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Engine speed fluctuates at high speed

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Old 01-22-2002, 01:37 AM
  #1
AC2
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Default Engine speed fluctuates at high speed

The transtion from low to high is good but when it reaches full throtle the speed fluctuates and does not hold a steady speed.

Fuel is getting to the carb and is steady. There are no air bubbles visible in the fuel line. Could it be the gas mixture is wrong? Carb is a Walbro and the engine is on the bench.
(Engine 2.3)

Any suggestions?

Thanks,
Aron
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Old 01-22-2002, 02:06 AM
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Default Engine speed fluctuates at high speed

Aron,
What is the engine?
Mike
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Old 01-22-2002, 02:16 AM
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Default engine speed

Aron,
What kind of engine?
What kind of prop?
What kind of igntion?
The last two questions can cause alot of problems.
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Old 01-22-2002, 05:31 AM
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Default Thanks Guys

I'm not sure what engine this is. It looks like a older G-38 but I have been told that it is not. (see comments below)

The carb that was on it was a single needle valve Walbro. The needle was for the low setting only. I could never get it to run at full speed because it would lean out so I had to keep from opening the throttle all the way. To solve this I put on a Walbro that is the same size but it has the low and high needle valve setting.

I noticed the needle valves are very sensitive and got it to run from low to high without hesitation but at full throttle, as stated the speed fluctuates.

I might also mention that the carb was used and I do not know the history of it. (I do have a rebuild kit for it)

The prop is a 18x6-10 and it has the CD ignition, spring starter.

The good thing is that now at least I can open the throttle all the way but the speed should be constant which it is not.

Good idle, transition.
Other than the mentioned problem it starts quick and runs good.

Thanks!!


Here are some comments made by a couple of Gentlemen who have seen a pic of the engine (I wish I knew how to attach pics on this site):

"The engine looks to be an Alpine converted chainsaw engine, either Model 330 or Model 380. Both had Walboro carbs with only a low speed needle adjustment and look very similar to what you have. That may not be correct since there are a lot of engines out there that look very similar to what you have.
Quadra looks similar as do a number of other brands. The crankcase design, side induction, rear exhaust, and the counterweight on the front of the crankshaft behind the flywheel helps identify the engine somewhat, but again, a lot of manufactures used the same design.The Model 330 was 1.9 ci (32 cc) displacement and the Model 380 was 2.3 ci
(38cc) displacement. You may have the Model 380, bore is 39.7mm (1.56"),
stroke is 30.4mm (1.2"). "

"It's not a G38, old style or new. What it very well may be is a 'Remington'? 41. I've seen one of these engines. same mount and starter config. I was told that Dick Bennett of B&B (http://bennettbuilt.com/index.html) got ahold of a production lot of these engines, originally intended as chainsaw/weedwhacker engines. He converted them for RC and sold them. One of
my club members said he was selling them for a good price at the Joe Nall meet a couple of years ago."
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Old 01-23-2002, 04:14 PM
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Default Engine speed fluctuates at high speed

The carb could be the problem. Walburo came out with a carb that had two high end jet ports inside the carb. I also have one on my G-38 and a local friend and chainsaw repair person told me that people used to burn-up chain saws when they would go into the mountains and cut wood, running them too lean. The manufacturer solved the problem by adding a safty port inside the carbs to prevent this. My buddy solved my problem by using a punch to close off the extra port. Any chainsaw mechanic should know how to do this. Another problem could be the timing. All mine are set at about 30 degrees BTDC. Is your engine in a cowl? The propwash could be presurising the cowl and also causing the problem.
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Old 01-24-2002, 01:31 AM
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The engine is on the bench and the carb says H L for the needle valves. I don't know how to check the timing but the engine starts up just fine. Would the timing affect the top end in this manner?
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Old 01-24-2002, 01:41 AM
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Default carburetor

There is a small hole in the cover of the carb..It's designed to let air at atmospheric pressure act on the diaphragm. If air is forced into the hole by the prop, it could cause richness and erratic running..It mostly happens in the air in a long dive, where the extra speed causes more air pressure on the diaphragm..Try holding a deflector in front of the hole at wide open throttle...
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Old 01-24-2002, 02:51 PM
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Default Engine speed fluctuates at high speed

Yes the timing can effect the top end only but is unlikly if you are not using electronic ignition. I have a FPE 4.2 that had the same problem at top end. It would start missing at full speed. Missing sounds different than four cycling because of being too rich. Does yours sound too rich or does it sound like it is missfiring at full speed? The carb's jetting is the other problem area.
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Old 01-25-2002, 01:39 AM
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Default Thank guys

I will check the carb tonight. It does not sound like it is missing. I wonder if the diaphram could be dry and need replacing?
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Old 01-25-2002, 02:01 AM
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Default Carb

If the diaphragm feels "crinkeley" it's too hard..
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Old 01-25-2002, 02:21 AM
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Default carb

yeah, it looks "crinckly" I have the carb kit and I am going to go through the carb. Any tips? Will probably just do a parts exchanage with all the parts in the carb.
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Old 01-25-2002, 02:42 AM
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Default Engine speed fluctuates at high speed

The diaphragm with the metal center goes on TOP of the gasket
The other diaphragm, on the other side, goes UNDER the gasket..
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Old 01-25-2002, 04:31 AM
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Default Engine speed fluctuates at high speed

Too much oil in the gas will cause pulsing at high speed Also.
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Old 01-25-2002, 05:58 AM
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Default mucho appreciato muchachos

rcign, Thanks! I noticed that. Got the carb all done and will check in the morning. I boiled it in soap and hot water to make sure every nook and cranny was clean once I took all the parts out.

But.....I think rcrmel has a good point. Here's what I did do: You know that homelite mixture they sale at the hardware store that you mix with a gallon of gas? i put two of those in my gas thinking it would help lubricate the engine better. (duh!)

I have another question: What should I set the needle valves at?
I think I read somewhere to screw them in all the way and back them out 2 turns as a starting point?
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Old 01-25-2002, 07:03 AM
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Default Engine speed fluctuates at high speed

One of my friends mixes 1 quart oil to 4 quarts gas..Runs fine.
Two turns out on both is a good place to start....RC.........
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Old 01-25-2002, 04:30 PM
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Default What???

One of my friends mixes 1 quart oil to 4 quarts gas..Runs fine.
Two turns out on both is a good place to start....RC.........


Wayyyyyy to much oil. Don't try that!

Try something like 32 to 1 for a new engine, or 50 to 1 on a broken-in engine.

Cheers!

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Old 01-25-2002, 05:56 PM
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Default Engine speed fluctuates at high speed

I don't use that much oil, either..BUT..My Zenoah 445 ran 7000 rpm on his mix, 6800 on my 32-1 mix, same prop, 5 minutes apart..24-10 Zinger, 6500 feet, 60 degrees temp...To each his own....
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Old 01-25-2002, 07:05 PM
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Default carb

Now the carb will not draw fuel??
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