Logo
FULL SITE MENU

My turn, Idle issue on a rochester 4 bbl 1986 350

kghost

kghost

Silver Medal Contributor
My friend just purchased a brand new 29 year old 18 ft checkmate with a 350 260 hp mercruiser.

only 20 hours of use and garage kept the whole time. Show room condition.

I have gone thru everything and it stats and runs very good.

I replaced the carb base gasket and needle and seat and accelerator pump.

NOTE: I did not remove and disassemble the entire carb. removed the top cover and replaced the needle and seat and acc pump and gasket only then lifted the carb up and slipped in a new base gasket.

Anyway once warmed up it will idle at 800 and run for sometimes 20 seconds, sometimes 10 seconds and then it will drop 200 rpms to 600 rpms. Give it a bit of throttle and it will repeat.

I have adjusted the cable to put positive pressure on the throttle and idle mixture screws work correctly and have been adjusted.

Any Ideas
 
Interesting Chris..........

So at idle it will be ok initially then a valve will stick causing the sudden drop in rpms?

would this issue be a slightly open/stuck valve will cause more fuel to be drawn in until it closes then idle returns to normal??

I am trying to understand what a sticky valve (which I have never come across in all this time) would do/cause.

Thanks
 
did you check to make sure the throttle plate-to-bowl screws were tight when the carb of unbolted? check the vacuum break and the short rubber hose for leaks?? 800 rpm seems a bit high for idle speed...make sure the primary nozzle isn't dripping at idle...
 
I did not check for loose screws on throttle plates.......carb has only 20 hours of use

The vacuum break hose looks brand new and I did not spray anything for leaks as I ran out of time Saturday when I was working on it.

I did not notice any gas coming from the accelerator discharge nozzles but will recheck

I agree 800 is too high...........but when adjusting lower it just would not do it normally it would just drop to or below 600 there seemed to be no in between.
 
Yes, was referring to the primary bore nozzles, in the middle of the triple venturi assemblies....especially @ the 800 rpms....if you get the "nozzles drip", then the idle mixture screws won't have any effect because the throttle plates are open too much....

And like Chris suggested, if the engine isn't mechanically sound, you'll never get a stable idle.....20 hours over 30 years leaves lots of opportunity for mechanical deterioration....
 
makomark said:
Yes, was referring to the primary bore nozzles, in the middle of the triple venturi assemblies....especially @ the 800 rpms....if you get the "nozzles drip", then the idle mixture screws won't have any effect because the throttle plates are open too much....

And like Chris suggested, if the engine isn't mechanically sound, you'll never get a stable idle.....20 hours over 30 years leaves lots of opportunity for mechanical deterioration....
Click to expand...

Anytime I see an strange idle issue I check the vacuum. A long sitting engine will do funny things when valves hang. Hard to set idle is one of them. Pouring some transmission fluid doen the intake while running will help with sticking valves. Running the hell out of the engine under a load will also help.


Put a timing light on it and see what the timing is doing when this is happening.

If vacuum checks, the one thing that will give you unexplainable issues is the distributor pick up. For the $ Change it anyway.
 
The ignition sensor is brand new, rotor is brand new. Previous owner replace with merc parts due to not starting.

Long story short the boat was not running when we bought it. I went with my friend to check it out. got almost $1000 off the price...lol

looks as if the fuel pump went bad from sitting, guy thought it was ignition so he replaced sensor, rotor and coil but but no go. had someone else diagnose the fuel pump. replaced the fuel pump and got it started. 10 years ago...

When we looked at it, it would not start and had not since the last time owner used it...10 years ago...he thought it was the sensor so he replaced it again within the last couple years with another brand new one but could not get it started and left it as is.

Turned out after we brought the boat back and I started to troubleshoot he had swapped out the coil with a non internal resister coil.
The coil was NG....replaced with a new internal resister coil and once we primed the fuel system she fired right off.

So the sensor is a new one and is the epoxied type (new style) with no use on it other than when we have run it for about 30 minutes total so far.

Also idle mixture screws work properly, as they are turned in the motor reacts like it should perfectly as it should!! and it reacts properly when adjusting them out...motor smooth's out very nicely and you can hear it!

I am going back later this week and will put a timing light and vacuum gage on it and see what's up.

I am leaning towards a float issue at this point...........but I will do a more thorough job looking at other things.......just ran out of time last Saturday...
 
Going back tomorrow night........Had to tear down my motorcycle last night to remove the heads.......need compression releases installed....ugg.......Buddy has the jigs/special tools and will do it over the weekend.....
 
I am leaning towards a float issue at this point...........
Click to expand...

I'm thinking that's why Mark mentioned; "make sure the primary nozzle isn't dripping at idle" ... or what I was calling the primary throttle bore air horn(s)!

Perhaps a high fuel level may be causing these nozzles or air horns to drip fuel while still within a throttle plate position that would normally allow for the low speed fuel metering circuits only.


.
 
kghost said:
I did not check for loose screws on throttle plates.......carb has only 20 hours of use

The vacuum break hose looks brand new and I did not spray anything for leaks as I ran out of time Saturday when I was working on it.

I did not notice any gas coming from the accelerator discharge nozzles but will recheck

I agree 800 is too high...........but when adjusting lower it just would not do it normally it would just drop to or below 600 there seemed to be no in between.
Click to expand...

