BollAero1.8 Running Gremlins

[pat_pending]

Hi Everyone,

I have got myself stuck on my BollAero build and could really use a hand. I was wondering if there are any 2stroke compression-iginition experts out there that might be able to give me some tips. this is my second IC engine after a Webster and it seems these little puppies are less forgiving. The build is partially documented here A pair of Boll Aero 18s

Fundamentally I am wondering whether converting the Chris Boll plans (BollAero 1.8 Gallery) from Imperial to Metric using the 'that's close enough' approach (particularly on drill sizes and fasteners) is the root of all my ills.

In short, the engine seems to fire up nicely if i squirt primer into the engine exhaust port / under the piston on the intake. It will start and rev nicely for 3-4 seconds and then stop. No matter how much i play with the needle on the carburettor the engine does not keep running, When i look at the intake, it seems to be spitting out of it quite a bit but It's hard to tell since there is fuel flying everywhere from squirting in the primer.

I have tested the carb on my Webster and it runs (though not that great to be honest vs. a vapour carb). Still, it seems to work.

The thoughts i have are:

1. There may be insufficient crank case vacum to suck in fuel when the inlet port opens. I am tracking down and fixing any crankcase leaks i can find (might add some gaskets).
2. My round-up/round-down to the nearest drill size might have screwed up the timing of the engine beyond repair. This is what i'm looking to rule out.

I have pulled-together the timing diagram and the diagram from the plans (attached) as measured as accurately as I was able with a rotary encoder. Would someone be able to tell me if this engine could run with the timings as such? I the fact that it runs on primer a sign of hope that this engine can be a runner rather than destined to the shelves of the non-runners? I Have hope!

Any input would be greatly appreciated. Thanks, Patrick

 

[pat_pending]

 

[scottyp]

Sounds like the engine is happy to run if it has fuel. If you hold you finger over the intake while turning it over by hand, you should be able to see fuel moving up the fuel line and not being pushed backwards. Sorry if that is to obvious, but that is how I tend to prime my small engines. That way, I have a good idea of how much fuel is in the engine to start with. Boll Aero is on my to do list.

 

[Tim Wescott]

It certainly sounds like everything is working except for fuel draw. In addition to Scottyp's request, could you show us a close-up picture of your venturi and one showing the engine in your test bench? You should have the fuel tank level with the venturi, the venturi shouldn't be too big, and there should be something necking it down at the point where fuel goes in -- in the Boll Aero, this should be the spraybar. If your venturi throat is too big, the engine just won't draw fuel. If you've made the needle too small, the engine won't draw enough.

It helps a lot to have the hole(s) in the spraybar pointing somewhere between 90 degrees to the flow of air and about 30 degrees off of pointing straight into the intake. However that usually only makes the difference between running oddly and running smoothly -- not running vs. not running.

Have you checked for primary (crankcase) compression? The fact that the engine will run for 3-4 seconds means it has good enough primary compression to draw air into the crankcase and push it into the cylinder. A badly leaking two-stroke will just pop and porble when you flip it, with air getting into the cylinder by chance -- if you're really getting a three second run out of it then it's definitely pumping air into the cylinder. However, if it's a bit leaky, or if it's getting air into the crankcase through unofficial channels that bypass the venturi, then that would cause problems.

If you are drawing fuel through the venturi when it's choked, then a further test is to prime it, start it, and then briefly choke it when running -- brief as in tap the end of the venturi with a finger, not brief as in putting your finger on it and saying "Hey look! I'm choking the engine!". If that keeps it alive, or makes it run longer, then you're just not getting enough fuel into the thing. I've certainly had engines that, on startup, required choking while they're running (at contests, much to my embarrassment -- there's nothing like starting a control line stunt run with the judges looking on and saying "WTF is he doing to his engine?").

If choking the engine makes it run then you definitely have a too-big venturi or a too-small needle valve. If you can elevate the fuel tank slightly above the venturi (half an inch would do) and it runs, then it's the venturi throat size. If you do this, watch out for fuel dribbling into the engine and causing it to flood -- the elevated tank leads to a steady flow of fuel, even when the engine isn't running. I'd only elevate the tank like this to verify what I need to fix, or if I wanted to say "look! it runs!" and then put it on the shelf.

