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ViragoTechForum.com Tech Help Only starts/idles on choke/dies when you give it gas(SOLVED)

#1:  Only starts/idles on choke/dies when you give it gas(SOLVED) Author: shmalphy Post Posted: Fri Apr 23, 2010 10:33 am

I have cleaned the carbs thoroughly 3 times. I have played with the pilot screws to no avail. I have checked for vacuum leaks and not found any. The bike idles perfectly on choke, but dies when I take it off, even after warming up for awhile.

What else could cause this? I am stumped. I have 122/47 jetting on both carbs, and aftermarket pipes, I think that might be it, but I am not sure. I want to narrow it down and eliminate all other potential problems first before I change the jets.


Last edited by shmalphy on Sun May 02, 2010 11:50 am; edited 1 time in total

#2:   Author: grazingazer Post Posted: Fri Apr 23, 2010 10:41 am

are you sure you haven't installed the pilot jets where the main metering jets should be

#3:   Author: shmalphy Post Posted: Fri Apr 23, 2010 10:56 am

The main (larger) jet goes into the emulsion tube, right under the needle, right? I am pretty sure I got that right...

I am likely gonna pull the carbs again later anyways to bench set the floats, I need a ruler that measures mm.

#4:  Re: Only starts/idles on choke, dies when you give it gas Author: coalstoves Post Posted: Fri Apr 23, 2010 11:17 am

shmalphy wrote (View Post): I have cleaned the carbs thoroughly 3 times. I have played with the pilot screws to no avail. I have checked for vacuum leaks and not found any. The bike idles perfectly on choke, but dies when I take it off, even after warming up for awhile.

What else could cause this?


You would not be the first who felt they thoroughly cleaned the carbs but missed something .

I personally feel based on my hands on experience that the pilot jet needs to be removed from the jet block and replaced if it has been so seriously blocked and the symptom you describe is a classic and repeated complaint of the Mikuni BDS model carbs on a 535.

I also feel that when contamination from stale fuel is so bad that there is a minimum amount of parts that MUST be changed in order to get to all the troubled areas fuel lays in storage situations, I will include links for these lists I posted in the KB at the end of this reply.

When I got involved with Viragos this was my first project and it did just like yours does.

I read everything here on the subject and everywhere else I could find info,, if you decide to try and short cut any of the steps or minimum needed parts then your outcome will be hit or miss . I cannot stress enough how important attention to detail is in this area of your bike .

My results were that we installed the carbs after rebuild and started it on a few cranks took it for a ride and dialed it in as to sync and mixture that was over a year ago and some 9000 mi without another single problem from the carbs .

I hope this works out fer ya, cause they are fun bikes to ride and the feeling of satisfaction is a real HOOT .

Link Parts

Link Carbs

Floats

#5:   Author: shmalphy Post Posted: Fri Apr 23, 2010 11:31 am

I do not have a 535, I have a 1982 xv750. That carb looks a lot different than mine...

#6:   Author: coalstoves Post Posted: Fri Apr 23, 2010 11:58 am

I'm sorry I don't know how I got it in my head you had a 535 my mistake .

Still the pilot jet, passage and mixture screw orfice are still the most likely culprit

#7:   Author: shmalphy Post Posted: Fri Apr 23, 2010 12:34 pm

Ok, sounds good

I came home for lunch and took a look at the float levels, and the front cylinder seems way low. I am gonna get a ruler on the ride home tonight, and set that to 19mm, clean out the pilot circut, and try again. I will report back on the results. Thanks for the advice

#8:   Author: shmalphy Post Posted: Fri Apr 23, 2010 6:35 pm

I changed the sparkplugs, and now it idles without the choke but still dies when I apply throttle. I am thinking the Seafoam is wirking through the crud...

I still plan to bech set my floats, but the auto store dodnt have a metric ruler...

#9:   Author: SeaComms Post Posted: Fri Apr 23, 2010 8:08 pm

19mm is close enough to 3/4 inch to not worry about getting a metric ruler (although you should still have a metric ruler in your tool box anyways since the rest of the world uses metric :))

19mm = 0.748 inches

#10:   Author: grazingazer Post Posted: Fri Apr 23, 2010 8:11 pm

make sure that you clean off the little screens behind the needle seats

#11:   Author: shmalphy Post Posted: Fri Apr 23, 2010 8:23 pm

i replaced those screens behind the needle when I did the carb rebuild kit.

Good to know I don't need to run out and look for a ruler first thing tomorow..

So what could be making it die when I apply throttle? I don't understand how that happens.

