Sooty plugs

Hello guyzs,
wondering if someone could think of something I haven't?

I have a 1986 Mercruiser 4.3LX with an Alpha One Gen 1 leg, fitted with 3in risers. 4BL Rochester, Thunderbolt IV ignition.

The issue the motor has is:

- numbers 5 and 6 are sooting up at low RPM
- numbers 1,2,3 and 4 are absolutely perfect.
- runs fine at higer RPM

I have changed the following:

- fitted 3 set of plugs
- changed plugs 1-4 into 5 and 6 (known good plugs)
- new set of leads
- changes leads from 1-4 into 5 and 6 (known good leads)
- new distrubutor cap
- new rotor
- different carby

I think I've exhausted ignition issue problems
I don't think its a carburettor issue as 4 out of 6 plugs are perfect and runs well at 2000rpm and above
I don't think its an air leak issue because the plugs would run lean and again the boat runs well at high rpm

I haven't checked the valve clearances. The engines has done around 10 hours work and the lifters have not been checked since. Now that the hydraulic lifters may have fully pump up and closed the valve clearances on 5 and 6?

I plan over Easter to do a leak down test...

Does anyone have any other ideas I can check out whilst doing this?

cheers
Dave

Re: Sooty plugs

If the lifters were adjusted properly at rebuild time they do not require adjustment again. If it was done wrong, they have been wrong all along and are still wrong. That's the nature of hydraulic lifters. You do understand that there are are cross-over passages in an intake manifold so it is possible to have carb problems that affect specific cylinders. Is the choke working properly -- more specifically, is the choke butterfly standing perfectly vertical on a fully warmed up engine. when are you checking plug condition. If its after a slow short run, that's wrong. Wide open for a nice long run. Chop the ignition and check plugs. No idle time. That checks the high end. If they are fouling only at idle, check idle mixtures.

Re: Sooty plugs

Silvertip said:

If the lifters were adjusted properly at rebuild time they do not require adjustment again. If it was done wrong, they have been wrong all along and are still wrong. That's the nature of hydraulic lifters. You do understand that there are are cross-over passages in an intake manifold so it is possible to have carb problems that affect specific cylinders. Is the choke working properly -- more specifically, is the choke butterfly standing perfectly vertical on a fully warmed up engine. when are you checking plug condition. If its after a slow short run, that's wrong. Wide open for a nice long run. Chop the ignition and check plugs. No idle time. That checks the high end. If they are fouling only at idle, check idle mixtures.

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Yes checked the choke, perfect, but an incorrect choke would effect the 4 perfect plugs. When you said crossover passages, they still come from the same place that the 4 perfect cylinders are fed so I don't understand your comment, sorry, please expain. The lifters were set dry (no oil in the engine, except assembly oil) and hydraulic lifters need oil pressure as well, meaning there could be slight variations in clearance, so again (sorry not trying to be rude?) but please explain. The plugs will soot up after about 10 minutes, then start a miss as they get really sooty, so I need to fix the lower speed problem, at high speed its good. The plugs soot up regardless of engine cold or engine hot. Plugs should good all the time?

Re: Sooty plugs

Is the soot oily or dry. If they're oily, you're looking at intake gaskets, valve seals, rocker covers filling with oil, or oil control rings. But I can't think of anything common to both cylinders.

Re: Sooty plugs

wrench 3 said:

Is the soot oily or dry. If they're oily, you're looking at intake gaskets, valve seals, rocker covers filling with oil, or oil control rings. But I can't think of anything common to both cylinders.

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Its definatley partially burnt fuel, dry, you could almost blow it of with your breath. Like the soot from a candle.

There is virtually no blowby from rocker covers and dipstick. Any fumes are black from the exhaust, not blue.
Engine was fully rebuilt 10 hours ago. Just 4 perfect plugs and 2 sooty plugs. Numbers 5 and 6 which are at the back of the block. Agreed, there is nothing to link the two?

Re: Sooty plugs

Silly question.. You have got the right plugs in (BR6FS), yes? Too cold a plug would cause exactly your problem.

Re: Sooty plugs

achris said:

Silly question.. You have got the right plugs in (BR6FS), yes? Too cold a plug would cause exactly your problem.

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yes, BR6FS, 3 different sets.

