Author Topic: The Stumble Free Guide  (Read 4460 times)

pinholenz

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The Stumble Free Guide
« on: October 13, 2014, 06:31:08 AM »
I'm pretty chuffed to have my XZ550 riding like a proper motorcycle at last. Lovely rich sound, great acceleration out of curves and no more stumbling when I need power the most. The bike still looks like a shitty rat bike, but at least I now have some incentive to start tidying it up after a couple of years fiddling to get it to this "happy rider" state.

This thread is  to  give some idea of how I solved stuff on my machine and to encourage others not to give up. In my case it boiled down to electrics, fuel supply and carburetion. Hopefully others will add their wisdom and experience to this post.

Electrics
1. Check coils according to the manual, clean up frame ground contacts, replace spark plug leads, plug caps and spark plugs. (I put in Iridium plugs)
2. Replace starter lead with a heavier guage to reduce voltage loss. (The TCI unit works erratically when cranking voltage drops below about 10 volts - never try to start from cold with your lights on!.)
3. And of course a decent battery and a stator/regulator that is charging it. To keep an eye on what was happening with the electrics, I slapped on a waterproof $10 Chinese voltmeter. - a great confidence booster
4. Others will recommend new fuse blocks, wiring harnesses and so on. - Not on my list of priorities at present.

Fuel
1. Fuel must be clean and rust free. At a minimum an inline filter is a must. Having a POR'ed tank is desirable. I haven't done my tank yet, but I put rare earth magnets in my carbs and filter to help suck up rust sludge. Works well.

Carburetion
1. In my experience, the biggest culprit causing bogging is air leaks into your system. Replace all vacuum hoses, especially those to your petcock and petrol pump. Make sure your YICS is airtight or throw it away and cap the ports
2. If you can find new ones, replace your carb boots or coat the old ones with several coats of liquid rubber to seal the cracks.
3. Totally strip your carbs, remove all jets. Thoroughly clean every part with a varnish solvent.  Then blow out all air passages with compressed air. Repeat! Access to an ultrasonic cleaner with solvent is a bonus.
4. Replace all O rings and the throttle valve seals
5. Replace all jets with new ones. I re-used my emulsion tubes OK. Reassemble carbs. Make sure the floats are set to spec and that the accelerator pump is working exactly as described in the manual.
6. Check that the vacuum flapper in your airbox is working OK  (do this initially by sucking on the vacuum tube)
7. (Adjust the valve clearances to the spec in the manual. - I haven't got the special tool required so I haven't done this important stage yet. looking forward to more improvements when it is done though)
7. Install the carbs into the bike. Setup up a pony gas tank. (I use an old motor-mower gas tank strapped to the headlight) Block off any vacuum ports that aren't needed.
8. Start bike and adjust carbs and idle screw until warmed up and running reasonably
9. Sync the carbs so that each carb draws the same vacuum at idle. Tweak the mixture screws for best idle, then recheck the sync for balance.
10. Install the airbox with its  vacuum and breather connections. Adjust the idle screw to bring idle back to up 1300 revs. Check your vacuum flap is working when the bike is revved hard. Shining a torch through the air intake helps to see if it is working ok.
11. Tighten down all joints, re-install tank,Take it for a spin. Whoop for joy or cry in frustration and start troubleshooting.

Jetting your carb. I am no expert here, but here is what worked for me.

Before refurbishing the carbs, I found that the stumble was reduced when I had the enrichening circuit (choke) just active.  I figured my idle was lean and probably my mid-range jetting was lean as well. Therefore I reduced my top Idle air jets down to 120's so that less air and more fuel was drawn in on each stroke. (The idle jets are  standard #60's) Next I increased my main jets size to 125 on the front cylinder and 130 on the rear to give the mid and upper range more fuel. I haven't tweaked the accelerator pump settings yet, but the actuator rod length is about 60 mm

Jetting will  be different for others depending on their exhaust setups. Mine has standard headers and mixing box and stainless steel mufflers. It is probable that my exhaust is fairly close to a standard setup.

