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Idle Speed High

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tached_out View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote tached_out Quote  Post ReplyReply Direct Link To This Post Posted: Jan/02/2018 at 4:17pm
Going back to the OP's original post. No you don't have two separate problems. You have two symptoms of a single problem. Boris is steering you in the right direction. Let me put it in other words.

The problem is, you don't have enough spark advance. You have the huge rpm swing from park idle rpm to drive idle rpm because you have too much of a curb idle (throttle opening) setting trying to compensate for lack of spark advance. 

That Thumper cam has a lot of overlap for street cam. It's going to have the idle characteristics of a more race oriented cam. You have to overcome that with more advance. There's no other way. Here's what you have to do;

In park, close the throttle more. Keep closing it until you get back down to about 800 rpm. Don't worry that it would likely stall if you put it into drive. In fact fact go ahead and put it in drive. Watch it stall. This is the baseline that you are going to work from. Now lets get tuning;

Idling at 800 rpm, vacuum connected, slowly advance the distributor. As you do this the rpm will increase. It does this because the engine is beginning to make more power with that throttle opening. As it does, close the curb idle to get back down to your 800 rpm. Continue doing this until it no longer stalls or drops more than about 200 rpm when you put it in drive. You may want to play with the idle mixture screws during this process to get the highest quality idle. The more you turn out the curb idle screw to close the throttle, the more the engine will respond to the mixture screws. 

This is now the ideal spark advance for idle in park and in drive. It doesn't matter what is indicated. It's what the engine wants, not some arbitrary number some sticker under the hood of the stock vehicle says. Now disconnect the vacuum advance and check that you don't have more than about 38 degrees indicated when you rev the engine until the spark stops advancing. If you don't, you are golden. Just advance it a little bit more until 38 degrees is indicated. If you do have more than 38 degrees when you check this. You have to get into the distributor and limit the centrifugal advance.

Now hook up your vacuum advance and enjoy.

As for the engine run on. It was caused by red hot carbon buildup from the crappy idle mixture compounded by a throttle opening giving it enough air to support combustion with out a spark.

Good luck.


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Midnight Rambler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Midnight Rambler Quote  Post ReplyReply Direct Link To This Post Posted: Jan/02/2018 at 4:27pm
Very well done.

'66 American 440 Convertible 290/M-40/AMC 20 3.15/PS/PB
'04 Jeep Wrangler X Rocky Mountain Edition 4.0 5sp
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WhatISit Quote  Post ReplyReply Direct Link To This Post Posted: Jan/09/2018 at 10:49am
Thanks, I’ll try this out when I finally manage to get a torque converter in there that fits. Ordered one from ACC (Boss Hog) and the base of the torque converter snout kept busting the seal on the trans because it was just too tall. Trying a Hughes converter today or tomorrow.
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letank View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote letank Quote  Post ReplyReply Direct Link To This Post Posted: May/08/2019 at 1:46pm
Originally posted by tached_out tached_out wrote:

In park, close the throttle more. Keep closing it until you get back down to about 800 rpm. Don't worry that it would likely stall if you put it into drive. In fact fact go ahead and put it in drive. Watch it stall. This is the baseline that you are going to work from.


I know this is an old post, but thank you. Doing this on a 360 w the TF727 on a full size wagoneer, stock intake and stock motorcraft 2150 , this is cal so not too many choices. I did a camshaft replacement (melling MTA-1) after some wipe out lobes, and it has been running fine, and of course I did some adjustments along the way.... choke, idle speed... and with the weather change, the idle speed was way too high at 1500 and stalling on first D or R in the morning, even after a full warm up.

Yes lowering the idle speed did the trick, btw I am running manifold vacuum advance

again thank you


Edited by letank - May/08/2019 at 2:00pm
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letank View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote letank Quote  Post ReplyReply Direct Link To This Post Posted: Jan/13/2020 at 6:48pm
update and a funny effect.... so I found the issue with stalling when putting in Drive or Reverse... the PO leaned the idle screws so much that the idle could now fight the transmission load, it was good enough with the wiped out cam for some reason, but after the cam break in, it was not that good, I managed to clock the first 400 miles that were mostly highway and not too many stop signs or stop lights without noticing the annoying stalling. But on the last trip it was getting really bad, that I had to do some diagnostics.

By sheer luck I found out when I flipped the choke flap close, the idle in park could keep up at less than 800 rpm, so I increased the mixture first by 1/4 turn, then another 1/8th and it is almost perfect. Also the main jets were #50, on the 2150 for sea level 55 to 56 are considered a good baseline, running with 56 and did a bunch of other contact cleaning such as the dreadful 3 prong duraspark coated with the green grease... of course upon removal the plug broke into pieces, time for weatherpack connectors
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