Why does fuel spit out the carb at full throttle? - RC Groups
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Aug 14, 2012, 10:01 PM
Registered User

Why does fuel spit out the carb at full throttle?

I have read and repeated Clarence Lee's explanation; that it is caused by to much
intake timing. But what if it isn't directly caused by the intake timing, perhaps it was
Clarence Lee's or a manufacture that found by trial and error, that reducing intake
timing eliminates the problem.

I have had problems in the past understanding Blair's and SAE terminology at times,
It all seems like it says the same thing. In this case, I think there should be a term for
how well an engine evacuates the exhaust. (Besides Blow Down Time (bdt) the period
from exhaust open to transfer port open, some people will call this dwell time.) Please
mention it if there is a definition.

For spitting fuel out the carb, I was thinking it was actually the scavenging efficiency
decreases at a faster rate than the delivery ratio. The scavenging ratio is going to
decrease with rpm, but I believe the scavenging efficiency is also effected by the
cylinders ability to empty itself after each power cycle.

What is your opinion?

I can model an rc airplane engine, and test all the possibilities with Blair's ModelNo2,
then plot the results.

Bill M.
Last edited by Capt. Quirk; Aug 15, 2012 at 12:35 AM.
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Aug 15, 2012, 12:21 AM
Who Dat!
GeetarJoe's Avatar
Basically because the air going into the engine doesn't flow smoothly but instead pulses. The air and fuel is sucked in when the port is open and has a certain amount of inertia, and pulls the air along that's behind it. Then the port closes and that air "bounces" against the closed port, as well as there is a certain amount of air puffed out by the closing of the port. This is also mixed with a small amount of fuel because of the inertia of the fuel flowing into the engine. Since the path to the engine is closed the only direction that this charge can move is out, it does.
Aug 15, 2012, 02:44 AM
Registered User
downunder's Avatar
It's very difficult to say exactly why fuel sometimes spits back because there are so many different things all happening at the same time. Some of the problem could be that an engine is running much slower than it's peak HP revs (almost no one runs them at peak HP revs) so crankcase compression is building up before the port closes causing a flow reversal. However the area of the port that's still open at this time is very small which would limit any reversal. Add to this that crank timings vary wildly between engines designed for the same (and even different) applications.

As a for instance, one of my CL engines has quite a late (55 ATDC) port closing time and it's only running at 7900 revs but spits no fuel at all. The only difference between it and an RC engine is that the venturi area is much smaller giving a higher velocity at the spray bar and the distance between the spray bar to the top of the inlet is longer than for an RC carb. The longer inlet may be just long enough to trap any spit back when the port next opens. A simple way to test this out would be to add a slight extension on a carb with an engine that spits back fuel.

As for terminology, dwell is the wrong term to use for the difference in exhaust/transfer port timings. It's almost universally called blow down time (or period) which effectively is the period when the engine evacuates the exhaust to a level where the remaining pressure is low enough for the incoming compressed charge to start flowing into the cylinder. An engine designed for moderately low revs will have less blow down time (degrees difference) because there's more actual time for the exhaust to escape. A high revving engine conversely needs a longer period (degrees difference) because of less time for the exhaust to escape. On average most engines have the transfer ports opening at ~60 degrees BBDC while the blow down is set by the exhaust timing which, on average, is ~70 degrees BBDC for slow revving engines and ~75 for high revving.

Dwell is really only used for the time a piston remains almost stationary around TDC for combustion to begin. The longer the dwell the more ignition can be retarded and allow for pressure to rise while the rod is increasing its angle to get more leverage on the crankshaft. The larger the rod length/stroke ratio the longer the dwell.
Aug 15, 2012, 09:37 AM
Registered User
A large carb hole will spit out the carb because it just can't take all the fuel getting pushed in. Also muffler pressure can still be pushing fuel through. It will maybe be less spitting if it is not run on pressure no matter what type.(muffler, bladder or backplate.) If the carb hole is too big, then it likely will not run well without muffler pressure. Most motors now have a fairly big carb to get more performance.
Aug 15, 2012, 09:38 AM
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I TOBOR's Avatar
Cap. take a look at the exhaust port and when the piston is at TDC note the intake port position, now rotate the crankshaft and if the intake is still open as the piston comes down that is the cause of the blowback/reversion. This is part of the overlap when the intake and exhaust ports are both open allowing the exhaust momentum to create a partial vacuum in the cylinder aiding cylinder filling. This stuff has an optimum rpm at which it all works in harmony, for example running too large a prop can make the reversion worse and running the engine faster reduces it. This is not as critical in a fourstroke.
Aug 15, 2012, 11:38 AM
T.L.A.R. eng
Fuel spitting of frothing at the entrence of the carb has more to do with fuel volume/weight than anything.
The air is being pulsed at a high frequency, the fuel has weight and doesn't start and stop so easily, so what we see as a result is the fuel being forced back out into the open. It has weight and momentum so it has to go somewhere. The next pulse of intake air will have a head start on a fresh supply of fuel rich air mixture.

