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| 150cc GY6 and Under Engine Tech GY6 and Smaller Technical Discussion Forum |
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#1
07-07-2012, 09:34 AM
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Strange one. Carb pressurized????
OK guys, this one has me puzzled.
Yerfdog with the Howhit 150 and Mikuni VM 26-606 carb. After you make a WOT run and let off the gas (fully closed) the RPMs start to decline as normal. But as it starts to come around 3000 RPM, bubbles form inside the fuel filter, and it actually forces the fuel back inside the fuel tank. And there it stays until you kill the engine or fiddle around with it until the pressure releases. My overflow tube is not clogged. I checked that, and occaisionally because of this problem, I have fuel forced out of that hose. The fuel tank vent is not clogged. I can actually remove the fuel tank cap, and it does not change anything. I know about reversion on motorcycles, but I have not heard it mentioned on these before. But even still, with the overflow being clear (and venting the carb bowl), how does it build up pressure inside the carb and force the fuel back into the tank???? Like I said, weird....... And I can get a video of it once I charge the batteries. 
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#2
07-07-2012, 09:47 AM
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Quote:
just grabbing at straws for any info that might suggest an intelligent answer. 
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kliff
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#4
07-08-2012, 04:59 AM
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Quote:
It's a possibility that it is reversion related, if it has a wild cam, and I really don't know which of the "A" series cams would really be called wild, or if the cam timing were on or 2 teeth off. I've just never experienced this problem, and since the engine has a non stock carb, I'm curious about what else is non stock. There has got to be a clue in there some where... You must be doing this test, with the engine unloaded, wheels off the ground, chain not installed...something, to see the filter. I'm curious how it performs under a load, on it's wheels. A video would be helpful, and anymore info you can provide.
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kliff
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#5
07-09-2012, 05:12 PM
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sorry for the delay guys. It was near 100* over the week end so i was out on the lake.
OK, here's my set up. UNI filter 6" PVC tube Miknui VM 26-606 carb 30mm aluminum intake manifold Stock Howhit head with home port work port matched head to intake manifold A11 cam NCY 58.5mm nicasil cyl and piston NCY CDI Bando coil Stock head pipe with a FMF slip on muffler. This set up ran great with the CV carb on it until I had that chain tensioner/ motor mount break. We installed a temporary mount, I made a WOT test run, which it did great. But at the end of the run i did a 180* on the short grass and it stumbled and died. After that, it never ran right with the CV again. I bought another new CV to replace it, didn't have any different results with it. Checked valve adjustment, but nothing changed. So I decided to install the VM carb and have good results except for the closed throttle after WOT pass problem explained above. Kliff, I saw the bubbles in the filter when the tires were off the ground. But I can see the filter from over my shoulder when running the cart. I'll get that video up soon.
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#6
07-10-2012, 05:20 AM
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Checki your valve clearances, and open them up justa a tad if they are under .006" each. The valves may be a little noisey at .oo6", but it usually helps keep reversion, caused by cam duration to a minimum. The cam induced reversion is usually minimized, with a cv carb, due to the vacuum operated slide.
If you can find a 28mm CV carb, you might want to try it again. CV's usually tune real well with a combo like you have. I ca't imagine why a broken chain agjusted would have affected carb tuning....had to be coincidence. As for the other carb...(CV) I'm at a loss. Unless you developed an air leak some where in the intake tract, by some sort of impact, when the chain adjuster broke. Just a few things to look at, hope some of it helps.
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kliff
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#8
07-10-2012, 10:02 AM
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Quote:
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kliff
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#11
07-10-2012, 11:01 PM
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Kliff thanks, valves are set at 5 thou
That CV did really well up until the 180* )spin out) I did. That's why I bought the new one hoping something in the old one failed. No such luck. So I installed the VM and it idles much better (smoother) with it, than the CV's do now. Coincidence for sure. Buddy of mine thinks it may be a vapor lock issue going on. I'll see if I can insulate the carb from the engine heat better and see what happens. Thanks guys
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#12
07-10-2012, 11:13 PM
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What was the temp outside when this happened. Vapor lock is one thing but the pressurising of the fuel is not a cause of vaporlock, and thats the part that bogs my mind. Thinking about it, the fuel bowl would have to preasurize then push the fuel up through the needle/seat back through the line and into the tank.
