hours later, and running low on flashlights, I am totally baffled. It HAS to work, but it doesn't! Looked over the ignition again for no reason I am sure, but I am at a loss. We have been drifting at night now for a long time too! So there is of course only one thing to do. I get a beer and think about it. (not the most popular move ever with the girlfriend). I am baffled. Had the carb totally apart at least 4 times and I just KNOW it is perfect. Then for no reason I can explain, I take the cap off the distributor and get my seriously exasperated girl to crank the engine (again) (thank god boats have many batteries!). AND I SEE THE PROBLEM! The brand new points with no hours on them has disassembled itself. The post the arm pivots on is no longer pressed into the base, but the spring holds it close enough so that it sort of snaps back in place once the engine stops! No way to fix it without new points, but I remember that months ago when I took it apart, I threw all the old ignition parts in a baggy and stashed it in with a bunch of other useless boat stuff. Found it, sandpapered the crusty green points, screwed them in, eyeballed the gap (in the dark!), and damned if it didn't fire right up! Unfortunately that pretty much used up all my good luck, leaving me none left for the girl, so we headed back to the dock, but they both purred like kittens all the way there!

The moral is - don't blindly trust new parts, and don't fixate on a cause for a symptom. All that excess fuel wasn't from the carb flooding, it was from all that spinning with no spark! I had just rebuilt that carb, and after taking it apart the first time, just couldn't believe that was the problem. And it wasn't! But I spent hours fixated on the wrong thing when all I had was a 5 minute problem.

My money says that is what you have - a 5 minute problem, so don't chase those geese too far down any particular road!

Thanks Mobius. I will report back on the 5th of Feb.