Loose or Tight Valves By speedracer (Read 25,069 times)

[speedracer]

We always here of doing a valve adjustment on our scooters. Maybe someone like JR can explain the effects of valves being out of adjustment such as:

Loose or tight intake valve. What would a rider notice in his scooter if the intake valve is too loose or too tight?

Loose or tight exhaust valve. Same question as above.

Thanks

[JR]

Basically the valves are one of the most critical parts of the puzzle as to a scooter engine running right. Sounds crazy but it's just a fact.

On a scooter one rarely hears someone say too lose but too loose and the valve timing changes, so the engine sounds rattly and runs rough. If you hear a clickity click then you're too loose.

Too loose on the intake valve also means it's not properly seating and one risks loss of compression and blow back of fuel gases to the valve cover area, extremely loose means no compression at all which means no start. Can even backfire.

When I say to loose on the intake valve I mean the rocker is not properly seating on the valve and it starts to hammer the end of the valve. Also that compression is lost because a non-opening valve on the intake side won't let in air so if a valve is closed all you would be doing is pumping a vacuum in the cylinder.


If the valves are tight, they just lift off the valve seat slightly, so they don't fully close. This just makes for leaking gases. The exhaust gas is the problem here. If it leaks past the valve continuously on each stroke, it heats up the valve (which needs to sit down on the cooler seat to shed some of it's built up heat) and so the valve stretches, which causes it to be opened more, which creates a worse leak in theory.


Also if an exhaust valve doesn't open from excessive clearance the only place exhaust gasses could go on the exhaust stroke would be down past the rings and into the crankcase.

Also if one runs too long with either valve too tight one wears excessively on the cam lobes.

But on a scooter if the valves are too tight especially the exhaust not only does it run rough but one will lose valuable vacuum for the fuel pump as well if you have the vacuum type.

Signs of tight valves is stalling, poor top end, dying at every stop, hard to start and loss of power and MPG.

JR

[speedracer]

Jr, Thanks for a very informative answer. I think this will help a lot of scooter riders by noticing some symptoms.

Now after reading what you said I have to wonder if my exhaust valve is too tight. I have great top end as I pushed it to 70 today. It idles smooth as well, and sure seems to be pumping enough fuel into the engine.

What concerns me though is after it has started from being cold, it will stall once at the same stop sign every time. This stop sign is about 1/2 mile from my home. The first stop sign it idles at about 2000 rpm's and off I go. Now if I don't come to a complete stop at the trouble stop sign I can keep on going strong. Here is the funny thing after sneaking by the stop sign where it always stalls, I can run it for about a mile where there is a traffic light. Now if the light is red the darn scooter will stall there. I can start it right up and from there on it will not stall again.

I have no clue at all what the problem would be. No matter what setting the A/F screw is at or throttle screw it will still find a way to stall once.

[oldputz]

Last valve adjust I thought the intake sounded a little loose, as in taka, taka,but have tightened up some though. And yes, no more stall outs at stops.

[JR]

How many miles now Speedracer? Mine had to adjust again at around 5500 miles and she now has over 9k and is still doing fine. I think the more miles the less they have to be done but who knows.

I just know when the signs show up I get ready to skin a finger or two! LOL

JR

[speedracer]

7000 JR and its so hot here in SC.

[JR]

Yea it's hot as hades here too, just came in from the garden already 94F and sweat a pouring!

7k yep I'd say time to get those knuckles dirty and if that exhaust valve is tight and you get her adjusted right, then look out she's going to run like a scalded ape! MY linhai didn't even peak until over 6k, runs stronger than when new!

JR

[speedracer]

JR have you found a way of lifting the scooter higher off the ground. If I could get the center stand 4 or 5" higher of the concrete floor it would make getting at that exhaust valve easier wouldn't it?

[mheinemeyer]

Hi JR... Your initial explanation was just fantastic. Any way of posting that in the TECH LIBRARY?

[bhinch]

Hi..guys and gals...the valve lash is required because our scoots do not have hydraulic lifters (cam followers) so we have to adjust them to .004 intake and .005 exhaust....this allows the valves to close completely thus giving us cylinder pressure (compression) intake...compression...power...and exhaust ...a 4 stroke engine....when the valves do not close completely we lose compression and on the exhaust valve a burnt valve could result...it is not overnight...but it can happen... the valves rotate everytime they open and close... most of the scooters come from across the pond with the valves too tight...no clearance...so they run bad or do not run at all.... no vaccum no fuel pump....so like JR said....this is a very important procedure to take care of....it shoud be on the PDI to be done first...this solves alot of the problems from the beginning.....enjoy the ride....

[JR]

Yes! We'll get all the input and then put it together in a good explanation and send it to the tech section for all to use.

That's what we all need to do put great info together like this and use for those who don't understand, a helping hand!

Thanks Speedracer for starting this thread!

JR