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Mercuiser fails at 2500 rpm

Rick, I just re-read you thread from the beginning.
You mentioned "rebuilt" engine. Can you be very specific as to which piston profile was used, and as to which cylinder head combustion chamber volume was used?
This must be correct for a Marine Cruiser engine.


Depending on the piston profile, Quench or NO Quench, and combustion chamber volume, 32* @ 2,500 rpm can be a recipe for Marine Load Detonation.


In post #20 you mention seeing an ignition advance of 32*.
When adjusting the ignition advance, be absolutely pro-active with regard to looking at the progressive advance curve and the TA (total advance) and at which RPM it is "full in". And yes.... even when setting a TB system. Don't leave anything to question! Those who Foo Foo the importance of proper ignition advance do not fully understand this!!!!!

Also, TA numbers are absolutely worthless without an associated RPM.
In other words...... if you read that your TA should be XX degrees, that info is incomplete.
You want to see a TA of XX* @ X,XXX RPM.

I would suggest that you verify #1 TDC and make sure that it is accurate. The PPS and degree wheel is the most accurate means of doing this!
Then mark off your harmonic balancer (in increments of 5*) up to approx 35* BTDC.
Now when you strobe your timing marks, you will set BASE advance at idle speed.
Now increase RPM as you watch the advance.
Make yourself a paper graph showing the Advance at each RPM increment up to the Full In RPM.
You will be looking for a rather linear curve up to the point of no further advance.


In post #20, you also mentioned setting your valves.
We do not set/adjust valves...... we adjust the rocker arm nuts as to set the cam follower plunger depth to spec.
I would suggest that if you question your procedure, that you go back through these using the 8 stop method.
Start at #1 TDC on the compression stroke.
Adjust the plunger depth on both the intake and exhaust. (follow the OEM specs)
Now rotate the crankshaft 90* and adjust #8 cylinder.
Repeat the process right on down the firing order until you come back to #1.


In post #23, you asked: "rather confused as to why she idles so much faster at 30 BTDC... any ideas on that?"
At BASE RPM, you should NOT being seeing 30*. You should be seeing BASE advance ONLY.
As Jack suggests, your ignition system provides a progressive advance as RPM increases.


Rick, your engine specs will provide a WOT RPM. This is a Test RPM ONLY.
With all else being correct (hull bottom clean, engine in full tune, gear reduction being correct for the engine, propeller pitch being correct) your engine must be able to reach WOT RPM.
If it does not, you will risk over-burdening it at ANY RPM!


Use the P of E from here on out.
Good luck to you.


.
 
Thanks again for the continued support in my foolish en-devour gents...
I had a great success yesterday... didnt lift the engine cover once...
Yes, it seems it was carburation causing the issues...
with the low jetting being so oversized, it was absolutely annihilating the plugs... it seems that through all the testing, it had killed a couple of plugs... even after cleaning them i found that they werent firing... so after re jetting, she was going better, but was only firing on about 6 plugs... i put new plugs in for yesterdays trip, and bang, straight up to 3500 rpm doing 28 knots... she still wouldnt reach wot, but i think that the prop might be a bit too big... im going to scale back from 17 to 15 and test her ... plugs are running far too white now, so also need to muck about a bit with mixture again, but yea, the primary jets were far too big, and so were the secondary metering rods.... think ive nailed most of it now!
 
You mentioned in post #1,7,and 9 that plugs were fouling,jets were too big along with metering rods issue, I'm glad you got it sorted out, post back after prop. change let us know how she does
 
Get the spark plugs a nice toasty brown as quick as possible. It wouldnt take much to burn the valves and piston tops. The best way to adjust the idle air needles is with a vacuum guage. Turn the needle out until the vacuum guage stops moving 17-20 inches mercury. then screw it in clockwise until it just begins to drop. Do both needles the same way a couple times to obtain the highest vacuum pressure at engine idle speed. You also use the tach for the highest rpm. Then check the needles are within 1/2 turn between the two. Count the turns in to lightly seated and then back out to where they were. If you change the base timing you need to go back and adjust the idle air screws. slightly rich is better than too lean.
 
Thanks mate... unfortunately, i have no vacuum gauges or such... Im pretty much going on plug colour and performance here... so wish me luck...lol
 
Just use your tach, turn the needle out for highest rpm and back in until it just starts to drop. Do both needles a couple times and then adjust idle speed and do it again. Be sure and disconnect the throttle cable and adjust it to fit once you have the carb dialed in.
 
Air fuel mixture screws only function at idle and just above idle.

Running lean is not a condition of idle mixture screws.

If you are running lean then you may want to up the main jets.

If you are running say 66 go to a 68 or 69. If you cannot find jets determine what size (diameter) a 68 or 69 is and drill it out.

