Another carb check and new tank
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| One of this year vacation thing-to-do was to test
the hovercraft and make it run. |
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Carb had caused some issues during latest
operations and it was at some point removed and overhauled with new
service kit. Previously it was just cleaned and put back together with
silicone gasket and old gear. At some point during spring 2004 or 2005
(can't remember exatcly) tank developed a leak causing 40liters of
gasoline spilled on the floor of my craft. Tank wasn't that good anyway
so no harm there, but little worried about the long term effects of
gasoline exposure. It was laying there for max. of two weeks. Stink of
gasoline has gone away now.
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| Before actually starting with the new tank, i
wanted to make sure service kit had improved the carburettor. It was
gone through and checked so many times in past without finding anything
that i was quite pessimistic about it. As a temporary test i bolted in
the carb and all the fuel and vacuum lines and filled the cooling
system. As a fuel tank we used just a canister with hoses going in and
out so no longer trips or test were possible, but brief test could be
done. Also intake air filter was missing at this point. Old one
(allthough being washable variety) had suffered so much damage it could
not be used anymore and being special order item replacement wasn't
easy to obtain. Allthough lot of debris in the air small test turned
out to be succesful. Engine responded quite well and i could take rpms
up with playing with accel enrichment. Service kit seems to have
helped. Next step new fuel tank. |
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I was planning to use plastic universal outboard
type tank, but i have fuel gauge sender installed as well as i need
fuel return line. These plastic tanks have just one built-in connector.
I could live with smaller size by using two of them and switch them as
needed. But this would make needed extra connections even more
difficult. Being on vacation i had time to source supplies and put
together a sheetmetal style tank i originally had with few
enhancements. Original capacity was 120liters (~32gallons) but due to
location of filler line could not be filled much over 70-80liters. Also
it was a single chamber without splash plates. It was laying on the
floor and most important of all it was just carbon steel without any
painting at all. Fuel tank cap was located outside the craft so if
there was a spill when filling it would not go inside the craft.
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I wanted to lift the tank off the floor this time
and therefore it needed to be smaller to fit under rear seat. I did
weight my options between aluminium, stainless and carbon steel,
but decided to go with carbon steel for easier welding. I could do that
home where good aluminium weld or nice finish on stainless requires TIG
weld and experienced welder - much more costs than material itself.
Besides i didn't want to spend much money beacuse i could still not be
sure if my craft would work properly under normal operation
conditions. These plates are sold in 1000x2000mm sheets and i
wanted to minimize cutting. But due to limitations of folding tool used
i had to make a compromise. I was aiming for 80liters capacity,
but ended up with ~62liters. I purchased 1,5mm carbon steel plate,
cutted and folded it to form a box of 250x250x1000mm. Edges that were
to be welded wera also folded to easier welding of end plates. I also
made a smaller box: 45x150x150mm. This will be the pickup location for
fuel line. I made couple of splash plates to be placed inside the tank
to dampen fuel sloshing.
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It took me couple of days to weld everything together,
but welding time was few hours. One needs to weld small passes and let
things cool to prevent warping. I started by slowly welding the
longest seam, forming 250x250x1000mm frame. I then measured center
location and made a hole to bottom of the tank. Hole is placed so that
welded edge of the frame is pointing up (old engineering wisdom: best
weld is non-existing weld). Hole is as round as i could make with angle
grinder, perhaps 100mm in diameter. I hammered edges down a bit and
placed the smaller box on the bottom side of this hole. Analogy is that
when fuel moves in the tank some of it always finds it's way to lowest
point: the pickup tray. The way the hole and box are dimensioned causes
majority of it also trapped there even if craft is tilted. Pickup tube
is then placed lower in the tank. I don't remember having problems
because of this, but it should help especially when running with less
fuel.
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After pickup tray was in place i welded in the
splash plates. They are 150mm high and placed in middle of 250mm
total height. Thinking back they could have been almost full height and
only open 1-2cm at the bottom. There are two of them in the tank.
Again, didn't experinece this as a problems before, but it's good to
dampen sloshing to minimize pressure build-up. Next the fuel feed and
return lines and fuel gauge sender. This was the only part i used form
old tank. Previously fuel gauge sender unit was bolted to attachment
plate with five 5mm screws to threaded holes, but since it was easy to
modify i welded 6mm nut to take bigger bolts and more threads. I
initially overtorqued one of the 5mm bolts, since attachment plates was
only 3mm. Now i don't have to worry about that anymore. Since tank was
now smaller i shortened the pickup tube accordingly made a hole on the
top pf the tank and welded in the attachment plate. One of the reason i
choose 1,5mm plate over original 1,0mm was easier welding. With 1,5mm i
could use higher setting and have airtight seam at once, where with
1,0mm you have to watch burning through material much more carefully.
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By far
the biggest compromise was the filling cap location. Before it was
outside the craft, now it's inside on the top of the tank. Had i taken
one extra trip to hoverport to measure and douple check the hull i
could have done it the proper way and make a neck for placing filling
ouside. But since i wasn't sure craft is going to work and all
this had already taken couple of weeks already i didn't want to do
that. I wasn't too keen on having those additionl weld seams there
either, so i put an piece old motorcycle fuel tank with cap there.
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After everything was welded together it was time
to test. I thought of using soap water in the tank, but hesitated
because pickup tray system that would trap some of that in the tank. I
used compressed air and our yard pool instead. Blocking all the
connections and feeding compressed air into the tank i could spot two
leaks, and issues with filler cap. Not too bad compared to first tank -
using thicker material paid off. I patched those two spots and they
were OK in re-check. I tightened the cap connection a bit with smaller
hammer and it helper some. But that connection must have been
originally soldered to tank itself and welding it had burned that off.
I tried to re-solder, but could get it clean enough for soldering.
I adjusted the welding machine bit and welded it around which
seemed to have done the trick. Problem solved. There is still some leak
from the cap, but tank never sees 5bar pressure as with
compressed air and replacement air has to go in somewhere. Next step is
to have some sort of surface treatment.
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Hot dip galvanizing was out of the
question because of pickup tray and being sheetmetal box. Preferred
method was to have it galvanised, but their pricing was just too much
for this even if i were sure everything works after i install new tank
not mention i still wasn't sure about that. Powder coating, not perhaps
the best way, but most cost effective at this time. Could do five
powder coated tanks with the price of 1 galvanised. Unfortunately
(or not?) their schedule delayd the project more. As it happens, i was
to pick up the painted tank next day i realized (while in the sauna)
that i eventually had forgotten to weld if legs to raise the tank off
the floor. Too bad, but i thought i could use some wooden blocks there
or add them and patch the painting. I called them the next day to ask
when i could pick it up. They promised to do the same day because they
had not done it yet. I did not realize it rigth away either we just
agreed when it was ready to be picked up. But as soon as i hang up the
phone i realized that this is the good chance to add the legs. I called
them back and had my father pick up the tank on his way home from work.
This was also better for them since the reason it was not painted yet
was difficulty to have i hanging due to lack of suitable holes. This
happened on friday so later same weekend added some flat bar to raise
the whole thing 30mm higher. Also put some holes to these legs to bolt
wooden blocks to raise it further. I left the the tank to powder
coating shop next Monday and it was ready on Wednesday.
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