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Carb sync? Never again.
This past Airventure I had the opportunity to talk with Hal Stockman (the Zipper big bore guy) and he shared with me his latest modification to the 912 engines.
He is removing the small balance tube between the intake manifolds and replacing it with a 1" diameter tube.
I just did this modification to my engine this past weekend and had the chance to test run the engine today. All I can say is WOW! What a difference this makes. The engine is now butter smooth from 1500 to 6000 rpm. A mechanical sync is all I did on reassembly and don't plan to do any more. This mod is something I've been thinking about for a long while, but didn't have the courage to do without a second opinion. Makes perfect sense to me and makes me wonder why these engines don't come this way from the start.
I forgot to snap a photo, but will do this tomorrow for the record.
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Re: Carb sync? Never again.
Yes, we or at least I would like more info and pictures. JImChuk
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Re: Carb sync? Never again.
Very interesting developement. Standing by for photos.
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Re: Carb sync? Never again.
Did you have to do any machining to the intake manifold, or was this simply a bolt on addition?
Thanks
Rodney
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Re: Carb sync? Never again.
That sounds interesting.
I am curious for the theory behind that modification.
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Re: Carb sync? Never again.
I did some machine work on the manifolds to position and tap 3/4 NPT holes for the fittings. I got 3/4 NPT to 1" pipe 90 degree elbows from Jegs, 1" thin wall tube from a local supplier and 1" heater noose and clamps from NAPA.
The theory behind the modification is to allow the two manifolds to equalize pressure or vacuume between them easier.
Photos later today.
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Re: Carb sync? Never again.
Quote:
Originally Posted by
Av8r3400
This past Airventure I had the opportunity to talk with Hal Stockman (the Zipper big bore guy) and he shared with me his latest modification to the 912 engines.
He is removing the small balance tube between the intake manifolds and replacing it with a 1" diameter tube.
I just did this modification to my engine this past weekend and had the chance to test run the engine today. All I can say is WOW! What a difference this makes. The engine is now butter smooth from 1500 to 6000 rpm. A mechanical sync is all I did on reassembly and don't plan to do any more. This mod is something I've been thinking about for a long while, but didn't have the courage to do without a second opinion. Makes perfect sense to me and makes me wonder why these engines don't come this way from the start.
I forgot to snap a photo, but will do this tomorrow for the record.
I have heard of this many years ago through some overseas sources... never tried it but you are the 3rd person I have heard glowing reports from recently..
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Re: Carb sync? Never again.
This is interesting to me too. I've been noodling on the theory and believe it makes perfect sense at the higher engine rpms because the intake flow is quite high and a bad carb balance may require a high flow thru the balance tube. However, down near idle the flows are very low and an imbalance would not need a very big balance tube to correct it, so I would think that the 1" balance tube would have very little value at near idle speeds. Thru the years of doing carb syncs I have noticed that if there is an imbalance it seems to always show up as far worse down in the very low rpms, but as you run the rpm up the imbalance seems to get better. I'm not sure how to explain this except that a small motion of the throttle cable near idle has a much higher percentage change of throttle opening than it does near WOT. Also, the "starting carb" circuit does not become inactive until rpms are higher. What this noodling is leading me to believe is that even if you go with a large balance tube, you should still occasionally do a carb sync at idle. Be interested in other opinions on this.
One more thing, I don't think there is much value in installing a balance tube that has an inside diameter any larger that the smallest hole thru the fittings on each end.
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Re: Carb sync? Never again.
Larry,
you and I talked about this a OSH and it makes total sense. Now you need to go through how you did it. I can get the parts, but I be scared to drill out holes. right now I be busy putting on tires and brakes and all that good stuff. this one will be my next thing. :)
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Re: Carb sync? Never again.
I ordered my kit from Hal (ph: 775-934-5714) today. Worst case, if it doesn't pan out, you make an adapter to go back to original design. Hal's kit consists of modified manifolds, the 3/4 pipe thread fittings, new crossover tube and all required parts for installation. When done, you send him your old manifolds. This may help with a rumbling kind of vibration when throttling back to land (3300 down to 2500 rpm) at airspeed above 90 mph. All in all, this is a fun airplane.
