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Let's talk about Valve Floating?

1587 Views 27 Replies 13 Participants Last post by  Kurlon
Today I was putting my finishing touches on my original CT70 build.

ATC125m Engine with S&S exhaust, Vm26 carb and matching TB intake. Running a Webcamshafts #3 grind cam on a good port, but not overly agressive.

First, can someone please explain the symptoms of valve floating in these engines?

I have never experienced valve float, so maybe someone can help me get this in tune...maybe it is not even float.

I have the pilot circuit good, running a 25 pilot. Needle in middle. No matter what main I put in, from 150 to 230, at about the 6-7k point, the engine starts rapping a bit. It is not a bog. If I go really slow on the throttle, I can keep accelerating a bit, but then it does it again.

Now, while it is sitting there without me on it, it revs right to the moon, but when you get me on it it does it around that 6-7k mark.

What I've tried. Well, I've tried all those jets. The original jetting was 15/150 before the cam and port. I've tried advancing and retarding the ignition. I've verified the valve timing is correct. It is getting gas just fine and I've adjusted the floats a few times as well.

So, what do you think this could be? Am I really experiencing valve float? From 1200-6000 rpm, this thing pulls your arms off..power wheelies on a CT70 without trying. But then it starts rapping. Let's see what this is.
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I dont think I've ever experienced valve float on one of these. I cant imagine it happening at that low an RPM. Does it not even rev past 7K under load? Not too familiar with s+s exhaust, have you tried a different pipe on this setup. Or that pipe with a different cam?
Originally the setup was a vm22 carb, stock head, and stock cam. It ran perfect for over 2000 miles. I wanted some more get up and go so I ported the head and installed a new cam and got a bigger carb. The exhast will flow this no problem.

It pulls like a train all the up to about 6-7 (it is a guess as I have no tach) but then it just pops and bangs. If I go really slow it will rev a little more. Under load is when this happens. It will rev out much more if you are just in neutral.
I've floated valves before, but I was also spinning 13k to 14k at the time. Think marbles in the top end and power loss, not the defined pattern of a rev limiter.

Silly question, are you sure the tach is reading correctly? Depending on the aggressiveness of the cam and the springs you're using with it I can see hitting a wall at 6k * 2 (12k RPM).

The other oddity I've run into that may or may not fit the pattern... Had an E22 head ported out enough that on a 107, it wouldn't run with a stock cam. You could get it to fire up but no amount of jetting changes would make it take throttle. Swap in a Z40, and the bike cam to life. I don't recall the details of Webcam's #3 grind so I can't tell if it's mild or wild.
Im wondering if your springs are long enough....where your motor wants to twist but the springs are bottoming pre maturely
Im wondering if your springs are long enough....where your motor wants to twist but the springs are bottoming pre maturely

If the springs aren't tall enough, but aren't spring binding at full lift you'll have reduced to no compression. The bike would run terrible if at all at idle, but come into it a bit with load and revs as combustion pressure would help press the valves shut better.

If the springs are binding, well, thats a short lived problem that'll show itself in short order.

Given the updated info that the motor ran like a top with a VM22 but won't take the 26mm setup, I'd be inclined to think bad carb? It's acting like it can't generate any vacuum, but it should be big enough and cammed to pull that 26 without this much trouble.
Well, I know it is not running 12k and popping. I am telling that by my top speed. Before I could run 55. Now I can't, but I have much faster acceleration and power at the lower RPMs.

Springs...I am using the stock springs in there right now. They are about as heavy as the dratv heavy duty springs with wider coil spacing. They are longer though. I didn't think they were binding, but who knows what happens at 6k or 7k.

As far as sound goes, I think that maybe marbles in a can could be the sound. It reminds me a little of an overly rich condition a little and its loud and poppy. And again, only under load. In neutral it cranks out fast and quick. Under load, no matter what jetting i put in there, it does the same thing.

I'll see if I can get some video so you guys can hear it.
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It may be float then, free reving you'll be able to wind higher than under load. What kind of ramps does the #3 grind have, steep or mild?
Pretty wild. Have a look at for the atc90/110/125
Yeah, I think you're floating them based on that info. They even suggest uprated springs for that cam. I think it's time for new ones, $20 for a new set, why not?
Kurlon said:
Yeah, I think you're floating them based on that info. They even suggest uprated springs for that cam. I think it's time for new ones, $20 for a new set, why not?

Well, the ones that are included in that kit don't fit is why I don't have them on. The inner springs are too small to fit over the valve seal, so I had to send them back. They still haven't refunded my money either.

The problem is I wanted to verify that I needed new springs before I find some custom application that I have to figure out to work. If it was a mount and go operation, I'd have em me.
Grounds are fine

blackjer said:
Check your ground wire.

All the grounds are fine...I didn't change up any of the wiring from before.
fatcaaat said:
All the grounds are fine...I didn't change up any of the wiring from before.

You never know when a wire can go bad. You dont want to spend hours looking at the engine to later find out it was a stupid wire. Only takes a second or two to check.

I had a loose wire before and it caused the same problems you are describing.
Your description kinda reminds me of an ignition problem, compresion pressure creating to much resistance for a weak spark to jump or maybe high resistance in the points (I hope your not running points, they are from the devil :) ) But I've also seen automotive application float just like what you describe, free rev fine but under load it couldn't rev past 4k. I've never had a problem with valve float on any honda motor I've messed with.

Just throwing out some ideas, hope it helps.
These things come to mind .

- Cam timing

- Valve over lap is too severe

- Ignition system is break'n down . ( coil , coil wire , plug ) not able to handle the flow of current on hard accel with a load to it .

- Kill switch / ignition has partial ground out or poor connections for maximum current flow.

- It revs so quickly now that your cdi box isn't made for such high r's ?

- Over carb'd .
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my guess is the carb, try putting the 22 back on just to see if it would go past the RPM its dieing at, if it does then you know it has nothing to do with the valves. At least if you do this you eliminated one possibly.
Motarded said:
Sounds like crappy tuning to me.

I wish it were crappy tuning on this one...that's easy to fix. I found a place that carries old-school valve springs and they had a set for my application. I'll get them in and see what happens.

On the tuning, again, I'll say that I've tried every main from 150 to 230 and at 220 I actually got some bog. So, I'm very confident that the tuning of the carb is correct or at least close enough that it would not do this.


New Plug...check


I'll try the old carb just to see what happens.
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I doubt it's over carb'd. If that were the case the bike would have a hard time at low speed and come into it as the revs climbed. This thing sounds strong on bottom, and looses it in the mid, so it's getting plenty of signal to the carb.
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