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PostPosted: Fri Sep 07, 2018 12:25 pm 

Joined: Wed Jan 18, 2012 10:36 pm
Posts: 166
My yz465 starts up and runs sharp, but after a lap or two, the rpms start taking a while to settle down. At that point, at a stop, if you snap the throttle and it revs right away but sort of floats back down to idle. It sounds all sorts of angry on the way back down to its idle.
Sounds like a lean conditon right? Maybe something sucking air, but only when warmed up all the way?
Maybe the jetting is too lean and works well for only until operating temp?
Thanks in advance!

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, TTR 125, 1994 KDX200 , 1978 Mobylette, 1985 KX350R-F91m (Frankenstein) and a Doodle Bug


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PostPosted: Fri Sep 07, 2018 1:39 pm 
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Joined: Tue Jun 07, 2011 6:37 am
Posts: 10460
Location: Rio Rancho, New Mexico
Do the standard spray a bit of WD 40 around the intake, or propane. Just don't burn down the bike. And with big singles, the hot crankcase messes up the fueling heating the mixture.

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PostPosted: Sat Sep 08, 2018 11:26 am 

Joined: Mon Feb 04, 2013 7:28 pm
Posts: 194
Location: West Coast
The big bore air hammer has a hard time overheating under racing conditions. I would suspect a small leak but your mainjet might be fine for trail riding or a couple laps but maybe a little lean for all out racing. I found my air hammers by lap 4 racing were starting to get hot and struggle but would be great on the trails. I had to go to a bigger main almost up to blubbering to keep them cool during the races. Middle of summer or a heavy mud race I always stepped up the main to keep from seizing. Diagnose leak test before messing with jet though. 465 is one of the best big bores ever made. You are a lucky guy!


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PostPosted: Sun Sep 09, 2018 7:08 am 
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Joined: Wed Jun 08, 2011 10:23 pm
Posts: 3821
Location: Colorado Springs, CO. USA
Check for air leak for sure -- and check the plug often during different conditions.
I had two 1982 IT465's I raced - very similar to the YZ. FYI both are prone to case cracks that can pull in transmission oil - but you'll know that pretty quick with fouled plugs.

I found that both of mine (and my IT175's) all do weird things at different altitude and temperatures. Dialed in great at sea level and then idle poorly at altitude but only when HOT --- when cool it's fine. Other strange things too. My 1990 CR500 has none of these issues.


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PostPosted: Thu Jan 10, 2019 10:01 am 

Joined: Wed Jan 18, 2012 10:36 pm
Posts: 166
Thanks for the replies!
Makes a lot of sense about the main jet. Ive been through several sizes, I started out big and kept trimming it down. To be honest, I cant remember whats in there at this moment. I was trying to clean the response up, and it worked, but I may have shot myself in the foot. And actuallly the last time I had it out was a cross coutry, so basically trail riding because im slow) and it still showed the same behaviour.
Ill check for leaks as suggested and if there arent any Ill go back up on the mains a few.

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, TTR 125, 1994 KDX200 , 1978 Mobylette, 1985 KX350R-F91m (Frankenstein) and a Doodle Bug


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PostPosted: Fri Jan 11, 2019 5:08 am 

Joined: Sat Dec 03, 2011 10:32 am
Posts: 600
Location: Jarrettsville, MD
You may want to plug up the openings and do a pressure/vacuum test as well. You mentioned it is pronounced when hot but it wouldn't hurt to check.

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