Stalling in forward gear

[Misty Star]

The engine runs fine but It labors when I put it in forward gear and stalls. Reverse works fine.
What needs work?

[Dave Neptune]

First welcome to the MMI Forum. Sounds a bit like a weak fuel delivery system or the carb is in need of attention. Did you try the choke to see if it had any influence on the stalling? Stalling in forward and not reverse is common as reverse has a gear reduction lessening the load on the engine.
A bit more info would possibly help too, like how long since a tune up, what kind of filter system you have on the fuel lines and do you have a fuel pressure gage? A fuel pressure gage will answer many questions and is good idea to install for future assessments regarding performance issues.

Dave Neptune

[Al Schober]

I agree with Dave. Do some diagnostics. Don't get obsessed with the fwd/rev thing - the engine isn't smart enough to know whether you're in fwd or reverse!

[roadnsky]

What would cause my engine to run with almost no power in forward, while it runs OK in neutral?
It also runs fine in reverse.


This condition is usually the result of low power caused by something that was done incorrectly during recent engine maintenance.
The reason the engine seems to run better in reverse is because the reverse mode has a 1.7 to 1 gear reduction.

Here are some examples of maintenance problems that we've heard about:

Spark plug leads installed in an improper sequence following a tune up!
Cylinders 2 and 3 tend to be reversed, and the engine will run fairly well in neutral, but will have practically no power in forward.

Assuming that there was absolutely nothing done to the engine since it was last running OK, check the following areas for potential problems:

1) Check for something hanging on to the prop.

2) Check basic integrity of the engine by a quick compression check.
For the purpose of diagnosing, a simple check using your thumb over each spark plug hole is more than adequate.
If your compression is sufficient so as to make it impossible for you to hold your thumb over each spark plug hole while cranking the engine with the starter, the cause of your power loss is not a major mechanical failure.
If compression is weak in two adjacent cylinders, or water is observed in any of the combustion chambers, the head gasket has probably failed.
If compression is "zero" in any of the cylinders, a valve has probably stuck open.
If you use a compression gauge, compression values over 80 psi are adequate.

3) Check for proper carburetor function. Normally, an Atomic 4 will accelerate almost "explosively" as you flick the throttle forward.
If there is any hesitation or mild backfiring, a lean condition caused by dirt in the main jet is the most frequent cause.
In most cases, a problem related to a fuel issue will manifest itself in neutral as well as in forward, but in mild cases, a lean condition might be missed in neutral, but will surely show up under load.

4) Check for proper ignition.
As in the case of subtle lean fuel mixtures, there are subtle ignition problems that can show up under load but not necessarily in neutral.
Plug condition is a good indicator.
If one plug is noticeably blacker (or oilier) than the rest, look for the relatively few things that can affect only one plug, such as the plug itself, the plug lead, or a cracked distributor cap.
If all the plugs are badly fouled, check things that can affect all plugs, such as a loose condenser, badly worn points, shorting coil (or oil leaking from the coil), cracked distributor cap, or a bad coil lead.

[Sam]

My bet is on the firing order as mentioned 1-2-3-4 vs the required 1-2 4-3. worth a quick check. After 48 years with the same A4 I have made the mistake a couple of times.

[Misty Star]

Thanks for all the information. I'll check the fuel line/filter first. The firing order is correct and the spark plugs have been replaced. I don't have a fuel pressure gauge.The last tune-up was about five years ago but the engine has not been used very much. I'll also try the pressure check with a thumb.

[TimBSmith]

I had this very problem when first bought Oasis. Carb rebuild and tuning air/fuel mix did trick. The deposits in the carb were both noticeable and lead directly to poor function. Good luck.

[Larry in Maine]

I just had the same problem with my A-4. It felt like something was caught on the prop but I checked that and it was OK. Recently had the carburetor replaced and it runs fine. You could also use a Moyer kit to clean and upgrade the carb, but I went with the replacement.