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#13
07-11-2012, 05:39 AM
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I got this PM from pud, on BN last night. This makes a lot of sense to me as well. Might be something you wanna investigate:
"kliff i read the problem a couple days ago of what xlint89 is going through with carb,i figured some one would have mentioned this but hasn't i have been goning through this with lawnower and weedeater its the dam ethanol in the gas trying to break down it will bubble pressure up and look water you have boiling on the stove i run stablizer in my gas all the time any more seem to help with the heat it makes it worse with the bubbling and pressuring up. my vent caps work but couple weeks ago ran weeder about 10 minutes at most stop to put new string on it figure i would go head and just fill it back up, engine was hot and than setting in sun opened cap blew gas all over me and it looked liike water boiling on stove what was left in tank. while pouring fresh gas in out of can in shade i notice it quit bubbling have had the same thing happen with mower running pressure force the float open brand new mower vent cap working gas was actually shooting out the carb 3 feet tank was squealing like a pig there was a kid burnt here last year with a mower no one will own up to it but i know what happen my new mower about caught fire,close any way i have been watching the tank you will see bubbles starting to come i never registered over at bm but i have a feeling thats whats going on .i wonder how close the carb is setting to engine just my thoughts pud" Help coming at ya from all over xlint...
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kliff
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#14
07-11-2012, 10:12 AM
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There could be something to that effect but I have never experianced it with ethanol blended fuel. It seems to me his thought is the ethanol is boiling off. If so the answer is no. Ethanol has a lower flash point then gas which is why the ethanol is used to achiev the rated octain. The fuels octain controls the burn of the fuel. Xlint89 may have a couple different problems one being vapor lock.
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#15
07-11-2012, 08:34 PM
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We use to put cloths pins on the fuel lines to stop.
Don't know if this worked or not but almost all the old one barrel carbs had them.Cast iron intakes and heat in motor compartment.Just a thing I thought of,took me awhile as old as I am.LOL
__________________
 [SIGPIC] Take What You Have and Build What You Need
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#16
07-12-2012, 07:03 AM
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I've had this happen before it was a plumbing problem with the vacuum lines on the carb. tube that runs from the valve cover was actually put on the carb vent thus pressurizing the fuel bowl. What vaccuum lines/ports do you have on the carb and what do you have them running to? That kind of pressure is not likely from vapor lock.
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#18
07-12-2012, 11:49 PM
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Wow, all kinds of great help on this thanks guys.
I do not have any vacuum hoses hooked up. The fitting on the intake manifold is capped, and the head beather tube is just hangin in front of the engine. (not hooked up to anything) Temps were near 90* when I first saw the issue. This is something that I just remembered. My fuel when sitting inside the carb becomes a very orange color from sitting. Although the fuel in the tank is bluish. (another odd one) Tom I too thought about the air and fuel pushing past the needle and seat which is really bazaar. I have to quit screwing around and get a video posted up for you guys. Thanks fopr the tips guys, I'll be looking into these when I get back to business on this thing. (hot days require use of PWC as riding season is short in Cleveland)
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#19
07-13-2012, 04:11 AM
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The orange fuel in the carb probably smells rank since it indicated bad fuel. Especially if using ethanol blended fuel. The bubbles could have been water and not air in the fuel. Due to ethanol being water based which with the corn ferments into alchahol. When the ethanol breaks down you get water. Change your fuel out with fresh fuel. Remember ethanol blended fuel starts to break down after 2-3 weeks unless completely protected from ambien and has not been sitting stagnent. WWW.pure-gas.org will show stations that carry pure fuel with no ethanol. The temp is not the problem since I have driven mine in 100+ temps the past couple weeks. The only thing I noticed is that when tank was full fuel seeped from vent tube due to the higher pressures in tank. Ethanol expands greater then gas so if using ethanol based fuel expantion pressure will be greater.
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#20
07-20-2012, 11:04 PM
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Thanks Tom, but I've been running ethanol fuel through this thing ever since i've owned it. Hasn't been an issue until now.
