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-   -   Loud bang and high RPM on start (https://www.vn750.com/forum/22-carbs-fuel-system/92689-loud-bang-high-rpm-start.html)

spbruski 04-06-2017 11:58 AM

Loud bang and high RPM on start
 
1 Attachment(s)
Hi !

So a little back story, I've had some electrical issues before which were fixed and I got about 70 miles of riding. Then one night my bike wouldn't catch and I ended up killing the battery after 15 attempts to start while rolling the throttle. I left for vacation for 2 weeks with the battery on a tender. I go and start the bike and I get it to start with the RPMs maxing out and a loud bang from right under my seat. This bang worried some of the neighbors and I didn't try to start it again. My best guess is that I've flooded either the carbs or the crank case. I've done some of the preliminary checks and everything checks out normal other than a weird mixture in my oil tank. So I'm guessing the fuel system is leaking into other things.

What are some checks that can be done to eliminate the carbs, petcock and how do I remove the extra gas from the engine block.

Thanks in advance !

michiganteddybear 04-06-2017 12:50 PM

well, 1st, thats not an oil tank. thats the coolant overflow (or reserve) tank. only a water/antifreeze mix should be there.

spbruski 04-06-2017 01:00 PM

Quote:

Originally Posted by michiganteddybear (Post 1170337)
well, 1st, thats not an oil tank. thats the coolant overflow (or reserve) tank. only a water/antifreeze mix should be there.

Sorry :/ Still new to all the parts and such.

How would I check to see if there is gas in the oil? Or how to would check if there is gas in the crank case?

Spockster 04-06-2017 02:11 PM

The first sign is the sight glass will show way over full if there's a fair amount of gas in the oil, assuming the oil level was correct before the problem began.

Second, the oil will smell more like gas than motor oil. On a hot engine, you may see white vapors coming off the dipstick as it is pulled out or just as the cap is removed.

When mine was over full and gassed, the shifter got a bit sticky after the engine/trans was fully hot. Would probably also do that if it was over full with oil only.

Can also put some in a glass container and let it settle out, gas should rise to the top.

Spockster 04-06-2017 02:16 PM

Check the vacuum hose from the petcock to the left carb, it should be dry, no gas in it. Check it cold and after running.

My bike almost always backfires if I open the throttle during startup.

Most VN750s will backfire on starts with a weak battery/low voltage.

Thorn 04-06-2017 04:27 PM

You may have found it already, but the oil sight glass is on the left side of the engine, near the bottom. It's a little glass circle.

Rolling the throttle usually messes up my start ... I have the best results choking and starting, then blipping the throttle once it's running to get it to smooth out. Rolling the throttle before it has started can lead to flooding.

A backfire might be unnerving, but not damaging. If working with a flooded engine it might happen as the fuel/air mix gets back closer to where it needs to be, so it can actually be a good sign.

You can check the petcock by disconnecting the fuel lines to the carbs (the side hoses on the petcock) while the lever is set to 'on'. No gas should leak out after the initial disconnect if the diaphram is good.

Gas in the oil after the bike sits for a while indicates bad seal at the float needle. You'd need to start that check with good oil though, if you've already got contaminated oil.

If your oil has gas in it you need to change your oil several times, running the engine between changes to clean it out.

Turning your petcock to 'off' and cranking the starter can help get rid of excess gas in the cylinder as well. Just don't burn out your starter by not giving it enough of a break between attempts. Then turn your gas back on and try again (allow some extra patience for gas to get back down to the carbs)

Pulling a spark plug out of each cylinder and running the starter can help push gas out of the cylinders of a flooded engine if it's really bad. I've only done this on smaller engines, not sure if it's a standard practice on motorcycles or cars.

spbruski 04-06-2017 08:59 PM

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So I propped my bike up on the center stand and let it sit. While that was just sitting there I went and did some visual checks, petcock was fine with no drip and totally dry. Then I massaged the boots to maybe free up some floats just in case any one of them were stuck. Also I checked the oil glass and I found something peculiar. Either my engine is defying gravity or I have some ****ty oil buildup. I only rode ~300 miles from the last filter and oil change. Then after 5/10 mins I closed the petcock and started the bike, I held the starter button for 3 seconds in 15 second intervals until the battery ran out of juice. It seemed to crank fine and freely. I dropped the battery on a tender and will retest again tomorrow after work.

Any other suggestions and checks while the time passes?

Thank you !

Thorn 04-07-2017 12:19 AM

The oil sight has a glass face, then behind it is a piece of metal to provide a backdrop for looking at the oil. Thats probably the silvery half-circle on the left in your picture. Mine has a complimentary one on the right side as well, but yours doesn't seem to? Regardless, you should see oil half-way up that metal piece between the high and low marks. It looks to me like you have extremely low oil. The black on the right side is just empty space reflecting no light.


"after 5/10 minutes I closed the petcock" ... do you mean opened? should be set to on/run... Does fuel come out of your petcock when a vacuum is applied to the center hose? Are you out of fuel? XD

Goofyfoot2001 04-07-2017 08:21 AM

If you still have the accursed airbox on, the function of that is to recycle unburnt gas from the cylinder into the hot pipes (California's idea), get rid of it (unless you aren't in a free state). Then adjusting your air mix per the verses. But like has been said, bikes will backfire with the throttle open on start. Sometimes when people are being dorks at the gas station I will backfire my sporty just cause. Scares the fuk out of everyone and guns come out and good times are had by all!

spbruski 04-07-2017 10:09 AM

Quote:

Originally Posted by Thorn (Post 1170529)
The oil sight has a glass face, then behind it is a piece of metal to provide a backdrop for looking at the oil. Thats probably the silvery half-circle on the left in your picture. Mine has a complimentary one on the right side as well, but yours doesn't seem to? Regardless, you should see oil half-way up that metal piece between the high and low marks. It looks to me like you have extremely low oil. The black on the right side is just empty space reflecting no light.


"after 5/10 minutes I closed the petcock" ... do you mean opened? should be set to on/run... Does fuel come out of your petcock when a vacuum is applied to the center hose? Are you out of fuel? XD


I put it on off so that no gas would be pumped to the carbs. I ran the test like that to run the flooded crank case and let the gas cycle through the system.

I've already ordered oil and a filter for the weekend. I'll send an update once I've done that. I have a full tank of gas :)


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