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From: Dennis on 19 Sep 2008 16:18
I recently purchased a '83 honda vt750c. When I got the bike, it was
leaking a small ammount gas from the carbs when leaning on the kickstand
after riding. I rebuilt the carbs and set the float levels and air/idle mix
to the factory specs. The bike runs real strong and idles perfectly.

I have noticed since it has gotten cooler ( 60 deg F ) the bike has a
stumble going from low throttle to full throttle when driving. It is most
pronounced when crusing under 2000 rpms and suddenly given full throttle.
When I do this, the engine stumbles for about a seconds before pulling
strongly. When the temperature outside is above 80 deg, the stumble
dissappears.

I have tried opening the air/idle mixture screw a bit in the cooler weather,
but it doesn't seem to make any difference other than at idle.

Would syncing the carbs eliminate the stumble? Any other suggestions?






From: Who Me? on 19 Sep 2008 19:09

"Dennis" <cctden(a)sbc_global.net> wrote

> Would syncing the carbs eliminate the stumble? Any other suggestions?
>

Maybe but probably not.

Running some good carb cleaner through, to get what you missed in the tiny
passages, might help.
Original Gumout Carb and Choke cleaner or Berryman's B12 are good ones. Put
twice the recommended amount in 1/2 tank of gas. If/when it starts to get
better, fill the tank the rest of the way.

And, yes, that mixture screw you have been turning is only for (near) idle.

HOWEVER............

I think the real key here is that you are trying to make it do something
it's probably NOT going to do.

Whipping the throttle WFO from 2K is asking a lot from a 25 year old
engine.......unless you are in first gear maybe. I'll bet it "jumps" just
fine from 4k........or maybe even if you take a full second or so to get to
WFO from 2K, instead of .2 seconds.

It is, after all, not a drag racer; wasn't when it was new. ;-)






From: Dennis on 19 Sep 2008 19:52

"Who Me?" <hitchhiker(a)dont.panic> wrote in message
news:agWAk.708$D32.295(a)flpi146.ffdc.sbc.com...
>
> "Dennis" <cctden(a)sbc_global.net> wrote
>
>> Would syncing the carbs eliminate the stumble? Any other suggestions?
>>
>
> Maybe but probably not.
>
> Running some good carb cleaner through, to get what you missed in the tiny
> passages, might help.
> Original Gumout Carb and Choke cleaner or Berryman's B12 are good ones.
> Put twice the recommended amount in 1/2 tank of gas. If/when it starts to
> get better, fill the tank the rest of the way.
>
> And, yes, that mixture screw you have been turning is only for (near)
> idle.
>
> HOWEVER............
>
> I think the real key here is that you are trying to make it do something
> it's probably NOT going to do.
>
> Whipping the throttle WFO from 2K is asking a lot from a 25 year old
> engine.......unless you are in first gear maybe. I'll bet it "jumps" just
> fine from 4k........or maybe even if you take a full second or so to get
> to WFO from 2K, instead of .2 seconds.
>
> It is, after all, not a drag racer; wasn't when it was new. ;-)
>

When I rebuilt the carb I soaked it in carb cleaner for a few hours, so I
doubt carb cleaner would do any good.

Maybe I'm just expecting "perfect" throttle response, which is probably
unrealistic for a 25 yr old bike.. I am pretty impressed, as it does pull
away from my friend's 04 Sabre 1100!! Go figure!


From: . on 19 Sep 2008 21:26
On Sep 19, 1:18�pm, "Dennis" <cctden(a)sbc_global.net> wrote:

> I have noticed since it has gotten cooler ( 60 deg F ) the bike has a
> stumble going from low throttle to full throttle when driving. � It is most
> pronounced when crusing under 2000 rpms and suddenly given full throttle.

There are usually three acceleration transition ports right where the
bottom of the throttle butterflies touch the bottom of the carburetor
body.

When the butterflies are closed, you can see *half* of one of the
holes, and that half a hole, combined with the single idle mixture
orifice about an inch further downstream supply *all* the idle mixture
you need when the butterflies are closed.

Now, when you just crack the throttle open a smidgen, the engine
vacuum drops off and there isn't enough vacuum to raise the vacuum
slide and it wouldn't matter if the slide would raise a quarter of an
inch or so when you twisted the throttle, because the upper portion of
the needle isn't tapered enough to draw fuel up through the needle
jet.

When you open the throttle less fuel air mixture is drawn through the
idle mixture port, but more mixture is drawn through the three
transition ports.

Adjusting the idle mixture properly requires that the mechanic know
about the transition ports and how they affect mixture just off idle.

If you have adjusted the idle mixture screws too far open, and then
adjusted the idle speed screw to compensate for an overly rich
mixture, the idle speed will race up to 3000 or 4000 RPM, especially
when the engine gets hot.

Soaking the carbs won't clean out the idle mixture passages, you have
to squirt aerosol carb cleaner through the pilot air jet and watch it
squirt vigorously out the pilot jet, the three transition ports and
the single idle mixture orifice.

And, it's necessary to put your fingers over some of the orifices to
get enough pressure to vigorously flush out those passages.

If I'm wrong, and all those passages are pefectly from your previous
efforts, you may have a tiny tear or pinhole leak in one of the vacuum
diaphragms.

Also, check the carburetor parts fiche at www.bikebandit.com to see if
there was a small rubber o-ring on top of the carburetor to allow the
engine to suck the air out of the vacuum chamber.

Those small o-rings sometimes get lost in the cleaning process and the
mechanic cannot understand why the vacuum slides don't lift...

From: Dennis on 22 Sep 2008 01:00
Thanks alot... I have a much greater understanding of how these carbs work.

>If you have adjusted the idle mixture screws too far open, and then
>adjusted the idle speed screw to compensate for an overly rich
>mixture, the idle speed will race up to 3000 or 4000 RPM, especially
>when the engine gets hot.

I currently curretly have them set at 2 1/2 turns out. If I open them to 3,
the idle is rougher and the hesitation is reduce. At 3, the bike idles at
~900 RPM a couple minutes after it is started and ~1200 when hot.


>If I'm wrong, and all those passages are pefectly from your previous
>efforts, you may have a tiny tear or pinhole leak in one of the vacuum
>diaphragms.


I was pretty vigerous in the cleaning. Soaked, sprayed, let sit, sprayed
again, compessed air... I looked on the fiche and I didn't see any
diagrapham.
http://www.bikebandit.com/houseofmotorcycles/showschematic/m2877sch3877 Is
the diagraphram part of 13?

Prior to rebuilding, the float level was to the point it was leaking gas
when on the sidestand. The idle screws were set at 2. The bike seemed to
have a little more power, and the idle was rough and speed varied quite a
bit based on how hot the engine was. At the time it did not have the
hesitation.

I currently have the float level at .3" ( float bowl mating surface to the
bottom of the float ). Prior to rebuilding it was .17"

Would raising the float height possibly help? I'm thinking .25? Would doing
this make the bike run richer throughout the throttle range?







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