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Discussion Starter #1
I was just wondering how soon the carbon should start to accumulate on the piston.
I just rebuilt 85 600 as you all probably know and only have about 50 miles on and there is no carbon on the pistons. To rich?
I am thinking about putting 220 mains in the carbs. It has 250's now and bogs. Stock jets are 260. 220 to lean? I had a 85 600 engine last year ran with 210's (Ran like hell for 15 min. than burnt down. Off course the airbox wasn't on. Hmm. Then put same engine in the sled I have in now (not bored though)
and ran 240's all last winter pistons still did not carbon up. Well I guess to sum it all up are the jets I have in now (240's) to rich and is this causing the pistons not to carbon up. Thanx for reading my life story and any help will be greatly appreciated.
 

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It may take longer than that. Carbon on the pistons isn't nessesarily good or bad although a little carbon is a good sign of not being too rich or lean. I have seen engines orn apart with thousands of miles on them and not a whole lot of carbon on them. I say, if you don't hear the engine detonate, it isn't too lean.
 

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watch the position of the e-clip and air screw when you change main jets, especially that much at a time! exact jetting im not sure but i'd get a carb book to start with.
 

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As you learned the hard way the air box is usually worth 4 sizes of main jet. I always advocate a set of EGTs .If you can find used, it will be the best two hundred dollar mod you can buy. Then you can dial in your jetting in all ranges and you have an instant eye on whats going on in your motor. I cant do without them anymore
 

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Discussion Starter #5
So what your saying hotrod is when I had 210 in the carbs without the box off I really was running like 180 or 190. wow if this is true. If I had the airbox on with 210 do you think it would have burnt down.
Thanks
 

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Willy the 600/650 motors were good at boging. Make sure you have a good tight belt. Most of the time the belt is what causes the problem. The type of oil can also be a factor on how much carbon is on the top of the piston. That sled should run pretty good with 240/250 main jets in it. Your main jet isn't causing your bog. Try adjusting your air screw on the side of the carb. Turn in = rich out= lean. My guess is the bog is coming from the belt. Riley
 

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Yes from my learnig and reading on the subject you would have been equivelent to 180/190. I have wrote this before but here goes.They did a dynotest on several sleds in the summer at 78 degrees. These sleds were jetted for -10 degrees. then they rejetted for 78 degrees and re dynoed. The results were suprising. 6% increase in engine HP. That is 6HP per hundred HP on your sled. And at the track it would be a wopping 3HP. That was a huge difference in temperature too. I suggest to everyone that without EGTs the power increase for a few jet sizes is just not worth it. If you use the recomended jet for your altitude, stock was 280s for 0 altitude and then lower by one size you will be safe and reletively efficient.
 

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Hotrod, when you talk about lowering one jet size, are you only talking about the main jet, or lowering the size of all the jets in the carb, and lowering the jet needle as well?
 

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Hi Tfin for the above paragraph I was just talking Main jets to avoid confusion in my attempt to show people what little gains you get from messing with your jetting. Since I use EGTs to keep track of my settings I can set all three ranges easily and safely. The Point is this. If you jet close to the factory setting and think maybe I can go one or two sizes down, based on the dynos above, for two sizes you will gain almost nothing because the gained only 6% by changing many jet sizes. The risk of being too low when the next really cold day comes along is not worth the tiny increase in power.
 
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