1979 AMC 304 Rough idle.

[jafo]

Before you fiddle with anything else, replace the ignition box. You are grasping at straws until that is done.

Good advice, but I've had the new one in for a couple days. Changing that plus fixing all the vacuum leaks as well as setting the idle mix screws smoothed it out. That's no longer an issue.

The popping and loss of power under load is still there. I just thought of something, I haven't checked the total timing. I have the initial timing at idle set, but not total timing. Does anyone know what the total timing should be at a given RPM? I'll look for that in the manual also. Hmmmm.

 

[mtnwhlr]

So not knowing how old you are I'm just curious have you ever owned something with a carb before? I see on here all the time young people who don't know how a carbed motor act's when cold because all they have ever had was fuel injection. I say this is because I hear "when it's cold" a lot in your posts.

 

[jafo]

So not knowing how old you are I'm just curious have you ever owned something with a carb before? I see on here all the time young people who don't know how a carbed motor act's when cold because all they have ever had was fuel injection. I say this is because I hear "when it's cold" a lot in your posts.

I'm 47. Yeh been around a few carbureted engines before. I've been around automotives before but haven't done this stuff for probably 10 yrs or more. I just have gotten out of practice and this jeep is knocking the rust off. That and being unfamiliar with anything other than Chevys, it's a new exoerience for me.

 

[jafo]

Ok. Sat down and read the manual last night. I learned some things. I didn't know as much as I thought I knew. I got a crash course in vacuum valving I never took the time to understand until now. I have two vacuum valves on this intake. One is thermal on the thermostat housing and one I'm not sure is thermal or not. I don't think it is. Anyways these two valves control the vacuum to the vacuum advance pot on the distributor depending on the temp of the engine. When the engine is cold, the valve is using direct manifold vacuum and after the engine warms up this thermal valve switches it to ported vacuum.

The thermal valve (they have another name for it) is broke on mine. When I was replacing vacuum line one tip pulled off with the line. So, I need to get another one and I might switch both out if they still make them.

I also found the total advance in the manual. It's two pages with graphs of the spark curves and what the initial timing should be and what the total timing should be. I just have one problem. They have three seperate ones for the AMC 304 engine. There is a number that is on each graph, I don't know what that number is for and don't know which graph to use.

Ideas?

 

[Hedgehog]

If you haven't got some sort of racing Dist. your total advance is likely in the ball park and more than likely not your problem ..... at least not yet. Your dist. is supposed to run on delayed advance. I.E. no vacuum at idle. get a vacuum gauge, they are about $20 and very useful for setting things up correctly. Plug it into your various ports to see what is what. My delayed port is on the drivers side in the front on my carb. (a 2150). The others open at various times and to be honest have little to do with the problems you are having. Remember one thing, A jeep engine unlike a Small Block Chevy is not a high RPM engine, they are meant to live on high torque at low RPMs. For instance my AMC 360 pulls like a bull up to ~4200 RPM's and falls on its face above that.

When you get your gauge use it to set your idle mixture. No "fiddling". If you don't have a tac. you will need one, it can be done by a sensitive ear, but a diagnostic tac. is the way to go. MY jets are se at ~1.75 full turns.

What did you get to replace the Dist. box that was fried? With your set-up I'd be extremely tempted to go with the Team Rush HEI modification that is pinned at the top. HEI will solve many problems you didn't even know you had.

The vacuum switches you are looking for are available. I've gotten a few at Willys Works here in Tucson.

 

[Posi]

At adjustment, it's setting at 15 deg BTDC now.

Set at 15° and the carb adjustment screws out by 3 turns would have me checking the engine vac readings for another possible leak.

If you don't have a vac gauge I would recommend buying one their inexpensive and a great diagnostic tool to have in your bag.

 

[Lumpy Grits]

You have a vacuum leak somedangplace. 3 turns out is a big clue here.

LG

 

[jafo]

I'll throw the vacuum guage on and measure every port. Icll post what I find. It doesn't run or idle like it has a huge leak. It may be a leak in just the right place maybe, but I redone some vacuum routing cutting out the TCS selenoid. I took it for a drive to a parking lot to trying get a feel for the clutch when it engages how much throttle is needed. I found that my rpms are around 1500 to 1800 to let the clutch all the way out. I noticed the last little bit of travel in the pedal hooks up really well and if I the rpms are not where they need to be it jeep jerks and engine coughs. What I'm saying is, that maybe the problem is me and my feet. I'm not slipping the clutch enough. I'm just going to put the vacuum guage on, and measure some ports and set the idle mix screws one more time with the vacuum guage and just drive it and see if it's an adjustment I need to make to the jeep or if there is something genuinely mechanically wrong. I want to eleimnate myself as a problem also. The more I stopped and took off in the lot the better I got a feel for the clutch relative to the throttle. So that's where I'm at thought wise.