#12:   Author: grazingazer Post Posted: Fri Apr 23, 2010 8:42 pm

gets more air than gasoline...needs a 14.5 to 1 ratio air to gasoline

Last edited by grazingazer on Fri Apr 23, 2010 8:43 pm; edited 1 time in total

#13:   Author: SeaComms Post Posted: Fri Apr 23, 2010 8:42 pm

To put it simply, its either getting too much air or too much fuel. Keep revving it until it just about dies a few times then check your plugs, see if they are dark and wet or dry and light/white.

I woulod hazzard a guess thought that its running lean since it was only idling on choke.

#14:   Author: shmalphy Post Posted: Fri Apr 23, 2010 8:46 pm

It now will idle with the choke off if I turn the idle up, but then it idles at about 4000 rpms when it is on choke.

I get that the mix is off, but it seems like it is getting too much gas? It idles and I can get the rpms to go up only by either choking it or covering the intake with my hand. The throttle just makes it dies, it will not raise the rpms

#15:   Author: grazingazer Post Posted: Fri Apr 23, 2010 8:53 pm

you need to re-examine your carbs and rebuild them properly if you can't discover any vacuum leaks...check the vacuum t under the gas tank for a major leak if your working on the xv750

#16:   Author: shmalphy Post Posted: Fri Apr 23, 2010 8:57 pm

I removed that T. I plugged the large outlet and went straight to the petcock on both sides. I sprayed all around, no vaccum leaks. I even changed the carb boots and that did not change anything.

I think it is because the float level is off. I checked it before, and it seemed ok, but it looked off today. I plan to remove and re clean tomorow, and set the floats at 3/4".

#17:  Re: Only starts/idles on choke, dies when you give it gas Author: MRBerner Post Posted: Sat Apr 24, 2010 1:53 am

I have been having this same problem with the xv700 I picked up recently. Except I cannot get it off choke without it dying. I haven't gone through the carbs yet though. I wish you luck and I will be following what you do so I can solve the problem with my bike!

#18:   Author: shmalphy Post Posted: Sat Apr 24, 2010 1:41 pm

took the carbs off again, blasted out all the orifices with cleaner and air. Put the pilot screws in 2.5 turns out. Set both floats at 19mm

It now will respond to throttle on choke, will not idle with choke off. I can rev it up and turn the choke off, but it bogs out and dies if it gets to the low rpms with the choke off.

what should I try next

#19:   Author: Matthew Post Posted: Sat Apr 24, 2010 2:19 pm

They like to be babied sometimes after a tear down.


how long of a warm up? shouldn't get rev happy on choke, let it idle like a car in the 60s 3-5min and leave it be tills its warm, then ride but still don't get rev happy, easy take off and low rev's, enough choke to keep it happy, once your 3-4 block out, beat that thing like ya stole it and run it up to redline between gears.

Mines very cold blooded but once hot its like a rapeape.

#20:   Author: Matthew Post Posted: Sat Apr 24, 2010 2:26 pm

Though I wouldn't push it, if it feel bad once warm, but wouldn't judge anything till nice and warm.

#21:   Author: packman Post Posted: Sat Apr 24, 2010 2:29 pm

I had the same issue with my 93 XV750. Once I got all the vacuum leaks fixed (as graz says) it was much better. Then I pulled the carbs (again) & found I had not cleaned as well the first time as I thought. I found that replacing the pilot screws & really cleaning the pilot circuit as coalstoves stated, was my fix. My pilot screws had been turned all the way in beyond soft seat & required choke just to get any gas (don't know how long it was like that). Good luck.

#22:   Author: shmalphy Post Posted: Sat Apr 24, 2010 3:48 pm

I think it seems like they are out of synch, I am gonna build the cheapo vacuum meter and try to set the idle that way. They are clean and there are no leaks...

http://viragotechforum.com/phpBB/viewtopic.php?t=9745&highlight=sync+hitachi

#23:   Author: coalstoves Post Posted: Sat Apr 24, 2010 8:31 pm

Just curious,
When you have the carbs off and cleaning them do you actually remove the pilot jet and then the screw and blow air and or cleaner thru the passage and made sure the jets open AND have you cleaned the Air jets in the diaphragm chamber that are hidden under the choke tube thingie ?

#24:   Author: grazingazer Post Posted: Sat Apr 24, 2010 8:38 pm

this may help you: CLICK HERE

#25:   Author: Xumi Post Posted: Sun Apr 25, 2010 12:52 am

SOunds like you've made some progress, which is good.

I'd re-check your float levels now that you have them benched, and you are correct - Syncing would be a great idea too.

Once you get through verifying your float levels, and sync, then I would do a dead-cylinder tuning next - remove one plug and ground against frame so that the bike runs on only one cylinder. Start bike, and adjust air/fuel mix screw for highest idle... Repeat for other cylinder.

Then if still not running decently, pull the spark plugs, take some good pictures of them, and post them back here.

#26:   Author: shmalphy Post Posted: Mon Apr 26, 2010 11:57 am

I gave up on the dual carbs. Synching is a nightmare. I can't even get it to idel, so how can I synch?

