Compression values mean?

[johnnynegaunee]

Please can anyone or two tell me what these comp. numbers mean:
cyl. #1) 125 lbs. with oil 175
#2) 137
#3) 142
#4) 112 lbs. with tablespoon oil 145
This 120 'o mine backfires bad about every 3-5 minutes and pops some more than that, I think the rings are bad, am I right?

Thanks!

 

[Haut Medoc]

Re: Compression values mean?

Backfire through the carb is either lean fuel condition, water in your fuel Or a bad intake valve.....
I would pullthe head & have it redone & see what that does for you.....
The rings are most likely worn on #4 for some reason.....​

 

[johnnynegaunee]

Re: Compression values mean?

Thanks!

 

[johnnynegaunee]

Re: Compression values mean?

A little more info: the cap, wires, rotor, and plugs are new and running tan near as I can tell. The timing and dwell were way out when I first bought the boat and discovered the backfire. The timing mark was advanced about 30 degrees, so it showed up under the top pulley, way out from the marker plate. The dwell was 48 degrees. I corrected both of these, the previous owner must have been doing them both by "ear". I had figured the corrections would cure the backfire, not so lucky. The fuel was fresh, from a new tank, quite sure no water in it. I haven't checked the carb. When I was running the compression test the coil did arc from the center post to the negative lead, but I was thinking it needed to bleed off somewhere, maybe this is not normal? Would the improperly set timing carbon up the valves so they don't seat right anymore? It's easy to start, pushes the 21' alum. starcraft islander about 24 mph.

Thanks

 

[Haut Medoc]

Re: Compression values mean?

You should ground the coil wire to the block using a jumper wire....
Your ignition is working fine.....
Hard to say about the carbon, but I am surprised that you did not hole a piston...
I would still have the head redone & see where that leaves you with regard to compression #'s.......​

 

[johnnynegaunee]

Re: Compression values mean?

Some more I didn't say: this happens at cruising speed, constant speed, I've not noticed it on accel or decel. Coil has been replaced, but not by me. I'll try pulling plug wires to see if it goes away. Again, thanks for input.

 

[Haut Medoc]

Re: Compression values mean?

All the more reason to suspect a valve train problem......​

 

[johnnynegaunee]

Re: Compression values mean?

I was just at Wally's and bought 20 bucks on those magic bullet additives thinking what the hell, if my problem goes away and I don't have to take the head off... I had the Marvel Mystery Oil in hand but got the Chevron techtron stuff instead because it's claims were more specific to the top and they had pic's of cleaner valves etc. I got some Chemtool spray carb cleaner that I've used successfully on carbureted cars. Haut, have you used any of these with success? Or is it just snake oil? I'm missin too much fishin.

Thanks,
John

 

[Haut Medoc]

Re: Compression values mean?

Snake oil, IMO......​

 

[HT32BSX115]

Re: Compression values mean?

Snake oil.

 

[HT32BSX115]

Re: Compression values mean?

Did you do a recent tune up? (plugs, points, condenser, cap, rotor etc?)

If so It sounds a lot like you may have a couple of wires switched.....I see it's a 4 banger.....switched wires is probably unlikely. It sounds a lot like ignition/timing problems.....


Did it always run like this or did it just "start out of the blue"?

Ok,

I just read the entire thread........Is this engine a bit worn out? and does it have a timing chain? If so, and you had to move the dist so much, I would suspect a slipped timing set.

 

[johnnynegaunee]

Re: Compression values mean?

Since I bought it, it has had this problem, seems to me I did check the firing order a while back but I'll recheck tomorrow. Fire order is stamped on the block I think, right?

 

[HT32BSX115]

Re: Compression values mean?

It usually is but that shouldn't be hard to determine.

If it was a V-8 I would say that you have a couple of wires swapped. Sometimes you can hardly tell on a V-8. On a 4cyl it's a little more obvious since you literally lose 1/2 of your engine if it happens. If you had 2 wires switched it probably wouldn't even start and there'd be literally NO power if it did!

I had the timing set slip in a Ford Y-block V-8 last year and I had to move the dist almost 90 degrees to get it to even run and it sputtered and coughed a lot!

I don't know anything about your engine but if it's worn out and has a timing chain it's at least possible that the timing set has slipped a few teeth. Even if you re-adjusted the ignition timing to compensate It might still run but it wouldn't run very good since the valve timing would be out.

Your compression numbers are for the most part enough to make it run. (my old 66 150 Mercruiser I6 had similar compression numbers and it ran fine....not as good as new but then it was only 39 years old when I sold it!)

While you're checking things you might remove the valve cover and ensure that each valve is opening and closing.

You might also have a look at the distributor underneath the points. Look particularly at the centrifugal weights that govern the timing advance.

And while we're talking about timing, does it have a vibration damper? Maybe it slipped. That would cause you to think it's out of time wen you checked it and when you re-timed it it would REALLY be out of time...


Cheers,


Rick

 

[johnnynegaunee]

Re: Compression values mean?

'72 120 is firing 1-3-4-2 same as stamped on the manifold. I had it out on the bay today, still have the backfire through the carb. I also notice some smoke/vapor blowing fairly vigorously from between the manifold and the engine. This was on top, right adjacent the carb. Could this be the source of the backfire? I'm use to seeing old dry land engines leak some from same so I didn't notice it, but now I wonder if water is percolating in somewhere it shouldn't. I see no water in the oil, none. Oil has been running cleaner than I expected, of course that is landlubber experience too. It does have a slight rocker tick too. I'm thinking as soon as the frost comes I'll pull the head off. Whadaya think? Unnerving to have it pop back through the carb like it is, not conducive to easy fishin'.

 

[Bondo]

Re: Compression values mean?

Backfire through the carb is either lean fuel condition, water in your fuel.....​

You seem to have covered the ignition,.........

Have you changed the Fuel Filters yet,..??...........

I also notice some smoke/vapor blowing fairly vigorously from between the manifold and the engine. This was on top, right adjacent the carb.

Ayuh,........ That could be a Problem............

 

[johnnynegaunee]

Re: Compression values mean?

Fuel Filter is new, pulling fresh gas from a new tank, 2 months old. Carb mix screws are 1.5 turns out, Rochester 2bl. carb. It's a head scratcher for this boy, take the manifold off? Toss a lit stick 'o dynamite into it?

 

[johnnynegaunee]

Re: Compression values mean?

The carb choke is opened by plumbing where hot manifold water comes up to a thermostatic "spring" adjacent the choke butterfly; looking down the carb throat I see a little bubbles of water inside the throat on that side of the carb intake. Can this cause the backfire I'm seeing? Also, I see vapors leaking from around the area above and to the rear of the third from the front manifold mounting stud.

Thanks,
John

 

[Bondo]

Re: Compression values mean?

It Sounds like your Manifold may have Rusted Out or something........

 

[delsol]

Re: Compression values mean?

my v-8 poped out of the carb. valves were too tight, compression test revealed bad compression in 2 clys
bob

 

[johnnynegaunee]

Re: Compression values mean?

"valves were too tight" what do you mean? My compression numbers from the start of this post:
#1) 125 lbs. with oil 175
#2) 137
#3) 142
#4) 112 lbs. with tablespoon oil 145
The old mare ain't what she used to be but the numbers I hear aren't too bad.