Round head fueling

[GuzziNZ]

What would cause a surge/stutter under load and full throttle from 4 to 5000rpm in a 850T?
From 5k on its clean all the way up to redline.
Starts with one tap on button and smooth idle and town running.
Bike had been running great, removed carbs when I had crank balanced and problem started after fitting.
Excellent compression and have changed complete ignition system.
VHB30. Standard Jets, needle at second notch, 10g floats set a 24mm. New red tip float needles.
Choke plungers have depressions marks in rubbers but no signs of over fueling.

[moto]

Surge and stutter seem like two different things to me. Can you describe the symptom in more detail?

[wirespokes]

The question I have is what was done before the symptoms arrived? It was running great, then the engine was torn down to the crank, rebuilt, and at the same time the carbs were overhauled. That's quite a lot to think with. Even though nothing but the crank balance was changed, in a way, everything was. So forget about finding what was changed by looking back and fixing it, since everything was.

It's highly unlikely the problem is electrical, most likely carburetion. Something in mid-range jetting I'd guess.

[GuzziNZ]

Is more like a stutter or goes fluffy if you want to be technical!
Borrowed a dyna system to rule out ignition and dialed up the cam timing again.
Had a closer look at the choke plunger,s and they are not that good. Have turned seals over and will give it another go.

[GuzziNZ]

Still no good. Only thing I am noticing apart from not wanting to rev is to get the best idle the RH mixture screw is only one turn out. Book says 2-2 1/2. LH is 2 out as per the book.
Also the exhaust gas out of the RH muffler is warmer!

[guzzisteve]

To find out what your carbs are doing check throttle settings not RPM. 1/8, 1/4, 1/2, 3/4, and pegged wide open. You also must hold it at those settings w/o moving the grip for 2-3 miles then cut ignition w/o changing the grip  position. Start with new plugs.
When you change pilot jet to a bigger size your idle mixture screw will be less turns out. This is all depending on what intake & exhaust flow you have.

Mark your throttle w/masking tape and lines on tape so you know where you are at not RPM

While riding and issue happens look and see where you throttle is, now you can fix the effected area.

A 50 pilot will be in the range in book for mix screws, a 60 will be 1 1/2 turns out down to 1.

[TOMB]

Don't overlook the fact that you could have put the idle jet and the main jet in the wrong location They both will screw into the locations.They could
You wouldn't be the first one that put him in the wrong location

TOMB

[moto]

If your problem is really at wide open throttle, no carburetor jets are involved except the main jet and the tip of the jet needle as it sits inside it. Idle jets, etc., mean nothing at full throttle.

You probably didn't completely change your ignition when you reverted back to the Dyna. Maybe you have a bad spark plug gap, or a bad spark plug, or two of each. Maybe you have bad coil wires or coils. Your problem is occurring at the highest load situation for your motor, that is, where it makes the most torque. This is where sparks have the most trouble being propagated and spreading through your mixture.

[moto]

If your problem is really at wide open throttle, no carburetor jets are involved except the main jet and the tip of the jet needle as it sits inside it.

I need to emend this. It is only the main jet that is governing the mixture at full throttle. The jet needle tip is sitting inside the needle jet, but its tapered end is small enough so that the annular area between it and the needle jet exceeds the area of the main jet. This means the main jet is what governs the amount of fuel that goes into the mixture. That was my main point.

[guzzisteve]

There is one thing it may be, maybe accell pump. It is in between the main and the tip of the needle. Maybe it needs that enrichment to work w/o issue
It continues to squirt till the needle clears the plate w/holes. Could be spring is broke then check ball does nill.

Trying to think outside the box , which this is way out.

[Dukedesmo]

I had a stutter/splutter on my LM2 that would occur only at or near to full throttle, it just appeared from nowhere after previously running fine.


Turns out there was a small amount of grit/rust in the float bowls, presumably came from the tank? It seemed that it mainly sat harmlessly in the bottom of the float bowl until, there was enough 'suction' (at full throttle) to lift it up and partially block the jet.


May not be the issue here as it sounds like the OP's carbs have been off and cleaned but don't discount it as mine had recently been overhauled and I think some loose crud had just been dislodged from the tank, maybe because of removal and shaking it about?


This was a few years back and I've been meaning to fit a proper in-line fuel filter (the small screens in the taps obviously aren't very effective) but I'll admit that I've not got around to it yet as the problem's not re-occurred so; out of sight, out of mind. 

[jacksonracingcomau]

What would cause a surge/stutter under load and full throttle from 4 to 5000rpm in a 850T?

Bike had been running great, removed carbs when I had crank balanced and problem started after fitting.
Excellent compression and have changed complete ignition system.

No need for wild imaginative theories IMHO
All clues are in this first post.
The only thing to ascertain is what exactly was changed during this process
Undo that and logic says bike will be running “great” as it was before
Only the OP knows exactly what was changed, complete ignition system only ?
Revert to the one that worked

[moto]

Did you re-gap your new spark plugs? The new plugs will have a gap of about .035 inch, while the spec is about .023 inches. If the ignition is trying to fire across a gap that is too large a problem will first appear at high load in the cylinders, as I said before. Sorry I didn't think to suggest the gap before. Probably you reset it.

[GuzziNZ]

Thanks for all the suggestions.
So just to clarify, plugs were replaced and gapped correctly.
Points were swapped out to Dyna and both coils changed although all coils ohm tested to spec.
Even tried it with mufflers removed and this had a slight effect of lowering the stutter in the rev range.
After testing again it really doesn't ever clear properly from 4 to redline under full throttle
On thinking back I may have felt this issue slightly as I was running in the motor but never really gave it full throttle under high load
I will try some bigger mains and see if I get a change and found access to a rolling road if all else fails.

[John A]

Try it without air filters. I had one that ran great except at those same conditions. It turned out that it was the air filters being too short.  The length from the end of the carb to the end of the air filter should be 2.5 times the diameter of the carb inlet. I got by with 2 times the diameter because there wasn’t enough room to go longer. It probably wouldn’t affect a standard engine but this one was tweaked.

[GuzziNZ]

Will try that also John A but it is running the big K&N oval with duel connection.

[John A]

An interesting thing I found was when I was testing without filters on I reached down to feel the air flow expecting flow. At wide open the intake air was puffing in and out and it was slightly warm. I had expected some suction.

[moto]

An interesting thing I found was when I was testing without filters on I reached down to feel the air flow expecting flow. At wide open the intake air was puffing in and out and it was slightly warm. I had expected some suction.

I suppose you were running a hot cam with a lot of overlap? If so, some of the exhaust has a chance to flow back through the intake port during the overlap. It becomes unimportant at high RPM, of course. Interesting observation.

[Moparnut72]

If you are getting that much back flow, can't think of the correct term, at the carbs and the engine has been disassembled I would check the cam timing.
kk

[guzzisteve]

The jetting I had in my 850T and Dunstals, K&N ovals, was 60 pilot, 150 main. Can't remember if I raised needle a notch, was 1977.