1974 4 cylinder 140 hp - distributor vacuum capped

[shizny]

Hi All-

I'm having some performance issues with my 1974 tri-sonic caprice (mercruiser 4 cylinder 140 hp). Initially, under load, the boat was a complete dog. The dash on the boat has a vacuum gauge and it was reading '0'. I traced back the line from the back of the gauge and found that it was disconnected and that there was an open port behind the carburetor. I plugged that port (because the line wouldn't reach very well which is why it probably popped off). Ran the boat and now I can get to about 2700 rpm @20mph. Much better than it was but still not where it should be. So, I was hunting around and noticed that the distributor has its vacuum port capped. Should I 'T' the port that I found behind the carburetor, run one line to the gauge in the dash and run another to the distributor? Could the capped vacuum port on the distributor be the power problem I am experiencing now ?

Thanks,
Josh

 

[ziggy]

something seems amiss to me.

The dash on the boat has a vacuum gauge and it was reading '0'. I traced back the line from the back of the gauge and found that it was disconnected and that there was an open port behind the carburetor.

so is the open port on the carb or on the manifold. it should be on the manifold. i've got a vacuum gauge too on my boat. it works as it should and if fed from the manifold. if the open port is on the carb. imho it must be the wrong carb. like off a car. as boat carbs don't have vacuum port generally.

So, I was hunting around and noticed that the distributor has its vacuum port capped.

imho, this is also amiss. boat distributors don't have vacuum advance. only mechanical advance. if your distributor has a vacuum port it is likely from a car too. this would be bad as it won't be marine rated for spark resistance. open sparks in the engine compartment of a boat are bad. the tend to create explosions if there are any stray fuel fumes.

Should I 'T' the port that I found behind the carburetor, run one line to the gauge in the dash and run another to the distributor?

what i'd do is get the proper marine parts for your boat. ie. not parts from cars.

Could the capped vacuum port on the distributor be the power problem I am experiencing now ?

could be. while i'm not a boat tech it seems to me if the distributor is from a car it may not have the proper mechanical advance needed for a boat.

 

[Alumarine]

The open port on the carb could have been for a hose that runs from the fuel pump to the carb.
In case the fuel pump fails it would dump fuel into the carb and not the bilge.

The distributor sounds wrong and besides being unsafe could be part of the problem.

Pictures would help.

I wouldn't worry about the vacuum gauge at this point.

 

[Rick Stephens]

Didn't those distributors have an angled vent fitting? Angled to prevent a spark jumping out.

You might pull the cap. If that hose fitting goes inside to nothing then it is just a vent.

Rick

 

[Silvertip]

If the distributor is wrong, then very likely the carb is also. Start with the basics. 1) proper distributor, 2) proper carb - rebuilt), 3) check dwell and ignition timing 4) perform a compression and leakdown test to verify overall engine health. All the tuning in the world cannot make a tired engine healthy again. It may be in need of a total rebuild. Under the circumstances, the vacuum gauge is of little use and unless you want to squeeze every possible MPG or GPH out of the engine, and know how to interpret a vacuum gauge reading, that gauge is just going along for the ride.

 

[thumpar]

I would also be checking the alternator and starter to make sure those are marine rated too. It sounds like someone just put whatever they could find on it.

 

[shizny]

Thanks for all the replies. Ziggy, the port I'm talking about is on the manifold and not the carburetor. I don't see any ports on the carb. I believe it's the original one. As for the distributor. I could have sworn that I saw that a vacuum line going to it in a mercruiser manual that now I can't seem to find. I've also been reading some posts that agree with you, Ziggy, on the boats don't have vacuum advances, but then somebody will chime in and say that is wrong and older boats can have vacuum advance distributors. Does anybody 100% know that vacuum advances are never the orignal part on these older mercruisers? The port I'm talking about on the distributor is on the bottom side and angled at 45 degrees downward. Maybe I'm wrong and it's for something else. I'll take a couple pics tomorrow and post them. Again, thanks for all the replies.

