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Engine idle speed hunting

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redmunk View Drop Down
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    Posted: 05 Oct 2018 at 21:03
Hi all,
Finally got my project v35II assembled! Started on the button once I’d turned the fuel on... It’s a V35 Imola with points ignition. 

So it idled a bit rough as expected, so I let it warm up and balanced the carbs. However, the idle speed hunts around and varies massively with no throttle input, from 1500-2500 with no pattern. Only seems to do it when hot though, and the carb balance didn’t help it. 

It does seem to be running rather hot, but the plugs say it’s running rich. And I’m not sure how hot Guzzis are meant to be! Water fizzles on the rocker covers after idling up and down for about 15 mins if that’s an indication?

Thanks for any pointers.

I’ve done the following things -
Carbs have been rebuilt as part of the project...cleaned, float height set, new viton needle and seat, new bowl/inlet pipe gaskets, new jet.

Checked for vacuum leaks in the rubber inlet pipe with the carb cleaner method

Ignition centrifugal advance mechanism rebuilt and lubricated with new points and condensers. I’ve set the timing to how it was pre rebuild, but haven’t checked it against the manual yet.

Removed the (brand new) throttle cables from the grip in case they were sticking.

Valves ground and clearances set and double checked 

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Brian UK View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2018 at 23:46
If you are running it at a fast idle for 15 minutes with no forced air cooling it will get very hot. If the plugs suggest rich, have you checked the choke plungers are seating properly?
Brian.

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jpc View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpc Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 08:35
Even though you've rebuilt it, the hunting is just what a loose ignition advance does.

Also easy to eliminate that possibility; put a strobe light on it and check that the timing mark doesn't wander in step with the RPM. You can waste a lot of time on carburation NOT doing that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote redmunk Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 09:41
Thanks Brian. I had a pair of decent blowers running across the cylinders, but that may not have been enough... Choke plungers are both brand new and the seats are clean. I was mentioning the rich running in reference that its (probably!) not running super lean and surging, but the carbs still need to be properly set up. It was missing the original air box when I got it so it’s running pod filters with bigger jets, needle still on the middle groove. I’ve built a new oil separator for the breathers so the top end should be happy. 


jpc - thanks, I had a feeling it was ignition related...I’ll dig out the strobe and see if it moves, and also check the advance assembly screws to see if it’s shifting. Thinking about it, it knocks just before it stalls so it may be too far advanced, which would explain the heat too. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 12:46
Certainly you do need to be sure ignition timing is correct and stable before looking elsewhere for problems.
Brian.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jefrs Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 14:01
I've run a little IR thermometer on a bike before now - an air-cooled motor can get to over 115°C in under 10 minutes of idling, which is overheating.  That means we don't have long to play with the engine before needing to switch off to let it cool down.

Rule of thumb, a petrol engine wastes 80% of its power as heat, which means a 50hp motor wastes 50x4 = 200hp, 200x750 = 150kW as heat. A typical kitchen oven is 5kW so a pair of fans probably cannot cope with that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jefrs Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 14:10
Originally posted by jpc jpc wrote:

Even though you've rebuilt it, the hunting is just what a loose ignition advance does.

Also easy to eliminate that possibility; put a strobe light on it and check that the timing mark doesn't wander in step with the RPM. You can waste a lot of time on carburation NOT doing that.


Very likely. 
If it is as I'm thinking, you have spring bob weights, they were replaceable, and the whole mechanism could get sloppy. These move the back plate, there should also be a return spring that gets weak and sloppy.

It's a simple system that works rather well when it's working properly. It not only increases advance with revs but also on acceleration.

There is a tendency to replace points with electronic. These need less attention but have generic A/R and don't always play as nicely.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote redmunk Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2018 at 18:43
Thanks all. Pulled apart the advance mechanism, cleaned my grease off and replaced it with a light oiling which helped the hunting - I think the grease was damping it. 

Set the static and advance timing against the manual and it runs a lot cooler and starts better now, the previous timing was way off. 

I had a pair of industrial blowers borrowed from work on the barrels and it was very hot after a fast idle before, now it’s well below 100C on the rocker covers. 

Now to balance the carbs again...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cylvabirch Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 2018 at 05:48
I found with my T3 that the float height was critical and had to be cock-on.
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