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High CHT Temp

I found my cht problem by comparing while running on one mag at a time. It turned out to be a timing issue. The problem ended up being so simple but it took me along time to find it. Now I am usually around the 360 to 380 mark. Dropped 30 degrees!
 
We checked the oil screen and it looked normal according to our mechanic. Just some carbon in the screen. Today I flew on one mag at a time in cruise for 3 minutes each. On the left mag the CHT dropped a degree or two to which seemed to go along with the 50 rpm loss, but when switched to the right mag the CHT dropped 20 degrees in about 1 minute with the samee 50 rpm loss. Landed and retimed both mags to engine to ensure that both were set at 25 degrees before TDC. Took the cub back up and did the same test...and the engine still ran cooler on the right mag. Left mag is brand new, right mag has some time on it. Any ideas that explain the difference in CHT temp between the two mags? Lowest CHT temp I got was 382 and that was at 2350 rpm on the right mag. Left was around 402 both was about 403-405.
 
Mag Timing

Besides external timing magnetos also have internal timing. It would not be surprising to see a mag with some hours on it have the internal timing be off a bit. It is not terribly difficult to adjust the internal timing, but you will need instructions and possibly a special tool to do it, depending on the make and model of magneto you have. A web search should turn up the instructions for your particular mag.

Dave Prizio
 
Don't confuse internal timing of the mag with setting the E gap.

Internal mag timing is the relationship between the distributor gear and the drive gear and is set on assembly. It won't move unless teeth depart from the gear. It happens but the engine gets quiet. The only way to check it is to take the mag apart and verify the correct timing marks align in accordance with the repair manual.

E gap is set with the points.
 
cruiser said:
http://www.eaavideo.org/video.aspx?v=972961455001 About 44 min into this he discusses high CHT and claims 2 causes of same, loss of cooling and a lean mixture. If lean mixture CHT will be high ROP and low LOP. You say it does not take much of a pull on the red knob to go lean, is that a hint? Good luck, Jim
Click to expand...

I understand. But how do you get more fuel to the cylinders on a carbureted engine when mixture lever on the carb hits the full rich stop? I've read some stuff about drilling out the jet on the vans aircraft forums. And any one know what would cause the CHT to be 20 degrees cooler when running on right mag vs left mag? I have to fly around at 2350 rpm to stay at 395-405 cht then when I need to climb soon as I hit 2500 rpm my temps go up to around 440-450 so I end up lowering nose and pulling back to 2350 rpm and climbing out at 300fpm at 420 degrees. Engine has 15 hours on it as of today. Running Aeroshell 100 Mineral. Oil temps at 220-230.
 
What can be done to the carb to enrich the mixture while at cruise power, when the mixture control is at full rich and against the carb stop? Is the knob on the back just for the idle mixture?
 
sburg58 said:
What can be done to the carb to enrich the mixture while at cruise power, when the mixture control is at full rich and against the carb stop? Is the knob on the back just for the idle mixture?
Click to expand...

The only thing you can do as far as I know is get a bigger main jet, or drill out the one you have.
 
sburg58 said:
What can be done to the carb to enrich the mixture while at cruise power, when the mixture control is at full rich and against the carb stop? Is the knob on the back just for the idle mixture?
Click to expand...

Take a drill bit a little bit bigger than the jet (nozzle) and ream it out then you'll get more fuel.
 
I adjusted the idle mixture so I get a small increase in RPM when I pull the mixture to full lean on shutdown. Should the same thing happen when at a higher RPM? When I pull the mixture slowly back at around 1800 RPM while on the ground there is no increase, the RPM just drops when the mixture is too lean for the engine to run. The field elevation is 2000 ft. Trying to decide if my engine may be running too lean. My engine also has high CHT's.
 
From my experience, if you have a very high temp, look for a big cause. You may pick up a degree or two here and there but when you find the cause , it will be a dramatic improvement.
 
sburg58 said:
I adjusted the idle mixture so I get a small increase in RPM when I pull the mixture to full lean on shutdown. Should the same thing happen when at a higher RPM? When I pull the mixture slowly back at around 1800 RPM while on the ground there is no increase, the RPM just drops when the mixture is too lean for the engine to run. The field elevation is 2000 ft. Trying to decide if my engine may be running too lean. My engine also has high CHT's.
Click to expand...

The only way I know to recognize adequate fuel flow is to check your temp spread between full rich and peak EGT. I was taught that 150* was the target temp spread. 100* and below is reason to pull the carb. My Cessna was getting closer to 125* spreads when a local shop complied with a service bulletin and increased flow. Coincidentally I now see 150*+ differential. The -12 hit the target from the git-go but when I made a comment about wanting to lower my CHTs my mechanic checked and subsequently adjusted the carb float. I'd like to hear what the pro mechanics look for.

I've never seen an RPM surge at shutdown in any carbureted airplane engine that I've owned. For what that's worth.
 
Last edited:
If one of my customers had a high CHT problem I would not drill out the main nozzle, ever. If the engine is running lean it will show other symptoms and there are other causes besides the wrong jet size. On a fresh overhaul, intake leaks are first on the list after verifying all the correct parts are installed to factory specs, on older engines, worn throttle shaft bushings will give lean at idle indications.