 

[pat_pending]

Hi Tim/Scott, thanks both for the helpful replies. I have attached pictures of my setup and the venturi. When I put a finger over the intake and turn, fuel gets sucked in but then seems to move backwards again later in the cycle (watching the bubbles moving in the tube). Fuel definitely makes it into the engine but there seems to be some backpressure also?

 

[stackerjack]

Looking at your venturi, there doesn't seem to actually be much space for the air to enter. Are your dimensions correct I wonder. Are there any leaks where the carb/air intake fix to the crank case? Maybe try assembling them with PTFE thread tape, being careful not to get any tape in the air/fuel entry points.
Have you tried an electric starter on it? I feel pretty sure that if you applied an electric starter, and keep tapping the intake with your finger, as a previous reply suggested, you would get better results.

 

[xpylonracer]

Pat

If the hole in the spray bar is 1 of 2 and they are both that size I believe that could in part the problem, make them bigger and set the holes across the passage of air.

xpylonracer

 

[stackerjack]

I normally set the spray bar so that, if it has 1 hole, it points into the engine. If there are 2 holes, then they need to be at 90 degrees to the airflow.
Jack

 

[Tim Wescott]

The hole in the spraybar does look awfully small. And it's pointed into the incoming air, which is Not Good.

@stackerjack, if anything, less area in the venturi should mean more fuel draw and a slower top speed. I'd suggest getting the thing running first, then dinking with a bigger venturi.

 

[Tim Wescott]

I normally set the spray bar so that, if it has 1 hole, it points into the engine. If there are 2 holes, then they need to be at 90 degrees to the airflow.
Jack

If there's one hole and it's pointed straight into the intake, then it's in a region of high turbulence. This can lead to uneven fuel draw. Hence, my suggestion to offset it 30 degrees or so. Anywhere from 90 degrees to straight into the venturi should work, though.

 

[andrewh]

Fundamentally I am wondering whether converting the Chris Boll plans (BollAero 1.8 Gallery) from Imperial to Metric using the 'that's close enough' approach (particularly on drill sizes and fasteners) is the root of all my ills.

You can be fairly certain that this is no part of any problem. - it’s a side port engine and designed to be very non-critical.
The running you describe is an engine which has no fuel following up after the prime is burned. All the people who have responded have suggested sensible ways to diagnose and rectify this.
I look at your needle valve assembly and my top-of-the-head thought is that the air passage is very small (but that would only increase fuel suction and decrease max power) and the fuel hole is VERY small. perhaps that is the problem.
I have a tin of needle valves from various engines - they are pretty universal and interchangeable (make that non-critical for sports running). Could you show us the drawing you worked to and the drilling size you used for the fuel hole or holes. If I can find them (!) I could measure a typical cross-hole, and would guess from memory that it’s around 1/16 (1.5mm)
As people have said, a single fuel hole should be anywhere from downstream ( pointing into the engine) to 90 degrees to the airflow. If there are 2 holes; somewhere near 90 degrees. Fortunately not critical!

You are so close!

 

[andrewh]

Hi, can I call you pat_?
The Venturi size from the drawing should be, I think 5.5mm dia, and the spraybar diameter 4.76. So that explains the tiny air passage we see in your photo.
The fuel hole should be #61 drill, which I make 0.039 or 0.99 mm ( I don’t know where you are in the world, so I don’t know what measurements you use (tho you did say metric) so a 1 mm drill right through the spraybar is pretty much what you need, set at about 90 deg to the airflow
Did you #61 it right thru?

 

[Rodrigo Castellanos]

Hi Pat!

Seems you are having the same trouble as me...

today gave it another try with no success...
so after cleaning i decided to disassemble to remove all that gray oil and found and issue...
my cylinder is a little loose over the crankshaft, so i decided to tight the carb against the cylinder, pushing it, and seems to i get the real breath when turn the prop... i will try to find a suitable o-ring to put between the carb and the cylinder and see how it goes.

