Excessive compression..

[frustratedboater]

Greetings all,
As from other posts I put up, I had experienced very rough idle in my Mastercraft, 87' Chevy 350, and had a pop/backfire sound inside the intake manifold while having a very rough idle. I double checked the firing order, rebuilt the holley carb completely (needed it anyway), and checked my point gap setting, so were solid all around.

I took a compression test in all cyls' and here are the numbers:
Cyl-1, 150
Cyl-3, 165
Cyl-5, 165
Cyl-7, 160

Cyl-2, 160
Cyl-4, 175
Cyl-6, 140
Cyl-8, 148

As you can see cyl's 4 & 6 have something going on. When I screwed in the tester in cyl #4, and cranked it over, it had much different sound than the others. It had a double cranking sound while cranking over and posted a much higher PSI.

The engine oils is amber clear and that motor is cooled by seawater pump through the hull. Without tearing it down yet, can anyone make some predictions as to what I'm going to find? Intake gasket? Head gasket? Just my imagination???:redface:

Frustratedboater-

 

[MikDee]

Re: Excessive compression..

An exhaust valve not opening will cause a backfiring into the intake manifold through the intake valve after it's firing cycle. Is there a lot of hours on this engine causing the exhaust cam lobe to possibly be worn out (round)? Or, another scenario, a broken intake valve spring, leaving the intake valve partly open during it's firing cycle?

 

[frustratedboater]

Re: Excessive compression..

What you're saying makes sense. Can I visually check for this with just the valve cover off? Others have said a possible lobe wear could be it...

The hour gauge has 172 on it, but looks to be replaced, it's not matching the mastercraft style.

 

[MikDee]

Re: Excessive compression..

Look to see if you're getting about 3/8" lift on all the valves when idling to check for a worn out cam. A broken valve spring, maybe hard to see? but maybe you can try turning the spring, if you can get that rocker arm at the top of it's travel. Or lossen the rocker, & then try to turn the spring, if it moves it's not enough spring pressure!,,, try this on both problem cylinders first, then I would check them all if you find something. When resetting valves I suggest running the engine at idle, & adjusting them 1/2 turn down from zero lash.

 

[JustJason]

Re: Excessive compression..

Eagh... i think your over analyzing your comp ##'s.... You said you rebuilt your car "coz it needed it". Why did it need it, and are you sure you got it right?

and checked my point gap setting

How about replacing the points (and condenser) and setting the dwell and timing, and not the gap.... And since that's a once a year tune up thing, when was the last time the engine was fully tuned? Plugs, wires, cap, rotor, points, all filters etc??


One last thing... depending on the Holley carb, and depending on the vintage of the parts inside of it. A misfire in the intake can blow out the power valve inside the carb and cause it to drip fuel. Are you getting any black smoke at idle?

 

[frustratedboater]

Re: Excessive compression..

To obtain an idle can be challanging. It's hard to dial in the timing when it's running rough like it is. Regarding why I rebuilt the carb- I had excessive fuel leak into the secondaries and was told that if the carb sits for a long duration of time, residue can build up on the needle & seats. Also the powervalve was slated to be the contributor as well. I'm not sure how long since it has been overhauled, so I spent the $39.00 to be sure. I got it going and no dripping and my floates before were not level at all as the instruction say they should. The powervalve was included in the kit, so I just replaced it while in there. I've saved the old PV just in case the contributing problem(s) are not the valve. After the rebuild the engine still had the same general symptoms.

As far as the smoke, my exhaust is almost non-existant. No smoke at all. I set the points at .018". My auto tech (fellow instructor) friend said that is a good setting. As far as the dwell setting....not sure how to do that. I did replace the cap, rotor, condenser, points, but not the wires. I gave it a spark check this morning, before the compression, and it jumped a good 3/8" to 1/4 gap on the tester and the same on all plug wires. I did, however notice a slight rusty-like deposit (real slight) on the 4 & 6 cyl's though (where I'm having compression issues).

