Mercruiser 140 backfire under load/high RPM

[Pupek]

Engine: 1984 mercruiser 140

Kind of long, but I didn't want to leave anything out...

First, while running on the lake last weekend, engine started bogging out at high RPM - limped into shore, pulled boat out. looked in valve cover found water mixed w/oil (white muck). Pulled manifold - water gushed out. Plenty of water in cylinders, but no hydrolock. It still ran at idle, but not at higher RPM (>3500). it backfired and lost power at higher RPM. Checked out manifold, found crack or rust-through into ports. Pulled head to check valves, rods, etc - it, looked OK, oiled cylinders, and manually cranked engine - moved pretty good. Changed out oil, replaced manifold. Started engine - runs OK, but still backfiring at high RPM. Found that exhaust shutter was gone, pulled lower unit found shutter blocking exhaust, removed it - tried again, same thing idles great up to about 3500RPM, then backfires. Checked timing - set to 6 deg BTDC - doesn't help. Spark is advancing as RPM rises. The backfiring is pop-pop-pop when I open the throttle slowly at high RPM, or if I bury it at low RPM. Ran compression test - got 150, 140, 145, 145. Replaced points, cap, rotor, wires, plugs. Same symptom. Removed fuel filter, same symptom, put filter back.
Pulled carb, adjusted float level per mercarb spec/bulletin - will try this in the morning.
Other info: brand new cylinder head installed beginning of last year (it rusted through to the outside).
Running out of ideas...

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Update:
re-installed carb after setting float level - no improvement.

Disconnected fuel line at fuel pump, ran a hose to a 5gal gas can - no improvement.

Other things I'm thinking of:
1. Centrifugal spark advance not working completely? It advances, but I don't know if it's enough, or too much / too early. Do the springs wear out? Can the spark advance mechanism be purchased separately from the dist?
2. Timing chain off by 1 or 2 teeth? Wouldn't I see this in the compression test?
3. Worn cam lobe? Any way to check this? I doubt this is it - I disconnected each plug wire, one at a time, and always get the backfiring. I doubt all cam lobes are bad.
4. Coil - have never replaced this, but I don't know how this would cause the problem - can it? I'll probably just go ahead and try it (it's 25 years old).

Here I am on the best weekend of the year out of the water - yeah, I'm spending the entire weekend on the boat, but it's not what I had in mind...

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Another update...

Checked the fuel pump, Merc manual says it should pump 1 pint in 30-45 seconds - it did.

I think I found the problem - as expected mixture is too lean. Ran a test where I put gas in a waterbottle, and poked a hole in the top so I could spray a small stream into the intake. Tried it at higher RPM, and the popping magically goes away, and runs like a champ - even when I accelerate quickly. Now I'm left with rebuilding the carb (again - did it last year).
I will tear the carb apart and clean it up, butI suspect I will see the same results after this. Some thought:
I have jets that are somewhat small - 1.45mm. Should I get 155's?
Should I just keep adjusing float level until it runs smooth? I currently have it set to 3/8" - maybe it needs to be 1/4"
Any other ideas?

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Another update:
cleaned up carb using acetone solution on metal only. Reinstalled - didn't help. Tried playing with float adjustment making it run richer. Even when too rich to run smoothly, I still get popping when accelerating or at high RPM - although I think its higher - maybe 4500. Tried running pressurized gas into carb with external tube/funnel - didn't help. Do I need a new carb? I hate to spend $500 on a 25 year old boat, and find it doesn't fix it. Any other ideas?

 

[cr2k]

Re: Mercruiser 140 backfire under load/high RPM

Your carb probably has some plugged passages that need a good soak and blow and even chasing with a wire.

From what you have told us I am 99% sure it is the carb.

Did you happen to check the fuel pressure and flow at high rpms?

A small inline glass or clear plastic filter will help you see if the fuel is flowing.

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Only checked fuel pump performance at idle as I stated - it filled up about 12oz in 10 seconds. I am also fairly confident that I have a carb problem - I suspect the power piston sticking in the 'up' position - like it should be for idle. Anyone have ideas on how to check this?

