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Author Topic: GV650 backfires at idle and sluggish acceleration  (Read 370 times)

Byrddawg

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GV650 backfires at idle and sluggish acceleration
« on: June 11, 2018, 11:11:37 PM »

Hello,
I know this is a long post, but if you feel you may be able to help please keep reading.  I bought this bike brand new over ten years ago, and even though I have put more than its worth back into it, I  have only had a few trouble free rides on it.  I really feel it is due to unethical and knowledgeable dealers and not the quality of the bike itself.  For reasons below I cannot afford or trust dealers any more, so if I can't figure this out with the help of you guys, I won't be able to sell it or enjoy it.
So thank you for your time in advance.

CURRENT ISSUES THE BIKE IS HAVING
The bike starts easy with no choke from cold (well 95F summer day cold) and idles strong, but has a very consistent, frequent, and noticeable "popping" backfire sound at idle and while giving it throttle in low rpm.  It also has sporadic sluggish acceleration.
 
Research suggests improper fuel:air mixture.  Before we go straight to the carbs, I have a couple of concerns that may be of importance.  They are POSSIBLE CAUSE #1 and #2 Below.

History of the Bike
I bought the bike brand new from a shady dealer who went out of business and sold his place to his mechanic who also went out of business a few months later. The bike had many problems and I had even more with them.  These were the only people to have worked on the bike. 
In 2009 I parked the bike (still with problems) for the last time.  The bike was now just a cool seat in the garage.

Spring, 2016 - I bought the service manual and decided I was going to have to do this stuff myself.  I started doing the maintenance checks from the service manual.  Which brings us to...
POSSIBLE CAUSE #1
Upon cleaning the fuel filter and rinsing out the fuel tank I noticed a hose from the bottom of the gas tank with a screw plugging it up. (picture below). 

Surely this is not factory.  My guess is this is supposed to be a vent hose?? and they plugged it up to prevent gas from spilling out of it while they were turning it over.  Then forgot to take it out?  OR... This was somehow part of their "screwy" fix for the recall due to the improper gas tank venting which caused stalling.  I wasn't sure so I left it.  Is it possible that the screw could be preventing the tank to vent?

I also saw POSSIBLE CAUSE #2
There are 5 hose ports/nipples under the air filter box, but only three hoses to connect and only one is capped shut.  Could there be a missing vacuum hose or something??  Picture below.


Carrying on, I ended up replacing all three calipers, rear master cylinder, flushed and replaced all fluids and filters.  Then I got a battery and some starter fluid and got it started.  It would start and run poorly if I left the choke on or held down some throttle, but die if I let it try to idle.  I figured the carbs needed cleaning, but I was timid about trying it.  After all, the bike was still pretty new (mileage wise) and I have only done garden equipment and old dirt bike carbs, never a multi carb system. 

Summer 2016, I found another dealer one town over and gave him a call.  While telling him about the issues the bike had, I also told him about the screw in the gas tank hose.  He said he had no idea why that would be there and during repairs, he removed it.  The information below is exactly as on the repair invoice:
Date: 05/18/2016
Mileage:8903
Running Problem Repair
 -new fuel petcock
 -new fuel pump
 -new carburetor boot, front
 -intake O-ring, front
 -new carburetor boot, rear
 -intake O-ring, rear
 -new spark plugs, NGK CR8EIX
 -carburetor cleaned
 -carburetor adjusted
Fuel Meter Function Repair
 -new fuel meter sending unit
Front Brake Repair
 -master cylinder bleed hole cleaned.

I drove the bike home and it seemed to run well.  Few days later it started losing acceleration power and sometimes even bogging down.  I took it back to him and told him not to make any repairs without notifying me of the diagnosis.  The next day he calls me up and says it is ready.  He cleaned the carbs again and charged me again.  We had words and went our separate ways.
Out of options and tired of getting screwed, I figured how much worse could I make the problem.  I decided to clean and rebuild the carbs myself using rebuild kits.  This did not fix the problem.  I then decided to go through everything the dealer said he replaced.  Turns out he did not tighten the front cylinder's spark plug when he changed them the first time.  This greatly improved the bike's performance, but still leaving it with the current issues of popping backfires and sporadic power losses since then. 

So that brings us to speed now, and 1 year and only 100miles on the odometer later, I am still suffering the backfires and power loss.


POSSIBLE CAUSE #3


Giving that a shady dealer kept charging me to clean and redo my carbs to fix a loose spark plug that HE HIMSELF installed and after some web searches, I am leaning towards the idea that he may have been turning adjustment screws on the carbs to fix the problem the spark plug was causing, and creating imbalanced, lean, or rich air:fuel mixture. 
When I rebuilt the carbs checking the dealers work, I did not make any adjustments, I recorded the settings in my service manual. The pilot screws were 3 turns out on the front carb and 3.5 on the rear. 
I do not know if that is the correct position. The service manual only says "STD" on the carb spec sheet  - seen below.


So I started searching...

