OMC 5.0 surges after 4100 rpms

[peanutbay1]

Hello;
This is my first posting but I have read the forum for quite a while and found much good information. My "88" boat has an OMC 5ltr engine. The problem is that the boat has developed an intermitant surge at 4100rpms,almost like if a plug was firing sometimes and not others. If you dump the throttle it will sputter out the carb and if you roll on steady she will pull strong up to 4100rpm and then after that slowly fall on her face or try to surge into full rpms (4600). Here is what I have done so far:
1) plugs,wires,cap,rotor,new petronix electronic ignition,coil.
2) rebuilt the rodchester 4bbl carb
3) new water/fuel filter,carb fuel filter and checked my pick up line.

The bottom line is I bought the boat last year and got the outdrive section dialed in but did not work much on the engine side but It had this surge condition then. With all this work I still have the same condition but the boat does pull much stronger than last year. A few questions;

1) Could this be a timing problem?
2) Any other paths to go down?

PLEASE HELP!!!!!:

 

[Bondo]

Re: OMC 5.0 surges after 4100 rpms

but It had this surge condition then. With all this work I still have the same condition but the boat does pull much stronger than last year.

Ayuh,... I'd continue prusuing the Fuel System,.... I think that's where the Problem is....
Fuel Pump,..??
Fuel Tank,..??
Fuel Lines,..??
Vent line,..??

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

I agree that it could be fuel related. I did verify,with clear fuel line and a separte inline fuel filter, that I had clean fuel with no debris or water in it. This also allowed me to witness fuel pressure but not measure it. Could a week fuel pump cause this condition??and if so,do you think I am running rich or lean at higher throttle? I also checked my fuel tank vent for being clear as I had one years ago that had a mud wasp nest in it and that made the boat run out of fuel..Could there be any type of bizarre electrical short that could cause this condition?? Thanks again for helping me,I am really glad to be on this forum and have people to share their knowledge with me.

 

[Bondo]

Re: OMC 5.0 surges after 4100 rpms

Could a week fuel pump cause this condition??and if so,do you think I am running rich or lean at higher throttle?

Ayuh,... The fuel pump needs to deliver a Solid 4/ 6psi to the Carb...
Fuel pumps have been known to Weaken, before they Die, Yes...
The Runability you describe sure Sounds like a Lean condition to Me...

I rather Doubt it's an Electrical problem,... But,.. I've been Wrong before....

Btw,... Welcome Aboard,... We're as Glad to have ya around, as You are to be Here...

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

O.K. we are honing in on a lean condition which leads me to several more questions. I noticed during my clear fuel line check that the pump would barley pump up to my inline filter and it would leave a big air bubble in the line at low rpms. When we went underway as the rpms increased the less the air bubble was which at high rpm was gone. If you came off high rpm the bubble would return. Could this mean the pump is weak? Does the (excuse my lack of the proper term) ball/check down at the tank pickup have anything to do with this meaning is it letting fuel run back in the tank?

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

Anyone have any answers to my questions from yesterday?? I would love to try some new repair ideas over the weekend.

 

[JustJason]

Re: OMC 5.0 surges after 4100 rpms

your fuel line could be getting airbound. A little leak may be hard to see but will effect WOT performance.
You can check everything by following this test.

http://forums.iboats.com/showthread.php?t=283269

If your using one of those cheap inline glass filters that west marine carries like this... http://www.westmarine.com/webapp/wc...Primary Search/mode matchallpartial/0/0?N=377 710&Ne=0&Ntt=inline fuel filter&Ntk=Primary Search&Ntx=mode matchallpartial&Nao=0&Ns=0&keyword=inline fuel filter&isLTokenURL=true&storeNum=6&subdeptNum=68&classNum=219

get rid of it.

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

The inline filter I used has been removed after I did the test so I am back to the remote filter and the carb filter all of which are new.The pick up tube in my fuel cell is solid aluminum and looks fine.Thanks for the test procedure!! I will get the parts tonight to build the gauge set up. I will read and re-read your test procedure and try it out. Thanks to all again,your help is greatly appreciated. This is awsome!!!!!!

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

O.K guys here is the update from the weekend. I did a fuel system test and it passed for leaks and fuel pressure. I really paid attention to the boat this weekend to accuratly describe what is happening. The boat is somewhat cold blooded at the start of the day,to the point that you must sit on the trailer at the launch with the boat nuetral throttled to about 1200rpms for up to 5 minutes. After that point she will hold idle and go in and out of gear. The first 10 minutes the boat almost performs normal but after that the boat will not get over 4000rpms and if you dump the throttle it will sometimes blubber back through the carb (not always). The condition I described as surging may have been misleading. What it sounds like to people on the shore is that my boat is missing or breaking up and what it sounds like on the boat is that you get intermitant clean electrical surges that allow the rpms to climb and that quick it drops down. If #1 represents my restricted rpms and #2 represents the climbing rpms this is what the pattern would sound like at +4000rpms...111111111221111111221111111121111111111111112222222221111111121122.... Another words it is not a steady surge like a fuel situaltion would be this is more like electrical. PLEASE HELP!!!!!

 

[MikDee]

Re: OMC 5.0 surges after 4100 rpms

If you rebuilt the carb, did you check to see if the power valve is working properly?

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

This is where I look stupid. I understand powervalves on 2stroke engines but I dont know what the powervalve is on my v8 engine. When I rebuilt the carb it was basically the excellerator pump,needle and seat and gaskets between the body sections. I obviously cleaned the carb with 2+2 very good but that is the extent of my carb rebuild.

