So Ive made a number of changes including new heads on my TH400 equipt 355 setup, the issue I have is in order for my car to even idle in gear without stalling when up to temp it has to be at like 1,100 rpm in park? also my afr mete reads like 13.5 to 1 in park but as soon as I put it in gear and the rpm drop to like 600 the AFR goes to like 14.7-15:1 I have eliminated any possible vacuum leaks. Any thoughts on whats going on or what im doing wrong? I have checked timing. the whole car jumps pretty hard when putting it in gear, I know theres a bushing that causes the rear to move when putting it in gear but it just seems like the trans is grabbing too hard right off the bat dragging the rpms waay down. The car did not behave this way before the roller cam install last year. the cam is nothing crazy..xr270hr comp cam.

 

The more and more I think on this the more I think its vacuum/timing related.. I went from the ported vacuum to manifold vacuum when I went to the new cam. the car wont idle worth crap on the ported vacuum with the new cam if I time the distributor to 36 degrees full in above 3000 rpm... last year I had my brake booster plugged into the manifold port instead of the one on the back of the carb at the base and it did the same thing whenever i stepped on the brakes the cars rpms would drop dramatically.. I think im losing vacuum advance when sticking it in gear due to the vacuum hose going to the trans.... I have to work out why this is happening.. I resealed my intake manifold yesterday and im confident I have no leaks there.. guess I need to double check that vac hose pipe to the trans. to make matters worse there are less larger vac ports on this carb than my old one, that had a large ported and manifold port.

 

Im wondering if maybe my modulator on the trans is bad?

 

Man still having tuning issues, I know this is getting old for you! m curious to what your base timing is? What's the timing at when it's idling in park with the vacuum advance connected, and what's it dropping down to while in gear?

 

ill be checking that as well as the modulator tomorrow. I retimed it today but didnt pay attention to what it was at idle with vac connected.

 

What'd your base timing land at with the advance disconnected? Mine ended up landing at 15. I'm thinking that you're having a big vacuum drop going into gear with the drastic RPM drop, and losing a lot of advance if your base timing is low for that cam. Or your advance may not even be working and is why your idle is dropping so much because the torque converter is straining the engine while it's not making much TQ with the low timing.

Hopefully someone else can confirm if my above theory is correct or not!

 

I don't think your AF reading are the issue. my car does the same. it is leaner when in gear at idle than in park. as stated above it sounds like it could be timing related.

 

What are the vacuum reading you are getting in park versus idle? I would guess that you have the throttle blades cracked enough in park that you might be exposing a little more of the transfer slot, and that is why the AFR is higher in park.

 

I once had a similar problem, the issue was I was having a large timing shift between park and drive.
Lars was very helpful in fixing it.
This can happen when you run manifold vacuum to your distributor vac advance, the issue is your vac advance can is pulling timing when you shift out of park, causing rpms to fall. You need to make sure your vac advance can meets the “2-inch rule” Lars emailed me a paper once with all the details to select the correct can.
you can test it by unplugging your vac advance, plug it so there’s no vac leak. Dial up your idle screw so it’ll run and set your rpm to 900ish in park, and shift into drive, see what happens. If it’s a gentle shift and there’s not a large drop in rpm then you have found your issue.

 

Hooray!~ Somebody who read and followed the advice based on the "2-inch rule" for vacuum advance! Yes, the rule is real, and following the guidelines regarding this is essential to proper timing and stable idle, especially with an automatic. For those unfamiliar with it, this info is outlined in detail in my "Vacuum Advance" tech paper, available from me via e-mail request.

With the regards to the OPs change in idle mixture between park & drive, this is completely normal. Fuel from the idle and transition circuits is drawn through the carb by engine vacuum - not by venturi vacuum. As manifold vacuum changes when you go from neutral to "drive," the metering signal to the idle fuel circuit decreases and the carb will lean out. The 2 things to do to rectify this are:

1. Select a vacuum advance control unit based on the 2-inch rule so that the vacuum change going from neutral to "drive" is minimized
2. Once this timing problem has been fixed, set your idle mixture with the tranny in "drive".