This thread is the only place I have seen a factory pilot screw setting.  It is just some dude saying I wanna try the factory setting of 2 and 1/2 turns.  So I thought I would give that a shot.  I do not have a Pilot Screw Adjusting Tool and quite frankly I do not see how it would be beneficial. 
So in order to adjust the pilot/ air mixture screw I...
 -take off the gas tank.
 -remove the air box.
 -pull the idle screw out of its clamp to give some slack in the carbs,
 -disconnect the hose at the fuel pump.
 -and loosen the clamps holding the carbs to the rubber boots

This allows me to lift the carbs up out of their boots and hold them slightly on the side so I can access and actually see the pilot screws turn to ensure I am keeping accurate positions without having to remove the throttle cables and other hoses still attached.

I set both pilot screws to 2 and 1/2 as suggested by the random guy, and put everything back together.  Upon starting, the bike seemed to idle fine but upon giving it any throttle it would bog way down.  So I killed it.  Tore it down again, and for kicks tried a half turn the other way.  Still wasn't good. So I go back to researching.

This guide, Mikuni Motorcycle Carburetor Theory 101, states, "If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed." 
Well the last jets to be put in my carbs were supposed to be direct replacements from a jet kit labeled for Hyosung GV650's.  So according to that guide I received the wrong jets or the air/pilot/CO screws are way out of spec.

Googling "BDSR39" from the carb spec sheet above, I learned the brand of carbs are Mikuni, and that the Suzuki SV650 have the same carbs.
I then got my hands on a Suzuki SV650 Service manual as show below.

The Suzuki manual is way more in depth, has sections on syncing and explanations that the Hyosung manual doesn't. The Suzuki manual infers that the I.D. Number stamped on the carb determines the jet sizes and settings.  As there are 4 possible I.D. Numbers of the Mikuni carbs for the Suzuki SV650, they each have different size jets or screw settings.  The factory stock air/pilot/CO settings on the Suzuki carbs are all above the "2 turn limit" suggested by the fore mentioned Mikuni Motorcycle Carburetor Theory 101.  So now I am thinking the guide may be wrong?

So I go look at my carbs to get the I.D. Numbers on them.  They both say "HP95 T622".  I am once again confused - they have the same I.D. # and according to my findings from the Suzuki manual, my front and rear carb's settings should be identical to one another, but the Hyosung Service manual says that the rear carbs main jet and jet needle are different from the front.  Nothing in the carb rebuild kits I got said that the kit was for a specific carb (front or rear).   So I can only assume I put the exact same jets and needles in both my carbs.
I also went to Mikuni website and found nothing about model BDSR39 carbs or corresponding I.D. #s. 

This is when I finally decided to seek you guys help directly.  My frustrations are helping my confusions.  I would like to try to fine tune my carbs myself, but as all the guides suggest...
 -I do not have an engine tachometer, and I have no idea what the last guy to touch the idle screw set the idle rpms at.
 -I also do not have a carb syncing tool, but I did watch youtube videos how to make your own.

I looked up tachs and sync tools, but scared the cheap ones won't be accurate, and I really cannot afford higher quality ones at the moment, nor wasting money on junk.

So this is where I am at.  I would desperately appreciate any help or suggestions.
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Christianleon266

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #1 on: June 11, 2018, 11:31:22 PM »

How do your plugs look? Rich, lean?

Where are your vacuum (the lines coming from your boots) going to?

When you ride, is there hesitation when you give it gas, bogging or popping, kind of a jerky ride like one cylinder isn't firing correctly...

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Christianleon266

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #2 on: June 11, 2018, 11:34:09 PM »

Also look up a bottle manometers. They are just as accurate as the tools.

http://www.xs400.com/threads/home-made-carb-sync.10009/
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1984 Yamaha Virago
1982 Honda V45 Sabre
1996 Kawasaki ZX6R
2009 Kawasaki Ninja 250
1984 Honda V65 Magna (Favorite)
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2007 Hyosung GT250R
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Re: GV650 backfires at idle and sluggish acceleration
« Reply #3 on: June 11, 2018, 11:48:44 PM »

I feel for you. Its just sucks when you have a great bike that all you need was a little knowledgeable people to assist in solving the issues.

First up, point one the fuel tank hoses. From what i can see on pictures of other gv650 carby fuel tanks, the hose that has a screw in it - that port isn't mean't to be there at all (unless its a california thing perhaps ???), most carby tanks have the larger hose (vent hose) and the petcock at the very back of the tank.
So blocking this off i would imagine is perfectly fine.

Second point, i'm not sure on the airbox pipes, i know the two at the front should be from the cylinder heads to catch blow by, the one next to the one you highlighted i'm guessing is an AIS pipe, but on a carby there is no idle speed air valve so i'm guessing its not needed and could be capped ??? Tho even uncapped it shouldn't cause you running issues because its not after the carbs.

Third point,  thats a lot of work on the carbs/intake area. The carbs themselves along with the intake boots seem to have been gone over so in theory they shouldn't be an issue.

