View Full Version : stalling motor

01-10-2005, 02:24 PM
when i rev my motor up and slow it back fast it will kill the motor does any one have any suggestions my motor has carbs.

east tx skier
01-10-2005, 03:14 PM
Are you idling or in gear when you rev it up (it sounds like you're idling, but wanted to be sure).

01-10-2005, 03:36 PM
for example when i back the boat off the trailer and i give it some gas to pull back then i move the throttle back to neutral when the boat gets off the trailer the motot will die. your probably think that the motor is still cold but it happens when the motor heats up to running temp about 170 although it does not happen so easily. the only thing i can think it might be is the idle is set to low. but to answer your question it will happen in idle or a neutral rev. can you think of anything it might be?

east tx skier
01-10-2005, 04:15 PM
I had my share of carb trouble, but if you're in an older boat, which it looks like you are, we're now running the same carb.

Two things to at least consider if it's not been a while since your last rebuild is (1) fuel slosh and (2) heat sensitivity. Both were problems commonly associated with my 4010 Holley Carb. Then again, your idle might be low. If you've got a manual, it should tell you the idle range. Mine is at about 615 rpm. If I adjust it to about 510, it starts to stumble.

01-10-2005, 05:04 PM
yours is 615 is that normal? my is about 400 so that may be the problem at least i hope i would hate to have to rebuild a carb.

Dan K
01-10-2005, 05:10 PM
If your idle isn't smooth it could also be a vacuum leak, I had a similar issue and I determined that my carb gasket was allowing air to suck in. I found it by spraying carb cleaner around the gasket slowly as the engine idled. If you get a surge it is from the cleaner being sucked in. either tighten the carb or replace the gasket. There are other sorces for vacuum leaks such as around the manifold.

east tx skier
01-10-2005, 05:47 PM
615 rpm is within the normal range for my engine, which, according to my manual is 600--800 for both the 351 and the 351 HO.

01-10-2005, 06:29 PM
400 revs at idle seems low for any kind of engine...

01-24-2005, 02:47 PM
how do you adjust the carb so it will idle higher?

east tx skier
01-24-2005, 02:57 PM
There is an idle adjust screw on the port side of the carb about half-way back.

01-24-2005, 03:15 PM
thanx East TX Skier i like the that pic "nobody f**** with the Jesus man"

east tx skier
01-24-2005, 03:50 PM
You said it, man.

"I see you rolled your way into the semis. Dios mio. Me and Liam, we gonn fark you up. You try anything funny. You pull a piece on the lanes. I'll take it away from you. Stick it up your arse and pull the trigger 'til it goes click."

01-26-2005, 06:26 PM
I agree with those who questioned your idle speed; it is too low. Also you may be running too rich as well. Adjust the idle speed first, then check you idle mixture screws, then finalize your idle speed.

01-26-2005, 08:13 PM
:( ,what........... :confused:

01-26-2005, 08:35 PM
Don't mess with the mixture till you get the idle speed back to where it's supposed to be. 400 is way low. Do this first and see what happens. What's the service history of the boat, at least as long as you have had it? Tune ups, major service, etc.

east tx skier
01-26-2005, 11:43 PM
Jim, what are the symptoms of an engine running either too lean or too rich? Asking to know how to diagnose if I'm running too lean after the intake manifold upgrade (better airflow could make it run lean, but have heard Holleys run rich to begin with).

Also, am I right that there are two mixture adjust screws on either side of the carb toward the front and turning them counter clockwise would make things more rich?

01-27-2005, 12:31 AM
But if the airflow is better(more and faster), wouldn't the air going through the venturies siphon more gas in? There's a thing called lean pop and rich pop when increasing the throttle fairly hard. You have probably heard this. Correct mix can be reached by trial and error, which is probably the most common way with a boat. At first, pull spark plugs frequently and look at the color of the deposits. Generally, CCW will enrich the mixture.

east tx skier
01-27-2005, 10:26 AM
Thanks. Vince at skidim said to go out, warm it up, and run it hard for about 20 minutes. Hole shots, WOT, and all points in between. He said I might also see some stumbling at WOT.

Do I have the location of these tiny screws right?

01-27-2005, 11:04 AM
The tiny screws for adjusting the A/F ratio at IDLE, are on the sides of the metering plates about 2/3 of the way down. You should start w/ them set at 1.5 turns out from "bottomed" and go from there.

JimN is right that if it is way lean, it will backfire through the carb, and if way rich there CAN be backfiring through the exhaust, though that is highly unlikely on a boat due to the cooling water in/around the exhaust. However, if you are getting this (backfiring) your mixture is WAY off, and the idle screws aren't going to do squat to fix it. You'll need to be changing jet's (probably) and powervalves (possibly but not likely).

