5.0 Merc inboard idle problem

[chrismurr1]

first off, im new to iboats.com but not new to boats or engines. I have been a guest on the site for some time and i look forward to hopefully getting some help from the many experts i see here.

history: 1975 33' Chris Craft Sportfish with twin 1984 Merc 5.0 (305) engines, Borg-Warner 1:1 trans straight inboard, closed cooling. both engines have good compression 155-165, thuderbolt IV ignition systems. New remanufactured Q-jet carbs 600cfm mounted on a wedge plate to keep them level. both engines are on new electric fuel pumps and currently fed from a new 12 gal fuel tank (getting original ones replaced). new Merc water separating fuel filter and new in line filter (right before the carb). port engine has stock merc q-jet intake, starboard engine has edlebrock aftermarket q-jet intake. Boat is in water.

Problem: Starboard engine wont idle on the idle circuit. i have double checked to assure that the circuits are not clogged with anything. i have the idle mixture screws at 4 turns out at the moment, however, to get it to idle well (950-1000 rpm) i need to have the idle speed set screw in all the way (which of course takes it off the idle circuit I believe). the engine idles a little rough at 800 in gear.

timing at 950 RPM is 8 degrees BTDC if i can get it to idle down at 700 its roughly 6 BTDC. the idle mixture screws can be closed at this point and have no affect which i am sure means im running on the cruise circuit. when i try to lower the idle speed screw RPM drops to roughly 500 and i cant stabilize it, even had the idle mix screws out to 6 turns but got nothing and if finally dies.

i did make a couple observations..... these new carbs are electric chokes, not hooked up yet and set very lean, almost all the way open. (i do warm the engine up to operating temp before trying to get the idle set right). the vacuum pull off on the starboard engine looks like it gets pulled back all the way when cranking (damn good vacuum) but the port engines pull off barely moves till the engine starts.

also i cant seem to fine any vacuum leaks at the intake or leveling wedge using starter fluid to see if the rpms jump.

the port engine is set to a 950 idle (800 in gear) with a 10 degress BTDC timing with the idle speed screw about half way in.

nether engine blows smoke or smells fuely at the exhaust.

sorry for being long winded but i know more detail is better when asking questions.

 

[WSUDERMAN]

Re: 5.0 Merc inboard idle problem

You mentioned that these are "New" carbs? When I installed the edelbrock carb on my 305 it needed some changes from stock. The stock settings where to rich so I had to make changes to the main jets and metering rods. I will look to see if I still have my notes. However, in case i don't, edelbrock's manual on this is very good. with lots of detail. (link below)

http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1409_manual.pdf

 

[Don S]

Re: 5.0 Merc inboard idle problem

950-1000 rpm is way to fast for an idle speed. They should be idling around 650 to 700 rpm in gear.
When was the last full tuneup (plugs, cap, rotor), and timing check?
If you can turn the idle screws all the way in and it runs best, then you have a leak somewhere.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

The reman company i bought these from does engine test them on a 305 (since they were specifically bought for the merc 305 engine). the test setting had them idling at 800 rpm with 21 inches vac according to their testing tags. when i installed the port engine, it fired right up from their setting and idled around 500, i brought it up to 950 so it would idle at 800 in gear. same with the starboard engine, except that it wouldnt run at all with their settings, figuring each engine is unique, i started by backing out the idle mix screws from their 3 turns all the way to 6 but still, no joy, i had to crank the idle speed screw all the way in to get to 950.

 

[WSUDERMAN]

Re: 5.0 Merc inboard idle problem

One more thing, The float levels where off from the factory so you should check those. Also, soapy water works well for finding small leaks since you can watch the bubbles move and get sucked in.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

so the 800 in the merc manual is out of gear. i can live with that. what is the best way to check for a leak... i would assume somewhere along the wedge plate.

New plugs were installed with the carbs. the cap and rotors were cleaned up with emery but not replaced. not sure how old but they arent pitted or worn. port engine has a new coil, havent tested the coil on the starboard.