Another thought - I just read another post where the guy tried different timing, distributor advance spring trials, even boring out the accel pump port in the carb, and finally found that the intake gasket was bad. I reused the manifold gasket from last time - it looked good. I'll try a new one since it's so easy/cheap, but I'll be surprised if this is the problem - any else suspect this?

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

I'm now out of ideas...

Replaced the carburetor - it didn't help - I still get 'popping' backfire and bog at high RPM - even when getting there slowly. I can't think of anything else that could cause this. Is it possible the valve springs are too weak, and valves are 'floating' open too long? The head is just over 1 year old. Another idea - I don't think the engine ever overheated, but if it did at some point, could it have damaged anything to cause this problem - I can't think of anything.

There seem to be a lot of posts with a similar problem, but I never see what finally fixes it. Do people just give up and part-out their engine in frustration?

Anything else to check???

 

[bigbrownbuku]

Re: Mercruiser 140 backfire under load/high RPM

check your distributor shaft side play it affects points at higher rpm, ignition misfires are often confused for bad carb(excessive side play, bad leads, weak coil etc.) you may have to play with the gap. also check your coil voltage is consistent (bad igniton switch in the run position). weak or erratic spark at wot will sometimes be easy to see with a timing light, if the spark isnt steady or drops out completely it will show.
if an engine isnt making enough spark it will give pretty similar symptons to a bad carb. try the wd40 test on the carb base and intake manifold to rule out air leak. a vaccum guage might also help. are you running at altitude?

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Interesting idea about distributor side play - how can I check this? Should I just try to wiggle it side to side and expect no side play?
I'm not running at altitude (500ft above sea-level).
Thanks for the new idea on the dist - this makes sense, and I'm so desperate, I'm well into checking things that don't make sense - even considering pulling the head (again) to see if anything looks odd.

 

[bigbrownbuku]

Re: Mercruiser 140 backfire under load/high RPM

ramp the points then check side to side and note the gap.

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Well I checked the distributor play, and there is a some, but how much is too much? I can push to the side of the dist shaft and change to gap from 0.022" to 0.032" fairly easily. Is this exessive?
Also, I'm looking at the valves - thinking maybe they aren't opening enough (compression test tells me they close fine). I tried measuring how much the valves open by hand-cranking the crankshaft (I have heard they should be avout 3/8"). Will this really work without 'loading' the lifters with oil? Either I have round cam lobes, or this isn't the way to check it. Wouldn't it make more sense to move the rockers off the pushrods, and measure the pushrod lift? Should this be 3/8"? Is tat the cam lobe height?

 

[jack gardner]

Re: Mercruiser 140 backfire under load/high RPM

while hav valve cover off ck 4 a broken or weak valve spring on an intake valve it could b floating at high rpm 3500 if is thu carb the popping it will b an intake. just a possibillity while ur checkin cam lift

 

[Silvertip]

Re: Mercruiser 140 backfire under load/high RPM

Popping is usually a lean condition and it can be caused by nearly everything you mentioned from a vacuum leak to clogged or restricted passages in the carb. Has the carb been blown out with compressed air after soaking and have you used suitable cleaning tools to get after the passages you can reach. At idle, you can spray starting fluid around the carb base and intake manifold to try and isolate vacuum leaks. If the engine picks up, you found the leak. The engine does not have a timing chain. It has timing gears and the likelyhood of this being a problem is slim as is advance weight springs and the advance mechanism. A timing light is all you need to verify there operation.

 

[Pupek]

Re: Mercruiser 140 backfire under load/high RPM

Finally found the problem...

While checking the valves, I measured about 5mm rise on the pushrods after disconnecting the rockers. This was for all pushrods except #4 exhaust (furthest back) - it moved about 1mm (if that). I also noticed that all of the pushrods could be pushed into the lifter, and it was springy. The last one was rock-hard. Pulled the lifters out and found #4 exhaust was sharply concave - the cam lobe is round. I'll have to now look into what it takes to replace the camshaft + lifters. I have a lot of reading to do, and I'll start another post if I need help with that,
Thanks everyone for all your help. This has proven to be one of the best (and most active) forums I have seen.