On 99% of bikes, the carb idle mixture screws are all usually 2 1/2 turns out from seated as a general rule. When the bike runs/idles, your mean't to screw in to the point of rough, screw out to the point of rough, count the turns and go half way.
But usually 2 1/2 turns is good.

V twin engines can sometimes use different size jets on the carbs, usually the rear runs a higher jet by 1 size i think. For example my XV1100 runs a 120 front / 122.5 rear main jets. Stick with what it recommends rather than going equal.

I'm pretty sure the Mikuni BDSR39 is a vacuum carb, so i'd remove the carbs again and check that the diaphragms (the rubber boots with the needle on them). If what i found about the carb on the sv650, the gv650 should have these also.
This is what i'm most interested in with your bike.
Is the popping/backfiring at idle ? from the exhaust or the intake ?

When i got my XV1100, It would backfire/pop on the intake side(and sometimes exhaust side too) during idle, the bike idled like crap, ran very poorly and unless was under full throttle, just felt bad. The carbs on this i had are Mikuni CV carbs with rubber diaphragms. When i stripped the carbs down, there was 2 noticeable issues. The idle mixture screws had gunk on the ends which had to be cleaned and the main issue was one of these rubber diaphragms had a split/hole in it. It was tiny, about the size of a pen tip. But this is enough to cause vacuum leaks internally of the carbs and not suck enough gas through the pilot jet.
Once this was fixed, the bike was a completely different beast.
Here is a picture of the patch i used on the carb diaphragm. Gives you an idea how little it was.


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umop-ǝpisdn

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #4 on: June 12, 2018, 01:15:21 AM »

Hi Byrddawg,

Epic post. Simply epic!  :o

It's going to take me a while to digest it (at work).

One point of note, about the fuel tank venting recall. If I recall correctly (and I usually do), there is a rubber seal in the fuel tank cap that if not perforated at some magical point, seals the tank and prevents air being drawn into the tank via the overflow. What happens is that the fuel tank becomes a vacuum and that the fuel pump struggles ever harder to suck fuel out of the tank, until eventually either the bike simply stalls, or the diaphragm tears and then the bike stalls.

Now, I think your suspicions about the blocked off hose are correct. If that blocked off hose is the overflow and the previous owner simply plugged it because they were getting some fuel leakage, I would suggest that, correctly notched rubber seal or no, you're going to get a vacuum situation inside your tank. Run the bike until it stalls and then open the fuel tank. If it sucks in air, you might try removing the screw and trying again. If the overflow is leaking way too much fuel, then perhaps the tank seal is imperfect, or was notched in the wrong place?

I know I am making a lot of suppositions, but you can test each step.

Run until it stalls, open the tank and if it sucks air in, then it's a venting issue. If it doesn't then I am wrong and look for alternatives.
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umop-ǝpisdn

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #5 on: June 12, 2018, 02:20:45 AM »

With regard to the Mikuni guide, it's probably written by a well meaning and likely very experienced netizen. It's up to you how you decide to interpret that. If it were me, I'd try both. At what cost a pilot jet? ..besides, what's the harm in trying something so minutely different? Have a go and compare both.

Idle can be affected by all kinds of things; elevation, fuel, air filter brand/cleanliness, spark plug temperature, compression, just to name a few. It's possible that in their part of the world, a size smaller jet and a lower idle screw setting worked better in their circumstances.

One more thing I noticed is the whole loose spark plug thing. ..now I don't want to jinx you, but how confident are you that he didn't strip the thread?
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spit051261

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #6 on: June 12, 2018, 01:38:27 PM »

If it was me , I would replace all the electrical components that could be causing this .
It is not easy to test HT coils and leads because they could break down at any time therefore better to just swap them for new .
At least then you have eliminated this part of the equation and if you still have a problem , you can then look at fueling which is a pain .
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bobe7316

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #7 on: June 12, 2018, 11:24:37 PM »

http://motorcycleviews.com/recalls/rec08v071000.htm

The above link will take you to the Hyosung recall due to the tank gasket. You can follow up some other recalls for Hyosung and other makes of motorcycle as well.

If your rubber diaphragm has a hole in it try to get a new one. I tried to mend one some years ago and it did not last. They are not expensive to buy.

Looking at your post I can see that the dealer that fixed your hyo did every thing that could be expected that a dealer/service mechanic would do. "You said that I drove the bike home and it seemed to run well". A few days later it started playing up again.

This proves that the motorcycle can be fixed if only for a few days, and even with a, as you say, loose spark plug. Check your engine to frame earth lead and the frame to negative battery terminal. Corrosion where the negative leads join to the engine to the frame and the frame to the battery can cause intermittent running problems.

Bob
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spit051261

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #8 on: June 13, 2018, 11:39:38 AM »

Is this some sort of competition as to who can write the longest post?
I want no part of it if it is  ;D
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umop-ǝpisdn

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Re: GV650 backfires at idle and sluggish acceleration
« Reply #9 on: June 15, 2018, 04:25:07 AM »

One of the longest posts would have been my hydraulics and universal harmony rant.
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