Recommendations to overhaul Marvel carburetor

[correamoon]

The C145/O300 differences are largely nomenclature. (Contenental switched from identifying engines by max horsepower to identifying by arrangement & displacement. The O300 largely is the C145 called by it's new name.) The carb folks likely have no knowledge of the actual differences, nor will the actual differences likely have anything to do with the carb model. In fact, it' unlikely the current Marvel folks can state the actual differences in their mentioned model numbers, particularly which differences are significant for C145 vs O300. For that reason, and for the fact that your engine is running "rough" intermittently and not continuously, the implication that a C145 should have an earlier carb model is not likely the culprit. The latest TCM manual for BOTH engines specifies the 10-4895-1 carb.

(The largest differences which might be instrumental are INsignificant, namely: Change in cam lift/timing and material of construction (cast iron vs steel cam) and lifters (steel vs cast iron) which change the relationship between manifold pressure vs RPM at low power and torque at high RPM. The fact is... your C145 has likely been overhauled more than once in it's lifetime and if so, and if the current Service Bulletins were followed, your C145 probably has the later cam and lifters.)

So, even if your C145 had a later carb with an early cam, the fact that the roughness is intermittent and not repeatable at a particular torque curve point makes the carb model question unlikely as a cause.

I should make a correction...my friend John the first one in point me the problem and he has hundreds of hours flying with the O-300 told me that I shouldn't say the problem is intermittent, according to him the problem is always there just sometimes is worst and sometimes the engine is quite smooth...which probably is true there has been some flights that after start has been very smooth and later slightly rough and smoother after leaned, other flights from the start the engine is running rough and when leaned smooths but not as much as in the "good" flights...so let's say the problem is not intermittent...just sometimes better sometimes worst...if the problem is the carburetor maybe the weather conditions are affecting and that is the reason some days has been smoother than others?

[GAHorn]

NOW... the first thing which jumps out at me is the float-level setting. Does it matter what flight attitude? (Nose down during descent, nose up during climb, nose up during speed-reduction, etc etc?)

[blueldr]

Is that the setting where you invert the carburetor lower body half with the float assembly exposed and measure and set the float level using a no. 7 drill bit for a feeler gauge?

[correamoon]

NOW... the first thing which jumps out at me is the float-level setting. Does it matter what flight attitude? (Nose down during descent, nose up during climb, nose up during speed-reduction, etc etc?)

No it doesn't and the mechanic checked the float and told me was working properly with the right adjustment, anybody knows the reason why some engines ran rough when they switch from 2 pieces to 1 piece Venturi?

[GAHorn]

Is that the setting where you invert the carburetor lower body half with the float assembly exposed and measure and set the float level using a no. 7 drill bit for a feeler gauge?

(I knew you did your best work upside-down and by feel!)

[jwsowles]

Not sure this is a help but I had a very similar erratic roughness with no pattern (unrelated to humidity, temp, lean/rich, rpm, etc.). Timing was good, plugs and wire were replaced and sent carb for rebuild (lot of time on it). Still rough. So went for rebuilding mags (also high time). Before my mechanic closed it up, I noticed that the condenser pigtails were loose even though screw was tight. Not sure if screws were too long or there was crud in the hole but the pigtail did not have a tight connection. Resolved that and after rebuild, ran/runs smooth. Could be coincidence and unrelated since also installed new mag parts. Unfortunately, I don't know for a fact that the previous connection was loose but it would seem to be possible. But that was my experience.

[sfarringer]

...... anybody knows the reason why some engines ran rough when they switch from 2 pieces to 1 piece Venturi?

I bet that if Precision Airmotive had known how to design a good venturi, they would have done it more than twenty years ago at the time that they foisted the one piece design on us.

[correamoon]

Finally my C-145 is running smooth!

Unfortunately I don't know exactly where was the problem!

