Backfiring on a '78?

[Anonymous]

Hi,
I recently picked up a 1978 GS750 from a garage, it had been sitting for a few months (gas in the tank), and upon starting it up (it would start up) it would make little pops and backfire as the throttle was rolled on. The more gas, the faster the pops, the louder the backfiring.
It only did this from the left pipe.
I cleaned out the carbs and still had the same problem. It now would idle beautifully, but once you would start to roll on the throttle, it would begin popping, and then around 3500rpm it would begin backfiring loudly.
I thought it might be from the old fuel in the tank since it had sat for a few months. I drained the tank completely, cleaned the petcock thoroughly, and put in new gas.
After letting it idle (again, beautifully) for a few minutes, I started giving it some gas. It took a few more rpms to get it to really start backfiring, but it started again. Now the loud popping is coming from both pipes.

I'm letting the gas sit in the tank overnight in hopes it will dissolve any crap still in the tank or whatever. Then I guess I plan on cleaning the carbs again, and then moving on from there.

In the meantime, I was wondering if anyone had anything similar happen with their bike, or know of a common problem or something.

Thanks so much for your help,
Sam J

[Anonymous]

does it still backfire after running it a long time? sounds like its just cold natured or you have some idle jets clogged.

[earlfor]

Re: Backfiring on a '78?

There are two things you must do. Replace the induction "O" rings (about 5 bucks) Replace both sets of points and set gaps and timing. (about eighteen bucks) Set the idle airscrew to 2 turns out and set the fuel pilot screw to 1 1/2 turns out. The bike will run just fine.

Earl

Hi,
I recently picked up a 1978 GS750 from a garage, it had been sitting for a few months (gas in the tank), and upon starting it up (it would start up) it would make little pops and backfire as the throttle was rolled on. The more gas, the faster the pops, the louder the backfiring.
It only did this from the left pipe.
I cleaned out the carbs and still had the same problem. It now would idle beautifully, but once you would start to roll on the throttle, it would begin popping, and then around 3500rpm it would begin backfiring loudly.
I thought it might be from the old fuel in the tank since it had sat for a few months. I drained the tank completely, cleaned the petcock thoroughly, and put in new gas.
After letting it idle (again, beautifully) for a few minutes, I started giving it some gas. It took a few more rpms to get it to really start backfiring, but it started again. Now the loud popping is coming from both pipes.

I'm letting the gas sit in the tank overnight in hopes it will dissolve any crap still in the tank or whatever. Then I guess I plan on cleaning the carbs again, and then moving on from there.

In the meantime, I was wondering if anyone had anything similar happen with their bike, or know of a common problem or something.

Thanks so much for your help,
Sam J

[earlfor]

Here's the proceedure for timing the 750. The points are located under the round cover on the right end of the crankcase/crankshaft.

The round 6" plate that both sets of points are mounted on (breaker plate) is held in place by three screws located at 12, 4 and 8 oclock.
Loosen the 3 screws on the breaker plate. Note that the screw at 12 oclock is in a slot. Rotate the breaker plate until the 12 oclock screw is centered in the slot. Tighten the three screws.

We will replace the left side set of points first (located at 9 oclock position) These are the points for cylinders 1 and 4.

Take a 19 mm wrench and rotate crankshaft until points are at widest gap. Unscrew hold down screw on pointset and remove points. Note the white wire that goes to the pointset and the placement of the insulator. The insulator must be correct or the points will be grounded out and will not work. Put the new points onto the breaker plate, set the gap to 14 thousandths, and lightly tighten the hold down screw. Install white wire.
Rotate crankshaft once again with 19mm wrench until points are at widest gap. Check that widest gap remains at 14 thousandths. Adjust and retighten holddown screw if needed.

Rotate crankshaft until rightside points are at widest gap (for cylinders 2 and 3) Remove old points and install new points and set gap at 14 thousandths. Note insulators on black wire installation. Rotate crankshaft with 19 mm wrench again stopping at widest gap. Recheck gap as you did with the left pointset. Adjust and retighten hold down screw on points if needed.