The best way to check YOUR a/f ratio is to read the plugs. You need to do this in a regimented manor, which I have posted on here before. You need to go run the boat at specific throttle settings, which focus on particular jets. You need to run the engine at that throttle setting for at LEAST 1 minute, then cut it off clean w/the key switch w/o slowing down first. Then pull and read the plugs. This will give you a very accurate reading of your a/f ratio for the circuit in the carb that has the most influence at that particular throttle setting.

EXAMPLE: If you got at WOT and check the plugs, and they are white, you're lean. But why? Is it the pirmaries, or the secondaries? You don't know, so you need to first run the boat at the highest possible throttle setting, WITH OUT cracking the secondaries. Then read the plugs. Maybe you're rich -let's just say for example. So you need to drop you main jet size until you get that correct and you'll HAVE to increase your secondary jet size and re-check at WOT, after you've straightened out the primary side. Of course, this is all a hypothetical example, and you may not have to do anything, but the fact isthat what ever you do, to do it right, it has to be done incrementally, and in a manor that best isolates each circuit of the carb. Follow me here? If you go out and just drive around willy-nilly, then idle back to the dock, and shut it off and pull plugs what circuit are you reading the plugs from? You have no idea.

Lasly carbs are not self adjusting at all. If you increase airflow, yes, the carb will draw more fuel, but not exactly proportionally to the air flow. Now please be advised!! I don't think your intake swap is that profound of a change that you a/f ratio will be affected much if at all, but the "more air so more fuel" assumption doesn't really work, accurately. So I'm not saying that you HAVE to go out and do all of the above. What I AM saying is that if you WANT to check you a/f ratio, the above is the only accurate and useful way that you w/your tools, will be able to do it.

Hey "Skimax", what is your question?

01-27-2005, 12:36 PM
I wasn't saying the mix would be richer or leaner, I was actually referring to the point that the mix maybe doesn't need much adjustment due to the new intake. What do you think about Doug's timing? Keep it at factory or go a little more advanced?

01-27-2005, 12:54 PM
I get what you're saying now, and I agree with you that it really may need little to nothing. But if he does desire to check, my previous post is how I think he needs to go about it.

About the timing, I'd say feed the engine what it wants. I'd set the timing during a WOT run, to achieve max RPM. I doubt it's going to want much different timing than what it wanted before, but I doubt stock was ideal before. It wasn't for mine, but I'm at high altitude so it's a little different for me. Is Doug's still stock? If so, I bet he'd see 50% of the gain he got from the intake, form some more timing.


east tx skier
01-27-2005, 02:52 PM
Great info, guys. Thanks.

For the record, I was getting the recommended top speed (GPS) and rpm at WOT before the intake swap. What I've been told (by both Vince and the head mechanic at SWCC) is, as you all said, more than likely, I won't have to do a thing as it's not a drastic change.

02-19-2005, 12:29 AM
I just joined the site and discovered this post. I also am encountering this problem. I have a 93 prostar 205 285 hp. It seems to idle fine once it is first started unless the engine has been running. If the engine is warm it seems to rev up and down in neutral. If I am out running and pulling skiers and come back to pick them up, it also revs up and down in neutral. Sometimes it dies, sometimes it revs up and down and acts like it wants to die. I also seem to get a lot of exhaust fumes. It runs like a top when pulling though. Any help would be greatly appreciated.

02-19-2005, 11:54 AM
What you're describing is symptomatic of a too lean air/fuel mixture.

It probably runs fine when cold due to the choke "covering up" the lean issue.

Get the engine warmed up to the point where it does the surging. Then adjust the idle mixture screws to achieve the best idle. To richen the mixture, you're going to have to turn the screws out, or counter clockwise.

Basically, you want to turn those screws to achieve the highest possible idle speed you can get out of it. First one screw, then the other. After you're satisfied that you've gotten the most out of it that you can, the idle speed may be too high. If it is, lower the idle speed using the throttle stop screw to the desired RPM. Doing this will make the idle circuit more "sensative", so now you can go back and double check the idle screw settings again, and you may get just a little more out of it. After that, do a final adjustment to your throttle stop for the idle speed and your done. That should do it.


02-19-2005, 12:49 PM
Lance- speaking of idle, what is the idle speed of your motor when it starts surging? 500 RPM or so? If it's that low, it needs to go up to at least 600RPM. If the tach needle is jumping around without corresponding to the engine speed, clean the distributor hold-down clamp and the surface that it mates with.

east tx skier
02-19-2005, 02:33 PM
Loewe, what carb are you running? If stock, the hot stalls could be a product of the type of carb. The surges on the other hand wouldn't have anything to do with that in my experience.