Boat engine dies under 1000 rpm's

[Longhorn21]

I posted about this same problem 2 weeks ago. My boat dies (not under a load) in neutral when I adjust my curb idle screw below 1000 rpm's. I have a 1987 supra comp ts6m. Since than I have tested the fuel pump, changed the fuel filter, and rebuilt my Holley 4160 carb. It's still does the same thing. This thing is driving me crazy. I've sprayed carb cleaner around the carb to check for vacuum leak and nothing. What else can it be. Can anybody help please.

[OLDSCHOOL85]

Where is your gas tank vent located. On most of the old supras the vent is located in the rear way to close to the water. Backing off the trailer or having alot of people in the boat can put the vent underwater. Water can actually clog up your carb, which is what mine did. I would check for water in your fuel. I went through everything on my boat before I realized I had a WHOLE LOT of water in my tank.

[Longhorn21]

That's one thing i have not look at. My tank is almost empty so I will check for water then hopefully it will run right cause I'm running out of options

[Longhorn21]

Let me ask another question. I set my spark plug gap wrong. It's set at .044 and it's supposed to be set .035. Will that make a big difference in the engine performance.

[rludtke]

No not a big difference.

How far out are your idle mixture screws set? The idle mixture could be incorrect, and cause rough idle and quitting. set you idle screw high enough to allow the engine to run, then tweek the idle air bleed screws. I would screw one in all the way, then back it out in 1/2 turn increments keeping track of how many. By 1 1/2 turns, it should run smoother, listen closely. If you reach 3 turns I think you have gone to far. Then do this to the other side. Now that both sides are close, tweak each one in or out (1/2 turn increments counting each one) as you listen. Give the engine what she wants, by turning the screws in or out from the ist setting until she runs even smoother and faster. Adjust the other side to match- it should run even smoother.

By now she should be idling faster. If so, reset you idle to 1000 rpm with no load. This should translate to about 600 rpm ideling in gear in the water.

Take a screw driver with you when you launch, so you can fine tune your idle-in-gear speed to 600 rpm.

[Longhorn21]

I've been playing around with the idle mixture screws and think I've got them set correct. It will idle fine at 1000 rpms not under a load but I've been trying to get it between 600 to 800 and it keeps dying. Is it suppose to be at 1000 not under a load or am I trying to set it to low

[Longhorn21]

I'm also having a problem with it not wanting to start back up if I wait 10 min or longer. If I start it right back up it will start but if I wait it turn over for a while till it starts up.

[rludtke]

1000 rpm not under a load is about right, but your real target is 600 rpm under a load. Set it to 1000 on the hose, then adjust it again to 600 (under load) if needed on the lake. I am not running a choke, so I have to be fully warmed up before my engine will idle that slow.

When the engine quits and won't restart, try cranking with the fuel line to the carb disconected to see if fuel is pumping. I am at a loss as to why it doesn't start, but at least this would rule out the pump as being the problem, or demonstrate that it is. Possibly it is losing its prime somehow.

What happens if it sits a day before you try to restart? < 10 minutes it will restart. > 10 minutes it won't. Is there a lenght of time where it will restart again?

[Longhorn21]

I've checked the fuel pump with the line off the carb and the gas shoots out a good solid stream on crank. If I let the engine sit till the next day it will start right up. I was at the lake and stopped the boat to swim for about 30 min and it had to turn over for about 20 sec before it would start. I wondering if the idle mixture screw were set to rich would it flood the engine enough for it to be hard to start cause I've checked the floats and their not flooding the carb and I just rebuilt the carb. I do smell gas when it's hard to start but it could be because I'm giving it gas while I'm cranking

[Longhorn21]

What could be wrong if it's losing a prime and how do you fix it

[rludtke]

If you think it is flooded when it is hard to start, try advancing the throttle to full open with out pumping it. Pumping will cause the accelerator pump to squirt more fuel into the throttle bore. Crank the starter, and when the engine catches, quickly retard the throttle befor the RPMs get out of hand. This will introduce air into the induction system and undue the over-rich condition.

How warm is it under the engine cover when the engine doesn't want to start? If the under cover air temps are high (high ambient plus high exhaust heat) then you are probably experiencing vapor lock. This is a condition were the fuel in the float chambers warms (after shut down) to the extent that it boils (vaporizes). The carb can not meter fuel vapor, so the engine will not start. Opening the throttle and cranking the engine will help new cold liquid fuel pump into the float chamber, and displace the vapors out the vent. This takes a little time (20 seconds or so). I suspect this is your problem.