Re: Sooty plugs

Hydraulic lifter should have been primed BEFORE installation and there is a procedure on how to do that. Intake runners are not all the same length. What may be good for one cylinder in marginal carburetion application, may not be ok for the others. Lastly, when you set hydraulic lifters you are not adjusting clearance. You are adjusting for zero lash. The action of the lifter maintains that zero lash. So even dry, the lifter plunger would be lowered by the amount 3/4 turn of the adjustment nut provides and hence zero lash is maintained after engine start up. The rockers may clatter a bit after intial startup but will be fine when they are pumped up. So I repeat -- if it was done correctly at build-time, it is right now. If not -- it ain't.

Re: Sooty plugs

Silvertip said:

Hydraulic lifter should have been primed BEFORE installation and there is a procedure on how to do that. Intake runners are not all the same length. What may be good for one cylinder in marginal carburetion application, may not be ok for the others. Lastly, when you set hydraulic lifters you are not adjusting clearance. You are adjusting for zero lash. The action of the lifter maintains that zero lash. So even dry, the lifter plunger would be lowered by the amount 3/4 turn of the adjustment nut provides and hence zero lash is maintained after engine start up. The rockers may clatter a bit after intial startup but will be fine when they are pumped up. So I repeat -- if it was done correctly at build-time, it is right now. If not -- it ain't.

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I will recheck them, it's a long shot I know, but everthing obvious has not yet seemed to work.
I will redo the lash and conduct a leak down and compression test.

thanks your comments
Dave

Re: Sooty plugs

Can you obtain a too lean condition with the idle mixture screws? (turn the screws in until the engine stumbles)

Re: Sooty plugs

wrench 3 said:

Can you obtain a too lean condition with the idle mixture screws? (turn the screws in until the engine stumbles)

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I would, but 4 of the 6 plugs are good. Only 2 are sooty. Idle mixture only affects idle. The plugs start to soot up around 1200 to 1500.

Re: Sooty plugs

Just trying to get an idea if the carb is over fueling.
I ran into a couple of GMs that had a lifter adjusted right at the bottom, from the factory. The compression would test fine but it would loose compression when running. But the customer complaint was a miss fire.

Re: Sooty plugs

Sounds like you'd be smart to do a running engine lifter adjustment.

Re: Sooty plugs

Since this is a Rochester Quadrajet and a replacement carb of unknown origin or condition, this may still be a carb issue. One might check the secondary air valve plates to see if they are opening way too soon. If the carb was rebuilt by someone that didn't have a firm grasp on this carb, the tension adjustment may have been incorrectly done or the little spring never connected and the secondaries are opening way too soon. They don't need to open much to begin overfueling. These are "air valve secondaries" so there is no physical linkage that operates them. Air pressure above the plates forces them open due to high throttle demand and therefore relatively high vacuum beneath them. Lots of things can affect what plugs look like when compared to others even when fed from a common source. If this situation didn't exist before the engine work, then something with the engine work is causing this issue. Ignition timing, valve adjustment, a couple of leaky cylinders that have not yet seated the rings fully. Pulling some oll down the valve stems due to bad or improperly installed valve stem seals or worn valve guides.

Re: Sooty plugs

Silvertip said:

Since this is a Rochester Quadrajet and a replacement carb of unknown origin or condition, this may still be a carb issue. One might check the secondary air valve plates to see if they are opening way too soon. If the carb was rebuilt by someone that didn't have a firm grasp on this carb, the tension adjustment may have been incorrectly done or the little spring never connected and the secondaries are opening way too soon. They don't need to open much to begin overfueling. These are "air valve secondaries" so there is no physical linkage that operates them. Air pressure above the plates forces them open due to high throttle demand and therefore relatively high vacuum beneath them. Lots of things can affect what plugs look like when compared to others even when fed from a common source. If this situation didn't exist before the engine work, then something with the engine work is causing this issue. Ignition timing, valve adjustment, a couple of leaky cylinders that have not yet seated the rings fully. Pulling some oll down the valve stems due to bad or improperly installed valve stem seals or worn valve guides.

Click to expand...

Just to clarify, the sooty is not oil, the plugs are neither oily nor stained with oil. The soot is definatley patially burned fuel.
If the secondary plate was opening too soon why would 4 plugs be good and only 2 plugs sooty? Wouldn't it be rich across all plugs?

I will move the distributor to slots and see if I can get 2 different plugs to soot up. Numbers 5 and 6 (the sooty plugs)are next to each other on the cap (and firing order). I'm not totally convinced they are not crossfiring. Compression check as well, but I suspect the motor is fine as it does not use or burn oil and the blowby is insignificant.

A new Rochester and my old Rochester have been used.