RPM rise on shutdown is sometimes hard to see, especially on a big Cont., I listen rather than watch the gauge. You should see a drop in manifold pressure easily if a gauge is installed. Throttle has to be fully closed with correct idle speed set.
 
I have high cht's at climb out as well. I was thinking of having the carb flow -checked . Would that answer the fuel flow question?
Also, I would like to know more about the e gap timing symptoms. If the e gap was off, but engine timing was correct, wouldn't it cause the spark to be weak resulting in lower cht's and possibly high egt's? Not a statement of fact, I'm just trying to learn a little.
Laz:)
 
flew the cub 2-3 hours today. OAT was between 50-60 degrees. 2350 rpm cruise was 375-385 degrees on CHT. 2500 rpm climb out was 405-410 CHT. These temps are much more reasonable but today was a solid 20 degrees cooler than the past few days.
 
If you have just the one sensor (on # 3) it's gonna show hot because that's normally the hot cyl, AND the exhaust pipe is very close to the sensor which will make it show hotter than it really is.

I put small aluminum "ramps" ahead of the # 1 & 2 cyls, which warms them a bit & cools the back two better. Also added a 2" strip to the lower back of the cowl to "suck" out more air. These helped me alot.
 
Lower cowl around the carb air inlet/filter sealed well? My CHTs dropped 10 degrees by closing a big gap that was allowing the lower cowl to pressurize.

Personal opinion--- you really need a probe on each of your cyls when working on cooling. Especially if you start building ramps or blocking off the front ones. I found that with ramps, my front CHTs went up more than my rear ones dropped. Was worth doing but I can see where it would be easy to go too far if you don't have probes on all cylinders.

On my engine the #4 runs hottest but that may be due to the rear mounted oil cooler.

I don't see where you mentioned if you have a bayonet probe or the spark plug washer type probe. The spark plug washer types always read a lot higher than the bayonet probes.
 
aviationinfo said:
Lower cowl around the carb air inlet/filter sealed well? My CHTs dropped 10 degrees by closing a big gap that was allowing the lower cowl to pressurize.

Personal opinion--- you really need a probe on each of your cyls when working on cooling. Especially if you start building ramps or blocking off the front ones. I found that with ramps, my front CHTs went up more than my rear ones dropped. Was worth doing but I can see where it would be easy to go too far if you don't have probes on all cylinders.

On my engine the #4 runs hottest but that may be due to the rear mounted oil cooler.

I don't see where you mentioned if you have a bayonet probe or the spark plug washer type probe. The spark plug washer types always read a lot higher than the bayonet probes.
Click to expand...

Bayonet probe.
 
Bb, I am working though a very similar issue w my o-320 that was top overhauled last March so I'm glad to see this conversation. for me, #4 is running 380-400+ in cruise depending on oat. Full power climbs are limited to a few minutes before temp climbs through 420 and i level out to cool. Other cyls are 290-330 using the same bayonet probe. We've spent a lot of time on baffle/cowl sealing, induction leaks, and timing, which seemed to have brought temps down to 370-390 in cruise, but now engine is difficult to hot start, and climbs are still limited before cht goes past 420.

Current theories are:
1. Mag or lead problem
2. Elusive induction or baffle leak
3. Cam problem (!)

My assumption with the fuel-flow and mixture discussion on this thread (and to a certain extent, mag timinng) is that temps would be high on all 4 cyls, which is not my case. With all the time ive spent on baffle and induction, Im beginning to worry about the cam...
 
907....did you plug the BIG HOLE down the front of the engine????? Good for 27 deg F....
 
I'm talking about the gap in front of the engine, past the flywheel and generator....
 
If you do close the gap between the engine block and the flywheel, you have to supply a new source of cooling air to the generator/alternator. You also need to check the openings next to the spark plugs, sometimes casting flash blocks those holes. Also check rocker arms for binding bushings, missing or blocked oil squirt holes...ramps in front of cylinders are good for 20 deg F, blocking the hole down the front 27 deg F, a defection vane over the rear cylinder to push more air down onto the fins another 20 deg F, a 60 deg 2 inch lip on the lower rear cowl exit good for 10 deg F. I finally wound up with 310 + OAT=CHT, not a bad place to be...but I started at 390....
 
Some good input...do you mind taking pictures of your cooling improvements? Was looking the plane over again yestersday and we still have prtty big gaps in a number of places. Big holes at the front of the engine on both sides, cowling support channels are open on the under-side, etc. I'll take pictures and post to give a better idea of our problems.

fobjob said:
If you do close the gap between the engine block and the flywheel, you have to supply a new source of cooling air to the generator/alternator. You also need to check the openings next to the spark plugs, sometimes casting flash blocks those holes. Also check rocker arms for binding bushings, missing or blocked oil squirt holes...ramps in front of cylinders are good for 20 deg F, blocking the hole down the front 27 deg F, a defection vane over the rear cylinder to push more air down onto the fins another 20 deg F, a 60 deg 2 inch lip on the lower rear cowl exit good for 10 deg F. I finally wound up with 310 + OAT=CHT, not a bad place to be...but I started at 390....
Click to expand...
 
Heres what our baffling looks like.

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582de445.jpg
 
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