Author Topic: Walbro carburetor operation  (Read 3766 times)

Online gbritnell

  • Full Member
  • ****
  • Posts: 2156
Walbro carburetor operation
« on: March 21, 2013, 12:53:16 PM »
 The subject of carbs has been discussed on this and other sites many times. Carbs ranging from home made, model airplane types, (simple and complex) to the ubiquitous Walbro carb. I and many others have offered information on what has been learned over the years to try and help fellow modelers with their engine fueling problems. I have used all of the previously stated carbs and have found what works best for me.
 The purpose of this thread is to add to and clear up information about using the Walbro brand carb. I say brand because in my experience I find 'import' carbs now being used that are almost direct copies of Walbros. These carbs are used on small gasoline powered devices ranging from weedeaters to chains saws. When they work properly they are wonderful, when they don't they can be very frustrating.
 I had found the following link to the operation of the Walbro carb and must say it's probably one of the best I have ever read.
A lot of time and effort went into this writing and I would suggest that anyone wanting to use one of these carbs should read it several times.
 The author's writing is related to using these carbs for RC airplane and helicopters so I want to add to what he has written to clarify using these carbs for our 4 stroke engines.
 First and foremost the available Walbro carbs usually are way too big in terms of venturi and fuel needle size for our applications. Usually when these are used a venturi bushing is needed to reduce the size. In doing this the main fuel feed port will need to be drilled into the bushing and located properly when it's installed.
 The next thing is the use of the internal fuel pump. This pump is used to pull fuel from the tank to the carb by means of the pulse provided by the positive and negative signals that come from the engine crankcase during operation. With 4 cycle operation this function isn't really available as most crankcases need to be vented to allow blow by and other internal pressures to be released so that oil is not pushed passed the piston/rings. All engines need this venting, even automobile engines have a PCV system.
 So what to do? One fellow that I spoke with built a dummy alternator for his engine and inside he made a small pump apparatus to provide the needed pulses.  Another engine builder that uses a Walbro carb pressurizes his fuel tank much similar to pumping up a camp stove or lantern tank. Other fellows take a totally different route and bypass the internal pump altogether. By using a low pressure electric fuel pump (2-5 lbs.) fuel can be supplied to the carb without needing any pulse setup. The fuel will flow through the internal check valves but generally they could be removed. Once fuel is supplied to the carb the vacuum formed by the flow of air through the carb operates the regulating needle valve to allow fuel to flow to the high and low needle valve. If the engine isn't running there is no vacuum. If there is no vacuum the needle isn't opened so even though the electric pump has pressure (low) if won't flow through the carb.
 The next issue is the size of the needle valves. These are made for engine that use a lot more gas than ours so in most cases if the stock needles are used the adjustments are very sensitive. In some cases there is no other option but just be reminded that when it comes to fine tuning your engine don't get frustrated, keep adjusting in very small increments.
 In conclusion there are quite a few fellows using these carbs.  I think initially they were used because they were small and had the necessary control features (needle valves etc.)  Is it worth the extra plumbing, pumps and modifications? You be the judge.
 I hope this helps out.

Talent unshared is talent wasted.

Offline Bearcar1

  • Full Member
  • ****
  • Posts: 709
  • Chicagoland Area, USA
Re: Walbro carburetor operation
« Reply #1 on: March 21, 2013, 02:39:18 PM »
Thanks George, I bookmarked the thread you pointed out so I can better return to it and digest the contained information as slow as my brain can handle it. However, you made some points that made the light bulbs go on regarding how these carbs handle the flow of fuel, thanks again.


Offline ScroungerLee

  • Full Member
  • ****
  • Posts: 299
  • Southwest Connecticut, US, North America, Earth
Re: Walbro carburetor operation
« Reply #2 on: March 21, 2013, 03:33:28 PM »
Thank you George.  I was wondering if it is possible to eliminate the diaphragm pump in this type of carb, you have given some good suggestions for that.

Mmmmm.... Shiny!

Offline steamer

  • Global Moderator
  • Full Member
  • *****
  • Posts: 10698
  • Central Massachusetts, USA
Re: Walbro carburetor operation
« Reply #3 on: March 21, 2013, 04:20:53 PM »
Thanks for sharing that George!   That is very helpful! :praise2:

"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline metalmudler

  • Full Member
  • ****
  • Posts: 57
Re: Walbro carburetor operation
« Reply #4 on: March 21, 2013, 08:21:09 PM »
Nice!   Way Nice!      Hey,thanks for taking the time ,shows im not just a nerd,  AND   Well said m8 :)
It's not done, until it's DONE... Even then there is something else to do to it !..