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Whats the deal with the decompression arm???


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Ok I'v just installed a new 488cc kit from BBT and I can't for the life of me figure out way this thing wont start.

First off this will be the 3rd new piston i'v put in this bike sense I'v had it "bout 2 years" and in the past I'v never had a problom with it firing right up after the rebuild. So the reason I tore it down was because my cylinder was DONE due to honda's one compression ring design and too many rebuilds "Oh and lots of sand riding LOL" but anyway I get it all back together and I'm damn sure the timing was wright cause I did it the same way as I always have "the other 10+ times I'v been in the motor" and never had a problem before but I try starting it up but every bout 3 kicks it almost seems like it locks up but it will barley move a little until it frees up and is ez again. I kinda think its when its coming up on TDC and the compression is at the most so I go looking on forums for this prob. and find that it's prolly my decompression arm clearance is prolly out of tolerance so i get it to TDC check that OH wow its way out, fix it and then notice my intake cam lobes where facing the exhaust. Duuuh well I start all over get it to TDC and well my cam was exact 180 off man I feel stupid, but now my decomp. arm is tight as hell "of course" fix that and now I'm thinkin OH yah it's good now it should fire right up. WRONG its worse now its like every or every other kick it gets hard as hell. All 225lbs of me can stand on it with out it moving but if i hop at all it will roll over and spin tell it gets to TDC again. I still think its my decomp. but everything looks fine nothing is broke as far as I can see. BUT I am at a brick wall now PLEASE HELP. :banana::banana:🤣:banana:

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Ok I'v just installed a new 488cc kit from BBT and I can't for the life of me figure out way this thing wont start.

First off this will be the 3rd new piston i'v put in this bike sense I'v had it "bout 2 years" and in the past I'v never had a problom with it firing right up after the rebuild. So the reason I tore it down was because my cylinder was DONE due to honda's one compression ring design and too many rebuilds "Oh and lots of sand riding LOL" but anyway I get it all back together and I'm damn sure the timing was wright cause I did it the same way as I always have "the other 10+ times I'v been in the motor" and never had a problem before but I try starting it up but every bout 3 kicks it almost seems like it locks up but it will barley move a little until it frees up and is ez again. I kinda think its when its coming up on TDC and the compression is at the most so I go looking on forums for this prob. and find that it's prolly my decompression arm clearance is prolly out of tolerance so i get it to TDC check that OH wow its way out, fix it and then notice my intake cam lobes where facing the exhaust. Duuuh well I start all over get it to TDC and well my cam was exact 180 off man I feel stupid, but now my decomp. arm is tight as hell "of course" fix that and now I'm thinkin OH yah it's good now it should fire right up. WRONG its worse now its like every or every other kick it gets hard as hell. All 225lbs of me can stand on it with out it moving but if i hop at all it will roll over and spin tell it gets to TDC again. I still think its my decomp. but everything looks fine nothing is broke as far as I can see. BUT I am at a brick wall now PLEASE HELP. :banana::banana:🤣:banana:

I would double check your timing. I would be willing to put money on it being off a tooth.

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Maybe the shims got mixed up....as far as the decompression arm if it is to tight I would say he shims are the issue. Plus if it was to tight it would kick over easier because it is holding the exhaust valve open.mine is set at. 025 in . Check your valve clearance. 011in exhaust. ....006in intake

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You checked them with a feeler gauge? The numbers you wrote were the tolerance? ....if the shims weren't mixed up set decompression to. 025 in (with exhaust valves properly set)... if you still have issues. ..check tdc one more time. Take plug out use something that wont hurt the piston to verify it truly is at tdc. Look at all timing marks including flywheel mark just to make sure...again

If valves are good, timing is good...

I would pull head and look the top of piston to see if the valves hit when the cam was in 180° off. ...if you have a dome piston I would guess you have bent a valve

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You checked them with a feeler gauge? The numbers you wrote were the tolerance? ....if the shims weren't mixed up set decompression to. 025 in (with exhaust valves properly set)... if you still have issues. ..check tdc one more time. Take plug out use something that wont hurt the piston to verify it truly is at tdc. Look at all timing marks including flywheel mark just to make sure...again

If valves are good, timing is good...

