Large RPM drop when car is put in gear

First things first: (I apologize for this lengthy post)

The engine is a basically stock 283 mated to an early 80's TH350. Its the TH350C trans, but the lockup function isn't hooked up. Car is a '56 Bel Air.

The carb is a reman Quadrajet with an electric choke, I've had it for week now. For the PCV setup, I have a PCV valve from a 67 Nova screwed into the rear of the carb where the power brake hose would probably go if I had them. A hose goes from this PCV valve to a port in the back of the block (Sort of looks like a gun turret) where I imagine a road draft tube would have gone on earlier blocks. On the front of the engine, I have a stock breather cap on the oil fill tube.

The distributor is a points type. Point gap is set to 0.019", and the timing was set to 10 BTDC with the vacuum advance disconnected. Plugs are AC Delco, gapped to 0.035".

The car idles smoothly at 900 RPM in Park or Neutral once its warmed up. (I haven't gotten the fast idle/choke set up properly yet). It drives beautifully, throttle response is crisp with not even a hint of hesitation.

However, when you put it in gear, it drops to 400-ish RPM. Yep, 400 or so. The odd part is that it's a steady 400 - it keeps on idling, and its fairly smooth. Rattles the car's panels and doors a little bit, but its steady enough to drive the car - I drove it around town yesterday and it never stalled at a stoplight or anything.

But, I don't think that RPM drop is normal. I don't remember my Camaro with a 350/TH350 combo doing that - I think I remember it idling around 700 in gear and around 900 in Park. My dad's '70 Olds does that as well - 900 in Park, 700-ish in gear. Verified that on Friday when I was at his house.

If I wanted mine to idle at 600-700 in gear, I'd have to have it screaming in Park.

So maybe I have a vacuum leak? My fuel pressure/vacuum gauge is reading 29 in-Hg without being hooked up to anything - so its broken and needs to be replaced. (Not surprisingly, none of the local auto parts stores had one on their shelves) I've sprayed carb cleaner around the base of the carb and the PCV connections, and the engine never seemed to pick up speed.

Now when I first installed the new carb, I used the carb gasket that the rebuilder supplied with the carb. When I used that gasket, I had such a colossal vacuum leak at the front of the carb that it sounded somebody had put a blower nozzle on a compressed air hose. Naturally, the poor motor didn't want to idle that way. After I swapped in a new carb gasket, that noise went away and the engine would finally idle. But I'm thinking that maybe its still leaking a bit there?

So, to make answering the questions easier:

1) What are some effective ways of determining if you have a vacuum leak, and locating where they are?

2) Barring a vacuum leak, are there any other things that could cause a large RPM drop when putting the car in gear?

Thanks!

 

Well if you have the PCV plugged into the back of the lifter valley (the road draft tube) and the breather on the front of the lifter valley ( tube in intake ) you have a problem with crancase ventlation. If you are going to do it that way block the oil fill tube 9 later tubes had a screw on cap).

I don't know that this is the problem with your idle but it is going to be a problem down the road. it sounds like you have a vacuum leak to me. Maybe not a major one but a leak, it could be that with the PCV just drawing air from the lifter valley and the open breather on the front of the lifter valley that it is not closing like it should. that would cause you to have a vacuum problem.

Try thins, have someone put their hand over the filler tube with the breather removed then put it in gear and see what happens. Or just bypass the PCV valve all together and pluf the hole in the back of the carb and put it in gear and see if that causes any difference, that will eliminate the PCV setup as a problem.

 

Try disconnecting the PCV valve........you may be pulling in too much air... Check the bottom of the carb for a fuel leak.....What vacuum advance can are you using?...it may be too much advance at idle....Disconnect it and try.....

 

First where do you have the vac advance hose plugged in at? Is it getting vac advance a t idle? Vac advance at idle will cause your issue. Second do you have the correct PCV valve for a 283 they are not all the same inside. One for a 305 would be OK. The rest of the PCV system is OK ,if you do not have breather on the valve cover you need the one on the fill tube to let fresh air in. A PVC off a say 400+ CID engine would flow more air than the correct one for a 300 CID engine. Try blocking the vac advance line off and see what it does ,could have a bad advance unit leaking air if you are connected to manifold vac.

 

Dropping from 900 to 400 is definitely a problem. For a stock 283" the book is 600 in drive.

Disconnect and plug the vacuum advance and PCV.

Set timing and dwell to spec.

Set parking brake and transmission in drive. Adjust carb to 600 RPM, then start tuning idle air screws. Get best idle on a dwell tach or vacuum gauge, then reset idle stop screw to 600.

Reconnect vacuum advance. Does idle speed change? If so that's a problem.

Reconnect PCV valve. Does idle speed change? If so, that's a problem.

jack vines

 

Thanks guys. Looks like I have a lot to check out, and I need to get a new vacuum gauge.

The PCV valve, if I remember correctly, was listed for a 327, but its been a few years since I bought it. I have the Chevrolet script valve covers, so no PCV holes in them.