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3 Attachment(s)
Re: Carb sync? Never again.
Here are the promised photos. I plan to get some better fitting clamps for the connector hoses. This is the proof of concept version.
Attachment 13858
Attachment 13859
Attachment 13860
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Re: Carb sync? Never again.
Is that smaller hose for a manifold pressure gauge? Looks pretty simple to do, might just have to do it!
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Re: Carb sync? Never again.
For those of you considering doing this, I would highly recommend looking thru the postings on this subject on Rotax-Owner.com. Click on the forums and then use the search feature, using Balance Tube as the keyword and Titles only. It will bring up about 80 postings which is interesting reading. What I got from it is:
- Results seemed to be somewhat mixed, with mostly good results, mainly at the lower rpms.
- There were several ways of hooking up the larger balance tube and fittings-some with bad results.
- Most of those with good results seemed to find that they needed to limit throttle opening to 95% or else the engine would start to miss at further opening; nobody seemed to know why this is.
- There was some concern about the intake tuning with a much larger intake system volume (can have a large effect on race cars).
- Most agreed that with perfect carb sync there is no need at all for a balance tube of any size.
- No one claimed that the larger tube would eliminate all carb sync forever, it just makes it easier and more forgiving of inaccuracies. A sync at or near idle still seemed to be a good thing to do.
- Most seemed to be happy overall; a few were removing the larger tube and going back to standard.
- There is an Italian company, I think, that was offering a kit to do this. I believe they use a spacer block in front of each carb with ports in it for the larger balance tube. It moved the carbs back an inch or two I believe.
Anyway, interesting reading; people have been experimenting with this for a number of years.
Av8r3400, curious if you have noticed missing/stumbling at 95%+ throttle opening?
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Re: Carb sync? Never again.
I have been watching this with some interest. The issues with carb sync are numerous, and mostly induced by the mechanics omission or error. Sorry, but I'm the mechanic most likely to be "That" guy. A sticky cable or poorly routed cable, a cracked or worn carb boot, a sinking float, a mis-adjusted float lever arm, a worn screw, a dirty anything etc. etc. etc. and you have a rough running Rotax. Stay clean, stay tuned, know your carb theory and fly with ease. I always say if it isn't stock, its wrong, but ... I see the Zipper kit guys are having a ball! the jury is out after about 5 years of testing. I"m not into wrenching like I was when I first started and frankly I just want to go flying. The constant tinkering with my motor and worry about its reliability just seems to increase as I add any variation to what the original engineer intentioned. And Here I am modifying the holy Cr@#p out of a motor so that I can make it fit in my plane. The only difference is, I"m not trying to improve performance, I'm just trying to make it fit and run in the parameters for which it was designed. Seems like airplanes just BEG to be worked on.
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Re: Carb sync? Never again.
Yes the small line is for a manifold pressure sensor.
I had the opportunity to put an hour of touch and goes on this afternoon. The results are in: The modification works wonderful. The engine is smooth through all of the rpm range. My nagging rough spot from 2500-3500 is gone.
Full throttle seemed to yield about 100 rpm more at static, with no roughness or missing. There are a lot of nervous nellies on fly-Rotax that refuse to believe anything but the party line from Rotax. I like my engine. A lot. I do believe there are areas that can be improved. This is one of them.
I had synced and synced my carbs with analog and digital manometers and could never get the midrange roughness to go away. Now it's gone. Smooth idle to 1400 with no chatter.
Very happy with the mod. No regrets.
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Re: Carb sync? Never again.
Nice, good to here things are working smoothly. :)
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Re: Carb sync? Never again.
I heard you could now go down in your idle without vibration, like 1200 with no problems.
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Re: Carb sync? Never again.
Quote:
Originally Posted by
Av8r3400
I like my engine. A lot. I do believe there are areas that can be improved. This is one of them.
Very happy with the mod. No regrets.
Keep in mind this mod is with a Zipper kit. Other results may vary.
