New 72bkk, Issue with Flooding Even After Warm-Up

[meniscus]

So, the previous 72bkk we put on the Icebear Maddog suffered from blowby about 600 miles after install.  I'm guessing the cylinder wasn't really round and the rings were not able to seat properly.  Luckily, there was no major damage and the magnetic drain plugs caught all of the metal pieces from the rings.

Issue: A new 72bkk was installed and we now have 100 or so miles on the motor.  It seems like even after letting the scooter warm up for several minutes (3-4 minutes), the engine tries to flood after full throttle.  It doesn't happen all of the time and it doesn't matter if you are easy on the throttle even after warm-up.

Symptoms:  After 2 or so minutes worth of warm-up, you can hear the idle drop (I'm assuming the auto/electric choke is disengaging).  After running the bike for about a mile and a half, it sometime makes a wheezing sound for a second or two, the power drops off, and the engine stalls.  Immediately after, the starter can't turn over the engine unless you use the kick starter twice.  Then it will start back up.  The engine turns over like normal.

If the bike is allowed to warm-up for 5 or 6 minutes, it doesn't seem to happen as often and this doesn't happen every time.  After the bike is really warm, you can do whatever you want and there's absolutely no issues no matter what you do.  If you turn the bike off and run it again in 5-10 mins., we can't replicate the issue.

Setup: 72cc BBK, Extra long case, 20mm carb (but measures 19mm), 28 idle jet, 88 main jet, stock cam, 6 gram rollers, and yes, the carb is completely clean (all passages).  After running for 10 mins, top speed with a 255 lbs rider is 40mph (verified by GPS) on flat ground.  Valves are set at .004 for both sides.  Yes, the piston is installed correctly with the intake on the top side.  Battery is new and has good voltage.  Stator is new and has good output.  Pickup sensor was measured and has the minimum gap recommended.  Coil is good.  CDI is good.  Running an Iridium plug.

Any ideas on what may be going on or how we might remedy this situation?  I'm assuming this is a flooding situation, but I don't know.  When we do a plug chop, the plug looks right and the motor response is completely smooth all the time, regardless of throttle input.  Unfortunately, we haven't had the tools to do a plug chop when the engine dies/stalls.

[snaker]

Checking your valve gaps would be effort well spent.

Did you do a ring gap/grind on the rebuilds?

Put a timing light on the flywheel

Possible that the normal carb circuits are over rich and being covered up by the auto choke during warmup?


Inspect the intake manifold for cracks

[meniscus]

Yes, I have checked the valve gaps, as indicated in my post. They are at .004 for both intake and exhaust. I did check the rings and they were fine on this set. I didn't check the last set. I know for sure that the camshaft is in the correct position. I would imagine that it wouldn't run well if it was off by even a tooth, especially after it is warm. As it stands, the bike runs perfectly once it has 10-12 mins of operation.

I'm sorry, I don't understand your phrase "normal carb circuits are over rich and covered up by the auto choke." Please help me understand.

I'll check the manifold, but I haven't seen any issues like that and the manifold hasn't been over-tightened.

[GrumpyUnk]

Can you set a trap, and record the wheeze noise you described? I haven't given it a lot of thought, but what if the variator/clutch/belt were sticking, not releasing as you come to a stop? That would load the engine so it would likely not idle as well as desired. They can make noises if there is something loose, but it's not a wheeze. Off the wall thought...
tom

added...

 Have you checked the float bowl vent?  It is possible that the bowl is being pressurized if it is not open to atmosphere, and forcing fuel up the jets, and then ... something... and the vent works again.  Maybe take a good picture of the rubber lines running around the engine... couldn't hurt.  You figure the carb is flooding the engine during this one situation.  Only source for fuel is the float bowl, IF all the other vacuum lines are connected properly, AND your vacuum petcock diaphragm doesn't have 'issues'.
tom

[meniscus]

Thanks Grumpy. It would be very difficult to record the noise since it doesn't happen often and only within the first 4-5 mins of running.

The variator/clutch/belt are not sticking. You can hear it spinning freely when the engine dies.
The float boat vent is clear. I checked all of the passages and cleaned everything without any difference.
The vacuum line and everything is installed correctly.   As I've said, after the initial, extended warm-up, the scooter runs without any issues at all.

My description of the wheezing sound may not be completely accurate. This symptom happens while the bike isn't completely hot, but the valve cover is hot to the touch (can touch for 1-2 seconds max). It can happen during acceleration or constant throttle at half or full throttle, it doesn't seem to matter.

Considering that the bike doesn't want to even turn over after this happens (requiring a couple pulses with the kick starter before the electric starter can operate), I'm inclined to agree with you about the atmospheric vent on the carb. Since the tank on a Maddog sits low and uses a mechanical fuel pump, driven by the vacuum created by the engine, I'm wondering if something else may be the issue. Obviously, this tank setup does not include a petcock.