It's impossible to tell when the engine was fully tuned. by an expert, since I purchased the boat (ran smooth and fine then), pulled the motor (to replace the flooring) and all I did was remove the cap and wires. I left the main distributor in just so I would have any timing issues later. The engine ran actually real smooth and quiet. This all happened when I put the engine in and added all the pumps, belts, and carb stuff back on. Real strange...

----------
"Now, the leak down test"" -- I have a leak down tester (from harbor freight-directions are vauge), but I've never looked into learning how to use it. I understand the concept about adding compressed air into the cly's and moving the pistons down slow to watch for sudden drop in pressure, but can you run me through the 'skinny version?' How do I get air into the cyl? Would it be on the compression stroke, or the exhaust movement of the piston? I'd appreciate it.

I hope I answered all the questions you've asked and hope I shed some new info.. I'm trying to save a buck on this unexpected issue, but thats boating!

~FB~

 

[Bt Doctur]

Re: Excessive compression..

points gap vs dwell reading
dwell is the time the points are closed to allow the coil to build the charge,the actual reading in degrees is the time the points stay closed and will vary with the points gap. If you feel the reading is that critical , install the points first, take a reading and then vary the opening to obtain the dwell reading you desire.

 

[frustratedboater]

Re: Excessive compression..

Very interesting.. Never knew the coil had to build up 'power'. Learn something every day!! Can I test this with an ohms meter? If so, how? That's the only meter I own...

 

[frustratedboater]

Re: Excessive compression..

Well I had some interesting findings... On the cyl that posted a 175 PSI, the rockers were real loose. I could wiggle them around and pick the rocker up about 1/4 inch from the valve stem.

#6 cyl, 140 PSI, the rockers were pretty solid to the feel. I also took a stab at a leak down test on #4 and heard air leaking all around the oil holes of the head. I wet my finger and dabbed it over the oil dip stick and the "bubble" didn't pop. I don't believe it's my rings. Could I be looking at a very worn cam, or a possible blown gasket?

~FB~

 

[Bt Doctur]

Re: Excessive compression..

Think of a coil as a sponge soaking up the power when the points are closed, the internal winding is producing a magnetic field around the the higher voltage winding .when the point go open this magnetic field collapses ,and the collapse causes the high voltage winding to discharge,like squeezing a spomge violently,only it`s directed out the top of the coil tower.

 

[frustratedboater]

Re: Excessive compression..

You put it in a good way I can understand. I'm a visual learner and when I can see it in a real world format it helps greatly! I can also understand how a dwell setting would matter. The duration of their opening (I think someone put it) would be the amount of the "water/energy" in the sponge. Cool..

Now I just have to figure out the loose rocker deal! I guess I have no choice than to pull the intake at this point...

 

[MikDee]

Re: Excessive compression..

Well I had some interesting findings... On the cyl that posted a 175 PSI, the rockers were real loose. I could wiggle them around and pick the rocker up about 1/4 inch from the valve stem.

#6 cyl, 140 PSI, the rockers were pretty solid to the feel. I also took a stab at a leak down test on #4 and heard air leaking all around the oil holes of the head. I wet my finger and dabbed it over the oil dip stick and the "bubble" didn't pop. I don't believe it's my rings. Could I be looking at a very worn cam, or a possible blown gasket?

~FB~

If your rockers were that loose, it must have been making a racket? Did you look to see if the valve springs were broken? Did you try to twist, or turn the spring by hand? on cylinder #4

The easiest way to tighten these up, is by running the engine, and setting them, Otherwise you MUST find TDC on this PARTICULAR cylinder, using the distrbutor rotor,,, (the crank TDC mark won't be there, to line up, or help you find it!) So, you can't go by that.

And did you set cyl #6 on TDC, using the distributor rotor, and the crank TDC mark as a guide (facing #6) on the cap, at this point you can loosen the rockers, and reset them at zero lash, plus 1/2 a turn down?

It's Simple to check the cam, you idle it with the valve cover off, and look to see if the rockers are moving enough to open the valves or Not!

 

[frustratedboater]

Re: Excessive compression..

I appreciate your advice on this.. Yes, it had a clattering, clicking noise along with the 'pop' in the rough idle. Sounded like it was running on six cyl's.

