A bit of a sputter.

Following my rebuild, I'm now mobile on my beautiful 72 Combat.
Runs well right through the rev range, however when going from about 2000 rpm to a big handful of throttle it "hesitates" for a second before accelerating well.
Spent the day with a timing wheel and found timing marks to be only one degree out. Retimed accordingly. Same result.
It may be worth mentioning that I have two brand new Amal carbs, tuned and syncronised with a manometer. Valve clearances are "spot on" . Tri Spark ignition.
Runs like a dream at all throttle openings.
Any thoughts ? Or, is this just it's "personality"

First off all, make sure your settings are excactly as the manual with the correct needle on the correct groove, correct slide cutaway, spray tube etc.
Pilot air mixture correct?
The bike should pull well from all revs cleanly.

Try 1/4 of a turn in on the air screws.
Cash

How do you tune with a manometer?

swooshdave said:

How do you tune with a manometer?

Click to expand...

You adjust the carbs so that the vacuum is eqaul at the intake side of the carbs. Fairly common tool for dual or triple carb syncing. I have one that consists of an adjustable venturi and a tube with a small ball in it that floats higher with more velocity. It has a flanged face with a neoprene gasket so it seals to the carb when pushed against it. All it really does is measure relative air flow at a given RPM.

rvich said:

swooshdave said:

How do you tune with a manometer?

Click to expand...

You adjust the carbs so that the vacuum is eqaul at the intake side of the carbs. Fairly common tool for dual or triple carb syncing. I have one that consists of an adjustable venturi and a tube with a small ball in it that floats higher with more velocity. It has a flanged face with a neoprene gasket so it seals to the carb when pushed against it. All it really does is measure relative air flow at a given RPM.

Click to expand...

So it balances the carbs, but how do you know the air/fuel mix is right?

I don't want to come off like I am the dual carb master...I originally bought my gauge in about 1972 to use on dual Weber and SU carb set ups and then used it on the Commando I had in those days. Truthfully I rarely get it out anymore.

You would have to assume or ensure that your pilot circuit is in good order and do a static adjustment on the screw, out 1-1/2 turns or so. Then sync the carbs to make sure the thottle linkage is matched. Then go back to running on one cylinder and tweaking the pilot screw for best idle. Others might have better advice. If you had a cylinder with crappy compression the test would be pretty useless I think because the "pump" wouldn't pull air as efficiently.

I think I have seen here somewhere that somebody replaced the balance tube between manifolds with a long piece of clear tubing that reached below the frame make a "U". Then by putting some fluid in there they could see if the carbs were pulling the same. This is a clever and easy idea. I also picked up a tip that if you reach under your carbs and hold a thumb and a finger on the idle screw of each carb that you can feel the slide tap them and tell if one cable is out of sync with the other. I liked this method enough that it is what I have been doing and it seems to be working for me. At first I didnt think it possible to reach but it is.

Sorry for the thread hijack!

Getting back to the purpose of this thread...how do your plugs look? I wonder if your needle adjustment is too rich. I put new needles in my carbs (Combat with 932s) and ended up moving the clip to the top groove because I was running rich. It was hesitating in a manner you describe if I recall correctly. I pulled my plugs in the last few days that have been in for about 400 miles and they are still a nice tan color with just a bit of carbon on the rim of the thread. Before moving the clip on the needle they were completely black and sooty.

Russ

Only time dual Amals should not be able to take a snap to WOT is off dead slow idle.
Manometer is used to both balance equal suck on carbs and get highest vacuum at idle which means both the ign time and carb tweaks. 1.5 pilot turns out is the ideal to shoot for and mostly gotten by adjusting float level for the base line of Everything Else. Float valve seat is finish nail tap easy to move cold.

Some have filed or installed cut away spray tubes to get more of an acceleration pump effect on low rpm throttle snaps. 'Knocking' them lower might have similar effect. They are just barely finger push tight in the pot metal bore.

Combat Col, I'd say your Carbies are very close, just a fraction lean. Big handfulls give big amounts of air, more air than fuel.
By all means check your plugs, but they may look OK. You would probably have to run the engine at 3,000 or 4,000 revs to check
their colour for this problem. New carbs = new needles and needle jets. Old ones were probably worn, this is why new carbs often run
leaner than old ones. Quick action throttle pieces can give the same problem. Snap it open, a whole venturi of air but too quick for the fuel.
Roll off a bit and she picks up. Roll on normally, A OK. Get it right though, as most of your cruising will be done between 3,000 and 5,000.,
that's where the needle is doing it's job.
My $.02 worth.
AC.

Finally got around to raising my needles, as suggested by AC. Checked compression and re - synced. Seemed a bit better on this mornings ride, acceleration wise. Started to miss a tiny bit after 100 mph . May need bigger mains. Currently running 220's. Altogether pretty happy with the way things are going. Man, these things are fast ... . Couldn't keep the smile off my face. Sheer riding joy ...
Thanks to all for suggestions and advice.

Once and if ya get the right size main jet and still crave more throttle snap response there are Amal race needles that have a guicker tapper but they tend to lower gas mileage by two ways, one puts out more gasoline with less effort and two it just feels so good to to do so often.

I'm trying to kept up with all the tuning variables too. Could you peek close at spark plug hooks and report where alo?ng it appears the slight color change of thermal staining. Near weld, half way to bend or at the bend or beyond.

Good to hear Col.
A bit of a miss at a 100, he he he, good on ya.
Does it improve if you roll the throttle off a bit , if it does, then you are too lean on the main.
If you find that you'r still not entirely happy yet with the mid range, you can try a thin washer (shim) under the
needle clip, to give that little bit more fuel, with out going a full step.
Low float level can also cause high rev lean out, you should be certain it is right.
AC.