:: Yamaha Virago XV Forums That Rock!!! XV1000 Front cylinder does not fire below 3000 (Solved)

#1:  Front cylinder does not fire below 3000 (Solved) Author: astro237 Post Posted: Wed Aug 11, 2010 10:47 pm

Ok my virago 1985 xv1000 was doing well for a while but now it only runs on the back cylinder below 2800 - 3000. This is what I have checked so far.

Battery - Good
Front cylinder gets spark
New Autolite AP63 plugs gaped to .28
compression 153 - 157 psi
I can not find any vacuum leaks
I have removed the AIS long ago

I put a new dry plug in front cylinder and bike starts fine on back cylinder and idles fine. I stop the engine and pull the plug and the plug is wet and smells of gas so I think it is getting at least some gas, but maybe not enough.

I go for a ride and it is gutless and single cylinder until I hit 2800 - 3000 RPM then it takes off and runs fine. I am sure the front cylinder is not running below 2800 - 3000 rpm because I can run the engine at 2000 rpm from cold start and keep my hand on the front cylinder or exhaust and it stays cold.

What should I check next? Pull the carbs? I have pulled the carbs before when I first got the bike last year and cleaned them well.

Oh and also some backfire both through the carb and exhaust below 3000, not a lot and not all the time but it's there.
Any help / ideas will be appreciated.

Last edited by astro237 on Fri Aug 13, 2010 7:21 pm; edited 1 time in total

#2:   Author: grazingazer Post Posted: Thu Aug 12, 2010 9:22 am

it sounds as though you may have an open circuit in your TCI
read: user LINK

#3:   Author: astro237 Post Posted: Thu Aug 12, 2010 10:56 am

I will take a look at the TCI but mine is different than the one in the link and is sealed pretty well. Also if there is an open circuit in the TCI would I still have a spark? I can put an old plug in the front plug wire and ground it and I see good spark from idle all the way through 3000 RPM.

#4:   Author: astro237 Post Posted: Thu Aug 12, 2010 5:28 pm

Replaced TCI with a spare I had that I am sure is good, no change.

#5:   Author: astro237 Post Posted: Thu Aug 12, 2010 6:23 pm

Here is one more data point. I just removed the rear plug wire and hooked it up to a grounded old plug and left the front plug connected. As expected I could not start or idle the bike.

I then removed the front plug and sprayed starting fluid into the front cylinder. I then reinstalled the plug and left the rear cylinders plug wire disconnected (but connected to an old plug that was grounded). With the starting fluid I was able to get the engine to start and run for a few seconds on just the front cylinder.

Unless I am missing something this is telling me that the TCI is good, the coil and the rest of the ignition circuit is good. I am leaning to this being a problem of not enough fuel being delivered by the carb at low rpm's. Am I crazy? Anything else I should look at? I really hate pulling these carbs. Oh and I did run a can of Seafoam through a tank of gas when the problem started.

#6:   Author: grazingazer Post Posted: Thu Aug 12, 2010 10:26 pm

#7:   Author: 1986 XV700 Post Posted: Thu Aug 12, 2010 10:29 pm

With what you just said, I am starting to suspect the following:
1. The carbs are out of sync very bad at idle and probably sync up better at about 2800 RPM.
2. There is an issue with the front carb's pilot circuit.
A. There is trash it somewhere.
B. The pilot screw has been tightened almost all the way up or has fallen out.

I'm thinking that your problem is related more to throttle position than RPM and after the front carb rolls out of the pilot circuit it begins to get the needed fuel.

#8:  Re: Front cylinder does not fire below 3000 Author: eaglebeak Post Posted: Fri Aug 13, 2010 12:37 am

My thoughts are centered around you pulling the front plug and it was wet and smelled of gas - but spark plug is firing and it will light up for a little while with starting spray.

That sounds like you're flooding the front cylinder at idle and low RPM. Flooding on the low end would wet the plug, keep the cylinder from firing, and cause some occasional popping or backfiring when rich mix lit up in the hot exhaust system. Starting spray is volatile enough to fire even a very rich fuel mix.

Since you have vacuum operated carbs, I would suspect either a leaking needle-valve/seat assembly in the front carb or a float setting too high. Either one would cause mild flooding on the low end that was enough to keep the cylinder from firing and wet the plug as a result; but as engine RPM increases, there's considerably higher airflow and fuel demand where the mild leakage wouldn't be a problem anymore - letting the mixture get more normal and light-up the cylinder.

Use the clear plastic tubing method in the picture below to check the float level in the front carb (doing the rear carb wouldn't hurt anything either). If there's any question about the level being at the seam of the float bowl as indicated, dropping it 1 or 2mm lower won't hurt anything. If the float level looks good, then I'd replace the needle-valve assembly - especially if you bought a "bargan" aftermarket set.

As 1986XV700 mentioned, going though the proper carb-sync procedure is a must as a FIRST step at 3K RPM, then adjusting the pilot setting on each carb is the final fine-tune.

Never assume anything when it comes to a vacuum leak. Since you've had the carbs off a number of times with old boots to the heads, use the starting spray (outside on only a warm engine) to spray around the carb boots to check for vacuum leaks each time you reassemble them since a crack can happen anytime during disassembly or reassembly.

Float Level.jpg
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Float Level.jpg

#9:   Author: Roger Ramjet Post Posted: Fri Aug 13, 2010 12:56 am

This will sound really freaking stupid , but happened to me , Unscrew the Sparkplug cap , clip a quarter inch or so off the end and thread the spark plug cap back on ... Sometimes moisture will get in and not only corrode wires but they loose their good Vroom Vroom capabities

#10:  Re: Front cylinder does not fire below 3000 Author: astro237 Post Posted: Fri Aug 13, 2010 7:20 pm

Ok I feel like a real idiot now after I told all you guys there was no vacuum leak my son pulls off the air filter pod and says hey dad here's the problem.

Sure enough I started bike and put my finger over the hole and the front cylinder fired right up.

Thanks to everybody on this thread for the help! Lesson learned, make sure you really cover all the basics first.

Thanks again.

#11:   Author: 1986 XV700 Post Posted: Sat Aug 14, 2010 11:06 am

Aren't you glad, in retrospect, that the repair was that simple? Glad you got it fixed. XV1000 Front cylinder does not fire below 3000 (Solved)

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