Creating a separate thread for this topic so people who might come here searching for this problem will have a clear place to read up on it. For those of you who have been following this problem in the various other threads that I have, you're probably not find much new here. I just need to organize my thoughts and make this more useful for those who follow behind me.

PROBLEM
Had an S & S 100" kit installed on my 2000 bagger, and followed the S & S guidelines for break-in, but was very concerned by my rear cylinder running 100 degrees hotter than my front for the entire duration of the first hundred miles, and thus I stopped riding it at that point until I could figure out and correct the cause. The front cylinder was running about 250 degrees, and the rear was running as high as 360.

HISTORY
The bike had been dyno tuned for this exact build months earlier, but due to piston galling within 500 miles of the 1st build, new pistons & cylinders were required.

Opinions in other threads here as to what caused this ranged from poor assembly, poor fitment of pistons to cylinders, defective pistons or cylinders (S & S replaced them for free, just in case), poor tune (too lean), cylinder washing (too rich), oiling problems, or improper break-in (too hard on the engine too soon). The cause has not yet been definitively identified and may never be. For the record, Harley-Davidson of Yuba City officially listed the problem as improper break in by the dyno tuner, but that's just one man's opinion.

TESTS I'VE RUN SO FAR
Compression Test
The front cylinder was 135 lb and tear cylinder showed 150. This engine has S&S easy start cams so I couldn't test compression the traditional way (cranking with throttle held open). Instead, I had to pull the connector from each injector one at a time and actually start the engine with the compression tester in place and blip the throttle a couple of times to get a max reading. I don't know how accurate this method is, but both cylinders have decent compression and are within 10% of each other so I'm satisfued with that for now.

Leak-down Test
perform a traditional leak down test and got 7% for the rear cylinder and 5% for the front, with the engine lukewarm so those numbers would have been lower if the tests we're done at operating temperature. Again I'm fine with that.

Visual Inspection of Cylinders
I used an inspection camera attached to my phone to look at both cylinders. There were some vertical marks on the cylinder walls but, after comparing this to one other properly running engine I've seen scoped in a similar way, and due to the opinion of some very experienced people here, these marks don't overly concerned me at this point.

Intake Leak Test
I used both propane and water (separate tests) around the intake manifold / cylinder head interface and had no change in engine sound or speed, so it doesn't seem to have a leak there.

Idle Temperatures
While doing the intake manifold leak test, I decided to let the engine idle in my garage and see how the cylinder temperatures compared over time. Tellingly, both cylinders rose at similar rates all the way up to 310 degrees, at which point I terminated the test so as not to damage the engine.

Temporary Enrichment
Using the buttons on my power commander 3 USB, I added 10% fuel to the entire RPM range and went for a 20-mile test ride. This had no effect on front cylinder temperatures, but did lower the rear cylinder temperatures by 25 degrees. The bike ran as one would expect for an engine tuned too rich.

NEXT STEPS
Since - at idle - the rear cylinder rises in temperature at the same rate as the front, and doesn't get hotter than the front at idle but only a cruise, that seems to suggest - as many people have postulated - that it's a tune issue; that it's running too lean.

Validate the Fuel Map
I have purchased O2 sensors, a dual AFR gauge, and the means to log data from the O2 sensors, the throttle position sensor, and the engine RPM. My next step is to install this and look for lean conditions that could be causing the rear cylinder to run hot.

Swap Injectors Front to Rear
if the data log doesn't reveal a lean condition, my next step will be to swap the injectors front to rear to see if the front cylinder gets hot rather than the rear.

that's where I'm at so far.

 

I've been following your threads on this, sorry for the problems. IMO you are approaching this in a very logical manner. Is the bike running well now? If temps are the same on both cylinders at idle, maybe it's just the fact that the rear cylinders always run hot when riding because the front blocks cooling air and adds hot air to the rear? I wouldn't think they be 100 deg different, but then again I've never measured them either. Not trying to be a dick here, but are you just a bit paranoid about this? I admit, I would paranoid too after the 500 mile galling you experienced on the last build. Swapping injectors is a good idea.How are you measuring front/rear temp? Are your temp measurements accurate? If the bike is running OK, maybe just ride it for while. Log your data, check map etc...; then ride it for a while.

Best of luck.

 

Thanks tappets, I'm trying to be logical about it, and I appreciate your comments.

From everything I've read, it's normal for the rear to run hotter than the front, by perhaps 20, maybe 50 - degrees, but in the opinions of some people who have a lot more experience than I do, 100 degrees is beyond the pale. Also, a cylinder head operating at 350 degrees seems to be of concern as well.

Am I being paranoid? I think I am, and you're not being a dick for suggesting it. I have a lot of time and too much money into this engine to have a "ride it and see what happens" attitude without first making sure everything is in order, because the result could be another grand or more out of my pocket. Also, next summer I'm planning to take this bike on a 6,000 mile ride from California through British Columbia, The Yukon and Alaska and I need not to have engine trouble creep up on me a few thousand miles away from home.

I'm measuring with a Milwaukee infrared temperature gun, and I'm pretty confident in its accuracy. I'm measuring about an inch in front of the spark plug.

If the temperature difference remains after validating the tune and swapping the injectors, then I'll do as you suggested and just ride it and see what happens.

Oh and, to your question about how it runs: It runs great as far as I can tell, but I've been very gentle with it so far, as I'm breaking it in per the S & S instructions. That said, once the temp on that rear cylinder starts climbing, I start getting what I believe is valve train noise.

have also confirmed that the left coil post is connected to the front cylinder.

I've also checked the temperature of the exhaust head pipes just outside of the cylinder head exhaust ports, and they are both in the 460 range at idle.

 

check the piston oil jets

 

Your new AFR unit will tell you what you suspect.

 

I imagine that if your running the 585 cam that your compression is at least 10.5:1 and if so your motor will run hotter than stock... My 96" Evo runs 320* without any issues. Your AFR meter should steer you in the right direction.. I also use a WEGO III.... Tune is spot on, confirmed on a Dyno as well...

 

Short and sweet, and a damned good suggestion that's crossed my mind numerous times. I've always had a nagging suspicion that this turn out to be an oiling problem. I asked the builder to check the piston jets before he installed the 2nd set of cylinders. He said he would but that they are hard to get off once the cases are assembled, and he's sure they're clear from the first build. Not sure if he actually did it.

I'm going to start a separate thread on this topic to make it more easily searchable.

 

I'm running 570 cams. Compression should be around 10.25:1. Good to know hear that you're not having troubles with your engine in spite of the higher temps. How many miles do you have on the build? I've got the Innovate Motorsport product.

 

For some reason I thought you were running 585 cams I didn't know you were on the 570 cam with a 40 degree intake valve closing on that Cam and at 10.25:1 plus being a 100" motor, yeah you might be producing too much heat. The tough thing to note is that you're not even sure what your CCP is because of the easy start cams. The one reason why I don't like easy start cams it makes it hard to diagnose and accurately pinpoint issues.
i have over 2k miles on this build and I run it hard... Still strong... I've had several hot rod builds in this motor from 80"to 96" from 10.2 - 11.2:1 for over 30k miles and never had heat issues maybe because of the WEGO III and timing being set properly..

 

I've learned a lot of lessons the hard way on this project, one of them being the down-side to centrifugal compression release cams. Like you said, inability to get accurate CCP complicates diagnostics.

Glad to hear your high temps haven't taken out pistons. With Harley killing the rear cylinder at well below 300 degrees to keep it cool, I worry about longevity. Once I get this sorted out, if the rear is still running very high 200's+, I'll probably install a set of fans to help keep things cooler, just to improve the chances of getting more miles from the build.