1600 rpm idle

[F250 Restorer]

17059547. Okay. This is impossible. All 4 throttle plates are seated. All v. ports are capped. Hot air choke passage was plugged. How is this thing getting air and fuel? I close the mix screws to no affect.

This was modified using Cliff's #3 recipe. Mix holes .090. Down angle .053. Float at 1/4". All air bleeds at .070. Jets 73. M.rods 45. Since I converted to electric choke and plugged the v., I added by-pass air of .060 per side to compensate for what it wasn't getting from choke v. I ran this carb w/o the by-pass air drilled, and got the same fast idle.

I don't understand how it is getting fuel and air.

[Cliff Ruggles]

What engine?  Specs?  How much initial timing?  Adding timing from the vacuum advance via manifold vacuum?

I've ran into quite a few engine combos that didn't like much timing at idle speed and if you added any with the vacuum advance you lost control of the idle speed.

If all the above is in order you simply have an air leak someplace or the throttle plates aren't fully seated without any light showing.  It has to get a LOT of air someplace to idle that fast.  If you rule out the carb start looking elsewhere.  Gasket under the carb?

I ran into one very similar situation recently on a tractor engine I did a carb for that was burned thru between the exhaust and intake.  Did a PERFECT job on the carb and the owner kept throwing it back in my face claiming he checked EVERYTHING else including compression, wires, plugs, timing, etc and it just had to be the carb.  Third time he showed up with it I wondered over and helped with the install.  Took my a few minutes but I discovered a HUGE hole between the exhaust and intake.  Had him buy a new intake instead of a 70 year old junkyard part and ALL the issues went away........

[F250 Restorer]

Thanks, Cliff. This is another I6. Ford 300, over-size valves, ported, cam 280adv, .480 lift, 112 lsa. I run 14* base timing. V. advance is plugged for now.

I have a sliver or crack of light around the p. throttle plates. I will try to adjust them. But even with that slight amount of air opening, that is not what would normally pass with the idle adjusted, right? So, as you said, there has to be a leak as well.

"I really like the sound of the engine with those small primaries."

Regarding the forum change: I think that is a good idea about a small charge for your service. However, it may be difficult b/c you are not the one turning the wrench on the QJ's, and the public can be difficult once $ is exchanged. Best of luck. 

[F250 Restorer]

It dawned on me today that in my infinite wisdom I drilled the baseplate for by-pass air. Tomorrow I will install plugs in the main body of the by-pass passage. Hopefully that will drop the idle.

I checked everywhere for v. leaks. Nothing.

[Kenth]

I added by-pass air of .060 per side to compensate for what it wasn't getting from choke v. I ran this carb w/o the by-pass air drilled, and got the same fast idle.

Has to be something else?

[F250 Restorer]

When I started the engine, echoke element removed, choke flap open, it idled at 800rpm. As the engine warmed up the rpm's increased, as though the fast idle cam were notching up in reverse.

Throttle plates are well seated. I think it is time to give up. Thanks for the help.

[73ss]

Maybe the advance weights & springs are adding timing when this occurs. Check timing when the idle is high.

[F250 Restorer]

I couldn't let it go. It bugged me that I couldn't find out why the carb was idling so high. So, I went back and tore it apart for the 3rd time.

I spent the day tracing v. passages through the 3 sections of the carb. Note: starting fluid sprayed in your eye isn't that bad.

I guess I had not searched the adapter and manifold because I had sprayed all around the base of the carb with starting fluid when the carb was running, and got no bump in the idle, so I figured there was no v. leak around the base. But on examination, I found this:



I was using a thin base gasket to save space. The passage that supplies v. to the choke pull off is exposed through this groove. I remember that the pull off was not working, and I thought at the time that it was bad. Now I realize that it was feeding the engine. Anyways, I think that is the problem. If I install it and it still idles at 1600 rpm, then the problem is beyond me. I'll find out.

[Kenth]

It is not uncommon for ill-fitting adapters to cause hard-to-find problems with the carburetor. Avoid adapters if at all possible.
Original intakes are better than what the aftermarket intake advertisers let people believe.

FWIW

[Bills64]

Intake manifold sealed good or a crack in it? Had a Edelbrock performer intake new and was chasing poor idle quality for some time and decided to clean both surfaces and set the intake on the heads without the gasket. It rocked like you wouldn't believe. Sent it back and put on an original cast iron intake (Pontiac) and no more issues.

[Cliff Ruggles]

The "U" shaped channel started showing up clear back in 1970 and was standard on most units by 1971.  It requires the later carb to intake gaskets that are longer in the front.

I'd also add that your baseplate appears to be from a turbo carb and looks like the vacuum to the PP is routed in from the front so it woln't be drilled to supply vacuum directly.  The front center vacuum fitting would need to be plugged (I prefer to remove them and seal the hole) and a vacuum supply hole drilled in the correct location like a non-turbo baseplate......

[F250 Restorer]

Hey Cliff. Thanks for the feedback. Yes, it is a turbo QJ. The baseplate has been drilled. You can see the hole in the photo below.



I think the .125 gasket would seal this opening, but why chance it? I put in a lot of work on this carb and don't want to be troubled by phantom v. leaks. I found this at Jegs. Notice where the edge of the adapter is in relation to the mounting holes on the primary side of the adapter. I thought about using one of the metal heat barriers sandwiched between two gaskets to seal that edge, but that seems iffy.



as opposed to this:

[Kenth]

This adapter might work for "U" channel Qjets to Holley intakes.

[F250 Restorer]

Hey Kenth. Thanks for the adapter info. I saw that adapter. I avoided it b/c of the price tag. It costs $120, whereas the Edelbrock open style one I posted costs $60.

Do you prefer the 4 hole type? Pls explain.

[77cruiser]

If you know someone handy with a tig you could build up that spot & machine flat again. I did one several years ago.