Author Topic: Did some good carb work today  (Read 4415 times)

vl5150

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Did some good carb work today
« on: April 26, 2015, 06:55:41 PM »
I just spend some time messing around with my carbs some more since I learned a bit more about tuning after doing my RD350's Mikuni VM carb mods.

Anyway on the Vision, I picked up the vacuum operated airbox flap late to the game and just slapped it on.  It was running rich as-is, to I lowered the main jets to compensate and all was well.  Anyway as I drive the bike more aggressively I notice the rich bogging in the midrange with really cranking the throttle.

I'm out of rod on the accl pump so that was that.  I did some more playing with the jetting, but I wound up finding an old post here on making a change to the vacuum lid. My vacuum lid rests on the little rubber pad when at rest and I figured it wouldn’t hurt to open it up to let more air in. So I loosened the 2 screws that holds on the diaphragm and moved it back just a bit so the flap is about 1/8' inch off the top of the rubber pad.  I still have a slight trace of bog when really getting on it in 4th but most of it is gone.  I'll see if opening it up a bit more gets rid of it completely, but I may not since it's not really bad at all.
Here's what I'm running
AIR jets 125/125
F. Main- 122.5
R. Main- 127.5
Pilot F- 2t
Pilot R-3.5t

Forgot to add these:
MAC exhaust
No YICS
Sea level w/ high humidity
Uni Filter properly oiled
« Last Edit: April 27, 2015, 10:35:12 AM by vl5150 »

vl5150

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Re: Did some good carb work today
« Reply #1 on: April 26, 2015, 09:31:20 PM »
I opened it up to about 1/4" and it ran at least as good, if not slightly better with only one very slight glitch on WFO after a shift so I'm going to button it up for now. Pretty clean carburation at this point. 

Rikugun

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Re: Did some good carb work today
« Reply #2 on: April 27, 2015, 09:39:22 AM »
Having revisited accell pump settings recently myself, I question how much more fuel is actually delivered at max adjustment vs. a more moderate setting. Once you get beyond a certain shortened length it seems the "over ride" spring wants to take over and limit further movement of the plunger. An older inflexible diaphragm may accentuate this phenomenon. Having said that I can't argue with success. Whatever it takes to get your Vision running well is what gets you riding.  :)

For anyone down the road reading this and looking for answers to their Vision's problems, how did you determine the mid range bog was a result of rich condition? Thanks for the informative post and including the details of what worked for you.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

vl5150

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Re: Did some good carb work today
« Reply #3 on: April 27, 2015, 10:34:40 AM »
I wound picking up a R5, RD350, RD400, and a old Honda FL250 Odyssey last year all with carb problems so I went to work and learned a bit more on what to look for. 
In addition I picked up a '84 Nighthawk 700s that while carb'd, feels like it's fuel injected it's so good.  So I figured the Vision was something less than optimal, although perfectly drivable.
So here's my experience so take it for what it is.

On the 2 strokes I just finished, it's easier to hear what's rich or lean but if you listen on a 4 stroke, it's there too.  Sound of a howling hound dog ("brahhhhh") is usually lean and the sputter or miss is rich.  Run with the gas off and you'll hear it as the float bowls drain.  Feel for heat pouring off the engine.  It shouldn't burn your legs to a crisp.

On the low end:
On the Vision, a (low end) lean condition results in a high idle "float".  So when you rap the gas in the driveway, it goes to a fast idle and doesn't return to the set idle RPM's.  With the stock main jets, I get this really bad with the 130 air jets.  127.5 air jets still operate lean.  Engine should return to idle with minimal float and for me the 125's on the top nailed it.  Lean will result in lots of heat pouring from the engine and will be noticeable to your legs.  I tried 135's and 140's on the top previously and it resulted in a hot engine.  I think the off idle stumble is lean on the pilot and when I went with the air 125's this went away with no other changes.

