Large rpm drop

[Charlesvolare]

I have a 904 behind a 360. 600 rpm idle in gear that jumps to 1100-1200 in park or neutral. I got the transmission and converter together used, but pulled it apart to check it out before putting it in and it was a fresh build, very few miles if any. I was told the customer had it in his build and decided he wanting an overdrive after initially driving his build, 100% believe given the state of the insides. I thought maybe the converter was too tight for my cam at first, so I went to install a higher stall and realized this is a lockup 904 and couldn't use the converter I bought.

The trans has a TCI Scat shift kit, and there is absolutely no felt lockup driving (that's why I thought is wasn't a lockup 904). I'm thinking that maybe the converter is trying to lockup at idle. Any thoughts on what to check? I've blocked the lockup on another 904 before and can do it again if I have to pull the valve body, but shouldn't it lockup normally even with a mild shift kit? Will using a non-lockup valvebody in a lockup trans do this as well?

It may be not locking up at all, might be just a bad idle tune and too tight of a converter, but a converter trying of lockup at idle sounds like a better scenario as well given the rpm drop.

Engine Specs:
360, 11:1, magnum heads, comp cam (specs inn picture), 750 holley (tuned with wideband, still slightly rich at idle I believe is idling off main circuit)

 

[Charlesvolare]

Are there any specific line pressures to check?

 

[RustyRatRod]

The first thing I would check would be there timing. Where is the initial timing set?

 

[Charlesvolare]

The first thing I would check would be there timing. Where is the initial timing set?

20* initial, 30* total, all in by 1500-1800. No vacuum advance. Limiting plate in distributor at 10* mechanical advance so I can change that if needed but it seems happy there. Can't run much more total timing with that compression and iron heads.

 

[RustyRatRod]

18* initial, 30* total, all in by 1500-1800. No vacuum advance. Limiting plate in distributor at 12* mechanical advance so I can change that if needed but it seems happy there. Can't run much more total timing with that compression and iron heads.

Have you set the T port synch on the carburetor? If the throttle plates are into the slots over square, that could do it.

 

[aaronk785]

I agree with RRR. I had to set my initial at 24 and 34 total to get my 340 to idle right. I had a tci street fighter that did the same thing. It was every bit as tight as the stock converter at idle and stalled about the same as the stock converter. Not a fan.

 

[Charlesvolare]

Have you set the T port synch on the carburetor? If the throttle plates are into the slots over square, that could do it.

At one point it was all set. Wouldn't idle in gear so I started messing with it and couldn't tell you what it's at now. I'll check when I get a chance. I was almost at the point of drilling holes in the throttle blades then decided to try a converter. Then realized I had a lockup. Then realized maybe it's trying to apply lockup all the time since I hadn't felt it ever lockup and I have no idea about the valvebody other than it's got a kit in it.

 

[RustyRatRod]

Just askin so don't shoot the messenger. How do you KNOW it's 11:1 compression? Are you 100% SURE? And I mean 100%. not 99%. What pistons?

 

[Charlesvolare]

Have you set the T port synch on the carburetor? If the throttle plates are into the slots over square, that could do it.

I'm almost certain it's drawing off the main circuit at idle with how rich it is. 12:1

 

[RustyRatRod]

I'm almost certain it's drawing off the main circuit at idle with how rich it is. 12:1

Yeah I think it is too......but I'm thinkin it needs more initial timing. Is it remotely possible you're reading the timing wrong somehow? Maybe the balancer is slipped? Marked incorrectly? Have you TRIED more timing "just to see"?

 

[Charlesvolare]

Just askin so don't shoot the messenger. How do you KNOW it's 11:1 compression? Are you 100% SURE? And I mean 100%. not 99%. What pistons?

Static compression, used an online calculator, but everything seems reasonable to me after checking a few different ones.

Measured everything myself. KB107, .060" overbore, piston .005" from deck measured with deck bridge, magnum heads cut .010" 62.5cc chamber, .039" felpro 1008 headgasket.

lol I had some downtime waiting on parts so I have a whole spreadsheet of everything imaginable

 

[RustyRatRod]

Measured everything myself. KB107, .060" overbore, piston .005" from deck measured with deck bridge, magnum heads cut .010" 62.5cc chamber, .039" felpro 1008 headgasket.

lol I had some downtime waiting on parts so I have a whole spreadsheet of everything imaginable
View attachment 1715457445

Ok. Good enough on that!

 

[Mattax]

At one point it was all set. Wouldn't idle in gear so I started messing with it and couldn't tell you what it's at now.