The vacuum advance is the one that came with the distributor - I really don't know much about it. Are there numbers on it I can check? Also, the vacuum advance is connected to a carburetor port. I'll check tonight if it gets vacuum at idle.

 

Sounds like the converter is trying to lock up

 

Yeah, I would go converter also, if there are no obvious leaks. Sounds like it runs just fine, so the converter is my guess.Partial lockup would also give you pretty good acceleration out of the hole if the motor makes decent power. When you come to a stop, does it feel like its bogging the motor at all?

 

I hadn't thought about the converter.

LeftCoastErik, well, other than it idling really low, I wouldn't say its bogging. Does come out of the hole pretty good though...well good for a 283 anyway.

 

Yeah, it reminds me of some issues we have with the high performance diesel trannies we build every day. Just the fact that it was a lockup converter could do it. May have a fluid bleed that is partially applying the clutch.

 

You may want to let warm up and set 500 to 600 and see if it idles ok.
If the engine does,nt idle well, some were along the line may have changed the cam,or you have a valcume leak.

 

George - good suggestion! I'll check that first.

 

Ok guys, I got home late so I didn't get to do as much as I wanted. I couldn't find anything lying around my garage to plug up the PCV valve and line with, so I'll do that either tomorrow or a bit later this week.

I also didn't have anyone to help me by holding the car still with the brakes while I check for suction on the oil fill tube. My parking brake is useless, so I had no way of holding the car still. I'll see if I can get someone to help me with this tomorrow or Wednesday.

I did attempt George's suggestion of lowering the idle. It seemed to idle decently (in Park) at 500-600 RPM.

I also managed to score a vacuum gauge and I connected it to an unused carburetor port. When I pulled the plug off the port, the idle went up quite a bit. It lowered back down when I plugged the gauge in. With a warm engine idling at approximately 900 RPM, I got 16 in-Hg on the gauge. It wasn't perfectly steady; it did flicker a bit between 15 and 16 in-Hg.

I don't really know enough about engines to know what this means. Is 16 in-Hg a good number? And should I be worried about it the gauge needle flickering a little bit?

Also, John, I disconnected the vacuum advance from the carb port it was on and hooked the gauge up to that port. I got 12 in-Hg there. So I do have vacuum at idle where I had the vacuum advance hooked up. Should the vacuum advance be hooked up to a place with no vacuum at idle?

 

You say mostly stock 283. What kind of Cam? In my first engine, I had a 278 advertised, .465 lift Crower cam in a mostly stock 283, with 350 heads, in front of a stock powerglide. It had about 7.4:1 compression if I remember, and made no powerdown low(or anywhere else, but thats another story) I had to Idle it at about 1000-1200 rpm just to get it to stay running at idle(4-500 rpm) I put a 3000 rpm stall converterin it, and it idled fine. wouldnt move till about 3,oo rpm either. I don't know much about lock up converters, but Like mentioned above, I would definately recomend checking the converter

 

Studebaker, honestly, I have no idea of the cam. I picked the engine up from someone that pulled it out of their car because God forbid you not have a chromed out 350 crate motor in your car. I really don't know how to determine anything about the cam?

 

Hopefully someone can chime in with a better method than my guess work here, but , does it lope hard. If so, the 16 inches of vacuum is on par with a performance cam. You have to differentiate from misfire from a failing cam. A vacuum gauge can help, the reading techniques are readily available anywhere online. Did the engine come with the turbo 350, or was it in front of a stick in the previous car. This makes a big differance with the little engines. do a compression test, to verify the engines health, as low compression can lead to a low vacuum reading also. Sorry if any of this is redundant info, just trying to help with what I learned the hard way when I was 17.

 

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I'd try it with ported vacuum, since it sounds like you have been using an unported source.

There are several threads that discuss this.
A popular opinion is only engines equipped with "emission" equipment were ever run with ported vacuum powering their vacuum advance, and that all engines really should use manifold manifold vacuum to run well. Some, maybe many of the holders of that opinion offer as incontrovertable proof an online statement by a GM engineer that ported vacuum only came into existence in the emission era, and ported vacuum would cause engine overheating and other maladies.

I try to ascribe to something Sherlock Holmes said to Watson (Dr John, not Larry). "Never theorize before you have data. Invariably, you end up twisting facts to suit theories, instead of theories to suit facts." In other threads I have provided carefully transcribed info and even scanned pages from 3 or 4 US auto manufacturers and period tech literature that it was common, very common in the 40s, 50s, and 60s to use ported (not direct manifold) vacuum for the vacuum advance signal. My posts were not intended to imply that manifold vacuum is not sometimes a good choice to power vacuum advance. They were intended to show why I think that whether due to inexperience or ignorance of earlier systems, or his obvious wish to simplify the explanation of vacuum advance theory to make it more understandable, the GM engineer's description of engine development history is simply not accurate. And although that does not automatically disqualify his "manifold only" proposition, I think it does soften his argument that ported vacuum can not perform well.

