Compression test results- WOT issues, 4.3L LX

[Baylinerchuck]

1990 4.3L LX with 670 hours. D428255.
Alpha 1 out drive.

Troubleshooting a WOT issue and lack of power. Performed a compression test, #1 117 psi; #2 120 psi; #3 112 psi; #4 120 psi; #5 114 psi; #6 115 psi. Taking into consideration that my gauge seems to read 5 to 10 psi on the low side as best as I can tell by comparing to a few other pnuematic gauges I have, are these readings normal, or a bit on the low side?

This motor has a quadrajet 4 bbl that I just rebuilt, so I'll be looking at that again.....
All the typical maintenance stuff has been done on this Chaparral 2000SL, plugs, wires, cap, rotor, timing, fuel pump cleaning, fuel line replacement, fuel filter install, pickup tube inspection, sea foam, oil and filter, shift cables adjusted. Barely will get on plane and runs at 3000 to 3200 rpm with throttle fully advanced. Oil pressure reads around 50 psi. Put stock prop of 14.75 x 21 back on. Verified secondary plates open, however top plate on secondary closed. If I manually open secondary top plate, rpm falls. Divorced choke working. What's strange to me is when cold, divorced choke pulls plate closed, engine idles at 1500 rpm. There is no high idle cam on a marine carb. When choke slowly opens as engine warms up, idle also slows down to 900 rpm at full open choke.

Am I right to be going back to the carb for further investigation? I have heard from auto mechanics that quadra-junk carbs can be finicky, ( their words).

Any help is appreciated. As info I bought this boat used and is my first I/O. i have two 4 cylinder Johnson outboards that I resurrected, but this 4.3 is a weeeee bit different,

 

[alldodge]

compression test, #1 117 psi; #2 120 psi; #3 112 psi; #4 120 psi; #5 114 psi; #6 115 psi. Taking into consideration that my gauge seems to read 5 to 10 psi on the low side as best as I can tell by comparing to a few other pnuematic gauges I have, are these readings normal, or a bit on the low side?

If I manually open secondary top plate, rpm falls.

I would expect to see pressures closer to 150 psi if not a bit more.

The top plate when manually opened allows more air in, and since the motor is not able to burn the amount of fuel mix, power drops. Kind of like flooding. The Q-jets are great carbs when adjusted correctly, and the vacuum secondary's work well when the motor is running good.

Suggest doing a leak down test to find out if its rings or valves

 

[Alumarine]

Was the compression test done with the motor warm, all plugs out and the throttle wide open?
If not, that and perhaps the gauge might explain the numbers.

 

[Oshkosh1]

Take an oil can and pump a couple of squirts of oil into the cylinders then take your compression readings again. THAT will tell you if it's the rings.

As far as the valves here's a fairly easy way to do it...no leak down tester necessary; You do need compressed air. Either a compressor or an air tank. Rotate the engine until the desired cylinder is at TD BD-C (Top Dead or Bottom Dead Center). This will ensure the cylinder is closed and sealed completely. Also, for this test leave all the plugs IN so that if the cylinder is not exactly at TDC it won't rotate due to the air pressure. Then, introduce 15-20psi of air. Now...use your ears. Listen for any hissing. Either through the carb or out through the exhaust. That will tell you if you have a leaking/bad or burned valve seat or bent valves. In your case with such low numbers across the board I don't think it's your top end. More than likely a tired engine with leaking/sticking rings or worn pistons. Now...even with those numbers you should still have enough power to get up on plane though so they alone aren't necessarily your immediate issue.

As far as that goes...I'd look at the carb, fuel pump or a plugged fuel filter.

Good luck!

Suggest doing a leak down test to find out if its rings or valves

 

[Baylinerchuck]

Great information. Used a squeeze bulb this afternoon in line just ahead of the fuel pump, no change in result so I don't suspect the fuel pump. Changed filter during carb rebuild and added an in line filter ahead of the fuel pump. I will definitely try the leak down test when I get some time, but will focus on the carb for now. To answer Marc's question the test was performed cold, with all plugs out, throttle closed.

 

[Baylinerchuck]

As info, the boat quickly gets to 2500 rpm and stays there with throttle pushed full open. It will slowly....painfully slowly climb on plane and rpms build to 3200. It stays at 3200 and runs smooth. It doesn't misfire, or drop off, just stays right there.

 

[nola mike]

Warm engine, throttle open will increase those numbers.