Sorry for the delay guys, here's a couple short vids to show you I'm for real. Here I am running WOT. (Sounds GOOD so turn your speakers up....) http://www.youtube.com/watch?v=pv9CasZrhKY&feature=plcp Here's the empty fuel filter after a WOT run. If you fiddle with the filter it will fill back up with the engine running. http://www.youtube.com/watch?v=y_J6tf29PqU&feature=plcp And here's the empty filter after another WOT run, but I shut the engine off. http://www.youtube.com/watch?v=wito12zwpO0&feature=plcp I'll try and get a video of the bubbles prior to the fuel leaving the filter. It was dark out when I shot these. Last edited by xlint89; 07-20-2012 at 11:06 PM.
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#21
07-21-2012, 06:24 AM
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You got a air lock! Fuel is leaving the filter faster than it can be delivered creating this air lock. Pushing down on the filter puts enough pressure in the line to release the lock. Use larger, longer and clear fuel line. larger line gives volume and serves as a reservoir. Longer line out of the tank keeps the filter from bouncing around. Secure the filter so it can't bounce around, that causes air bubbles in the fuel. Clear line allows you to observe the flow,
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#22
07-21-2012, 08:02 AM
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Thanks Ckau, but I don't think this is the case.
It's not being used up and can't supply more fuel, it's actually being forced out of the fuel filter. When i get a video of the phenomenon, you'll see what I mean. The fuel lines I'm using are 5/16" (already too big to properly fit over the carb fitting) I have to really tighten down the clamp to prevent a leak. My hoses were longer, I shortened them because I first thought I was making the fuel go up-hill. I cut them down so that the filter is angled toward the carb. Obviously didn't work.
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#23
07-21-2012, 08:10 AM
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In an effort to correct my problem, I drilled the carb overflow/vent line fitting and passages larger. Hoping to bleed off the pressure inside the carb bowl. It may or may not have worked to some degree????
I say that because now I haven't seen the fuel pulse out the overflow hose like I've seen before. This is really baffling though. Obviuosly the bowl is not full allowing the float and needle to drop. Then this unknown pressure (which shouldn't be here because it's vented) is escaping through the needle and forcing the fuel out of the filter. Kart runs quite well as you can see in the video.
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#24
07-21-2012, 10:07 AM
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I have put every scenario together as to what could be causing your problem. If you have a vacume gauge connect it to the intake. See what kind of vacume you have at idle which will be hard to read due to only a single cylinder it will be all over the place. Then acellorate the engine and see what happens to the vacume. As you know it should go to zero when punched. Let us know what you find.
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#25
08-10-2012, 04:22 PM
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OK, got back to this one yesterday.
Sorry Tom, forgot the vacuum gauge. What I did do was swap back to the stock cam from the A-11, thinking something may have changed in the valve timing after my "donut". (last time it ran correctly) I also found a huge exhaust leak at the slip on muffler and header pipe connection. Sealed up the leak, swapped cam and didn't change anything else. It sucked the filter dry after the first lap, wasn't really pushing it hard either. (warm up) Just like in the video, if you pull the filter down in such a way to allow the trapped air to exit into the fuel tank, it will fill up the fuel filter. So i did that, and it never really emptied it again. Maybe went as low as 3/4 of the way full. Made about 6 more laps, but it never went dry again. This is weird.......
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#26
08-14-2012, 10:00 PM
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OK, so I went out again today.
Tom, just like you thought, the vacuum gauge needle bounced all over the place. I saw the needle pause at 5, 8, and 20 inches momentarily as it idled. Mostly near 20 inches when you revved the engine. Tried out a new (correct) needle for the VM 26-606 carb. The one that came with it was just one the guy had laying around. Took off slightly better, but was missing power at WOT. Installed the "old" needle and gained the top end back. Fuel filter never got 1 air bubble in it today. Probably did about 20 laps with no issue. Only differences today: VERY full gas tank Low 70's temperature outside Still using stock camshaft. I'm going to have to try putting the A-11 cam back in and see if the problem comes back. (remember, it did do it once with the stock cam installed)
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#28
08-15-2012, 05:47 PM
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I didn't notice it doing that. I can retest if need be.
It's supposed to be hot here again tomorrow. I'll see if I can get up to the Kart and take her for a spin. If it does act up, then we'll know for sure it's a weather/heat related issue.
