Motor dieseling; Timing issues

[rocky388]

Hello,
I’m having an issue with my 1988 mercruiser 5.7 260hp thunderbolt IV ign system dieseling while turning the motor off. Usually there’s a sputter or 2 before it finally dies. I read one of the potential causes could be timing. So out on the water today I attempted to adjust timing. While the boat was running, I let it get to operating temperature, while in neutral (not in gear) rpms are at 750 and I adjusted timing to 8* btdc. In neutral in gear rpms drop to 550 and timing is around 6* btdc. Is this correct?

My confusion comes in because I thought I read rpm speed should be set while the boat is in neutral in gear; if I attempt to make any of these changes in gear, I can get rpms to 750, but taking it out of gear brings the motor to 1100 or so rpm, with timing 12* btdc, and the motor sounds like its racing at this point, and it’s not easy to get into gear without bring the throttle further forward. This seems very incorrect.

As it sits now, I made my adjustments to 750 and 8*btdc while in neutral out of gear. However, this did not solve my dieseling issue. My next thing was going to be to adjust the fuel/air screws on the carb, but I wanted to make sure my timing is correct before bringing more factors into this. Looking at the adjustment screws on the carb, they look the screw heads are rectangular plugs, is there a special socket meant to turn them?

Another side question: Is it normal for timing to “hunt and surge” while the motor is warming up? Once the motor was warm I was able to adjust it to a steady 8*, while the motor was warming up I noticed there was some variation to the timing; it would not hold steady.

Thanks

 

[Bt Doctur]

A lean condition will lead to dieseling ,also a high engine temp. Without knowing your engine serial number or style of carb, the mixture screw question cant be answered

 

[rocky388]

I'll report back with the serial number when I head to the boat later today. It's a 4bbl Rochester quadrajet. Is that the proper procedure for setting idle and timing, or is that dependant on serial number of the motor as well? I did replace a bad thermostat. I wasn't sure what temp was in, after researching it seems like it was a 160 possibly. All a local parts store had was a 195. The motor still runs at a steady 160 like it always has. Could the higher temperature thermostat be the issue? My understanding was that the thermostat doesn't really come into play unless the motor is starting to overheat. Thanks.

 

[Bt Doctur]

sure hope you didn't put the 195 in there.
~~ Could the higher temperature thermostat be the issue?
Without a doubt!
~~My understanding was that the thermostat doesn't really come into play unless the motor is starting to overheat. Thanks.
nope, therm keeps the temp constant to get good fuel economy, and water vapor/condensation out of the oil

 

[MikDee]

First thing, unless your boat is closed freshwater cooling, you need a 140 degree thermostat! The cooler you can keep it the better. I read that salt water starts to crystalize at that temp. Next, they do make a reasonable aftermarket tool with 4 flexible head designs on it, one of them is square, I've had it for yrs. That's what you need! Also, adjust your mixture screws out, to reach the highest idle speed you can, your engine will appreciate that!

 

[rocky388]

I did put the 195 in there. I'll run out and get a 140..Could the 195 (despite never getting passed 160 on my gauge) damaged anything that I now need to inspect? I'll get the 140 in there, and looking through the manual it seems idle rpm needs to be set in gear to 650 - 700..then take it out of gear (which would mean rpm would probably be around 900) then set timing to 8* btdc. After all that I'll check the mixture screws. That sounds about right? (Provided this can be answered without knowing the serial number, which I have been yet to get)

 

[MikDee]

Set the mixture screws First, so you have a stable idle!

 

[thumpar]

I don't think the 195 thermostat would damage anything but it could definitely cause the run on issue which could damage things. My manual says 143 for brass and 160 for stainless. I guess it depends on if you are in the brine or not.