Next, I will drop the plugs out and inspect the colour to make sure I am not running too rich and wasting gas.
« Last Edit: October 16, 2014, 06:41:02 PM by pinholenz »
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QBS

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Re: The Stumble Free Guide
« Reply #1 on: October 13, 2014, 11:21:47 AM »
Most excellent!  Congratulations on your success.  Thank you very much for taking the time and trouble to report your findings.  This post provides a wonderful starting point for any Vnary  who is in, or is just beginning, the process of sorting out his or her machine.  Interesting to note that much of what you report regards the common basics of engine tuning, that is, solid well sorted electrics and the elimination of vacuum leaks.  Regarding vacuum leaks, all are very easy to address, with the sole exception the butterfly seals.  That one is reported to be really scary.  Your jet finding are particularly valuable.  Of course, the magnet tip is gold. Thanks again for the report.

pinholenz

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Re: The Stumble Free Guide
« Reply #2 on: October 13, 2014, 02:51:58 PM »
Regarding vacuum leaks, all are very easy to address, with the sole exception the butterfly seals.  That one is reported to be really scary.

Patience is your friend when it comes to the butterfly valve seals. Get a new flat chainsaw file to file off the splayed ends of the screws. If any excessive force is needed to get the screws out, support the shaft from underneath with a dowel of wood about the diameter of a broom handle. Its all over if you bend the butterfly shaft. That is scary. Over the years I have collected a couple of spare carb sets from a 550 and a 400 just in case.

I made a brief video of how I tackled the butterfly valve seals here: 

http://vimeo.com/70672209

(You may need to cut and paste the link into a new tab if the link doesn't connect; - the forum settings don't allow videos to be inserted into posts)
« Last Edit: October 13, 2014, 03:06:01 PM by pinholenz »
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hoverhead47

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Re: The Stumble Free Guide
« Reply #3 on: October 13, 2014, 09:26:39 PM »
Hi,
 I've just got my XZ back on the road, club permit and all.  I kitted my carbs because when beginning this task I had fuel pouring out the overflows and tapping the carbs didn't unstick the float needles.  The tear down was uneventful and plenty of carb cleaner was used.  I noted the the sizes of the jets didn't match completely those supplied in the kits.  I now have  030 pilot air jets where the 020 jet use to live.  I might consider putting them back if I can get them cleaned enough and the new ones prove to be unsuitable. They seemed to have a bit of bluey green corrosion on the inlet side.

I set the mixture adjusting screws according to the count I recorded when I dismantled them.  When it came to adjusting the mixture adjusting screws on the engine I found it near impossible to adjust the rear one due to the back engine mount.
I've got Carbtune balancers and I've managed to get the same vacuum (within a graduation or two) but I'm not sure I know what I'm listening for when I make adjustments to the mixture screws.
Also, which carb do I make master to adjust against or is this not relevant?

Any advice, thanks

pinholenz

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Re: The Stumble Free Guide
« Reply #4 on: October 13, 2014, 10:45:37 PM »
Yeah, the rear mixture screw is a bit of a bugger. I ended up buying a long skinny electricians screwdriver with a good quality blade and shaft. Then I stripped all the insulation off the shaft so it would squeeze  in past the rear engine frame. Using a torch to light the slot helped reduce the risk of damage to the screw slot.

The proper way to adjust the mixture would be with an exhaust gas analyzer. I haven't got one. Instead I screw the mixture in till the engine starts to run rough, then I screw it out counting the turns until it starts to run rough again. Then I screw it in for half the number of turns that I screwed it out so that the adjustment ends up half way between rough spots. Repeat for the other cylinder.

Its not a very refined technique but my motor seems to be happy with this adjustment. Its neither too lean, and being at risk of overheating, nor too rich and being at risk of fouling plugs. Assuming there are no air leaks, the result should be an even idle without spluttering, and a healthy "well rounded" sounding exhaust note.

Readjust your idle for 1300 revs and recheck your carb sync. Adjusting your mixture will affect the vacuum balance.

