5.7 strange carb issues

[Rick Stephens]

Interesting conundrum. I don't know anything about how a weighted advance system would work on a boat motor. The advance curve is not nearly as predictable as a marine programmed electronic module will give you. And may be designed for a car or pickup in road conditions and not a constant load motor like we have in boats. Your video definitely indicates you have a lean sneeze, which is either your throttle pump isn't pumping, your carb is too lean in the midrange section - depends on what carburetor is installed, or the ignition is way off.

Like MadProps, I failed a Thunderbolt module and chose to install the Delco Voyager EST in my boat. It has been perfect for years.

 

[JayEll87]

Looked back a couple posts and it looks like you have quite the cobbled together ignition setup...
It looks like the previous owner was using a mechanical advance ignition setup... which means the thunderbolt module probably doesn't exist in your engine compartment as it was probably mounted to the old distributor...

I'm guessing they ran into a situation similar to the one I just had... My thunderbolt module failed and its a small fortune to replace... However, instead of switching to the Delco electronic ignition, it looks like they went backwards in time...

Also, that might explain why your new MSD distributor didn't fix anything, because if you wired it in to the existing harness, theres no module there to control the advance.

My recommendation at this point would be to follow what I did, and replace your ignition system with a Delco EST system. It was very easy to install and worked perfectly...

Indeed the ignition setup is quite a mess, I agree Thunderbolt module does not exist so the advancing is done with the mechanical advance inside the distributor only. I asked advice from local V8 guys and quite many have used mechanical advance which have worked fine. Probably not as fine as the original thunderbolt but there should not be this kind of issues I'm currently having.

I'll keep in mind this Delco solution, but as I spent a fortune to the new MSD Marine distribution (as it's marine I suppose it should work? ) I try desperately (or stubbornly) to get this set working. It worked originally but something went wrong..
I presume if timing curve is matching the one in below link, it should be ok?
https://forums.iboats.com/forum/eng...-mercruiser-thunderbolt-ignition-systems-work

Interesting conundrum. I don't know anything about how a weighted advance system would work on a boat motor. The advance curve is not nearly as predictable as a marine programmed electronic module will give you. And may be designed for a car or pickup in road conditions and not a constant load motor like we have in boats. Your video definitely indicates you have a lean sneeze, which is either your throttle pump isn't pumping, your carb is too lean in the midrange section - depends on what carburetor is installed, or the ignition is way off.

Like MadProps, I failed a Thunderbolt module and chose to install the Delco Voyager EST in my boat. It has been perfect for years.

Carburettor is a 2bbl Mercarb which has been opened many times now. That's "good" to know that sound is lean sneeze type as I wasn't sure. At least there is a good stream of fuel when throttle is added so I presume accelerator pump is working correctly. How can I adjust midrange section on Mercarb?
Ignition I'll still have to study, but it follows the above shown curve also in real life, if the curve is not correct then there might be an issue also on this..

 

[achris]

Since the engine was previously 'messed about with', it may be that whoever put the mechanical advance distributor in :facepalm: also changed jetting to get it to run properly with that ignition system. You may need to pull the carb and check all the jet sizes.

Check the carb number and make sure it's the right carb (other things have been changed, you never know.)
Here's the full spec for the carb...

Chris.......

 

[Jcris]

Very interesting thread,
Hope you get this resolved

 

[tank1949]

FUBAR! Is the electric fuel pump a marine grade pump for that motor or has someone substituted an automotive pump. Go back to marine!. Have you pulled gas tank pickup tubes and examine internal filters not to be clogged up? I owned a 2900 proline whose former owner allowed gas to set in tanks for over a year and it caused issues. Some tubes have internal screening that act as a filter before gas reaches canister filter. Two of the Proline's were clogged up.

 

[JayEll87]

Right, carburettor opened again and everything inside is acc to table informed by achris.

Fuel pump is Carter marine pump and after that I have holley pressure regulator with pressure gauge and fuel pressure is adjusted to 6 psi. During acceleration and sneezing there’s no indication of fuel pressure dropping so in that respect I think pump is working correctly.
And again, I only have issues during acceleration, when boat gets on plane and stops sneezing she works fine and does not have any hesitation after that. WOT is approx 4500rpm and as she works fine with high revs and high fuel flow, I presume she fuel delivery is sufficient

 

[JayEll87]

One question of the spark plugs. Manual indicates three possible types, two of which are in below picture. MR43LTS and BPR6EFS. BPR has a lot more thread than MR and it would then be quite a lot deeper in the cylinder.