The engine is usually tuned for maximum power, so this effect normaly isn't an issue. Unless of course we are looking for fuel economy.
Aug 15, 2012, 10:08 PM
Registered User
earlwb's Avatar
I had the understanding it was called "reversion" where the engine tended to spit fuel out of the carb, normally there is a vapor cloud "standoff" occuring just at the mouth of the carb too. But with model airplane engines the vaport cloud gets blown away causing the spitting to appear. The carbs do not care which way the air is flowing, so the jets will flow fuel in either direction.

Two stroke engines tend to behave similar to the 4 stroke engines in that the valve overlap where both valves are open is similar to the ports all being open at the same time. The pressure pulse and or shockwave affect it in the same manner. Also as mentioned already the air/fuel mixture tends to bounce when the valve closes stopping the intake flow. That bounce is quite similar to the water hammer effect in the plumbing of large buildings and large water mains pipes too. The air/fuel mixture doesn't want to stop, but it has no choice, but in the process it tends to bounce back out due to inertia.

There is a good explanation of it here http://www.circletrack.com/enginetec...ake_reversion/
and here too

My Harley Davidson can really show a pronounced reversion effect from the odd firing arrangement of the V-Twin engine. When I go cruising up a long mountain road at slow speed, the carb can actually start dripping raw fuel out of the bottom of the air filter, from all the gas vapor condensing and collecting there in the standoff vapor cloud just outside the mouth of the carb.
Aug 16, 2012, 08:36 AM
Registered User
I TOBOR's Avatar
Earl, there is a slightly different reason for your Harley doing this, I'm sure yours is carbed. When the rear cylinder is filling, the air/fuel mixture has to make a sharp left turn to enter the rear intak valve. Then as the rear intake valve closes and the front intake valve opens the fuel/air mix must come ot an abrupt stop and turn right to enter the front intake valve. This is the main cause of the famous Harley shake at idle. The early fuel injected ones did not do this, so after early test riders complained, the skake is programmed in to the ECM. The fuel injected bikes don't have that shake because the injector only injects fuel while the intake valve is open.
Aug 16, 2012, 08:55 AM
Registered User
earlwb's Avatar
Yeah, like I said HD has a rather odd firing arrangement for that. But the air/fuel winds up having to jump through hoops to get into the combustion chamber. HD did have to program in a shake at idle so the bikes "look good" at stop lights, etc. HD also put in counter balancers and they had to unbalance the engine slightly so it shook better too.

I remember one time going up a long hill and the guy in front of me was smoking a cigarette and holding it in his right hand on his knee, right next to the air cleaner. I was thinking this is going to go bad. Sure enough, the raw gas ignited and he stopped, jumped off the bike and it fell over and we all watched it burn. Of course way out in the middle of nowhere, there is no water, no fire extinquishers, etc. It did garner a good crowd of spectators though.
Aug 16, 2012, 01:30 PM
Registered User
Did anybody give consideration to the pressure feed to the fuel tank pushing fuel to the carb (and the carb spray bar). Yes, we know that the pressure drop pulls some of the air fuel mixture down the intake throat, but could not the pressure feed also contribute to fuel spit back? I mean the flow is generally unregulated (exception to Iron Bay and Cline systems) and rises with the increased pressure input to the tank. So that the worst of conditions are at max revolutions - max flows, max pressure into the cart needle valve, etc.
Aug 16, 2012, 11:45 PM
Registered User
downunder's Avatar
Originally Posted by Chip01
Did anybody give consideration to the pressure feed to the fuel tank pushing fuel to the carb (and the carb spray bar).
Yep, aspeed mentioned it back in post #4 and I have to admit that was something I'd never considered either but seems a distinct possibility. Kind of makes the mention of my CL engine in #3 invalid (or possibly proves the point??) because it's not using muffler pressure. Added to my little black book of things I've learned .

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