Low Compression causes on V6 Looper

[proaudioguy]

I have under 30psi on all 6 cylinders. The engine is new to me. It's cosmetically in good condition, had some carb issues which I cleared up in a few hours. It runs rough, but I think there are some electrical issues too. I plan to methodically go through it all (I have the manual), but I started with a compression check and it came out very low. The first comp tester I tried read zero on on cylinders and I assumed the gauge was bad cause it was brand new. I got another one and it runs 25-30 on all cylinders. I shouldn't have purchased the engine. Now what do I do to fix it? I have the manual. I have a machine shop nearby. I can buy a cherry picker. I know how to remove the power head and how to wire the engine, etc. Could this be something as simple as reed valves? The engine coughs during idle. I hate to tear it down if it's something simple. Gasket kits alone are very expensive. Any advice appreciated.

225HP 1990 V6 Looper

 

[jbjennings]

Re: Low Compression causes on V6 Looper

I still think you have a bad compression tester. I just can't imagine the odds of having an engine with only 30psi on all 6 cylinders. I also think you have some carburetor problems, and as you said, no telling what else. The "cough" at idle is a pretty classic symptom of a lean, stopped up low speed circuit in the carbs. I really think you need to do some more checking of the compression before you go further. Actually, I don't think it would even crank at all with 30psi on all 6, even thought I hear those v6 OMC's are low compression engines from the factory. I'm thinking I've heard folks having good runners with 60psi on all cylinders.
Then again, I don't know much about a v6 motor. So hopefully someone more knowledgeable will come along and see your post.
Good luck,
JBJ

 

[boobie]

Re: Low Compression causes on V6 Looper

I've got 5 different compression gauges and if I check the same cylinder on one mtr I'll get 5 different readings. If the mtr runs , get some BRP Engine Tuner, FOLLOW directions on the can and decarb it. If it improves do it again and then check compression with yet another gauge. Don't tear it down before doing this.

 

[metriccrescentwrench]

Re: Low Compression causes on V6 Looper

How many rpm's is the engine turning when you're checking compressison? If you have a weak battery or an aging starter you can see artificially low readings.

As far as "normal" compression numbers, it seems that around 80 to 90 is typical. A few weeks ago I finally found an explanation of why loopers show lower compression than other engines. They have an additional small idle relief port above the exhaust port which aids in starting and idle quality (by effectively altering port timing at lower rpms). As rpms increase, more and more of the exhaust gas exits through the exhaust port (because of it's small size the idle relief port can only flow a small amount of gas), so port timing in effect returns to normal.

 

[Haffiman]

Re: Low Compression causes on V6 Looper

Before digging into carbs and throttle bodies, check out the engine temp and thermostat system. Buy an IR gun and check the cylinder for even and correct temp. Un-balanced temp between the two banks will make it run rough. Sticking and leaking thermostats and thermostat membranes are common problems on those engines.

 

[JB]

Re: Low Compression causes on V6 Looper

All six cylinders went bad by exactly the same amount and at the same time?? I don't buy that, not one cotton pickin' bit.

It is normal for those big sixes to compress 60-90psi, but only when the test is done correctly.

I suspect that you are doing something incorrectly.

Do it like this. . .exactly: http://forums.iboats.com/showthread.php?t=391323

Let us know, and good luck.

 

[Borgey401]

Re: Low Compression causes on V6 Looper

I don't even think that motor would run at that low of compression. Bring the gauge back and maybe switch to a different brand? I would def. decarb the motor also before you tear it down.

 

[daselbee]

Re: Low Compression causes on V6 Looper

I would like to see that before you do anything to it. PM me with your phone number, and I could come up there to look it over if you want.

 

[proaudioguy]

Re: Low Compression causes on V6 Looper

I warmed it up this time. Made no difference on the autozone gauge. My dad dropped off the adapter for the old circa 1970s Mac tools gauge. It read 85-90 on all 6. I'd like to believe the mac.

What causes the sneeze?

Plan is to pay down vacation debt. Get new carb kits (although I am confident they are good after i cleaned them and stole parts off the old carbs that were rebuilt late last year). Get the dva out and check all the voltages. Buy a power pack because both of the ones that came with the engine have chopped wires. Get a regulator cause the 3rd used one still shows symptoms of being bad. Hoping the dva shows me the stator and timer base are good. They both passed the ohms test. I have some fiberglass reeds brand new. Wondering if I should put them in. Company says they will last for years and will give better gas mileage. Thoughts? Manufactured by Chris Carson Marine.