Interesting Saito Problem Solution

I've been using the Saito 4 strokes for the last 3 years with the Beringer designed Sportster and Sukhoi models. I used the 56 with the Sportster and then switched to the Sukhoi last season. I loved the way the 4 stroke ran. This years new and slightly modified Sukhoi was build around the Saito 72. At the start, the engine ran ok, but I sure wasn't impressed with the quality of the run. The engine was set up with the UHP venturi set-up and started out with the #2 venturi which is .275". I also used the clunk tank suggested by Brad Walker. The engine just seemed off at times and you could hear a slight wavering in it's output. I tried all sorts of props, plugs, venturis, fuel and even changed back to a metal uniflow tank. Nothing seemed to make it any better. About this time I was talking to Paul Walker and the subject of timing came up. He too was experiencing some difficulty with his #2 engine. After a complete tear down of the engine it was discovered that the timing gear was off by 1 cog causing the valves to open or close (I don't remember which) 7.5 degrees from where they should. Putting the engine back on the test stand did seem like I'd cured the run problems but once it was back in the plane it still didn't seem right. I still could sense and hear a wavering in the power output of the engine. With the season just about gone for me I started in a desperate attempt to solve the problem. Thinking that there was some sort of strange harmonics being set up by the model I decided to put a 56 into the plane to test out the model/fuel system combination. It ran like a top and showed me the wisdom of chosing the 72 for the larger Sukhoi as opposed to the Sportster. About to give, up I decided to give it one more college try and wondered if there was anything else I could change. Last Thursday I put the metal uniflow tank back into the model and removed the UHP venturi from the engine. I had cleaned the spray bar and needle assy. numerous times but this time I decided to take it out and redrill the venturi assembly for a ST Large NVA like I used in my 60's. Friday morning I met Keith Varley at the field and from the first instant I knew the problems with the 72 had been fixed, The engine ran perfectly from start to finish every flight, Now after 4 months of fighting the thing, I can finally get to some serious flight trimming.
I've inspected the OS NVA that came with the UHP venturi and connot fine anything out of the ordinary. Any ideas?

No idea what may be wrong with the NVA, but were I you I would be wondering if the timing is really right or if 7.5 degrees of really matters that much.

Alan,

So you went from an OS NVA to an ST?

If so, the ST NVA is a smaller diameter (if I remember right) than the OS giving you proportionately more choke area. In effect, you went to a larger venturi. It also may be that the ST, for whatever reason, has better flow than the OS you have. A burr or something?

The new NVA had a larger diameter than the OS. It might be a DS copy. I went up in venturi size to compensate. Can't find anything wrong with the OS assy.

I have oftne wondered how the ST needle would work. I think it is something I might try on mine too. It just seems wimpy at times.

Something to think about.

From reading threads about 4 strokes in the past, I get the impression that the fuel draw is marginal. Since 4 strokes make less power than 2 strokes run flat out, the 4 strokes used in Stunt are flown more on the ragged edge of maximum power, using wide open venturis. By contrast, 2 strokes make so much power, they're slowed down deliberately, allowing the use of small venturis (with much more fuel draw). The fact that 4 strokes only suck for 180 degrees every 720 degrees (a simplification here, of course) might have something to do with it (a 2 stroke sucks 180 degrees every 360 degrees).

Hence the experiments with low viscosity oils, etc. to make the best use of the limited fuel draw. I'm thinking a pump might be better, or even the use of a plenum chamber under the venturi.

Going to the larger needle valve has two possible effects. First, it chokes down the venturi area, increasing fuel draw. Second, if the diameter inside is larger, it makes for less flow resistance for the fuel.

>draw. Second, if the diameter inside is larger, it makes for
>less flow resistance for the fuel.

This may be the real answer, as he said he opened up the venturi to compensate for the larger needle. So what if we plumed the tank with the next size tubing up from 1/8, used large fuel line and a larger needle. Wouldn't this decrease the fuel flow resistance?

I don't understand why people don't realize that it is possible to run a 4 stroke rich by opening the needle up? The needle is the restriction, not the tubing. I still doubt that fuel viscosity of the fuel has much to do with anything (otherwise it would be impossible to run a 4 stroke in cold weather when the fuel viscosity is higher than in hot weather).

