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TOPIC: 71 Chrysler 55 hp compression issues

71 Chrysler 55 hp compression issues 3 months 1 week ago #144752

So I’ve gotten the water flow issue resolved I believe. Went to start it and I’ve noticed that the last couple times I ran it it started very hard, compared to when I started this project anyway. Put new plugs in it, cranked it and wouldn’t fire. So I pulled them and the top one was literally dripping with gas. Cleaned it off, finally got it running. Won’t idle, runs rough. I pulled the plugs again. Top one looks new. Soaking wet. The bottom one is clearly firing. It’s showing signs of combustion, and there is a noticeable temperature difference between the 2 cylinders. So I ran a compression test. Top and bottom both 90 psi. Which is low according to the manual. Pulled the head. Gasket shows no visible signs of damage. I’m at a total loss here. I’ve been a mechanic my whole life but never really messed with 2 strokes so this is a whole new world to me. I’m lost, disgusted and just ready to give up. But my heart and my wallet won’t let me just yet. So any direction I could get would be very much appreciated

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71 Chrysler 55 hp compression issues 3 months 1 week ago #144759

  • ed-mc
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I can't recall, but does the fuel pump come off of the top cylinder or bottom? The reason I ask is, if the fuel pump is taking pressure pulses from the top cylinder, and the fuel pump diaphragm is leaking, it'll dump a lot of fuel into that cylinder. Then it won't run. So that's something to check. Do you have a strong spark on #1 cylinder? Another thing to check.

The 90 psi reading is likely due to inaccuracies of your compression gauge. 90 psi is enough to run this engine. The critical thing is that both cylinders are perfectly even. It's extremely unlikely for both cylinders to be having serious ring/piston problems, and have identical compression. Maybe in theory, but not in my experience.

So now you're gonna have to install a new head gasket; you can't re-use the old one. If you have a piece of glass, or a very smooth metal surface, lay down a sheet of medium-grit sandpaper and surface the head. Thus making sure it doesn't have a lot of high and low spots that will compromise the head gasket seal. Or take the head to a machine shop and have it surfaced. A lot cheaper to do yourself, just takes a bit of time and patience.

Anyway, those are my thoughts about that. The engine only has one carb, so no multi-cylinder/multi-carb confusion here. If the carb is running the bottom cylinder, then the top should run, too.

Anyway, I'd look at excess fuel out of the pump, and spark quality. You can compare the spark on #1 to the spark on running cylinder #2, top should be as good as bottom. If the spark is orange or reddish, it's not enough to fire the spark plug under compression.

HTH...........ed

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71 Chrysler 55 hp compression issues 3 months 6 days ago #144770

Your 100% correct. I packed everything up that night and started posting and researching. I somehow stumbled across a post somewhere that stated the same thing about the fuel pump. Went back out and took it apart and low and behold it was split.
I called the man who’s been helping me some to order the parts and he as well said the same things about the compression.
I work in a building with a machine shop in it and they checked the head. Said it was about 2 thousandths out. They surfaced it for me and now I’m just waiting on the parts. Hopefully she’ll be back to life soon. Thanks for your response.

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71 Chrysler 55 hp compression issues 3 months 4 days ago #144795

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Patrickseastar wrote: Your 100% correct. I packed everything up that night and started posting and researching. I somehow stumbled across a post somewhere that stated the same thing about the fuel pump. Went back out and took it apart and low and behold it was split.
I called the man who’s been helping me some to order the parts and he as well said the same things about the compression.
I work in a building with a machine shop in it and they checked the head. Said it was about 2 thousandths out. They surfaced it for me and now I’m just waiting on the parts. Hopefully she’ll be back to life soon. Thanks for your response.


Glad you figured out the root of the problem. It can be a difficult one to diagnose as it's a lot easier to troubleshoot a "not enough" fuel condition; "too much fuel" is not so obvious.

Must be nice to have ready access to a machine shop! .002"-out is not horrible but better to have that head nice n' flat.

If you're in an area with salty or alkaline water, I'd recommend putting some kind of salt barrier on the head bolts before reinstalling. I use Permatex No 3 aviation-type non-hardening gasket dressing. You can get a brush-in-bottle of that at most any auto parts store. Just brush on a very thin coating on threads and bolt shank.

Here are some "good words" I purloined off the InterWebs about torquing the head bolts:

"Chase the threads on the block and bolt, clean well, oil the threads and torque to 225 inch pounds, starting in the center and working out in larger circles." (of course don't need to oil the threads if you're coating the bolt with gasket dressing)

And it's better to tighten in stages; either 2 or 3. i.e., tighten 1st round to around 112 inch-lbs, 2nd to 225. Or, 1st round 75, 2nd to 150, 3rd to 225. Tightening in stages helps to even out the "crush" on the head gasket as you torque 'er down.

Don't use any gasket sealer on the head gasket or surfaces of the head or block. Just make sure the block is scraped free of any gasket residue and block/head are degreased before reinstallation. A razor-blade-type paint scraper works great for those type of surfaces.

Attached is a diagram of the recommended head bolt tightening sequence.

HTH............ed
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