 

[MikDee]

Re: OMC 5.0 surges after 4100 rpms

This is where I look stupid. I understand powervalves on 2stroke engines but I dont know what the powervalve is on my v8 engine. When I rebuilt the carb it was basically the excellerator pump,needle and seat and gaskets between the body sections. I obviously cleaned the carb with 2+2 very good but that is the extent of my carb rebuild.

Well, on the Qjet, when you remove the top cover (air horn) the power valve is a small spring loaded piston, located right in the middle (behind) the two front barrels, and controls the fuel flow going through the jets, by regulating the main jet metering rods. It is held in place by a small snug fit plastic bushing, AND the top cover. It has a vacuum port, & a spring, underneath it that regulates fuel flow according to engine conditions. It needs to be free moving in it's bore, and can be easily checked, by pushing down lightly on the metering rod hanger against the spring pressure, if no movement is found, it can easily be removed, by popping out the bushing, then checked, cleaned, & reinstalled. It is a key item to check when rebuilding a Qjet.

Also, have you sprayed carb cleaner through the front, & rear jets, to make sure you're getting fuel flow?

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

I know the exact part you are talking about. We checked the measurement of the metering needles to the air horn yesterday just to see where they were at. Should the arm that holds the metering needles feel springloaded or just free almost like a float are would? Yes I did make sure all the jet pathways were cleaned. My idle circuit needles had a very thin layer of black on them and we had to clean those out but other wise the carb was actually fairly clean when I did the rebuild last week. So just to clarify,are you thinking that I still have a fuel issue,and if so do you think rich or lean?? Thanks for helping me,I really appreciate it.

 

[MikDee]

Re: OMC 5.0 surges after 4100 rpms

It still sounds like a fuel issue. The arm should move down with light pressure on the spring, and spring back freely by itself. Have you blown carb cleaner thru the front, & rear jets, in the bowl, then the upper part of the primary jets in the air horn, and checked your rear hanging metering rods?

One more thing, is your flame arrestor clean? take a good look

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

I basically took the carb apart and cleaned everything as best I could. The metering needles were cleaned and functioning properly as well. I checked the air cleaner for obstructions also. I make sure my mechanicals are very clean and maintained. It sounds to me like I have a carburater issue. I may have made the error of approaching the rebuild too simply. I have the boat going to be run on a dyno on wednesday morning so my mechanic can witness what the boat is doing and mabye offer some help. The boat actually runs very good under 3800rpms but when you start kicking in the secondarys that when she starts to go away. I actually watched the carb while my brother drove the boat and down the venturies was an good high pressure spray of fuel but at the secondary ports the fuel flows like a small water fountain under the butterflies. The fuel actually chills the butterflies (through eveporation,Iguess) to the point that they build condensation on them. Any thoughts???

 

[MikDee]

Re: OMC 5.0 surges after 4100 rpms

I basically took the carb apart and cleaned everything as best I could. The metering needles were cleaned and functioning properly as well. I checked the air cleaner for obstructions also. I make sure my mechanicals are very clean and maintained. It sounds to me like I have a carburater issue. I may have made the error of approaching the rebuild too simply. I have the boat going to be run on a dyno on wednesday morning so my mechanic can witness what the boat is doing and mabye offer some help. The boat actually runs very good under 3800rpms but when you start kicking in the secondarys that when she starts to go away. I actually watched the carb while my brother drove the boat and down the venturies was an good high pressure spray of fuel but at the secondary ports the fuel flows like a small water fountain under the butterflies. The fuel actually chills the butterflies (through eveporation,Iguess) to the point that they build condensation on them. Any thoughts???

Is your needle, seat, and float level good? Are there metering rods hanging off the secondary bracket in the middle, that lift when you open the top butterfly?

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms

I did not measure the float level.(my bad!!!) The measurment for the metering rods is actually about .060 short of the dimension that my book lists. I was afraid to change this dimension as that would drop the rods further into the jets thus leaning the engine out. The metering rods are hanging off the bracket correctly and move freely. When I rebuilt the carb I took great precaution not to change any adjustments but now I am wondering if it was never correct as the engine did the same thing last year when I bought the boat. Since this whole tune up occured,this year, the condition has worsened to some degree and in other aspects the engine seems to run stronger so I think I am in the right direction just not there yet.

 

[peanutbay1]

Re: OMC 5.0 surges after 4100 rpms (BAD NEWS UPDATE)

Re: OMC 5.0 surges after 4100 rpms (BAD NEWS UPDATE)

UPDATE;
My boat mechanic ran the boat on a dyno and it only produces about 115hp at 4100rpms. His charts list it at 175-200hp.Even thought the engine is smooth and quiet in the hull,out the hub it misses and bobbles with any load put on the motor. His opinion is valve issues. I will be doing a compression and leak down test tonight to confirm but it sounds bad. I must be honest a 23foot boat the pushes 9persons and itself right up on plane and runs 35mph smoothly blows my mind that it is junk. I would have bet the farm it was a simple issue. Not that it matters but does anyone have any thoughts....

 

[Bondo]

Re: OMC 5.0 surges after 4100 rpms (BAD NEWS UPDATE)

Re: OMC 5.0 surges after 4100 rpms (BAD NEWS UPDATE)

I would have bet the farm it was a simple issue. Not that it matters but does anyone have any thoughts....

Ayuh,... If the compression,+ Leak-down tests are OK,... I'd go back to the Carb....

 

[MikDee]

Re: OMC 5.0 surges after 4100 rpms

I know, I'm going off in another direction, but check your spark plug wiring order for a crossed wire. Then if that's okay, take a look at your spark plugs to see what they look like, too lean, too rich, too clean (water intrusion) or a broken, or faulty plug, you can do all this just before a compression test