Compression numbers

"Please can anyone or two tell

"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 pop some more than that, I think the rings are bad, am I right?

Thanks!"

"Maybe, but not necessarily.

"Maybe, but not necessarily.
I would check your plugs, ignition, coil, timing, and carb before checking her in for major surgery.
Rod"

"I agree with Rod. Other than

"I agree with Rod. Other than comp test, how does it run between backfires and pops and how hard is it to start?
Cracked,damaged or moisture in distruber cap will cause backfire. Damaged plugs,wires and coil can do the same. Other than checking timing with timing light, the next easiest thing is to look at dist cap for cracks, moisture, carbon tracks etc. then pull each plug to inspect for fuel or oil fouling. They should look dry with the porclin (SP) insulator tan or brown. One or two dirty plugs could isolate proplem cylinders/ wires. All plugs dirty could be coil,carb,timing etc. Check the area around the top of coil wire for signs of arcing, burn tracks, cracks etc.
Please forgive me but I tend to ramble...
Compression test with low numbers is not necessarily a bad thing. It just gives a hint to how the engine is aging.
Adding oil to the test determines if rings or valves are cause of low number. If oil to the test causes NO increase to test numbers than the fault is in the valves which is BAD. Slight increast in numbers just indicates rings are getting weak which is not a big deal. However, two cylinders next to each other with very low numbers (50 psi) indicates head gasget blown between cylinders, BAD but cheaper to fix than burnt valves.
Your comp test is just fine, motor is happy. Check out the dist cap, plugs, wires and coil.
Just keep track of your observations."

"A little more info: the cap,

"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"

"When does it backfire? All th

"When does it backfire? All the time, idle only, accelleration only? Timing not advancing would backfire; check dist. mech. advance. Cheap plug wires crossed could crossfire and backfire.

Have you tried pulling one plug wire at a time to isolate the backfiring cylinder? I'd suspect cyl. #4. Run a couple of cans of SEA FOAM; one thru a tank of fuel and (engine hot) one-half can poured REAL SLOWLY thru the carb with the RPMs up to keep it from stalling and the other half can in the oil to clean out any sticky valves."

The arcing should never happen

The arcing should never happen under any condition. Best to replace coil.
Tan plugs is a good sign.

"The backfire occurs at cruisi

"The backfire occurs at cruising speed every 3-5 minutes generally. I'll give the seafoam a shot, change the coil."

"Just a footnote about the arc

"Just a footnote about the arcing.
A hairline crack could be the reason for arcing or at one time some moisture collected at the center post allowing the arcing to neg post. Once arcing starts, it will leave a carbon track which will make arcing more severe with time. Cleaning may or many not remove the carbon track but nothing will fix it if there is a crack.
As a serious safety point, any gas fumes in the engine compartment could cause a fire or explosion from any arcing! A backfire thru the carb could blow fuel which the fire arrestor on top of the carb is to prevent. Have a second set of eyes and ears and fire extinguisher when next test run to look for arc and until all arcing is eliminated. Run with engine compartment open and blower running and run blower a minute or two before starting the engine.
No matter the reason for backfire, arcing must be eliminated.
Keep us updated"

"Appreciate the help, the bfir

"Appreciate the help, the bfire is pretty unnerving and a head scratcher too, I thought the timing and dwell were the source. I'll start with the coil, it only occurs on the water under load for several minutes at least. All this screwing around is sure cutting into fishing time!"

"John;
Another thought.
I ha

"John;
Another thought.
I had an 898 a few years back that would backfire every once in a while for no apparent reason. After many false starts, someone suggested that it might be a hung or sticky valve. I put some Marvel Mystery oil thru it and presto; it never happened again.
You could try the same, or Sea Foam, which I understand also works well.
Just a thought.
Rod"

"Was that MMO put in the gas o

"Was that MMO put in the gas or in the oil? 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 MMO 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. Bought some Restore too but I think I'll take that back unless I hear positive feedback . That along with a new coil."