The bluey-green crud at the top of your pilot air jets will probably be from a damp atmosphere or the carbs being exposed to some water.

All the best.

« Last Edit: October 14, 2014, 05:16:05 AM by pinholenz »
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Re: The Stumble Free Guide
« Reply #5 on: October 14, 2014, 02:11:28 AM »
Hi,
 I've just got my XZ back on the road, club permit and all.  I kitted my carbs because when beginning this task I had fuel pouring out the overflows and tapping the carbs didn't unstick the float needles.  The tear down was uneventful and plenty of carb cleaner was used.  I noted the the sizes of the jets didn't match completely those supplied in the kits.  I now have  030 pilot air jets where the 020 jet use to live.  I might consider putting them back if I can get them cleaned enough and the new ones prove to be unsuitable. They seemed to have a bit of bluey green corrosion on the inlet side.

I set the mixture adjusting screws according to the count I recorded when I dismantled them.  When it came to adjusting the mixture adjusting screws on the engine I found it near impossible to adjust the rear one due to the back engine mount.
I've got Carbtune balancers and I've managed to get the same vacuum (within a graduation or two) but I'm not sure I know what I'm listening for when I make adjustments to the mixture screws.
Also, which carb do I make master to adjust against or is this not relevant?

Any advice, thanks

With proper vacuum gauges like the Carbtune, you can adjust the screws for maximum vacuum - then balance with the sync rod and repeat till you get no change. No master as such, but one will always end up going higher than the other, so you will need to back that off to make them both the same  Finally richen both by 1/4 turn.  That's the same as shooting for highest RPM but way way easier.  The RPM deal can be very tricky with both cylinders running.

Accessing the rear screw is difficult - but much easier if you have an extra long, quite fine screwdriver.  Mine is about a foot long with about a 3mm shaft


hoverhead47

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Re: The Stumble Free Guide
« Reply #6 on: October 17, 2014, 05:30:32 AM »

With proper vacuum gauges like the Carbtune, you can adjust the screws for maximum vacuum - then balance with the sync rod and repeat till you get no change. No master as such, but one will always end up going higher than the other, so you will need to back that off to make them both the same  Finally richen both by 1/4 turn.  That's the same as shooting for highest RPM but way way easier.  The RPM deal can be very tricky with both cylinders running.

Accessing the rear screw is difficult - but much easier if you have an extra long, quite fine screwdriver.  Mine is about a foot long with about a 3mm shaft
Just so I'm sure, the air mixture screw leans out when screwed out and enriches when screwed closed, yes?
I had a fiddle with them last night and I found the the front cyl liked the screw turned out 4 or more turns, revs picked up quite a bit but I didn't get quite the same response from the rear.  I did however manage to increase the vacuums on both after a while.   
Thanks.

pinholenz

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Re: The Stumble Free Guide
« Reply #7 on: October 17, 2014, 05:55:32 AM »
I found the same with the front cylinder - turned the screw out about another 3 1/2 turns from the setup recommendations and it improved heaps. The rear cylinder starting setup is at least two turns less than the front cylinder.

I THINK the screw in will give you a lean condition (less petrol) and the screw out gives more petrol for the same volume of air at idle. (Rich) My understanding is that the Idle air jet (top jets) controls the idle air circuit and the mixture screw controls the idle petrol circuit after it has been through the idle jet. Someone correct me if I am wrong please.

I am sure that maximum vacuum is not optimal setting. That would occur when the minimum volume of petrol mixes with the maximum volume of air That would be a lean mixture.  Make sure that you richen the mixture after you find maximum vacuum (as POD suggests) to avoid overheating issues.
« Last Edit: October 18, 2014, 12:33:17 AM by pinholenz »
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Re: The Stumble Free Guide
« Reply #8 on: October 17, 2014, 03:39:25 PM »
That's right Pinholenz, IN=CLOCKWISE=LEAN, OUT=COUNTERCLOCKWISE=RICH  It's the opposite of most motorcycle slide carbs.

