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Valve or Carb issue?

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Valve or Carb issue?

PostAuthor: ApartmentMechanic » Sun Feb 02, 2020 2:55 pm

I am a new owner of a 93 Yamaha Virago 750. The bike is a bit rough and has about 46k miles on it. I know she has been dropped (not by me) and there are some problems. What I am currently working on is the backfire through the carbs and a run away throttle. My thoughts are it could be the valves? The things I know for sure:
1.) Carburetor cleaned thoroughly and rebuilt. Replaced main, idle, and needle. All passages cleared and there are no tears in the diaphragms. I could not get the air screws out because the tops are buggered up.... and I’m hesitant to drill them out.
2.) Battery is new and the charging system is working. Good spark at the plugs!
3.) No chugging sound or anything of that nature from the engine. I believe the timing chain to be fine.
4.) The air emissions system is probably not sealing right (possibly this is the problem?)
5.) I have not attempted to adjust valves yet so I’m not sure if it’s in spec.
6.) I’m attempting to get it to idle without the air boxes. Most bikes I have worked on dont run great without it but, one cylinder should not spit more than the other.
7.) These are the Mikuni Carbs and I really hate pulling them out!

Any experience or advise would really help me know if I’m going the right direction!
https://youtu.be/Cxb4Cq6WJTk
Last edited by Jake on Sun Feb 09, 2020 10:00 am, edited 1 time in total.
Reason: (SOLVED) removed from topic. Problem is ongoing.

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Re: Valve or Carb issue?

PostAuthor: ApartmentMechanic » Sun Feb 02, 2020 3:57 pm

Fixed!

Overthinking it all. Pinched off one part and turned the idle way down... Now she purrs [album][/album]

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Re: Valve or Carb issue?

PostAuthor: drainplug » Mon Feb 03, 2020 12:14 am

Now all you gotta do is change all the fluids (don't believe the PO) and adjust the valves
The wise gardener shakes the tree
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Re: Valve or Carb issue?

PostAuthor: bstig60 » Mon Feb 03, 2020 11:13 am

:yup: The symptoms you describe are typical of a vacuum leak.
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Re: Valve or Carb issue?

PostAuthor: Flyingdog » Tue Feb 04, 2020 2:39 am

:yup: :yup:
Good you got it runn'n ...ok.

FYI:
ApartmentMechanic wrote:4.) The air emissions system is probably not sealing right (possibly this is the problem?)


Any additional air could/would tend to "lean it out"..causing to carbs to hiccup that badly thru the carbs. 90% Virago owner ditch all that non-sense anyway. Just a good way to eliminate any potential leaks/malfunctions down the road.


ApartmentMechanic wrote:1.) Carburetor cleaned thoroughly and rebuilt. Replaced main, idle, and needle.


This would also point to carbs not set right. Lotta little things that can be off just enough to cause your hiccups (thru the carbs).

ApartmentMechanic wrote: a run away throttle.


This is what really jump out to me. If, after a thorough spray'n of carb cleaner, on a running engines, does not reveal any change in RPMs (vacuum leaks as Bill said), then I'd be looking towards the carbs, themselves, as being on the lean side (RPMs hanging/hunting points to being "lean").

Rule of thumb:
LEAN: Hiccups/sneeze'n/backfire thru carbs and/or RPM hangs after letting go of throttle.
RICH: Backfire'n thru exhaust.

While valve lash check'n/adjustments are always good maintenance..highly doubtful this would cause your particular problem. Valves rarely get tighter, to cause problems, due to the pounding those things take. Even with the locknut loose..angle of threads don't tighten.
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Re: Valve or Carb issue?

PostAuthor: ApartmentMechanic » Wed Feb 05, 2020 12:25 pm

Update: I can get her to run still but the popping is mostly while it warms up. I have not yet completely sealed the AIS system so, that’s definitely on the menu. I have sprayed carb cleaner around the boots and found no change in rpm. When I put them on I noticed there wasn’t an O-ring so I cut a gasket out and found that it is sealed nicely. I am concerned the front and back cylinder are not working “evenly” still and this is why.

When I go to feel the pipes the rear is very hot as you would expect but, the front is not even close to the same temperature. It is hot and I’m sure is firing. I’m still suspecting a carb issue but I need to do a compression test to rule out a deeper issue.

Thanks for putting the Valve issue to bed (It will get adjusted when I change fluids)! I’m confident the previous owner had no idea what they were doing so, I have to go back and look at everything. Once I get the bike going forward my next usual step is making sure it can stop. Probably will do brake lines and pads if I can get this other stuff sorted cheaply

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Re: Valve or Carb issue?

PostAuthor: ApartmentMechanic » Wed Feb 05, 2020 12:29 pm

Also, it backfires through carbs AND pipes. So it must be richly running lean...... :bg: :bg: :bg:

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Re: Valve or Carb issue?

PostAuthor: Flyingdog » Wed Feb 05, 2020 3:48 pm

ApartmentMechanic wrote:I am concerned the front and back cylinder are not working “evenly” still and this is why.


Yep..I'd be putt'n "carb sync" on that "to do" list, too. Do'n a complete electrical troubleshooting regime could also show something.
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Re: Valve or Carb issue?