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#29
08-16-2012, 07:28 AM
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1.) Is that filter on backwards? Just curious.
2.) When you push that filter down and it begins to fill up, is it by chance pinching off the hose that is on the engine side of the filter (thus cutting of this pressure that the engine may be sending back through the carb and up the needle/seat?
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#30
08-16-2012, 09:35 PM
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1. Arrow on filter pointing towards carb. I would assume that's fuel flow direction.
2. I don't believe the hose is kinked/pinched off when pushing the filter down. It's not that hard you have to push the filter for it to fill back up. It's almost like a trapped air pocket. Once it's released the fuel flows fine. I didn't make it out to the buggy tonight. Truck needed a new battery.
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#31
09-04-2012, 05:38 PM
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The kart has been running pretty good with the stock cam installed. (no lack of fuel issues any more). So I decided to re-install my A11 cam and see if the problem comes back.
The kart seemed less responsive and VERY doggy. It pulled like a slug, but would start to come alive at the end of my test track. I did notice one time that a decent sized air bubble did form inside the filter, but didn't run it dry like it had before. The lack of performance alone was enough for me to re-install the stock cam. Once it was back in, the kart felt pretty good again. Did a max speed run with the stock cam and came up with 34.5 MPH. Pretty close to what I had recorded before. But well below the max speed I recorded with the A 11 previously (38.9MPH) I'm kinda thinking something is wrong with the A11 cam? Or maybe I stretched the timing chain and it doesn't seem to affect the stocker as much? The performance issue all started after I did a 180* in the short grass and it stalled out right after. The flywheel seems to match up with TDC of the piston, so i don't think it's a sheared woodruff key problem. Anyone know a test for checking the timing chain for wear/stretch?
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#33
09-04-2012, 07:39 PM
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Don't forget you got a timing chain tensioner. unless your chain has stretched to the point the tensioner is fully extended then the type of cam makes no difference.
The biggest beef I got about gy6 cams is there is no specific specification for the cams , only a number, no duration, lift or overlap specifications so you really don't know what or how a particular cam affects the motor. Only way to know is to put your particular cam under the dial indicator. Even manufacturers have their different idea of what makes a torque or hp cam. The same number cam from separate sources can be a different grind! The machining tolerances are so sloppy two cams with the same number from the same source can read different It's possible, if your A11 has a high lift that you may have busted a valve spring when you stalled. A broken inner spring would not be noticeable unless you were looking for it. The valve would still function but would react sluggish resulting in your lack of performance.
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#34
09-04-2012, 08:18 PM
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Your correct ckau. I have found in an A-12 cam one from Stu and other from MRP. one had a 3 deg overlap and the other a negative 7deg. overlap. The thing with cams is the more that are ground on production the centerline gets off track then when the sprockets are pressed on one may have a 5 deg. or more timing retard. But this is the same to a point with any major cam company even comp cams or like. However they will have a better quality control to where after say 10 cams the stock to cnc is recalibrated. Custom cams are more precise since they will be ground not in production mode. The Taiwonese cams are better then the Chinese in this respect but as a whole are not persay like a Hot Cams for their cams spec out very close to one another. I personally degree all my cams so that I get the most performance but with these cams since the sprockets are pressed on it makes it very challenging and time consuming for the sprocket will retard or advance 5 degrees with as little as 1/64th off. The A-11 is mainly a higher lift cam with not much more duration then a stock cam the A-8 will have about a 7 deg. overlap. So none of the A series cams are anywhere radicle they are all sport grinds basically or mild grinds.
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#35
09-05-2012, 03:56 PM
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Oops, well I did forget about the chain tensioner.
It doesn't make sense why the A11 didn't improve my perfromance as it once did. Granted, I do have the Mikuni VM 26 -606 installed now VS. the stock CV carb, but I don't see why that wouold make that much of an issue. It didn't before. I understand rejetting for the cam and all, but I didn't really have to change anything the first time to feel the benefits gained from the A 11. There's something staring me right in the face, and I can't see it. Ckau, thanks for the heads up on the valve spring, but those are fine. I actually swapped back to the (home ported) OEM head after I did something dumb with my NCY head.
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