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Big RPM drop when going from Park to Drive

goathearder

goathearder

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Hey Everyone,
My Blazer is a 73 with a 350/350 drivetrain. When in park, it idles at about 1000 RPM however when I put it into drive, it drops down to around 550 or 600 RPM! This seems a tad excessive to me, I am used to a 100-250ish RPM drop on previous carburated vehicles.

I did some reasearch on the internet and most everyone seems to say timing and fuel mixture are the usual culprits on this so I checked my timing tonight and have it at 14 degrees advanced (12 is stock but 14 seems to be the best for me).

I also checked the idle settings and the fuel mixture on the carb and that seems to be really good as well so I don't think either of those are causing the issue.

I did notice something curious when I was checking my timing though, If I have the vacuum advance on the carb disconnected (vacuum line plugged of course) the motor idles around 750 RPM in Park and when I put it into drive it drops to about 500-550 RPM.

It seems very odd to me that I don't have the same amount of RPM drop with or without vacuum advance on the carb. It seems even more odd to me that the RPM seems to drop to the same setting irregardless of whether the vacuum advance is connected.

Would there be something I don't know about that is essentially forcing the motor to drop a certain amount? Something I could be overlooking?

Thanks,
John
 
Maybe somthing in the tranny?? Torque converter??? Something causing alot of stress somewhere when put into drive???
 
is vacume currently ported or manifold ? try switching it and see what happens
 
Have you checked for vacuum leaks? That could be a cause of the high no load idle of 1000 rpm or so.

Vacuum leaks are strange beasts. Causing all sorts of problems. My old Wagoneer ran like that; high idle, dropped to a reasonable idle when in drive, before the vacuum leak was fixed. 3 miles of vacuum line later... LOL!

And as has been mentioned, the vacuum to the dizzy should be ported. Essentially, you probably don't have any vacuum going through that line until the throttle is cracked a little. If you do have vacuum at idle to the dizzy, it's either the wrong port on the carb, or the throttle is opened too far just to idle.

Good luck,
Clay
 
Now dont turn this into a ported non ported argument lol. I run mine to manifold vac and it runs fine. I really didnt notice a differance. I say replace all the vacuum line. Stuffs way cheap
 
Ok, to reply to all the questions:

1) The only mod I have done to the motor is a Performer RPM intake manifold and a new coil. The PO may have done other things but I think the motor is pretty stock. It does have an edelbrock carb on it though.

2) Vacuum is currently manifold I believe. I.E. at idle, there is definitely vaccum on the hose I have connected to the vacuum advance.

I did some more research and it looks like switching to ported (no vacuum at idle) might take care of this since in my testing, when I had no vacuum connected to the dizzy the drop was only around 250 RPM. I will switch that tonight and see if it makes a difference.

3) I did a basic check for vacuum leaks and didn't find any but some of those hoses look like they are older than me so I should probably go through and replace a bunch of them. I think there might be a small vacuum leak at the brake booster also but I can't track it down for sure.

Thanks for the input guys!
 
No one has mentioned that 1000 RPM at idle (at operating temp) is too high.

Start by fixing that. Auto trans cars should be at 750 RPM at idle at operating temp in P or N. Should be about 550 RPM in gear.

That thing must be banging into gear at that high of an idle.
 
goathearder said:
Would there be something I don't know about that is essentially forcing the motor to drop a certain amount? Something I could be overlooking?
Thanks,
John
Click to expand...

Yes, it's called 1st gear! :)

I agree with 496, of course it can't stay at 1000 RPM in gear with the stock converter, or your truck would be moving forward at that point. You probably have to hold the brake down to pretty firm to keep the truck from moving.

goathearder said:
I did some more research and it looks like switching to ported (no vacuum at idle) might take care of this since in my testing, when I had no vacuum connected to the dizzy the drop was only around 250 RPM. I will switch that tonight and see if it makes a difference.
Click to expand...


Do not switch the vacuum to ported vacuum, you are just taking a step backwards doing that. There is a reason your engine runs 250 RPM higher with the vacuum advance plugged in, because it likes it! Keep your vacuum advance plugged into full manifold vacuum, and turn your idle down to 750 RPM like it is supposed to be, and your problem should go away unless you have vacuum leaks or something somewhere.
 
If I drop any lower than 1000 RPM idle in park the way it is now, it drops below 500 in drive and doesn't want to keep running.

Hopefully switching to ported vacuum gets rid of that huge drop so I don't need such a high idle in park.
 
goathearder said:
If I drop any lower than 1000 RPM idle in park the way it is now, it drops below 500 in drive and doesn't want to keep running.

Hopefully switching to ported vacuum gets rid of that huge drop so I don't need such a high idle in park.
Click to expand...


Retarding your timing at idle (ported vacuum advance) will most likely make it stall easier. If that's not the case, you probably have a vacuum leak or something else going on, which it seems like since you are saying you have to idle at 1000 RPM in order to be able to put it in gear. Unless your cam is not stock?
 
The cam certainly might not be stock but I have no way of telling. I will say this, considering I have 35 inch tires and 3.07 gears the motor pulls way better than I would expect the stock 350 to pull.