PostAuthor: ApartmentMechanic » Thu Feb 06, 2020 3:27 pm

Well... I synced the carbs.
SUCCESS!!!

Turns out they were way off... The front was around 300 while the back was hovering around 6-700!!! I used my ears to slowly dial it in and noticed on my vacuum gauges that I could not get the numbers to match. Then I decided to check the gauges to see if my eyes and ears were getting the same thing.

Turns out Fleabay is not the best place to buy quality tools. Once I swapped the lines they were both spot in and it stopped the backfire issues!

Pro Tip: Never trust cheap tools from China**

** Disclaimer: I am trusting parts of the same broken tool. From China.

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Re: Valve or Carb issue?

PostAuthor: Flyingdog » Thu Feb 06, 2020 3:44 pm

ApartmentMechanic wrote:Well... I synced the carbs.
SUCCESS!!!

:GJ:
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Re: Valve or Carb issue?

PostAuthor: ApartmentMechanic » Sat Feb 08, 2020 2:37 pm

Back to the drawing board. I thought the Carb Sync was it but, that was not it.. I’m thinking it’s a combination of things and is going to be hard to pin down. Tried to start her up today and same issues again... Gassed out the apartment complex garage and every time I open the throttle it would choke out. Still wants to run on the back one cylinder more than the front. it’s a good thing this bike isn’t my daily

Going to check wiring and make sure it’s good as well but the spark seems solid and the plugs look new. It is sparking good and the wires look okay, I’m thinking CDI/ECU could be faulty?

I did cap the AIS system off completely and found a very easy fix!!!! 1/2in copper cap with some of the rubber tubing cut off (the stuff that enters the left side AIS) and then you push it into the elbows. I used a screw driver to press it down all the way. it looks clean and fits very tight. No air leaks there...

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Re: Valve or Carb issue?

PostAuthor: Flyingdog » Sun Feb 09, 2020 5:02 am

ApartmentMechanic wrote: I’m thinking it’s a combination of things and is going to be hard to pin down.


Unfortunately..this is oh-so-true to a certain point. But that's where bike ownership really shines. Air..Fuel..Spark..is all ya need to make an engine to work. Simple, right?

ApartmentMechanic wrote:Still wants to run on the back one cylinder more than the front.


What makes you say this? Are the cylinder temps different? When the bike "is" running...can to remove the spark plug caps and tell the difference in cylinder firing?

ApartmentMechanic wrote: Gassed out the apartment complex garage and every time I open the throttle it would choke out. Still wants to run on the back one cylinder more than the front.


I would say this still points to carb problems. But not absolute. (Sadly) IE: Something is still plugged, carbs unbalance, floats still not set correctly or even sticking (rubbing on float bowls/float pin not hang'n like it should on the float's tang..etc etc etc). I've even take'n carbs apart after being told they're supposedly in perfect shape.. and a jet is missing.

ApartmentMechanic wrote:Going to check wiring and make sure it’s good as well but the spark seems solid and the plugs look new. It is sparking good and the wires look okay, I’m thinking CDI/ECU could be faulty?


Virago's units are TCI...Difference between TCI vs CDI? Think of it this way..the first letter: T--transistorized vs C--capacitor. One could stop yer heart if shorted out. Back in the old days, when cars "used" to use points/capacitor in their distributors..you could get a nasty Zap..from the capacitor..cuz it stores up a charge, before collapsing, to send the spark to the plugs. But if you're getting spark..doubt that's the problem.

Long distance diagnose'n is sketchy at best. Just hang in there..doesn't sound tooo bad..just a little elusive to pin down, at times.
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Re: Valve or Carb issue?

PostAuthor: ApartmentMechanic » Sun Feb 09, 2020 8:41 am

TCI is my main suspect now-

The float heights were set and all jets are new, clean, and good to go. When I slowly rev the engine it will kinda climb up then choke out (not firing on all cylinders evenly and backfiring). The rear cylinder decided to stop working and then the front started?! I’m basing this off of the heat being produced at the exhaust pipe. Sure will pull the plugs to verify my findings.

When I went to investigate the TCI (thanks for the correction) I noticed the battery box had been removed before and there wasn’t a single bolt holding it in place. Then as went to unclip the unit, the white tabs had been broken making the plugs come out easily (Thanks previous owner). My goal at the time was just to clean up all the connections and make sure they have good contact.

After a light sanding and wipe down the symptoms appeared to get worse with trying to run! I want to try another unit that I know is works. The one I want is An aftermarket model for $300!!! I think I’m going to reach out to a local salvage yard first and see if they have any salvage bikes with a possible working unit. As my next step.. Then I will see if i can re-solder all the connections in the unit/replace capacitors if I buy a new one and it works. Maybe I can get the old one to work and get some money back somewhere

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Re: Valve or Carb issue?

PostAuthor: Jake » Sun Feb 09, 2020 10:06 am

This thread was prematurely marked as (SOLVED).

I removed (SOLVED) from the title and the four posts made afterward.
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Re: Valve or Carb issue?

PostAuthor: mark75 » Sun Feb 09, 2020 7:56 pm

ApartmentMechanic wrote:Fixed!

Overthinking it all. Pinched off one part and turned the idle way down... Now she purrs [album][/album]



... :con:

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