 

[bbook83]

I have a 140 that I bought new in 1974. It has never had a vacuum advance line on the distributor. I have not seen any others with vacuum advance. Several years back, I bought a rebuilt carb from the sponsor of this forum that had at least 3 vacuum holes that I had to plug. It appeared to be an automotive carb not properly converted to marine. I agree with Silvertip that the vacuum gauge is of little value.

 

[thumpar]

Boat distros have weights that swing out as it goes faster to advance the timing.

 

[Bondo]

Does anybody 100% know that vacuum advances are never the orignal part on these older mercruisers?

Ayuh,.... That's Right, No vacuum advance distributors are used on boat motors,.....

 

[fishrdan]

Is the "port" on the distributor a simple 90* fitting or a vacuum pot? The 90* fitting would be a vent and not an issue. Cars use vacuum advance to increase economy while under low load, 99* of the time boat's are under a heavy load, so no use for a vacuum pot (big thingy in pic).

2700RPM WOT... Check the mechanical advance in the distributor, are the weights swinging free and springs working, not rusted or broken. Remove the distributor's points plate and you will see the weights and springs of the mechanical advance. An "advance timing light" can help to check the distributor's timing curve, so you wouldn't even have to touch the distributor to see if it's advancing properly.

Agree with others to check dissy, carb, alt, and starter for being "marine" rated. Car parts are 1/3 to 1/4 the cost of marine parts, but can blow up your boat....

 

[shizny]

Ok, I'm a complete idiot. It wasn't the distributor that had the capped port it was the fuel pump. I'll upload pictures. Pretty sure that it's the original fuel pump too.

 

[ziggy]

It wasn't the distributor that had the capped port it was the fuel pump.

well that's good. no vacuum port on the dizzy. + a third port on the fuel pump. i'm gonna guess that the oem fuel pump was auto style. ie. a weep hole for when the fuel pump rubber ruptures. i've had two old boat. '72 and '75 L6's. both have had just in and out fuel pumps. w/weep hole. auto style. from what your describing it sounds like ya got an after market fuel pump. w/third outlet for when the diaphragm ruptures that it feeds it back to the carb. there by not spilling fuel into the engine bay where it becomes a explosion hazard. , my guess for having it capped off is that there was no provision for a fuel line feeding back to the carb in the mid '70. there may be at the top of your air horn on your cap a cap that dumps just below the choke plate. this is for a 2gc carb. if plugged. ya can put a nipple on it and feed the third outlet from your new style proper marine fuel pump back to the carb. if the fuel pump diaphragm ruptures. it'll feed the fuel back to the carb. which is proper and usgc approved, these days.

 

[shizny]

No Title

Here are the images I took yesterday.

 

[thumpar]

The fuel line from the pump to the carb should be a solid metal line. There should also be a clear line from the pump as a sight tube.

 

[ziggy]

There should also be a clear line from the pump as a sight tube.

yep. and your almost there too. remove the brass plug just below the flame arrestor. you have a great pic of it in the first pic. replace that plug with a nipple and then uncap your extra capped off line from the fuel pump and sent to in a clear tube up to the new nipple at the top of the carb.

looks like the 2nd cap off is next to the carb is on the manifold. use it to feed your dash vac. gauge. while a vacuum gauge is'nt a wealth if info mostly. it most certainly does tell you something. to see some charts of vac. gauges. goggle interpreting a vac. gauge. you can tell the general health of the engine via a vac. gauge.

 

[Alumarine]

It looks like there is some sort of an adapter plate under the carb?

 

[Rick Stephens]

It looks like there is some sort of an adapter plate under the carb?

That's just a steel base for the carb. It is part of the carb itself screwed into the bottom of the carb body.

 

[Fishermark]

The fuel line from the pump to the carb should be a solid metal line....

Rubber line is okay as long as it is Marine type A-1.

 

[thumpar]

Rubber line is okay as long as it is Marine type A-1.

Yea, mine is A1 but on that it would have been solid and I doubt whoever changed it used A1 especially considering the other parts.