Sputtering Under Hard Acceleration

Those are good filters, but while they stop most all junk, they also can get plugged pretty easy. I can't remember, do you have a Air Compressor? If so remove those filters and blow through them backwards to see if they are holding some junk.
If you have full advance at 4K RPM, and the problem is 6-7K, the advancer should be ruled out. Capacitors usually will make it crap out all of the time, not just at a certain RPM range. But another thing to consider on you Points set up would be Dwell Angle, which is point gap. This can change drastically under RPM do to point "Bounce". This is from a weak spring on the Point assy. Does you DVOM have a Dwell measurement capability? If so check your dwell angle, set the point gap so the Dwell is correct, then rev the motor watching for a big change. The Dwell should remain constant with just a couple of degrees. This Dwell Angle is the Time of Charge for the Ignition Coils, so is quite important.

bluej58 wrote: Because of all I have read about problems with inline fuel filters screwing with gravity fed systems,
I abandoned the idea and am putting all my trust into the petcock filter because the screen is very fine and I figure anything that passes it through will make it past the jets and get burned up.

Interesting. My petcock screen has a small hole in it. I'd need to replace that if I were going to run without inline filters. I'll take it for a ride without the filters just to see if the problem goes away.

Motor Head, I do have an air compressor, but the filters are brand new. So I don't think they're clogged.

I do not have a dwell angle tester, but I might know a buddy who does. Maybe I'll swing it by his place.

Did this problem just come on and has the bike ever ran good. If the problem suddenly came on without you changing anything then I would leave the carbs alone and look at the ignition system. Carb jetting does not wear out. I have had the same symptoms occur on various vintage bikes and the problem was low voltage to the coils. There is enough energy in the coils to fire the plugs correctly with low cylinder pressure but when the cylinder pressure get higher, (WOT or accelerating hard)the coils cannot produce the energy to fire the plugs. What bike are your working on?

This is 1979 KZ750. I've owned it for about two years. I've been riding a lot harder than I used to, so I'm not really sure if this is a new issue or a long standing one.

I replaced the coils with new Emgo ones shortly after buying the bike.

Trey, if you are using the "coil mod" you might check the relay. I had some intermittent current loss problems and traced it to the relay cap comming loose. Fixed it like this

and have not had any problems (with this) since.
Good luck,
Jim

Is the battery good, fully charged, and holding the charge?
If any doubt whatever, would charge overnight and have it load-tested.

If not already done, would assure good ground connection where the negative battery cable attaches to the engine.

With engine off and ignition switched on, what voltage is actually being received at the coil primary terminal?

With ignition switched off, what is voltage between the battery posts?

Is the air filter fresh and clean?
And nothing blocking the intake (such as shop rag, gloves, etc.)?

Good Fortune!

My petcock screen has a small hole in it. I'd need to replace that if I were going to run without inline filters. I'll take it for a ride without the filters just to see if the problem goes away.

Motor Head, I do have an air compressor, but the filters are brand new. So I don't think they're clogged.

I do not have a dwell angle tester, but I might know a buddy who does. Maybe I'll swing it by his place.[/quote]

Unless you still have a bunch of junk in your tank, running without an in-line filter to see if it is the cause shouldn't be an issue. Good Idea to run one, i run one, but you don't want a restriction. I run an Automotive filter, metal not plastic, no restriction, it can feed a 500 cid motor under 6 psi of pressure. So a gravity flow is ok. I had one of those other Motorcycle filters and it would starve it under hard running.

Just a quick update. It's not the fuel filters. I pulled them out and went for a test ride, there was no change. I still get the stuttering and misfire under hard acceleration.

I'm leaning towards it being the timing advancer, but I've got some other issues to work out before I come back to this. I'll keep you posted.

Does the bike have the stock coils & spark plug wires? How about the battery? Do you check it regularly?

+1 on suspecting weak ignition (weak spark), for whatever reason.

If not already done, would install brand new spark plugs (not-R type), and solid metal core plug wires; and eliminate resistor caps (if being used); all toward minimizing resistance in the secondary loop.

Would also perform ohm check on secondary loop.

And would measure voltage being actually received at the ignition coil primary terminal, which should ideally match voltage measured across the battery terminal posts. As it's critical that the battery is good, fully charged, and holding its charge, and that the full charge is being received at the coil.

Good Fortune!

I haven't checked the voltage recently, but the ignition coil (and wires) were replaced with a new Emgo coil last year. At the time I remember having good voltage at the coils, but I should test again.

Battery is about a year old and has been recently tested.

Topper wrote: I haven't checked the voltage recently, but the ignition coil (and wires) were replaced with a new Emgo coil last year. At the time I remember having good voltage at the coils, but I should test again. Battery is about a year old and has been recently tested.

Are suppressor type plug wires fitted (as opposed to solid metal core)?

Are resistor spark plug caps fitted (which can go bad and create excessive resistance)?

Resistance in the secondary loop should be measured to determine whether it's within the specs as provided in the FSM.

Excessive resistance in the secondary loop may result in stumbling while under high speed acceleration, caused by a weaker than needed spark under that condition.

(Sorry to sound like a broken record.)

Good Fortune!