Cheers from Mexico
Rodrigo

 

[stackerjack]

Here is the drawing of the spray bar, plus dimensions.
Jack

 

[pat_pending]

I normally set the spray bar so that, if it has 1 hole, it points into the engine. If there are 2 holes, then they need to be at 90 degrees to the airflow.
Jack

Hi, the spray bar has two holes (It's drilled all the way through) and they are at 90 degrees to the air flow.

 

[pat_pending]

Fundamentally I am wondering whether converting the Chris Boll plans (BollAero 1.8 Gallery) from Imperial to Metric using the 'that's close enough' approach (particularly on drill sizes and fasteners) is the root of all my ills.

You can be fairly certain that this is no part of any problem. - it’s a side port engine and designed to be very non-critical.
The running you describe is an engine which has no fuel following up after the prime is burned. All the people who have responded have suggested sensible ways to diagnose and rectify this.
I look at your needle valve assembly and my top-of-the-head thought is that the air passage is very small (but that would only increase fuel suction and decrease max power) and the fuel hole is VERY small. perhaps that is the problem.
I have a tin of needle valves from various engines - they are pretty universal and interchangeable (make that non-critical for sports running). Could you show us the drawing you worked to and the drilling size you used for the fuel hole or holes. If I can find them (!) I could measure a typical cross-hole, and would guess from memory that it’s around 1/16 (1.5mm)
As people have said, a single fuel hole should be anywhere from downstream ( pointing into the engine) to 90 degrees to the airflow. If there are 2 holes; somewhere near 90 degrees. Fortunately not critical!

You are so close!

Thanks for the reply. Ill post more details of the carb later today when I can sneak away from my desk. I seem to recall the hole in the spray bar being 1mm but I will re-check. I have the Webster running on that carb so I know it works (but the Webster is much larger displacement so probably 'sucks' through the venturi a lot more).

Could there be any other leaks e.g between the bottom of the cylinder liner and the crank case be a cause of the fuel being pushed back down the fuel line that I mentioned above? i.e. when I turn the prop fuel gets sucked in but then seems to be being pushed back again? Could there be some other blowby effect to worry about?

Sorry for all the NOOB. Questions. This thing will run!!!!!

 

[pat_pending]

Here is the drawing of the spray bar, plus dimensions.
Jack
View attachment 119691

Thanks Stackerjack. Yes the hole is 1mm. The spray bar is 3.4mm and the venturi is 5.26mm.

 

[pat_pending]

Hi Pat!

Seems you are having the same trouble as me...

today gave it another try with no success...
so after cleaning i decided to disassemble to remove all that gray oil and found and issue...
my cylinder is a little loose over the crankshaft, so i decided to tight the carb against the cylinder, pushing it, and seems to i get the real breath when turn the prop... i will try to find a suitable o-ring to put between the carb and the cylinder and see how it goes.

Cheers from Mexico
Rodrigo

Good luck mate. Hopefully we'll get them running at the same time!

 

[pat_pending]

Hi, can I call you pat_?
The Venturi size from the drawing should be, I think 5.5mm dia, and the spraybar diameter 4.76. So that explains the tiny air passage we see in your photo.
The fuel hole should be #61 drill, which I make 0.039 or 0.99 mm ( I don’t know where you are in the world, so I don’t know what measurements you use (tho you did say metric) so a 1 mm drill right through the spraybar is pretty much what you need, set at about 90 deg to the airflow
Did you #61 it right thru?

Hi, yes the hole is 1mm. The spray bar is 3.4mm and the venturi is 5.25mm. I had the spray bar set at 90deg.

 

[pat_pending]

Thanks for all the replies above. One thing that came to mind going through the replies was this:

Could there be any other leaks e.g between the bottom of the cylinder liner and the crank case be a cause of the fuel being pushed back down the fuel line that I mentioned above? i.e. when I turn the prop fuel gets sucked in but then seems to be being pushed back again? Could there be some other blowby effect to worry about?

Thanks. Patrick