I did look for broken springs on cyl #4, where the loose rockers were, and tried to move them around but they seemed pretty tight. I didn't think about looking at the rotor as it fired on #4, or #6 to determine the TDC position on the piston. I did hold my finger over the plug hole as I turned the damper while both valves were still on the compression stroke. It seemed very hard to turn over the motor this way with my 1/2" breaker bar and all the plugs removed.

I looked at the threads on the rocker nut and they both seemed secured about the same position as the others. meaning I don't believe they backed their way out. I will try to get TDC and tighten the nut down. Knowing all the rocker play I saw, I don't believe their will be enough threads to lash them up. The nut will bottom out and I should still have play-but I'll try. Seems like my lifters just collapsed!

It appeared to be running smooth and fine last fall, then all the sudden all this. Very strange to (possibly) suddenly loose cam lobe lift.

 

[MikDee]

Re: Excessive compression..

Then you best take the intake manifold off, and see what the Cam, & lifters look like, they could be collapsed, there's definitely something funky goin on here?

 

[JustJason]

Re: Excessive compression..

Eagh.... did the studs start to pull out? Knock the rocker out of the way and pull out a pushrod and see if its bent. If neither of those 2 things happened there should be enough thread on the stud to bottom out the lifter completely.

 

[frustratedboater]

Re: Excessive compression..

Thank you for the advice. The studs look to be normal and the same height as the others. I am going to perform the valve lash adjustment and go from there. After talking to numerous individuals, I believe that I may have collapsed lifters as you mentioned I may have. Thinking about how smooth the motor ran just last fall and compared to how it's running now, I'm placing my chips on the two lifters as the culpret. There's not a ton of hours on the motor and don't think the cam is worn out that much to give me the play I'm experiencing. I borrowed a Chilton's Automotive Service Manual from school that gives me the adjustment patterns and will hope for the best as I run through the motor.

 

[MikDee]

Re: Excessive compression..

If you do have collapsed lifters, I don't think they're gonna fix themselves, I would pull the manifold, and check things out, especially the camshaft, the lobes could be worn out (worn round, instead of any lift).

 

[frustratedboater]

Re: Excessive compression..

I was hoping not to have to pull the distributor and intake to avoid additional gasket costs, but I think you're right. That will tell the tale of the tape there! I was looking at an automotive forum where they talked about hydrating, or soaking lifters in oil before assembly. If I do have collapsed lifters, and thats all, can I bring them back to life? If not, should I replace all 16? If my cam lobes are visually worn out, do you recommend just cam & lifters, or should I include a new timing chain as well? Thanks...

~FB~

 

[MikDee]

Re: Excessive compression..

I was hoping not to have to pull the distributor and intake to avoid additional gasket costs, but I think you're right. That will tell the tale of the tape there! I was looking at an automotive forum where they talked about hydrating, or soaking lifters in oil before assembly. If I do have collapsed lifters, and thats all, can I bring them back to life? If not, should I replace all 16? If my cam lobes are visually worn out, do you recommend just cam & lifters, or should I include a new timing chain as well? Thanks...

~FB~

Hydrating, Soaking in oil, I dunno?

Your best bet is to replace the timing chain, & gears, if replacing the Cam, it's not that much more work, or money for the security. Just take note of one thing I found when you do this: I've noticed when you line up your timing marks on the gears, next to one another, on the small block Chevy, you are usually set to fire on Cylinder #6, with the TDC mark lined up on the crank pulley. This happens to be the exact opposite cylinder of #1, so drop your distributor back in with the rotor facing #6, and cold adjust those valves relating to this,,, Then crank your engine over 1 turn clockwise, and adjust the valves relating to #1, then you are set to go. I like to do the final valve adjustment, after cam break in, with the engine warm, & idling

 

[frustratedboater]

Re: Excessive compression..

Thank you so much for offering your help with this. It's nice to be given a little heads up before diving into this. I will start tonight little by little. If you lived in DE, I'd offer to take you out skiing, or a day on the Chesapeake when this things done! I REALLY appreciate it,

~FB~