Lars

 

So I played around a bit more today, changed the modulator on the trans old one was fine but the rubber elbow was kinda loos and ratty, anyway same behavior so I threw a vacuum gauge on it. in park (vac connected) I was seeing below 45 and it was in the latent timing zone instead of green... It didnt go green until I turned it to see about 30 degrees on the timing at 950rpm in park? the rpms kept picking up if I went beyond that as well as vacuum increased while staying in the green. I have to double check but I think I set my vac can up for either 10 or 12 degrees of additional total timing If I time it for 36 degrees max with vac disconnected and then reconnect vac its in the latent timing zone on the vac gauge and I even sometimes get some run on when shutting the car off.

 

this is what I feel is going on.. the vac drops while in gear and kills the vac advance. the car wont idle without the manifold vac plugged in if I have the distributor timed for 36 degree all in this has been one of my issues.

 

How do I request these papers? email you or pm you with my email? Thanks.

For some reason if I time the car to 36 degrees mechanical with the vac hose disconnected and plugged the car wont even idle without stalling? its like something is off or im missing something. with my old smaller cam I ran ported vac and the timing setup was great with the same distributor.

 

Is this your cam?
XR 270HR Hydraulic Roller Cam for OE Roller SBC Advertised Duration 270/276 218/224 @ .050". .495/.502 lift 110 LSA

Report your vacuum reading INCHES ( RPM idle in park and in gear) Try blocking the wheels, with emergency brake on , to eliminate brake booster.
What is your timing at idle without vacuum advance?

 

I will check today. I believe my vac gauge at the moment is 46 in park at like 900 rpm not sure what measurement it is though. EDIT 17 inches in vac but I need like 30degrees advance at 900rpm to get this.

 

Amount : 46 centimeters of mercury (cmHg)
Equals :18.11 inches of mercury (inHg)

 

I was literally editing my respose at the time to commented this but yeah thats roughly what I have... what do you expect I "should " have? I am wondering if I should restrict the amount of mechanical advance and make up for it with more vac advance? this way I wont get as much of a timing drop when going from park to a gear? I have this gauge and to get the "normal motor" zone while in park I need a lot of advance for some reason? im beginning to question my adjustable timing guns accuracy here because my numbers just seem off.

 

Need to know answers to above questions . Is it the original HEI distributor? Don't worry about total timing at this point.
Is your gauge not dual scale and at least 2.5 to 3 inch diameter

 

no not original dist, I just pulled it apart and discovered I had modified one of the weights (must have been some tech tip I saw 10 years ago,) this allowed too much mechanical advance.
Anyway I put a set of performance replacement weights in and now I get more mechanical timing at idle while still only getting a max of 36...
So the engine idles much stronger without the vac advance on manifold (i may have to check the mechanical at idle since I lost the number I jotted down)... if I connect the vacuum advance up to manifold vac the idle rpms and vac increase and the vac reads just over 17 inches and the total advance at 1000 rpms in park is a crazy 46!... if I unplug the vac it drops to just over 15inches.

My concern now is if I rev it to say 3500rpm I now see the total advance up around 60! I think at this point I need to dial back the total allowed vac advance to only allow a total of about 48 right?

on the plus side the car only drops like 350rpm now when going from park to gear with manifold vac connected and the engine doesnt act like it wants to stall. I was able to bring my park rpms down to like 900.

 

There should not be any mechanical advance stating at idle. Probably need heavier springs on the mechanical advance. Then you can set the idle timing to around 10 degrees. The idle timing and mechanical advance needs to be set up first, and then you deal with vacuum advance.

Example : 10 degrees initial advance. Mechanical advance starts above idle rpm sloping to 30 degrees at 3000 rpm to 34 degrees total 4500 to 5000 rpm
Vacuum advance adds to mechanical advance for 45 to 50 degrees at cruise rpm