Also confirm secondary butterfly is opening properly. A 2 barrel carb will work just fine on that engine so if the secondary's are not opening you will not get maximum performance.

Also with regards to prop pitch.

if you are running a 17 and only reaching 3500 rpms then going to a 15 will only get you to ~ 3700-3800 rpm. you will still be 1000 rpm below specification.

I would confirm tach is reading correct and double check final gear ratio.
 
Yea, valid points... getting jets here is proving a bit of a hassle... I should have 69's in and i have found 70's, but i dont think the primary metering rods are correct specs.... so till i can find a parts source, im going to be farting against thunder here... if i drill my 70's bigger, theres no going back, and i might not be able to find more in any hurry, so ill first try source the metering rods.
i will check the tach... it is a brand new faria, and i think its ok, because in neutral it will rev up to 5000... only in gear it is restricted.... so far, everything i have replaced i have got spot on manual specs... its only the carb that is not to manual specs, and each time i get the carb closer to spec, it goes better, so I pretty sure its those primary metering rods, and the hanger (which i cant bloody find the right part for!!!) anyway, the sea and wind are up and down like a honeymooners bum at the moment, so i can look around for a bit...lol
 
Just re-checked my carb work, and discovered i miss calculated the hanger dimensions inches to mm... i need to raise it another 2 mm to be on f hanger spec, (manual says not to bend it, but im out of options here till i find parts, and Ive already bent it up 1.8mm and its going better).... and im going to pull the primary metering rods out and check their size tomorrow.... they were matched to a .075 jet, so i figure having replaced the jet with a 70 and not having changed the metering rods, it must be as lean as an ethiopian on diet... is my reasoning correct on that? (please bare in mind im a carpenter, so i have no idea how this **** works... im just trying think like a bloody carburetor...lol)
 
The primary metering rods (very small and thin) attached to the weird device with a small plastic "cover" and with the funky spring holding the rods in place.......... have you removed this assembly (pryed it up out of its location) and if so was there a spring underneath it?

The way to tune is to replace the spring under the holder with a different rate spring........But I would not say that is your issue at this point.

If you are running 70 jets no need to do anything.

Can you get all the part numbers of that carb... there may be a couple different locations so get them all and I can get all the specifics on it.
 
So now you are actually learning stuff about your boat other than how to throw off the dock lines and turn the key. Very good! You must live in Oz or something cz it's too cold here in the USA to be boating. If you need specific parts, let me know I will get them and fedex them your way.
 
Bahahaha.... I said im a carpenter, not an Idiot...lol... yes i have removed the power piston, and yes there is a spring in the chamber, and I could tell the difference between a primary and secondary metering rod with my toes in the dark by now.... lol. (i have to admit, 6 months ago, I had no bloody idea how a 4bbl carb worked or anythings names in it!!!) ( Im quite proud of how much i have learnt from sticking my fingers into it) lol
I will measure up the primary metering rods (very small and thin) (yea, im being a dick, but its hell funny..bahaha) tomorrow, and see if they are .042 inch... (i doubt they are though), and yes please, I would really be so glad if you could point me in the direction of a big pile of carb parts for sale... ill get a dam shopping cart for it....lol... unfortunately who ever rebuilt the carb had the great idea to sand off the serial number... so im in the dark there... all im running on is instinct and trial and error and mercruiser manual specs!!! I know it is the on with the part number stamped vertically on the throttle side of the carb body, so its not the oldest model....
What I do need is a F hanger, and two 0.069 jets and 0.042 primary metering rods.... thats a definite need ... i can feel it in my water!!! lol
If you could put me onto some of those, ill take back all the bad things ive said about you over the years....lol thanks mate...
 
Hell yea... i could recognize any bolt out of that engine, and draw up a wiring diagram with my eyes closed... can even recite the torque specs and head torque sequence off by heart.... i dont count sheep to get to sleep, i swallow whiskey to try drown the firing order that is imprinted in my head....lol... yea mate, I have learnt a huge amount on this one!!!
Yes, I do live in Aussie land, (although it skipped summer this year it seems... weather is really ****, but im still gonna go boating!!!)
Much appreciate the fedex offer, and I will thankfully take you up on it as soon as i have double checked the parts i need...

Have to say, you guys have been such a great help... thanks again to everyone for flicking all your knowledge and experience this way..

o2batsea said:
So now you are actually learning stuff about your boat other than how to throw off the dock lines and turn the key. Very good! You must live in Oz or something cz it's too cold here in the USA to be boating. If you need specific parts, let me know I will get them and fedex them your way.
Click to expand...
 
