Topic: Tuning the transition from idle to... um, not idle

[grumpy]

I put a hot-ish cam (webcam 41a* ) in my K3 & now it always burbles right at the point where the slides just start to open.
Not a big deal when accelerating from a dead stop - it's fine if you twist the throttle a ~ quarter or more.
Where it's an issue it on the freeway - you know how when you're rolling along in traffic you'll coast for a bit then give it just a tiny bit of throttle to maintain 65mph? That's where it burbles. Makes for a choppy ride...
Clears up the instant I give it a twist but that doesn't work out too well when there's a car 15 feet in front of me.
I've tried to fix it with the air screws but it doesn't really help.
It idles fine, BTW. 
Floats are set to spec with an official Honda slide rule and evened out with a sight tube.
So what's next?
can't be the needles, right?
Is it just something I have to live with because of the cam?



 
*yeah Mark, I know you like Megacycle but this was free!

[flybox1]

what do a set of brand new plugs look like after a 3-5 minute idle?

[harisuluv]

zero to 1/4 throttle is dominated by idle/slow jet and the mixture screw.  When you get to 1/4 (which is where it cleared up) it's mostly needle until 3/4.

So if air screws have no effect, I suppose you could try bumping up to a 42? 

That is assuming you started with 40 and all other variables are known good, synced, etc.  And I'm basing this on you saying that the only thing that changed was a camshaft change.

[grumpy]

what do a set of brand new plugs look like after a 3-5 minute idle?

duh, yeah I should have checked the plugs. Will do.
then we'll see about new slow jets.

yeah, all else is basically stock, points, airbox -
EXCEPT!!  it has a Hondaman mild porting / blue printing job so now as I think about this, I bet the vacuum pressure at idle it on the low side - but I had the cam before the porting job & the problem seemed to come with the cam.


 

[TwoTired]

Maybe Honda can help?

[DH]

http://www.cyclexchange.net/Engine%20Tech%20Tips%20Page.htm



Anyone ever read this? Scroll down about 2/3 down the page or so to the part
about carbs and performance. They don't  mention anything about cams tho..
Pertinate or no?

[grumpy]

well, I'm certainly not gonna start filing away at the slides...


TT - I think we spoke about this a few years ago... I think it was you who pointed out that while riding in the situation above the engine is turning 5,000 RPMs and therefor sucking in a whole lotta air - or at least trying to. So if the slides are shut then the vacuum pressure must be huge and the little bit o' air that is getting in has to be screaming through the throats - so then gas must be coming up the jets like a drinking fountain even with the needle all the way down into the jet. So then it must be running super rich.
Whereas at idle - with the carbs positioned exactly as above - the engine is only doing +/- 900 RPM and so the vacuum pressure must be way less, which means less gas and a proper mixture.
Right??

[TwoTired]

Do you know what the cam timing numbers are for the cam and how they are different from stock?  Back in the day, putting hot cams in V8's ruined the smooth idle and the vacuum at idle because of valve overlap and early intake lift when the piston was still coming up.  The cam didn't really work until the RPM's were up, and the race guys didn't care about idle or streetability of the engine.  There's a real reason why race cams weren't put in factory street vehicles.  It's called "trade-offs".

You do realize that the vacuum at the carbs is pulsed and you usually have to average the pressure pulses for a vacuum reading.

The carb responds to both air velocity/volume AND the "vacuum" generated by the falling piston in the cylinder.


Anyway, burbling is always a sign of over rich, in my experience.  Does adding a bit of choke make it worse or better?
Are all the spark plugs showing exactly the same deposit patterns?

Further, if you are going to fine tune the carbs for street driving, you really need to mark the throttle so you know exactly which position it is in when the problems occur, as that will tell you which area of the carb that needs tweaking.

You HAVE already done full power WOT testing to ensure the main jet supplies the correct maximum fuel, right?

[grumpy]

   
Intake 1st number, Ex 2nd

Valve Lash (Inch):
.004"
.005"
Valve Lift (Inch):
0.335
0.335
Valve Lift (mm):
8.51
8.51
Advertised Duration:
275°
275°
Duration @ 0.050":
242°
239°

Choke doesn't really do anything until about 1/2 closed then it bogs - certainly no "improvement spike" before it bogs, if you know what I mean.

HELL YEAH I've done WOT checks of the mains! That's the fun part! I "test" that throttle position as often as possible!

It occurs to me that I haven't go through the carbs in a long time. And it could likely use a tune up. I'm gonna do all that & then put in new plugs and do some chops...

[TwoTired]

   
Intake 1st number, Ex 2nd
------------------------Stock Numbers
Valve Lash (Inch):
.004"
.005"
Valve Lift (Inch):----------
0.335------------------------0.315°
0.335------------------------0.295°
Valve Lift (mm):
8.51
8.51
Advertised Duration:
275°------------------------215°
275°------------------------220°
Duration @ 0.050":
242°
239°

When you degreed your cam, where was it set relative to TDC/BDC?
Analysis so far....
The stock cam began opening the intake @ 5° before B.T.D.C.  The new cam has 60° more duration.  How much earlier is the intake valve opening?  Realize that opening the valve earlier while the piston is still on the rise, reduces the average intake draw, particularly at lower RPMS.  Additionionally holding the intake open after the piston has past BDC, also reduces the draw.  The cam you have chosen relies on the mass velocity at higher RPM in order to provide volumetric benefits.  At low RPMs the cam profile actually hinders volumetric efficiency.

Choke doesn't really do anything until about 1/2 closed then it bogs - certainly no "improvement spike" before it bogs, if you know what I mean.

That and the burbling say it was too rich at the throttle setting where it occurs.

With the cam profile change, I'd theorize that some intake charge is being spit back into the intake manifold at low RPM, making the next draw cycle too rich. I also need to know what the throttle position is where the complaint is realized.  I can't can't say more without knowing the cam/valve timing relationship with cam/lobe position.  But, as it stands, you may be able to improve low end response with a cam timing change/refinement.


Could you state the current exhaust and induction set up?

[grumpy]

cool, thanks TT

I set the cam per the manufacture's specs: piston at TDC (used a piston stop to find it) and cam locating grooves lined up with horizontal split in cam bearings.

HM341s and stock air box (K&N filter) if that's what you mean.

as for throttle position it's maybe 5 or 10 deg of rotation - just at the point where the slides begin to rise.

[TwoTired]

Maybe you have to live with it.   Unless you do a finer degreeing the cam process.

You could try to make the pilot circuit leaner.  Either use smaller pilot jets, turn the air bleed screws way out, or make the air jets larger.

Wait, I should ask if you are using Keihin pilot jets or aftermarket.  I've seen aftermarket jets that use a different emulsion tube hole pattern than Keihin.  This could certainly change the emulsion ratio and may not allow the air screws to properly compensate.

The risk of making the pilot circuit leaner is throttle response wheeze if you twist the throttle too far, too fast.  (due to no accelerator pump)  I get the impression that you can whack the throttle and never get wheeze or stumble.  Just a bit of burble which clears up by itself with more throttle position twist.

Normally, you can only twist up to one half of remaining throttle without getting stumble/wheeze when at low RPM.  (top gear from idle RPM.)  If you can do full throttle rotation and still get acceleration, it's another sign that the pilot circuit is too rich.

[grumpy]

I think I have keihin slows. need to confirm, though.
It'll still do the typical stumble if I go past half throttle from idle.
I'll give it a full tune & carb cleaning & start with the air screws again.


You're right about the it using mass velocity at high RPMs. That thing PULLS once it hits 6,000.