Carb problems continued

[nyamwegi]

Okay I'm gonna try to give as much coherent info as possible so maybe yall can see what's going on and give me some helpful advice:


1975 350:

I rebuilt the Rochester 4 barrel, checked the float level, I'm pretty confident it has all been put back together right.

The previous owner ( my uncle) had apparently had a manual choke lever installed inside the cab so you can pull it closed when it's cold, open it up when the engine gets warmer. I'm guessing this interferes with the vacuum diaphragm choke pulloff.

The truck fires up and seems to run pretty strong at idle. I've adjusted the idle mix a couple of different times, right now I have both screws backed out about 3.25 turns.

The only thing that goes wrong is when I start driving it, when I'm going down the road and put on the brakes, the truck dies as it comes to a stop.

In response to this, I've tried speeding up the idle with the adjuster screw. Seems like even when the idle is fairly fast (no tach so I don't know what rpm), the problem persists.

I'm leaning towards it being a problem with the choke, though I've adjusted it across the range that it will open to see if it will fix the problem and no luck so far. Once the engine gets warm the choke should be almost fully open to allow air in, right?

I know that if the torque converter isn't doing its job it will kill the engine when you apply the brakes, but it seems to work fine when you start the truck and put it into gear with your foot on the brake (doesn't die)

I'm kinda at my wit's end on it. I hate to just go buy a new carb if the one I have can be salvaged. Right now it's just a truck to mess around with since I'm a senior in college and don't have a ton of money/time to put into it.

[joesgarage71]

 when you come to a light and it dies does it start right up? does the carb flood/leak after it dies? if so you might change the float.
 Steve

[gbskunk2006]

If the problem only occurs when the engine in cold then you should probably try fixing the manual choke, or let your engine warm up a little longer before going anywhere.

As for your idle speed, make sure you're setting that when you're in drive and not park (e.g. have a friend step on the brake while you're making adjustments).  A used tach or RPM guage will be a good investment if you plan on tuning it properly.

Good luck.

[nyamwegi]

Yeah it does start right back up after it dies. I see what you're saying about setting the idle speed. My only concern is that I've tried it with the idle set well above what it should be in park and it's still a problem. I've set the float a few different times and right now I'm pretty sure it's right.

It actually seems like the problem is far worse when hot, which is why I've tried making sure the choke is getting opened up enough, but that doesn't help.


I was reading up on it a little bit...is there any way a vacuum leak could do this? How do you check for those?

[Lt.Del]

there any way a vacuum leak could do this? How do you check for those?

Yes, wd40...spray around gaskets/hoses/etc when engine is idling low.  If the wd40 hits a vacuum leak and the fluid goes into the intake, the rpm's will increase suddenly..then you've found the leak.

[nyamwegi]

I can definitely look into trying that.

Wouldn't a vacuum leak also mean problems with idle when the truck wasn't moving?

Listening to it idle you wouldn't think anything was wrong with it...

[gbskunk2006]

By increasing the idle you may be masking the problem.  If you set your idle RPM to spec and it runs rough or dies then you probably have a vacuum leak somewhere.

SgtDel offers some good tips for finding leaks.  Alternatively, if it's within your budget try replacing all of vacuum hoses, in particular the ones that connect your your PCV valve.  Those hoses will over the years become brittle or cracked and tend to not seat properly.

Good Luck!

[nyamwegi]

I may just have to throw in the towel and take it to a mechanic. 


can't seem to make any progress on it.

[Donut]

What condition of tune are you in?  What condition are the plugs and wires?  Have you checked the timing?

Why did you need to rebuild the existing carb.  Were there symptoms beforehand?

Do you have the correct base gasket for the carb?  The wrong one can cause massive vacuum leaks.

The ignition system and fuel system are closely related, if one doesn't work correctly the other one might not. 
The reason I mention these things is I've been chasing a shuddering problem on acceleration in my truck.  The carb was running rich and couldn't lean it out.  The truck would also die in gear unless I upped the idle.  I was blaming the carb and a vac. leak. Turns out, 1 cracked plug, 1 wrong plug, and 3 that weren't gapped right stacked the odds against me.  The thing would idle like a top, but under load....

[nyamwegi]

The truck sat for a while after some kids put water in the gas tank via water hose.


My uncle turned it over to me and after I got out there I decided it probably be best to rebuild the carb.


The inside of the carb did look pretty awful. In all honesty it's a possibility I did something like chose the wrong base gasket to put on there. I just picked the one in the kit that looked like it fit right.

As far as having ignition problems I'm not too sure cause the truck seems to idle fairly well in park and seems to pull fine driving down the road.

I would love to fix it all myself but when I start looking into setting a carb up properly by the book the measurements and adjustments all look like you need to work for NASA in order to know what's going on.

[Lt.Del]

you ever think that the reason your uncle parked it was because of the rough idle?  meaning, it may have been a problem prior to your carb experiment.  Could be cracked intake, block, head, etc... that would have same symptoms as you describe.  Something as simple as not having correct or old antifreeze would do that.  i'd do compression tests on cylinders next

[nyamwegi]

I took it by this guy I know who had a vacuum gauge and used to work on cars for a living.

Apparently the vacuum tube that goes from the carb to the distributor has very little vacuum, which I'm guessing is ruining my spark advance and timing issues.

He adjusted everything on it the best he could but said it would probably be best for me to just get a holley or edelbrock to put on it rather than rebuilding the quadrajet.


Any thoughts?

[Donut]

What were the readings on the vacuum gauge?  Also what was the needle doing?
http://www.users.bigpond.com/ergoff/vac1.htm

Your distributor vacuum could be off ported manifold vac. instead of full vacuum (I think GM did this for emissions).  That "could" be why the reading was low.

After the water incident, did you change the fuel filters.

You could buy a new carb. (FWIW, a reman. Q-jet would probably be cheaper.) but if that's not the problem, it won't fix it.

[Lt.Del]

I bought a remanned 750 cfm qjet from PAW (Performance Automotive Wharehouse) when I threw my 383 stroker in the truck, and she runs so smooth; https://secure.pawengineparts.com/shoppingcart/showpage.cfm?catid=135

[nyamwegi]

Yes sir I did change out the fuel filter. I am a little confused as to how, after rebuilding the carb, there wouldn't be vacuum coming out of one of the tubes. He didn't really talk like it was because of a leak....