Another TD 20 idling question

[crosbyman]

to  load up the crankcase with fresh fuel  &  then feed  the cylinders  with that load of fuel vapors   on the downstrokes      the crankcase vacum  caused by the   upstrokes  must  be strong enough suck in fuel from the carb.

if air seeps in from bad crankcase seals ,  the ” lowered”  negative pressure needed  to suck fresh fuel  from the carb .   will draw (suck) less fuel  vapors  ….  less fuel =less energy to idle  properly.

see video

(1852) Finding a vacuum leak – YouTube

something to look into after  you check the easy stuff …

 

 

Joining AOMCI has priviledges 🙂

• This reply was modified 3 years ago by crosbyman.

[Bob Gansen]

I hate to bring this up. Is there a way to check crank case compression? Some of these motors were run at a gas oil ratio of 50:1, ruining the crankcase bearings and crank case compression would leak out, causing no idling. I so hope I’m wrong. This motor does not have crank case seals or roller or ball bearings.

Bob G

Robert S. Gansen

[Tom]

Yes.  With the cowl off, you should see it squirting into the hole in the Y-shaped casting.  If you don’t, remove what looks like a big screw in plug at the bottom of the carb and clean out the check valve.

T

[Tubs]

Check the fuel line to the low speed Y fitting for blockage.
Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[lloyd]

New rings alone will not solve a compression problem if the cylinders are also worn badly.  TD’s rarely have compression as bad as the numbers you listed because they ran on 16:1 fuel mix and the worn out original primer seals put the motors out of service long before the rings and cylinders wore out. It’s possible that someone ran the motor without sufficient oil and wore it out.  If you properly installed the new rings and LIGHTLY honed the cylinders, it is also possible that the compression will improve when the new rings get seated. But for sure until your compression is better it is not going to idle “good” or at all.

[slim60]

Thanks crosbyman for these informative videos. I’ll save them for spring. My Merc 450 needs a tune up. I’ll use a torch to check my TD 20 although it has low compression.

• This reply was modified 3 years ago by slim60.

[Fastwin18]

lloyd wrote:

New rings alone will not solve a compression problem if the cylinders are also worn badly.  TD’s rarely have compression as bad as the numbers you listed because they ran on 16:1 fuel mix and the worn out original primer seals put the motors out of service long before the rings and cylinders wore out. It’s possible that someone ran the motor without sufficient oil and wore it out.  If you properly installed the new rings and LIGHTLY honed the cylinders, it is also possible that the compression will improve when the new rings get seated. But for sure until your compression is better it is not going to idle “good” or at all.

I’m aware that there are several causes for low compression. Your post got me thinking though, because I did hone the cylinders lightly to get the glazing of. Once I had it running yesterday I kept it running for about half an hour, then let it cool down. This morning I retorqued the cylinder head bolts to specs and did a new compression test. The results are 74.5 for top and 77.6 for the bottom cylinder. I hope this rules out too low compression.

Surprisingly the bottom cylinder has higher compression now than the top. The previous 2 times I measured it, the bottom cylinder had the lowest compression, as one could expect.

[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 tried that this morning, and I could see a mist of fuel coming out of the hole. So it worked. Then the plot thickened.

For good measure I removed the low speed needle from the engine, cleaned it, blew out the hole with compressed air, and remounted it. Now the primer pump will not work anymore. It builds up pressure, I need to press it harder than earlier when it worked, but there is no longer fuel coming out of the hole. So there definitely seems to be a blockage somewhere, or I have made a mistake when remounting the low speed needle.

So right now my money is on the low speed fuel line (as several of you suggested). Especially since the compression seems to be ok. I’ll get back to it later today and keep you posted.

[aquasonic]

Try removing the low speed needle, and push the primer with the low speed needle removed. Sometimes this will clear an obstruction.

Improvise-Adapt-Overcome

1 user thanked author for this post.

Fastwin18

[Fastwin18]

aquasonic wrote:

Try removing the low speed needle, and push the primer with the low speed needle removed. Sometimes this will clear an obstruction.

Up early huh??

Thanks for your suggestion, will certainly try that this afternoon!

[Author]

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