 

[NHGuy]

I went through this very problem. Here's what I recommend. 140 degree stat, don't forget the marine brass rivet gasket. The rivet makes contact between the t-stat housing and ground. So your gauge will be true.
725 idle rpm, it will fall to 650 in gear, in the water. That's fine, idle it as low as it will run smoothly. Higher idle allows fuel to feed your hot motor which exacerbates the dieseling. Try a cool down before you shut it off, but once adjusted it won't be needed. To cool before shutoff run the motor at high idle for 30 to 60 seconds- maybe 11 or 1200 rpm. The added coolant flow will help stop the dieseling. If it does diesel turn the ignition back on, the motor will resume running. Cool it again, then shut it off and immediately shift it into gear (either fwd or reverse) that should stop it. Don't allow the dieseling, bad for hard parts! Obviously, your boat is tied up during this.
The "book" adjustment is 8 degrees BTDC, Mercruiser says your mixture screws need to be turned down til she stumbles then out till it runs poorly and then split the two extremes. I have found that I tend to blacken my plugs this way so I'd say find the stumble and then go half or 3/4 turn out from there. Now reset the idle to 725 and recheck the mix. Repeat til it is adjusted to correct mix and low idle at 8 degrees and with 140 degree thermostat, engine warm.
It all works together.
The springy carb adjuster tool is essential. It makes setting the mix screws a breeze, probably a $12 tool on ebay, and $15 at the store.

 

[rocky388]

Looks like with all the replies I was able to get this sorted out (hopefully).

I couldn’t get the serial number off the rocker arm cover, it’s too faded to be legible. Not sure if there’s a stamp somewhere else on the motor that I can get the serial number from.

I went straight to the marina and bought a 140* t-stat. I did not install the gasket with the brass plugs on it, I will swap it out though. There’s also an o-ring and cork looking gasket that are both the diameter of the t-stat, the old one did not have them on, so I didn’t put them on. Do they go above/below the t-stat? Is it possible they were included in the kit but I don’t need them for my motor? With the new 140* t-stat the motor didn’t get above 120*, which seems colder than I’ve ever seen, but it’s possible the boat never had the right t-stat in since I took ownership, even before my 195* swap out.

I tried setting the idle in neutral to 800ish (which had the idle speed screw turned almost all the way in, which seems very wrong)at 8*BTDC in order to give me an in gear neutral idle of 650ish. This sort of worked, I was still only at 600rpms, but shifting in and out was difficult, and all around this just seemed wrong. I set it back to what it was before: 750rpms in neutral out of gear, at 8* which gave me 550rpms in neutral in gear and it ran like it should. Turned off perfect with no dieseling. I turned it on and off through different scenarios about 6 times and each time it shut off like it should. Hopefully this solved the problem.

Thanks for all the replies. Upon doing this work I realized my tachometer on the dash is out of whack. I’m hoping I can clean the contacts on it. I tried turning the screw back and forth to clean the contacts like I’ve read on here, but that didn’t seem to fix it.

 

[rocky388]

I should add, I did not touch the carbs mixture screws. It has been running fine prior to the dieseling issue which I credit to timing and the 195 t-stat. I'll probably end up going to the boat by myself tomorrow should I/can I adjust these screws then? I'll be by myself, which makes adjusting anything underway impossible..do these adjustments need to be made in gear? Should I just leave it alone if it runs good..if it aint broke..?

 

[rocky388]

I should add, I did not touch the carbs mixture screws. It has been running fine prior to the dieseling issue which I credit to timing and the 195 t-stat. I'll probably end up going to the boat by myself tomorrow should I/can I adjust these screws then? I'll be by myself, which makes adjusting anything underway impossible..do these adjustments need to be made in gear? Should I just leave it alone if it runs good..if it aint broke..?

 

[Silvertip]

First: The thermostat is closed during engine warm up and may in fact may stay closed at idle and slow speeds. It opens when water temperature reaches the thermostats "open" temperature to allow more water to pass through the engine. It is there for performance and economy purposes. Use the correct temperature thermostat.
Second: You do not adjust timing to achieve a specific idle rpm. Nor do you set idle speed to achieve a specific timing number. You set ignition timing using the proper procedure (in base timing mode for most electronic ignition systems). You then set the idle rpm with the idle speed screw, with the throttle cable disconnected. If you attempt this with the cable connected you are working against the cable which should already be tight. When idle speed is set you next adjust the throttle cable. If carb idle mixture screws need adjusting you also do that with the cable disconnected as you will likely need to readjust idle speed when done. Idle mixture screws have no effect on high speed operation and if you don't understand how to make that adjustment you are advised to either "read" or keep your fingers off them.
Next: Dieseling is generally caused by idle speed that is set too high and not allowing the engine to idle for a couple minutes after a long high speed run. Yes, timing that is advanced to far, faulty fuel systems, over heating and carbon build up can all cause this but those are less likely causes.