The root cause may be exhaust manifolds that are hotter than they should be. This could be indicative of a worn raw water pump impellor. If you think it is notably warm under the engine cover, or if the exhaust manifolds are to hot to touch when this occurs, then I think you should replace the impellor and, while your at it, the thermostat. This is a simple job, and about 50 bucks from ski dim. Change your impellor and thermostat before every season.

[Longhorn21]

The exhaust manifold is hot to the touch, is it not suppose to be. When I'm playing around with the carb if I bump into it it will burn my arm. I just assumed it was suppose to be that way

[Longhorn21]

Another question for you if my impeller's were bad wouldn't it show on my temperture gauge, cause it reads about 150 all the time. I'm not really sure but i would think if my water pump or impeller's were causing insufficient water delivery it would start over heating which it's not.

[Longhorn21]

Does anybody know if your engine will overheat if the impellers are bad cause I don't mind buying a new impeller and thermostat but I don't want to if I don't have too. I'm trying to figure out if the problem is vapor lock are not

[OLDSCHOOL85]

When my impeller went out, most of the blades plugged up the exit side of the water pump and wouldnt allow water to pass. I put a water hose right at the filter for the water pick up and nothing would go through. I took off my water pump and sure enough, no blades were left. I also had a blade make it through and got hung up in my spark arrestors at the rear of the boat. I'm not saying this happens all the time, but this is just what mine did. If your impeller is gone, your boat should over heat, mainly at idle.

[OLDSCHOOL85]

I think Rick is on the right track. Also check for exhaust leaks around the manifolds. Exhaust leaks under the hood can choke out your engine. While at cruising speed you are pulling in fresh air. When your idleing, your not.

[Longhorn21]

thanks guys I'll take a look at it tomorrow.

[rludtke]

Hey Longhorn, any luck?

Your exhaust shoudn't be hot enough to burn you, as they should be constantly cooled by water from the raw water pump. The raw water pump needs to be able to deliver enough water to support both cooling systems, the engine crankcase cooling and the exhaust cooling systems.

The impellor is rubber, and they simply will not last long- consider it sacrificial. Replace it at the start of every season just to be sure, as so much depends on it.

I experienced the exact problem on the last day that I used my boat last season, but didn't try to fix it when I put it away. I noted that it was hotter than normal under the hood, so I made a mental note to be sure to replace the impellor. I was worried that parts migh break off and get caught downstream, and plug the exhaust up. What I found was a impellor fully intact, but with evidence of wear. The new unit completely fixed the problem.

[Longhorn21]

No luck yet, I will replace the impeller when I get the $$$ but it's not overheating so it's got me scratching my head. Another thing what size power valve do you have in your carb. I just rebuilt mine and it came with a 65 and a 75. I put the 65 in because that's what was in their before. My vacuum reading is between 20 and 23, so I would think it would need a bigger power valve. Holley says it should be half of your vacuum reading. Any suggestion.

[fastab]

Are you running points still, if the dwell isn't set right it will throw the timing off and be hard to start warm as well as idle issues.

[Longhorn21]

Yeah I'm running points but had my timing, cap, points, wires, solenoid, and spark plugs done last Aug, so can't imagine that the timing off. It better not be I paid a lot for the work. Not sure what the dwell is. I appreciate all the reply, I'm working on it, just running out of money.

[rludtke]

Discount Inboard Marine (Ski DIM) has your impellor for $27.95.

http://www.skidim.com/prodinfo.asp?number=RP061015

[fastab]

Dwell is the best way to set the points, it is a electronic measurement. You would first set the gap on point with a feeler gauge and then check dwell and tweak them from there. Points can burn out if the key is left on and the points are partially open it will arc and were the points. New or old. I believe the spec is 28-32 degrees for the Dwell on that unit. A good multi-meter will have a setting on it to test dwell, it will only take a minute to do.

[oward1202]

Hey Longhorn. I was reading about your problem because I am having the same issue with my 1985. It seems like you have done more investigating than I have. I just bought mine this year and have just been dealing with it. It only seems to be happening when its hot. I have learned a few things here that I definitely need to check. Keep me posted on what you find with yours and I will do the same. Hopefully we can get them both fixed.