Checking compression and timing on 83 750 twin.

I'm fast approaching the end of my rope here with this bike. Last year it ran great but in the fall it started backfiring while tooling down the road. This year it does not want to run at all.
Here is a listing of what I have done so far.
1)New coil.
2)New plugs
3)New Plug wires
4)New Battery
5)Cleaned and rebuilt carbs.
Fuel is fresh and clean. I'm guessing it may have slipped time or I may have a valve going south on me.
Any suggestions and ideas are appreciated.

If not already done, would (1) do compression test (remember to hold throttle wide open), and (2) also check valve clearances (regardless of compression test figures).

A too tight valve can cause loss of compression upon engine attaining normal operating temperature. And a too tight intake valve may allow backfiring through the carb.

Regarding ignition, would assure having a good fully charged battery with solid connection where cable from negative terminal attaches to its ground at engine or frame. And measure voltage across battery terminals, and also measure voltage at coil to determine extent of dropped voltage between battery and coil. There's often a loss of voltage due to old wires and connectors (loose or dirty) along the line between battery and coil. Even with perfection from coil to plugs, insufficient voltage supplied to the coil often results in poor spark.

Good Luck!

And is that fresh clean fuel reaching the carbs (thinking maybe a dirty in-line filter or clogged petcock)?

Addendum -- when checking battery negative cable ground, would also measure voltage between positive terminal and some metal part on the engine (such as the head), which will help assure good connection where negative cable attaches to its ground. Compare this voltage reading to the reading across the battery terminals. Hopefully both are the same.

When you rebuilt the carbs, did you set the fuel level according to the manual (using a hose to actually measure it)?

Have you synchronized the carbs using vacuum gauges?

82 and 83 750 twins have electronic ignition that triggers off the crank, and it's not adjustable. The only way the ignition timing could change enough to cause your problem is for the crank to break, and I've never heard of a single 750 twin crank breaking.

Cam timing is not going to change without serious mechanical damage having occurred, and you'd know it when it happened. And again, so rare as to be non existent.

Got spark?

Got fuel? (plugs wet with gas?)

Check the valve clearance, it's not that hard and proper valve clearance will make the bike run better, and be cheaper to run (as in prevent damage to motor).

I have an 82 motor in my 79 750 twin, takes two kicks to start cold, half a kick to start warm. Eight hours to do a complete tune up (fuels levels, new plugs, check and adjust valves, synch carbs, adjust clutch) to get to that point.

Yep the carbs are supposedly synched (hired out that job :blush: )with the floats in the factory set spots.
Plugs wet, coil hot spark is a white color.
I'm a maintainance mechanic for BF Goodrich tires so while I don't do gasoline engines I'm well versed in MIR (make it run) if I have access to the correct settings. So what are the valve clearances? What kind of compression readings should I be getting here.
I had a Kawi shop manual for this bike .... but it decided to leave or something. So it would be greatly appreciated if you could help a dummy out.
Thanks
Wild.

My 750 twin manual is in the shop right now, I'll try to remember to bring in tomorrow.

Compression has been 150-170 psi on the 750 twins I've checked, my current one is at 170 psi (82 motor with ~7k miles).

A 750 twin will start, idle, and run on one cylinder, I actually rode one that way.

Just sticking in a pair of new plugs is often enough to get it running.

If the carbs were recently synched, then it must have been running, unless they only did a bench synch.

When you leave the petcock in the Prime position for an extended time, does gas run out of the carbs?

This one I have seen two or three times:
Is there any possibility that small critters have filled up the exhaust with nuts in the fall or over the winter? Strange as it sounds, I have run across that 2-3 times, the exhaust being totally packed with nuts. And the motors wouldn't start. since if the old air can't get out, new air can't get in

Compression is 165 on both cylinders. The valves looked fine too. I'm guessing I'm back to the carb again.

Wildh2oskier wrote:

Compression is 165 on both cylinders. The valves looked fine too. I'm guessing I'm back to the carb again.

If not already done, would first address question of getting sufficient voltage to coil.

Good Luck!

Power to the coil is correct.

OK it fired and ran.... in the garage. Of course the strong smell of an engine running rich was burning my eyes in a matter of seconds.
Heres the deal. It started with a bump of the starter, not like it used to where the engine had to spin up to speed and slowly the engine would wake up.
Once it started it idled rough at about 2500 rpms. then it died. I restarted it and kept it running with small throttle blips. I kept it running for a couple of minutes and allowed it to die.
Now it won't do anything again.
Suggestions???

Wildh2oskier wrote:

OK it fired and ran....in the garage. Of course the strong smell of an engine running rich was burning my eyes in a matter of seconds...Once it started it idled rough at about 2500 rpms. then it died. I restarted it and kept it running with small throttle blips. I kept it running for a couple of minutes and allowed it to die. Now it won't do anything again. Suggestions???

Could be excessively rich mixture quickly flooding out new plugs.

Would first assure correct service fuel level in floatbowls, using clear plastic tube test. If floatbowl fuel level is too high, will cause over-rich mixture even with everything else being perfect.

May also result from sticking float, obstructed floatbowl overflow circuit, dirty or obstructed air filter, or starter plunger stuck open.

Good Luck!

ok. How do I perform the clear plastic tube test? And how do I adjust the float levels if that turns out to be the problem?