I had seen this mod talked about on other forums and had considered doing this to mine before this thread showed up. Sounds like this is more fuel to add to the should do list.
I would also love to do the Zipper mod but dollars and cents don't allow at this time. Thanks for the report.
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Re: Carb sync? Never again.
Quote:
Originally Posted by
Av8r3400
Yes the small line is for a manifold pressure sensor.
I had synced and synced my carbs with analog and digital manometers and could never get the midrange roughness to go away. Now it's gone. Smooth idle to 1400 with no chatter.
Very happy with the mod. No regrets.
Thank you sir! This is also the RPM range that I can't seem to get the 'shake' out. The roughness in that RPM, especially when I am over all the trees around my strip, is never comforting. If I can relieve this I would be a much more ardent Rotax supporter. It just bothers the bejeesus out of me when I am landing and the motor is rough.
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Re: Carb sync? Never again.
This midrange roughness at 2500-3500 rpm that many of you talk about has always puzzled me. Most of you are experienced Rotax owners with many more hours than I, so I don't doubt for a minute what you are saying. What puzzles me is I don't ever see any sign of it on my 912uls with 500 hours on it now, and there are others out there with my same experience. Being a meticulous engineer type, I carefully synced the carbs mechanically and pneumatically from day 1 and have never had a roughness problem. In fact my sync hardly ever seems to move or get out of adjustment-the last time I needed to readjust slightly was 300 hours ago; it has stayed as perfect as I can get it for 300 hours without touching it.
I'm not at all trying to imply that those with this problem are not doing the sync carefully enough; there must be something else going on that my engine fortunately does not have. The fact that your roughness shows up in a narrow range of rpms seems to eliminate things like prop balance or a partially sinking carb float. Could it be your throttle cable is not smooth thru midrange? Have you totally removed the throttle springs so that the cable does not have a consistent tension thru the stroke? I am using the lightest McFarland springs.
I believe at these rpms is where the carb idle circuit makes the switch to the main circuit; could it be some deposits are causing the switchover to be rough?
The large balance tube seems to solve whatever the underlying cause is and that is a good solution and it also seems to give you a smoother idle at lower than normal idle speeds. The lowest I can get a smooth idle is about 1600 rpm.
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Re: Carb sync? Never again.
In doing some research on the subject of a balance tube I found the following on the Rotax Owners web site: An individual there wrote::
((Hope it's ok to post this: it's a probably a better explanation than I could write. I apologize if this is not proper etiquitte.))
"Built and added a 1 1/2"D. balance tube to my 912S and it is remarkable how much difference it made in the smoothness at low RPMs. This is not new technology and has been used on intakes and exhaust systems since the forties to smooth out low end vibration. It works.
I saw it advertised recently by an italian racing equipment firm called FLYGAS but they wanted something like $1200.00 for the package. (cost me about $100.00 to build). It does not increase HP, I don't know yet about fuel consumption. Its advantage is to reduce vibration and improve manifold airflow.
Here's what happens. Firing order on the Rotax, and most other boxer engines, is such that when one cylinder is firing, the other cylinder on the same side is on a compression stroke which means that NO air is flowing into that manifold. This inrush of fuel/air mix which has suddenly stopped has nowhere to go except through the balance tube and into the other manifold which is calling for air. The Rotax balance tube is woefully small for this task, even though it is great for idling. You can see this starting and stopping of airflow in the wildly bouncing of the vacuum gauge needles during carb balancing. Because all of this is happening in fractions of seconds, most of the air in the balance tube never actually goes anywhere, but bounces back and forth in the large tube. "
If this explanation is accurate, then the it appears that this modification is more a function of the style of the engine ( ie: boxer) rather than the displacement of the engine. The zipper kits only add to the cubic inches of the engine.
Got a text from Hal and Jay this morning. Cost of their conversion is $300.00 plus shipping plus your old manifolds. I have no connection with Hal or Jay, am just posting this for the information of our group.
Well, I did buy the 114HP Zipper Kit from Hal n Jay and I can report I'm very pleased with it.
