Confused...PLEASE HELP

78kztwin wrote:

The other day my 78 750twin started up on the first try. Now it won't start a week later. Good battery, good points/condenser, charging system good and coils. Only difference is I put on a new balancing hose, hoses for the overflow nipples on carbs and made new gaskets for float bowl screws. also put inline fuel filters on. Doesn't seem like the carbs are pulling in fuel, but I checked fuel flow in lines and is good, even put lines on without filters just in case...nothing, motor will crank but no ignition.


so I'm almost 100 percent sure that my float bowls are filling up and not emptying so no more fuel is getting in...any suggestions

Carbs don't pull in fuel.
Gasoline gravity flows into float bowls.

Gasoline in the float bowl supplies the carb circuits.

The piston intake stroke sucks fuel mixture from the carbs into the combustion chamber.

Have the plugs been pulled and inspected immediately after attempting to crank to determine whether they are dry which would indicate no fuel mixture being sucked into the combustion chamber?
If plugs are damp with fuel mixture, reported problem might be caused by something else such as poor compression or poor spark or incorrect ignition timing.

While the plugs are out, have they been held grounded against the engine and observed for fat blue sparks while spinning the engine over?
If no spark, would try brand new spark plugs.
Could also do static timing check.

Good Fortune!

Engine needs three things to start and run, fuel, air, and spark, in the proper amounts and at the proper time (assuming the engine is in good condition mechanically).

Pull plugs and lay them against the motor then kick/crank the motor to see if you have good spark.

Put the plugs back in and apply the "choke" fully, then kick/crank it over a few times. Remove plugs and see if they are both wet.

Either you won't have spark (my guess) or you won't have fuel.

Check the shutoff switch on the right handlebar switch pod to make sure it's in the "Run" position.

Do not ever open the throttle while trying to start the bike, not if you want it to start anyway.

Since a 750 twin will start and run on one cylinder, my guess is either the Master Switch is in the wrong position, or you accidentally unplugged the red wire that powers the coils (next to the backbone under the tank.

steell wrote:

Since a 750 twin will start and run on one cylinder, my guess is either the Master Switch is in the wrong position, or you accidentally unplugged the red wire that powers the coils (next to the backbone under the tank.

I checked the wires for the coils, both connected and switch is correct. I just don't ubderstand, it fired perfect last week and now nothing mechanically is different and it won't fire, just cranks but no ignition...is it possible for the timing to be off from just sitting?
I'll check the spark tomorrow, gotta work and watch my daughter.

78kztwin wrote:

I checked the wires for the coils, both connected and switch is correct. I just don't ubderstand, it fired perfect last week and now nothing mechanically is different and it won't fire, just cranks but no ignition...is it possible for the timing to be off from just sitting?
I'll check the spark tomorrow, gotta work and watch my daughter.

Not to be a dick or something... but if nothing was mechanically changed - it would be running :laugh:

Start at the beginning with what Steell said (and others). You need to use a process of elimination to correctly and quickly identify the issue, then rectify it. Any good mechanic knows this.

Your issue (barring a sudden head gasket failure/compression loss, or timing change - highly unlikekly) is SPARK, or AIR/FUEL not being delivered in the correct proportion, or at all. Period.

Now there can be alot of reasons for this to be happening.. like acidentally swapped plug wires... coil connections... hose connections... carb holder air leaks... blah blah. Everything you touched - needs to be thoroughly inspected again.

Off point - The engine wont flood if set to prime on a petcock, unless your carbs are shit.
and... the 750's will only start on one cylinder if there in really good condition, cause mine wouldnt :laugh:


Dont get flustered - we realize its hard to ubderstand.

Best of luck.

Nope, timing is not going to change. But it may have been on the verge of dying because the points screws were loose and letting the points close, when you shut it off.

One coil, two wires, one plugs into the red wire that comes out of the main harness under the tank, the other goes to the points.

Take the points cover off, rotate the motor until the points are closed, use a screwdriver to flick the points open and see it there is a spark (be careful to not ground the screwdriver).

The plastic plug caps on the ends of the plug wires are generally 5k ohm resister caps, and they go dad after a few years.

Got a meter? Measure the resistance between the two plug wires, should be 17k - 20k ohms, if not then take the plug caps off and measure the resistance directly from the plug wires.

Oh yeah, keep in mind this is a wasted spark system, one bad plug will keep both from firing, disconnect one wire and you lose both.

simple thing is to do the checks I suggested earlier.

You need to use a process of elimination to correctly and quickly identify the issue, then rectify it. Any good mechanic knows this.

Unless you want to waste hours upon hours

Your issue (barring a sudden head gasket failure/compression loss, or timing change - highly unlikekly) is SPARK, or AIR/FUEL not being delivered in the correct proportion, or at all. Period.

Now there can be alot of reasons for this to be happening.. like acidentally swapped plug wires... coil connections... hose connections... carb holder air leaks... blah blah. Everything you touched - needs to be thoroughly inspected again.

Swapping plug wires on the 750 twin doesn't make any difference

Off point - The engine wont flood if set to prime on a petcock, unless your carbs are shit.
and... the 750's will only start on one cylinder if there in really good condition, cause mine wouldn

Prime position didn't show up on 750 twin petcocks till at least 1982, before than it was ON/OFF/RESERVE.

As long as you have spark at both plugs, a 750 twin will start even messed up (as in carb flooding), heck I even took one for a ride on one cylinder just to see what it was like.
Serious lack of power

TeK9iNe wrote:

78kztwin wrote:
Dont get flustered - we realize its hard to ubderstand.

I knew I should have corrected that...I'm gonna figure this out and hopefully rippin down the street by the end of the year, then I can tear down to finish my project, gotta make sure it's running good before I put all this work into it.

Patton wrote:

...While the plugs are out, have they been held grounded against the engine and observed for fat blue sparks while spinning the engine over?....

78kztwin wrote:

...I'll check the spark tomorrow....

No Fear! :laugh: