Leak Down Test for 454's

[spartonboat1]

I have been chasing a loss of power for a while now. starboard engine runs about 200 less than port. Starboard got carburator rebuild, valve stem seals and general clean up. new plugs etc. 12 Degrees BTDC. Just checked plugs on 1357 since easy to get to . they are a nice brown color and even. I can find nothing wrong with this engine. nothing we have done has made any significant difference.

Here is another odd thing. plugs in port engine are one season old, pulled the 2468 since easy to get to and 2 was dark brown, others were dry black. smokes at idle, carb needs rebuild. Any how this engine is stronger than the one with new carb and all other work done. Again both engines have nearly identical compression and cylinders are within a pound or 2 even.

Wish I knew more about these things. Learning every time I work on it.

Planning on giving port engine (the good one) same job as the starboard one. carburator, pull intake, clean up any sludge, pull valve covers and clean up and sludge. change the valve stem seals, clean up lifters, push rods etc. bead blast intake and valve covers and paint them up. Etc.

Can anyone tell me if these are "pure" carburated units (no electronics, only pure fluidics), or are they one of the past 10-20 year hybrids with "dancing needles", or some other electronic units. I had the hybrid carbs on cars, and if any sensor and/or transducer starts to lose calibration, it will drive the computer crazy that controls the carb and create all sorts of strange symptoms. Hard starting, loss of power, surging, etc. So I am curious about the carb.

 

[Maynard Rupp]

They are old fashioned big block Chevy truck engines with Quadrajunk 4 barrels. If you go to "everything else" and look at the latest page under HOF party~~~, you will see a photo of our engines.

 

[krush]

They are old fashioned big block Chevy truck engines with Quadrajunk 4 barrels. If you go to "everything else" and look at the latest page under HOF party~~~, you will see a photo of our engines.

He's referring to computer carbs that were on the early 80's cars. Quadrajet had some crazy stupid computer carbs.


Sparton, they are just regualar "carbs". Quad's have "dancing needles" (metering rods) that move up and down due to vacuum. Holleys and the like have powervalves. Quads are great carbs.

More specific info on the carb and parts (ignore the high-perf stuff).

http://www.florida4x4.com/tech/quadrajet/500cid.com/Quad.htm

 

[MikeP]

A compression test is useful but only tells you one thing - compression. What this, in turn, tells you is not as straightforward as it may seem.

For example - let's say we have two identical 454 gas engines. We'll ignore any differences between individual cylinders for this example and say one engine shows all cylinders with 150 psi compression and the other shows all cylinders 160psi. Factory manual says 145. Which one is better? The 160? Actually,You can't tell from that test.

Your 160 compression engine could EASILY have more cylinder leakdown than the 150 and, therefore, less power and efficiency. You can't know this without a leakdown test. A compression test is half a test. Actually, I'd suggest a compression test is 1/3 a test with the leakdown providing the other 2/3 and lot more useful information. Compression tests are handy because they are quick and easy. Their best use is just like oil samples - useful when done periodically to detect changes. When your engine goes from say 140psi to 160psi over a couple of years (and they often do), that engine is not magically more efficient.

What is happening is that carbon is building up on the piston and the valves, reducing combustion chamber volume and increasing static compression. However, at the same time, blowby has increased past the rings due to normal (or abnormal) wear but this is masked compression-test wise by the increase in compression within the combustion chamber.

Again, I think compression tests are great but NOT as a sole engine diagnosis tool. As an aside, the typical specs for compression is that the difference between the high and low cylinder is +/- 10%. Frankly, that's TERRIBLE and if an engine rebuilder can't give you an engine with a MAX difference of 5psi between the high and low cylinder, find a new engine builder. If you are looking at a performance engine rebuild, than that figure should drop to 2psi and I'd call that 2 a minimum acceptable standard with 0 being what I'd expect.

 

[34Hatt]

Steve it could be difference in the props have them prop scanned and have the Tach's been checked??? Also check them through out the total range!!!

 

[67hat34c]

Dan, digital tachs on helm jive with the analog tachs on the flow scans.

hand held digital jives as well.

4 blade set were sent to General Propellor who called and they are done, they took another 2 " of pitch out. Will install them once we pull it out and have bottom done. will see what happens.

Mike, if we were getting blow by wouldnt it be using oil? She uses none and no smoke.

I plan on taking back the crappy leak down guage and buying a better one and check both engines out.

Mike, will seafoam remove all the crap or will we have to pull the heads. We have seafoamed it to death and no apparent change in compression.

 

[MikeP]

Well, blowby is relative, of course. To have NO blowby is impossible from a practical standpoint but, as you noted, in a "normal" engine, blowby is usually a non-issue if it is not consuming oil. (You can also have oil consumption via valve seals/guides that is not related to blowby at all.)

Frankly I will admit that IF there is no oil smoke or noticeable consumption and the compression checks are in a reasonable range, I wouldn't worry about it at all.

Re Seafoam - I have never used the product so I can't say how well it works. I'm generally not a believer in any additive that claims it can remove carbon from rings/pistons/valves. I have disassembled a lot of engines where the owner used these products and have never seen any conclusive evidence that it made any difference.

 

[captddis]

Would you give a example of a real world marine gas engine that has good compression and excessive blowby? I have been working on marine engines for 35 years and have never seen a case like that. Likewise I have never seen carboned up main engines. Underloaded generators yes engines no. In the case of the generators they had low compression and blowby.
No mention of vacuum gauges, they can tell a whole lot and only take seconds to connect.