I asked local guys about this and the said that correct is BPR6FS, which has pretty much the same thread lenght as the MR43LTS. I now installed the BPR6FS plugs, was this correct choice? I haven't started yet so it's still possible to fix if it's not

 

[Rick Stephens]

Long thread goes into Vortec heads. I run the MR4LTS in mine. MR43T is the short pre-Vortec plug of same temp. The alternates you listed are same plug, different manufacturer.

 

[JayEll87]

Damn it.. Would that have something to do with this issue then as all plugs in engine were with short threaded?

 

[achris]

One question of the spark plugs. Manual indicates three possible types, two of which are in below picture. MR43LTS and BPR6EFS. BPR has a lot more thread than MR and it would then be quite a lot deeper in the cylinder.

I asked local guys about this and the said that correct is BPR6FS, which has pretty much the same thread lenght as the MR43LTS. I now installed the BPR6FS plugs, was this correct choice? I haven't started yet so it's still possible to fix if it's not

BR6FS are for pre-Vortec heads ONLY. And they are the same as MR43T, not MR43LTS...

Your serial number, 0M365765, should have Vortec heads. You should be using BPR6EFS or MR43LTS. Confirm you have Vortec heads. Count the intake manifold screws. Should be 8, and they should be vertical (non-Vortec screws go in at 45 degrees, and there's 12 of them).

Chris...........

 

[achris]

Just had a look at the previous photo of the plugs. I don't know how we missed it, but these are the WRONG plugs for your engine!




This is what the right plug will look like...



Chris.......

 

[JayEll87]

Yes so it is Vortec engine, I'm sure of it.. 8 bolts in intake vertically.

So I'll get a new set of plugs then.. Funny is that these plugs installed originally was told to be changed before winter by previous owner.. As he was my friends friend I did not have any doubts.. Well, now I do as the plugs do not look half year old..

I'll get a set of BPR6EFS plugs and give it a try. Luckily weather has been awful for the last 2 weeks so I've not missed anything

 

[JayEll87]

Plugs changed, did not hit the pistons
Meanwhile I noticed that few fuel hoses were not in too good shape anymore and I changed them with 10mm (3/8") hoses. After this pressure regulator showed 3psi fuel pressure only, thought it was the regulator and changed it, no difference. Thought it was the fuel pump and changed it ( Carver P4070 ) and no difference. So now I have all fuel lines and all equipment between tank up to carb completely new and pressure is stuck in 3psi and engine is barely running obviously due to low fuel pressure. Tank is full of new fuel and it does not make a difference.

I've understood that carburetted engines are picky on fuel line size, originally lines were 5/16" so could this have effect? I personally don't have any other explanation in my mind but before I redo these, it would be good to know..

 

[Rick Stephens]

A trick that I think came from achris is to T your vacuum gauge before the pump and see if you have a blockage between pump and tank. Like AntiSyphon.

 

[JayEll87]

I actually removed the tank dip tube and at least by blowing through the pipe there was no blokages.. Haven't used any vacuum gauge though..

I presume one thing to test the anti siphon is to take hose off of it and place it in remote tank?

 

[JayEll87]

Right, took off the dip tube, had o closer look and there was no ASV at all and pipe was clean / open all the way.
So I tried to use different hoses, with and without filters before pump, tank filling cap open / closed, with and without pressure regulator, engine running / not running and used remote tank and pressure did not increase over 4psi after pump at any time and obviously engine did not run even idle too well.
Called two local Mercruiser shops and they did not have too many magic tricks available but basically both thought that new pump is shot..

 

[JayEll87]

Update again despite there's not too much to update. Opened carb again and float arm was bent in a way that needle was not closing at all, adjusted it again and put it in place. After fuel pump started pressure increased to 4psi and fuel was constantly dripping from venturis. Opened carb again and float arm was again bent?? I was extremely careful when I placed the carb halfs together so I don't understand how the arm could be bent...

I try to get another carb for testing from working boat and see if it makes any difference.

 

[JayEll87]

I know I'm not supposed to come back to old topics but as it's winter season here now I have nothing better to do
So I took engine off the boat and started opening it, the "real" reason for all this(hopefully) was found from right hand side head, which was cracked between 3 and 5 cylinders enabling water to enter both cylinders, which also looked quite poor..

As per Mercruiser manual, stainless head gaskets should have been put together with "perfect seal" and these had none. There were some rust found also from left hand side exhaust ports but it does not necessary mean that water has entered there.
Anyway, block, crank, cylinders, pistons all looked fine so new heads and putting all together properly. Maybe this summer would be more successful Thanks all for help and have a better year 2021.