Testing Gas Flow on 82' Wellcraft Sterndrive

[foste1cc]

How can I go about checking the gas flow from the tank to the carb? I'm trying to chase down an issue of bogging out/spits at certain RPMs. If I disconnect the fuel line - post mechanical fuel pump - and turn the engine over, would this be an okay way of testing if the right amount of fuel is getting to my carb? Or do I need to pull up my floor panels hiding my tank and physically inspect the lines? I'm running out of things to check on this motor, and I'm starting to think it's not the motor, but the fuel supply feeding the motor. So just looking for a way to rule out fuel delivery issues. TIA!

 

[foste1cc]

Just in case anyone's wondering what's been done, here's a list
It's a mercruiser 470

I've been through the carb 3 times - clean as a whistle and float is set
New condenser - points were still in good shape - point gap set
New plugs and gapped
New plug wires
New ignition coil
New fuel pump and fuel filter, and of course new gas
Timing was verified at 8 BTDC

She fires right up and will idle all day, but will start spitting/back-firing and bogging out at around 1800rpm when I'm giving it throttle.

 

[JimS123]

If it idles you got fuel flow. That's an early aluminum engine, and not one that is highly regarded. Check compression next.

 

[Scott Danforth]

What was dwell set to?

 

[Lou C]

check the anti siphon valve on the fuel tank, it's what the fuel line from the tank to the fuel pump is connected to at the tank. They can corrode and restrict fuel flow, or get clogged with junk from the tank
what's the fuel from the fuel filter look like?

 

[mr 88]

A remote 5 gallon jug with hose hooked up to wherever you deem easiest junction pre fuel pump . Engine runs OK, you backtrack from the fuel line or start at the sending unit on top of the fuel tank. Runs the same , at least you've ruled out fuel delivery as the issue.

 

[foste1cc]

Point Dwell is set at 30 degrees
The fuel filter looks good and clean
Compression is at 112 +/-3psi across all 4 cylinders ( I know it's a bit low, but they're even, and it's not "bad"compression either. Doubting this is this issue for bogging out over 1800rpm)
Today I'll start by pulling up my floor board cover, revealing my tank. I'll visibly inspect the lines to check for deterioration and look for and inspect the ant-siphon valve as well.

Really hoping it's something dumb, and an easy fix.

 

[foste1cc]

check the anti siphon valve on the fuel tank, it's what the fuel line from the tank to the fuel pump is connected to at the tank. They can corrode and restrict fuel flow, or get clogged with junk from the tank
what's the fuel from the fuel filter look like?

I'm really hoping THIS is my issue!

 

[Scott Danforth]

your compression is really low for the 470.

 

[foste1cc]

your compression is really low for the 470.

Asking Google "What should compression be for merc 470" gives:

AI Overview
Learn more

For a Mercruiser 470 engine, a good rule of thumb is that each cylinder should have compression of 130 PSI or higher. While the specific range can vary slightly depending on the engine's condition and age, a general range of 110-115 PSI is considered acceptable for a 470. No cylinder should read less than 100 PSI. Additionally, no cylinder should have compression less than 70% of the highest cylinder. For example, if the highest cylinder has 150 PSI, the lowest allowable pressure for any other cylinder would be 105 PSI.

 

[foste1cc]

I don't disagree that the compression is low, as stated, but I feel this is unrelated and insignificant for checking fuel delivery, and ruling specifically fuel delivery, out of the equation.

 

[Scott Danforth]

Asking Google "What should compression be for merc 470" gives:

AI Overview
Learn more

For a Mercruiser 470 engine, a good rule of thumb is that each cylinder should have compression of 130 PSI or higher. While the specific range can vary slightly depending on the engine's condition and age, a general range of 110-115 PSI is considered acceptable for a 470. No cylinder should read less than 100 PSI. Additionally, no cylinder should have compression less than 70% of the highest cylinder. For example, if the highest cylinder has 150 PSI, the lowest allowable pressure for any other cylinder would be 105 PSI.

you may want to check the manual vs trusting the AI google

 

[kenny nunez]

Be sure to pull the suction tube from the tank and push out the fine mesh screen that is inside it, quite often that is where the restriction is. You might be able to blow the screen out if you have a compressor that delivers 140-150 psi by blowing into the fitting with the anti siphon valve removed. The tube should unscrew if you do not have a compressor. Leave the screen out.

 

[airshot]

If you have the floor up, why not just replace the fuel line with a good quality new fuel line, the cost is minimal for piece of mind.

 

[bajaman123]

When did it last run correctly? Is this a new-to-you boat? As in, has it EVER run correctly for you?

 

[foste1cc]

Was already thinking of replacing the fuel line and anti-siphon since i'll have the floor board up right after my last post haha, so it's on the shopping list.

The boat did run at WOT when I first bought it 3 years ago. I only had it on the water 3 times before winterizing. During the few times I had it out, I was very unimpressed with the supposed 170hp.., I felt like it was slow getting to plane, and only topped my 20' at 25mph. Now, I can't even get up to 10mph.

I wanted to switch to the pertronix electronic ignition, but ended up frying it somehow, so I went back to points and condenser. It's mostly been tested in my driveway with muffs the last 2 years. But every time I've gotten it on the water for a test drive, she boggs out once I get past the no wake zone. On top of the list I mentioned in my 2nd comment, I've also been through the lower unit 3 times. Nothings binding, it shifts F/N/R with ease. I've just been running out of things to check, because it DID run. And everything's switched back to the way it was. I'm becoming clueless..

However, I still have yet to pull up this floor board and inspect my fuel delivery. However, since I do plan on replacing it all, I can finally rule out fuel delivery, and hopefully that fixes my problem.

 

[mr 88]

I would install a electronic distributor before putting a Petronix system in . See them online for 120-150.00 , which is less money than the Petronix set up .

 

[Lou C]

If you maintain them points can be quite reliable and just carry spares….ive been boating with a points distributor for over 20 years with no on the water breakdowns…if you get a Petronix or an electronic full distributor replacement buy a spare module….

 

[Scott Danforth]

The boat did run at WOT when I first bought it 3 years ago. I only had it on the water 3 times before winterizing. .

I wanted to switch to the pertronix electronic ignition, but ended up frying it somehow, so I went back to points and condenser.

How old is the fuel?

Many of us have had pertronix failures

Your compression is low, so you have nowhere near the original 170hp. My guess with the low compression, you are in the 120 - 130 hp range

 

[foste1cc]

The fuel is 5 months old, and had stabilizer in it. After inspecting my fuel line pickup at the tank, the line is clean with no rot. Still looks brand new inside. I also have no anti-siphon valve. It goes from the 90 on the tank, to a strait barb with no internal workings.

The fuel filter is also brand new. It's part of my mechanical fuel pump which I bought new - and tested the diaphragm before installing.

I understand the compression is low. I did check my manual, and it said the same numbers as the AI overview I posted above. I don't see how I could go from WOT before, to not at all, like the flip of a switch. I'm no guru, so I could be completely wrong. It is an issue I'm planning to address, but I'm not there yet and I'm saving it for a new thread.

I know I mentioned that the points were in good shape. "I" never actually looked at them. I was taking my helpers word for it since they have a mechanical background. He's becoming of age and I guess doesn't see the best anymore, because they were pitted and corroded quite badly.
So I just installed the new points - I gapped at .025". My book says .022 in the specs, but says to set new points .003" wider - for the consideration of new wear.

I don't think I'll get around to resetting my dwell and timing until tomorrow. So I'll post back soon.

Thank you ALL for the help/criticism/suggestions/advice so far!