 

[Posi]

Set the idle at 800 and hook the gauge to full vacuum. Post up your reading and how the needle responds.

 

[jafo]

I got side tracked a little. I'll get the results posted up soon.

 

[jafo]

Ok engine warmed up, idle about 800 rpm and from the main T vacuum port on the front of the intake measured 18 inch Hg of vacuum, steady. If figured it would be higher. Usually around 20 Hg and over? I do think I may have found out where the main leak is. The new base plate gasket. The new one was different but looked like it would work. But after spraying around the baseplate of the carb with carb cleaner, it instantly choked it down. As soon as the carb cleaner evaporated the idle came back up and smoothed out. This would explain the idle screws setting so high. It's gwtting too much air through the base plate. I thought about some things that was said here and got.me to thinking about why the idle mix screws would need to be out so far, it's getting too much air.

So I think I'll try to sneak the old gasket back under it and see what happens. Then look for a new gasket tomorrow.

 

[Lumpy Grits]

Jafo-What's the altitude(ASL)where you're do'n this?
LG

 

[Torxhead]

Jafo, also consider making your own gasket, since you can use the old one to trace the mounting holes.

 

[jafo]

LG, had to look that one up. Elevation is 910 ft. Kansas city area.

Torx, these are about 1/4 inch thick gaskets. What it is, is I also run one of those stock aluminum spacers. So it was actually the spacer base gasket.

Here is what happened after puting the old gasket back. I turned the idle screws back down to seat and brought them out two turns each. Would't stay running. So added 1/2 turn each. Ran better. Then watched the vacuum guage and one more half turn out it idle up a little vacuum settled on 18 inch Hg. so I tried one more half turn and it started to idle down and lost vacuum a little. Went back in a half turn and got 18 again. So it ends up with three turns out to get 18 inch of vacuum. Idles around 900 rpm. I'll test drive tomorrow when I take my wrong center caps back.

 

[Lumpy Grits]

At 910' ASL. with a stock cammed engine, you should have a higher vacuum reading than what you posted.
I have 18"hg at 2800' ASL, where I live.
LG

 

[jafo]

At 910' ASL. with a stock cammed engine, you should have a higher vacuum reading than what you posted.
I have 18"hg at 2800' ASL, where I live.
LG

I'm running out of ideas. When the rpm is raised, it shoots up to about 25 or so, idle is 18. This is coming of a main port on the imtake. So yeh I agree.

So, I've been looking at the vacuum line for the brake booster. I wonder if there could be a leak there. Either in the brake booster or along the booster vacuum hose. I might try unhooking that tomorrow and throw the vacuum guage back on. There just isn't too many more places to lose vacuum orher than maybe intake manifold gaskets on the lifter valley side. I sprayed down the outer intake gaskets and nothing happened. But if it's leaking internally, I.would think the plugs would show that.

 

[Torxhead]

Usually, when the brake booster is shot it will kill the engine when you depress the brake pedal. You could also replace all the vacuum lines, just to make sure. You could also be losing vacuum thru the charcoal canister. You might want to do a compression check, just so you know what it is.

 

[jafo]

Usually, when the brake booster is shot it will kill the engine when you depress the brake pedal. You could also replace all the vacuum lines, just to make sure. You could also be losing vacuum thru the charcoal canister. You might want to do a compression check, just so you know what it is.

I've replaced all but a few lines. I have eliminated a couple also. I might as well run the compression test tomorrow first. It's the big one. Should ha e done it days ago.

 

[jafo]

Ok. Had to go buy a new compression checker. But now here is what we have. They are all with in range, manual says max variation between cykinders is 20 psi. But it also says minimum 140 psi per cylinder. So not the greatest but not much between with one at the minimum at 140 psi.

1- 130
3- 135
5- 135
7- 133

2- 133
4- 133
6- 133
8- 140

 

[Lumpy Grits]

Was the throttle opened in the carb, when doing the comp. test?
LG