How can a twin cylinder engine run rich?

Geo-TH,In · Dec 21, 2021

How can a twin cylinder engine run rich, produce black exhaust, when it only has one carb with fixed jets, one cylinder is firing and the other one isn't?? After about 5 seconds the exhaust was normal and it started hitting on both cylinders.
Ran good for the rest of the day.

Eureka I think I know the answer.

I'll post back my guess after I hear your diagnosis.

Janicholson · Dec 21, 2021

If just started, the richness of a non firing cylinder can make the exhaust look radically black/rich. Spark plug fouling or sticking valve. Jim

Geo-TH,In · Dec 21, 2021

Jim you the man.
The only explanation I can come up with is a sticking intake valve on the cylinder that isn't firing. Or carbon is holding the intake valve open. An open intake valve allows air to be pushed backwards through the carb on the compression stroke. The air is picking up extra fuel for the firing cylinder.

So what can be used to clean up the carbon inside the cylinder or free up an intake valve?? Or both??

Old Ford Mechanic · Dec 21, 2021

old · Dec 21, 2021

As I said before ATF mixed with the gas

504 · Dec 21, 2021

Get it running full bore, and start pouring Marvel Mystery Oil down the carb. Stop before you kill it a couple of times. Then pour it in till it dies,then leave it set for a day or two. Clean the spark plugs and run it again,that should get the carbon out of it. Put some MM in the gas tank too.

T.R.K. · Dec 21, 2021

gas in the oil will do that too. Only it won't clear up.

Geo-TH,In · Dec 21, 2021

How can ATF remove carbon from inside the combustion chamber?

old · Dec 21, 2021

ATF is very high detergent so as it gets burned it also cleans

old · Dec 21, 2021

ATF works just as well and cost half the $$

rustred · Dec 21, 2021

as with any carboned up engine you mist water in the intake. oil does nothing. years ago in the 70's and 80's GM had about a 10 oz can of stuff you pour down the intake while running at 2000 rpm. you would swear it was a rod knocking but it was carbon on top the piston. after the treatment the knock was gone. ATF goes in transmissions.

Steve@Advance · Dec 21, 2021

What I don't understand is why would a properly jetted, healthy engine carbon up in the first place.

That was more a problem back in the good ol days of bad gas, crude fuel delivery, poorly formulated oil, poorly machined engines that consumed oil from the day they were born.

Excess carbon in the combustion chamber is different from gummy valve guides.

And what was the purpose of products like Rislone? Was it to free gummy valve guides, or clean contaminated hydraulic lifters swimming in unfiltered non detergent oil?

All I can think of, especially on an air cooled, possibly clogged fins causing hot spots cooking the oil. Any oil leak will result in dirt packing. Might be worth pulling the top cover and taking a look.

That's also why synthetic oil works so well in air cooled engines, more resistant to turning into sticky tar deposits from hot spots.

rustred · Dec 21, 2021

the air is picking up extra fuel for the cylinder , ? how can that be when the fuel needs a vaccum to be drawn into the cylinder. so what can be used to clean up the carbon inside the cylinder.... water is what u use.

VicS · Dec 21, 2021

Think my x mark has a 2 bbl carburetor

GlenIdaho · Dec 21, 2021

You don't say what type of engine it is, but I have Kohler 25 hp twin cylinder that does the same thing. I suspect that when the engine shuts down and sits here is some residual oil or fuel that seeps into the cylinder so that when it is restarted it smokes heavily and within a few seconds clears up. It doesn't do it all the time.

Janicholson · Dec 21, 2021

Here is a interesting easy solution to the valve sticking idea. Pull the plug wires and ground them with used plugs or??. Then crank the engine listening for two compression events as it rotates. If it sounds like a single cylinder, it is. Then take a compression reading.
If it sounds like it has 2 compressions, (as it should) put one plug wire back on. If it starts, immediately take that wire off and ground it and put the other wire on. If no start, swap the plugs from one hole to the other to see if the no start follows the plug swap. Jim

kencombs · Dec 21, 2021

More info on the engine would help. late vtwin is much different that an old flathead of any brand.

Black smoke is not necessarily proof of a rich running engine. No spark on one cylinder would result in all the fuel from that one being expelled as black, unburned fuel. On some ignition systems a single cylinder failure is fairly common, intermittent ones too

John M · Dec 22, 2021

You are exactly right. My 23 does the samething, but George is always right.

merlynr · Dec 22, 2021

(quoted from post at 20:23:29 12/21/21) You don't say what type of engine it is, but I have Kohler 25 hp twin cylinder that does the same thing. I suspect that when the engine shuts down and sits here is some residual oil or fuel that seeps into the cylinder so that when it is restarted it smokes heavily and within a few seconds clears up. It doesn't do it all the time.

My Ferris zero turn does that sometimes and no oil consumption.
This Ferris is carbed and my old jd was injected with quicker starts.

How can a twin cylinder engine run rich?

Geo-TH,In

Well-known Member

Janicholson

Well-known Member

Geo-TH,In

Well-known Member

Old Ford Mechanic

Well-known Member

old

Well-known Member

504

Well-known Member

T.R.K.

Well-known Member

Geo-TH,In

Well-known Member

old

Well-known Member

old

Well-known Member

rustred

Well-known Member

Steve@Advance

Well-known Member

rustred

Well-known Member

VicS

Well-known Member

GlenIdaho

Well-known Member

Janicholson

Well-known Member

kencombs

Well-known Member

John M

Well-known Member

merlynr

Member

Similar threads