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High RPM's


Rocky671

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I was on my way home from work on the freeway and i felt my gas pedal sink in.It was verry sensitive.I pulled over come to find out the bracket on the throttle cable broke.I replaced it But,Now In neutral and in gear my rpms shoot up to 3k+ and my gas pedal is not alighned with the brake and clutch pedal like before.Only way to get the idle down is to put it in gear and hold down the brake and hold the clutch half way.

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  • 2 weeks later...

ok guys.Sooo i went out and got a new weber 38 carb.Buuuut,still Everytime i start my truck my rpms sky rocket past 2k almost 3k.I checked the pedal,my throttle cable is not to tight,hose from distributor to carb is hooked up.Any ideas???

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Well there is a fast idle on cold engines. On the Hitachi it's ,.800-2,200 RPMs at the start and reducing as the engine and choke warms. The choke will be on and the mixture very rich. Extra air is added to increase the engine RPMs and speed the warm up.

 

So what happens when the engine is warmed up????

 

 

 

 

The hose from the distributer has no bearing on this. It's ported vacuum from above the throttle plate and not intake vacuum. On a warm and  properly set up engine I have NEVER see RPM go up if an intake hose is off or there's a vacuum leak. ALWAYS the RPMs drop.

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Honestlt this is my 3rd 720 and is the 1st time dealing with this issue.Never had a high idle like this.i have to press the gas a couple of times for idle to drop and the keep pressing it until its warmed ao it doesnt die out.once its warmed up a little bit it idles at about 15

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Again, check the basics first.

 

Since it had this problem before you installed the new carb, I'm guessing it's either in the linkage or you have a massive vacuum leak. The fact that you have to keep pressing the pedal after it warms up tells me that it's probably a vacuum leak. Could be timing too, but if this was ok before, then probably not.

 

If those check out, then it could be the electric choke adjustment.

 

I don't know where to go with a question like this. There are so many possibilities and so many variables, it's hard to give a simple answer. If I had a feel for your mechanical capabilities or some more history on the car/truck, those would both help guide us to a solution.

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If you 'kick' the gas the rpms will drop because the choke unloader is activated. Give your truck a chance to warm up.

 

Once warmed up, adjust the idle speed up to keep it going.

 

 

 

 

Possibly a vacuum hose is leaking.... or the idle is just too low to begin with.

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This is a Weber now, does it have an adapter plate?

 

If everything else is good check your PCV valve, I had one go bad once, it run up past 2000rpms, when I put my finger over the hole it dropped to normal idle.

Yes it does have an adapter.Stupud question.Would not having an air filter kause this problem also

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The PCV valve can fail anywhere between open or closed. Specially if you have an oil burner that clogs it up. It has a valve that is pulled open against a weak spring by manifold vacuum. It is open pretty much any time the engine is running. Naturally more air is pulled in during high vacuum conditions, such as idle and deceleration.

 

The PCV valve is technically a vacuum leak that is compensated for by adjusting the idle mixture screw. Blocking it off during idle should force a slightly rich mixture. At all other speeds this 'leak' is irrelevant as the amount of air entering is proportional to vacuum not engine speed. If the RPMs go down when blocked off, maybe the mixture was too rich to begin with and the extra air makes it rev up.

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The PCV valve can fail anywhere between open or closed. It has a valve that is pulled open against a weak spring by manifold vacuum. It is open pretty much any time the engine is running. Naturally more air is pulled in during high vacuum conditions, such as idle and deceleration. The PCV valve is technically a vacuum leak that is compensated for by adjusting the idle mixture screw. At all other speeds this 'leak' is irrelevant as the amount of air entering is proportional to vacuum not engine speed.

Okay.So do i play around with the idle screw???Also the other daybj was spraying carb cleaner around the 4 corners of the base and all around everytime i sprayed rpms shot up.Is it possible for a bad gasket,to tight or to loose???

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Yes.

 

You have a vacuum leak and too much air is getting in making the mixture lean. Carb spray adds to the air leaked in restoring the air/fuel mixture and raising the RPMs.

 

Try snugging the nuts.

 

If a Weber there are two gaskets and there is also the risk the adapter is already cracked from tightening.

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if you have the cheaper thin carb adapter(cast) if one bolts them down to hard they may crack. theya re different versions so Im just guess which one you have . I cracked one and I gave my spare away to anoher guy and he cracked that one. I went with the tall 1 pc unit. on a L16. On a L20 or taller motor the tall aircleaner might hit the hood. They sell a 1.75 in tall aircleaner if needed

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if you have the cheaper thin carb adapter(cast) if one bolts them down to hard they may crack. theya re different versions so Im just guess which one you have . I cracked one and I gave my spare away to anoher guy and he cracked that one. I went with the tall 1 pc unit. on a L16. On a L20 or taller motor the tall aircleaner might hit the hood. They sell a 1.75 in tall aircleaner if needed

Thank u banzai510...I will change out my adapter plate.

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The two piece adapters are notorious for coming loose. To keep this from happening, use loctite on the threads of both plates. And then use lock washers on the nuts holding the carb to the adapter.

 

Before you tear into it, put a wrench on the carb nuts and see if they are loose.

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  • 4 weeks later...

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