opinion

by Clyde Poser in 912 / 914 Technical Questions 12 years ago

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Re: opinion

by Roger Lee » 12 years ago

Roger Lee

126 5.13k 4.58k

Hi Clyde,

Syncing at another rpm will not fix your problem. You most likely have an air and or a fuel flow issue. That needs to be found.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: opinion

by Roger Lee » 12 years ago

Roger Lee

126 5.13k 4.58k

Hi Clyde and Mark,

I sync about 60 sets of carbs a year. By using a set of gauges I can usually tell you if the carbs are partially clogged and which one, the bowden cable on one has too much friction in its sheath verses the other cable, the cables are too far out of adjustment to do a sync, which carb I want to adjust and how much to adjust it. Learning the ABC's and diagnostics with a set of gauges just works better than the electronic ones. Maybe the electronic ones can split a hair, but once you hook the balance tube back up you just overrode any hair splitting. The gauges tell you more if you know how to interpret the readings. Unfortunately this only comes from syncing lots of carbs and seeing lots of issues or a good instructor willing to take his time and explain.
Syncing the carbs at 15" or 5" of vacuum won't make a difference on the adjustment between them and it is determined by how far out of sync they are and the flow through the carb. It is all about fuel/air flow through the carb and a sync at 15" (i.e. idle) or 4" (i.e. high rpm setting) of vacuum will put them in sync at that rpm range. The issue with a low rpm sync is and like most instruments you should not use them within the top or bottom 10% of their scale or work load. 2200-2500 is in that range and the idle circuit and once you get above the 3500 rpm you are completely out of that range and operating on the main jet and needle. You don't really care what the vacuum is because that will change with other factors, but what you do care about is the relationship between the two barbs air flow and then compare ones vacuum against the other. The actual number doesn't matter, just that they are the same when you are finished.
With having your engine running in the mid range of rpm then you have a better representation of the air flow through the system where 2500 rpm is too low on the scale to represent a true flow through the carb. Try this test and do it with gauges. Try syncing at 2200-2500 rpm and then run them up to as high an rpm as your gauges will accommodate. Then try and set the sync at let's say 3500-4000 rpm and then run them back up again and see which stays in sync at the higher rpm where you spends most of its life. If there is a vacuum difference then it will be much smaller. The second place it spends a lot of time is down in the idle range of 1700-2200 and that's why we need to also sync at that range. You can't change and make a perfect sync at every 100 rpm (maybe close), but we can pick the best place and most common rpm range where we play at most of the engine's life. Any little difference between the rpms the balance tube will help out. I Feb. we just discussed this in class and the low rpm of 2500 was too low to make an accurate sync for the high rpm side.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: opinion

by Clyde Poser » 12 years ago

Clyde Poser

17 19 22

I with draw my original post. Since I am only getting instructions on carb syncing and nothing useful in the way of diagnosing why there is a power loss and plug fouling on an engine it is a waste of time.

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Re: opinion

by Dennis Richardson » 12 years ago

Dennis Richardson

1 153 169

Hi Clyde
Diagnosis is difficult when you are not 'parts in hand' but looking at the picture you supplied and reading what is available my first thought is an air leak on the 1-3 side of the intake. Something is disrupting the fuel/air balance on this side of the engine. It is also possible that you have an exhaust system imbalance, are 1-3 on a common exhaust?
I would be real interested in hearing what you find. Many years ago we had rotax engines that would have lean burn failures on one cyl of a two cyl/single carb set up. Turned out that the production models of the intake manifold produced a finish that was subject to creating a sonic barrier, the sand cast prototypes worked fine.

Dennis

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Re: opinion

by Roger Lee » 12 years ago

Roger Lee

126 5.13k 4.58k

Hi Clyde,

I understand your frustration, but everyone's suggestions are just trying to cover all the bases and not leave anything out in an effort to help you. Leaving one thing out may be the cause and no one knows what you may or may not know about your engine. The answer is probably in these post. You have to start somewhere and check each individual item and when there is nothing left and no matter how improbable the solution seems to be it will be the answer. It most likely isn't an electrical issue. I said most likely because these issues are usually fuel or air related. The electrical issue should have shown up other places and not just limit you to just 4800 rpm without any signs of it being somewhere else.
If the carbs are balanced then let that part go. The post on carb balance gauges was for other peoples edification too and to show they can be a useful diagnostic tool.

Stick with it and you'll find it.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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