454 Low Vacuum + Flutter?

[RayRay70]

My Mercruiser 7.4 454 vacuum reading seems low, and the needle flutters *I think* like it has worn valve guides. It has the stock cam + stock pretty much everything else, except electric fuel pump and regulator. Engine serial# 0D390851 . Marine Quadrajet carb.

Idles pretty steady at 600 rpm with ~5 lbs fuel pressure. Runs great at all speeds. Suffers Quadrabog when I drop the hammer, but that's probably a choke issue I'll be working on. Should vacuum be closer to 20 hg?

Short video of vacuum reading.

Thanks.

 

[Scott06]

My Mercruiser 7.4 454 vacuum reading seems low, and the needle flutters *I think* like it has worn valve guides. It has the stock cam + stock pretty much everything else, except electric fuel pump and regulator. Engine serial# 0D390851 . Marine Quadrajet carb.

Idles pretty steady at 600 rpm with ~5 lbs fuel pressure. Runs great at all speeds. Suffers Quadrabog when I drop the hammer, but that's probably a choke issue I'll be working on. Should vacuum be closer to 20 hg?

Short video of vacuum reading.

Thanks.

Yes it should be higher 17-20 ish. Have you done a compression or leakdown test? Wondering if a valve is hanging up or if you have an intake or carb base gasket leak.

If you had worn valve guides I would expect oil smoke on start up and oil fouling on spark plugs

 

[Scott Danforth]

timing affects vacuum at idle

 

[RayRay70]

Yes it should be higher 17-20 ish. Have you done a compression or leakdown test? Wondering if a valve is hanging up or if you have an intake or carb base gasket leak.

If you had worn valve guides I would expect oil smoke on start up and oil fouling on spark plugs

Thanks, Ill run those tests and post the results.

 

[RayRay70]

timing affects vacuum at idle

Thanks I'll check timing and post results.

 

[kenny nunez]

Keep in mind that there is more vacuum when the boat is on land, the back pressure when the boat is in the water on the exhaust will change the vacuum reading.

 

[RayRay70]

Keep in mind that there is more vacuum when the boat is on land, the back pressure when the boat is in the water on the exhaust will change the vacuum reading.

Didn't realize that, thanks. Do you think it would be as low as I'm reading out of water?

 

[kenny nunez]

I had a MK4 330 hp 454 ”V” drive in. 24’ Campbell, I could get 14-15 ins. On the trailer. Roughly 1/2 of that in the water.
google the procedure for adjusting the secondary air valve spring on your Quadrajet.

 

[Bondo]

My Mercruiser 7.4 454 vacuum reading seems low, and the needle flutters *I think* like it has worn valve guides.
Idles pretty steady at 600 rpm with ~5 lbs fuel pressure. Runs great at all speeds. Suffers Quadrabog when I drop the hammer, but that's probably a choke issue I'll be working on. Should vacuum be closer to 20 hg?

Thanks.

Ayuh,..... Welcome Aboard,......... So what's the Problem,..??..??

 

[RayRay70]

Ayuh,..... Welcome Aboard,......... So what's the Problem,..??..??

Hey Bondo, thanks. I guess I don't know for sure what this engine's vacuum is supposed to read at idle, and I'm worried about the flutter in the vacuum gauge indicating an issue that I could exacerbate by continuing to run.

I guess I'm hoping for a marine-454-whisperer to either tell me the readings are fine, or tell me they aren't, what they should be, and how to fix it.

I don't want to blow up my motor

 

[Tail_Gunner]

Your over thinking here. Make sure your drawing vacuum from the intake not the carb. Make sure all the vacuum ports are freshly plugged..aka no leaks.

Still bad low readings, dump a few cans of seafoam down the carb, let the thing soak under high heat. Yes snake oil clears up sticky vales.

Check your timing.

If your still low after all that go to orileys get a compression tester. Pull a plug and take a read...actually more that just one.

Low readings squirt some oil in the cylinder...readings go up... uhh oh bad rings.
All of the above should take a hour or so


Ne

 

[Scott Danforth]

bog is probably either air valve or accelerator pump

a compression test will tell you if you have sticky valves.

 

[Tail_Gunner]

Perhaps a more complete guide towards using a compression test.

Wet Compression Test Results Explained

What is a wet compression test? It's important to understand the results of a cylinder compression test. The procedure and results are easily explianed here in this article about cylinder compression and wet cylinder compression testing.

www.freeasestudyguides.com

 

[RayRay70]

Update: I used a zip tie on each end of the vacuum hose going from the vacuum break to the carb and voila. On the bright side I can do anything I need to a quadrajet, now.

Thanks for all the help narrowing this down.

 

[dubs283]

On the bright side I can do anything I need to a quadrajet, now.

Yep

Then totally get it to top performance level. We're talkin straight up hp, torque and economy!

GM makes amazing products that transcend decades. Can't wait for folks to add information about all their decrepit cars they've kept alive, somehow....