RCU Forums - Idle Issue

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- - Idle Issue (https://www.rcuniverse.com/forum/giant-scale-aircraft-general-467/11652532-idle-issue.html)

I have a new DLE-35 installed in a giant scale Taylorcraft on floats. I have run 3 tanks of fuel through the engine to help with the break in. I haven't maidened it yet until I get all the smaller issues with the set up dealt with. After I was satisfied with everything I reattatched the cowel and prop. The last thing I checked was the high and low rpms. Well the owners manual says the low end idle speed should be 1,600! At that speed I would have great difficulty controlling it when taxying out for take off, never mind the danger in starting it with such a high rpm on water! So I tuned it down using my radio to 400 rpms. Later I can tune it up depending on how it reacts in the water. I know low rpms should not be run for a long time because of plug fouling but control of it during slow water manuvering is important. The manual says that the top end rpms are 8,500!! The problem is the digital tach mounted on the fuse tells me top rpms are only approx 950. I used a hand held tach to check the numbers again and they were the same. At full throtle on the ground, at 950 that engine pulls very hard ( I tied the fuse to a tree for safety) and the prop wash leaves me no doubt that it will lift off the water with no problem. Still, the reserve rpms 7.550 seem to be hiding somewhere. I can't imagine using full throttle, it might pull the wings off!! Still,emergency power is critical in some situations and I don't know how to adjust it here. All throws are correct and I am using fresh fuel 91 octane with a mix of 30:1. I have invested too much time and money to leave anything to chance. I'm not new to flying RC but I am new to gas engines and giant scale power so any help you can offer here would be greatly appreciated.

Your numbers are quite confusing. My first guess is that your idle is actually 4,000 rpm and your full throttle is 9,500 rpm. Even the 150cc and 170cc engines I have owned were not capable of a 400 rpm idle and an idle that low would be even less likely with smaller engines. It may help to know what prop size you are running. The 91 octane gas is also not nesessary and has zero benifit over 87 octane unless it is a pure gas that has no alcohol or smelly additives. Our gas engines just do not have a high enough compression ratio or aggressive timing curve that would make high octane nesessary.

What diameter and pitch prop are you using?

As noted above:
1. You are mis-reading your tach. It is clipping off the final zero, so 400 is really 4000, etc. Make sure, if it has a setting for number of blades that you do have it set correctly for your prop. Usually 2 blades. Your idle needs to come way, way down.
2. If all you ever do is run a very low idle, you might see the problems with plug fouling, but for all normal RC operation, including long taxi's on water, it will not be an issue, ever..
3. If your top end is 950 (ie 9,500) you very likely have too small of a prop. As asked, what diameter and pitch are you using?