In the midrange:
Regardless of when I did with any of the jets, my bike ran rich in the midrange at WOT.  1st gear would be clean, then shifting at 10K at WOT would result in multiple stutters that I took at rich.   Once the stutters cleared up, it would rev to redline so it was more of an interruption in the RPM range.  Lengthening the pump shot rod made it worse and I couldn't shorten it anymore so after living with it for a while I started looking at the actuation of the flap and this seems to have the biggest impact on the midrange on my bike. Opening the flap at rest allows the vacuum to raise it enough to supply enough air in the midrange and this is the cure for the rich condition- just give it more air.  The Vision can go from horribly lean in the low end to horribly rich in the midrange and I think this fine tuning is what doomed the bike since it's really sensitive to a few mm's of movement of the flap.

For the top end, if it takes WOT and pulls to redline while making power in all gears you should be good.  I see the Vision as being less sensitive to main jets as the air box opening is probably limiting air flow at 10K anyway.  Air delivery via the pilot air jet and tuning of the flap are critical from 2500-7500 RPM's and in the meat of where you normally drive, but the main get you past this and on to redline..  If you give it WOT and it stops pulling before redline, you're lean.  If it stops pulling before redline with gurgling noises, then that's the un-burnt fuel and you're rich.  Look for this between 8500 and 10000 RPM's

Like I said, this is my path and everyone has to take their own.  It's the method of finding stasis on these carbs so while helpful, naming jet sizes probably won't work 100% for you.  These downdraft's are sensitive to the air jet and the flap tuning that are unique to these carbs.  Pilot screw adjustment and fuel main jet is about like any other carb.

At this point, I have the carb tune about 99% as good as the 100% perfect stock Nighthawk and that's not a bad feat for a downdraft.  I'll drive it some more and report back if I find any problems after I put more miles on it.

One more thing--I drive the same loop around town that allows me to test all of the carb circuits.  Only when I'm happy with that, will I take it on the highway. 
« Last Edit: April 27, 2015, 10:41:04 AM by vl5150 »

fret not

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Re: Did some good carb work today
« Reply #4 on: April 28, 2015, 12:56:33 AM »
We would be interested in how your adjustments have effected your fuel economy, both around town and on the highway.

Thanks for your ability to dig in and get stuff done, and to report it here. :)
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

vl5150

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Re: Did some good carb work today
« Reply #5 on: April 28, 2015, 07:37:56 AM »
I'll start pulling stats on my next fill up.  It may take a while because I spread my riding across a bunch of bikes and I really don't like taking the Vision on the highway.

Rikugun

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Re: Did some good carb work today
« Reply #6 on: April 30, 2015, 06:15:49 PM »
So you are currently at the shortest possible length on the accell rod adjustment? I second the request on economy figures when you get them. I have a set of MACs I've not yet installed but may have to at some point. This info could become very useful then so thanks again.


Quote
At this point, I have the carb tune about 99% as good as the 100% perfect stock Nighthawk and that's not a bad feat for a downdraft.
I'm familiar with both 650 and 700 Nighthawk carburation and anything close to that is quite good.  :) 

It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

vl5150

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Re: Did some good carb work today
« Reply #7 on: May 01, 2015, 08:19:18 AM »
Cool..glad I can help. 

Yes, my rod is at the shortest length.  I may try a few turns out, but I'm happy with the bike's performance as-is.  I did another 18 mile loop the other day and it feels like it's "on the pipe" at all RPM's and pulls strong when shifting without delay.  I did do the other mods earlier (frommy other posts) to get the cooling under control with the thermostat and the thermo switch so my temp gauge once warmed up does not go past the middle of the green zone.  Before it would go into the red when at long stoplights and traffic jams and this would affect throttle response.  I really don't think you can jet this bike until the temps are stabilized if you're having that problem.  I live in South Florida where it's generally warm and humid so this may be different depending on your location.

My MAC story--
I really like the MAC pipes for me price, but they can me improved.
The MAC exhaust sounds "tinny" and loud stock because they provide unpacked baffles (or at least mine were).  Not the end of the world, but I carefully removed the baffle on both pipes and packed the muffler with FMF muffler packing.  The muffler sounds much more controlled when packed properly.  You take away that echo between the baffle and pipe.

Rikugun

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Re: Did some good carb work today
« Reply #8 on: May 01, 2015, 05:13:03 PM »
I remember your cooling system mods and agree when the engine flirts with the red zone fueling  suffers.  Regarding the MACS, a loud exhaust note is a matter of personal preferences and I don't prefer it. I've read posts where members have packed the MAC mufflers to civilize them a bit and that's what I'd do too. 

