C6 Transmission Stall converter question

[Wbruiser2]

What I thought was a fuel or ignition issue because of the fluctuation in RPM is now what I believe to be a stall converter issue.
The problem: Cruising at say 60mph at 1900 rpm the truck feels like something is grabbing and releasing. Imagine if you will, riding with severely warped rotors and coasting at about 10mph. I can replicate this scenario while holding the brake while in R or D and reving up to about 1500rpm. The RPMs will fluctuate back and forth between 1100 or so.
The previous owner said it had a 2000rpm stall converter and a .471 lift cam.

I believe the stall converter is too big for this set up and the weight of the truck. It has the Ford 9" rear end with what ever gears it came with.

Does anyone have experience with C6's and Stall converters that can enlighten me and or recommend a better converter?

 

[SuperCab]

2000 seems unnecessary for a mostly stock application...

For anyone who doesn't quite understand the whole stall speed deal, this puts it in pretty plain terms:

http://www.bankspower.com/techarticles/show/10-understanding-stall-speed

stock stall would probably be in the 1500 rpm range, but I don't know exactly...

I don't imagine this would be the cause of your problem, but I'm not an expert...

How old is the transmission?

 

[DNFXDLI]

Here are the factory numbers for a 1979 C6. I agree Matt, sounds like other than a converter problem.

 

[Wbruiser2]

2000 seems unnecessary for a mostly stock application...

For anyone who doesn't quite understand the whole stall speed deal, this puts it in pretty plain terms:

http://www.bankspower.com/techarticles/show/10-understanding-stall-speed

stock stall would probably be in the 1500 rpm range, but I don't know exactly...

I don't imagine this would be the cause of your problem, but I'm not an expert...How old is the transmission?

I was told it was rebuilt about 3k miles ago... I just took it out for about a 30 minute drive and when I got back I checked the fluid and it was good but I noticed my fuel pressure gauge right before the carb wasn't reading anything and before, at idle anyway, it was between 4 and 6psi. I shut the truck off and the inline fuel filter was basically empty and I could see fuel draining in to it. I have questioned the fuel pump because I looked it up and what I have on there looks just like the one off the 6 cyl (filter canister on the bottom)... Obviously it works but I wonder if it's not pumping enough or if there is a different in the arm that's keeping it from pumping like it should. I don't know... It runs and runs good except for the annoying miss, fluctuation or whatever the heck it is..

 

[primetime]

I had a TCI Breakaway in a C-6. They are rated for 1000 rpm over stock stall speed. It never acted like that. I think you are looking at a fuel issue. Have you tried opening up your idle mixture screws? At steady part throttle the carb actually operates off the idle circuit and a lean condition can cause a stumble or surge.

 

[Wbruiser2]

I had a TCI Breakaway in a C-6. They are rated for 1000 rpm over stock stall speed. It never acted like that. I think you are looking at a fuel issue. Have you tried opening up your idle mixture screws? At steady part throttle the carb actually operates off the idle circuit and a lean condition can cause a stumble or surge.

Yep sure did per Holley's instructions anyway. Maybe I just need to turn them out some more and see what happens...

 

[SuperCab]

I'd be sure you are getting fuel before you mess with the carb.

The fuel pump should be a $20 piece... may want to just replace it.

There could be a hole in your fuel line, allowing the pump to suck air...

Maybe the in tank sock is getting plugged...

Lots of possibilities... Diagnose first, repair second.

 

[Wbruiser2]

I'd be sure you are getting fuel before you mess with the carb.

The fuel pump should be a $20 piece... may want to just replace it.

There could be a hole in your fuel line, allowing the pump to suck air...

Maybe the in tank sock is getting plugged...

Lots of possibilities... Diagnose first, repair second.

As I said in a previous post, it's getting fuel to the carb... Since then, I took the suction line off the pump and cut an inch off and during this, fuel continously came out (like it was siphoned) so I'm sure there is nothing clogged. As far as the chance of a hole goes, all that I can see looks good. And of course there are lots of possibilities, that's why I posted the issue here hoping that someone with some actual experience can help me narrow it down. In the end I'm sure it's going to be something simple. The hard part is figuring out what the previous owner did or didn't do. Or I bought a POS...