Check this first.
Take the carb off. See how much transfer slot is exposed on the primary side.
Also if any is exposed on the secondary side - shouldn't be. But sometimes people wil l crack the secondaries open to get more air at idle and then they go to far.

Write this all down and then go make some reference notes.
Turn the idle positon screw (stop screw) so it just touches the throttle.
Write down how much, if any of the t-slots are exposed.
Give it a 1/4 turn and measure the t-slots again.
Keep going until you have a reference up to .040"

Set the t-slots at .020" or .025" and put the carb back on the engine.

Get it running. If it's too low of rpm to stay alive, give more more throttle and keep track of the number of turns.
If you can't keep it running with the t-slots in the .020 to .040 range, then adjust the intial timing and idle fuel mix.
If it needs a little more air to get rpm up - then crack the secondaries. If that works - then you can drill the throttle plates.

 

[Mattax]

I'm almost certain it's drawing off the main circuit at idle with how rich it is. 12:1

Main circuit flowing takes a throttle that is way too far open at idle to get enough velocity past the boosters.

 

[Demonracer]

I agree with above post, I think you need more total timing, maybe 34-36*.

 

[RustyRatRod]

Check this first.
Take the carb off. See how much transfer lsot is exposed on the primary side.
Also if any is exposed on the secondary side - shouldn't be. But sometimes people wil l crack the secondaries open to get more air at idle and then they go to far.

Write this all down and then go make some reference notes.
Turn the idle positon screw (stop screw) so it just touches the throttle.
Wite down how much, if any of the t-slots are exposed.
Give it a 1/4 turn and measure the t-slots again.
Keep going until you have a reference up to .040"

Set the t-slots at .020" or .025" and put the carb back on the engine.

Get it running. If it's too low of rpm to stay alive, give more more throttle and keep track of the number of turns.
If you can't keep it running with the t-slots in the .020 to .040 range, then adjust the intial timing and idle fuel mix.
If it needs a little more air to get rpm up - then crack the secondaries. If that works - then you can drill the throttle plates.

All great stuff......but generally, what makes one need the drilled throttle plates is the camshaft having a lot of duration. That one is kinda small for that is what I was thinkin.

 

[Mattax]

That takes an throttle that is way too far open at idle to get enough velocity past the boosters.

Also idle mixture of 12 to 13:1 at idle is, or was, normal. It only became 'rich' when emissions goals became a priority over mechanical efficiency.

 

[Mattax]

All great stuff......but generally, what makes one need the drilled throttle plates is the camshaft having a lot of duration. That one is kinda small for that is what I was thinkin.

That's what I was thinking too. That's why I put it as the last resort. Only do it after seeing if timing and t-slot would take care of the big difference in N to drive.

 

[Mattax]

That one is kinda small for that is what I was thinkin.

My first 340 had a comp cams 280H in it. Flat top p[istons in the hole. Not knowing other ways, and with an old school stock elim racer as my mentor, I drilled the throttle plaates of my 3310 to get the idle speed down. Worked great until I tried drive up a gentle hill., LOL. So we stuck some wires into the idle air bleeds to get the off-idle corrected. Eventually got the machine shop to turn some small bleeds and pressed those in.

Now I know that more initial timing and a little fiddling with the idle mix screws probably would have brought the throttle plates into the proper range. And then no messing of the airbleeds would have been needed either. No holes in throttle plates of my current version of that carb, even witha slightly more radical cam.

 

[AJ/FormS]

I had a Hughes HE 2430 AL FTH,(224/[email protected], 270/276/110+2),
in my LA360, also with KB107s, at zero deck.
but with Edelbrock heads at 10.9 Scr
And the 750DP on an Airgap.
Best dang combo I have ever had.
This combo idled at 650 with 12* initial, and Transfer slots about square, all other air blocked except the PCV.
But I gotta tell ya; my lifter preload was just .020. and my combo had the A833 manual trans, so I did not have the idle-drop into gear. But the combo would pull itself on hard level flat pavement down to 500 rpm with a 3.55x2.66=9.44 starter gear.
Your cam has Looooooooooong acceleration ramps, making it idle like a big cam. Forget trying to tune the idle with an O2 sensor, it will lie,lie,lie and likely finally soot up and quit working altogether. 224/230 is a great sized street cam. But those long ramps make it just a bit tougher to tune the idle.