I bought a new carb and a new set of Bendix and the idea was to change the carb do a test flight and then the mags, after change the carb and start the engine and testing the mags on the pre flight check we got a bad drop on he left mag...back to the hangar we found oil inside the mag (this mag was checked before and looked good)... So we changed the mags and went up and the engine runs just wonderful, full power, smooth at any regime (not giving me ice indications anymore so maybe something was making ice in the carb or maybe just better weather?).

Unfortunately I will never know the exact issue but I know was the carb or mags, or a combination...prop needs some balance which is the next thing but I had a big smile flying durning the last weeks in California with 7760C now I am back to China for another airline "combat" tour can't wait to go back to CA!

Thanks to everybody for the support and ideas I learned a lot!
Leonardo

[edbooth]

Congratulations on your new airplane and it sounds like you are taking all the right steps in getting checked out and making sure it is airworthy. It is normal for these engines to run slightly rough after start but they should run smooth within a minute or so. Prop imbalance manifest more as a vibration or slight shaking as opposed to engine roughness. Your mechanic has probably checked but make sure when you move the mixture to idle cut off that the rpm rises slightly before cutoff indicating mixture control adjusted properly. Roughness in flight or during ground ops might be the result of carb ice and this is a common problem in California where it is so dry that pilots are lulled into complacency until it sneaks up on them. Experts will tell you: "impossible" If it ices up it will be obvious and will quit unless you apply carb heat. I have heard of cases involving intermittent carb ice including occasional roughness. You might try applying carb heat when you experience roughness and see if that helps. Some old-timers will tell you that partial carb heat can help smooth engine roughness under these circumstances. (carb ice can occur up to 70 degrees plus with sufficient moisture in atmosphere) Chris at Lycon told me they have a bench in house to perform work on these carbs.

You bring an interesting point as my carb temperature indicator shows low temperatures too often...and I respect them and use carb heat all the time even immediately after take off, according two very experience pilots/mechanics my temperature indicator could be working wrong but on the ground before flight is showing the right ambient temperature...is anything that can be wrong in the carburettor that can be creating more ice than it should?

The prop position was checked and adjusted, the balance was checked just by hanging the prop and marking the position when rotated I know is not the best way but was the only one we had and looks good but I can feel is not perfect as you can feel and slight increase in vibration when turning right, which is not the same of the roughness.

I do get the 50 RPM increase when cutting the mixture but as I said the problem is intermittent.

Good thing about this problem is I am learning a lot, also all mechanics agree that is a very solid engine and the roughness must be something minor that nobody could find so far...

Thanks for all the comments.

You might check the carb Venturi also. Some time ago they went from a two piece venturi to a one piece. Some people had truoble with rough running engines with this "fix". I still have the two piece and check it every annual.

[GAHorn]

...we got a bad drop on he left mag...back to the hangar we found oil inside the mag (this mag was checked before and looked good)... So we changed the mags and went up and the engine runs just wonderful, full power, smooth at any regime (not giving me ice indications anymore so maybe something was making ice in the carb or maybe just better weather?)....Leonardo

Wed Feb 11, 2015 8:21 pm

...I also had an occasional rough engine about ten years ago which was traced to leaking oil seals in my magnetos. It was intermittent and virtually impossible to duplicate. Slight oily-ness inside the points cavity ws cured with new seals and no more problem.

It was very difficult to believe it was oil inside the mags...but cleaning them and installing new oil seals cured my intermittent roughness.
The event occurred with Cleo and Louise in the Wyborny's B-model alongside as a flight-of-two, as we departed for the 2002 Las Vegas convention. During climbout it became very rough and we landed at nearby Llano, Tx and looked into it. After a delay troubleshooting to no avail...we started back up and it ran smoothly.
So we departed and flew on to refuel at Midland, then on to Ruidoso for two overnights, then on to Deming for fuel, then on to Las Vegas ... and it ran fine all thru that. Then, in Las Vegas on the fly-out it got rough intermittently, and the mechanic there found evidence of oil inside both mags (which had been installed only 4 years previously, but had an overhaul date 4 years prior to that. He replaced the oil seals and the problem never reoccurred.

Glad you found it.