what to do next?

[JLAfarms]

I have a 1991 20.5 foot montery with a 5.0 mercruser. here is the prob the motor is fouling the rear plugs after an hour or so of boating.Front plugs are fine maybee alittle lean but the back plugs are just black with soot on both left and right sides.<br />A little history for you. Last season had the same prob so I replaced the plugs, drain feul and cleaned tank had the carb overhauled, checked compresion. still did it every trip we took. so this winter I put a performer intake and an edelbrock 1409 carb, new coil, plugs (br6fs), plug wires distributor cap and rotor,feul filter went to the lake in cool weather boat ran great. Now in the heat with the kid being plulled around on skis the boat is good for an hour on fresh plugs and then it starts missing!!! I have also recently had a cooling prob the thermostat was stuck open, I fixed it and motor tmp is 160. I also only run super unleaded fuel. What do i do next?<br />Thanks ,<br />Jerry

 

[Laddies]

Re: what to do next?

Sounds like the intake manifold gasket is leaking--Bob

 

[djvan]

Re: what to do next?

I am not endorsing it by any means, but small block chevys are kind of known to foul the rear plugs when an engine gets worn. Usually oil fouled. You could try spark plug spacers, I think most auto parts stores would have them or could get them. I've never used them but have known others that have. It might be worth a try.<br /><br />DougV>

 

[jeff13601]

Re: what to do next?

I think you need to do another compression test and see how much those back cylinders vary from the others

 

[JLAfarms]

Re: what to do next?

well I was working on the boat and I found a elbo on the back of the distributer cap looks like there should be a hose on it?? I took my finger and held over the hole and I think I can here the motor speed up just a bit, but I might just be reaching for something at this point!!<br />What is the elbo for??<br />thanks,<br />Jerry

 

[Laddies]

Re: what to do next?

It's a vent

 

[JLAfarms]

Re: what to do next?

well i did check the intake and it appears it is not leaking and I went and bought a compression gauge and will test the compression in a little while. Jerry

 

[JLAfarms]

Re: what to do next?

checked all my cylinders and all read 150lbs of compression so now what do i check???

 

[moosejaw]

Re: what to do next?

Sounds like a serious vacuum leak to me.

 

[jeff13601]

Re: what to do next?

how many air adjustment screws are on your carb?

 

[JLAfarms]

Re: what to do next?

there are two air adjustment screws right in the front. 1409 carb. I pulled the plug after the last time out and cilinder 1 2 3 and 5 looked pretty good but 4 6 7 and 8 had exsessive amount of carbin to the point of dry fouling 7 and 8.Could it be that i am having low voltege issues with my ignition and I should add a msd ignition box and coil to the boat?

 

[Don S]

Re: what to do next?

I would try that comprssion test again, and try it this way.<br /><br /> http://www.boatfix.com/merc/Bullet/97/97_25.pdf <br /><br />Did you do a vacuum gauge test? Not just look for a leak.<br /><br />

 

[JLAfarms]

Re: what to do next?

don, I did the compression test just as your post indicated as you know I have a 1409 carb and a performer intake and the only placr I can tell to chek the vacuum would be by pulling a plug on the intake. Does this sound right? And I just got a set of metering springs from edlebrock and changed them to the blue springs which should lean the motor out two steps leaner than it was. I do have a vacuum gauge and will check it. Is the vacuum test to check the intake or what?? Jerry

 

[Don S]

Re: what to do next?

Replace the manifold plug with a hose barb fitting to hook to your gauge. <br />The gauge checks everything that effects vacuum.

 

[Buttanic]

Re: what to do next?

I had a similar problem in a Camaro with a 5.7. About every 700 to 800 miles it would oil foul the rear plugs. Looked at all the same things as recommended to you and even changed the oil seals on the valves on the back cylinders and the intake manifold gasket. As a last resort I replaced the stock AC plugs with Bosh platinum and never fouled a plug again. I don't think you have a vacuum leak because that would lean out the mixture not cause a rich mixture that would carbon foul a plug. Does it have a quadrajet carburator. The plugs on the bottom of the trottle plate are notorious for leaking fuel into the manifold on these carburators. Fix is to seal them with epoxy.

 

[JLAfarms]

Re: what to do next?

I did a vacuum test and here are the result, at idle 14hg and at two thousand rpm it is 18.5hg. my vacuum gauge indecate it is calabrated at sea level and to add 1 hg for every 1000 feet of elevation. I am sitting at 2750 elevation so this should ad 2.7hg to the above numbers.<br />Buttanic I am running a brand new 1409 edlebrock carb with a new performer intake!!<br />Jerry

 

[JLAfarms]

Re: what to do next?

well i am going to the lake and will post how it did. Jerry

 

[JLAfarms]

Re: what to do next?

It had no change in performance so I think I will just change the engine. thanks for all the input!! Jerry