 

[SuperCab]

I just took it out for about a 30 minute drive and when I got back I checked the fluid and it was good but I noticed my fuel pressure gauge right before the carb wasn't reading anything and before, at idle anyway, it was between 4 and 6psi. I shut the truck off and the inline fuel filter was basically empty and I could see fuel draining in to it. I have questioned the fuel pump because I looked it up and what I have on there looks just like the one off the 6 cyl (filter canister on the bottom)... Obviously it works but I wonder if it's not pumping enough or if there is a different in the arm that's keeping it from pumping like it should. I don't know... It runs and runs good except for the annoying miss, fluctuation or whatever the heck it is..

From what you said here it sounded like maybe it wasn't getting fuel all the time. My mistake.

 

[Wbruiser2]

From what you said here it sounded like maybe it wasn't getting fuel all the time. My mistake.

The truck hasn't ever stalled or died but it does fluctuate, surge, or something to that effect. So I believe it's getting fuel but the question is, is it getting enough constantly? I have to assume it does because I can go WOT and it doesn't hesitate. I'm getting a vacuum gage today and will start over again with the basics. Maybe the carb adjustments are off a little or something is up with the advance. I sucked on the tube and it moved but it was like trying to pull a golf ball through a straw...

 

[SuperCab]

The truck hasn't ever stalled or died but it does fluctuate, surge, or something to that effect. So I believe it's getting fuel but the question is, is it getting enough constantly? I have to assume it does because I can go WOT and it doesn't hesitate. I agree... probably getting fuel then. I'm getting a vacuum gage today and will start over again with the basics. Maybe the carb adjustments are off a little or something is up with the advance. I sucked on the tube and it moved but it was like trying to pull a golf ball through a straw... That's normal. As long as it moves, you're ok.

replies in blue.

Also, is your carburetor pretty clean? having water or other junk in it can do all kinds of things...

Have you checked for vacuum leaks? Vacuum leaks can cause all kinds of hard-to-track-down problems...

 

[Wbruiser2]

replies in blue.

Also, is your carburetor pretty clean? having water or other junk in it can do all kinds of things... Yes it's clean but the previous owner did say he got some "bad gas" when I mentioned a vague miss and I have ran two bottles of Heet and put a half tank of 93 in it.

Have you checked for vacuum leaks? Vacuum leaks can cause all kinds of hard-to-track-down problems...

Yep checked and if there is one I can't find it.

I hooked the vacuum gage up to it today and it said my timing was too late. I retarded it until it was good then checked it with a timing light and I think it was around 8btc. I then adjusted the carb. I haven't driven it yet but when I did my test in the garage it fluctuated maybe 100rpm. And it sounds like its got a cam in it now. I can't wait to drive it, I got my fingers crossed...

 

[Wbruiser2]

I thought I would update this post. I still have the problem but have narrowed it down and back to the carb. I've learned the symptons I have described here are actually an off idle stumble. After setting the timing and all kinds of other stuff I discovered that the top washer on the accelerator pump nozzle (squirter) is missing. I ordered one and hope that will fix the issue. Either way, I will provide an update. Don't you just hate when your reading a thread that describes a problem that your having and it doesn't have a post saying whether it was fixed or not? There's a lot of them here...

 

[Fellro]

That is a common problem in many forums. Thanks for keeping us updated.

 

[Wbruiser2]

Update: I got the washer in and put it on. I went ahead and reset the timing using a vacuumn gage and timing light. It ended up being about 16degrees with 15hg. The stumble was gone.. I put everything up and was feeling pretty good. I can't drive it yet because my seat is out and in pieces plus waiting on a new seat cover. About two hour laters I go back out and start it up again and the stumble is back. Long story short, the previous owner had the PCV valve hooked to the rear manifold port. That is where I had the vacuumn gage connected. I plugged that port and the stumble is gone.

 

[SuperCab]

Great! Glad to hear you fixed it and thank you for updating the thread for future reference...

 

[Wbruiser2]

Update: I got the washer in and put it on. I went ahead and reset the timing using a vacuumn gage and timing light. It ended up being about 16degrees with 15hg. The stumble was gone.. I put everything up and was feeling pretty good. I can't drive it yet because my seat is out and in pieces plus waiting on a new seat cover. About two hour laters I go back out and start it up again and the stumble is back. Long story short, the previous owner had the PCV valve hooked to the rear manifold port. That is where I had the vacuumn gage connected. I plugged that port and the stumble is gone.

I made a typo... not 16 degrees, I meant 6 BTDC.