Seem like every source I read on this subject reports a major improvement in idling performance of the engine.
Hope this sheds some light on the subject.
Regards
Rodney
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Re: Carb sync? Never again.
I like that explanation! it makes sense to me and I may be more inclined to make that mod ... after I fit the current engine to my plane. Yes, I know, I said the jury is usually out on a mod after about 5 years of field data, but, it sounds like we may be closer to about 40 years of data. I would like to do some historical research. ... after I fit my current engine to my plane. Did I say I was changing engines?
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Re: Carb sync? Never again.
For the record my modification is to a 1" diameter balance tube.
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Re: Carb sync? Never again.
Hi folks
This is slightly off topic but pretty close. Today I was changing oil and decdied to check the carb balance on my 730 hour 912UL. It has been at least 4 years and 400 hours since doing this. Runs smooth but let's check.
As soon as I connect the Carbmate the engine begins running rough and both carbs start flooding.
The carbmate operates well when vacuum or pressure is applied.
I t doesn't leak.
Remove it and engine runs fine.
I use a vacuum hose as a balance tube with a tee for manifold pressure gauge.
It's been this way for many hours.
Appreciate any suggestions
Cheers
Don
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Re: Carb sync? Never again.
I would try a dual manifold pressure gauge or a different CarbMate.
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Re: Carb sync? Never again.
A Carbmate does not allow any air to pass back the forth like a balance tube does. That`s why after syncing the carbs the engine always runs smoother when the Carbmate is removed and the balance tube reconnected.
The issue is that my engine does not even want to run unless the balance tube is installed!! What does this tell me?
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Re: Carb sync? Never again.
To balance th carbs you must disconnect the balance tube and attach the two vacuume tubes to the manifolds and the carb mate.
When you disconnect the balance tube you then see the actual balance of the now totally independent carb systems. Your carbs are out of sync, this is why the engine runs rough.
The engine will alway roughen if the carbs are out of sync and no balance tube it present to equalize them. This is the essence of this modification, to allow free breathing between the manifolds.
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Re: Carb sync? Never again.
It`s possible I did not explain the situation well.
The engine runs great with the balance tube installed. And has for many years and hours. Smooth at all RPM`s.
When I connect the Carbmate it floods and runs really rough. In fact it won`t even keep running.
I wouldn`t think the carbs would be that far out of sync when it has been running so smoothly all along.
You may be right, though! I will start from scratch with a manual sync and adjust it from there.
Thanks for the suggestion.
Cheers
Don
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Re: Carb sync? Never again.
Quote:
Originally Posted by
kitfox2009
I will start from scratch with a manual sync and adjust it from there.
Thanks for the suggestion.
Cheers
Don
Don I have been in your plane and agree it runs smooth at all rpms. I would be hesitant to start from scratch with a manual sync. Check with the vacuum gauges first but when it runs as well as it does proceed with caution.
My 2cents worth.
Wait Canadian got rid of the penny.
My nickels worth:)
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Re: Carb sync? Never again.
hook up the carb mates to the fittings in front of the carbs. you take out the little screws and hook up there. take the balance tube and block both ends. I used a straight hose between the two, when I balance I squeeze the hose off. if blocking the two manifolds yields a rough engine then the two sides are not close, need to go full throttle and check adjustment to the carb stops. then try again. if it's from hooking up the guage then go where I suggested and try again.
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Re: Carb sync? Never again.
OK, here's an update for anyone interested.
Today I started from scratch.
Checked all cables for free movement.
Set throttle at minimum idle and adjusted idle stops for .004 clearance.
Then closed the idle stop screws about two thirds of a turn.
Set the mixture screws open one and half turns.
Left the balance tube connected. Started engine and warmed up to operating temperature .
Carefully adjusted both idle stop screws to give a smooth running engine at desired RPM. I set mine at about 15 or 1600 with throttle pulled completely back.
Then I shut down and connected the Carbmate directly to the balance tube nipples.
Restart but do not idle the engine. Keep it above 2500 or so and run up to 3500 or 4000 . Adjust the cables to put the GREEN in middle at the most sensitive setting through these high RPM s This may take a few restarts and fine adjustments but it is worth it.