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Re: The Stumble Free Guide
« Reply #9 on: October 18, 2014, 07:38:28 AM »
A reliable rule of thumb on most carbs is if the screw is on the engine side of the venturi, it controls fuel and CW is leaner. If the screw is on the air filter side of the venturi, the screw controls air. Another giveaway is fuel screws typically uses O-rings to seal whereas air screws do not.
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hoverhead47

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Re: The Stumble Free Guide
« Reply #10 on: October 18, 2014, 07:52:31 AM »
I've found a couple of vids on the subject.  I thought this guy explained it the best.
http://youtu.be/YxpQjDQzX7g

pinholenz

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Re: The Stumble Free Guide
« Reply #11 on: October 18, 2014, 02:43:11 PM »
Nice find, his explanations are clear and would have helped my trial and improvement tinkering no end. Whilst the Mikuni down drafts are a totally different mechanism to the slide carbs,  they still have exactly the same function and so the broad principles are the same.

This guy is good as well and does a thorough job of explaining jetting changes needed for non standard exhausts and air intakes.
http://www.youtube.com/watch?v=y836ZPjvoMI

At the end of the day it isn't magic, its just physics, logic and problem solving. For newbies such as me, half the battle with our carbs is finding where the jets are and then sticking to a naming convention for them. Haynes isn't helpful here. I still don't know what the "1.8 Main Air Jet " is, and calling the butterfly valve as the "No. 120 Throttle Valve" is just plain confusing when the jet sizes have similar numbers.

It would be nice to have an animator do the same for our carb types to explain their workings as in this great example.
http://www.youtube.com/watch?v=wyspAHrMbb8
 
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hoverhead47

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Re: The Stumble Free Guide
« Reply #12 on: October 19, 2014, 08:56:26 AM »
It would be nice to have an animator do the same for our carb types to explain their workings as in this great example.
http://www.youtube.com/watch?v=wyspAHrMbb8
Here's one with graphics the explains the downdraught automotive carburettor that we have AND it's the right vintage!!
http://youtu.be/q_XX3QpKsZM

pinholenz

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Re: The Stumble Free Guide
« Reply #13 on: November 09, 2014, 04:18:55 AM »
I mentioned that I had a slight off throttle hesitation when I open op the throttle from idle. I figured that this might be because the idle mixture was too lean. To test this I opened up the enrichener circuit (choke) a little, and the hesitation became less. I figured there are several ways to adjust for this.

1. Screw the mixture screws out to enrichen the idle fuel mix
2. Adjust the accelerator rod to ensure it activates earlier by squirting gas down the throats of the carbs
3. Put in smaller idle air jets so that there was less air (and hence more gas) with each stroke at idle.
4. Increase my idle speed so there wasn't such a big difference between idle and mid-range carb operation.

From my optimal mixture settings (half way between rough, running lean and rough, running rich) I screwed the mixture screws out 1 1/2 turns (ie anticlockwise) Bike did not like it, Idle was too rough. A 3/4 turns out from optimal was better and gave less hesitation.

Next I lengthened the accelerator rod by 2 turns - even less hesitation

Then I increased the idle speed from 1300 rpm to 1500 rpm -better still. Nearly there.

I am running 120 idle air jets at present so I have ordered a pair of 110's and will try again. More out of curiosity really. Hesitation is 90% gone.

I am starting to wonder if the bogging experienced by XZ owners is because Yamaha leaned out the Mikuni carbs to meet US EPA regulations at idle and this was at the expense of transitional carb performance? So far I have increased the main jet size and my gas consumption has only slightly increased, but (subjectively) performance feels better. With original main jets my consumption was 4.7 litres per 100 Km, now it is 5.0 litres per 100 km.
« Last Edit: November 10, 2014, 03:48:49 AM by pinholenz »
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Re: The Stumble Free Guide
« Reply #14 on: November 10, 2014, 01:22:57 AM »
Good write up. I'm a fan of richeming the idle circuit and have been running 120 pilot air for years. Do you wNt to borrow an EGA to help dial it in?