So like has already been said, get your transfers set right, and the mixture screws about in the center of their working range, the PCV working, and then set the idle speed with Idle-Timing, and maybe a touch of idle-air bypass. Once the transfer slot is ballparked at square,to slightly taller than wide, try not to increase more than ~.020 as per Mattax recommendation. Just set the idle-speed with timing. I would ballpark 600/650 in gear and up to 100 more in Neutral. I would run the idle-timing between 12 to 16 degrees to get the speed. If it doesn't work down to 12* with the transfer slots as above, and up to 1.25 turns out on the mixture trimmers, THEN, I would suspect a faulty TC but not before.
The transfers are your primary low-speed circuit. The primary job of the mixture screws, is to be idle trimmers. If your transfer exposure is too little, you will find that the trimmers have to be opened to compensate. If the transfer exposure is too much, you will find the trimmers want to be too far closed. Holleys should be 1/2 to 3/4 turn open. Idle timing is your primary idle-speed control; with bypass air your back-up tool.

The more initial timing you run, the smaller will be the transfer slot exposure, and the greater the tendency to stall when you put it into gear. So then the tendency is to increase the idle speed in gear, to increase the idle air, which also increases the transfer exposure, so then you close up the trimmers, until it's happy..... and then you get the high neutral idle. The correct thing to do is to start over with less initial timing NOT more, and to make absolutely sure the timing does not begin to advance until after ~1000 rpm. Sometimes you have no choice but to introduce idle air from an alternate source... but I didn't have to with my 223/[email protected] Hughes cam.
The more idle-timing you give her the higher will go the rpm. This seems like the thing to do, but it's NOT. Forget thatchit. You can keep going to 30, or maybe even to 40 degrees like that, but it's all wrong, cuz it destroys the transfer slot sync.
Who cares what the timing is below the stall-speed; the engine is never required to do much of anything down there so let the timing be what the engine wants, not what YOU think it should have.
I tell you what; you put a 2-stage timing-curve in there to hit ~25/28* at 2800, and slow it down to hit 34* at 3400 to 3600, and you might be able to burn 87E10 in it. This will give you a 2* window to get the powertiming right.
But if you target 20/25 at 2000, 35* by 3000, you will be buying premium gas for No good reason. Let the engine choose the timing.
I mean if you had a slanty, you might want to run best gas and force the timing on her, cuz you know, they don't have a ton of torque to spare. But a hi-compression 360 already has a preponderance of torque, and there is no good reason IMO, to hunt for it below the stall-speed.
Happy HotRodding

One more thing,for a streeter, install a 10.5 PV in that Holley. Whatever is in there now, if it ain't a 10.5, it's wrong,lol. Mine ran 70/78MJs, to 72/80s at 930 ft elevation, thru TTIs and dual full length 3" pipes. I ran 3.55s

 

[Oldmanmopar]

What is the brake stall on the converter. If its under 3000 you found your problem

 

[Wyrmrider]

magnum heads
advance similar to 302/308 or aftermarket heads
which do not take as much as older LA Heads
what's your quench
whats your compression
lots of good tips here
how do your plugs look
wide band?
Is that shift kit ok with lock up?

 

[yellow rose]

I'm almost certain it's drawing off the main circuit at idle with how rich it is. 12:1

I doubt it's pulling off the mains at idle.

Stop STOP using the O2 to get the tune correct. You are trying to live up to a number.

Learn to read plugs first, and then use the O2 to tune with.

I'm doubting you are at 11:1 and if you are, that isn't the cam I'd have picked.

What plug are you using?

 

[Charlesvolare]

I doubt it's pulling off the mains at idle.

It was just a thought, I have many of those as I'm sure you do too

Stop STOP using the O2 to get the tune correct. You are trying to live up to a number.

Something wrong with science? Lol I'm not using it to tune my idle, just stating that's where it was landing, adding to info that may help. Not gonna completely disregard the second law of thermodynamics, but I'm not too concerned how rich it needs to be to idle decent. If I can lean it out some and good, if not oh well kinda deal.

.
Learn to read plugs first, and then use the O2 to tune with.

Plugs look good, I can pull and take a picture of I need to

I'm doubting you are at 11:1 and if you are, that isn't the cam I'd have picked.

Some calculators put it at 10.8:1 if it makes you feel any better. I'm not sure what to say, I can send to my build sheet. Put it together myself, I'll swear on all I've got the measurements are correct.

It's an amazing cam, zero complaints at all- very streetable and plenty of power.originally I thought it was around 9:1, so we speced the cam for that. Had detonation problems running timing anything I've 32* on 93 octane. Butted rings and broke a ringland, pulled and rebuilt. Made sure to measure everything like I did putting it back together.

What cam would you have gone with?

What plug are you using?

Autolite 3923

 

[fishmens67]

Vacuum leak ? 20 degrees initial timing should be close give or take a couple degrees.
Factory Holley 750 settings should be close enough for a better generic tune than you have.
I believe you have a vacuum leak that is altering the tune.