Thunderbird Cath style Hull 22 ft I/O mercrusier GM

[thunderbird cath]

I purchased this Boat for $500.00 I hooked a new marine battery to motor , the engine will turn over. I could not get to motor to fire . I Purchased new spark plugs and replaced them all , still nothing , hooked an external fuel tank to the carb , nothing Found the carb was not letting fuel thru to the motor . Removed carb ,poured half a cup of gas down the hole where the carb was the engine fired off and ran for about 3 sec. I did a compression test on the cylinders 3 of the cylynders read 60 psi the other were below 18 psi.

Checked the shoe around the outdrive there was no cracks,

I dont Know where to start?

Thank you for reading my post

 

[ezmobee]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Moving to I/O repair.

 

[mkast]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

I did a compression test on the cylinders 3 of the cylynders read 60 psi the other were below 18 psi.

Time for some major engine work or replacement. The compression should be quite a bit higher, with the lowest cylinder within 10% of the highest.

 

[Fishermark]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

I dont Know where to start?

As noted, it sounds like you are in for a complete engine rebuild.

BUT... the best place to start here is to post a few more details. What engine, etc.

How did you do the compression test?

By the way - welcome aboard!

 

[thunderbird cath]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

As noted, it sounds like you are in for a complete engine rebuild.

BUT... the best place to start here is to post a few more details. What engine, etc.

How did you do the compression test?

By the way - welcome aboard!

Ty I did the compression test with a compression tester that i got from Sears i did not do a wet test how bad is a complete rebuild if i have a boat mech friend that will do it for free as far as installing stuff

 

[thunderbird cath]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Do you think it would be better to Buy a new block or have the other one machined out . and buy the rebuild kit

 

[Fishermark]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

You haven't said what engine you have yet. Everyone here needs a lot more information before anyone can tell you that you need a new engine. At least based on what little info we have so far.

For the compression test for instance - regardless of the brand of tester - did you remove all the plugs and let the engine revolve several times?

 

[thunderbird cath]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

The motor is an In line 6 GM Mercrusier, I belive its a first generation,
I did let the engine turn over several times with all of the spark plugs out and i did the test twice to make sure got the same reading . is there certain markings anywhere i should be looking for, to give me some better info.

 

[Bondo]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

is there certain markings anywhere i should be looking for, to give me some better info.

Ayuh,... Look for the Shop Manuals up in the Adults Only section of this forum,...
You need manual #1...

Btw,... 100psi compression is considered Minimum...
I'd check out the hull for Rotted wood before I spent a dime on the motor...

 

[Fishermark]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

I agree that you need to completely look over the boat and evaluate it before "sinking" a lot of money into it.

The good news? You have one of the simplest, most dependable engines ever built - in my opinion. It is the Chevy 250, straight six. (It's possible it may be a different engine - there was more than one straight six... but by far the most common is the Chevy 250.) I have rebuilt a couple of them in the past year or two.

But one thing at a time. Look the boat itself over with a fine tooth comb - especially the transom and stringers. See if they are solid. If you don't know, or aren't sure how to do that, find someone who does.

It has been said that the most expensive boat you can buy is a free boat. (Sometimes even one that costs $500. )

 

[nitrosteve]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

You also might consider checking the timing chain it may have jumped timing and the valves aren't fully closed on the compression stroke. Good luck with it.

 

[Fishermark]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Just a note - the straight six doesn't have a timing chain - only timing gears.

 

[nitrosteve]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Ok check the timing gears. Better yet maybe don't check that if you snapped a timing gear you would more than likely have 1 cylinder with the valves closed and all your are low comp.

 

[mkast]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Ok check the timing gears. Better yet maybe don't check that if you snapped a timing gear you would more than likely have 1 cylinder with the valves closed and all your are low comp.

How do the valves in one cylinder affect the other five cylinders?

 

[Fishermark]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Ok check the timing gears. Better yet maybe don't check that if you snapped a timing gear you would more than likely have 1 cylinder with the valves closed and all your are low comp.

Steve - as they say, sometimes a picture is worth a thousand words. Here's a shot of what the timing gears are like for the Chevy 250:

Unlike an engine with a timing belt or chain, there is no way for the timing to "slip". With a gear set like this, it either works or it doesn't. If the gear set breaks a tooth the chances are good the entire gear will break out completely and if that happens, it will generally destroy the engine.

 

[nitrosteve]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

They don't but he has low comp on all the cylinders and chances are if he rolled the key in the timing gear set or shattered a gear at least 1 or 2 cylinders would still have the valves closed this would allow those cylinders to build compression. To be honest im not sure what prompted your question I never said one cylinders valves would effect the other five. I hope Ive explained my logic more clearly for you.

How do the valves in one cylinder affect the other five cylinders?

 

[nitrosteve]

Re: Thunderbird Cath style Hull 22 ft I/O mercrusier GM

Steve - as they say, sometimes a picture is worth a thousand words. Here's a shot of what the timing gears are like for the Chevy 250:

Unlike an engine with a timing belt or chain, there is no way for the timing to "slip". With a gear set like this, it either works or it doesn't. If the gear set breaks a tooth the chances are good the entire gear will break out completely and if that happens, it will generally destroy the engine.

Thanks for taking the time to show the gear set. I would agree its highly unlikely he would roll the key or break a gear set without basically destroying the engine. But stranger things have happened.