"I hear the Chevron stuff is g

"I hear the Chevron stuff is good, so by all means.
I used the MMO in both the gas and oil. About 1/2 can in the oil and the rest in about 1/2 tank of gas.
Like I said, it worked for me.
Rod"

"I think you should hold off p

"I think you should hold off putting additives in the engine until you fix the arcing problem and see if the poping and backfire goes away with the arcing going away.
All 4 cycle engines both automotive and marine develop carbon, its a fact of life. By adding stuff to the oil to remove carbon, the first thing that happens is the 40 weight oil is reduced to 30 weight or less depending on how much additive you pour in. Second, pouring additive in the fuel to remove carbon on valves causes all the crud that you loosen up to run thru the combustion chamber. This crud can get on the rings and further score the cylinders and IF any particles happen to get trapped between the valve and valve seat, this will cause white hot gasses to leak thru the seat and burn the valve/s
There is no way to know how much carbon is on the valves without removing the heads and looking. If you are not burning oil, that is, you are not having to add oil very often, you should have normal..ish amounts of carbon deposits found on all 4 cycle engines. Most of the carbon is from unburned oil deposits getting past the rings or down the valve guides. One quart of oil usage per oil change is ok. With engine warmed up and running, remove oil fill cap and check amount of blow-by coming out of oil fill hole. Some visable vapor is ok but vigourus puffs of light smoke/vapor is a sign of possible blow-by past the rings which could cause excessive amounts of crank case oil to get past the rings and burned in the combustion chamber....causing carbon build up. No or very little blow-by but oil usage could be oil running down the vave guides.
My bent opinion about most makers of additives is their prime function is to remove money from my pocket.
Dont let this babble confuse you
Keep us updated on results of new coil."

"John
I am with Bill.
Your c

"John
I am with Bill.
Your coil most likely has a hair line crack in the tower area, throw it away and fit a new one,
before you blow up your boat.

Then try the additives.

As far as I am concerned, although not good, your compression readings are Ok for an engine that has seen a few hard hours but there should still be some life in the old girl yet.

Check your mechanical advance as well to make sure that you are getting full advance at a smooth rate as you increase the revs, you could also check to make sure your float level isnt to low.

Cheers
Peter C"

"The arcing was only seen with

"The arcing was only seen with the center lead ungrounded at the time of the compression test, later when I realized this it went away. But the coil is new. So it should advance under load right, but what dwell should it be at under full throttle? ('72, 120,4cyl.) The backfire doesn't occur on acceleration, but at level cruising speed. I did notice that the rotor can rock slightly on the end of the D-shaft with the cap off. What/where is the float measured from? It doesn't use oil, I didn't see any blowby at the cap Bill. Thanks for all the tips. I'll be soon checking if the new coil does it on th water."

"John

The coil has nothing

"John

The coil has nothing to do with your advance curv, that is controled by the movement in your distributor base plate as influenced by the weights below the plate and if fitted the vacum module on the side of the distributor.
You have an initial timing point of so many degrees that you set the timing to, then you have a maximum timing advance.
Check your specs in your manual and it will tell you a figure of maximum advance, at a particular RPM, Put your timing light on the engine and rev the engine to the supplied figure and see if the timing advances to match the specs.

As far as the float measurement it varies from engine to engine and carby to carby, you need to set it to the height prescribed for your particular engine with your particular carby and your manual should be able to give you that info.

Cheers
Peter C"

"I had it out on the bay today

"I had it out on the bay today, still have the bfire through the carb. Mechanical advance appears to work ok with timing light it advances smoothly. I also notice "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. Change the manifold gasket? Have the head done too?"

Is it the crank case vent? Th

Is it the crank case vent? There should be one somewhere with a plastic tube to the carb. cover.

"No, there is no crank case ve

"No, there is no crank case vent that I can see, it's a '72 engine. The oil fill cap is one of those tall dome shaped ones that are vented to the air. I thought this era engine didn't have a PCV valve.

Thanks,
John"

