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New head & hi-comp piston...Jetting Help Pls

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I have an '04 Honda CRF250R... I sent the head out for a professional valve job, and installed a new Weisco 13.5:1 piston. After putting everything back together, the engine will bark, but not run. This is a stock carb with stock jetting, and stock pipe. Prior to the head and piston, the bike was all stock and ran fine (until the valves couldn't be adjusted anymore). To be sure I covered my bases, I dumped the gas, pulled the carb and cleaned it. Installed new plug, cleaned the air filter. At this point, I'm thinking the only thing left is to change the jetting for the higher compression?

I should also mention we are at sea level to 2000 ft, with 40~80RH.... I'm hoping for some base line jetting suggestions, or other info.....

Thanks.

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The 13.5 is not that much harder to start.

It does not require more or less fuel because of higher compression.

You probably have to bump start it, then adjust your fuel screw.

Try raising the idle screw a bunch and then try and start it.

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Triple check the cam timing.

Thanks, I keep coming back to timing too..... I've checked and rechecked, then rechecked the timing. I have a matching pair '04's, so to be sure, I checked it against my other bike. I even replaced the timing belt tensioner. At 13.5:1 I still should be able to run premium gas. I dumped the tank and filled with fresh gas.... I haven't tried to pull start it with the truck (yet).....

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Even 14 to 1 is not a big deal to kick. I find higher compression engines more eager to 'light off' if set up right.

Was the stator out?

Flywheel off?

Is/does the plug come out wet with fuel?

How long was the bike apart? Was the carb completely emptied of fuel? Was the slide out of the carb?

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Even 14 to 1 is not a big deal to kick. I find higher compression engines more eager to 'light off' if set up right.

Was the stator out?

Flywheel off?

Is/does the plug come out wet with fuel?

How long was the bike apart? Was the carb completely emptied of fuel? Was the slide out of the carb?

Just the head was removed, and the bike had been sitting over the winter, but I always drain the carb. I took the carb off and removed the jets, but not the slide. I also removed the float. Blew it all out with carb cleaner, and reset the air/fuel screw back to factory as a baseline (2 1/4 out). There is a small amount of fuel that is venting, but that's after kicking, and kicking, and kicking, and kicking. Yes, at some point it is flooding, so eventually the plug is wet. The head was off for about a month.

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I'll assume you get a strong, repeating spark, correct?

That still leads to cam timing or valve clearances.

Yes, to the strong spark... The new valves are shimed to spec.... Timing (was) also my thought, but I've mostly ruled it out. Theses don't have points, and the electronic timing hasn't been changed, nor has the flywheel been touched, which leaves the camchain, tensioner (which I replaced), and the cam sprocket. When I pulled the head and cylinder, I was careful to wire up the cam chain so it didn't move. When I put it all back together, I made sure the timing mark on the crank aligns with the mark on the (rightside) engine cover, and the the cam sprocket marks are horizontal with the top of the cylinder. The piston is at TDC on the compression stroke, cam lobes facing to the rear. To be double sure, I checked it against the alignments on my other bike (since I was adjusting valves and had it open). So I'm not sure how it can be the timing....?

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Timing was refering to the cams. The only time the ignition timing gets out is when the flywheel is not installed properly and either the key slips out and the flysheel shifts or the key breaks due to the flywheel not sitting 'square'. Sometimes too, a mechanic has removed and reinstalled the crank position sensor and not put it on right, that too, will screw up the spark timing.

Most engines have multiple marks on the flywheel. Some are for TDC, some are for cam timing. You sure you have the right marks?

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Timing was refering to the cams. The only time the ignition timing gets out is when the flywheel is not installed properly and either the key slips out and the flysheel shifts or the key breaks due to the flywheel not sitting 'square'. Sometimes too, a mechanic has removed and reinstalled the crank position sensor and not put it on right, that too, will screw up the spark timing.

