:: Yamaha Virago XV Forums That Rock!!! XV750 82 XV750 front cylinder not firing (SOLVED)

#1:  82 XV750 front cylinder not firing (SOLVED) Author: mkelsho Post Posted: Thu Jun 03, 2010 2:22 pm

My xv750 misses under 3000rpm. At idle its like the front cylinder isnt even firing. If i give it slight throttle, and put my hand over that cylinders exhaust i can feel the random bursts of heat and compression, but at high revs, the engine doesnt skip a beat, but takes its sweet time returning to idle. On top of that, i get a constant backfire/popping from that cylinder. No matter what, even at idle. I can stop the backfiring by setting the pilot over 4 turns out, but doesnt effect the sputtering. Ive tried everything, ive checked for vacuum leaks, with carb spray and propane, removed the mcv, put the mcv back on (the only thing that concerns me about that is on the vacuum line there is a T connector with one end of the connector has a pin hole, and the other two ends are normal size, im not sure if the pin hole goes towards the carb, the mcv, or the petcock), cleaned (x4)/sync the carbs, new o-rings on manifolds, wd40 all connections, moved the ground wire, switched the coils, replaced spark plugs/wires, brand new battery. I thought it might be the battery, so i hooked up a jumper pack to the bike with no change. I also checked for spark, it is constant. I thought maybe weak spark, but even with a weak spark, i shouldnt be getting cold air out of the exhaust. I havnt checked compression, but i assume if it were mechanical, i would have problems at all rpms. If i give it a little choke, she runs perfect. Im not sure what else to do, this has to be fixable. I need help please

Last edited by mkelsho on Mon Jun 07, 2010 2:06 pm; edited 1 time in total

#2:  Re: 82 XV750 front cylinder not firing Author: michel Post Posted: Thu Jun 03, 2010 5:26 pm

You don't seem to have an electrical problem so your problem is with the carburetor.

I know you said you cleaned them but I suspect a jet somewhere is plugged or the diaphragm has a hole in it.

Are your valves in spec?


#3:   Author: grazingazer Post Posted: Fri Jun 04, 2010 8:57 am

check the end of your coil wire where it enters the coil...a corroded connection at that point has been known to cause the problem that you are having

#4:  Re: 82 XV750 front cylinder not firing Author: michel Post Posted: Fri Jun 04, 2010 12:16 pm

Greetings grazingazer. I had the issue with the coil, but symptoms are a bit different. the miss fire happens across the rpm range and its on and off until the electrical cable snapped and then the cylinder was dead, period.

Also mkelsho claims his spark is OK.

So idle jets or diaphragm? (or something else?)


#5:   Author: mkelsho Post Posted: Fri Jun 04, 2010 3:12 pm

i know electrical better than carbs, i wish it was that simple. last night i got it to run good, but i constantly had to adjust the pilot screw. i rode it around for an hour, then parked it for 2, then went to ride again and she ran like crap the whole time. im starting to think the carb is just no good. i think im gonna reverse the carbs and see if i can get her running, and if i have the same problem, but on the rear cylinder, then ill know it is the carb. since i have them off for the tenth time, ill inspect the carb again, the last time i cleaned it i blew compressed air in every hole in that thing, there has to be something im overlooking. i wish i had colortune. maybe ill do it tomorrow, but i dont want to screw it up even more, the dartmouth building supply poker run is on sunday ---mike

#6:   Author: anaman51 Post Posted: Fri Jun 04, 2010 3:30 pm

Those Hitachis provide dozens of places for a spoogeball to hide, and a cursory cleaning won't find it. The jets all need to come out and be independently cleaned with compressed air and carb cleaner, with the first removed being the pilot jets (so the O-rings are not damaged by the carb cleaner). Take your time and check everywhere, make sure the slides go back in the proper carb body---it matters! The needle jets are slightly different sizes, and the number one jug, the back one, gets the Y-23, and the front jug (#2) gets Y-22. Also, see the New Tech Tips for answers on your MCV removal and what to do with all the loose ends.


#7:   Author: mkelsho Post Posted: Fri Jun 04, 2010 4:10 pm

anaman51 ok so im not really sure who touched the carbs before me, but i put it back the same way i disassebled. all the parts look new, so i suspect the previous owner did a rebuild. when i first got the bike, the pilot jet was loose in the bowl, the same carb im having problems with, and there was no manifold seal. now i know of the 2 jets your talking about, the pilot jet, (has the smaller hole) had no seal/washer/o-ring, on either carb. on the main jet, (with the larger hole), has a metal washer. is there supposed to be an o-ring on the pilot jet? im not really sure about the slides in the proper carb. i also have jardines on my bike, but i dont think thats my problem im sorry for the way i put things, but carbs are before my time, and i didnt really get what you were saying. if it were a fuel injected car, id be all set --mike

#8:   Author: slipknotking490 Post Posted: Fri Jun 04, 2010 5:28 pm

having the same problem with my 82 750 im running striaght pipes could the lack of back pressure be the cause of the problem would re jetting the carbs help

#9:   Author: anaman51 Post Posted: Fri Jun 04, 2010 5:51 pm

Slipknot, anytime anything is changed in the process that begins at the air filter and ends at the tip of the tailpipe, it will affect the way the bike runs. Now, Jardines shouldn't require re-jetting, but straight pipes, that's a different story. Since I have no experiece at this, you'll have to search around for some help with the re-jetting, in a new thread.

Mkelsho, since you have the carbs off the bike, go ahead and pop the tops on the slide containers and pull out the slides. Carefully remove the needles from the inside bottom of the slides with a screwdriver. Do not mix them up! Each needle will be marked at it's top end with either Y-22 or Y-23. They go where I indicated, with the larger needle (Y-23) going to the back jug. When you reassemble, make sure the diaphragm's sitting where it's supposed to sit. There's a small tab on the outside edge of the rubber that fits into a depression on the rim of the hole. Line them up and keep them that way while you screw the lid down.

As to the pilot screws, I'm not talking about the pilot jets, but the pilot screws---they are different items. The screws are reached from the outside of the carb at the underside rear of the carb body, right next to the bowl. There will be a small vertical cylinder on each carb, possibly closed off at the bottom with a brass cap which must be carefully drilled through and then removed before the screws can be accessed. These screws are what have the O-ring on them, and they are the mixture screws for fine-tuning the idle mixture. See the diagrams in the In-Depth Hitachi article in the New Tech Section. These must be removed and cleaned religiously. Re-set them at 2 1/2 turns out from a soft seat when you reassemble. This is where a lot of crud can hide, and making sure they're clean is important!


#10:   Author: mkelsho Post Posted: Mon Jun 07, 2010 2:05 pm

so i took the carbs off again to clean one more time. the only thing i did differently is took a can of wd40 with the built in straw (the straw is fatter), took off my pilot jet, put the straw in there, (the fatter straw made for a nice seal), and began to spray. i have a 1st gen carb, so i have two pilot holes where the butterfly is, an oval and a circle. wd40 was coming out of the oval hole and thats it, so i blocked it, then sprayed again. i got nothing, then a couple of spurts, then a bunch of foam (last time i ran compressed air throught it). i put everything back together, and she runs better than id ever thought possible. id like to thank those who gave me advice. id also would like to thank wd40 and there fat little straw

#11:   Author: slipknotking490 Post Posted: Tue Jun 08, 2010 10:48 pm

my problem ended up being bad gas go figure XV750 82 XV750 front cylinder not firing (SOLVED)

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