Motor wants to die...
 

'67 GTO 400/th400. Motor wants to die whenever I come to a stop at full operating temp. This just started after I got the exhaust done and installed new plug wires (MSD 8.5mm) plugs are AC r45ts and gapped @ .045. I have checked the vacuum line to trans and it's fine. It shifts fine with no slipping. If I put it in neutral the idle picks up and runs fine. Last year I changed the torque convertor but again this just started. I replaced the fuel filter and have checked for vacuum leaks around carb (Edelbrock 750) and everything appears fine. Trans is connected to manifold vacuum and carb to ported. Again everything was fine until the exhaust work and plug wires.
Any ideas or help will be greatly appreciated. Thanks.

ER

What distibutor are you running? HEI? My 400/T400 did that twice and it was the module in the HEI both times. Ran fine until it got up to operating temp and then it wouldn't hold an idle unless you gave it a little gas with your foot. Ran fine at anything other than idle. What heads do you have? Those plugs are for later model heads aren't they?

Might make sure there are no fuel lines laying on the manifolds or something causeing like a minor vapor lock by boiling the fuel.

Does it sputter to a stop, or just flat out die.

A friend also had a problem that his idle was set way to low, and the choke was makeing up for it, but as soon as he got to a temp that the choke disengauged his motor would not idle.

I am far from a mechanic, but just something I encountered before.

The dist is the Summit GM HEI and it will idle all day long even being hot, it's just when your driving and come to a stop. The heads are 670. Which plugs do you recommend for these ? The cam is the comp 280 h.
It kind of sputters and dies. The carb is the 1407 w/manual choke which is wired open. We can get by without the choke in Texas. The idle is at 800 rpm. The fuel lines aren't touching the manifold or laying on anything.

It acts like it's still in gear. I was wondering about the vacuum modulator. Can they cause this ? Thanks for the replies.

ER

I thought those heads needed a gasket type plug. R45s. Go to the parts store and get a new 4 pin module.

David, I'm not sure what 4 pin module you mean. I will get new plugs but these survived the heat of last summer here in TX and never had any trouble like this. The color looks good on the plugs as well. I did have to regap them a while back. The were installed at .045 and checked them a few months later and all were at .055 or greater. Can plugs cause something like this ?

ER

That distributor is made by Proform, I think. I uses a standard GM HEI 4 pin module. It's located under the cap. Look also at the weights in the distributor. Make sure they move freely and that the springs aren't stretched out of shape. I think the plugs you've been using are designed for later heads. Made in the 70's. Not sure what year. Earlier heads use a plug with a gasket, such as the R45S.

This may seem obvious,but needs to be asked,was the carb readjusted after the exhaust work was done?

Any changes in the exhaust system could have easily affected the carb settings...

Go back to tuning 101,check and reset your timing,then re-adjust your carb,idle mixture/idle speed etc.,and see how it runs then,you may need to adjust the carb further with rods or jets or such if it still exhibits issues.

It's entirely possible you bumped your distributor position doing the plugs or something too.

The 670's use the (R)45S "gasketed" style plugs like David mentioned.

The T in the R45TS plug stands for tapered seat,like David mentioned that is a later model plug design ('72 and up).

The first thing that comes to mind is the carb's float level.

I did adjust the carb after the exhaust work was done but I am waiting for my friend who has a digital timing light to get back in town to reset the timing. All I have is the blinking light.

I replaced the vacuum modulator on the trans ($11.49 @ NAPA) and drove around for about 30 min and the problem "seems" to have gone away. I don't want to get my hopes up that that's all it was but it appears to have made a difference. The way it was acting like it was not downshifting and hung up in gear kind of led me to this. I will keep my fingers crossed that this was it and recheck my timing when my buddy gets back.

Thanks for all your help.

ER

whats wrong with using your "blinking light" they are far more accurate than a digital whizbang. Get it out and check the timing, definately check the advance springs, if you have the flimsy advance springs installed replace with one heavy and one medium advance spring.
Also I noticed in your first post you mentioned having the carb connected to ported vacuum? whats that about?

I was going to suggest a vacuum leak & the fact that replacing the vacuum modulator has improved/fixed the problem poinys to a vacuum leak. If the problem is still there, I would try plugging vacuum accessories such as the brake booster, vacuum advance line etc & see if the problem disappears. Look for perished rubber hoses etc. Sounds like something got disturbed/damaged during the exhaust job.

Problem not completely solved. I ran it some more last night and it stumbled a little but not quite as bad. I did get my timing light out and the timing seems to be erratic. I looked at the dist springs and they do seem to be real loose and not returning the same place every time I'll have to address that today. I also am trying the advance on a manifold port and that seems to have helped a little as well. Thanks for all the help so far and keep the ideas coming. I'll get this figured out eventually with yall's help.

ER

These guys have held my hand through all kinds of issues. You'll figure it out!