Remove the carbmate and reconnect balance tube.
My engine seems to now run smoother than it ever has and that is saying something!
After considerable time attempting to balance with the carbmate attached through ALL
RPM ranges I have come to the conclusion that it is more effective to get a smooth idling engine by adjusting the idle screws with the balance tube connected. My thinking is if the ide is SMOOTH that is more important than where the green light is!
Anyway did a few circuits in 30C Temps tonight and it performs super smooth through all RPMs.
This a long winded explanation that may not be by the book and may not work for everyone but my old 912UL likes it.
Thanks for comments especially from Mangy that really got me thinking.
Cheers
Don
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Re: Carb sync? Never again.
Ok. I had to try this balance tube thing. Been hearing n reading about it for over a year.
I have never been able to get my engine to idle smoothly much below 1800 rpm
I made one from 1 1/2" pipe just for the heck of it.
Wheels, I don't know if this is old technology or new technology, but IT WORKS.
I installed it this afternoon and started it up. Let it warm up, but hey when the oil starts out at 90 degrees it doesn't take long. Started backing down the throttle.
Finally hit the low speed idle stops at 1400 rpm. Super smooth. I can't hardly believe it.
Am going flying this evening before the sun goes down, but for now am really pleased.
Rodney
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Re: Carb sync? Never again.
I'm in the process of redoing my fox. I'm turning it into Badassfox. I installed the Desser 27 1/2 tires. I'm installing the T-3 spring set for the tailwheel. currently the 8inch wide tire from Matco is being built, should be here next week. I put on the balance tube last night and it is real nice. flew it this morning and yes much less vibration. the stick is nice and vibration free. I like it. coming into land there was no vibration. I generally come in real short and pull back to about 500rpm and I didn't feel anything this morning, I thought the engine quit, but it was running. now those tires. makes the look on the plane as well as ground ops is way cool. I sit up much higher. ha, watch out I'll eat you up. haha.http://www.teamkitfox.com/Forums/album.php?albumid=493
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Re: Carb sync? Never again.
Rodney,
Did you drill out and retap the manifolds, or did you put and adapter to the factory port?
Pictures would be helpful! :)
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Re: Carb sync? Never again.
I used the existing ports. I wasn't sure that would work,but I thought I would try it and let the engine tell me if it liked it. So far it seems to work fine. Am headed to the airport shortly to fly. Will pull the cowl n take some pictures
Rodney
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Re: Carb sync? Never again.
If interested, you can go here and look at my setup for the balance tube.
http://www.teamkitfox.com/Forums/album.php?albumid=494
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Re: Carb sync? Never again.
http://www.rotax-owner.com/en/rotax-...hro-carb#16469
Just chucking this out there and don't know if it's useful or not. I'll be getting the 912iS, so none of those nasty carb things for me :D
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Re: Carb sync? Never again.
What if you used a piece of 1 inch water or fuel rubber hose, with no metal tube?
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Re: Carb sync? Never again.
I don't actually know why the tube is metal except for the nipple that is on some of the metal tubes used for MAP. My old tube is metal and has no Manifold Pressure attach point. So I think If I had a rubber tube, it wouldn't matter. But I have a question.
If a person used a large diameter crossover tube and simply adapted the size for the existing manifold fittings without making them larger, As Rodney did, why wouldn't that do the same as enlarging the fittings? I understand that the air in the crossover tube is largely static, so the benefit is not in "bernoulli, or venturi, principle but rather from the larger mass of air in the tube giving a "buffer" area to the molecules in transit from acceleration to static. Have I been watching to many re-runs of Star Trek, or am I actually ... close?
P.S. My mom used to say "better to be judged a fool than to open your mouth and remove all doubt." ... I never listened to her. Maybe I should have.
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Re: Carb sync? Never again.
Quote:
Originally Posted by
mr bill
What if you used a piece of 1 inch water or fuel rubber hose, with no metal tube?
The Aluminum tube is lighter, that's why I used it.