_JOE_

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I was doin a little web surfing today and came across some pics of a Keihin FCR carb with a build-up of dirt under the carb cap. I was shocked at the amount of junk that accumulated under there. The thoery was dirt is being sucked in through the vent lines and then passed into the engine. The dirt was accumulated directly above the choke/hotstart passages. Seeing as how I am extremely anal about maintenance and still have had issues with intake valve tightening I decided to have a look. Holy crap, there it was. A bunch of crud build up. I wish I would have known about this about 60 engine hours earlier.

Anybody else see this in thier FCR carb? There is a few options to relieve this issue. Some compaies make a filter kit that the lines plug into. I have also done some digging and found that people are using rc fuel filters with the screen replaced by air filter foam. I think I will be trying this option as I have a couple already and they are only three bucks a pop.
 

Rich Rohrich

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Of course it's going to suck in dirt. It's a tube that is open between the the atmosphere and the internals of the carb. Think about how a carb actually works (atmospheric pressure PUSHES fuel through the jets) and it's clear that unless you filter the air (and you have to do it without causing a pressure loss or you are going to change the fuel curve in a BIG way) you will always pull in some dirt via the vents, and it's always been that way. Two-stroke or four the carbs all work on the same principle. This is nothing new.

The good news is it's usually only really fine dirt that will get passed as there is only a small pressure differential that exists in the carb and the actual flow is minimal. Guys who ride dusty areas tend to have the most trouble with it.
 

_JOE_

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Do you think there is much potential to increase valve life by eliminating the dirt coming in through the carb Rich?
 

whenfoxforks-ruled

Old MX Racer
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Clean and inspect. Any figuring to short cut will compromise the function of the carb. IE., any filter or adapting to the air box, will result in changing the flow characteristics of the carb. Possibly another hot idea to detune the monster 450? Clean and inspect. I never KNEW where this debris actually came from. I was figuring the cable or choke seat. 1 intake getting tight, bad valve and or seat. Could be sucking dirt, could have been bad to start with?
 

Rich Rohrich

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_JOE_ said:
Do you think there is much potential to increase valve life by eliminating the dirt coming in through the carb Rich?

Probably couldn't hurt. There are a number of catch cans made for supermoto racing that properly collect all the carb vent tubes into a common can. Using one of these would give you a much greater path for the dust to drop out before it ever made it to the carb.

Check the supermoto section at www.mxsouth.com SME Racing is a good company for these parts. If MX South doesn't have them you can get them from www.motostrano.com.
 

_JOE_

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I'm interested to see if the valves hold up any better without the carb suckin in all that dirt.
 

_JOE_

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That was my train of thought. Dirt being titanium valves worst enemy, I would think even a very little bit would be bad for it.
 

whenfoxforks-ruled

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Honestly, I think the valve guides and rings to be hurt the worst from a dirt slurry sandwich. Joe, when you put new valves in did you have the seats cut? How many sizes from stock are you at? Clutch side intake?
 

_JOE_

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I'm still running the stock the stock valves. Both intakes had tightened when I replaced the piston over the winter. I was expecting a little better wear. The bike only has 60 some hours on it, and it really isn't that dusty in my area. The air filter gets cleaned every ride. Why did you pinpoint the clutch side intake valve?
 

whenfoxforks-ruled

Old MX Racer
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On both my sons hondas, it tightened first. The 450 ended up with a bad seat and consequently a bad new valve job, they did not get cut, just lapped. The 250 valve job was gold! 60 hours on a 250, pushing it a little? I am surprised at the kibble whites lasting a real long time after rebuilds, I have heard of none dropping off. Greg at Service likes telling the tale of hondas built in safety factor, stock ti valves just cup out and runs like crap when it is past time, guys with stainless, they fall off into the combustion chamber! Like I said though, I have heard none of this from Kibbles! What shim size are you at, over stock?
 

_JOE_

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I had to go .003" and .002" smaller shims on the intakes. The exhaust valves were in spec but at the bottom so I bumped them up to the top so I wouldn't have to mess with them again. I thought sure they would stay in spec till the second piston swap. The plan is to go KW s/s once I have to shim again, but I was hoping to finish this season before that happens. I will be riding more this year, maybe 80 hours or so. My local service guy says he will cut the seats for 110 bucks using a manual cutter. Is this sufficient, or should they be done on some sort of mill or something? I can r&r the valves and stuff myself. I just have this fear of it costing $1k+ to fix the head up before I hit the 100hr mark. :bang: If that's the case this is the last performance 4t that will find it's way into my garage. :|
 

Ol'89r

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Joe.

If I were you I would not reuse the ti valves. You have a lot of time on them and it would be pushing it to reuse them. Go with the Kibblewhite setup and you won't have to worry about dropping a valve. Plus the Kibblewhites can be lapped in and re-ground if necessary.

