OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Victa Imperial 2-stroke Engine

I have a technical question(s) that I hope ODK engineers and members
can help with. I am writing a draft for the Victa Imperial lawnmower.

It's about the Victa engine that powered these machines from 1972.

Context:
For whatever reason, Victa introduced this machine in 1967 and powered
it with a Kirby four-stroke. No two-stroke version.

It would take another 5 years before Victa finally powered their Imperials
with their own Victa 2-stroke. This was a very special version of the Series 70
Mark 2, with the then standard decompressor, but with two unique features.

(1) The carburettor was a modified G3 - making it ungoverned.
(2) A cylinder head spacer was added.

My questions relate to the head spacer. Why was it added?
- Given that the governing of the machine was via human being, did the spacer
make the engine more responsive to hand throttle?

I do not have the technical expertise to answer this question.

Cheers
-------------------------
JACK.

A spacer between the top of the cylinder and the underside of the cylinder head is used to reduce compression ratio - on cars it used to be called a "compression plate". In the case of cars, there were at least two reasons I know of for it being done. In the case of the Ford "Y-block" V8, it was done on engines fitted to Australian-assembled cars because our fuel back then was lower octane than that required by the engine as normally built. The other reason I know of was to reduce power, because some driveline component wasn't up to the job otherwise. I won't give an example of that because it implies a criticism of the vehicle involved.

In the case of a reel lawnmower, I can only make pure guesses as to why it would have been done. One possibility is that the mower, which may have been based on a design centred on a smaller Villiers engine, might have been considered over-powered with an engine designed for a much less efficient rotary mower. Another is that the spectacularly poor-running Victa 2 stroke benefitted from weaker power pulses which might have de-emphasised the fact that they occurred so irregularly.

The victa series 70 was a very good running engine, shame the imperial was rubbish. Even though official power fidguers were never published I would estimate from experience the 125cc in stock form was 4.5hp or thereabout, a lot more than required on a 14" and 16" reel mower.

The engines in the imperials are great, the only people who have gripes with them either lack maintinance or have a problem with the company in general, unfortunately the lack of a slippable clutch and very fast drive ratio made it a horrible machine to use. I have 2 of them, they have been in disuse since I aquired four scott bonnar 45's

Hello ODK Engineer Grumpy & MrMckay,

Thanks for the input, and I believe some progress is being made.

Grumpy, your first guess is less convincing in that the Imperial was powered
for years with a Kirby 4-stroke, and that they were made in sizes up to 24 inch,
and with an optional trailing seat. This was a chassis designed to be up-scaled
and take more horse power.

Your second guess does make a lot of sense to me; in that the Imperial did not
have a heavy blade holder and the operation of the centrifugal clutch may have
required weaker power pulses for smooth operation at idle and clutch engaging speeds.

Mr Mckay, some great and helpful points there.
Could I just clarify your reasons for saying the Imperial was 'rubbish'.
From my understanding, you are saying this, not on the basis of build quality, but
on the basis of an unforgiving on/off clutch and too fast a landroll speed. Is that right?

I appreciate the input.
-----------------------------------
JACK.

Thanks Jack, I hadn't known that the Imperial was made in 24" towed-seat configuration. A key consideration with centrifugal-clutch mowers is getting a stable idle speed, which is a challenge for an un-governed 2 stroke of simple design. A heavy flywheel, long stroke, and low compression might be a good combination to try when that was the objective. Reduced compression is by far the easiest of those to achieve. Do you know whether that engine had a special heavy flywheel as well?

Hi to Grumpy and CyberJack,
I have a feeling that the Imperial may have been fitted with what was called the Victa Power Unit....available in 125cc and 160 cc Series 70 engines....See the brochure that follows....

By the look of the parts list, the centrifugal clutch centre was mounted on an 'auxiliary flywheel', Part 98 in the exploded diagram.

Yep, the auxiliary flywheel was needed to hold the clutch shoes.
I don't know how heavy it was, but there would have been some flywheel effect.
I'm still just asking the question - why the spacer was needed.

Deejay, not really.
No Victa Power Unit had the special low compression spacer.
That appears to be unique to the Imperial lawnmower.
Victa Power Units were really destined for non-Victa applications.

Cheers
------------------------------
JACK

Here's a photo courtesy of Gizmo:-

I think Grumpy was on the money with the smoothness argument.
The 1973 catalogue seems to say the most (that I can find).
Interestingly, they recommend BP Outboard Zoom (50:1).

- runs smoother
- runs cooler
- longer life
- needs less oil

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=62024#Post62027

All very interesting.
-----------------------------------
JACK.

Yes, I'd agree. Power, which was not needed for the task at hand, was sacrificed for smoothness.

