Gz250 starts but dies on throttle - Wrist Twisters
 
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post #1 of 13 Old 06-09-2015, 06:45 PM Thread Starter
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Gz250 starts but dies on throttle

I'm working on getting my friend's bike tuned up, so I pulled and cleaned the carburetor. Pretty standard cleaning; pulled and soaked jets, replaced seals on main jet and carb bowl, replaced float valve, and blasted all passages with carb cleaner.
Put it all back together and the bike fires to life and idles without issue, but applying ~half throttle causes it to die. I suspected needle valve issue, so I tore it apart again but couldn't see any issue. Just to be through I had my buddy do the same, and he gave it his seal of approval, but the problem remains.

Oddly, it looks like others have had this identical problem before: http://www.justanswer.com/motorcycle...ine-won-t.html

Anybody have any ideas?

Added relevant info: the bike ran pretty well before the teardown.
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post #2 of 13 Old 06-10-2015, 07:36 AM
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Checked for leaks? How old are the o-rings? Intake boot in good shape? Is the needle shimmed where it was before?

if you love your motorcycle, set it free.. if it comes back and hits you.. you highsided
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post #3 of 13 Old 06-10-2015, 10:48 AM Thread Starter
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Quote:
Originally Posted by beefsalad View Post
Checked for leaks? How old are the o-rings? Intake boot in good shape? Is the needle shimmed where it was before?
The bike is an 06, so the rubber all seems pretty good. It was off and back on three times, but i suppose it's possible there could be a leak at the boots - I'll have to check tonight. I replaced the rings, and I didn't at all disassemble the needle/diaphragm assembly so the needle should be exactly as it was prior to cleaning.

Just like in the posted link I also redid the petcock. There shouldn't be anything wrong with it (flow checked out at least with prime option) but I can't shake the nagging feeling that it's somehow related. I think I'll test with an external reservoir just to be sure.
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post #4 of 13 Old 06-10-2015, 11:16 AM
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This is a fairly common problem when rebuilding / cleaning CV carbs.

First, all those tiny little drillings in the carb body, such as the fuel feeds just below the throttle plate, must be cleared by using high pressure air blown against the normal direction of flow: in other words starting in the venturi area down into the float bowl area. This makes sure no contamination is lodged in the smallest part of the drillings.

Second, make sure the slide runs free. In this case it rides in slots in the carb body which are quite accurate (read tight), and even the smallest amount of foreign substance can prevent the slide from rising, starving out the fuel. Even if the slide moves fairly easily when pushed up by hand, this is applying considerably more force than is usual with pressure differentials. Clean these with carb cleaner soaked lint free cloth for the slide bore and white pipe cleaners (to more clearly see if anything is being removed) twisted together to make firm contact with all areas of the slots. DO NOT USE ANY KIND OF ABRASIVE ON THE BORE, SLOTS, OR "WINGS" ON THE SLIDE! This can cause more friction, making the problem worse.

Third, there is an o-ring between the main jet holder and the needle jet -- make sure it is properly installed / there at all. If it is missing or damaged it can cause a leanout.

While I have not specifically worked on this specific carb, I have worked on hundreds of others and I have found that they are pretty much prone to similar maladies.

Hopefully this will help.

Rob

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post #5 of 13 Old 06-10-2015, 12:13 PM Thread Starter
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Thanks Rob for the advice. I did replace the o ring on the holder, so I don't think that's an issue. I checked the slide and there did seem to be a small amount of rubbing, so I'll give that a good cleaning. And unfortunately I don't have access to a compressor right now, so I'm going to have to make due with carb spray and a canned air duster.
Is there any kind of gas resistant lube that might help the slide?
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post #6 of 13 Old 06-10-2015, 07:28 PM
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Absolutely avoid any sort of lubrication on the slide! Even something as light as WD40 can seize a slide practically sold for the simple reason that it can trap particles and hold them until they imbed in either the slide or carb body, trashing the piece. Absolutely clean is your best, no, your only bet. Of course dirt can enter, but as long as it's relatively mobile it is not a problem.

Good luck.

Rob

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On the other hand, if it has not been done never assume it is impossible to do it.
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post #7 of 13 Old 06-11-2015, 09:42 AM
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For the most part, I agree with Rob. While you don't have a compressor, it's worth finding someone who does and using it for 5-10 minutes. Barring that, strip down the carb completely, removing all rubber pieces, get a pot of 50/50 water and lemon juice boiling and stick the stripped carb in there. Boil it for about 10 minutes or so and then take it out to cool down. Once cool enough to handle with bare hands. Use compressed air to dry everything out.

If you still have problems with the slide and there are no scratches, burrs. Etc, you can try a dab of ATF, a trick an old carb rebuilder taught me and works very well.

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post #8 of 13 Old 06-12-2015, 09:38 AM Thread Starter
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So it is with mild embarrassment that I admit I found my problem. During disassembly the needle jet popped out without me seeing it's orientation. I tried putting it back in both ways, and it only appeared to seat one way. Since I was willing to try anything at this point (and I had realized that that was the only thing that may have changed) I decided to try it flipped. Lo and behold, it seated perfectly snuggly the other way around. Pop it all back together and she revs quite happily. I'll go ahead and chock this up to a learning experience. :rollseyes:

But now I think I have a bigger issue; taking a look at the oil showed it to be higher than where I filled it to just a thousand miles ago, and a quick sniff reveals the unpleasant aroma of gas...
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post #9 of 13 Old 06-12-2015, 01:04 PM
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If the needle seat isn't replaceable, I've had luck giving it a light polish with a qtip chocked in a drill. Learn from my mistake...bench test the !$%^er before cramming it back into place and hoping it works.

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post #10 of 13 Old 06-12-2015, 01:14 PM
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Quote:
Originally Posted by EscapedLabRat View Post

But now I think I have a bigger issue; taking a look at the oil showed it to be higher than where I filled it to just a thousand miles ago, and a quick sniff reveals the unpleasant aroma of gas...
Is it possible that during the idle issue you were dumping a lot of fuel and it got into the oil? You may want to change the oil and run it for a while and see if the oil level increases again.

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post #11 of 13 Old 06-12-2015, 02:01 PM Thread Starter
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Thanks beef, but she's running pretty happy now.

Quote:
Originally Posted by marylandmike View Post

Is it possible that during the idle issue you were dumping a lot of fuel and it got into the oil? You may want to change the oil and run it for a while and see if the oil level increases again.
So this is actually very likely since it sounded like it was drowning instead of leaning out (immediate rpm drop and not spike), but I'm concerned since when I checked it out for her before purchase I noticed that the oil was a tad high and there may have been a slight whiff of gas. I wrote it off as careless fillup and an overactive imagination, but now I'm a bit concerned. My immediate plan is just to change the oil and keep a close eye on it, so hopefully your theory is correct and it'll be fine.
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post #12 of 13 Old 06-12-2015, 03:02 PM
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Don't worry about it just yet. Do an oil change as suggested with cheap oil. Run it a bit too wash out the gas and then change it again with your preferred oil and filter. If you start to hear/feel the main bearings knock then it's time to get worried, but it would take a lot of fuel over an extended time to make them go bad.

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post #13 of 13 Old 06-18-2015, 07:56 PM Thread Starter
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So I'm 90% sure I know where the gas came from now. If the rings were shot bad enough to flood gas in just a couple thousand miles, it would also be belching blue. What I noticed during the rebuild was that the airbox had almost a full cup of gas in it, and I had to fix a leaky petcock. I finally put two and two together; the petcock was most likely still allowing fuel to flow after being shut off, so gas flooded out of the carb and into the airbox and engine where it slipped past the rings.
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