Really high RPM after carb rebuild

[DMully]

Hi all, I read through different posts on this site and didn't find an answer.

I rebuild the carb on my 77 Bronco (I was told by Mike's Carb that it's a 2100) and it starts easily but the RPM shoots really high. The butterfly valves appear to be closed. I disconnect the choke, pulled the high idle screw out so it's not touching and disconnect the throttle linkage (as well as pulling the idle screw out so it's also not touching) to make sure nothing was keeping the valves open. Still same issue.

I capped off every vacuum port I could find on it.

I sprayed brake and carb fluid around the base and did not hear any change to the RPMs. I had to do this quickly because it's revving so high. I hold the top/choke valve almost all the way closed to control the RPM.

I made sure that the carb to EGR spacer and EGR spacer to intake gaskets match the old ones.

Can it be related to the Power Valve or the float level? How is it revving so high if the butterfly valves are closed?

I'm stumped so I took this video of it starting. This is from a cold start this morning. Help!

https://drive.google.com/open?id=1X2VZt2WH63W9EmRnCQXAABc9ZMlL68vB

 

[Broncobowsher]

Did you back off the idle speed screw? You mentioned the fast idle on the choke isn't the issue, but the regular idle speed screw. Can't judge too much just by "it looks closed", Half a turn makes a huge difference in idle speed. Unless it is backed off so far it isn't touching, it is holding the throttle open. Never expect the idle speed to be set after a rebuild.

 

[blubuckaroo]

The video doesn't show the throttle lever. I can see a zip tie around the linkage.

Is the idle speed screw or the anti-dieseling solenoid holding the throttle open a bit?

Also, I've had the mechanical linkage bind and hold it open. Give the throttle lever a push in the closed position to see.

 

[langester]

That zip tie is strange for sure. Might make sure its not hanging the linkage up. You might check the cold idle adjustment as well, it might be holding the throttle open too.
Good luck!

 

[DMully]

The idle screw is backed out very far so that it can't touch the linkage. The zip tie was there (and I put it back) when I bought the vehicle. It's holding some other linkage (from a vehicle with an automatic?) that just flops around if I remove the zip tie. I took some video with the zip tie removed so that you can see. No change in performance with or without it and it doesn't impeded movement of the throttle linkage.

Notice in this video (at 40 seconds in) that the RPM drops when I pull back (i.e. close) on the throttle linkage. Therefore, it appears that the butterfly valve might be slightly open. However, it doesn't look that way when I took it off. See second video...

https://drive.google.com/open?id=1TtXacDSlFFzd_L8FISWWGTFIL5Jm4xjA

I took the carb back off and verified the valves are closing. There is a small amount of play that could leave the valve open but the spring connected to the bottom of the linkage should hold it fully closed. There were two springs on it prior (and one now) so maybe it just needs more pressure to fully close. Perhaps a new/stronger spring will help keep it closed.

https://drive.google.com/open?id=1ziaQJM7zXqJdIryrl_J6VELEQn-7NIPz

Hopefully, the videos help...

 

[B RON CO]

Hi, there are a few things to check.
Your original choke has been converted to and electric choke, and the original stove tube is probably now a vacuum leak.
Most guys do not remove the butterfly plates and throttle shaft during a rebuild. If you took it apart you may have either mixed the butterflies up, put them back upside down (an edge is tapered) or misaligned one or both on the throttle shaft. That could explain why the throttle does not want to close alll the way.
It looks like the zip tie on the lever is for a tranny kickdown. I would just leave it off.
For safety you should run two return springs, in addition to the one on the throttle shaft.
It seems to me that something is keeping the throttle from fully closing, and another spring is not the answer.
Good luck

 

[Slowleak]

I would check the fast idle cam screw on the choke to make sure it’s not holding the throttle open.

If the throttle plates were removed from the throttle shaft, they may have been reversed which can cause an air leak.

What happens when you turn the mixture screws in? How about the timing, is there any chance it may be advanced too much?

 

[DMully]

B RON CO, you were spot on... I did not remove the butterfly plates during rebuild but I did remove/clean the screws. I took the carb back off and noticed that if I closed it gently, it would hang up just a hair before closed.

I loosened the screws on the butterfly plates and attempted to adjust them (trial and error) until it closed smoothly. I also put both springs on the linkage (I only had one on there).

The RPM is slightly high but it's not crazy like it was.

Next issue... white smoke is pouring out like a banshee. I thought it might be the Seafoam I poured into the intake when carb was off but that should have already burned off. I'll need to start a new thread on that one. I'm praying it's not a gasket...

Thank you all for your help and I hope my experience helps someone else!

 

[DMully]

Much better...

https://drive.google.com/open?id=1cUSmUYToUnb-zlV6Vu1w3DaAIT70hbp7

 

[DirtDonk]

Gettin' there!

A couple of observations in addition to what the others have already said.
You can't really check for vacuum leaks (especially small ones) when the engine is revving up like that. Some might show up, but most will be totally overpowered by the high-speed and different mixture of the higher rpm.

The choke slowing the engine down like that is normal, because you were literally taking it to the "choking" point where there just was not enough air allowed into the carburetor to support the engine at that rpm/throttle setting.

Glad you figured out the throttle blades. Common issue if things have been messed with. But if all was well, you should literally be able to let the engine die by turning down the idle screw. By the time the screw is off of the throttle lever completely, the engine would have stopped running.

So you're close. But still not all the way.
What is ignition timing at that lower speed? Was it all working well before, or could the timing have been messed with a little before all of this started?
Disconnect the vacuum advance to see if that changes the idle speed.
Check the EGR valve for function too, as it's a direct line into the intake. I believe this type on the Bronco would take exhaust gasses directly from the intake re-route it to the carb? Or is yours different with a bit tube going somewhere? If it's this type, and the old valve is stuck open, you may have a really big vacuum leak there.
I'm not an EGR expert on these things though, so someone else may have to provide more info on that.

Good luck.

Paul