It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

hoverhead47

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Re: Did some good carb work today
« Reply #9 on: May 02, 2015, 09:31:33 AM »
Thanks for placing your data up here vl5150. I'm also trying to find the setting and performance I remember when I was riding my XZ regularly over 18 years ago (it's been left in the shed since then). I replaced the 120 air jets back after having installed the 130s that came with the rebuild kits. I believe that the mid throttle stumble was accentuated by the 130s. I have even vacuum balance on these but I am still struggling with the pilots. I've found that too far out and the idle remains high and too far in and there is sometimes a big miss when cracking open the throttle. I've also found that the pilot on the front cyl has a greater effect on the speed than the rear one. I'm heading out tomorrow on a club run and hopefully I will return with only a small amount of fuel in the tank so I'll be able to remove it and apply my Colourtune plug to see how the adjustments are affecting the combustion. I wish I had two so I could do both cyls. at the same time. I did note the pilot screw positions before I rebuilt the carbs and I don't recall them being quite as far open as you now have (2.25 and 2.5 if I recall).

vl5150

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Re: Did some good carb work today
« Reply #10 on: May 04, 2015, 05:45:53 PM »
Hoverhear47: I did some testing using the 120's and the main jets had to be reduced to keep it in balance.  It took running 112F and 115R on 120 air jets to clear up the stumbling on most throttle positions.  I couldn't clear it up 100% on wide open and that annoyed me (this was due to the flap not opening up enough).  I still was able to take the bike on several road trips over the winter with that jetting.  But leaning it out will get the most HP and that's what I think I did with the 125's and going back to stock jetting.  It's making all the right sounds when accelerating and more importantly decelerating.  It should sound like F1 car.

As I think about these carbs, they do have a needle after all- it's the flap.  The flap timing is what will give you the correct transition from pilot to main- the exact function of the needle on a regular carb.  So the whole ball of wax rests on where the flap starts in it's travel.

fret not

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Re: Did some good carb work today
« Reply #11 on: May 05, 2015, 12:55:05 AM »
So, maybe the flap should be mechanically operated rather than vacuum operated. ???
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

Rikugun

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Re: Did some good carb work today
« Reply #12 on: May 05, 2015, 08:52:36 AM »
So, maybe the flap should be mechanically operated rather than vacuum operated. ???
Using vl5150's analogy, that would be like the needle on a set of slide throttle carbs - some of which had a bog when the throttle was suddenly opened from lower RPM's. Having it "vacuum" operated gives some buffering effect like that of a CV carb. Fine tuning the flap's opening point or preload may be analogous to altering the spring strength or enlarging the through hole on the slide of the CV type to cause it's rise to be quicker.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

fret not

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Re: Did some good carb work today
« Reply #13 on: May 06, 2015, 01:05:18 AM »
 :o ??? ::)
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

Rikugun

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Re: Did some good carb work today
« Reply #14 on: May 06, 2015, 08:09:02 AM »
Well, that's my theory anyway.......     :)
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

vl5150

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Re: Did some good carb work today
« Reply #15 on: May 20, 2015, 09:12:35 PM »
Update;  I did some high speed testing tonight and the results are good.  Pulls strong all the way through the gears and in 5th past burying the speedo and beyond.  I'd estimate a hard pull all the way to 100MPH.  If I had to nitpick, I'd say there's a wee bit of a rich condition when pulling WOT to redline and rolling too fast to WOT after the upshift.  I think my pump throws a bit too much gas, but since it's running so good in the other 99% of conditions, I think I'm good to go and can roll a bit less under max acceleration.  That probably can be tuned out though by opening the flap a bit more, but I'm not going to spend the time on something that minor.  I have to say the bike is much more sporty and fun to drive for me now.  The only problem is my spirited driving is exposing the weaknesses in the suspension, lol. 

Rikugun

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Re: Did some good carb work today
« Reply #16 on: May 21, 2015, 07:41:47 AM »
Congrats on getting your Vision not only up and running, but running well.  :)  It sounds like you've got her running about as well as a Vision can run and then some.  :)

Modify the front springs (or replace w/aftermarket), find yourself an '83 rear shock and you'll enjoy your spirited rides all the more. Thanks again for the update.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan