Won't Rev

[GuzziNZ]

I'll try to keep this short.
After full recon of 850T the motor was running great but because of gearbox leak and not being happy with engine balance I stripped it down again.
Now that it is back together motor will not take full throttle under load past 4000.
Can get it past 4k with light throttle on the flat.
Stripped carbs and everything looks perfect.
Tested coils, check points gap. Reset timing at idle and double checked full advance at 4500 rpm.
Everything seems spot on but still no joy.
Any ideas?

[pehayes]

Have you used a strobe light to inspect ignition timing at higher rpm?  Can you see the system advance and retard as rpms change?  Mechanical advancer could be stuck/dirty.

Something limiting fuel flow at higher rates?  Fuel cap vent plugged?  Tank or carb filters plugged?  Hose kinked?

Patrick Hayes
Fremont CA

[moto]

This sounds like bad ignition timing at high RPM. The spec says to check the timing with a strobe at 6000 RPM. It makes a difference from 4500 RPM. Another thing to be aware of is that the static timing conflicts with the dynamic timing in the sense that the proper dynamic timing makes the static timing too advanced. Dave Richardson in Guzziology wrote about the too-limited advanced curve of the distributor. (I confirmed his observations.) Time to get out the scope and set the timing at 6000.

[GuzziNZ]

Yep, using a good quality strobe light.
Bike has two taps and cross over fuel lines so don't think its fuel.
The 850T is 8 deg and 34 deg which it hits at 4500 and I always set timing at full advance ignoring idle.
I used this setting last time with no issues but could try to see what happens at 6k.
Hard to tell but seems to advance at the seem rate on both cylinders.

[n3303j]

<SNIP>
Hard to tell but seems to advance at the seem rate on both cylinders.

The cylinder ignition timing differential is established by point cam and point location. There is only one mechanical advance serving both cylinders. If point location and gap are correct each cylinder should fire at the appropriate flywheel mark.

Therefore you can check for advance at either cylinder since you are checking the component common to both cylinders.

[Bulldog9]

I'll try to keep this short.
After full recon of 850T the motor was running great but because of gearbox leak and not being happy with engine balance I stripped it down again.
Now that it is back together motor will not take full throttle under load past 4000.
Can get it past 4k with light throttle on the flat.
Stripped carbs and everything looks perfect.
Tested coils, check points gap. Reset timing at idle and double checked full advance at 4500 rpm.
Everything seems spot on but still no joy.
Any ideas?

Granted a completely different motor, but when I had this problem with an 8V Norge, I had the same exact problem. After months of going back and forth to Moto International and having Micah and Jason play with it, a recommendation from Pete Roper to check cam timing showed that the cams on the right jug were off by one tooth. Not enough to make contact, but exactly what you are describing. I'm not sure if you got that deep in the motor, or if the motor would run if the cam and both cyls were off, or even if you could run the motor without valves and pistons touching, but it is worth a check to ensure the cam and crank are properly aligned. Could also be ignition timing.

[John A]

An air filter can cause those symptoms as well.

[guzzisteve]

Check full advance at 6000 rpm, the point it stops advancing.

I looked & was wrong, 4500 is good  I check at 6K rpm, that's me.

[wirespokes]

The rule of thumb is - the problem is the last thing touched.

It has to be something in the "strip down and reassembly" since it was fine before that.

What was done to the engine in that process? I would have balanced the carbs to get it running smooth. Or was it some other 'out-of-balance'?

Since you can get it running past 4K with light throttle, I suspect it's something to do with the carbs. It's getting a weak mixture, slow flow to the float bowls, accelerator pumps. I don't suspect the pumps because it's highly unlikely both suffered the same time.