I would pull head and look the top of piston to see if the valves hit when the cam was in 180° off. ...if you have a dome piston I would guess you have bent a valve

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Correct tdc and cam timing= piston at top of bore, all valves closed

Cam installed 180° out = piston at top of bore, intake or exhaust open piston can hit the valves.

My guess running a big bore your gona have a dome piston therefore piston to valve timing becomes more critical

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Correct tdc and cam timing= piston at top of bore, all valves closed

Cam installed 180° out = piston at top of bore, intake or exhaust open piston can hit the valves.

My guess running a big bore your gona have a dome piston therefore piston to valve timing becomes more critical

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Correct tdc and cam timing= piston at top of bore, all valves closed

Cam installed 180° out = piston at top of bore, intake or exhaust open piston can hit the valves.

My guess running a big bore your gona have a dome piston therefore piston to valve timing becomes more critical

If the cam was exactly 180 out, the vavles would still be closed at the top of the stroke.. Now if the cam was 90 out, the valves would be open at TDC..

The crank spins 720 degrees for every 360 the cam spins, So the cam being 180 out, you are just changing the compression stroke.. Only reason it would make a difference, is if your bike doesn't spark every rotation..

On my crf50 it makes no difference, I ran mine 180 out for over a year.. It sparks every single rotation, so it just fires on the opposite stroke and compresses on the opposite stroke then it did factory 👍

a 450 might not run 180 out, but shouldn't affect the piston hitting the valves at all... Someone please correct me if I'm wrong🤣

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Basically what I'm getting at

Is your cam and piston are at TDC, your piston goes down and back up and spins the crank 360 degrees and now the piston is back at TDC, and the cam is 180 degrees off its originall position.. Now the piston goes back down and coomes back spinning another 360 degrees spinning the cam another 180 degrees putting it back to its original position.

The piston is in the same place when the cam is at TDC, or at 180 degrees out, on the compression or the combustion stroke. 👍

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Jetting is off. When you go bigger you usually need to leaner......Due to more vacume by the piston, pulling harder on the jets. Lean out the pilot and main 1 size for starters. Check your diaphram also to make sure thats in good shape...

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OK 1st: the +#'s were + what it was suppose to be, like +1 loose or -1 tight.

2nd: idk what the deal was with it being 180 off but it was and I know for a fact that the valves didn't hit the piston cause after I get it timed I turn it over by hand about 5 times threw to make sure theres no abnormal sounds or feels "and there wasn't" it was perfectly smooth.

3rd: when I got home I checked the timing 5 times with a straw in the cylinder and its dead on but there slack in my decompression arm so i was measuring with out pushing the slack up so I readjusted it and the "hard as hell to kick" problem is gone but it still wont start 👍 It will pop and fire here and there but wont get even close to an idol or even rev up, the only time I can get it to do anything is when I kick and kick and pull the hot start and it will sputter idol for about 1 second die then kick and kick and nothing again. The only thing I can think of now is I must have my jetting all F'ed up. Cause when you pull the hot start lever it leans out your starting mixture so if it would want to run when the hot start is pulled then its to rich

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Just saying if the cam is not timed correctly. .it does have the potential of the valves hitting the piston.....

Oh most definitely, you definitely want the cam timing 100% spot on

but if its 100% perfectly 180 out I don't think it would cause any issues, just confusion when setting timing 👍

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and I can tell I'm getting plenty of gas, I can smell it and every time i'v pulled the plug sense i'v been trying to start it, it has been soaked so I bumped my pilot down to a 145 smallest I'v got. "had a 147" and leaned out my screw to 1 turn out and dropped the main from a 168 to a 162. what do ya think? PLEASE help very new with jetting. 👍

🤣

Also I know I sound stupid by the timing situation but it was late when i threw her together like I had so many times I must have just looked past that some how.🙂

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Check your diaphram also to make sure thats in good shape...

please explain this a little better this might be y its just flooding the hell out of it.

🤣 You guys r awesome thanks for all the replies. 👍🙂

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