1979 gs750e carb questions

[Anonymous]

i've got a 1979 gs750e, it's running a bit rich and when i play with the air screws to make the intake fuel leaner,(to get a good looking sparkplug) the bike backfires through the carbs (not sure which one) a couple of times on a 20 km run. the bike also seems starved of fuel on the highway. i've decided to rebuild the carbs, my question is, "what size jets should i find in the carbs?".

i've had the bike for 2 years and i have never been able to rev past 7000 rpm in top gear, the engine starts cutting out. the exhaust system is a four into one black coloured unit with a black muffler attached. i installed pods on the carbs hoping this would help with the rich fuel mixture, but it did not.

i adjusted the valve clearance 4000 km ago and this helped alot with cold starting but not the fuel mixture.

is there a "ggod guess" as to what size main jets to install on this bike when i'm rebuilding the carbs?

i noticed at one supplier on the web includes main jets (i think) with the rebuild kit for sale there, is it possible these jets will work ok?

18 years ago i had a 1975 honda 750 (i'm pretty sure it was a 75) and i purchased a 4 into 1 chrome pipe for it (hooker i think), i popped the exhaust pipe onto the honda and it worked (and sounded) great, no need for jet adjustments etc.

is it possible to do this with a suzuki gs 750? if the jets that are in the bike now are too large, should going back to stock make the bike burn it's fuel properly and hopefully rev higher under load?

one more question, what do the screws that are painted yellow under the carbs do and can they be adjusted?

thks

jim long

[earlfor]

Re: 1979 gs750e carb questions

I have a 79 750E. Stock jet size is 102.5. Suggested jet size if installing a 4 into 1 V&H is 116. From experience, the the stock 102.5 jets that come with the carb rebuild kits are too small to run with a V&H 4 into 1 because the plugs will run ash white lean.

Adjusting the fuel pilot screw and the air screw will result in a good idle, but will have almost no effect on overall mixture and the color your plugs are. They are both idle circuit adjustments and their effective rpm range is aprox 0-1500. The screw underneath the carb that protrudes from the float bowl is the fuel pilot screw. It should be turned out 1 1/2 turns.
The screw on the top of the carb is the airscrew, it should be turned out 2 turns. Turning out the fuel pilot richens idle mixture with more fuel.
Turning out the air screw leans mixture with more air.

Jetting may not be your problem. I have often said when everything else is correct, then it is time to look at the carbs. Unless everything else is correct, it is not possible to properly adjust carbs.

Some causes of black/rich/fouled plugs:

Too small a spark plug gap
Incorrect ignition timing
Points worn and gaps incorrect
faulty condensors
excessive resistance in spark plug caps
fuel level too high in float bowls
Aircleaner element dirty or too heavily oiled


I suspect installing pods in hope of increasing airflow to combat a rich mixture did not help because the problem isnt caused by your carb jetting.

I wouldnt jump in a throw $100 at carb rebuild kits before checking a few other things first.

Earl

[Anonymous]

i had the carbs professionally cleaned last year. i assume the floats were adjusted properly. i replaced the points also last year, gapped them a couple of times. the timing has never been adjusted with a light as i don't have one. the carbs were balanced on the bench when they were cleaned. the bike will rev up 9000 in ist 3 gears and seems to accelerate ok but i haven't been riding since i sold the honda many years ago so i'm not sure if the bike is working like it should be.

i'll remove the carbs and check the float levels and take note of the jets. i'll take your advise and hold back on the carb kits as they cost $111 cdn.

i find the bike revs a bit high on the highway and i would like to drop the rpms. would you suggest a larger sprocket in the front or a smaller sprocket in the rear? i believe the bike has stock gearing (rear sprocket is 43 teeth) i can't see the front sprocket.

the bike revs around 5500 @ 120kph (our acceptable speed limit in canada) and is very loud at this speed.

is it possible to set the ign timing close without a timing light? i tried trial and error with the timing and the only difference i came up with was a bike that was hard on the starter (too far advanced?).

thanks for your advice, i appreciate it very much

jim long

[earlfor]

The book says to adjust your float levels to 23-24 mm. I have mine set at 25 26 mm. The plugs burn a medium brown. The float height is measured with the floats held upside down and is the distance between the metal gasket surface and the bottom of the float (which is on top if the carbs are upside down) :-) I would check float heights first as the mechanic may have set them to the 23mm spec and I know that is too rich.

You do not need a timing light to adjust the points, all you need is a continuity light and if you dont have one, it is easy to make one with a 12v bulb, and a couple of wires. Even easier if you have a 12 volt socket that already has wires on it. :-) I posted instructions on gapping and timing a while back and I'm just going to paste them here rather than rewrite it.
Just ignore the part about replacing the points.

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 srews 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.



You stated "the carbs were balanced on the bench when they were cleaned" To balance a set of carbs, they must be on the bike and the engine must be running. Balancing is to balance the engine vacuum
through the carbs. If your mechanic thinks you can balance a set of carbs that are sitting on a workbench, I would find another mechanic.


I have aproximately stock sprocket gearing (converted to #530 chain) and I run 4300 rpm at 60 mph/96kph. If I remember correctly, the #630 sprockets were 15/41. I'll have to dig them out to be sure though.
(I wonder where they are) :-) I wouldnt make a sprocket change before the engine is running satisfactorily as it would change characteristics and confuse things.

Earl