Technical Got another question I've been scratching my head about: idle speed

[nzbobc]

Several months back, I replaced the carb in addition to to doing a bunch of other things on the motor. The motor starts immediately, accelerates, and runs great.

The one thing I can't understand is that as things settle in, I've had to turn in the idle speed screw on several occasions as it gets to the point where it idles too slow. It does this whether the engine is cold or hot. I've checked the idle mixture over time and it's set in the sweet spot.

Any thoughts as to what's causing the idle to continually drop over time? From the photo, you can see that the screw is set pretty far in. I've had to snip off the spring a couple of times to turn the screw farther in. Back when I first did the work, the idle speed screw was about 5 turns in.

The carb is a 28IMB (on a 650cc).

 

[fiat500]

Bob, you're sure to get the technical answers regarding the carb from elsewhere. As a piece of lateral thinkng I just think it's worth checking the tappets are still correctly gapped. When a similar issue happened to me a few weeks ago, the problem disappeared like magic when I adjusted the clearances.

 

[the hobbler]

Morning Bob;
Following on from Peter's suggestion, what cam have you got in your engine and what clearance are you adjusting the tappets too?

 

[Bambino]

I found the following on another website - it may be of interest.

The best setting for clearance is specified by the manufacturer, if set perfectly the valve will be seated during the time the base circle of the cam is in contact and will open the valve as per the calculated duration just at the right time.
Too much clearance = reduction in opening of the valves,valves open late, the rocker arm with the follower comes in contact a bit later and in turn opens the valve later .Also it will create the characteristic noisy clatter after heating up almost similar to a diesel engine.
Too less clearance = valves remaining open for a longer time,loss of power.
Extremely tight = valves not closing completely during the base circle contact and allowing the hot gas to keep flowing and burning the valves. intake valves can also get burnt like the exhaust valves.
The behaviour and feel of the engine and the exhaust note will clearly indicate if you have got it right ..
A good test would be to let the car idle till it gets hot.
After getting hot this clearance begins to play a more significant role.
During idle:
-If the idle remains constant and smooth you are good to go.
-If the engine begins to stumble or the idle becomes rough it means you didnt leave enough clearance
During driving:
If the idle is smooth and there is no roughness it means we can now drive off and do another check.
-If there is loss of power felt then it means tappet is still a bit to tight, try for the next bigger (+ .0xx mm) gauge.
-If there is that noise again then it means you are still loose ; go for the next lower setting (-0.xx mm) gauge
I suggest you go for the setting in 2 stages :1st make sure the feeler gauge just fits and slides a bit tight, if this does not work then go for the next .00x setting but this time let it slide smoothly.
Also always remember to inspect with gauge again after tightening the locknuts just to make sure it didn't shift.


Chris

 

[nzbobc]

Thanks for that guys, once the weather improves a little bit, I'll check and adjust the tappets then go from there.

Hobbler, to answer your questions, I'm setting 0.20mm (inlet) and 0.25mm (exhaust) when cold (It's a 650). I don't know what I have for a cam as it was modified by a previous owner. All I know about it is what the previous owner wrote in his advert: "Hi-lift cam (good low end torque)."

 

[the hobbler]

Afternoon Bob;
Those setting sound about right---they are what I set my 35/75/75/35 cam to.

 

[Bambino]

My 695cc engine has a 43/77/77/43 cam and the clearances on a cold engine are 0.20mm on the inlet and 0.25mm on the exhaust - the same as yours.
Chris

 

[nzbobc]

So, I found the reason why my idle speed kept changing: The throttle lever was rotating about on its shaft that goes thru the body of the carby (and which the choke plate attaches to).

While in the photo, the edges of the hole that the shaft goes thru look flat and parallel (these flat edges should sit flat against a flat part of the shaft), there's a big gap between the lever and the shaft. It looks like it should be a snug fit between the two.

I can't snug the outside nut down much to take the play out of the throttle lever as the choke plate starts binding.

Anyone know where I can get another one of these throttle levers (with the ball end)? Or a way of fixing this one (JB Weld, maybe?)

 

[the hobbler]

If you can't get a bracket with a ball already on it, get a 'non-ball' bracket and a separate ball-end from 'Burlem fuels'--they are the supplier of all things 'SU', and sell the ball-ends as separate items--there is just enough clearance on the bracket to bolt on one of their 'ball-ends'--I know, because that is how I have done my throttle link.

 

[Bambino]

Anyone know where I can get another one of these throttle levers (with the ball end)? Or a way of fixing this one (JB Weld, maybe?)

Hi Bob,

The throttle levers are different between the IMB series carbies. The angle of the slot is different, for what reason I'm unsure - see the attached photo.
I have one from each of the 24 and 28 carbies (neither with the ball attachment though) and as I'll never be using my 28IMB again, if you send me a PM with your email and street address, I'll pop the correct one in the post to NZ for you.
BTW, how is the spindle?

Chris

 

[nzbobc]

Chris,

Thanks! That's great!

I'l send you a PM...

 

[Bambino]

Posted today - let us know how it goes.

Regards,
Chris