compression psi

E

emmit

Junior Member
What is a decent psi number to look for when testing a model a engine? I just brought out a 29 aa from a 45 year sleep and have gone through the basics of it, all seems well. Cleaned carb. Oil up the cylds. New wiring, timing gear, plugs, wires, dist., coil, cleaned fuel tank all new oils, checked over starter, gen. And all else i could think of. It starts and will idle( rough) but acts as if shes not getting fuel, it spits it back out the carb. Timing seems o.k., not backfiring. But advancing the timing on the steering colum makes it quit. Any ides. Iwant to check compression, but need to make an adapter to fit the gauge i have first. What numbers should i look for? Thanks
 
most stock motor will have around 55# compression . i would run it , drive it , for a few days then check compression . gives it time to get the rings loose & reseated . i just use the compression guage with the big rubber nose .
...................... steve
 
I'm going to stick my neck out and say this. Compression ratio X Atmospheric pressure. I was told that a lot more goes into it than that, but it's a good place to start. So it's like this;
Atmosperic pressure + 14lbs Depending on your altitude, but we're being rough here.
Comp. Ratio 4.2 So it's 4.2 X 14 equals around 60 lbs ( Yeah, I know my math is sloppy) Considering all the variables, like worn rings and sticky valves you'd be lucky to have 60 lbs in all 4 cyls.
If it seems like fuel, it probably is. Disconnect the fuel line at the carb. and see if it flows, not dribbles.
Terry
 
regarding the engine quitting when you advance the spark...check the pigtail wire under the top plate in the distributor. Its likely its shorting out against the side of the distributor as you advance it.
 
I second the advise about checking for shorts in the distrib. You also said "timing SEEMS fine" which means to me that you did not go through the (easy) proper timing procedure to ensure it is ok. Use Marco's timing to do it properly. Google "Model a timing" and look for the website from A Barnyard.

Here is the link:
http://www.abarnyard.com/workshop/timing.htm

Do the easy free things before anything else
 
Re. compression, It's most important that all cylinders have the same, or very close to the same, compression. A low one or two indicates leaking valves, a leaky head gasket, or worse. I've had Model As with 50# or less, even in all cylinders, that ran great.
 
Elrod has a good point, i did not go through the dist. Yet, but will order some goodies for it today. She will only let me get to 1/4 throttle before she dies out. Went through the carb. Twice, its fine. Did notice i lost all spark yesterday, cleaned the points and she took off again. Running the same but spark was back. Its been timed twice so far and its on. Hopefully i'll get things sorted out when i get the dist. Redone. THE SPARK ADVANCE DOES NOTHING BUT MAKES HER DIE OUT?? WEIRD
Thanks
 
I still say it's the timing (just like mine) It was just the play in the distributer that my timing was off by.
 
THE SPARK ADVANCE DOES NOTHING BUT MAKES HER DIE OUT?? WEIRD Thanks[/QUOTE said:
again, check that pig tail wire between the two plates. Its a very common problem. Its either shorting against the spring or the side of the body. It could be loose, or the tab could be bent so its making contact to ground, or the insulator is missing or the insulation is worn so as little as one thread of wire is contacting the ground somewhere. Another thing to check is that the rivets on the point block are tight. If they get loose, the point block can move against the plate when you advance the spark and if the insulators are broken, shorting out the points.
Click to expand...
 
You must log in or register to reply here.
Back
Top