Dual Carbs, Idle Options

[cpavlenko]

Starts right up with good idle, but after a good drive when stopping for red light, it wants to start dying and drops idle real low. Like 500 or less rpm. My idle on the Weber 40's when cold is about 1200 rpm and and smooth. Any thoughts out there. 1.7 built to a 2.2. Engine has almost 600 miles since build. Other than this problem, it runs great. Oh oil temp after good run bout 210 to 220 degrees in 95 degree weather.

[nditiz1]

What linkage?

[cpavlenko]

What linkage?

Crossbar linkage that came with Webers.

[nditiz1]

That could be your problem if they are not in good shape and setup correctly.

As the engine bay heats up it will expand the parts. Usually this would make the idle faster, but i have also seen it the other way. See if when it starts to do it you can feather it and pop off the linkage. This will tell you if it is the linkage.

When you do get to a stop and it wants to die does reving it bring it back to 1200 or only driving? Could also be float levels are they set properly with the gasket. Could be the heat is affecting the fuel pump if it is old or worn out.

Start with the linkage and we can go from there.

[michael7810]

Couple thoughts. The linkage shouldn’t be an issue if both stop screws are set properly. Even if the cross bar is affected by heat the throttle plates can’t close if the screws are set. Is the idle mixture set while hot? Maybe try setting the mixture screws after a run when the idle is low. Are the carbs synced? Is the centrifugal advance sticking when cold and retarding too much when hot (check timing at idle cold and see if it changes when hot). Are you using vacuum advance, if so make sure it isn’t advancing the timing at idle (check hot and cold). As said above it could be the float setting.

[cpavlenko]

Couple thoughts. The linkage shouldn’t be an issue if both stop screws are set properly. Even if the cross bar is affected by heat the throttle plates can’t close if the screws are set. Is the idle mixture set while hot? Maybe try setting the mixture screws after a run when the idle is low. Are the carbs synced? Is the centrifugal advance sticking when cold and retarding too much when hot (check timing at idle cold and see if it changes when hot). Are you using vacuum advance, if so make sure it isn’t advancing the timing at idle (check hot and cold). As said above it could be the float setting.

I'm thinking jets, got 60's in the carbs now, going to try 65's. 60's might be starving for fuel when hot at a stop.

[Mark Henry]

Couple thoughts. The linkage shouldn’t be an issue if both stop screws are set properly. Even if the cross bar is affected by heat the throttle plates can’t close if the screws are set.

Wrong.

[thelogo]

Cables stretch . espcially with new builds/ installs

Same thing with new mountain bikes

So linkage may not be bad ... But just not broke in then adjusted ..enough .... Cable Needs to be taught ?

[michael7810]

Couple thoughts. The linkage shouldn’t be an issue if both stop screws are set properly. Even if the cross bar is affected by heat the throttle plates can’t close if the screws are set.

Wrong.

How could the linkage force the throttle plates closed when the stop screws on the carbs are set to touch the linkage at idle? Wouldn't the force need to be so much that the screws and/or the throttle plate shaft bends? Seems unlikely there would be enough force to do that.

[Ansbacher]

I was never able to get a good stable idle over the whole temperature range until I replaced my raggedy intake gaskets where the intakes meet the heads. Any air leak there causes all kinds of grief. Check your gaskets and replace if unsure of condition.

Ansbacher

[nditiz1]

Couple thoughts. The linkage shouldn’t be an issue if both stop screws are set properly. Even if the cross bar is affected by heat the throttle plates can’t close if the screws are set.

Wrong.

How could the linkage force the throttle plates closed when the stop screws on the carbs are set to touch the linkage at idle? Wouldn't the force need to be so much that the screws and/or the throttle plate shaft bends? Seems unlikely there would be enough force to do that.

It depends on where the linkage is installed. If the linkage is on the opposite side of the stop it is feasible to force one side of the carb past the stop thereby closing the butterfly. If you have ever worked with dual carbs and no linkage you can easily see this by force the shaft from the non stopper side. The rpm's will go down.


Upping the idle from 60 to 65 may just mask the issue if it is in fact related to an intake leak or in my case a cylinder to head leak. In order to diagnose properly you will need proper equipment. You may want to temporarily install a AFR meter to gauge if you are in fact going lean. If you are 12-13 at idle then you shouldn't need to go up an idle jet.

As someone already mentioned check your intake gaskets. These could easily be creating a lean condition if air is getting by.

Lastly, which distributor are you running and did you set the timing when the engine was warm/hot?

[cpavlenko]

Couple thoughts. The linkage shouldn’t be an issue if both stop screws are set properly. Even if the cross bar is affected by heat the throttle plates can’t close if the screws are set.

Wrong.

How could the linkage force the throttle plates closed when the stop screws on the carbs are set to touch the linkage at idle? Wouldn't the force need to be so much that the screws and/or the throttle plate shaft bends? Seems unlikely there would be enough force to do that.

It depends on where the linkage is installed. If the linkage is on the opposite side of the stop it is feasible to force one side of the carb past the stop thereby closing the butterfly. If you have ever worked with dual carbs and no linkage you can easily see this by force the shaft from the non stopper side. The rpm's will go down.


Upping the idle from 60 to 65 may just mask the issue if it is in fact related to an intake leak or in my case a cylinder to head leak. In order to diagnose properly you will need proper equipment. You may want to temporarily install a AFR meter to gauge if you are in fact going lean. If you are 12-13 at idle then you shouldn't need to go up an idle jet.

As someone already mentioned check your intake gaskets. These could easily be creating a lean condition if air is getting by.

Lastly, which distributor are you running and did you set the timing when the engine was warm/hot?

My mechanic has been working on 914's since 1970. We are just working out the bugs, and we decided to go with the jetting first. I still have a percolation problem after adding phenolic spacers at head to manifold. there are no leaks. So where going to go through a slow process, and figure this out step by step. oh so much fun.