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My 1974 450 desmo wont run cleanly after about 1/4 to 1/3 throttle opening. It seems to misfire and the more you open the throttle the worse it gets, until at full throttle it is totally bogged down and hardly running. Below that throttle opening it starts, idles and runs really well up to about 4500rpm which is usually where the problem starts. The bike was stored for about 20 years while life got in the way. About 2 years ago I started the process of getting it running again and for the last 6 months I have been riding it a couple of times a month sorting out issues as i go. Until now I haven't been revving it much while i worked through other issues, but now I need to sort this out......

As far as i'm aware the head and cams etc are stock.

The carb is a 29mm square slide with the following:
Main 135
Pilot 50
Slide 60
Needle New V7
Needle position 2
Needle jet New 262T
Float height 24mm. The float height was about 21mm - changing to 24 which I did today made no difference.
New carb gasket kit.
Inlet manifold gasket was not new but it was unused when I started to get the bike back on the road 2 years ago.
Fuel tank has been flushed and fuel taps are working ok. Fuel flow from taps to carb is fine in 1/4" fuel hose.

A week ago I installed a Sashce ignition to replace the original Electronica ignition. It made no difference.
Still using 6v electrics. Battery is putting out 6.3-6.5 volts.
I installed a new plug about 1500 kms ago. That didn't make any difference.

I'm leaning towards fuel being the issue, it just feels more like a fuel issue. But I am open to any suggestions!!!

Cheers
Dan

According to the supplement to the factory workshop manual, all the square slide Dell'Orto VHB carburettors fitted to 450s had atomisers (needle jets) 265T.
Confirm free flow of fuel to float bowl - no restriction at filler cap breather or taps.

Thanks Jordan,

Thats interesting about the needle jet. Mick Waker says 260T for the 450 Desmo. Phil Hitchcock sold me the 262T and the V7 needle together (no V8 needles were available at the time) and advised they should work ok, possibly just have to adjust the needle position. But if that was a problem I'd expect just a midrange (say 1/4 to 3/4 throttle) effect and at wide open it would be ok. But it gets worse at wide open.
I have checked the flow to the carb. I think I mentioned that it was fine, and the taps have 1/4"barbs so there's a good flow.
But it sure does feel like a fuel starvation thing so I'll go over it again.....

Cheers
Dan

I'm glad that you mentioned that the new ignition made no difference, as I'd have sworn that this is an ignition issue. Did you use a new coil with the Sachse?

The other thing I would do is try to fit the correct needle jet and needle, just to eliminate those from the equation. What about valve clearances? If the valves are not closing properly that would cause the problem you have and desmo cams were notorious for being ground wavy around the closing lobes.

Jordan wrote:According to the supplement to the factory workshop manual, all the square slide Dell'Orto VHB carburettors fitted to 450s had atomisers (needle jets) 265T.
Confirm free flow of fuel to float bowl - no restriction at filler cap breather or taps.

The 1974 user manual lists 260T, the 1970 UM has 265T.
I have 2 different parts catalogs. Both say 265T.
Needle is always V7, not V8.

???

Long shot, but if you could borrow a known good performing carb from another well-running similar bike, that would sure simplify diagnosis.
I wonder if there's a mistake with the cam gear timing?

What fuel are you using ?
Is the plug wet or dry, black oily soot or whiteish ?
Restrictive air cleaner ?
Have you stripped and cleaned all the carb jets and orifice s?
I assume your tank is steel not fibre glass?

Regards
Graeme

All air/fuel passages clear? Atomizer replace with same type (bleed or non-bleed)?

Thanks evryone for your thoughts - very much appreciated!
Duccout wrote

I'm glad that you mentioned that the new ignition made no difference, as I'd have sworn that this is an ignition issue. Did you use a new coil with the Sachse?

The other thing I would do is try to fit the correct needle jet and needle, just to eliminate those from the equation. What about valve clearances? If the valves are not closing properly that would cause the problem you have and desmo cams were notorious for being ground wavy around the closing lobes.

Yes I fitted a new coil with the ignition. It was supplied by Sashce.
Ducwiz says V7 is the correct needle anyway, and the 262T jet should work, albeit there might be a requirement to change the needle position a bit to fine tune it.
Valve clearance is something I hadn't thought of, that makes sense and is worth checking. They were set more than 20 years ago by Phil Hitchcock but the bike hasn't done many miles since then. It was running ok when it was parked up.

Thanks Ducwiz for the needle and jet info. Seems like either would work and only fine tuning between the two. This is quite a significant "misfire/loss of power"

Graeme wrote:

What fuel are you using ?
Is the plug wet or dry, black oily soot or whiteish ?
Restrictive air cleaner ?
Have you stripped and cleaned all the carb jets and orifice s?
I assume your tank is steel not fibre glass?

Fuel is 91 or 95 I forget which I last put in. Upper lube added. Not ethanol. Fuel is as per Phil Hitchcock recommendation. Apparently the additives in 98 foul the plug.
Dry and black. Its indicating running rich but that could be a bit misleading. I have 5 kms of dirt road before i hit the tarmac so I'm coming home at 3000-4000 rpm. Which is not the trouble zone. A mate suggested a plug chop in the trouble zone so thats on the hit list.
Air cleaner is the small K&N type directly on the back of the carb. No trumpet. Good thought and easy to test - next time I'm on the tarmac, I'll take it off!
I have stripped and cleaned all the carb jets and orifices 3 times in the past fortnight. the last time the carb was reassembled i also installed a new gasket kit.
The tank is steel and as I may have previously mentioned its been flushed. No crap is blocking fuel flow. With the float bowl removed and 1 fuel tap opened fuel pisses out. the float and needle work ok to stop the fuel flow. Same with the other tap.

It will likely be something very simple that I've overlooked. The valve clearance idea was not something I would have thought of.

Cheers
Dan

Jordan

Long shot, but if you could borrow a known good performing carb from another well-running similar bike, that would sure simplify diagnosis.
I wonder if there's a mistake with the cam gear timing?

Re borrowing a carb, I would if I could! I'm a bit of an orphan in this neck of the woods. I do have some other singles but they are all awaiting attention so I don't have a good performing carb on any of them!
I need to think about the mistake on cam gear timing, but my initial thoughts are that it would be having a noticeable effect all through the rev range wouldn't it? This problem cuts in fairly reliably at around 4500 rpm. If you are running downhill and don't exceed about 1/4 throttle you can get to 5000rpm.

I'm going to try hooking the ignition directly to the battery and bypassing the wiring loom in case there is some sort of short impacting the ignition. Sashce say this can happen.