 

[shoestring]

Re: 5.0 Merc inboard idle problem

my hokey way of doing things would be to flip flop the carbs and see if the problem moves with the carb or stays with the engine. might seem odd but i've done odder things.

 

[Don S]

Re: 5.0 Merc inboard idle problem

1000 rpm is also too fast to adjust the idle mixture. You MUST slow it down.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

ok, sounds like a plan. i am using the thick 1/4 inch gaskets under the carb that came with them and .063's under the wedge plate. i will do that test tonight or Saturday when i get out to the marina and see what i come up with.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

yes, i am going to bring them down to 800 first and then check timing and idle mix which i will also do when i get out there tonight or Sat. im starting to wonder if those foam 1/4 inch gaskets are any good or if i should just do .063's under the carbs too. it did seem to me that the think ones have alot of squish in them.

 

[Don S]

Re: 5.0 Merc inboard idle problem

Bring them down to 700, that will be close to what it should be when in gear and in the water.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

i see your logic, very well i will do that.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

ok, so... i put all new 1/6" thick gaskets on both engines (under the wedge plate and carb). i found that on both engines, the wedge plate wasnt completely seating like they should and i didnt trust the 1/4" gaskets under the carbs as they just have too much give. after bolting it all back down i have the following results....

Port engine purrs like a kitten at 800 RPM and just a smooth at 700 in gear. 8 Degrees BTDC. now i do have to adjust the idle speed screw in just a little to do that which is perfectly acceptable, however, i can turn out that set screw off the throttle, close the plates completely, close off the idle mixture screws completely and still idle at 600 rpm............ holy cow batman, is that just some awsome vac or is the 600's too small for these engines (i think not since i should be able to run them on 450's, technically). i dont have any smoke of any color from the exhaust so its not over fueling... just thought i would make mention of that in case anyone has any input on that.

starboard engine... the problem child.. with the appearent vac leak gone, i can get it to run just like the port engine EXCEPT that i have to turn the idle set screw in about half way to achieve it. timing at 8 degrees BTDC @ 800 RPM, the engine seems to respond nicely to a snap of the throttle but on a gradual throttle increase, the initial bump off idle takes it from 700 to 1200 (in gear) and i have to tap it back down to 1000 where i like to be in the no wake zone. it could be the cable or the controls as there is a good 1/2" play at the control. when im down at 700 in the channel and idling along, the engine sounds like its running on 6 cylinders but it never tries acts like its going to die or anything, just seems really rough sounding, like it has a radical cam in it. that being said, i am going to replace the plugs, wires and coil this coming weekend. i dont particularly care for the cheap champions that are in it now and the wires and coil are of an unknown age. when dpressing the wires down onto the cap and coil to make sure they were seated well (engine running) i got a nice little pick me up from the coil so i think the wires may be on the way out if not gone.

both engines ran beautifully at 2000 rpm out on the gulf pushing us along at about 8 mph on the chartplotter. so no complaints there.

i would appreaciate any input or thoughts anyone may have on my next move (right or wrong).

 

[Don S]

Re: 5.0 Merc inboard idle problem

close off the idle mixture screws completely and still idle at 600 rpm............

Very simple, you still have a leak somewhere if you can close off the mixture screws and it still idles.

 

[chrismurr1]

Re: 5.0 Merc inboard idle problem

just wanted to give an update and thank everyone for their assistance. turns out there was a gremlin in the system so to speak. after going thorugh the paces several times, i decided to connect the chokes to their power and get them set, upon giving the cold engine a pump of the throttle, i hit the key and she fired right up and raced to 5000 RPM. shut it down quick, went and checked the linkage.... all looked good so hit the key again and right back to 5k. after going back down (knowing that there had to be a throttleplate open somewhere, i disconnected the throttle cable and took a gander down the secondary bores and low and behole, the plates were held open about half way.

whats worse, the culprit holding them open was the return spring that ran under the secondary linkage. for now, i removed the spring and BAM... instant gratification on both engines... hot them set at 700 RPM, 8 Degrees BTDC and the air/fuel mix screws actually cause a response, so they are set now too.

fancy that, a freakin spring...