DonM

A little back ground first. I bought the truck for cheap, knowing it will need some work. Good project for me and my coming of age son (16). Its an 86 4x4. It spun a rod bearing shortly after getting it. SO, We rebuilt the engine. but before it spun the bearing I did notice that the truck had a surge while at cruising speed. Ok. I rebuilt the engine. Well, still has a surge at cruising speed. So I figured it had to be in the carb. So I rebuilt the carb this past weekend. It still has the surge at cruising speed (by surge I mean almost a bucking at lower speeds). Plus now it idles a little ruff (it did not before the carb rebuild) and trys to cut off or does cut off on decel. It seems to me to have a vacuum leak. I can turn the idle/air mixture screw all the way in and it does not choke off. Oh yea, I also just replace the intake gasket thinking it make have a vacuum leak there.
I am really frustrated with this thing now, lol.
I have done a search on here, but did not find anything pointing to a "fix"
If there is, I am sorry I missed it. Just shoot me the link and I will go from there.
Thanks for the help in advance

xxxtreme22r

Your right, if adjusting the idle mix screw does nothing, you have a vacuum leak. Most common cause is incorrectly routed vacuum lines or a leak ate the carb base gasket.

You can liberally spray some carb clean around the base and see what the engine does. If it wants to die or rev up, then you know the leak is there.

Also another trick, is, pull the pcv and block it off. If engine wants to either die or lowers rev, your vacuum leak is either in the oil pan, timing cover or valve cover.

More uncommon, but another hidden source could be the brake booster. Do the same thing with that line as the PCV valve.

DonM

I have replaced the base gasket with the ones that came in the carb kit. I had to remove the originals from the phanelic spacer. The top gasket looked the same, but the lower was a little different. The original lower had two small hole that allowed vacuum to enter the passages in the spacer for the PCV port. The new gasket does not have the holes, just open passage to the PCV port direct from the secondary opening.
I have trued all the base plate and main body surfaces when I rebuilt the carb. When I spray carb cleaner around the base of the carb on the back side (throttle linkage side) I get the rise in RPM. I have checked the throttle shafts and all are tight. This is the reason why I replace the intake gaskets. I have blocked off the PCV and no change. The secondary choke pull off is bad, so that line is removed and blocked. Can not find a replacement one yet
As far as I can tell, all the vacuum lines are routed correctly. At least via the diagrams I have found so far.

Thanks for the pointers. Any more Ideas. This thing really has me stumped... I will check the booster in a few. I have not checked that one yet.

xxxtreme22r

How did you true up those sealing surfaces?
Did you use any sealer on the carb gaskets? You shouldn't have.

Your leak is there if you spray carb cleaner and idle raises, however IMO it's a small one. A larger vacuum leak would actually cause a lower idle when spraying carb cleaner.

Could still be the throttle plates even though they seem tight. That's a rather common issue I often forget about.

What have you set your timing to? Should be o degrees with the engine at no more then 700-750 RPM with both vacuum lines blocked off. And that 0 degrees should put you right at the middle of the "window" on the distributor. If the adjustment bolt close to the end either end, then the distributor is off a tooth. I doubt that it's a tooth off since the problem was there before and after the rebuild, but doesn't hurt to check it anyway.

Another common issue that has been popping up from time to time on here is the crank pulley being shifted where the timing mark is not aligned on it properly since it's a two piece unit. The v-notch should be lined up with the keyway.

How old is the fuel pump?

DonM

No silicone. I resurfaced the items in my mill. The timing is right on and a new balancer was installed when the engine was rebuilt, as the old balance has spun. I did run the engine after the rebuild, before messing with the carb.
The issue I had before the carb rebuild was just the surge, tht was present before and after the engine rebuild

xxxtreme22r

I don't think that small vacuum leak would cause a surge you would feel. Is this an auto or manual trans? Maybe the problem isn't the engine itself but in the trans? Especially if it's an auto.

Is a surge or a bog down? Meaning does the truck want to all of a sudden slow down or speed up?

DonM

It's a manual. Best way to describe it, is its almost like an engine miss, but not a sudden hit, it drages or surges. Like when you are driving too slow in a higher gear, but not as violently

xxxtreme22r

Fuel filter and fuel pump new? Plug wires? All your engine grounds good, specifically the one on the head in the back attached to the firewall?

DonM

New filter, new pump. Ground is connected. New plugs, wires, cap and rotor installed at time of engine rebuild. Lol. Like I said, I am stumped.
Thinking I may have to try a different carb. That's the only thing I keep coming back to

DonM

What should I be looking at at far as engine vacuum reading and PVC valve. This thing is sucking extremely hard though the PVC valve. I can block It and the idle Improves. I can leave it connected and block the vent on the front of the valve cover and it will suck my finger down on the vent. I think it's pulling too much vacuum though the PVC.
It has a new aftermarket on in it. Number 2184. Which way do I need to go for less vacuum on the PVC?

xxxtreme22r

PCV1139 should be the correct one according to the autozone site. What do you mean by idle improves? I thought you said earlier it has no affect?

DonM

Sorry, my mistake. When I checked it earlier the choke was still on. Yea, it makes no difference after the engine is warmed up.

DonM

Ok, so this is where I ended up. I spent about an hour fine tuning the float level. Truck still ran a little ruffer than before at idle, but when driving and coming to a stop it would return to idle fine (not try to cut off). After a trip to the store I get back in the truck and it runs like crap again. Get back to the shop, look and the float level is now way too high. And yes, still has the surge while driving. Only conclusion now is having to replace te carb. I think that will be up to a new owner. I am getting rid of it. Don't want anything that I have this most trouble with. Thanks for the help though

igotit

i understand your aggrevation don,id walk away for a few days....get a fresh start on her then,before giving up.