Most engines have multiple marks on the flywheel. Some are for TDC, some are for cam timing. You sure you have the right marks?

The CRF has two timing marks... The marks on the Flywheel are for setting the EI at the factory. There is an access port on the right side; removing the plug exposes a sprocket with an alignment mark, for setting TDC/cam. That mark aligns with an arrow on the engine housing, and the cam sprocket aligns with horizontal marks that align with the top of the head. Align the three marks, with the piston at TDC on the compression stroke (cam lobes facing to the rear) and you are set correctly. This is also the engine index for checking and setting valve clearances.

I may go back and pull the carb and spray it out again.... It's a long shot, but it's possible there could be debris that I either misses, or got in there, while I was putting the carb back together, or during re-installation. However, after cleaning all the orifices and blowing them out with air, I did fill the bowl and bench tested it to make sure the accelerator pump was working; it was.

It's a PITA to pull the plug with the tank on..... But after several repeated kicks (lots of them) I am venting fuel, which tells me: 1). the accl pump is pumping fuel. 2) the carb is sucking fuel through compression.

At this point, I'm stumped.....

Edited by William1

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Sounds like you have somewhat of a handle. So many have no clue when setting cams, pure gueesing, very scary.

Keep in mind, engines are simple. Air, fuel, compression, spark and they will run. Fuel new?

When starting, new spark plug (gap checked), no throttle, choke on?

Hot start closing fully?

Was the slide ever out? Release plate in square side down?

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Make sure the carb inlet, on the air filter side, and its jets are lined up on the air filter boot. You might be blocking the slow air jet, and then not much is going to happen.

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check the sticky in the 450 forum for setting your cam timing.

use the left side, right mark. that is the timing mark. Then check that the hash marks are level with the head. I believe there is also an arrow on the cam tower those hash marks line up with also.

what is your jetting? have to inspected the hot start?

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Triple, quadruple check the timing. Release the tension on the tensioner and turn the motor over a few times and check the timing again. The bike should still start even with slightly higher compression and stock jetting.

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Thanks guys.... I've pretty much been over all this, and more than once.....

Some of the answers to the various questions:

- Nope, I haven't taken the slide out

- The carb is fully seated in the boots, both at the airbox and engine. At least it is seated in the notches in the boots correctly.

- Plug gap....check

- Jetting is stock from the factory

- Gas is fresh, 92 Oct....

I'm with you guys on the timing, but geeez, I've gone over and over it.... I'm not ruling it out, but I'm 99.9% sure it is set correctly. I'm going to tear it apart again on Sunday and start from scratch.....(again).

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Well, some success.... I removed the cam, and re-indexed the piston to TDC, then reinstalled the cam to make sure the timing was set. Removed the carb and cleaned it again to make sure. Set the air/fuel screw to factory. Reassembled..... I was able to get it started after a great deal of kicking.... It finally fired off, but it's running rough and won't idle. I think at this point I will take it to the shop to have it fine tuned....

I can't say for sure, but I think it was timing.... One thing to note when setting the timing, initially there is no tension on the cam chain when indexing the cam and crank. When the tensioner is installed I noticed the cam was off by a tooth. I may have missed that the first time around. Lesson learned, check the marks after the tensioner is installed....

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I noticed that when ever i tightened the tenstioner, the cam sprocket would spin( i believe CCW) a little no matter how many times that i thought i got it right. Then i decided to install the cam with the left cam marks one tooth off in the opposite direction, so when the tensioner tightened and spun the chain, i would be lined up with the head. This worked for me. Others might get different result

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When checking timing with the tensioner removed, simply rotate the cam/chain clockwise lightly with your hand to tighten the drive-side chain tension while making sure the flywheel hash mark is still aligned with the case with the camshaft still aligned properly. This will ensure proper timing when you install the tensioner as you're simply taking up the slack in the "already loose" side of the timing chain.

Edited by Eddie8v

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