Idle Speed Hanging - MFI Engine

[Rosco_NZ]

Sometimes the idle takes a second or 2 to come back down .. it can hang at 2500-2800. A quiet flick on the cross bar and it drops back down. I’m suspecting the cross bar bearings in the plastic stacks … did anyone ever try replacing them?

[Jonny042]

I'd pop the balljoints off the linkages and check the entire system for something binding. There are a huge number of things that could be hanging up in the system, but they're pretty easy to isolate. IMO the needles in the stack aren't all that likely to be an issue, although they're a common size of metric bearing (from memory, a 16mm OD, 12mm ID) and easy to change, But probably best done off the car, so that's a can of worms, potentially.

[Jonny042]

BTW - As set of these tools in very useful for popping the linkage joints off. They should come off easily but it needs to be an even pressure pulling straight out to get them to pop.

[Henry Schmidt]

Link rods/ ball ends, throttle bodies and distributor, in that order.
The needle bearings are easily replace but probably not your issue.

Remove and lube each ball end, check throttles for shaft play and retard the timing slightly. The distributor springs get weak over the years and the flyweight can hang up leaving too much timing to return to idle.

[Henry Schmidt]

After thinking about your issue for a few minutes I remember seeing an issue that you might want to check.
The throttle bell crank on the peddle assembly has bushing that will deteriorate over time. Some have been replaced with bronze bushings but if the original bushing come apart, the bell crank can hang up in the tunnel.

[Rosco_NZ]

Thanks for all the ideas ... however the car has been through a long restoration ... pedal box new, all ball sockets & arms new & lubricated, the throttle bodies have been overhauled and move freely. I do recall the cross bar bearings being poor when reassembling ... but weirdly it seems a lot happier after a 350 mile drive home .. managed 22mpg on very light (65 mph cruise) throttle!!

I'll keep an eye on it .. guess one of the stacks needs to me removed to do the bearings ..

[Rosco_NZ]

Thanks for all the ideas ... however the car has been through a long restoration ... pedal box new, all ball sockets & arms new & lubricated, the throttle bodies have been overhauled and move freely. I do recall the cross bar bearings being poor when reassembling ... but weirdly it seems a lot happier after a 350 mile drive home .. managed 22mpg on very light (65 mph cruise) throttle!!

I'll keep an eye on it .. guess one of the stacks needs to me removed to do the bearings ..

[Henry Schmidt]

New information....handy.
If the crossbar was re-plated it's not uncommon for the extra thickness to challenge the crossbar bearings.

EDIT:

Quick comment: As a company that restores MFI intake systems we use a huge number of new ball socket rod ends.
In resent years we are seeing a large number of new ball-sockets than fit overly tight. Given that most throttle body restorations
include plating of the butterfly lever arms and new sockets are installed, we are seeing tolerance issues.

Good rule of thumb: some times new parts don't work. When looking for problems with new assemblies, never assume that a new part
functions as expected.

[targa72e]

Had the same issue. Checked all the MFI parts and could not find a issue. It was distributor weights hanging up. replaced the distributor and problem was fixed.

john

[Rosco_NZ]

Good line of thinking Targa … but I’ve locked the distributor and running the curve in software .. no vacuum retard on this scooter ..

[Jonny042]

Originally Posted by Rosco_NZ

Good line of thinking Targa … but I’ve locked the distributor and running the curve in software .. no vacuum retard on this scooter ..

And you did state that flicking the crossbar gets it back to normal idle. Obviously you don't have a fuel cut solenoid either!

Henry has a point about the new parts not fitting and tolerances. Sometimes I feel like I spend all my time trying to make a part that should work, work.

[Rosco_NZ]

The cutoff switch is there … doesn’t seem to affect the hanging. I’m thinking I need to replace the bearings … everything pointing to them … or at least rule them out.

[jackson51]

Have a wool blanket and fire extinguisher handy… then, recreate the idle hang up in your drive way.
Spray a lubricant (lightweight silicon grease aerosol is my current favorite) on the suspect parts or area.
Might confirm the issue or location.

Also, you noted replacing parts. Did that include all spiral springs on the system? They might be misaligned, binding, or not providing enough resistance? Maybe a rod was bent or twisted slightly during the restoration?

Generally I run H. Schmidt’s suggestions first.

[Rosco_NZ]

Hasn’t been hanging as much last week or so … The cross bar return spring was replaced and lubricated with the build. I have the bearings on the shelf … think that’s my next project. Don’t think I’ll be going wild with flammable spray in the hot engine bay …

[roblav]

For me, this issue was usually the mechanical advance mechanism in the distributor. It gets hung up in advance and won't retard at idle.