Now I'm wondering if the fuel tank is not vented properly with the gas cap. It's just a theory, but there's no way to test without running the scooter without the gas cap on. That doesn't seem practical.

Could a fuel tank vent be the culprit and only occur for a while after the scooter starts-up (during full warm-up), but not occur once the engine is completely hot (after 5-8 mins. or so)? I don't know. I would think that if it was the fuel tank vent at the gas cap, it would have problems even well after the scooter is run. The scooter can run for a half hour or more without any issues after.

My only other theory to try is to replace the auto choke/enricher, to see if that makes a difference at all.  The only thing I can think is that sometimes it sticks partially open and when we get the scooter restarted, it manages to fully close the second time.

Any more ideas?

[GrumpyUnk]

With the engine running, when the problem occurs, does the engine sort of keep turning but slowing down? It could do that if it is not getting fuel, more or less running out of gas effectively.
The starter being unable to turn the crankshaft is curious. When you use the kick start, does it turn over normally where you can feel the engine go through its rotation? You should be able to feel intake-compression-fire-exhaust cycles as you kick. I use my hands to feel the strokes and know it is actually doing its thing.
If you were to get too much fuel, hydrolocked, it could prevent the piston from cycling to the top of the compression stroke. Liquid fuel is not compressible, so would resist rotation a lot. Manually cranking could force the liquid out past the rings or valves if kicked slowly. You would likely be able to smell fuel in the oil if that happened.
The vacuum fuel pump I have is not very good. If I don't run the engine regularly, I have to crank the engine for over a minute(guess) before it starts to work. If the poppet valves are leaky or STICKY or get blocked partially open by debris, it could cause the pump to intermittently fail. Maybe take the pump apart and insure there is no crud internally that would make the valves leak...
tom

[meniscus]

Yes, there is a loss of power and starts to slow down, but the engine continues to run for a few seconds before it dies. It almost feels like a bog when you have too large of a jet or the needle is set too high. However, it doesn't bog or hesitate any other time, regardless of conditions or throttle input.

I haven't tried kicking slowly after it dies. I just kicked it normally, but didn't have the brake depressed any time I did it. There is some resistance, but not excessive. You can feel it turn over normally after the first kick. For interest sake, I did drain the oil and there was no smell of fuel. Also, there's no smell of fuel at the oil catch can when I check it.  Also, if you turn the engine over by hand with the flywheel, everything feels normal and smooth (the valves do not hit the piston either).

When I reassembled the engine with the new BBK, I checked the rockers and they were moving freely. I did use a fair amount of engine assembly lube there and coated the bearings for the camshaft well when I reassembled. I haven't noticed any debris at the valves.

I am familiar with the issues associated a mechanical, vacuum driven fuel pump. It's still original, but I do not believe it is suspect. This issue happens when it is colder outside (48 degrees) or when it is warm outside (83 degrees) and it happens, regardless of the humidity outside. Incidentally, it can happen on the same day, when there's 3-4 hours between when the bike was run last. The fuel lines are routed away from the engine and have a continuous grade from the fuel pump under the plate under the seat down to the carb.  I would think that if it was a fuel pump problem, it would happen at different times and in different weather, not just show the symptoms during this one time during warm-up.

The bike is no longer at my garage, so I am limited when I can test different things and operation. I did order a new auto choke, just to rule that out. I've yet to test the bike since my friend lives a half hour away. I believe the new auto choke comes-in on Saturday.

I'm running out of things to try or consider and I am hoping that the new auto choke will either resolve the issue or change the symptoms/behavior. I haven't noticed a knock or anything like that. When the engine stalls, the engine gets a little louder before it dies, but it doesn't thump like hydrolock when it cuts off. However, I am more familiar with this sound on older carburated cars or trucks with much larger displacement. I'll try to go out to youtube to see what hydrolock may sound like in a QMB139 engine. I don't know if I will find anything since most of the videos I see are either misdiagnosed or not described properly. If you know of any videos that have this or other sounds that are similar, please let me know. I will be happy to review them.

The only other thing I can think to share is that this only happens when the engine is under load and it isn't immediately when you drive away (even after a couple mins. of warm-up). As mentioned before, I've let the engine warm-up to the point when you can't keep two fingers on the valve cover for more than 1-2 seconds. Sorry I don't have an actual temp, my infrared thermometer quit a couple years ago and I haven't replaced it.

