Gt750A won't rev past 3k

[Vince]

Hi guys, I have a good mate who has a '76 Gt750A which has suddenly started playing up. It won't rev past 3k. It just splutters and pops.
We think it's the centre cylinder as revving it on the centre stand and feeling the exhaust ends, we get a 'popping' feel as it is revved to 3k only from the centre pipes.
Carbs have been cleaned and the jets checked.
Plugs all look ok. Centre one replaced to no avail.
Inlet rubbers are perfect.
Timing is checked ok.
Points look excellent.
What are we missing? Can we check coils with a multimeter?

Any help gratefully accepted to keep my mate from topping himself!!

[GT750Battleship]

Hi,choked exhaust system...Carbon build up in pipes,baffles,exhaust ports,filthy air filter or no filter,how much smoke is it making ? crank seals could be shot !! Is there enough fuel reaching the carbs,petrol cap no breathing etc PS coils very rarely give problems,I had a wiring harness problem awhile ago,had similar symptoms ,lack of power etc,found a voltage drop with the meter,only showed 10.5 volts ! from memory a previous owner had done something dodgy with the regulator !! These bikes also need a good battery,will run a little erratically at low revs with a dying battery from my experience

Cheers,
Roger

[Vince]

Thanks Roger, fair bit to go at there.
I'll post back when we get to the problem.

[pearljam724]

Do a ohm resistance test on all 3 coil leads. One coil may be shot or you may possibly have a bad spark plug boot. Replace all 3 boots. You can buy a good set of 2 for roughly $10. One resistor inside the boot may be too oxidized. Creating a weak spark. You can easily test that with an ohm meter if you would like. That will tell you 2 things. That either that coil or boot needs replaced. Second thing I would look at is your points / timing. If the carbs have been reassembled correctly. It's an electrical issue. Battery, coils, points, plugs, ignition, etc. Electric and timing is the culprit to cylinders not firing or firing well enough. All of those things are electrically related to one another. The battery is the number one culprit to these bikes not running properly. The motor will simply not run well with a battery anything less than very good. You have to properly test the battery before, during and after the motor has attempted to turn over. Also at idle and higher revs than idle. Simply charging a battery is not testing it. It could have a dead cell. You have to test for that using a multimeter. Just because these bikes start and run doesn't mean that battery is any good. They won't run well without a very good battery. Drop cylinders, poor power, won't rev up right, etc. Having just bought a new battery doesn't mean squat either.

[tz375]

If you are certain that only the center is firing, I would look at the leads to the coils and points. It could be a lead has melted on the pipes or something pulled or vibrated loose.

Are you getting spark on all 3?

[Vince]

If you are certain that only the center is firing, I would look at the leads to the coils and points. It could be a lead has melted on the pipes or something pulled or vibrated loose.

Are you getting spark on all 3?

Bike starts and ticks over on all 3. It's just when you try to rev it past 3k it pops and splutters on middle cylinder.

[Craig380]

Are the carb diaphragms OK? If one is split, the bike will start and idle fine, but opening the throttle and trying to get it to rev up will have hardly any effect. If it is just one split diaphragm, the other two cylinders will work fine but the cylinder with the faulty carb will start to miss and splutter as it's not able to keep up.

You can repair splits temporarily with the flexible rubberised superglue you can get from WH Smiths.