spyder79

Hello Everyone!

I just stumbled into this thread while searching for something else, and was captivated by the subject and how it has unfolded, enough to read every post. I have a Tower .46 in a Sig Sealane and another in a Ace Seamaster. Both have been very reliable engines for me and have quite a bit of time on them. I have had a bit of the problem described, specifically the flat response of the high-speed needle. I believe it was due to some incorrect low-speed adjustment on my part, which I managed to sort out by trial and error. Overall, the Tower .46's have been powerful and reliable for me, right out of the box.

The evolution of this thread has prompted me to comment on a few things. I hope that maybe I'll be able to add something useful to the mix. First, I should mention that in the 3 years I've been flying RC, I've had little reason to mess around with these carbs, aside from careful adjustment and cleaning, so this thread really got me to thinking and got me to take time to really examine the carb in question. I've been turning wrenches for most of my life, from chainsaws to heavy equipment so I've spent my share of time tinkering with various types of carbs.

What I noticed right away is that there is a lot of similarity of operational theory to carbs used in some motorcycles, snowmobiles, and even those used on certain English cars. Many of these applications employ more sophistication, but they have a common attribute. I'm talking about the tapered-needle-in-the-jet arrangement. The thing that I find important here, and that nobody has directly stated so far, is the basic theory of this type of carb and how it applies to the subject of this thread.

The tapered needle concept is important because it continuously adjusts the fuel flow in proportion to the throttle opening, thus preserving a predetermined fuel/air ratio. At low throttle settings (idle to midrange), the clearance between the sides of the needle and the interior of the spray bar barrel determines the amount of fuel delivered. The taper of the needle allows more fuel to flow as the throttle barrel is opened and the needle is withdrawn from the spray bar. The air and fuel remain in proper proportion.

As the throttle is opened more the amount of fuel flow is greater. If not for the high-speed needle, the fuel flow would soon be excessive for the amount of air intake and the mixture would go drastically rich. But the high-speed screw works as a FUEL VOLUME LIMITER, so that when the throttle is open (WOT), the high-speed screw assumes full control over the mixture. This is assuming that the needle is fully withdrawn from the spray bar, or nearly so, so that the full amount of fuel passed by the HS needle is allowed to pass from the spray bar into the venturi. If the needle (which is being called the low-speed needle) is adjusted so it is still substantially inside the spray bar at WOT, it will be restricting the fuel flow and the high-speed needle will be ineffective, or nearly so. Opening the high-speed needle will have little or no effect because there is a downstream restriction, specifically the LS needle still blocking the spray bar. This accounts for the situation where the HS needle becomes fairly insensitive and has to be opened excessively to get a rich enough mixture at WOT (someone mentioned 5 to 7 turns!).

In a scenario where the LS needle is turned out excessively, allowing a lot of gap between the needle-end and spray bar-end, there will be plenty of fuel available at WOT providing that the HS needle is adjusted rich enough. The HS needle will retain the proper sensitivity since it will resume its proper role as a fuel limiter at WOT. The problem arises then at lower throttle settings because there will be excessive clearance between the sides of the tapered LS needle and the bore of the spray bar, establishing an overly rich mixture. This could also cause excessively rich mixtures at throttle positions somewhat lower than WOT because the needle will have left the barrel of the spray bar early. If the HS needle is then adjusted (mistakenly) by trial and error to achieve a decent transition, it will be running lean at WOT, possibly accounting for the flameout under load.

With these concepts in mind, it can be seen that the exact size and taper of the LS needle is carefully calibrated to deliver the proper amount of fuel at all throttle positions. But it should also be noted that LS needle position should be indexed in relation to the spray bar if is to perform as intended by the designers. Thus the measurement of the clearance between the LS needle and the spray bar at WOT. There is a little "wiggle room" in this measurement to achieve best tuning under varying conditions, but to stay within the original design parameters, it should stay fairly close, otherwise the transition between LS control at low throttle and HS control at high throttle openings will be compromised. After thinking about all this a bit, I remembered reading the owners manual for an old OS engine that lists a specific value range for this clearance. I would have to go through a bunch of paperwork to find it, but suffice to say that it exists.

With some of the other carbs I talked about earlier, particularly snowmobile carbs, the tapered needle is secured in the throttle slide with a C-clip that fits in a groove in the top of the needle. The needle contains up to 5 grooves to allow the needle to be indexed at varying heights to produce mixture adjustments for temperature and altitude. In many cases, other needle are available with different diameters and tapers to compensate for extreme conditions, or for engine modifications. As with the RC engines, making these alterations will affect the mixture and transition point from idle to WOT.

Back to RC engines and carbs, it can be seen that anything but the smallest adjustments of the LS needle will affect the idle mixture of course, but also the throttle transition point (between LS and HS needle operation), and possibly the WOT mixture (if the LS needle is leaned too much).

I think it is truly excellent that so many people have contributed to evaluating and solving this problem, coming at it from different directions and finding unique answers. I'm not trying to take sides with my observations, merely trying to fill in a few blanks. It seems to me that several people have danced on the edge of this in their posts, but I thought it was worth a bit of effort to explain the mechanics involved. Maybe it will help someone merely to understand their carb a little better. If you take exception to my comments, well...ROCK ON! Everyone is entitled to an opinion. Admittedly, most of my experience comes from other 2-stroke engines, as well as SU and Stromberg carbs, which are considerably more complex. Yet, it is easy to see that the same basic concepts have been used in the GMS and Tower carbs.

Regards to all,
Don