Tight intake valve

[Fistfullofthrotl718]

Can tight intake valves cause false lean symptoms? Or will it just make the bike run lean period? Lean symptoms such as popping, "blip" the throttle and a tad bit of a long time to return to idle, header starts to glow quickly, diminishes when riding around the block but still emanates a faint glow. I ordered a jet kit but its not going to be here for a while. I'm picking my shims up friday. Wondering if I'm going to have to grab a few random jets also.

Elevation - Sea level

Jetting - stock

Temp - between 65-70f

Humidity - 50-75% (today) weather will be similar to all this on Sat when I want to go but I'm not sure what the humidity will be then.

Shim clearance (current intake) - .07, .10, less then .03 (smallest feeler I have is .03)

[ronbuell]

A lot of people have looked at this, but no one has attempted an answer.... so here goes.

I'm thinking it would cause a lean condition since some of the charge would regurge back up into the carb with an intake that is open during compression stroke. Engines that have tight intake valves typically do not start easily and will have a black streak into the intake tract showing some burning of fuel in the intake rather than in the combustion chamber. On a high performance engine, the engineers typically tune the cams for large overlap and timing that favors high power in the high rpm range. When the valve clearance is out of spec. then the intake pulses going into the combustion chamber are interrupted and you don't get a complete intake charge into the cylinder.... so yes it would result in a lean condition in that sense. But the popping you get is the intake charge firing up the intake into the carb too. I had a 79 KLX 250 that had a terrible habit of "caughing back" up the intake at low rpm. That was the result of a bad intake tract design catching up with the intake charge speed at low rpm. The only way to fix that one was get the head ported and a lot material removed. The Yamaha is already a 3 valve design with a lot of flow area. So when some of the intake charge can go backwards, it causes bad stuff to happen.... it will eventually stop running if the valves will not fully close. I've heard some of them run when you could actually hear a churping sound coming back up into the air filter box. When they get like that, you have to fix the clearance because its about to stop running.

[Fistfullofthrotl718]

I noticed no one really wanted to touch this one either. I wasn't sure if maybe i was just asking a stupid question. I mentioned before that its one thing to be able to replace a part, and it's another to understand how it works. I can't tell you how thankful I was to see a response to this question. I couldn't find my answer anywhere. Not only did I get a yes or no answer, but more importantly, to me, I got the "because....." Is there place to find answers to these tech questions short of course at MMI? lol. Anyone suggest and good books on the subject? The service manual is great for the dis assembly and reconstruction but again, its just the how, not the why... Thanks again Ron for the info. It's much appreciated!!!

[BIG_FAT_&_SLOW]

Dude don't look at the header to ever explain jetting or anything. In my past experiences when my intakes have been tight it generally made the bike harder to start when cold but then would run normal the rest of the day. If you have identified that a valve is tight don't try to do any jetting work until the valve is shimmed correctly. I always tried to shim to the loose side of the spec also. One thing to keep in mind is that if you have to shim a valve then you are going to have to pay more attention to it going forward. Once I've shimmed a valve it is usually just prolonging the replacement of that valve. So just keep an eye on it.

I forget...what are the specs for intakes isn't it .04-.06 your others are to loose

[BIG_FAT_&_SLOW]

Oh yea hanging idle is a sign of being to lean on the pilot. Turn it out in quarter turn increments till it goes away. Rule of thumb more than three turns out means you need next pilot richer. What year is your bike

[Fistfullofthrotl718]

Its an 05. I just shimmed the valves this morning. Am I mistaken on clearance? Exhaust is supposed to be .17-.22mm and intake .10-.15mm cold measurements? My air screw is already at 3.5 turns out which I know means I need a richer pilot. Bike started first kick even before I shimmed the valve, which from all the other posts I've read about this, is a little odd. Then again, I'm brand new to the 4 bangers so this could just be nominal. My main concern is this. I'm just afraid that changing the jetting might make the bike run better, but just be masking another underlying problem. I just got the bike and don't know who did what as far as maintenance is concerned. Of course most people would say, yea, I ALWAYS do my maintenance on time, but you never really know unless you look for yourself I guess. I'm just not sure what to be looking for.

On a side note, if your valves were "peened" over, is this something that is visible with the naked eye? Or is it something that can only be detected with the proper measuring tools? When I cleaned my head and piston of the carbon, I took a look at the valve heads. They seemed to have a consistent taper and I didn't notice any mushrooming.

[BIG_FAT_&_SLOW]

Sorry I always do valves in inch measurements! If you got your valves in spec then just move on and keep an eye on them at regular intervals. Just work on the jetting independently it won't mask any other problems. What pilot is in your bike now a 42?

[ronbuell]

You can do what I do. I got a caliper from Harbor Freight and a set of multiple shim sizes. Then I converted the spec range to inches. Should come out to between .007" and .004". You insert the shim and install the bucket and cam, and measure the gap with the feeler gauge. Insert the smallest feeler gauge and feel for loose or snug fit and go from there. If the gauge is loose, move to a larger feeler until you find the actual or near clearance. A tight clearance needs a smaller shim, and vice versa. Note the feeler you used, then measure the shim that is in there. Vwa La, the shim that is nearest that total is your shim. You may have to go one size either way, but do go on the loose side since they will close up a little as they wear in. Good luck.

[2grimjim]

In my experience, valve clearance does affect carb jetting to some degree. Even if the clearance is within the limits of adjustment. Thats why I always recommend valves be checked before making jetting changes. Obviously, if the valve clearance is too far off the bike is hard to start or wont start at all.

The bad thing here is this; once your valve clearance goes to zero (or close enough that you are have cold starting problems) it's time to pull the topend off and replace the valves. You might be able to re-shim and get a few more rides, but the valves are done with.