THE ABOVE COVERS SETTING THE POINTS GAPS




Next is the timing:

Timing for cylinders 1 and 4 (the left set of points)....................
With 19 mm wrench, rotate crankshaft while looking through the timing window and align the mark that looks like F1| with the timing mark on the engine case.
(the line following F1 is the mark (|)

note: some bikes will have a stationary timing mark on the engine case and some will have a mark on a stationary timing plate that can be seen through the timing window)

Connect a continuity lights wire lead/clip to the white wire clip on the left pointset. Clamp continuity light probe tip to ground on the bike or battery negative terminal.

Turn ignition switch to on (do not attempt to start bike, just switch ignition to on)

The continuity light should be on.

Loosen the three screws on the breaker plate located at 12, 4 and 8 oclock. Rotate breaker plate until continuity light just flickers out.
Retighten the 12, 4, and 8 oclock screws on the breaker plate.

Unclip continuity lead from left pointset.



For the Right pointset
Rotate crankshaft with wrench until you see the timing mark F2| become aligned with the stationary timing mark on the engine case.
Connect continuity light lead to black wire on right pointset.
The ignition should be switched on and the continuity light should be on.

The right pointset is mounted to a breaker half plate.
You will see that you can loosen the mounting screws on this breaker half plate and this will allow you to move the right pointset. This will adjust the timing without changing the points gap. When you rotated the complete breaker plate for the left pointset to get the continuity light to flicker out, thats what you did. You adjusted the timing on cylinders 1 and 4 without changing the gap on their pointset.
Unfortunately, the whole breaker plate being rotated moves BOTH sets of points, so that is why you must set the left side first and that is why the right side is mounted on its own independent half breaker plate.
SOoooooo.... :-) Loosen the breaker half plate on the right side and rotate as needed to cause the continuity light to just flicker out.
Tighten the half plate back down when satisfied with the adjustment. :-)

Remove continuity light and HOPEFULLY, start bike.



Earl

[Anonymous]

Wow, thank you for taking the time to reply!!
I'll try all this today if I can get the parts. I'll let you know how it goes.

Thanks,
sam

[earlfor]

There were two types of pointsets used on your bike. If your pointsets are held in place with a single screw, they are Nippondenso points. If your points are held iin place with two screws they are Kokusan. The two styles are not interchangable, so you must get the correct type.

Earl

Wow, thank you for taking the time to reply!!
I'll try all this today if I can get the parts. I'll let you know how it goes.

Thanks,
sam

[earlfor]

You might want to also put new spark plugs in too. NGK B8ES gapped to 28 thousandths. :-)

Earl

Wow, thank you for taking the time to reply!!
I'll try all this today if I can get the parts. I'll let you know how it goes.

Thanks,
sam

[Anonymous]

Earl,
Went in to the shop this morning and followed your instructions, had the bike running perfectly within 20 minutes.

WOO!


Thanks again!!
--Sam

[earlfor]

YeHaaa! Another no runny, no go'ee bites the dust. :-) :-) Good going Sam! One of the reasons I suggest replacing the induction "O" rings is that even though replacing points and setting timing alone will normally get it running decently, if you have a leaking "O" ring/rings, you will never get the carbs adjusted as they should be, vibration levels will be high and idle mixtures will be impossible to set right.

Earl

Earl,
Went in to the shop this morning and followed your instructions, had the bike running perfectly within 20 minutes.

WOO!


Thanks again!!
--Sam

[peterpressure]

One question!

Set the idle airscrew to 2 turns out and set the fuel pilot screw to 1 1/2 turns out.

Is the fuel pilot screw the 4 jets inside the 4 lower Carb baskets ?

Idle airscrew is the big screw on the outside of the carbs in between carb 2 and 3 ?

I just acquired a 1978 GS750 and replaced the battery and cleaned the carbs, but it backfires and sputters above 3k RPMs and the Spark plugs are black. I am not sure if they were already black when I got the bike, so I will clean them off and follow your same instructions and see what happens!

Replace the induction "O" rings (about 5 bucks) Replace both sets of points and set gaps and timing.

Thanks for the info!!