venturi size

[bob whitney]

what is a good venturi size with a randy smith needle assbly. to start with in a Retro 60

[bob whitney]

a little help here please

[Steve Helmick]

Low rpm engine, big prop, I'd make a .270 and a .290 for a .157/4mm spraybar and see how it goes. Most .60's can use bigger than that, and it's easy enough to drill it out bigger or stack on some pantyhose nylon mesh. It'd run with 5/16" venturi bore on suction.

I've run an OS .25LA on a .270 with 4mm spraybar and it worked fine. I've also used a .312 with 4mm spraybar on a K&B .40 #4055 and it ran ok, but still didn't have enough power to suit me on a Humongus.  Steve

[GERALD WIMMER]

Interesting no answers, hmm I would say from a guess start small (6.5mm) for a soft break and go bigger till it runs away then come back.

Regards Gerald
Now I have started the ball rolling you can all give more educated answers....


  Ahh Steve bet me to it by seconds

[Lauri Malila]

Bob,

Same size than the original Venturi, spraybar diameter is same as Yuriy's.
The only improvement you can make is to replace the plastic venturi with a similar but aluminium one. With that, the case deforms less when you tighten the spraybar nut. L

[bob whitney]

Thanks guys that puts me right in the middle  Rad

[Lauri Malila]

As a rule, if you change any factory parameter of a Retro, it will very very rarely become better. Only exceptions are a better quality needle valve and sometimes some work with bearings. L

[Brett Buck]

As a rule, if you change any factory parameter of a Retro, it will very very rarely become better. Only exceptions are a better quality needle valve and sometimes some work with bearings. L

    Orestes had a useful improvement, but, as a general proposition, to have any hope of making any engine or airplane design better, you had darn well better be able to get it to work as well as possible stock first. And this is not intended as an insult to anyone or anything, but, to be entirely honest, it takes someone of the caliber of an Orestes Hernandez or similar to even know what needs to be changed, much less how to accomplish it.

    Bob is doing the right thing, asking how to get it back to equivalent of stock.

    Brett

[Lauri Malila]

    Orestes had a useful improvement, but, as a general proposition, to have any hope of making any engine or airplane design better, you had darn well better be able to get it to work as well as possible stock first. And this is not intended as an insult to anyone or anything, but, to be entirely honest, it takes someone of the caliber of an Orestes Hernandez or similar to even know what needs to be changed, much less how to accomplish it.

    Bob is doing the right thing, asking how to get it back to equivalent of stock.

    Brett

 Yes Brett, Orestes did some work on the muffler. It gives less carbon buildup and some extra power which is sometimes welcome, especially with the .60.
Then there is some work I learned from the great Franco Ballesio, mainly with bearings and some minor lapping to improve thermic stability. But those require special tools and skills, so maybe this is not the right place to go into details.
My point was, that as far as it goes with cr, Venturi, fuel, plug and prop, better stick with the original.
I may have an original Venturi somewhere. I'll try to find & measure it. L