O320 rough idle

[hcccs]

Originally Posted by snopercod

Hans: Did you find the problem?

No.
Lycosaurus suggested earlier that I must have a intake leak and a club member thought so too so I had to check again. I have also removed the carb and magnetos. I uppgraded the magnetos with new distributor gears according to SB15 (?). The distances between the old gear electrodes and the distributor house electrodes were 0.35mm and with the new 0.15mm and I thought that would make things better but it didn't. The engine started more willingly but nothing more. An old experienced mechanic in the club helped med and while I sat in the cockpit and kept the RPM at 850 he screwed on the idle mixture screw and he said the engine did not reacto to anything he did so he said there must be something with the carburetor. I have taken it apart twice and upgraded it with the new, blue epoxi floats and checked the idle channels. Next step is to check spark plugs and ignition wires. They are all new so I haven't paid much attention to them.

[dougweil]

Hans... our local RV guru Tom Berge saw your issue and he has some ideas. He asked that I post his contact info so you could drop him an email directly if you like. His email is: tom@rvtransition.com

[hcccs]

Originally Posted by Lycosaurus

My o-320 gives me a MAP of about 11 at 800 rpm. Yours is showing MAP of almost 15, which I suspect would indicate an intake leak. I would be inclined to check that area again, including any connections such as the MAP hose connections, carb/bracket/sump gaskets.

I had repetitive leaky intake gaskets, but solved that with an o-ring kit from one of our VAF posters/advertisers,Ross from SDS. http://www.vansairforce.com/communit...d.php?t=161030

About 1 year and 100 hours with the SDS o-rings. Works well so far.

I'm back again with my rough idle O320. I had the magnetos checked by a air mechanics shop and they were fine. A friend suggested I had a look at the throttle shaft and bushings. I did so and the bushings were quite worn so it is most likely leaking through there. The carb has come off now again for the Nth time. I've ordered a new shaft and I'm making new bushings and I'm confident that this has been the problem. What about lubrication? What do I use to lubricate the shaft/bushings?

[jrs14855]

You should buy factory bushings and they need to be reamed after installation. They are held in place by locktite.
If you have a wood prop and high compression engine, standard 160 hp or higher compression it may never idle very smoothly. Mine with 10-1 pistons requires about 900 rpm before it smooths out.
You need to get an overhaul manual for the carb and follow the procedure for the throttle shaft bushings.

[Lycosaurus]

Originally Posted by hcccs

... the bushings were quite worn so it is most likely leaking through there. The carb ...

Great find Hans. Let us know how it works out.

[lr172]

Originally Posted by hcccs

I'm back again with my rough idle O320. I had the magnetos checked by a air mechanics shop and they were fine. A friend suggested I had a look at the throttle shaft and bushings. I did so and the bushings were quite worn so it is most likely leaking through there. The carb has come off now again for the Nth time. I've ordered a new shaft and I'm making new bushings and I'm confident that this has been the problem. What about lubrication? What do I use to lubricate the shaft/bushings?

That is a good start. A carb that does not respond to adjustment of the idle mixture screw indicates a problem of some sort. These carbs use a needle in the fuel delivery side. You want to crank it down to force it more lean and see if it will lean out and wether the current set up is rich or lean. bad throttle bushings would make it lean and hard to get rich. If you can observe an RPM rise while leaning it out, that points away from bad throttle bushings. If you crank the screw all the way in and it is still running, you have other problems, like a leak or bad float bowl needle/seat.

[hcccs]

Originally Posted by Lycosaurus

Great find Hans. Let us know how it works out.

The new throttle shaft is installed together with new bushings. I ran the engine today and it ran slightly better but the original fault is still there. Another thing is that it does not react to the mixture screw settings and does not rev up when I lean to near cut off. There's a leak somwhere and I can't find it. Back to square one again I suppose.

[VA Maule]

Quite a perplexing problem your having. Seems you've addressed every field addressable option and sometimes a field repair is not possible, and the suspect component has to be sent back to the manufacturer or their authorized representative. Since only they have the appropriate test equipment to find the defect .
I make this suggestion from experience. Had a similar head scratcher, tried absolutely everything everyone recommend with out success. Gave up on fixing it myself, sent it in for overhaul / repair . Turn out to have had a casting defect, hairline crack in the carburetor body allowing a vacuum leak that only could be detected by bench flow & vacuum test .

[Pilot Dog Ship]

I had the symptoms of an induction leak and it took me a long time to find and even then it was by accident.
Despite there being no soap bubbles from any of the potential leak areas when a vacuum-cleaner discharge hose was gently held up to the carb inlet and after changing of all the gaskets at the cylinder head, I elected to change all of the intake ?rubbers? that seal the join between the pipes extending from the oil sump to the individual ?riser? elbows attached to the intake port on the cylinders, more as a prophylactic measure than a suspected leak source.
It was when I was twisting one of these rubbers to get it off the lower pipe that I discovered the pipe itself was rotating where it was swaged into the sump.
And that was the leak source.

[N941WR]

It looks like you need a second gasket between the cable bracket and the carb.

This setup needs a gasket between the sump and the bracket and one between the bracket and the carb.