1982 GS750T cranks but won't start

[Joe]

My 1982 GS750T cranks but won't start. It has the airbox, 140 main jets, stock pilots, aftermarket 4 into 1 exhaust, and a new battery. I just rebuilt it from the ground up, and I'm still trying to get everything figured out.

[Rich82GS750TZ]

Those main jets were 112.5 from factory. Why the 140’s? Higher flow pipes alone do not require rejetting. Going up a size or 2 is somewhat typical, I think, as jetting was a bit lean out of the box. I’m no carb expert, but that big of a jump, I would think would be flooding the engine. Mine runs great with the stock jetting. You may find the 750 16 valve manual and the 750ET supplement found on BikeCliffs Website useful. Link in my signature.

Welcome to the forum. And, we like pictures.

[pdqford]

Are the spark plugs wet or dry?
Is there fuel in the float bowls?
Is there spark at the business end of the spark plugs?

[rphillips]

Cranks but won't start??? Does it try to start (hit, sputter, poot, cough, pop,) or just cranking, cranking, & cranking & nothing? Mains do seem rather large, but shouldn't affect starting. One size larger pilots probably would make it start a little easier & warm up a little quicker with less choke.

[XRRRX]

140 main jets sounds like someone re-jetted the carbs for velocity stacks. One thing I have found in the past is the resistor plug caps being bad. You can unscrew them off of the ignition wire and check them with an ohm meter. The should all measure near the same, typically 5K or 10K.

[Joe]

140s are mine for pods. I put the air box back on and didn't rejet back to stock. Stock pilots are 42.5. New everything for the ignition system. Just cranks, even with starting fluid. Plugs were dry soot, presumably carbon fouled. I replaced them. Runs down my Li-ion really fast.

[tom203]

neither intake system nor main jet size will effect starting

Go back to the basics ….read this link

http://members.dslextreme.com/users/storagecliff/images/electrical_odd-n-ends.html#A17

[pdqford]

140s are mine for pods. I put the air box back on and didn't rejet back to stock. Stock pilots are 42.5. New everything for the ignition system. Just cranks, even with starting fluid. Plugs were dry soot, presumably carbon fouled. I replaced them. Runs down my Li-ion really fast.

No fire even with starting fluid, I’ll take that as an ignition problem, but would feel better if you had laid a spark plug on the cylinder head and cranked it.
Did this ground up rebuild have any impact on cam timing or valve lash?

[chuck hahn]

You need FOUR things to make it run, so whats missing?
FUEL TO CARBS
AIR TO CARBS
SPARK ON ALL CYLINDERS
COMPRESSIONN ABOVE THE VERY MINIMUM AS LISTED IN SERVICE MANUAL.

Compression issues may be from tight valves. Adjust them per the service manual specs. If compressions low after adjustments do a leak down and listen for air passing the rings. Place your ear in the oil fill hole and listen. To check leak down on valves listen into the airbox or throat of carbs to see if leaking past intake valves. Listen at the muffler for exhaust valve leakage.

EDIT... Proper timing is important too... BOTH CAM AND IGNITION PLATE. Yes i know its transistor and no points. bring number 1 to TDC on a compression stroke and see where the timing marks line up.

[Rich82GS750TZ]

Rebuilt from the ground up. Did it run before?

[Joe]

I didn't make sure it ran before I tore it down. My mistake. It's my first motorcycle. I did time it according to the manual. It did ride down the road when I finished it. It does have compression and spark. I just rebuilt the carbs and put mechanic in a bottle in the tank.

[pdqford]

I did time it according to the manual..

I’m trying to follow along, but do you mean you timed the cams according to the manual?
Or did you time the ignition? Maybe you did both?
Perhaps you could give a somewhat detailed description of just what your “rebuilt from the ground up” entailed.

[Joe]

I have since powder coated the wheels and lower forks and brakes and stuff.

[Who Dat?]

In post #11 you say you rode it after you rebuilt it.

Then you say you rebuilt the carbs and now it won't start.

Your troubleshooting should start with the carbs.

Did you do a bench sync?

Did you preset the idle speed?

What are the pilot screw settings?

[Joe]

They are only bench synced. Pilot jets are 42.2 stock. I don't know the fuel screw setting. I don't know how to preset idle speed.