Rick71 said:
Bahahaha.... I said im a carpenter, not an Idiot...lol... yes i have removed the power piston, and yes there is a spring in the chamber, and I could tell the difference between a primary and secondary metering rod with my toes in the dark by now.... lol. (i have to admit, 6 months ago, I had no bloody idea how a 4bbl carb worked or anythings names in it!!!) ( Im quite proud of how much i have learnt from sticking my fingers into it) lol
I will measure up the primary metering rods (very small and thin) (yea, im being a dick, but its hell funny..bahaha) tomorrow, and see if they are .042 inch... (i doubt they are though), and yes please, I would really be so glad if you could point me in the direction of a big pile of carb parts for sale... ill get a dam shopping cart for it....lol... unfortunately who ever rebuilt the carb had the great idea to sand off the serial number... so im in the dark there... all im running on is instinct and trial and error and mercruiser manual specs!!! I know it is the on with the part number stamped vertically on the throttle side of the carb body, so its not the oldest model....
What I do need is a F hanger, and two 0.069 jets and 0.042 primary metering rods.... that's a definite need ... i can feel it in my water!!! lol
If you could put me onto some of those, ill take back all the bad things ive said about you over the years....lol thanks mate...
Click to expand...


No need to change main jets. One jet size will do no good either way, up or down.


http://quadrajetparts.com/rochester-quadrajet-c-128.html

http://www.carburetor-parts.com/Rochester_c_31.html

http://www.carburetion.com/quadrajet.asp



Here are three sites for parts

Here is a site with hanger, rod specifications.


http://www.carburetion.com/rodshang.asp
 
I ordered the tuning kit from Jegs. It has everything including instructions. I had to detune an Edelbrock for this 5.0 and it was super easy.
 
Rick, what I'm about to say does NOT negate what Kim, Jack or Johnny have suggested. They are giving you good info. Take heed!

Keep in mind that your Marine Engine does NOT want to run lean. A lean F/A will lead to high cylinder temperatures and eventual damage....... especially
with the piston profile used by GM in the SBC Marine Engine.
Also, your Marine Engine does NOT want to see an aggressive Ignition advance...... and again, especially with the piston profile used by GM in the
SBC Marine Engine.
You will want to be conservative..... perhaps 28* @ no less than 3.2 K RPM.
Until this is sorted out, I would stay out a sustained 2k to 3k RPM range for any duration.


Re; Kim's post #66

kimcrwbr1 said:
Get the spark plugs a nice toasty brown as quick as possible. It wouldn't take much to burn the valves and piston tops.
100% agree.

The best way to adjust the idle air needles is with a vacuum guage. Turn the needle out until the vacuum guage stops moving 17-20 inches mercury. then screw it in clockwise until it just begins to drop. Do both needles the same way a couple times to obtain the highest vacuum pressure at engine idle speed. You also use the tach for the highest rpm.
again...... 100% agree.

Then check the needles are within 1/2 turn between the two. Count the turns in to lightly seated and then back out to where they were. If you change the base timing you need to go back and adjust the idle air screws. slightly rich is better than too lean.
Yes!
Click to expand...

Please note that the low speed F/A adjustment affects low speed fuel metering only. Once the primary throttle plates have taken it above low speed, the low speed circuit drops out, and the engine will now begin to pull from the high speed metering circuits of either the primary and/or secondary throttle bores.
This is NOT to say that this of no or little concern...... it is of concern.
And use a Vacuum Gauge as Kim suggests.
You may also want to verify your tachometer!


As for WOT RPM, the engine MUST be able to make the OEM spec'd WOT RPM. (see your OEM specs for this)
WOT RPM is a test rpm only, and should NEVER be sustained for any duration.

Your cruising RPM should be in the 3.5k to 3.8k +/- range ONLY while the hull is up on step and "then some".
The "then some" ensures that the hull is not only planing, but is on top of the water giving least resistance while staying below the WOT RPM.
You will find your "sweet spot" once this is all sorted out.

If you cannot reach WOT RPM now, you will be over-burdening the engine at ALL RPM.
I would suggest pitching down ASAP, and then testing again.


Good luck!


.
 
Rick71 said:
Just re-checked my carb work, and discovered i miss calculated the hanger dimensions inches to mm... i need to raise it another 2 mm to be on f hanger spec, (manual says not to bend it, but im out of options here till i find parts, and Ive already bent it up 1.8mm and its going better).... and im going to pull the primary metering rods out and check their size tomorrow.... they were matched to a .075 jet, so i figure having replaced the jet with a 70 and not having changed the metering rods, it must be as lean as an ethiopian on diet... is my reasoning correct on that? (please bare in mind im a carpenter, so i have no idea how this **** works... im just trying think like a bloody carburetor...lol)
Click to expand...


You can drill new holes if there is material to do so.

I have seen hangers with multiple holes. Of course they were tuning the carb for a NON OEM build.
 
Ran into a similar problem on 79 SeaRay.....have some one run boat and slowly close the choke flap if engine starts to pick-up check the fuel pressure and float. My problem was a weak fuel pump and carb running lo n supply.
 
