Can’t get idle speed below 1300 RPM

[steveg]

Hi Patrick/Steve,

Patrick,
First, 1/10" = 0.1" or 2.54 mm. I mis-spoke about the needle step and really ment the to say the line on the needle that you set to the bottom of the piston.

Steve,
I agree that setting the needle a 1/10" deeper into the piston would be equivalent to dropping the valve 1/10" to increase richness. However, that would be at run and not at idle where the butterfly is closed and the HD idle is supposed to be solely dependent on the setting of the slow idle needle valve adjustment. Maybe there is a reason this works that I don't understand and the reason I qualified the level of my knowledge and what this action would do.

Ray(64BJ8P1)

It would be at idle, too. All the fuel supply comes through the main jet. The slow running valve admits a small flow of air/fuel mixture that bypasses the closed butterfly.

 

[RAC68]

Steve,

Then, am I correct in concluding that raising the pin would increase richness of the air/fuel mix at idle set by the slow-run needle valve?

Ray(64BJ8P1)

 

[steveg]

Steve,

Then, am I correct in concluding that raising the pin would increase richness of the air/fuel mix at idle set by the slow-run needle valve?

Ray(64BJ8P1)

IMO it would. Unlike, say, Webers, there's no separate idle jet.

 

[tahoe healey]

I have th opposite problem. I live at 6500 ft and have set my idle at 800-900. When I go to sea level the idle goes to 1200. If I crank down the slow idle 1/8 turn I'm good to go until I head home to the mountains. Often I leave it at "mountain" setting if I'm only down there for a day.

 

[Bob_Spidell]

I used to 'tune' for altitude, now, with a little margin, I usually don't bother. I just run a bit slow and rich above 6K feet or so.

Side note: I can spend a day tuning my engines per 'the book,' but then I always seem to need some tweaking on the road. When I rebuilt my engine in 2016, I tuned and tuned and tuned in the shop--with ColorTunes and flow meters--then, when on a road trip it started dying at idle. Opened the butterflies a little and richened it up a little and it sits at 700-800 all day depending on temperature, altitude, etc. I'd guess every engine is just a bit different, and likes a different tune.

 

[bob hughes]

I had a similar experience with my BJ7 after rebuilding the engine.
It had never idled properly since I bought it in 2002, but I found that my dizzy had an odd feel about it, the moving plate wobbled, removing the fixed plate revealed that the star spring helping to hold the fixed to the moving plate had lost an 'ear' so I overhauled the dizzy including the vacuum advance mechanism and lo and behold the car can idle at 750 once warmed up no probs.



Bob

 

[SteveHall64Healey]

Update on this: the throttle linkage is not a factor as the idle speed remains high even when the linkage is disconnected.

I went back to basics and did the full set up from scratch per the procedure from Steve Byers and the SU workshop manual. The idle speed comes down to a more reasonable 700 RPM. Test drive tomorrow.

 

[SteveHall64Healey]

The issue here turned out to be ignition timing. The engine builder told me that they’d set timing to 15 DBTDC, but when I put a timing light to it, there was more than 40 degrees at idle. The engine ran well expect for low-end hesitation, so I suspected the pulley was installed incorrectly as I never heard anything resembling pinking from the valve train.

After some effort with a piston stop tool, I validated that the pulley was indeed installed correctly. Upon reducing the idle speed timing to 15 DBTDC, the idle speed dropped happily to 500 RPM and runs quite smoothly. The carbs are now set to normal levels on slow-run valve and idle fuel, and are balanced for air flow.

Steve

 

[Randy Forbes]

How's your (ignition) timing? That too has an affect on idle speed.

The jet bridge isn't so easily defined as a measurement number; a slight flat spot on one (1) of the needles will render that method useless. Besides, that has negligible effect on idle speed anyway, its purpose is mixture strength.

Are the throttle plates being held open by any of their adjustment screws? Or choke linkage not fully releasing?

Static Ignition timing was set by the rebuilder at 10 BTDC. Will check throttle and choke release....

The issue here turned out to be ignition timing. The engine builder told me that they’d set timing to 15 DBTDC, but when I put a timing light to it, there was more than 40 degrees at idle. The engine ran well expect for low-end hesitation, so I suspected the pulley was installed incorrectly as I never heard anything resembling pinking from the valve train.

After some effort with a piston stop tool, I validated that the pulley was indeed installed correctly. Upon reducing the idle speed timing to 15 DBTDC, the idle speed dropped happily to 500 RPM and runs quite smoothly. The carbs are now set to normal levels on slow-run valve and idle fuel, and are balanced for air flow.

Steve

What a surprise, I never would've guessed!

But hey, look at all the other neat things you got to check/verify in the process