I am building a PVC man, and getting this carb, since I currently have a first gen Hitachi...
http://www.bikebandit.com/mikuni-round-slide-vm-series-carburetor?WT.mc_id=1484958&CAWELAID=338444899

The price is right, and I like the design a lot better, it is just like the dirt bike carbs I am used to.

What should I use for an airfilter, and throttle cable? I also can not figure out what stock jets come on that carb. Does anyone know what jets I should order for it? I don't want to wait for shipping twice.

#27:   Author: Xumi Post Posted: Mon Apr 26, 2010 12:39 pm

Sorry to hear you're giving up on the dual carbs - I prefer them myself, but to each, his own.

If you ever face the same situation again, you can do a bench sync with the carbs off by using a business card under the butterflies, comparing the force it takes to drag the card out from under... adjust so that the drag is the same. Also, people have reported using a machinists square to measure the bottom of the butterfly to the outer edge of the carb - making those match... Or even just eyeballing it will get you close enough to start tuning.

Then dial in the mix, then do a final sync and final mix adjustment.

#28:   Author: shmalphy Post Posted: Mon Apr 26, 2010 1:21 pm

I will try again while I wait for parts to come in, because if I can get the duals to work I will put them on my other bike, which needs a carb.

I have read through about 100 threads on carbs, tried repeatedly, but seem to be getting nowhere. I am so frustrated I just want to flatten the stupid carbs into the pavement with a sledgehammer . I have already spent $50 on two rebuild kits, $20 on an air filter, $20 getting Allen bolts and carb cleaner, so I don't want to ditch the duals, but I would rather be riding than spending week after week doing the same thing over and over to no avail.

I tried to get them to open at the same time when they were on the bench, but I later learned that is not a good idea.

I will take the carbs off again and try the business card trick, and also use a machinist ruler. If they don't idle after that, I am gonna sell em on eBay...

#29:   Author: Xumi Post Posted: Mon Apr 26, 2010 3:24 pm

I wish you the best luck! And where did you buy those Allen head screws / carb cleaner - I want to stay away from that place.

#30:   Author: shmalphy Post Posted: Mon Apr 26, 2010 3:58 pm

16 s/s metric allen bolts $8
16 s/s lock washers $2
can of cleaner $5

after tax I have a few singles and some change, so I rounded up, factoring time and gas to drive there...


At any rate, I know the carbs are out of synch, so I plan to take them off and bench synch. I also suspect the diaphragm is leaking, there are 2 very tiny cracks on the edge. I tried sealing it with vaseline, and they didn't even seem big enough to make a difference. Is there a way to tell if they are sealed?

Basically, I am gonna give the duals one more chance, and I also want to make sure the carbs are the problem before I go and buy a brand new Mikuni. I also might switch to manual petcocks just to eliminate the possibility the petcock are an issue. (I had an extra pit bike petcock lying around that I sold a couple months ago, but for $10 it seems to be worth it to make a more simple system) I am also going to test spark plug gap, compression, and valve clearance.

#31:   Author: Xumi Post Posted: Mon Apr 26, 2010 4:20 pm

Vaseline probably wouldn't seal them for long.

Liquid electrical tape, or use rubber cement and a balloon as a patch works well. have also used super thin packing tape (the cheap kind from USPS office) successfully.

And though you need to fix/replace the diaphragms, that wouldn't cause you to not idle anyway. That would cause misfire/fuel starvation when cracking the throttle off of idle.


Easiest way to test petcock is with engine off, to put them in the ON position, and pull the line off the carb... no fuel should come out. Same with Reserve. in Prime, fuel should flow easily. Then when you can, start the engine with what's in the carb bowls, and you should get fuel coming out of the fuel line in the on and reserve position. Also make sure that fuel never comes out of the vacuum line.

Alternately, if you have a vacuum pump you can apply a light vacuum to the line and use that instead of a running engine.

Also - did you verify your float levels after your last settings change??

If you have a fuel deliver issue, it will show up as a low fuel level in the bowl, and will cause fuel starvation - which is what it sounds like.

#32:   Author: shmalphy Post Posted: Mon Apr 26, 2010 4:24 pm

if it is a fuel delivery issue, it would be solved by manual petcocks, right?

#33:   Author: Xumi Post Posted: Mon Apr 26, 2010 4:43 pm

[quote="shmalphy";p="203343"]
if it is a fuel delivery issue, it would be solved by manual petcocks, right?
[/quote

Maybe - could be Petcocks, fuel line, fuel filter or float level.

Better to troubleshoot the issue and find out what is wrong before just throwing parts (and $$) at it. You'll save both time and money this way.

Troubleshooting is best done by dividing the problem in half... You know you aren't getting enough fuel to the engine (because it works with choke on, but not with it off).. so where is the half way point of fuel delivery to the engine?