Thanks again everyone... definitely going to sort out that running lean asap as im sure i overdid it last run!!!
will be a while before i get her in the water due to parts and **** weather, and the fact i might need to get my ass off the boat and go earn some money...lol, but I will let you all know what changes and how they worked once out and about...
Cheers
Rick
 
Right... ive ordered all the dodgy carb parts as per manual specs from carbs unlimited.. (thanks for that O2Batsea)
couldnt find a F hanger, so I went with G hanger which is only 0.005 height difference... also ordered new secondary plastic cam as im not comfortable with any old looking working parts on this carb... if 0.005 inch is enough to affect the performance, then im bloody sure that will be worn and out of whack...
Tools down till parts arrive... 6-10 days...

In the interim, heres a little laugh for all you fellas.... 20 hour run in is complete so its oil change time.... captain twat over here gets a bright idea of sucking the oil out the drain plug.... so i made a dummy drain plug able to fit a hose which goes out through the bung plug, and into a sealed 15 liter plastic drum, and then another top mounted sealed port that i can attach the vacuum cleaner onto..... hee hee hee.... im so clever it just has to work...
slid a small oven tray under the engine for small spills and slipped the drain plug out, finger in hole, then fitted the dummy drain plug... all going well, but starting to get slippery... quick grab for the drain hose, and there isnt enough space for me to use the force needed to get it on the drain plug with slippery hands.... that was first fail.....
but no problem... now im wedged halfway under the engine and cant get out, with oil starting to fill up the catch tray.... ha ha, no problem, i just have to stick the hose into the drain tray and hold it there while the vacuum does the work.... im in agony, but its working, and doing it fast enough that no spillage has even happened yet.... fail no 2, the vacuum was more powerful than i thought, and totally sucked the sides of the container in against the force of sucking thick black oil, so about 2 litres got held in the container and the other 3 litres went straaaaaight into ole faithful vaccy.....luckily the poor bastard thing managed to last till it was down to the final trickle of oil before it died a horrible, oily death, and spewed oil all down my driveway....but better on the drive than in my bilge!!! bloody wish i had filmed that stunt.... i could have made money off of it on youtube.... hahahaha
Lesson learnt from that.... DONT BE A TWAT!!! lol
hope that gave you fellas a chuckle for the day, cos i cracked up laughing half way through at how f ing dum i had been...
now im signing off to go mop my drive before the landlord sees in and boots my miserable ass out of the house.... lol
 
Rick71 said:
Right... ive ordered all the dodgy carb parts as per manual specs from carbs unlimited.. (thanks for that O2Batsea)
Click to expand...


Wrong person who gave you that information......Just sayin........................reread the post with the links........



What parts did you order.

please include a list so we can see what you are replacing.

Going from a 70 main jet to a 69 main jet actually will be leaner so I hope you under stand this. The difference is most likely only a few thousandths of an inch.......like .005"

The oil dip stick tube should have a garden hose male fitting on it to attach a manual oil pump. If you are not aware of this, the Mercruiser factory dip stick tube goes all the way down to the bottom of the oil pan. ~ 1/4 inch from the bottom maybe less. So sucking the oil out thru the dip stick tube is the best way to get it out. Been doing that way for almost 30 years.

Also there are 12 volt oil pumps that feed thru the dip stick into the oil pan.
 
ordered 69 jets (i am aware they are leaner,)
42 primary rods to match the jets as per spec
g hanger
hanger cam
new piston spring,
needle and seat

i know the jet will be leaner. its is supposed to be 69 jet with 42 metering rod... i have 70 jet with 46 metering rod...i dont want to end up with the right metering rods and then be needing to order the jets... rather toss $10 away in the name of having it incase.

as for the oil... yes i know you can use the dipstick hole,... i tried doing that with a hand pump when i first got the boat, and it was not working....im shelling out so much on parts, i have to be a bit tightass on some things, so improvisation came into play and oil pump tools can come later.... and it entertains the neighborhood when I do dum ass things....lol

kghost said:
Wrong person who gave you that information......Just sayin........................reread the post with the links........



What parts did you order.

please include a list so we can see what you are replacing.

Going from a 70 main jet to a 69 main jet actually will be leaner so I hope you under stand this. The difference is most likely only a few thousandths of an inch.......like .005"

The oil dip stick tube should have a garden hose male fitting on it to attach a manual oil pump. If you are not aware of this, the Mercruiser factory dip stick tube goes all the way down to the bottom of the oil pan. ~ 1/4 inch from the bottom maybe less. So sucking the oil out thru the dip stick tube is the best way to get it out. Been doing that way for almost 30 years.

Also there are 12 volt oil pumps that feed thru the dip stick into the oil pan.
Click to expand...
 
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