[GrumpyUnk]

I think you are running out of fuel in the float bowl. The pump depends on pulses from the intake manifold to work the diaphragm. It does not take much to make it flutter the diaphragm, and pump the fuel. If you are running at WOT, I think you may not be producing useful pulsations, so the pump becomes less effective.
If you note the problem after running hard for a few minutes that may be a clue. Or not. One machine I have uses a pulse pump and has the tank underfoot. That one does not like to start after it has sat for a few days, and needs cranking for a minute or so. Seems longer. I have no idea where the fuel in the bowl could go as there is no evidence of leakage and I don't see how it could siphon back to the tank.
You might try blipping the throttle when you start to feel it slow down. If it is lacking fuel, it will have no effect, except that the pump may get more actual pulses from the intake with the throttle closed. The accelerator pump should shoot a jot of fuel into the engine as the throttle plate opens from closed, and if you lack fuel, AND there is fuel in the bowl, it should possibly help.
No matter, it seems that something is marginal or it would happen more regularly. You could try a gravity fed tank to supply fuel and see if that eliminates the problem.
tom

[meniscus]

This is a very helpful explanation. I did try blipping the throttle before and it actually made the bike die faster.

I wish I had a see-through float bowl, like the ones used on the Lectron carb. For that matter, it would be great if there was a Lectron carb that was suitable size for the QMB139 engine.

As a side note, the symptoms on this bike happen even at 1/3rd throttle. Once it runs again, you can go right back out and have it happen again if you don't give it a couple mins. idling in place. Also, it idles fine in place, even if you raise the rear tire and apply throttle. In other words, this only seems to happen under load.

You bring up some interesting points. I had replaced all of the fuel lines and vacuum line previously and I inspected them all when I was doing the rebuild. I just can't seem to place why it would only happen at this time and not any others (during operation). Keeping the bike in the garage, it doesn't matter if the bike and fuel is warmer than outside or if the fuel is colder than the outside temp when you take it out.

[meniscus]

Well, I stand corrected. There are transparent float bowls available.  I'm not sure it is a direct fit for the CVK carb for QMB139.

[meniscus]

OK, let me ask some other 'uninformed' questions (forgive my inexperience with these small carbs, I only know the big ones from cars and trucks back in the day):

• We know that the symptom occurs before the engine is fully hot.  If we assume that the float bowl is running out of fuel, then could it be that the mechanical fuel pump gets warm after it runs for a while, assuming that the heat comes from the vacuum line?
  
• If it is a lack of fuel, then why wouldn't gravity help draw fuel down the fuel line into the float bowl?  I'm assuming that your first suggestion to check the vent for the carb is the reason why it wouldn't?

Based on your comments, I believe it would be prudent to check the vent on the carburetor again.  Maybe it is clear, but still has some gunk that prevents it from venting properly until the carb or the fuel warms up going to the carb.  Thoughts?

Could this symptom be in any way related to the diaphragm for the needle?  Everything there is good and is aligned with no damage.  I figured I'd ask this question as it is also an easy replacement.

[GrumpyUnk]

If you have some hemostats and a long-bladed screwdriver, you could try clamping off the fuel line when the problem occurs, (preventing the float bowl from refilling) and then use the long screwdriver to open the float bowl drain. If no or little fuel drains, you know you are lacking fuel. OTOH, if you get a normal float-bowl full of fuel to drain, you know you do have fuel supply.
Something is functional, and then not, and once not, it will rehabilitate and start working again after a rest. If you had a gas cap that had a sticky vent, you would have similar symptoms. If there was a vapor lock, ditto. I have had intermittent vapor lock using current fuels, more often hotter weather. If your symptom is happening regardless of hot or cool temperatures, then I don't have an answer at hand.
tom

[meniscus]

Thanks for the input and suggestions. I'll probably be able to put my hands on the bike this weekend and see what may be going on.

[meniscus]

Just a quick update. We swapped the auto-choke and we cleaned the pilot jet. Bike was very difficult to start and once we got it started, it died within about 30-seconds. After further investigation, we found that there was a very, very weak spark at the spark plug. We believe this is unrelated to the overall issue and both the coil and the pickup from the flywheel were checked previously with no issues.

New spark plugs are on order and the bike will be tested again in the coming days.

[GrumpyUnk]

I may be repeating myself, but. Did you check the 'jet' built into the float bowl? The tube that sticks down from the upper half of the carburetor is the pickup for the 'byvalve', the enrichment device that takes the place of the choke. The tube sticks into a 'well' built into the float bowl. The well is filled with fuel through the tiny jet built into the side of the well. If clogged, it will not allow fuel to flow when the enrichment is desired, and it will be hard to start, and likely want to stall right after start-up.
I used a straight pin to poke through to be assured that the jet was at least open. You can use choke cleaner with a 'strat' to push fluid through. If it flows and starts to gurgle up the well, then it's open, and needs nothing else.
A help to a cold-hearted carb is to twist the throttle grip 3-5 times. Each cycle will operate the accelerator pump(if equipped and it's working) and squirt a bit of fuel into the carb inlet to give a richer mixture upon startup. If your byvalve is not working, doing that successfully(improved start) will give a clue that the engine needs more fuel which the byvalve may not be providing.
I think it is called the byvalve because it bypasses the throttle control. It is a tiny carburetor all in its own, with air inlet, jet(in the bowl), and throttle controlled by the heat-up thingy that pushes down when expanded to cut off flow through it.
tom