High revs

sogood

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May 11, 2016
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I find it difficult if not impossible to get my revs at idle down to the recommended 650/700 on my 5.7 Mercruiser with a Rochester Quadrajet. I've reduced the throttle idle screw right back off the throttle arm and still it idles at 900+ RPM's.
There are several things I want to do but they all require the engine to idle at the proper speed, so, is there a common/usual reason why an engine might rev too high? Th choke is functioning properly and fully. The air cleaner/ flame arrestor is clean and clear. Would adjusting the timing help? I originally set the static timing using a timing light, but without the idling issue sorted, so could my timing be a little too far advanced or retarded. The ignition is thunderbolt without the need to ground any wires etc.
Any thoughts appreciated.
 

Rick Stephens

Admiral
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Aug 13, 2013
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Take the control cable off to be certain it is not holding the carburetor open.

Worn out shaft holes can also add a lot air through an uncentered plate. As can any foreign material in the carburetor or misaligned parts, like loose plates.
 

harringtondav

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May 26, 2018
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Worn out shaft holes can also add a lot air through an uncentered plate.

Yes. Air leaks can cause high idle. Make sure your carb-to-manifold joint is tight and the gasket is in good shape. I can't imagine any vacuum take offs in a marine carb, but make sure if there are any they are either capped well, or the hoses are good.

Also a loose intake manifold, or a loose vacuum supply plug in the manifold could cause it.
 

kenny nunez

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Jun 20, 2017
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Any time I install a Q-jet I start by holdding the throttle and secondary wide open and gently tighten the mounting hardware while opening and closing the throttle. Make sure that the gasket has the hard spacers that the bolts-studs go through to keep from twisting the throttle plate.
If the engine has ever backfired through carburetor the throttle shaft or plates on the secondary could also be damaged. Remove the carburetor and check if the secondary plates are seated by holding it up to the light. You may have to remove the throttle plate to re seat the butterflies.
 

sogood

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May 11, 2016
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Many thanks for the responses. The carb has been rebuilt and all seems to check out but I will certainly look into any possible air leaks. I don't know if it's relevant, but on start up, it tends to blow quite a bit of black smoke for a while, until it warms up. searches tend to suggest that this is a symptom of rich running, but until I get the idle right, setting the mixture screws properly is a non runner. I've also read that the smoke can be attributed to ignition timing being a bit retarded, which is why I mentioned the possibility of it being a timing issue. Thanks again and I will update.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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that is 900+ RPM while in the water and in gear?

verify the throttle cable isnt holding it open

if you did the rebuild yourself and you used screws from the carb top on the carb base, they are longer and will bottom out prior to clamping the gaskets leading to a vacuum leak.
 

sogood

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May 11, 2016
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The carb was rebuilt when I got it and I've had no issues with the top screws that I know of. The throttle arm/ cable is adjusted to ensure that it sits home nice and snug, with the throttle closed fully. I'll get back at it by the weekend and look into any air/ vacuum leaks. Thanks for the input.
 

harringtondav

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May 26, 2018
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The carb was rebuilt when I got it

This is a long shot. I've never been inside your carb model, so I don't know if it is possible for someone to get the primary and secondary jets mixed up. Or install the wrong primary jets. Too large intake jets will cause a high idle.

I rebuilt my pal's 4BBL weber off of his 350 mag. I removed the 'same' weber from my 4.3 and installed his to verify the rebuild. It ran fine, accelerated fine, but the idle was very high. I'm positive it was his larger primary jets.

Is this a carb you've been using, or a new/reman? Possibly you have the wrong carb.

Someone smarter than me on the forum can lead you to the Rochester data that will show the correct jets, etc for your engine application.
 

Bondo

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The throttle arm/ cable is adjusted to ensure that it sits home nice and snug, with the throttle closed fully.

Ayuh,..... So yer sayin' that you can back out the idle adjustment screw to zero, 'n it still idles at 900 rpms,..??

Yer black smoke when cold is a misadjusted choke,.....
 

sogood

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May 11, 2016
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361
Yes, all the way out and it still won't help, it still idled high.
 
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