2 stroke carburettor conversion?

Hi Folks,

I am rebuilding an old 2 stroke scooter and am in the process of converting a four stroke carburettor to a two stroke one. Basically I need to drill a hole into the (alloy) carb intake and fit a metal (brass) nippled tube as a feed for the two stroke oil. The old carb has a short pipe of approximately 3mm diameter with a nippled end and appears to be a friction (sealed) fit rather than being tapped. Any suggestions as to the best way to go about things (are there tubes available similar to this in modelmaking circles?).

Mike

Mike if its an old two stroke motor, wouldn't the oil be mixed with the petrol? so you can use the modern carb with bigger jets. just a thought. Alan.

It's a 1986 Yamaha engine and it has an oil pump which feeds directly to the carb intake. The pumped supply (from the oil tank) is metered by the throttle opening as it is linked to the accelerator cable. I was thinking of just pre-mixing the oil/petrol but I have been told that the pump needs to be completely removed to do this. The two stroke carbs (with the inlet pipe on the induction side) are like rocking horse manure to find these days so I was going to go with the readily available (new and ridiculously cheap) 4 stroke version and drill and fit a pipe to the inlet.

Mike

Can't believe it! Looking for a pic, I found just the thing on the bay!!!

http://www.ebay.co.uk/itm/121163423224?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I supose that I could have made one but by the time ............. etc!

Mike

Edited By Mike Teaman on 07/07/2014 20:41:42

If the pipe is just 3mm OD the tolerance for a press fit is going to be hard to achieve or quantify at home (2.992-2.996mm ?)

Personally I would put it in a jig under the drill press or mill, drill it to 2.9, then ream quarter (maybe half) to 3mm and press it all the way in using a 2.5mm drill bit or silver steel with a drill stop as a shoulder to do the pushing - this should keep it nice and straight - I might even pop the brass nipple on ice and I would use a dab of Locktite for sure.

Educated guess and keen motorcyclist, not a metallurgist.

Edit* I see it's 3.5mm but you get the theory hopefully, I would also be sure to remove any sharp edges off the nipple, and perhaps spin it up in the lathe or drill chuck and add a small chamfer to help wedge in, 0.5mm x 20° perhaps?

Edited By Russ B on 07/07/2014 20:59:24

Excellent advice. I was thinking along the same lines myself. I need tocheck if there is already a drilling. Often they were in the castings (for 2 stroke use) and capped off for the 4 strokes.

Until my new carb arrives, I can't check it.

More later,

Mike

Mike,

For future reference: eurocarb appears to be the genuine successor to the original UK importer of both Dellorto and Weber. ... Lots of useful parts and diagrams on the site.

MichaelG.

Edited By Michael Gilligan on 08/07/2014 07:11:22

Mike,

A word of warning about those press-fit pipes.

In the late '70s I bought a Reliant Scimitar Coupé [which used the Ford V6 Essex engine] ... When I first tried to insure it, the Broker panicked, because there had been some nasty fires. The Weber carb, which sits atop the manifold, in the vee; used a press-fit pipe for the [pumped] fuel inlet ... and some of them come loose !!

When I tugged gently on the flexible hose, the brass pipe pulled out.

... This could easily have saved my life ...

The standard fix was to drill and pin, but I actually used a small self-tapper in the carb body, locating into a dimple in the pipe ... all assembled with a smear of Loctite.

Yours is only passing oil, so it's probably not an issue, but do beware.

MichaelG.

Thanks again for that. I may change plans again (Oh Dear!) as I need to modify thebolted flange fitting on the inlet, to a tube fitting. I have enough space to fit the feed pipe to this steel tube instead of the carb (some two strokes feed directly into the reed valve cavity via a tube to the mounting plate). A friction fit with some circumference centre punching and Loctite should hopefully do the business.

Mike

As I'm replacing a VM22 (2S) with a VM22(4S), I shall compare them. Thanks for the info.

Mike

I compared the two throttle valves and they were identical. I fitted the inlet tube as suggested with slight chamfer/interference fit with loctite and a blob of araldite around the whole thing (externally obviously) for good measure and the whole thing is performing well. Many thanks for the input.

Much appreciated,

Mike

My experience of fitting a Mikuni two stroke carb onto a four stroke engine was that the emulsion tube design is quite different. This did not prevent me managing to tune it up to give quite reasonable results, but it was not trivial...I had to hacksaw off a piece of the emulsion tube that stuck up inside the intake partly surrounding the needle. I still had to be careful about opening the throttle too suddenly at low revs, but then it was a rather large bore carb for the size of engine so such things are to be expected.

John

Its a long time ago since I owned a SAAB 95 2 stroke and I put SAAB 2 stroke oil in each tankful. A later model had an oil pump to put the oil straight into the engine but my aquaintance found it was unreliable and he went for the oil in the petrol as the preceding models. It did not affect the engine in any way at all.

I bypassed that model and bought a petrol V 4 SAAB 96 which went like a bat out of hell. I always regret selling that car for a Ford Estate.

Clive

Being a child of the mid 80's I feel like I've missed out on so many elaborate engines, with almost every mass produced car engine being an L4, with the occasional L6 or V6 and a few deviations to the motorbike scene throughout the 90's - bar the sad loss of many 2 strokers =(

The only things that stand out in my memory are Volkswagen's 2.3 V5 (nothing really special) and the long standing but slightly fragile Wankel from Mazda. I have good memories of big 2 strokes as I fortunately grew up in the garage with my head between a Kawasaki H1 and a H2 (mums and dads). I know now Aprilia are closing the curtains on the final models in its 2-stroke range.

It all feels a bit sad and homogeneous and I'll be doing my absolute best to get my hands on a few special bikes during my lifetime and doing my bit to keep them alive! I'd like a rotary... but who wouldn't....... and a big 2 stoker along with a couple of classic race bikes, one 2, one 4 stoke.

I believe there has been a bit of stir about Norton producing a new Rotary engined motorbike - I'm not sure if they'd run in to emissions issues with the tip lubrication but I guess Mazda do ok!

.

Great car the V4 ... loved mine too.

FreeWheel >> Left-foot braking, and Gear-Change at the same time

Less regret at selling it ... because that's when I got the Scmitar Coupé.

MichaelG.

.

P.S. ... I wanted to put twin Webers on it, like this one, but the Insurance Company wouldn't even give me a quote !!

Edited By Michael Gilligan on 11/07/2014 15:51:33

You are spot on correct John. I have compared the emulsion tube! It seems that 4 stroke to two stroke conversion is OK so long as the drip feed oilway is used to the intake manifold (ie after air/fuel is mixed). The different design for the two stroke mixes some air with the fuel before it is released into the main carb airflow.

I have had a gentle rideout to test the setup but my air filter was a different size and in a slightly different position, so couldn't be fitted to the intake on the new carb. The mixture was therefore a little lean, which became more evident higher up the revs. New filter ordered, so awaiting the postie!

Mike

You lot do know the V4 in the SAABs was a Ford Taunus...

Neil

.

Yes

Hi Neil, Yes to that and I also believe it had a hollow crankshaft, the oil ways were tubes put through the cranks.

The brakes were also innovative as it was maybe the first tandem braking that i know of. Fuel economy was excellent at 44mpg. The brakes also had a swing caliper, it did not close the pads straight in but had a tilting action. maybe that helped braking.

Indeed the freewheel was great but you could not use it in slippery or snow conditions. It cost me £420 new in 1966.

Clive