CHT's and timing (e-mag / p-mag)

[Scott DellAngelo]

Drilled the jet to #38 and it is much better. I put the timing back at TDC and still hovered around 400 degrees with cruise CHT's this morning. Added the gear leg fairings and wheelpants this afternoon and went for a flight. I noticed the CHT's were down a little. Perhaps the 20-22 mph I picked up helped my cooling a tad??? I am very glad I drilled the jet because now on 1, 2, and 4 I can see a pretty good EGT rise from rich to peak (150 or so degrees). 3 is still a bit odd and runs much higher to begin with and does not have much change when leaning (maybe 75 degrees)?

Scott
#90598 - N598SD Flying - 12.6 hrs

[AlexPeterson]

Scott, it sounds like you have done some good work so far. Do take the several hours it takes to read through all of John Deakin's writings on Avweb. Mixture is a remarkably powerful variable on RV's (doesn't matter much on over-cooled low end spam cans) with respect to CHT's.

In my plane, mixture set to about 50 ROP will deliver CHT's of about 400, while 30 to 50 LOP, everything else equal, will deliver CHT's of about 330. Really. The IAS will be about 5 less or so, while fuel flow in this case will be about 1 to 1.5 GPH less. I have cruised at 22", 2300rpm for about 750 hours now, at fuel flows of 7.1 to 7.5 gph. This delivers about 160 knots TAS. Other RV's flying side by side routinely burn around 8.5 to 9. Electronic ignition is generally necessary at the lower MAPs, as is fuel injection, although there are some cases where people can run LOP without fuel injection.

If you gained 10 to 20 on the airspeed with fairings, it will make a noticable difference. I can almost tell if I'm in a shallow climb by watching the CHT's.

Do you know the MAP above which the Emags do not advance beyond baseline? I'm sure there is an advance table which doesn't kick in until MAP drops below some value, maybe 25 or 26 inches, I don't know.

[casper]

Originally Posted by Scott DellAngelo

Drilled the jet to #38 and it is much better. I put the timing back at TDC and still hovered around 400 degrees with cruise CHT's this morning. Added the gear leg fairings and wheelpants this afternoon and went for a flight. I noticed the CHT's were down a little. Perhaps the 20-22 mph I picked up helped my cooling a tad??? I am very glad I drilled the jet because now on 1, 2, and 4 I can see a pretty good EGT rise from rich to peak (150 or so degrees). 3 is still a bit odd and runs much higher to begin with and does not have much change when leaning (maybe 75 degrees)?

Scott
#90598 - N598SD Flying - 12.6 hrs

I'm a little confused as to why you would heat your engine up with bad timing and then cool it down with more fuel.

[Scott DellAngelo]

I put the timing back to where it "should be" because frankly it seemed to run like **** retarded 5 degrees. I figured I would try going back. Besides I only flew once with it back 5 degrees and it did not really seem like it brought the temps down much anyways. The fuel seemed to help, which it should. I would rather run better and be a little higher on the CHT's. Also I am 9:1 compression so that should add some heat. I spaced the #3 baffle with one more washer today but now I have to wait for flying weather to test. My one flight with fairings installed seemed to help bring the temps down some too. With ambient temps of 80 something they were below 400, so I will fly some more and maybe call it good.

Scott
#90598 - N598SD Flying - 12.6 hrs

[Scott DellAngelo]

I will further update this one to say that I went ahead and drilled to #36 after still not being able to get what I thought was enough EGT rise on cylinders 1 and 3. I think it is perfect for my setup. Climb temps are down more too. Now my hottest CHT's I see are more like 375 (unless leaning) where before I still got 400. Also spacing the baffle near the #3 cylinder about 3/16" (3 washers) my #3 CHT is very close to the others now. I think I am done fiddling with that now.

Scott
#90598 - N598SD Flying - 28 hrs

[collinsj57@adelphia.net]

Got the same problem, as outside temps rise my CHTs are getting up in the low 400s. I to think I am running lean, did you have to replace the gasket on the carb after splitting it and what about the tabbed screw locks, New? How long did it take. My EGT,s are running > 1400 on 3,4 and about 1200 on 1,2 when climbing out ? and I am planning on drilling the jet this weekend.

[Scott DellAngelo]

I reused everything. I just slightly bent the tabs in order to remove and the gasket seemed fine. I have had it apart twice. As far as time, once the carb is off it only takes 15 minutes or so until it goes back on. Perhaps 30 the first time. Most of the time is uncowling, and the R&R of the carb and airbox. Just be careful when splitting the carb. Gentle pull and/or wiggle it apart and once apart it will be obvious why (plunger for accelerator pump and the mixture tube sliding out of the "cylinders" they are in). Also don't mess with the accelerator pump linkage on the outside like I initially did the first time, as it can stay put.

In my case my CHT's are now running cooler than they ever did. They run somewhere around 350 when full rich. My 1 and 3 cylinders peak near the same time at a little over 1300 and full rich are a little over 1150.

Scott
#90598 - N598SD Flying - 29 hrs

[BruceMe]

It looks like you worked out a number of "common" RV engine problems all at once. I really enjoyed re-reading this entire thread... Something to keep in mind as I do Phase-I.

Thanks,

-Bruce
N254MM (reserved)
RV-4 not quite airworthy yet