The Fuel in the float bowls.


Start with verifying that your float level is ok using a clear tube hooked up to the drain valve, and compare it to the flange level. Should be about 1-2mm below the flange.

If you dont' have enough fuel in the carbs, you have a fuel delivery issue or your float level is set wrong.

If fuel level is good in the bowls, then you have a clogged idle circuit (idle jet, air jet, related passageways), or a vacuum leak.

Not saying that Syncing the carbs is a bad idea, or that running a manual petcock is a waste of time (I prefer them, hands down).. but check these things first, because you can check them with the carbs on the bike.

If the problem is fuel delivery to the carb, the single carb setup won't solve anything.

If it's the float level, then replacing the petcock with manual won't fix it.

Find out what is wrong and then decide how you want to fix it. - There are often several solutions to most problems, once the problem is understood.

#34:   Author: Xumi Post Posted: Mon Apr 26, 2010 4:51 pm

and btw - yes, I know you checked most of these areas before, but you also made changes, so it's best to re-visit, and verify that what you think is working... actually is.

#35:   Author: shmalphy Post Posted: Mon Apr 26, 2010 7:34 pm

I was actually just gonna put my xr50 tank right on the bike and connect it to both carbs with a T and see if that worked...

I have brand new fuel lines, scrubbed the fuel filters at the petcock with carb cleaner and a toothbrush, replaced the filters at the needle valve, and just set the float levels at 19mm, so I suspect either the vacuum leak I can't find or petcocks.

#36:   Author: Xumi Post Posted: Mon Apr 26, 2010 8:22 pm

You set the floats to 19mm, but you still don't have confirmation that you have adequate fuel in the bowls.


Using the xr50 tank might work, if it's petcock has enough flow to keep up with the engine when running, which might eliminate the petcock from the equation. But not knowing if the petcock is big enough, I would say that if your symptoms change but the bike still isn't quite ok, then that's a good indication of a petcock problem.

#37:   Author: shmalphy Post Posted: Mon Apr 26, 2010 8:32 pm

I think it is big enough because the 920 has only 1 petcock feeding both, and I have seen a few people using the xr50 petcok on their 920's.

I want to eliminate everything else from the equation but the carb, and if I cant fix the carb, replace it.

Unfortunatly it was raining and I had to file taxes today, so I couldn't mess with the bike, but I will be back out tomorow to bech synch the carbs and clean again, and if they still don't work, I will try the 50 tank at that point. I will check spark plug gap, too. If that still don't work, I will be back here again...

#38:   Author: Xumi Post Posted: Tue Apr 27, 2010 12:53 am

Good luck, bro.

#39:   Author: shmalphy Post Posted: Sat May 01, 2010 1:37 pm

well i built the synch tool and she runs now. I went to take it for a ride, but she stalled, and the battery is dead from starting it so many times....

I am just gonna let it charge up and then try again. It definitely idled at about 1000 with no choke, and has even vacuum pressure. I got it really close at 3500. I had the petcocks on prime, and hooked the lines from the tool straight to the boots

It still seemed to stumble at low throttle, i need to fine tune the pilot screws i think. I have them both at 2.5 turns out now.

#40:   Author: Xumi Post Posted: Sun May 02, 2010 4:08 am

Glad to hear about the progress - good luck with the rest of the tuning.

A dead battery can definitely cause issues - I hope that you have better luck tomorrow.

#41:  Re: Only starts/idles on choke, dies when you give it gas Author: shmalphy Post Posted: Sun May 02, 2010 8:02 am

the battery charged up to 16v, is that normal? Yesterday, had it about 12.4, then started it, it went down to 12.0 and then stalled and would not restart, so I charged it overnight.

How do you know if the battery is junk? I am about to go test it out now, hope the neighbors are awake

#42:   Author: shmalphy Post Posted: Sun May 02, 2010 11:49 am

well, I just took her for my first ride!!! WOW that is a great feeling. In the end, I just needed to tune the carbs with a manometer, I made one for under $5, and it still needs more tuning, but I am very happy to have finally taken a ride. I did 2 miles, came back and made adjustments, and then did 2 more. I need to do a few more adjustments, and it should be good.

Thanks everyone for the help, I really appreciate it!

#43:   Author: Xumi Post Posted: Mon May 03, 2010 12:05 pm

Congrats on getting it running!! Persistance pays off again.

As far as battery = 16v - I would take a hard look at your battery charger and consider replacing it.

If your bike made it run that high, then you need a new voltage regulator.

Normal charge for a battery is 12.5v - 14v. Anything above that will damage the battery. Do it enough times or for long enough and you'll need a new one.
ViragoTechForum.com Tech Help Only starts/idles on choke/dies when you give it gas(SOLVED)


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