Slightly Rough Idle on very Hot Days-Normal?

[JR_VIC]

I've got a 92 Ski Nautique 19'6" with the HO 351 PCM. I've owned since was new and this year run all new fuel lines front to back, completely rebuilt the carb changing all internal parts after soaking for one day in the vat, Edlebrock intake, new fuel/water filter, rnew 1700 amp Odyysey battery, new style 3 blade acme prop, new style Hi-Torque PCM starter, upgrade the GT-40 100 Amp alternator, cleaned and dielectric grease all plugs and connections, tightened ground wires, etc. etc. etc............................... Two years ago installed new 8mm spark plugs and PCV valve. Anyway, on very hot days when it is in the mid 90's outside and we have been running for a while we may sit with the engine off for maybe an hour. Upon restarting it will start right away but idles a little rough with RPM's going up and down maybe 100 or so. No stalling or hesitation just idles rough. I have read others talk about vapor locking. Is this what it is? After the boat runs a while and shut it off and re-start it back up 5 minutes later there are no problems. It just occurs on hot days when it has been sitting for a while w/engine off. I thought about the silver heat wrap the race car websites sell to wrap around the starter for heat protection. Would this help to wrap around the fuel lines for heat protection? Maybe some type of carbon/aluminum heat shield that mounts under the carb to keep heat away from the carb? Other than that the boat runs flawless with strong acceleration and top speed of 45 MPH. Any thoughts would great.

[DavidF]

RE: Slightly Rough Idle on very Hot Days-Normal?

Does the exhaust smell like it is idling rich when the problem occurs? Is the exhaust blowing black smoke?

[joelhern2]

RE: Slightly Rough Idle on very Hot Days-Normal?

My boat was doing the same exact thing this past weekend...and I did have black smoke coming out of the exhaust. whats the deal?

joel

[JR_VIC]

I did not notice any Black smoke or any smoke for that matter. It did not smell rich to me. I have heard before that black smoke means it is running rich but I did not notice any. Someone mentioned to me it may just be simply vapor locking the fuel and most likely nothing at all is wrong with the boat. It only does it only very hot days after the boat has been run for an hour and then sit with the engine off for maybe and hour. The engine and all of it's componenets are very, very hot at that point and it may just be boiling the fuel. A friend recommended me to Sumitt Racing and on their website they have all sorts of heat deflection shields/wraps that are made out of an insulated aluminum material that reflect temps up to 2,000 degrees. I bought a fuel line wrap and the same material made into tape and I am going to wrap the entire fuel line up to the carb for heat insulation to see if that cures the issue. Sumitt also has an aluminum heat shield plate that mounts between the carb and intake I am looking at. Maybe I can mount that same heat deflection/wrap on the bottom side of the alum. heat shield to keep heat off of the carb as well. We'll see what happens.

Any other suggestions are appreciated.

Thanks!

[TRBenj]

Has the boat has always been in this hot climate, and has it only started doing this recently?

[joelhern2]

In my case it has only started doing this recently. As far as being hot...it has been in the upper 80's to mid 90s ever sine summer got into full swing. The black smoke is what has me perplexed????

[DavidF]

The black smoke tells me that the the needle valve in the carburetor is not seating properly and letting too much fuel past at idle. Hence the engine is flooding at idle and thus poor idle and black smoke.

JR_VIC: 90+ degrees in the summer is nothing. Here in TX we see 100+ degrees in summer and never an issue with vapor lock. Marine engines just do not run hot enough to cause vapor lock unless a fuel line is up against the exhaust manifold as it exits the cylinder head. Something else is going on and wrapping the fuel lines with a heat deflector will do nothing in my oppinion. Somethings going on with the carburetor, but I do not really know what it is. i would say some electrical ignition component is acting funny due to heat soak, but if that was true, the symptom would not clear up after revving the engine.

Sorry I am not more help, but I will be interested in hearing the solution once you or someone finds the problem. Might think about a carb rebuild.

[bobchris]

there realy isn't a problem you only need to give it a little more gas to bring up the RPM's for a minute and then it should idle fine. You could adjust the choke a little to help it out.

There is no way that it can be a vapor lock issue so doing anything along that line is a waiste of time and money. If you where having the issue 5 minutes after it was turned off then maybe but doubt it. BUT since it is sitting there for an hour the engine is cold and the choke is coming on and needs tweaked a little. Your timing could be tweaked a little if it's advanced alittle over factory specs then retarding it a degree or two wouldn't hurt. But you realy don't have a problem.

Do you have to give it a little gas when you first start the motor for the first time of the day, Why wouldn't you have to give it a little gas after it cools off?????

[JR_VIC]

I just rebuilt the carb with all new internal parts and 99% of the time it runs flawless. I can't remember how long it has been doing this but it seems like the last 5-7 years. It only does it during the really hot months so I kind of forget about it because it does not do it in the Spring, Early summer, Fall, or even winter...just during July/August/September as I am in Texas just north of Houston and it can reach high 90-100 degrees in late summer.

I have to disagree on the motor cooling down that quick. If I open up my engine compartment after running the boat for 30-40 minutes and then let it sit for 45 minutes to an hour the engine is very hot to the point you can't even touch the engine or heads/valve covers. Even the carb itself is very hot. The other day I even touched the raw water pump after the engine was off for about an hour and it was even hot. I have since heard this is normal because compared to a car that has more ventilation from the front of the engine and heat escapes from below the the car as well compared to a boat there is no ventilation of heat from the engine once the boat stops as heat can not as easily disipate under the closed up engine cover. The engine is cooling properly while running as it will usually run 150-160 degrees most all of the time.

Someone mentioned heat soak in the electrical. I am still running the original PCM pro-tec system so I hope I am not running into that system dying on me yet again it has been doing it for the last 5-7 years. I changed to an Edleborck intake about 5-7 years ago so maybe that has something to do with it. I'll recheck the torque on the intake to rule that out. I may just be too picky because overall it does run flawless.

[bobchris]

your a tade picky and realy off base, a car motor will run 60-80 hotter than your boat will, any engine will heat soak. if you want wrap the phuck out of the fuel lines and what ever else you feel needs to be insulated from the heat because all your doing is wasting your money and time. The carb/induction system needs fine tuned by someone that knows what the **** they are doing and your problem is solved.