The truck has no issues idleing at 500 RPM, in fact it idles there just fine when in drive. The only reason it has to be at 1000 in park is because it drops so far when I put it into drive. If I have it at 800 in park, it drops to 400 or less in drive and doesn't want to stay running.

If I could get it to be around 750 or 800 in park and then 550ish in drive, that would be perfect.
 
Well, PROBLEM SOLVED! Switching the dizzy to ported vacuum was the answer. Now I idle in Park at 750 and in Drive at 525 which is perfect.

Thanks for the help!
 
And now you wont waste all that gas at idle with it not highballin at idle:thumb:
 
That doesn't make any sense, something else is probably going on that you haven't found yet. Or the A/F ratio is off with the timing at idle, and the lack of timing covers it up.

Plus, in the first post you said it was weird, because no matter what RPM you idle at it always drops to the same RPM, but now you are saying it has to idle at 1000 or it will die in gear? So which is it?

I'm not trying to be rude I just feel like you more than likely covered the problem up rather than fixing the real problem. But I suppose if you are happy with the way it runs then that's all that matters, so maybe it's not worth investigating it further.
 
Last edited:
folkenheath said:
I'm not trying to be rude I just feel like you more than likely covered the problem up rather than fixing the real problem.
Click to expand...

No worries. Let me claify my original post a bit as I could see where it would be confusing.

My idle and everything was all out of whack so I decided to start at the beginning and time the motor and tune the carb.

I disconnected the vacuum (manifold vacuum at the time) from the dizzy, set the idle to 750 RPM and then set the timing to 14 degrees advanced (stock is 12). Once I got that set up, I adjusted the fuel mixture on the carb till I had 21 inches vacuum at the 750 idle and considered the fuel and timing to be good.

At that point, I checked what happened when I put the truck into drive and the RPM dropped to around 500, which was acceptable.

Then, I plugged the vac line (manifold vacuum) back onto the dizzy and the idle in park jump up to 1000 RPM without me even touching the idle screw. When that happened, I dropped it into drive, expecting the RPM to fall 250 to 750ish however it fell all the way back down to 500ish again.

I then dropped the park idle to 900 and removed the vacuum line and the idle dropped to around 600-700 and when I put it into drive, the idle was 400ish and the truck wanted to stall out.

Because of this, I had to move the park idle back to 1000 RPM or the truck would want to stall when you put it in drive. Keep in mind this was WITH manifold vacuum connected.

By switching to ported vacuum (i.e. no vacuum at idle) the RPM magically dropped to 750 RPM at idle in park. When moving into drive, it now sits at around 525 rpm which is perfect.

My theory, and it could be way off, is that somehow, when I had manifold vacuum connected to the dizzy the extra advance was somehow opening the throttle blades slightly so that they weren't sitting on the idle stop anymore. Then, when moving into drive and placing a load on the motor, the throttle blades fell back onto the throttle stop, putting the rpm around 500 ish. That is why irregardless of manifold or ported vacuum, the idle was falling to 500ish, because that is where the throttle stop was set.

In any case, the factory settings are 12 degrees advance at 700 rpm idle in PARK and I am really close to that so I am happy.
 
The reason that happened is you set the fuel mixture on the carb with the advance disconnected. I agree on adjusting timing with the advance disconnected, although I adjust it at 3 - 4K, not idle. However, before you adjusted your idle rpm and A/F ratio you should have plugged the vacuum advance back in. (to manifold vacuum). The reason the RPM jumped up is because the engine wants the advance(it has nothing to do with the throttle blades, they are not connected in any way), However, it effectively makes it leaner with more advance, so the A/F ratio needs to be adjusted with the timing the way it will be when it's running. Usually with full advance at idle and the proper A/F ratio it will run better and more stable with full manifold vacuum than ported, I haven't seen an engine that didn't. That doesn't mean it's not possible but probably not common.

If you want some good info on vacuum advance...go to maliburacing.com, there is a stick there in the technical forum about it. Good to know info.

On the other hand, if you have to pass emissions you may have trouble with full manifold vacuum, ported vacuum makes the engine run warmer at idle and reduces emission readings slightly at idle. Which is why ported vacuum was ever used in the first place, for emissions.
 
Interesting, if I have some time this weekend I'll move it back to manifold vacuum and screw around with the fuel mixture to see if that makes any difference.

I am still confused at why adding advance (making it leaner) would cause the RPM to increase though. If you have the throttle stop in a specific place doesn't that control what the rpm is? Why would the rpm be different depending on the fuel mixture?
 
goathearder said:
Interesting, if I have some time this weekend I'll move it back to manifold vacuum and screw around with the fuel mixture to see if that makes any difference.

I am still confused at why adding advance (making it leaner) would cause the RPM to increase though. If you have the throttle stop in a specific place doesn't that control what the rpm is? Why would the rpm be different depending on the fuel mixture?
Click to expand...

The RPM doesn't change because the fuel mixture does, the RPM changes because the timing does, the fuel mixture is just another side effect of different timing, it will burn more or less efficiently. The same amount of fuel is still being dispensed relative to the air.

The timing change is what is causing the RPM to change.

Make sure your vacuum can doesn't have a leak in it.
 
Ah ok, now it makes sense. I'll fiddle around with it some more and see what I can get out of it.
 
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