I would recommend having the seats cut with a Serdi or have them ground. Hand cutters are only as good as the person using them. If you have seen the guys work and if he comes highly recommended and if he has a new set of cutters, you may be ok. But, if the cutters are a little worn or have been mistreated, he could ruin your head.

Hand cutters can chatter and when they do you have to cut or grind the seat deeper to get the chatter out. This results in valve shrouding and can disrupt the flow in your head. I have had heads in my shop that guys tried to cut the seats with a hand cutter and completely ruined the head.

The dirt in the fcr carbs is a big issue out here in dusty ol' California. There are several companys that make filters for your breather lines. A good test is to hold a cigarette next to the breather lines while the engine is running and watch them suck the smoke into the carb.

Just my $ .02
 

_JOE_

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That's what worries me. The guy has a great reputation, but what if the tooling is old or he's having an off day when he does it? I think my best bet is gonna be to send the head to Eric and have him rebuild it with the Kibblewhites. Seeing as how this was the first shimming and they weren't extremely tight, I should be ok to run it till they tighten up again right? I don't have the extra cash to do it now or it would already be apart. Did I read that right when you said 60 hrs was lot?
 

whenfoxforks-ruled

Old MX Racer
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If it is the first adjustment, if you are with in 4 shim sizes of stock, it may well last a while. Projected ring life 15 hours, not changing the piston is not a real good idea in my book. Once you have been in you can estimate exactly how long you can get out of it. Running the rings and piston long will distort the bore. And check that big end bearing with measuring tools! I hope you fall into money and can Eric do some voodoo on the porting! Cam would be nice. So is the high compression stuff, but that is another matter.
 

_JOE_

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I just changed the top end with the Wiseco 13.5:1. Have 1.8hrs on it now. My concern is that the valves won't make it through this top end. I'de rather bite the bullet this winter and do it all at once. I changed the valve springs when I did the piston. I'll link to a thread with pics of the engine. It showed some skirt wear on the piston. I think I'll be pulling it down around the 60hr mark again, hopefully the wiseco wears a bit better. I did switch oils after the rebuild to see if it wears any better. I'm very happy with the power with the higher compression. I hear the 06 has good top end porting and I'm really happy with low end, so I'm not sure if I'll mess the porting. I know you can find a good bit of low end by epoxying the ports. I guess I'll talk to Eric about it when the time comes. Thanks for everyones input on this!
 

whenfoxforks-ruled

Old MX Racer
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The worst that can happen is it will run like crap, hard to start. Hopefully? Best of luck on that adventure. Eric did have a 14.8 piston, more low end. When you ask Eric for mo power everywhere, and race gas. It makes insane power. The valves, as long as the person does respectable work. Them pics told a story of one rich 4 stroke. I know a guy with a wide range o-2 sensor, and the plug.
 

Ol'89r

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_JOE_ said:
. Did I read that right when you said 60 hrs was lot?

Joe.

It all depends on your riding style. If you are a fast intermediate or semi-pro then 60-80 hours is a lot. If you are a beginner or novice you could probably get away with more time. This decision can only be made by you. If the valves have only been adjusted one time, you could run them again and check the clearance often. If the clearance starts to close up faster than normal, change them asap. If you can't afford to buy the Kibblewhite set-up right now just replace the ti valves with new ones. They are about $35 bucks each and will give you more security than running the old ones.

If you bite the bullet and go with the Kibblewhites, you won't have to worry about it. I personally would rather spend the time and money to do it right than have to hold my breath every time I rode the bike. If you do lose a valve it will cost you big time.

In regard to the dirt in the carb. Dirt will cause more problems than just wearing out the rings and guides. The coating on the ti valves can be worn off from dirt entering the carb. Once the hard coating is gone, the valves will cup, (sink into the seats) and wear excessively. If your clearances start moving faster than normal, this is most likely the cause.
 

_JOE_

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WFFR, I have no interest in running race fuel and I can't use all the power the bike has now so I think I'll stick with the 13.5:1 and pump. I used the seat of the pants dyno to set the main. Any leaner and it lost top end pull. What's the negatives of running it rich?

89'er, I just rip it around the woods and play in some gravel pits and a little mx here and there. I don't wring it out alot. I'm probly average to a little below average speed. It has alot more bottom end now with the high comp piston so I try to take advantage of that and keep it in the midrange as much as possible. Hopefully this will help prolong the life. I've gotten pretty proficient with checking the clearances so I don't have a problem doin it every few hours. It only takes about 20 minutes. So far so good after almost two hours. The issue of the ti valve coating being erroded by dirt is why I wish I had known about the dirt coming in through the carb earlier. I guess only time will tell how they are going to wear in the future.

Indy, I saw that in a recent isuue of Dirt Rider. Looks like a decent setup. I'm gonna give the rc fuel filters a try as I already have a couple and they're pretty cheap. We'll see how they work.

Thanks again for the input guys. I'll keep ya posted on how the little red rocket holds up. :cool: :ride:
 
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