>
>>draw. Second, if the diameter inside is larger, it makes for
>>less flow resistance for the fuel.
>
>This may be the real answer, as he said he opened up the
>venturi to compensate for the larger needle. So what if we
>plumed the tank with the next size tubing up from 1/8, used
>large fuel line and a larger needle. Wouldn't this decrease
>the fuel flow resistance?

Brian Eather and Joe Ortiz from Aussieland have already done that and there is a significant improvement in performance (in their opinion). I do not think they have the thinner fuels available to them.

I have some 5-32" fuel tubing I just bought the other day and a large clunk. Doug has an Eather tank with the large clunk that we have never tried.

What some people fail to realize is that the 4 cycle does run signifacantly richer at high loads and low RPMs. Even if the venturi is smaller. The fuel consumption with a 14" Zinger with 5" of pitch will be much higher than say a 12" 3.75, even when the latter has a larger venturi. The 4 strokes just draw fuel better at richer settings (loaded). This is not to say the engine is set "rich", it can be set at peak in fact, but the engine will be RICHER due to the lower RPM and load.

This is no different than a 2 stroke.

The only thing I have notcied is that fuel draw becomes more of an issue when run up on a higher RPM. When a Saito 72 is run under 9000 RPM it draws fuel very well, and is very forgiving. Not so true when trying to run a higher RPM setup.

All in all, there are goods and bads to both setups.

Why Alan's engine would not run on clunk tank is a mystery to me. My guess is that it had nothing to do with the tank or the needle, but instead had to do with matching the prop to the venturi.

Also, anyone who says that thinner fuels do not make a difference in 4 strokes is completely and utterly misinformed.

Bob, it would also decrease the overall pressure. Probably not a good thing.

I do like Isky's idea of a plenum chamber "under" the venturi. It may be that increasing the volume just under the venturi would have a possible effect.

Hmm...

I just use a fuel tank designed to keep the head pressure up. Seems to work, but I'm always looking for ideas to maintain or increase the overall fuel pressue in the system.

'zilla,

Now I'm convinced. You are, in fact, the secret, evil twin brother of the Dirt. AH HA!!

>'zilla,
>
>Now I'm convinced. You are, in fact, the secret, evil twin
>brother of the Dirt. AH HA!!

I thought we were friends...

I think it was the needle. I have often thought of trying the ST needle because of the finer adjustment you can get from it. I have also heard it has a more gradual slope on the tapper. I know it helps the ole FPs like no tomorow.

He already tried all the props and venturis. Why would it not work the OS and then work with ST? He said he matched the venturi to the ST so it would be the same as before.

Why would you think it was not needle?

Alan,

This is my second year of running the Saito 72 with UHP venturi set-up. My engines have always run dead consistently on the UHP venturi set-up but a few weeks ago before leaving for Muncie I began to experience inconsistent running. After trying different plugs, fuel & tanks all to no avail, I finally discovered the problem was in the OS needle valve assembly. I changed to a brand new replacement OS needle valve & spraybar assembly and instantly the inconsistent running disappeared. The problem was due to wear in the needle thread allowing the knurled body of the needle to develop sideways play and allowing the tip of the needle to move around inside the spraybar. If you can feel any sideways movement or rocking of the knurled body with your fingers then it's time to change the needle valve.

There seems to be differences in the thread tightness of new OS needle valve assemblies. The one I'm currently using had a very tight thread while another new one I have is no where near as tight. Regardless of how good the thread tighness is when new, the thread wears with use. I also found that changing venturis too often makes the thread wear even quicker. In the near future I plan on changing all my OS needle valve assemblies for ST types as you have done.

Regards,
Joe Parisi

In the near future I plan on changing all my OS
>needle valve assemblies for ST types as you have done.
>
>Regards,
>Joe Parisi

Hey Joe,

Somebody please recommend a better quality needle than the OS max and we will switch. I simply do not think there is a better one on the market for overall quality control and consistency.

I had considered recommending to Steve that we make the the next generation of manifolds for a sprinkler type needle.