How did the bowl area of the carb look when you had the air horn removed...super clean? Some dirt? Anyway, I think I would recommend removing the carb, disassemble it, inspect, good cleaning, compressed air thru all the passages, reassemble. This would eliminate the carb in regards to being the cause of the idle issue which, good chance that the carb is the problem.
 
Ok Been busy so here is the update

I removed the carb, definitely a fuel related idle drop. When I went back and started it, it idled at 800 or so and dropped off very suddenly and as far as I am concerned it is fuel related.

I tore down the carb completely. Extremely clean..........virtually new inside.

I blew out all orifices, removed all jets screw in plugs etc. I found one orifice that when I blew it out it gurgled a bit.......kind of odd as the same one on the other side did not....

Reset float height to spec,

As I reassembled it I noticed that the accelerator pump when depressed would not immediately return to its at rest height.

I removed it and changed the spring ( I have many spare parts from many carbs) to a stronger spring.
I am not sure what a slow returning accelerator pump would do to idle but that is all I found. Nothing else out of wack...........

I also epoxied the 4 plugged ports at the bottom of the bowl just for piece of mind.

I am going over tomorrow afternoon to reinstall and retest.

Oh and before I removed the carb I checked the timing.

Rock solid at 8 *BTDC

Will update Friday.
 
There are no vacuum leaks.........

No worn throttle shafts as carb has only ~ 20 hours of use.
Base timing (only timing) No jumpers needed TBIV, 8*BTDC

Thanks for the adjustment procedure..............been adjusting carbs for well over 40 years............

I have thorough knowledge on rebuilding these including having the repair tools to rebush the main throttle shaft. Have done several over the years.



If I had a vacuum leak the idle would be higher than normal and the air fuel mixture screws would not respond as they do......Exactly as one would expect.

Of course I could miss something but I feel it is a fuel related issue........

Will know more Friday
 
Did not hook up a vacuum gage last time I was there but I am bringing one tomorrow.

I can assure you there is no Vacuum leak. Not sure if there is a valve sticking though but the motor is quiet as a mouse and at 800 rpms idles so smooth you could put a glass of wine on it and barely a ripple............
 
kghost said:
Did not hook up a vacuum gage last time I was there but I am bringing one tomorrow.

I can assure you there is no Vacuum leak. Not sure if there is a valve sticking though but the motor is quiet as a mouse and at 800 rpms idles so smooth you could put a glass of wine on it and barely a ripple............
Click to expand...
Ripple wine Eh!


kimcrwbr1 said:
What is the vacuum pressure at engine idle?
Click to expand...

Vacuum Pressure?

Yes, I had wine tonight.
 
You giving me sh!t here............

or are you making a statement.........all comments and suggestions are welcome
 
kimcrwbr1 said:
You can use the vacuum guage to adjust the idle air needles...... you want the highest lean vacuum pressure.
Click to expand...

kghost said:
There are no vacuum leaks.........

Thanks for the adjustment procedure.............. been adjusting carbs for well over 40 years............

If I had a vacuum leak the idle would be higher than normal and the air fuel mixture screws would not respond as they do...... Exactly as one would expect.
Click to expand...

I believe that Kim was suggesting to take a vacuum reading (manifold pressure) for the purpose of adjusting the low speed fuel metering circuits only, and was not suggesting a check for a vacuum leak.
Yes/No???
 
Yes I agree and understand......

My view is I can adjust the air fuel mixture well enough by ear.........especially on a V8 with thru hull exhaust...

I have used vacuum and tachometer to adjust both in the past and not much difference for me than by ear.

Not for everyone but as I said I do have a bit of experience with this....

Chris suggested a vacuum check to see if there may be a mechanical issue such as a sticking valve...
 
I have not reinstalled the carb yet.
Based on the idle screw adjustment before I removed it I would say it "was" normal.

I will have more feed back tomorrow. I am going over tonight to reinstall and test.
 
Just got back from installing the carb

No real change

vacuum is steady at 19 inches at 800 rpms
18 at 650 rpms

Timing is 8 btdc at 650 rpms

If you raise it just above 650 rpms then rev it up it will settle immediately at 800 rpms

if you adjust to 640 -650 rpms it will settle right back 650.

If you raise the rpms to just above 650 approaching 700 it will on its own jump to 800 and then back down on its own...


My friend said just under 650 and leave it for now.

I have never come across anything such as this.
I have no answers.....

One question earlier was can I see gas dribbling out of the two "nozzles". Yes at idle there is a very small but steady dribble of fuel.
I would say this is normal.

When the accelerator pump is activated the "nozzles" just above the main ones shoot fuel immediately

Motor appear to run perfect on the hose so I am not sure what to do from here.

Friend want to run it and see how it goes......
 
WHat is funny is he is actually very experienced also...

A while back I posted pictures of a carb I was working on for a 1970 big block convertible Chevelle..........well it is his.

He has a garage that he sells used vehicle out of and he and a few other get to use his garage to work on our own vehicles..he has a lift!!

He has done a lot of motor work and all....he knows I know mercruiser so he has me work on his boat......good deal cause get to use his lift to work on my truck...
 
Back
Top