 

[Avenger]

I too was wondering how much carbon it takes to make up for bad rings. Unfortunately I don't have the luxury of being able to do that kind of research.

Vacuum gauge? So I know when to change the bag?*

*Diesel humor

 

[67hat34c]

did vacuum check a while back but did not have any specs to compare it to.

idle vacuum, acceleration etc.

Maynard do you have this info?

Mike, Seafoam is something most on this site swear by. poured directly down carb throat. Have also used ATF in the past. Would guess it may work better on cleaning the intake valves.

May pull heads but dont want to if I can help it. Why break a perfectly good seal.

Think we will wait and see when I change the props.

 

[Avenger]

I would never pull the heads off an engine unless I knew for sure that I absolutely had to. I've got customers who do it all the time and it just boggles me.

Everything you've explored so far says that the engine internals are good. The only thing I can think of internally, and it's a stretch (there's a pun in here, wait for it) is cam timing. IF your engines are counter-rotating, then your port engine's cam is probably gear driven for opposite rotation and your sbd engine's cam is chain driven. Given enough hours, or those classically crappy nylon-coated timing gears, the timing chain may have worn which has the effect of "stretching" (there it is) the chain. Or the nylon has made its way into the pan creating excess slop in the chain drive. Other than that it has to be a fuel or ignition problem, if it's an engine problem at all. Chances are that bad cam timing would show up in a compression test, and almost certainly on a vacuum gauge.

If you want a quickie check for this, pull the distributor caps and bar the engines against rotation. See how many degrees the crank can turn before the distributors move. If the stbd engine has significantly more movement than the port engine it may be time to pull the timing cover off.

 

[captddis]

one note, if the engines turn outboard, the stbd has the gears and the port is auto rotation/ chain. If you have BW 1.9:1 gears they turn inboard and the port is then opposite. Std auto firing order is 18436572, opposite 12746348.

 

[captddis]

I too was wondering how much carbon it takes to make up for bad rings. Unfortunately I don't have the luxury of being able to do that kind of research.

Vacuum gauge? So I know when to change the bag?*

*Diesel humor

the bags must only be on 3160 Cats.

 

[Avenger]

one note, if the engines turn outboard, the stbd has the gears and the port is auto rotation/ chain. If you have BW 1.9:1 gears they turn inboard and the port is then opposite. Std auto firing order is 18436572, opposite 12746348.

This is why I use phrases like "port engine is probably gear driven." I'm glad somebody has actually seen these things firsthand.

 

[Avenger]

the bags must only be on 3160 Cats.

Q: What's the difference between a Hoover and a Harley?

A: The location of the dirtbag.

A repeat, but a funny one Or was that a crack about letting the CAT out of the bag?

 

[captddis]

You know the one about the Harley having the intake too close to the exhaust?

 

[67hat34c]

starboard is the counter rotating and has the gears so i guess timing gear problem is not the problem. Besides I do not believe there are nylon gears in the standard rotation engine, these are crusaders based on the GM industrial block, not a mercruiser based on automotive block.

In our quest for proper engine performance we found many small problems with both engines but nothing has made a significant difference with the exception of finding the distributor nut on the starboard engine loose and the distributor backed itself to TDC. When that happened it would hit about 3600 then die back. At the end of last summer we did the carburator, new lifters, valve stem seals and cleaned up an sludge in the valley and under rocker covers/top of heads. changed oil in that engine and put in 10w30 rotella. since that time it has been used a few times, guessing 30-40 hours and oil remains clean and as of the last off shore trip, no oil use.

 

[MikeP]

I can think of many auto engines I worked on that showed good compression but excessive blowby. It is actually quite common. When we bought it, the 350CID Crusaders in our previous boat showed compression in the 165 range yet the engines burned some oil due to blowby. This happens because of carboned-up piston ring grooves which do not allow the combustion pressure to properly seal the rings when the engine is running. A static compression check shows fine because there isn't really all much pressure in the combustion chamber compared to the pressure in the chamber when the mixture is ignited. The compression rings (top two) function by expanding due to the pressure of the combustion gases as the gasses pass between the upper surface of the ring and the back of the ring (within the ring groove) and exert considerable outward pressure. If the rings only functioned by there own "springiness," they wouldn't work at all in operation.

However, from a practical application, unless the engine is pumping oil, the blowby present is normally of little concern.

If you are really interested in leakdown testing, here is an excellent article that walks you through it - it's the best one I found after doing some searching and it pretty much mirrors the method we always used: http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/index.html

 

[Avenger]

You know the one about the Harley having the intake too close to the exhaust?

Yes I do, and I'm not going to repeat it here. I'm already in enough trouble with the ladies on this site from my comments in the "corporate greed" thread.

Now I've probably alienated all the Harley riders on the forum too. I think I'm going to shut up now.

 

[captddis]

Again the question was marine gas engines, not hot rods. If the engine has compression it should be able to make power. I have never seen a gas marine engine where carbon was a problem. Only on generators. Yes a leakdown test will tell if it is valves or rings, SO what? if it is either the heads have to come off. Once off it should be obvious as to which it is. All engines have blowby to some extent, A crankcase pressure test will tell if it is excessive and it is checked at WOT. The leakdown is checked static.
As far as diesels, yes they are tested by doing a leakdown test and a crankcase pressure test, In fact most diesel manufacturers do not publish compression specs. but that was not what we were discussing.