"Yep, you have a pcv valve.
I

"Yep, you have a pcv valve.
I assume when you say manifold, you are talking about exhaust? I would not think a leak here would cause a backfire thru the carb. However, a leaking exhaust can cause a explosion or fire, best replace the gasget.
With a new coil and still backfire, even tho you have a new dist cap, take a look at it again for any moisture. It is possible for fumes to come up thru the dist shaft due to bad seals or a plugged pcv valve would allow crank pressure to work its way up thru the dist shaft.
I have purchaced new caps with cracks from the factory. After you are sure the dist cap is good...
Now, as for the backfire thru the carb. Is it more like a spit similar to opening a fizzed up beer can? It could indicate a lean carb. Vacume leak thru the carb base gasget is typical. It is also easiest and cheapest fix when dealing with carbs. Using a can of spray carb cleaner and engine idling, give it a quick squirt around the base gasget and be careful spray only the base gasget area. If the base gasget is leaking, the engine will increase rpm's and then slow back down. If that happens, tighten the carb mounting bolts and test again. You may have to replace gasget if tightening fails to stop the leak. If rpm's dont increase than the problem may be internal to the carb. A bubble of water rolling around the float boal partially plugging the main jet (water will not pass thru the main jet easily) or a partially pluged power port gizzy, sticking float seat, low fuel level in bowl. A small bubble of water or crud can be blown away from the main jet due to the back pressure of the "backfire/spit" and eventually work it way back again for a repeat.
If you have never rebuilt a carb, it is bet to give it to carb guy.
If you get as far a rebuilding the carb and you still have a backfire problem than you have performed all the cheapest and easiest tasks, coil, wire, plugs, dist cap, pcv etc all that is left would be a funky valve. One of 16 and very difficult to decide which one. A rebuild of 2 heads is the most expensive and a total head job in a shop could be around $500 if they use all the old parts add another $100-$150 for a gasget set to reassemble the heads, manifolds,etc.
Taking the heads to a shop, they will rebuild every part of it to factory specs hence the $500. Do it yourself, you can do nicely by taking it apart, cleaning everything and if the valves are not burned, just hand lap them and reassemble for the cost of head gasget set. Only special tool needed is a valve spring compression tool you can probably rent from a tool rental. Oh, you will need a torque wrench for the head bolts, intake and exhaust."

"John & Bill:

I looked

"John & Bill:

I looked at the engine parts diagram and it did not show a PCV valve, only a filter/oil filler cap.

John:

I would go with your own suggestion to change the exhaust manifold gasket. Inspect it for hairline cracks before reinstalling it. Consider replacing the intake manifold gasket along with the exhaust. Check the intake for cracks also. Did you try the MMO or SEA FOAM in the oil? Did it help? The low compression on #4 cylinder may be an indication that the exhaust valve is in need of work. It could be the cause of the occaisional backfire. Plug wires are good and not crossed? You can verify if you have cross firing of wires by using a spray bottle of tap water and misting the wires while it idles and watch for it to stumble. Good luck.

If you are concerned about the blowby gases in the engine/bilge area since they contain carbon monoxide among other exhaust gases, you can change the filler cap to one with a hose vent connection and run the hose to the carb. cover where it will be ingested and burned."

"I put some Chevron Techron in

"I put some Chevron Techron in the gas ran about half of it through without noticing any change. The spray bottle on the wiring sounds like a good tip, I had planned to pull plug wires to see if I could isolate, but when I was out I completely forgot this step. The thing is pre-PCV it has a tube from the carb lower to the valve cover which is open. It has the dome shaped breather cap of the era. The intake and exhaust on this 4cyl. look to be integral. Plug wires, cap, rotor, and points are new. The coil is new. I do want to clean up the air around the engine but just now I'd like to find the source of the bfire, not taking apart more than I need. In this rust encouraging climate my experience with old machines is that I have wrenched on things I thought were broken only to actually break them myself. Do you think R&R'ing the manifold gasket will do it?"

"John:

I agree with you whe

"John:

I agree with you when it comes to taking apart an old part to check for a problem. Bill said to use carb spray to find out if you have a vacuum leak around the carb base or intake manifold. Why don't you give that leak test a try? WEAR GOGGLES and have a fire extinguisher at hand. Do it once while the engine is cold and just started and then again when it is nice and warm. Expansion of hot manifolds may cause it to only show up on a hot engine.

I have always used a small canister propane torch (unlit) with the valve slightly open and SLOWLY move the nozzle as close as I can get to joints, gaskets, vacuum hoses and connections. It works quite well; if you get near a leak the RPMs will increase.

What does the #4 spark plug look like compared to the others?

Do you have a vacuum port or nipple you can attach a vacuum gauge to it? Look on the intake manifold for a plug you can remove and install a nipple. It will tell you if there is a vacuum leak, too rich or lean carb mixture, sticking valve and of course the amount of vacuum your engine produces at idle. Not expensive."