 

[Scott Danforth]

Great information. Used a squeeze bulb this afternoon in line just ahead of the fuel pump, no change in result so I don't suspect the fuel pump. Changed filter during carb rebuild and added an in line filter ahead of the fuel pump. I will definitely try the leak down test when I get some time, but will focus on the carb for now. To answer Marc's question the test was performed cold, with all plugs out, throttle closed.

You have an outboard squeeze bulb on an I/O? That is not legal, or safe. Not to mention a major fuel restriction

The issues you describe could be fuel system related. When you went the the carb, did you clean ever passage with a fine wide?

However the low compression numbers could be contributing.

 

[Baylinerchuck]

You have an outboard squeeze bulb on an I/O? That is not legal, or safe. Not to mention a major fuel restriction

I used the squeeze bulb simply to prove the fuel pump was keeping up with the motor. The carb is hard plumbed from the fuel pump and I didn't want to destroy my new fuel tubing to install a pressure gauge. A gentleman with the screen name Bondo mentioned doing this in another thread, and honestly I believe this is a great test. The squeeze bulb will not be left there permanently.....that would be pretty silly.

I cleaned the carb with carb cleaner using compressed air on all passages. The fuel filter in the carb was completely full of white powder, which was aluminum corrosion from the fuel pump. The filter was replaced during rebuild. The fuel pump was also cleaned and put back together. I used sea foam in the first 10 gallons of fuel.

Thank you for the great suggestions and help. It helps me to decide what to focus on. Much appreciated!!

 

[Oshkosh1]

Are you sure your butterfly's are opening properly? Make sure your secondaries are linked correctly. Sounds like they may not be performing at WOT. The primaries are trying to keep up with the demand but can't provide enough air. Read your plugs as well. What color are they? Post a pic here...one of my other hobbies is old British sports cars with dual carbs and can usually read the timing and mixture with a pic...

 

[Baylinerchuck]

Butterfly linkage is correct. Both primary and secondary are open when throttle is fully advanced. Top plate over the secondary operates on vacuum and does not open. If I manually open that plate engine rpm falls off at WOT. Going to look at fuel level in the bowl today and fully inspect the carb again. I have trouble posting pics, I guess my iPhone is too high resolution for here. The plugs are brand new, and still look that way. No black or brown on the anode or electrode, or the insulator. They still look new.

 

[Baylinerchuck]

Took top off carb. Verified all passages clear. Float was set to 1/4", adjusted to 9/32" per data sheet. Also adjusted air valve spring. Seems like the float adjustment went the wrong way to help with this problem. I'll have to wait to test her out, no more time this weekend. RAIN.

 

[Baylinerchuck]

Ok just an update, problem is still not solved. Took the carb apart again for the third time and did find one of the idle off jets blocked. I tried everything to clear it aside from drilling out the plug on the bottom of the carb. I ordered a reman. carb. from National Carburetor Inc. While I was waiting for that I also replaced the thermostat and sleeve. I bolted the carburetor on and really had no noticeable change in performance. Still my max RPM is 3400 at WOT position. I get on plane better due to installing a fin on the out drive. I also now have two hesitations while advancing the throttle with this reman. carb.

This problem is really baffling the crap out of me. I am running a 13.75 X 21P prop, so I guess I can down size the pitch to a 19P and that will help. I know that's just a band-aid though. Maybe the motor is just too tired for this 20' boat.......suggestions?

PS I'm really missing my outboards at this point.

 

[alldodge]

With the compression numbers posted your motor is tired, it probably needs a rebuild. If you do a leak down test and turns out to be the valves you might get away with just freshening the heads

 

[NHGuy]

I'm sorry to be so late to the thread, but I think I can point you in a good direction with the OE Qjet. I'd use the factory one BTW since it will have the right jets and etc.

Take a look down the throats with the secondary flaps held open, have someone operate the throttle with the engine off. The primary throttle blades open first and all 4 should be vertical at WOT.

Since your engine won't pull higher rpms you might have a problem with metering. The metering rods rise out of the jets and allow more fuel when you open the throttle. If the metering rods are stuck or somehow messed up you only get low speeds. Does the engine pop or sneeze?
The wrong gasket could have caused this too. So when the carb is apart verify the holes on the gasket line up with your body. The kits come with gaskets that cover different versions, not all are needed.