 

[NHGuy]

I concur with Silvertip, and yes recheck your idle and timing back and forth til both are correct. Although it sounds as if you are just about there. ps. on your ignition system there is no base mode to set for timing, just shoot and set.
To verify the mixture take a look at your spark plugs. Black buildup means the fuel is too rich, white is too lean. Tan is just right.

 

[rocky388]

So here's a bit of an update:
The dieseling issue is slightly better, but still there. Currently the 140* degree t-stat is installed, with the proper brass rivet gasket. Idle rpms out of gear are 750rpms, I set timing to 8*BTDC.
As suggested, and after reading, I decided to adjust the mixture screws to obtain the highest RPM, and then adjust idle rpms and timing to spec in hopes to solve the dieseling issue.
The issue: turning either mixture screw all the way in or out has no effect on the motor what so ever. This seems very wrong.
I’ve been reading on ‘non-responsive q-jet mixture screws’, and it seems there’s 2 main causes; vacuum leak, or too high of an idle.
I sprayed carb spray all around the carb to attempt to pinpoint a vacuum leak. Spraying the motor did not change the way the motor ran, therefore I am somewhat ruling out a vacuum leak.
I adjusted the idle lower, which still left the mixture screws non-responsive.
What next? I do not know the last time this carburetor was rebuilt, if ever. Should a rebuild solve all these issues, or is there a possibility of an underlying issue outside of the carb?
I was also thinking of doing ‘dunks de-carb method’, to rule out the possibility of carbon buildup causing the dieseling, but now I think I have a bigger issue with the non-responsive mixture screws. Once the carb is sorted out, is there any reason not to de-carb a sterndrive this way? (basically a temporary fuel tank with a fuel/seafoam mixture ran through at 15 minute intervals)

 

[NHGuy]

Yow, the mixture screws went in til they stopped? And the motor ran the same? That's really odd. Since you do them one at a time, the one that was not turned down had enough supply to continue running the motor.
Do a re adjust with the mix screws. Start out with them 2.5 turns off from softly seated, then turn them in til it stumbles, then back out to a mid point. BTW, you are not trying for highest idle. If you had a gauge you'd be trying for highest VACUUM at idle. The adjustment is to get the mixture right between rich and lean. Trying to push idle speed through the mixture screws will probably put the adjusters too rich.

 

[havoc_squad]

The adjustment is to get the mixture right between rich and lean. Trying to push idle speed through the mixture screws will probably put the adjusters too rich.

I believe you mean the inverse, trying to push idle speed up to the highest rpm's will actually put the idle mixture adjusters too LEAN. Eventually you get to the point where its so lean the engine chokes and dies, but most people don't adjust this right.

Too rich on idle mixture screws makes the engine run rougher and will stall it out if too much.

The best procedure for idle mixture adjustments from scratch is reset them to Mercruiser specs and get the engine running as close as possible to what is requires if setting on the muffs. You could probably use a vaccum gauge to assist with this.

To make sure it is right, the final idle mixture adjustment should be done in the water under load per Mercruiser specs.

 

[NHGuy]

Well, call it what you will but he should not be trying to add mix to increase idle speed. On that we agree.

 

[achris]

What should happen when you play with the mixture screws is you have about 1/2 turn either side of the 'sweet spot' and the engine should try and stall out... If you can run them all the way in and out with little to no change, pull the carb and clean it... There is something very wrong... May even be something like the secondaries are being held open slightly... But certainly the mixture screws (at 1000rpm and lower) will have a definite effect on the engine speed...

 

[rocky388]

Latest update:
Rebuilt and cleaned the carb. I noticed the float that was in there was not set to the spec height. Ran the boat for awhile today, and had no dieseling aside from at the end of the day parking the boat in my slip, but it was a very quick "putter" before the engine died. I think this improvement is due to the new float set at the proper height not flooding the engine as was the issue before? I also noticed during the rebuild that the secondaries were not opening at a full 90*, they were just shy of that. I corrected that issue as well.

At any rate, the mixture screws are still not responsive. Anyone have any ideas? Again I sprayed carb cleaner to test for vacuum leaks, and found none. Other than the dieseling issue which appears to have gotten better, but not resolved, the boat runs great. Plenty of power, quick hole shot, starts right up, responsive throttle, idles good. However I feel if I can solve the unresponsive mixture screws perhaps that will solve the dieseling issue.

Is it possible to have a vacuum leak, and not suffer any performance losses?