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trackdom

Well Known Member
Hi,
My engine is a O-320 D2A lycoming, carburator is Marvel 4-SPA. It has been overhauled, and now Time has come to start. All is fine except when trying to reach above 1500tr/mn...above this RPM engine is very rough... I hav? been told that could be due to air intake not well dimensioned...During these tests I don't use engine cowlings.
Thanks
 
Did it run ok prior to carb overhaul? (Helps to isolate the problem to the carb.)

Have you tried leaning it while running above 1500 to see if it improves? If so, it would indicate running too rich.

Do you have a manual primer pump? If so, you could try slowly advancing the plunger with the engine running above 1500 to see if is smooths out (would indicate running too lean).

Does running the boost pump affect it? Restrictions in the supply line to the carb could cause it to go lean due to the fuel level in the float bowl dropping with higher demand.

Are you sure it's the carb? Magneto timing/setup could cause issues. Have you tried it with a magneto check, first left only and then right only?

All are 'shots in the dark', but are relatively easy things to try.

Charlie
 
most carb'ed engines have poor air fuel distribution. This should only cause roughness when you try to lean it. I would be looking for something more significant. Have you confirmed that you have full traval on the mixture arm? Timing is possible, but would have to be way off. Please define how rough it is and elaborate on the type of roughness. It could be many things.

EDIT: I assumed the engine was overhauled. If just the carb o'hauled, I would be looking at the mixture arm travel and an incorrect float setting.

Larry
 
Last edited:
29h8wl.jpg
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As you can see There is a diameter difference . One is 6 mm and the other is 8 mm. Could this restriction cause ? too low fuel flow?
 
Probably not

At 1500 RPM's you are not putting all that much flow through the carb such that the smaller diameter should have an impact. However, do change it to the right size.

Vic
 
trackdom said:
29h8wl.jpg
[/IMG]
As you can see There is a diameter difference . One is 6 mm and the other is 8 mm. Could this restriction cause ? too low fuel flow?
Click to expand...

Not at 1500 RPM with a properly operating fuel pump. On that note, have you confirmed your fuel flow prior to first start? You should run your aux fuel pump while disconnected from the carb to flush out debris and confirm a strong flow. are you seeing a black exhaust during the roughness? You could have got junk in the needle/seat if you didn't flush the system and that would cause it to run very rich (black exhaust).
 
lr172 said:
Not at 1500 RPM with a properly operating fuel pump. On that note, have you confirmed your fuel flow prior to first start? You should run your aux fuel pump while disconnected from the carb to flush out debris and confirm a strong flow. are you seeing a black exhaust during the roughness? You could have got junk in the needle/seat if you didn't flush the system and that would cause it to run very rich (black exhaust).
Click to expand...
I am going to run the aux fuel to Check the flow.
 
Fuel flow has been tested , and I found 150 l/hour with aux fuel pump.
Camshaft has been changed During overhaul...Could it be misaligned ?
 
trackdom said:
Fuel flow has been tested , and I found 150 l/hour with aux fuel pump.
Camshaft has been changed During overhaul...Could it be misaligned ?
Click to expand...

That would certainly cause rough running. Do you have a reason to suspect this? They are pretty simple to properly align. I would think an improperly adjusted float is a more likely error. Did you look for black smoke at 1500? You can also try aggressively leaning at 1500. If the roughness goes away, you are too rich and likely caused by the float (improper adjustment of float or debris in the needle/seat interface.

Larry
 
This morning I checked camshaft alignment, it 's ok.
Carburator has been cleaned up and Checked by a professional.
I don't see what is missing...
I am charging the battery and Will try again...
 
trackdom said:
This morning I checked camshaft alignment, it 's ok.
Carburator has been cleaned up and Checked by a professional.
I don't see what is missing...
I am charging the battery and Will try again...
Click to expand...

The most likely candidates here are Fuel (too rich, too lean, etc.) and spark. I would double check the spark wiring and mag timing. Go back to the beginning and confirm you have the mag wired properly. Determine Cyl 1 and follow it, all the way through the firing order on both mags. Then re-set the timing on both to be sure that it is what you expect. Be certain #1 is on the compression stroke, etc.

Just because the carb was overhauled, doesn't mean that you didn't get debris stuck in the needle/seat interface. This is a new build, correct? They likelihood of debris in your new fuel system is high. Because you didn't initially flush it, that debris passes the needle/seat and can stick in the rubber tip of the needle. The test is simple. If you can eliminate roughness at 1500 by leaning, you are too rich and the float level is most likely to cause this.

Larry
 
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