Here are links to 2 pages from the 1959 General Motors/Rochester carb publication "The ABCs of carburetion."
http://chevy.oldcarmanualproject.com/carb/UM900/abc01.htm
http://chevy.oldcarmanualproject.com/carb/UM900/abc08.htm

It says "There is one more job the carburetor does, though somewhat indirectly. In most modern cars, the carburetor bore just above the throttle valve is the location of a vacuum port leading to the automatic spark advance on the distributor."

A port slightly above the throttle plate/valve will not be exposed to manifold vacuum at idle. That is ported vacuum.

In 1959 the EPA was not even created yet ( that was 1970 ). I doubt "most modern 1959 cars" were plagued with overheating at idle. But I'll bet consistent smooth idling and acceptable throttle response were on every engineer's check list.

 

Has the motor maybe had the heads swapped? I fiddled wit a 283 in a truck that had just been changed over to an automatic. It idled perfect, ran smooth and had decent power but every time you put it in gear the rpm dropped way low. Went over everything I could think of and finally did a compression check and all cylinders were even but way lower than what they should have been.

 

Wow.......landseaandair, you may have just nailed it.

I say that for two reasons.

For one, I had been using this engine for 2 years mated to a manual trans, and it never gave me any issues. Just swapped to the automatic and I'm still tinkering to get everything right.

Number two, and I had totally forgotten about this, is that the heads have been swapped. I remember coming across that when I first got the motor, but I had since forgotten about it.

The heads on it have the 3986339X casting number, which if my research is correct they are 74cc heads that went on 307/350 motors. I'm sorry I didn't include this info in my original post, I had just forgotten totally about it.

I'll run a compression check tomorrow and post the numbers. Not used to checking 8 cylinders - I'm used to checking outboards with 4 or less!

I will also try the other suggestions given here; everyone has made valid points.

 

Be nice if the mystery is solved that easy. That motor stock is probably around 8:1 and about a full point lower with big chambers, a bit of cam would bleed even more off. The one I did really had me scratching my head since it had early heads. Did have a cam but not much, pulled the pan and it had a good chain installed straight up. Cam could have been ground retarded, ring gaps lined up? Never really figured it out.

 

Found this:

http://www.secondchancegarage.com/public/186.cfm

Scenario 3 is almost exactly what my engine is doing on the vacuum gauge. My cam is an unknown, but I wouldn't say it lopes hard. At least not like a clattering dragster cam. Might lope a little harder than stock.

I'll try and find a ported vacuum source for the distributor to try that.

And, of course, hopefully today I can try out the other suggestions too.

 

My first issue is why are you idling at 900RPM in the first place? For a mild engine like that, you should be 750 max or even less.

 

Greg, no particular reason. I can set it to 750 without any issues, but I'm sure it would die if I put it in gear.

 

Try opening up the idle jet metering tubes from .035 to ..039. Need special drill bits to do this. http://quadrajetparts.com/rochester...ents-idle-pickup-tubes-035-orifice-p-291.html

 

One thing is for sure....nothing more annoying than a car that is fighting the converter.

The other guys might be right about the advance and other engine details but my first guess is the converter is just not right for the setup. Maybe you need a little looser converter. If you foot-brake it, what RPM can the engine reach?

 

Greg.....not too sure what you mean.....you mean holding the brake pedal down and revving the engine up?

 

Update for those helping:

I didn't attempt readjusting the idle mixture screws yet. But what I did do is disconnect the PCV valve and cap it with a rubber plug, and plugged the end of the line that went to it. Fired the car up and put it in gear - still idled really low in gear. Nothing changed.

Reconnected that, and pulled the vacuum advance line off and capped the port on the carb. Fired the car up and it was idling lower in Park - somewhere around 6-700 vs. the 900 it was at before. So I reset the idle to 900 just to keep all these tests consistent, and put the car in gear. Still idled at the same 400-ish RPM it did before.

Is it normal for the idle to lower when the vacuum advance is disconnected?

Now one thing I did notice is that the base of the carb, at the front of the engine, feels cold when its running. Cold like air is rushing past it. But I couldn't detect any leaks with my finger, and spraying the area with carb cleaner didn't change the engine tempo at all. So I can't tell if its a vacuum leak or not? This is the same area that had an incredible leak with the base gasket the carb rebuilder supplied.

Also, I'd like to apologize if my questions seem stupid, but I have to learn somehow! I'm just trying to sort out what my next moves should be, and if this is an engine or torque converter problem.

 

If you set the idle to 600-700 and you put in gear and stays at 400 rpms.
There is no problem the trans likes that rpm.

 

First off, i think 400rpm is ok as long as the engine does not stall out and you can accelerate ok from a stop. Although 500 and even 600 may be better (?).

You stated that you haven't got the choke/hi-idle sorted out yet and i think that's the problem. As you probably know there is a separate fast idle adjustment on this carb. 700 in neutral/park is ok.

i also have this transmission.

 

P.S. i'm not real positive with quadrajets, but i think a good ballpark would be:

fast idle 1200, or possibly higher.

mid-step 900

curb idle 6-700 in neutral/park, 5-600 in gear.

But really, it's what the engine/trans combo likes is what's important.