Another TD 20 idling question

[Fastwin18]

So I’ve fixed the leak along the exhaust cover.

It starts, runs (reasonably) smooth at high speed but simply won’t idle down. As soon as I move the throttle below the start position (see picture below) it starts sneezing and dies. Starting after that is difficult, unless throttle is move to much higher position.

Some background information:

• Motor has new rings and head gasket, compression of lower cylinder is 67 psi, upper cylinder 70 psi. Not very high numbers, I know, but it should idle down with compression like this right?
• Carb has been thoroughly cleaned
• Check valve works properly in one way
• Good spark on both coils
• Motor does not start very easy, but as said runs good at high speed
• High speed needle needs to be out 1 and a halve turn to run well, seems more than the proposed 3/4 turn
• Low speed needle is set to 5/8 turn out

My gut feeling says it’s starving of fuel, but I can’t quite put my finger on what’s wrong. Any suggestions would be appreciated

[frankr]

Is the carb butterfly properly closing at slow speed range?

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Fastwin18

[Fastwin18]

It closes when the carb is of the engine, not sure if it does when it is on.

[nabmd]

In my experience, the low speed needle is very sensitive to how open it is. I have mine set at 5/8 turn open from gently closed. Even 1/8 turn more or less made a big difference in idle performance. Other more knowledgeable people will chime in.

[lloyd]

67 and 70 PSI not good enough for good idle. Both cylinders should be over 70 psi for decent idle.  75 to 77 psi is ideal.

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Fastwin18

[Fastwin18]

Thanks lloyd, that’s what I was afraid of. You mention that it shouldn’t idle “good”. Mine doesn’t idle at all, I guess it dies at a third throttle. It needs to run a fairly high speed to keep running. Would that be consistent with these compression values?

[Tom]

When you prime it, can you see fuel spraying into the idle hole in the front of the motor?  (Is it getting any fuel to the idle circuit?)

T

[Fastwin18]

Tom wrote:

When you prime it, can you see fuel spraying into the idle hole in the front of the motor?  (Is it getting any fuel to the idle circuit?)

T

I can hear it squirting. Would that be visible through the front hole?

[aquasonic]

When everything is clean and working properly a small spray of fuel will spray from the port when you push the primer. Your high speed needle adjustment at 1 1/2 turns from gently seated is unusual. As you stated 3/4 to 5/8 is more of an average adjustment, but every carburetor/needle combination is unique. The sneezing is a sure sign of a lean condition. Have you tried richening the low speed needle? nadmd mentioned that the adjustments are very dependent on each other, so an adjustment to the high speed needle requires an adjustment to the low speed needle. Is the entire fuel system clean from the tank and tank filter, right down to the check valve and low speed fuel supply line?

Regarding the compression numbers, as LLoyd stated the 67-70 PSI is on the low side for this model. I owned a TD-20 that I put a ton of time into and could never get it to idle. The top end power wasn’t good either. That motor had 70 PSI on both cylinders. I finally blamed the problem on low crankcase compression from excessive wear. After a different powerhead was installed, that motor ran strong and idled down very nicely. The replacement powerhead had approximately 72-73 PSI compression.

It’s definitely worth looking into fuel supply/adjustment issues before concluding that the powerhead needs a replacement. Hope you figure it out.

 

Improvise-Adapt-Overcome

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Fastwin18

[Fastwin18]

That’s very helpful aquasonic.

I have been fiddling with the low speed needle but will try if richening it more will improve things.

The entire fuel system has been cleaned, but I’ll go over it again. I will also have another look at the head gasket. I was actually odisappointed by these low compression numbers after having replaced the piston rings and head gasket. Of course there can be numerous other reasons for that, but I want to rule out the gasket. The head has been lapped and is flat.

[Author]

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