2000 2400 SCR Surging at High RPM - Maxum Boat Owners Club

jlpete · 08-11-2011, 05:33 PM

I have a 2000 2400 SCR that while idling and at speeds of less than 4000 rpm's seems to run fine. When I try and open it up past that it seems to surge or hesitate. The RPM's drop and then raise again. If i really hammer it and dont back off the throttle it will stall the boat. Has anyone seen this? I just bought the boat a couple of weeks ago and i know it was sitting for awhile. I changed my fuel seperator and cleaned the filter at the carb and it seemed to help for a little bit and then it went back to surging. I pulled the filter at the carb again and it seemed to solve it for a little while. Is this all related to my fuel line having stuff in it. Or could it be something else? Also what is a good cruising speed on these boats? I seem to do about 32mph at 3900-4000 rpms is this about right?

seapuppy · 08-11-2011, 07:18 PM

jl
welcome to the zoo.....first off..that is indicative of a weak accelerator pump....not getting enough fuel shot into the venturii while the throttle opens and causing a leaning condition....it could also be indicative of a slight vacuum leak....so...get a squirt bottle full of soapy water....with the engine idling...spray aorund the base of the carb....if there is no change at ALL to the idle...then it's definitely the acc. pump....

SP

jlpete · 08-11-2011, 07:50 PM

Thanks i will look into that.

ss3964spd · 08-11-2011, 08:03 PM

What engine, and does the engine have fuel injection or a carb (Im guessing carb).

As a test of the accel pump, if you are at idle and open the throttle quickly and far, does it hesitate then? Or is it only at upper RPM's when it hesitates.

When at higher RPM's and it starts to surge (speed up, slow down, speed up, slow down), will it continue to do so until you back off?

mcoffey · 08-11-2011, 08:17 PM

You stated that you replaced the water seperator and cleaned the strainer at the carb and "it helped". You then mentioned pulling the strainer at the carb again and it "seemed to solve it for a little while". So I have to ask, was it full of junk both times you did this or was there nothing noticable at all, just seemed to help after it was back together? Definitely seems to be starving for gas, could be just a bunch of junk clogging it up or could be what SP stated in the acc. pump. If you are seeing junk in the strainer and it runs good after cleaning it, you need to flush out the entire system (tank, lines, filters, carb.). Could just be dirt and such but could be the ethanol gummed up on you due to too much water in it (doesn't take much or too long of a time).

jlpete · 08-11-2011, 08:34 PM

Sorry guys it is a 5.0 mercruiser carb. If I back off the surging stops and i can re accelerate. I can put it in nuetral and rev it up to about 4800 until it cuts out but i think that might be the rev limiter.

jlpete · 08-11-2011, 08:36 PM

And there is some residue in the strainer not much but definitely some!!

seapuppy · 08-11-2011, 08:42 PM

Quote:

Originally Posted by jlpete

Sorry guys it is a 5.0 mercruiser carb. If I back off the surging stops and i can re accelerate. I can put it in nuetral and rev it up to about 4800 until it cuts out but i think that might be the rev limiter.

you don't have a rev limiter in the carbed version.....

surging (reving up and down) is usually water in the gas....but total hesitation or flat spotting from acceleration....is caused by the carb leaning out...which is the accel. pump...

SP

jlpete · 08-11-2011, 08:45 PM

Thanks I will let you know if it helps!

jlpete · 08-16-2011, 04:08 PM

Ok so i pulled the air horn off and looked at the acclerator pump, i didnt change it because i couldnt find one locally. i then reassembled the carb and filled up with gas and also put in some fuel clean. I ran with out any problems for about half a tank as soon as it got around half it started cutting out again. Do you still suspect the accelerator pump? I have one on order so will change it anyways. Just making sure this new info didnt change anyones thoughts?

ss3964spd · 08-16-2011, 05:31 PM

Quote:

Originally Posted by ss3964spd

As a test of the accel pump, if you are at idle and open the throttle quickly and far, does it hesitate then? Or is it only at upper RPM's when it hesitates.
When at higher RPM's and it starts to surge (speed up, slow down, speed up, slow down), will it continue to do so until you back off?

Can you answer the Q's above?