The secondary air door spring tension can be adjusted without removing the carb. But it's easy to screw it up and loose the spring catch which requires removing the top and reassembling it. If you decide to reset the tension as a last ditch try, make sure to hold the existing spring tension with a small straight blade as you release the spring tensioner bolt with your torx bit. The spring should be tensioned to 3/4 turn from zero (contact).

If there is a restriction in the idle tubes they should be removed and the passage they sit in cleaned. It's the biggest pain in these carbs, and the most helpful step too.

Go to quadajet.com or something similar to familiarize youself with how the tubes look. They are about 3" long with a restriction down at their bottoms, and there is a collar at their tops which you access next to the the primary venturi's.
Get a set of flat tip punches, cheap at Harbor Freight. Tap the inner tube down a little bit, like 3/32". Get a deck screw and grind off it's tip a little. put the screw into the collar and turn it enough to grip the collar. Get some side cutters and then a bit of flat metal, maybe a metal ruler. Use the cutters to grip the screw and the flat piece as a protector for the soft aluminum carb body. Leverage the tubes and collars out of the body.
Now reclean the carb body. Don't forget to be sure the tiny holes in the primary and secondary bores are clear.

As I said above when you reassemble it, that secondary air spring needs to be 3/4 turn from where it contacts the stop. The 4.3 barely gets any secondary activity, at WOT it's able to pull a little bit of air, but the spring has to resist the engine air flow the correct amount or the engine will over fuel and you get the infamous quadrajet bog.




If the carb has a little metal tag on the front get the numbers off there and look it up. That gives you the original build settings. There are lots of ways you can tune the jetting and metering. I'd avoid that though and look up how Mercruiser jets the carbs for 4.3's. You will need to know the metering rod, and spring sizes. And you will need the jet sizes.​

I know that's a lot of detail, but those carbs are awesome. They just need to be set up well.

 

[Baylinerchuck]

Thanks so much NH Guy for your post. I wish I could have bent you ear two weeks ago!!

I bought a remanufactured carb from National Carburetor.....these are factory set to the motor you specify, so I bought one setup for a 4.3LX. I still have the old one but was going to send it back as a core. I guess I'm starting pretty fresh on this one. This carb came with a tag with the vacuum it pulled and idle rpm while on their test engine. They say they test everything prior to shipping. It smelled like gas so I'm confident they did.

I adjusted the tang on this carb so the secondaries open a little later than was set from NCI. The secondaries still open fully at WOT. I need to test these changes and see if it made any difference.

The old carb did not pop or sneeze at all. Just like the newer carb I just put on, the engine runs strong to about 2500 rpm then just stops accelerating. The new carb runs stronger up to 3000 rpm and gets the boat on plane better, but the engine doesn't rev beyond 3400 rpm in the water. The new carb seems to make the engine stronger but has 2 noticeable dead spots where the engine studders while advancing the throttle. I may have to call NCI and see what they say, as I can't go into the new carb without voiding a warranty.

So looking at your comment on the secondary top plate not getting much activity in a 4.3. When I was testing the old carb while running at WOT I tried to push open the top plate and the engine lost rpm, stumbling pretty bad. The top plate doesn't come open at all by itself at WOT so I definitely don't have an over fueling issue.

If I remember correctly the main jets in the old carb were stamped 66. I haven't shipped the old carb back to NCI so I could verify.

All Dodge is leaning toward a tired engine being my problem, and sadly I'm beginning to think he's right. I have a few more things I'm going to try and some tests to run. I really would like to get another season out of this motor. I have about a month of testing left in the season......we shall see. I'll post updates here as I progress.

 

[Rick Stephens]

Might look at running a vacuum gauge test. Can give interesting insights to motor diagnosis.

 

[Baylinerchuck]

Might look at running a vacuum gauge test. Can give interesting insights to motor diagnosis.

I agree Rick. I just picked up a pressure/ vacuum gauge from Harbor Freight. Now I just need to find some time.....

 

[NHGuy]

Go down to a 17" prop. It still won't make 4600 rpm probably though. How big of a boat is this, and what is the drive ratio?

 

[Baylinerchuck]

Go down to a 17" prop. It still won't make 4600 rpm probably though. How big of a boat is this, and what is the drive ratio?

20' chaparral 2000 SL Sport.
Alpha 1 gen 1 ser. 0D464664. I'm not sure of the ratio but assume it's the right one for the 4.3l.
Motor ser # D428255.

Thanks for your interest in helping me out!! Much appreciated.