Some interesting 'selling points' in that lot!

Availability of BP Zoom 50 would have been a bit questionable through mower shops, though. AFAIK it was usually [but not always] a bulk product, sold 'at the pump', by boat marinas and a very few BP service stations. Whereas Zoom 25 was usually mixed from concentrate, at the point/time of delivery from BP, in mower shops.

It may be of historical interest to describe how this worked, so here goes.
Our shop kept a 44 gallon tap drum of BP Zoom 25, on a wheeled stand. We also had a 12 gallon drum of Zoom Concentrate on hand. When the 44 was getting low, we would use a BP-supplied aluminium dipstick to measure the remaining contents, and calculate the required top-up volume [IIRC the volumes of Concentrate and petrol were read directly from the dipstick markings]. Then it was a matter of phoning the local BP distributor, to place the order.

When the fuel tanker arrived, we would pour the appropriate volume of Zoom Concentrate into the 44, then wheel it outdoors. The tanker driver would set his discharge meter to deliver the corresponding quantity of Standard grade petrol, and pump this into the 44.

After 40 years, I can't recall the exact mix ratio used; but it was NOT 1:25, rather it was a lesser proportion of petrol. That is, the Zoom Concentrate was pre-diluted to some extent. I'd say this was to ensure that pumping the petrol in on top of the concentrate, provided sufficient agitation to achieve thorough mixing.

i have 2 of these. i remember running one of them when i first got it and still remember how fast it was it scooting around.a bit too quick for me. my second one which ive never tried to start looks as though it may have been modified as it has an extra cog and chain to reduce the gearing somewhat. pretty sure its a home mod and not standard. (i could be wrong though?)

Hello Contributors,

Gadge, that's great information about BP Zoom. It will probably find
itself in the History Forum with a BP Zoom brochure at some point.
It's good historical stuff. Thank you.

-------------------------------------------------

Gizmo seems in agreement with MrMckay about the Imperial:
that it's clutch was not amenable to slippage, and the ground-speed was
too fast. I think that's sort-off right.

I know this topic needs to be flushed out and expanded, but I want to
articulate the basis for what I think about this in draft here.

Ground speed - These high ground speeds seem to be part of the
design philosophy dating back to 1902. Historically, the advertising
suggests that it was a boast and a feature to have a quick speed.

Why?: the two historical reasons were labour costs and fuel costs.
Power mowers were beyond the reach of most folks early 20th Century.
If you had a power mower, the servant would operate it.
As an example, Atco advertising for their Standard machines would
boast how much fuel would be used for X area of lawn, and how much
lawn area could be mowed in Y amount of time. It appears to me that
the argument was never primarily about operator comfort.

Given that most power reel mowers arriving here were of British origin, we inherited
that design concept in Australia, even though we had a less-rigid class system.

Clutches - The Imperial's primary clutch is a small centrifugal one.
The small shoes require significant RPM to engage the drive. This exacerbated
the transmission ratios. Other machines - like the Ransomes - had much larger
centrifugal clutches that were engaged at lower RPMs.

The second issue is the landroll clutch itself. My argument here is that
lawnmower designers followed the design philosophy from automotive practice -
One 'let the clutch out' to engage drive. Slipping the clutch was frowned
upon, because the mechanism would 'burn out'.

The Imperial's plate clutch has a pressure plate using 3 strong springs.
This inhibits sensitive feeling of the clutch hand control. The operator is
encouraged to just use the clutch as an on-off switch. Ridiculous!
The Imperial, again, exacerbates the problem by the un-ergonomic position
of the lever and the rotary-style handlebars. The first Imperials had a
much better handle bar of Ransome's design.

I am trying to solve a riddle about who first thought of reversing the
notion of 'letting the clutch out' to 'letting the clutch in' on lawnmowers.
This has made all the difference to operator comfort on machines like
Scott Bonnar's 45. That 'hair-trigger' clutch still uses plates (like
the Imperial) but the clutch is 'let in' by sensitive hand inputs. It's
variable speed clutch will happily slip all day. What company first used
the hair-trigger clutch? Most likely not Scott Bonnar.

In summary, it's not transmission ratios that make the Imperial a bit awkward
for some. Although, Personally, I love reel mowers where I can use just enough RPM
to do the job at a leisurely pace. A quick pace is not good for me.

The Imperial uses a combination of two types of clutches that, through their
particular designs, take away from the enjoyment of turning grass into lawn.

All very interesting.
-----------------------------------
JACK.

A highly insightful article on the Victa Imperial 2-stroke here: -
https://www.outdoorking-forum.com.au/forum/u...9/re-various-victa-brochures-i-have.html

A History Record is now available for the Victa Imperial: -
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=72511


This topic is now Closed and Locked.