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T3 will not idle when hot

Series4

Series4

Cruisin' Guzzisti
Joined
May 11, 2009
Messages
367
Location
Melbourne, Victoria
OK folks here we have a problem that defies my diagnostic capacity.

Bike is a bog-standard '75 T3 with 28k miles. It was running perfectly up until recently (because the chrome was starting to lift) when I replaced the cylinders with new 850cc Gilardoni kits. At the same time the valves were replaced (still standard size) and the inlet ports were opened up slightly to match the manifolds. The engine retains the standard coils/points ignition.

The bike rides and performs very well with one exception; once it reaches full operating temperature it will not idle. Pull up at a set of lights for example and over perhaps a five to ten second period (with gearbox in neutral) the idle speed slows until the engine stops. It doesn't suddenly cut out, it just slows down and dies (I know how it feels...). While the engine is cool or warm it idles perfectly but once hot it just won't. We've (me and Brad Black, Guzzi and Ducati spanner extraordinaire) been able to replicate the problem on the stand as well as on the road. On the stand we've been watching the timing and the mixtures and the numbers stay within range all the way down until the engine stops. There's no significant fluctuation in the mixtures and the timing doesn't suddenly go awry. It's had the problem over a number of tanks of fuel, each from a different service station. Once the engine stalls it starts again right away on the button and will idle happily with the throttle cracked open just a fraction.

  • To date what we have done to try and resolve the problem:
    Dismantle and check both carbs (no issues found)
    Battery voltage is perfect both static and at all engine speeds
    Drop the idle jets from 50 to 48 (recommended by the cylinder head man). Otherwise all the jetting's standard and unchanged
    Synchronise carbs (perfect)
    Check the distributor is advancing correctly (OK)
    Ensure there's nothing sticky or jammed in the distributor mechanism (OK)
    Swap complete distributor (from another low-mileage T3 that was running well)
    Check for and rectify air leaks (one very minor one rectified)
    Change coils (from another low-mileage T3 ditto)
    New plugs, leads and caps
    Coils are now powered via relay, each with its own direct feed
    New condensors

So, the problem appears to be heat-related but for the life of me I can't what the heat's affecting? And what's changed as a result of the engine work to cause the problem? The only thing we've found is that the LH carb only reacts to large adjustments of the idle mixture screw, but the mixtures are fine on both sides.

Peter (who'd pull his hair out if it were long enough)
 
Have you re torqued the heads since the rebuild which you should do and then adjust the valves on a stone cold motor.
 
Yep, twice. Bike has now done about 1,000km since the rebuild and the problem does not appear to be related to ambient temperature.

Peter
 
Series4 said:
I'll check the clearances again tomorrow. I've been going by the book i.e. 0.22mm, is that correct?

Peter
Click to expand...

Yup, but I run all my roundfins at 5 and 7 thou inlet and exhaust and even when super-hot the just chug along.

When it dies is it sudden? Are the points correctly set and the timing right? Does it happen on one side first and then the other, or both at the same time?

Pete
 
It just sort of fades to a halt, not favouring either side although the RHS responds better to carb adjustments. The points are correct (have been on both distributors we've tried) and the timing is correct all the way down to when it stalls, we watched it with the timing light. And the TDC and timing marks on the flywheel have been verified.

I test-rode a Breva 1200 today. So nice...
 
Pete mentioned the choke plungers. If the cables are tight, it could be flooding out a warm engine. Have you checked them yet? Also set the idle to about 1,000 rpm. Most of the old bikes have the idle set too low anyway. The problem sounds fuel related since she will restart immediately. If were from heat related electrical problem she would have to cool to restart.
 
I'll look again today when I'm doing the valve clearances. That was one of the first things we suspected but the air/fuel ratios do't show excessive richness from sucking fuel or leanness from sucking air.

Peter
 
I bet it's the idle mixture screw and the idle speed screw. Does it idle right off when cold? ...warm? If it does, it's too rich. When hot, try resetting these screws per spec, then turn the idle screw in, slowly in small increments, (to increase idle speed) equally on each side till it maintains the idle. Afterwards check the balance again.

If that does not work take a ride till the engine is hot. Then shut her down, replace the plugs with brand spanking new plugs. Start her up but do not throttle it, let it idle and as long as it does then read the plugs. A leaky choke valve should be apparent. I haven’t seen one of these fail yet.

Does anybody know if the spark plug temperature range affect the spark at idle?

Rafael
 
Re: T3 will not idle when hot - fixed?

Well, I don't know exactly what's done it but the Old Sow seems to have got over her 'issue'. This morning I re-torqued the heads (just 'cause I was there anyway), re-set the valve clearances and wiggled the choke cables. Burned some rosemary as an offering, scattered the ashes to the breeze and nailed an anchovy to the garage door.

Went for a ride and the old tart was now idling too high (porca scrofa!). Tweaked the mixture and air screws and now all seems to be well. TIme will tell, the sword of Damocles is still hanging over her head. Maybe I should have offered her money?

Anyway, thanks to all for your responses. There's a good side effect from all this, my spare coils and distributor have gone towards getting a Le Mans back on the road after its Rita failed.

Peter (who's really, really taken by how good the Breva 1200 is. Trophy wife, maybe?)
 
Ditto on the 1000RPM idle. My T3 tends to lose idle RPMS when at altitude, so I've taken to setting it a bit higher (1000) where I live near sea level so it doesn't die at altitude. Seems to me I recall it also doing that when really hot, but its been a while since I had the idle lower.
 
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