With the engine at idle, when you open the throttle more air is admitted into the cylinders but the additional gas that is needed with the extra air lags behind. In other words, the air starts flowing before the gas does. This creates a lean condition and the symptom is a hesitation when you open the throttle. The hesitation will vary according to how much you open the throttle. It could hesitate just a little with small throttle opening, or a lot (or simply dies) with large opening. There simply isn't enough gas mixed in with the air to support combustion of the mixture.

It is the accel pumps sole mission in life to mechanically inject a shot of gas to cover up the lean spot, thus allowing sufficient time for the gas flow from the carb's primary metering circuit to catch up with the air flow. Movement of the throttle = movement of the pump. If you don't move the throttle the pump isn't pumping. So, if you are at a steady cruise speed and the engine suddenly looses power and you haven't touched the throttle, it isn't the accel pump causing the trouble.

Given that it ran fine for 1/2 a tank the problem isn't the accel pump - An accel pump isn't going to sometimes work, other times not.

Dan

jlpete · 08-16-2011, 07:44 PM

While in neutral it hesistates also. It lets me get to about 4600 if i gradually raise and starts pulsing if i open it up fast and hard at about 3600? So if not the accelerator pump what would be a good second culprit? Im considering just buying a new carb but dont want it to not fix the problem.

ss3964spd · 08-16-2011, 08:04 PM

Hmmm, so while in neutral, when you quickly open the throttle it will hesitate but if you open the throttle slowly it does not hesitate? Is that correct? And once you get it to a high RPM it starts surging (speeding up/slowing down repeatedly) - correct? At what RPM does the surging start? 4600 is pretty high - if the engine has electronic ignition it may also have a rev limiter so you could be bumping against the limiter at 4600.

Is it also correct that after you took the top of the carb apart, cleaned it out (was there anything in in?) and put it back together, filled the carb with gas and added fresh gas to the tank, it ran completely fine through a half a tank of gas?

jlpete · 08-16-2011, 08:43 PM

If it does have a rev limiter then i would suspect that is what i am hitting. Yes after taking apart the top half of the carb and reassembling it ran perfectly at WOT I was running about 4800 RPM's and 38MPH. I ran all day saturday and then started off fine on sunday until i got to about half a tank.

ss3964spd · 08-16-2011, 09:17 PM

Well, if that's the case, then it suggests that the accel pump is fine but you do have some kind of fuel delivery issue. Could be a partially clogged vent line, gunk/junk in the tank obstructing the in-tank pick up line, possibly a weak fuel pump.

I think I'd fill it up again and see if the problem goes away, then comes back when it hits 1/2 tank.

jlpete · 08-16-2011, 09:31 PM

If it does have a rev limiter then i would suspect that is what i am hitting. Yes after taking apart the top half of the carb and reassembling it ran perfectly at WOT I was running about 4800 RPM's and 38MPH. I ran all day saturday and then started off fine on sunday until i got to about half a tank.

jlpete · 08-29-2011, 09:32 PM

Ok so I did a little tinkering and put in a newer used fuel pump and carburetor. The boat ran like a champ, until half a tank again. I also changed the fuel vent. so my last option is the fuel pick up being clogged is this a simple check. Do just pull the hose off and undo the nipple and see if i have an issue? I really love the boat until I get down to a half a tank!!!!!!!!

ss3964spd · 08-30-2011, 12:29 AM

I would do absolutely nothing else except fill the tank again, run it, and see if the problem goes away and then comes back at 1/2 tank. I mean, don't change filters, carbs, pumps, underwear, socks, nothing. Fill it and run it. See if the symptom is consistant when it hits 1/2 tank.

Since you have experimented with a different carb and fuel pump it's probably safe to say that whatever the problem is, it is before the fuel pump. Since you changed the vent that leave only the fuel line and the in-tank pick up.

Dan

mcoffey · 08-30-2011, 12:53 AM

Sounds like the pickup tube may have a crack or hole in it about half way down. When fuel in tank goes below this, it causes it to suck air a bit and cannot deliver quite enough fuel for WOT.???????

And as Dan stated, don't even change your sunblock or swimsuit

jlpete · 09-05-2011, 02:33 PM

Ok, so i took your advice and filled her up and took a very long cruise. It started off suispect only allowing me to get to about 4400 rpms and then started cutting out. as the trip went on my rpm range decreased. so little that i had to limp home from seattle at about 1800 rpms. I attempted to fill the tank again before my journey home but no lluck. any other ideas?