"First of many thanks to you G

"First of many thanks to you Guy, Bill, Pete, and Rod for all the input. I sprayed some water on the wiring to see if it would sputter, then around the carb base to see if I could see any disappear. I didn't see/hear anything happen. Engine runs fairly smooth until it gets to 350 rpm (trolling it down). Only backfires above 3K r's. All the plugs look clean, #1 has a light black coat up inside the porcelin but the electrode appears clean. The manifold leaks some above the exhaust? port on #2.
This is right above and to the rear of the 3rd manifold stud in from the front. 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 few little bubbles of water inside the throat on that side of the carb intake. Can this cause the backfire I'm seeing? The oil looks almost clear hasn't used much, but I haven't really put more than 10 hours on this year trying to track down the backfire. I don't want to get stranded, rather not give the Coasty's something to do. I do have a vacuum gauge around here somewhere."

"John

A leaky manifold gask

"John

A leaky manifold gasket can give you problems with backfiring so what you have to do is ask yourself what have I checked and disscounted already.

Many troubleshooting guides are in the form of a flow chart, its a good way to work, write down everything that could possibly cause a missfire on a list, start with the easy things like plugs and points and finish with things like sticky valves and lifters.
Now go down the list and cross off every item that you have ("DOUBLE") checked. Going through the postings so far you are running out of things to look at.

I am still suspect of a loose spark somewhere, is it possible for you to run your engine at night with the engine cover off and see if you have a light display happening around your plugs, leads and cap while the thing is popping ?

Just a thought, I have found electrical problems with the HT system before by doing that.

If you end up totaly discounting the electrical system I would be very suspect of the manifold gasket. Cooler air being drawn in accross the exaust valves through manifold leaks can cause backfiring.

As for the water droplets forming in the carburetta that can be a normal reaction at high revs in the lower venturi level of the carby and will not be an issue.

You have a good problem here, unfortunatly you need to keep digging.

Cheers
Peter C"

"Sunday night I will give it a

"Sunday night I will give it a go in the evening at dark looking for sparks. Trouble is I can't make it backfire in the yard here with muffs, only under a good load on the bay. I guess I could still see a voltage leak if I look, so that'll be my plan. After that I'll take off the manifold? "Cooler air being drawn in accross the exaust valves through manifold leaks can cause backfiring" You've first hand experience of this Pete? The backfire happens every 3-5 minutes of run time once its warm and up in r's. More than me eating a bean and cabbage casserole."

""The manifold leaks some

""The manifold leaks some above the exhaust? port on #2." Is this the exhaust manifold or intake?

"The carb choke...hot manifold water comes up to a thermostatic "spring" adjacent the choke butterfly; looking down the carb throat I see a few little bubbles of water inside the throat on that side of the carb intake."

Pull the choke housing off and check it for cracks or leaks. This leak may become a gusher at 3,000 RPM, thus the backfire occurs. I would change it to an electric choke."

"John

You can have the bean

"John

You can have the bean and cabbage casserole mate, thats all yours.

In an automotive application I have come across the problem with a manifiold gasket on a car many years ago that was causing popping in the manifold, I would not of called it a serious blow the air cleaner off backfire, but simply a popping and as I cannot hear your problem it makes it hard to know wether we are talking the same type of noise.

The trouble is that a marine engine operates under a different set of circumstances to an automotive engine but as I said in my last posting you are running out of things to check.

Have you adjusted the valves at all ?

I dont know what octane fuel you are running in her but you could also try running a higher octane rating fuel in her as well.
If she is a bit old and carboned up and your octane level is a bit low in your fuel it would not be helping either.

One thing to look at in your diagnosis is engine temp, the problem doesnt exist till it warms up. What happens as the engine warns up, the manifold expands and thats when a leak may occur, or the choke comes off and causes the mixture to lean out to what is supposed to be a correct running mixture.

Maybe you should try and run the engine with just a small amount of choke still on when the engine is warm and see if the problem is still there.

Its an interesting one so I will continue to think about it for you and see if I can come up with any more possible solutions ("guess's").

Regards
Peter C"

"My 888 has a pcv, valve. All

"My 888 has a pcv, valve. All other 888's I've seen had one also. Starbord side rocker cover has the typical rised oil fill cap with the wire filter screen. Port rocker cover has the PCV with hose going the the carb base plate.
Oh well, Im outta here."

"It looks like its the exhaust

"It looks like its the exhaust port that leaks some, puffs out near the third stud. I checked it out under the dim predawn light and I see no sparks or lights around the wiring suggesting a voltage leak. I think I'll take a peak into the carb next I have her out and see if water is getting into the carb while under way. I did think about an electric choke when I saw this setup with the water tube. I don't see any water in the oil though, none."