# honda gx670 vtwin mystery



## grubba76 (Jul 2, 2017)

honda gx670 vtwin....was working perfect, turned it off for the night then went to start it next day and it ran rough and backfired and somtimes thru flames out exhaust....fuel was put in day before as it gets used everyday. had oil, filter, fuel filter changed 2 weeks ago...spark plugs changed, carby cleaned, new coils set right, kill switch was disconnected to rule it out...worked out its the left side as testing with a temp gun the right side got to 180c in about a minute (and climbed from there) left side only getting to 50c in same time...pulling right plug made it run rougher, pulling left side made no diffrence....was thinking sticky valve? just pulled head off and cant see the problem as its clean and doesnt feel stick...valves are black but no carbon build up....exhaust valve would have about 1mm play in stem and doesnt have the cleanest seal like intake does but still looks like its sealing....compression test on right gets to 120psi quickley. left side gets to 90psi quickly but will go to 110psi if i keep cranking a few more times......pistons are clean can still see alloy...quick spray and wipe its like new..using choke makes no diffrence....need this for work and im lost


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## AVB (Nov 28, 2014)

When you the kill did you do it at both coils or some single point connection?


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## grubba76 (Jul 2, 2017)

single point connection after the diode....where it connects to ignition key....2 wires come of coils to diode, then from diode it is one wire to ignition key. both coils are getting spark testing with spark tester and timing light.


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## AVB (Nov 28, 2014)

Anyway then try removing the kill wires at the ignition coils and test. The diode maybe be leaky causing flakey coil timing. If still doing it then replace on the cylinder that didn't make a difference unplugging as it may be failing under compression load.


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## grubba76 (Jul 2, 2017)

i will try that...do you mean replace coil or one wire from diode connect back to coil? coils are brand new from honda.....i have no idea and was gonna try removing wires from coil but read if you do it can cause one to burn out other one....some said it will others said it wont....was scared to risk it as coils were $330....am i just reading bad info? is this true whats your thoughts? thanks for you time.


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## AVB (Nov 28, 2014)

Ouch on the price tag. Must some super special coils but of course I don't have type numbr or serial to verify but here is an example one the coils that some the GX670 uses and probably all of them used the same coils.

https://www.jackssmallengines.com/j...in-gcam-1000001-to-gcam-1049999/ignition-coil


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## grubba76 (Jul 2, 2017)

im in australia, nothings cheap haha, $10 for a spark plug lol.... no one carrys parts for these had to order from another start express.


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## AVB (Nov 28, 2014)

Ahh. I have dealt with similar costs on shipping to NSW. Cost an arm and leg to ship.

From what I see in the ipls there no reason that shouldn't be able to remove the kill wires; Just be aware you must kill the by an alternative method such choking death or pulling the plug wires. 

I think the reason for the different coil part numbers is the installed plug wire length.

The reason I thinking that the diode pack might be bad is that diodes do getting leaky and cause strange problems such as triggering both coils as the same time.


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## grubba76 (Jul 2, 2017)

thanks will give that a go..it has fuel soliniod so easy to shut down.


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## grubba76 (Jul 2, 2017)

i tried removing those wires from coil, nothing changed.


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## grubba76 (Jul 2, 2017)

it ended up being the plastic jets on top of carby....i sprayed cleaner thru them and it was going thru but not enough.....the hole is tiny...i used a bit of wire off a wire brush and poked it thru and seen the tiniest bit of junk come out and then rub the wire against the walls. popped them back in and running great now, better then it has in a long time...thanks every one for the help.


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## AVB (Nov 28, 2014)

And tnx for letting us know you got it fix and what you did as may help others too.


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