The unfindable vacuum leak

[elliott10]

Well ive read every thing i can find about the common vacuum leaks and I'm still having trouble.
I recently installed a trollhole card and dui ignition just because i wanted to update that stuff. And the truck ran as bad or worse than with the stock setup. I finally started finding small vac leaks and fixing them as i went. It's running much better but still has a stumble and "shaky" acceleration. So i pulled the intake and checked for warps and cracks and none found. I have an old header from way back so i used 2 felpro gaskets sandwiched together. Back together and its a little better but still not 100%. Yesterday i called @Trollhole and he walked me through the carb tuning process just to be sure i had it write and it is tuned correctly. Ive sprayed carb cleaner over the whole damn truck at this point and I'm not getting the slightest rpm increase anywhere. I'm at my whits end with this. It idles perfectly smooth but driving i still have the off idle stumble and the sort of shaky feel as i accelerate. But if i pull the choke out its runs and aclcelerates off idle or off of a gear change smooth as silk. Ive sprayed all over the intake top and bottom, no rpm change. I just cant seem to figure this out.

Any help would be greatly appreciated

 

[LAMBCRUSHER]

I had a similar experience once. One by one, cap every vacuum sink until you get a noticible change in engine behavior. This is done idling. I found a vcv that vented the dizzy was leaking internally- no outside evidence; no amount of carb cleaner sprayed would have shown it. Once I unplugged it's source and capped it the engine came to life- it was day and night. Plugged the vcv back in and back to running like crap. A test of the sucker showed it was a straight hole thru it, back to the air cleaner port the other line went to. Put in a new one and retuned the carb. It had been put out of tune mistakenly by me trying to compensate for the extra unmetered air... you might not have a dizzy vent on the dui, but you prolly have a couple of vacuum sinks you could remove from the equation. Hth

 

[emac]

Timing? If you unplug the idle solenoid when idling does it stall? It should. If it doesnt, the carb gasket may be causing the butterfly to stay cracked open a bit adding extra fuel to the engine. In other words the idle circuit isnt working properly and you are adjusting a system not working properly.

 

[elliott10]

@LAMBCRUSHER im not sure what the vcv is. Mine has been desmoged. The only vac I have in the dizzy is the vac advance line.

@emac my timing is set to the bb at the bottom of the inspection window per marshalls advice. And yes if I unplug the i. S. It immediately dies. And it's getting strong voltage to it running, 13 volts.

 

[65swb45]

When was the last time you adjusted the valves? What is your vacuum reading at idle?

 

[elliott10]

Adjusted the valves about 500 miles ago but they were all just a hair off spec. So no dramatic changes there. I don't have a vacuum gauge yet unfortunately.

 

[kruisinkid]

i can tell you from personal experience that i have a fully desmogged rig with a trollhole carb and dizzy and it runs incredibly well. zero hesitation or stumble. i have never heard one good thing about a dui dizzy here. always creating problems. i would dump the headers and dui dizzy. im assuming you are running a stock air cleaner. what year is your truck? a desmogged 40 is one of the most straight forward engines ever i gotta believe. yes it did take me awhile to get the carb and timing zero in. like several hours. it was also my first car ever, let alone a carbed one. what shape is your brake booster in? pcv valve and grommet? are the spark plugs the kind that Mr. T intended?

 

[elliott10]

@kruisinkid its a 78. The booster is probably stock. Pvc and grommet is old. It's oily so I couldn't really say how old. I did new plugs and wires when I did dui. Gapped correctly

 

[65swb45]

Adjusted the valves about 500 miles ago but they were all just a hair off spec. So no dramatic changes there. I don't have a vacuum gauge yet unfortunately.

I recommend you purchase one.

 

[emac]

You could try advancing the timing some and see if it gets any better. For some reason mine runs better advanced more than spec. Easy to try.

I also run the trollhole dizzy and carb. No issues once I got them dialed in.

 

[whitey45]

Vacuum leaks are only an issue at idle, I have found that a little advance helps, and if you run premium gas, the old tractor will come alive!! A vacuum test at idle will tell you the health of your engine. Sounds like no accelerator pump off of idle?

 

[thebigredrocker]

If the assumtion is you don't have vacuum leaks and your valves are near spec, a big a$$ accelerator pump squirt is important. Stack as many small washers on top of the accelerator pump arm and under the cotter pin. If that cures the sputter you can bend your linkage so it's shorter, then you wont have the redneck looking washer setup.

 

[elliott10]

Ill get a vacuum gauge today. What should the readings be.

It's just strange that it runs so much better with the choke pulled out. It's not that it has a lot more power but that it it just accelerates so much smoother and doesn't have that little stumble off idle or gear change.

 

[emac]

Mark your dizzy where it is, then advance it a little listening to the idle. Find that sweet spot. You may need to adjust the idle. Go drive and see if it helps. If it does, then make sure it doesnt ping under load. If it doesnt help, put it back where it was.

 

[thebigredrocker]

Another thing to do is shine a bright flashlight down the primary side of the carb at idle. If you can see ANY fuel spray from your brass main nozzle, you've got a problem.

Also, while shining the flashlight in your primary side, actuate the throttle and see how well your accelerator pump squirts. It should BLAST fuel into the carb, not just send a gentle stream like a peeing cherub.

 

[LAMBCRUSHER]

and spend some time reading thru the archived threads regarding vacuum levels and tuning. alot of the questions you ask have been answered like eleventy billion times. not trying to cop-out, just saying you also need to invest into your own education...

 

[thebigredrocker]

Here's my 71 F at an idle speed of 700rpm's. It was parked in a field in 2001, so my guess is, it doesn't have fancy new rings and pistins

 

[elliott10]

And if the vacuum is low that means a sizable vacuum leak somewhere? I'm just so stumped why it runs so much better with the choke out but i cant find a vac leak anywhere.

 

[Strait Cruisin]

You might want to check out your secondary ignition circuit as well, a plug wire with higher than normal resistance will cause an intermittent high load cutout, the effect is like a plug is gapped too wide. I would assume you got a new cap with the dizzy but you did twist and wiggle all the wires around and they can be somewhat fragile with old age. Kind of like me, now that I think of it....

 

[kruisinkid]

that looks about right. i feel like i remember that ideal number is 17" in the 650 to 700 rpm idle range.

*places your vac will leak:
*brake booster internally - disconnect and plug hose when testing
*brake booster hose. that insulation on it if its stock could be hiding a crack
*pcv grommet, valve and its hose
*mating surface from carb to carb isolator
*mating surface from carb isolator to intake manifold
*vac hose from carb to dizzy
*dizzy itself
*valve cover gasket
*air cleaner cover gasket
*mating surface of intake manifold to head. having 2 felpro gaskets means you can keep tightening those nuts ( i hope rather than bolts) almost forever. i would go back and retighten with a warm engine.
*crack in the "base" of your intake manifold
*hose from air cleaner to valve cover

if your header was never machined with the intake manifold, i can almost guarantee its leaking there. i dont think anyone here has ever been able to just slap some headers on and go.