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On a ported cylinder I saw the a/f becoming leaner, but this was an FI. With a carb the additional air intake draw more gas. For the squish, if it is too tight for the amount of squish band radius and angle, I believe it can loose power. It can also loose power if it can't hold the compression. I think you have the right idea to give a thicker base gasket a try. Can you increase it by only 0.1mm?
 

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Discussion Starter · #222 ·
Hey Daag
As I haven't noticed any loss of power, I'm not ready to use a thicker base gasket yet. I can go .005 or .0015" thicker if needed, but will wait till I see what the timing does.
 

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Well, I've got the motor on the bench and discovered the carb boot has a crack in it, so it could have been lean. The carb boots for this sled have a reputation for splitting and I keep a spare on hand. I've replace it 3 times for cracks over the years so I'm a little bummed I didn't catch it. It was last winter but if I remember, I did flip the choke on an off a little while running to check for lean and also sprayed ether around the engine looking for leaks. I didn't notice anything on the EGT either, The boot is definitely cracked so these tests obviously didn't catch it.

I did a leak test and the engines still tight, so that's good.

Sightation

You indicated you use this engine for racing? Out of curiosity, what are you using for a carb? Mine came with a vm34. I've heard of guys going to a 36mm but also heard you might lose some low end performance. Haven't really considered it with fitment issues.

Do you think the compression is low enough that I can get away with taking .010-.015 off the head to tighten up the squish some?

Again I appreciate the help and comments.

Steve
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Looking back at this thread. Niko overseas who Races his 277 used a 38mm Mikuni Flat Side, with Head at 14.5cr and he turned it 9,200rpm with a Modified 670 Tuned Pipe.

277F = 268.8cc turned 9200rpm at 93% VE = 54.1hp and makes 81.2cfm!
277F = 268.8cc turned 9200rpm at 94% VE = 54.7hp and makes 82.1cfm!
277F = 268.8cc turned 9200rpm at 95% VE = 55.3hp and makes 83.0cfm!

Did you Measure your Intake ID?

Did you Measure the ID of your 34mm Carb?

My Intake sitting on my desk the ID = 37.6mm.

I recently bought a Mikuni 38mm Flat Side New off eBay for $81. The ID = 37.87mm and OD = 46.9mm! So very thick Walls, so you could Bore it for a True 38mm or overbore it to a 40/42mm.

The Mikuni 34mm Flat Sides New off eBay are cheaper Average $40. So I'm wondering if you couldn't just overbore a 34mm to a 36mm, or 38mm!

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More evidence 38mm Carb is too big for a 277 TRAIL SLED.
Mikuni has a lot of experience with carburetors. They published a tuning manual advising customers not to use a carb too big for the application.

There is a chart showing how too big of a carb will hurt low end performance.
Also a reference chart of proper carb size per hp from experience.
http://www.mikunipower.com/Manuals/VM_Manual.pdf

Surprise!!! - Carb experts agree with the real world results Rotax has seen
380HO= 28.6 hp/cylinder = 30mm on chart and real sled
550F=39 hp/cylinder = 34mm on chart and real sled
FULL RACE 277 50hp = 38mm on chart and real sled
The 34mm carb on Steves Tundra should be good for over 40 hp.

For great low end pulling power Skidoo Alpine II used a SINGLE 30mm carb on the 503. The Nordic 60 and its Lynx twin used a SINGLE 32mm on their utility 503 sleds [email protected] rpm

TOO BIG of a Carb hurts low end performance on a TRAIL Sled
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Discussion Starter · #225 · (Edited)
My freshly groomed test track.
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CDIs
I've read that dialing in the squish can lead to a quicker burn which needs the timing retarded some. I was hoping this might make the less aggressive RMS CDI perform better. I hit the track with both the RMS and the reprogrammed OEM CDI. 3 runs each at 300', 600' and one at wot.

RMS , OEM
38.6 , 39.3 at 300'
40.5 , 42.2 at 600'
42.6 , 43.6 at WOT Note that the aggressive OEM held rpms throughout while the RMS CDI's dropped off 150.

I wasn't all that impressed with the top end of either one though the OEM CDI clearly performed better. My plan at this point was to put a .005" thicker base gasket in and run the reprogrammed OEM CDI. What's nice about this little motor is I can have it out, on the bench and opened up in 45 minutes.

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This is the piston after roughly 6-8 hrs of run time. Of note is the leakby on the left. The Oring was compromised which might explain some of the performance loss. The wider squish band might also play a part in top end loss? It does seem a little more lively on the bottom end, but that's hard to quantify. I think I will need to play with jetting and clutching a little.
I hit the test track again after replacing the Oring and base gasket with a thicker one.
With the OEM CDI
42.0 mph at 300' (I believe this is the best I've seen)
44.9 mph at 600'
This was at 8 degrees so it's likely a little leaner, but also with slightly lower compression with the thicker base gasket.

Piston ring
With the RMS CDI, I didn't see any damage in roughly 4 hrs of use. After the testing with both CDIs, there was a little damage on the exhaust side which I'm guessing was from the tests with the modded OEM CDI. I'm hoping the .005" thicker base gasket lowers compression enough that the aggressive OEM CDI will work.

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Here's plug chops with both CDIs at WOT using brand new #10 plugs. Can you guess which is which?


.
 

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Discussion Starter · #227 ·
You're right Daag. To me, it looks like it's richer then the less aggressive RMS one. So it would seem that it would be even leaner yet with the original conservative timing, but I was running a 200-210 MJ with that without issue... Seems odd then that I'm running a 240 now. I will probably run it this way for a while and pull the motor for inspection again.
 

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What do I win? lol

I wonder if we are seeing the further advanced timing working to provide a better/cleaner burn. Could this be due to the timing that more closely matches the larger squish area in newer sleds? Or maybe it is a combination of the ignition timing and squish area that better matches the tuned pipe you have?

I was glad you gave a thicker base gasket a try. It was only a hunch, but one that stemmed from my own compression testing. My original plan was to take a deep dive this past summer, but I ended-up looking into charging batteries and straightening crankshafts.

I think this would be a good time to plug-in the numbers to get a rough estimate of the MSV. Do you still have the Aaen combustion chamber to try?
 
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