The unfindable vacuum leak (1 Viewer)

[emac]

If your choke is pulled out then you are getting some increase in fuel mixture, but you are also getting fast idle. The fast idle is probably masking the stutter. Why are you convinced you have a vac leak?

 

[elliott10]

Just from working on old stuff over the years if closing the choke helps its usually an indication of a vac leak, but i am far from a mechanic. I drove it a few miles today with the line to the booster clamped shut and it didn't make a noticeable difference so i assume the booster is good. Im heading to get a vac gauge now.

 

[kruisinkid]

happening under load or also in neutral?
warm engine or cold engine or both?
could be metal shavings in bowl. not uncommon on trollhole carbs. edited: no longer a problem. good to hear. these carbs and dizzys are amazing.

 

[elliott10]

The stutter is only on take off and after a gear change so under a load. Both cold or warm engine but it isn't as bad when the choke is closed when its cold and warming up.

We have 4 big chain auto part stores in my town and not a single one had a vacuum gauge so ill be ordering one from amazon

 

[Trollhole]

happening under load or also in neutral?
warm engine or cold engine or both?
could be metal shavings in bowl. not uncommon on trollhole carbs.

That was one batch 3 years ago. We do a ton of inspections.

That being said check and make sure you are getting a good stream out of the accelerator pump. You can confirm this with the engine off.

 

[kruisinkid]

edited my post marshall. must have been a big batch. i remember there were quite a few issues with them back then. certainly not trying to bash your gear. it and you helped me IMMENSELY.

 

[iptman]

See if you can find a local shop with a smoker. They shouldn't charge too much to just plug it up and run for a few minutes. Might be worth it saving you time chasing the leak down with trial and error.

 

[elliott10]

So i played with it for a couple of hours tonight, pulled the breather off and the carb and found that the front pass side carb bolt wasn't tight in the manifold, fixed that, went over all the bolts on the manifold and checked that they were all tight, one was a little loose but not much. Re checked all the vacuum lines, everything good. Put the carb back on and tightened everything up. Re checked the carb tune via lean drop, checked the timing, its set to bb at bottom of window, had been set at 3/4 to bottom of window per marshalls advice, just changed it to see if it was any difference. Took off down the road and it's exactly the same, no change at all. Backed the idle mixture screw out one full turn and took off again. Maybe a slight slight improvement. So i back out another full turn, not any change so i screwed it back in. Advanced the timing back to 3/4 window, no change. So i advanced it up till the bb was gone out of the window, no real change i can notice. So i stacked a few washers on the accelerator pump, which i checked before i did that and it has a health squirt of gas, that helped a little off the idle but it goes back to a stumbley acceleration after a second after the peddle is pushed. All that being said i still have the issue of all the stumble and hesitation being gone if i pull the choke. If I'm running along at say 2000-2500 rpm going up a hill (under a load) its will accelerate and isn't like it has no power but if i pull the choke it immediately smooths and has a noticeable increase of power.

Also i replaced the rubber grommet at the pcv today and the last non new vacuum hose.

Also i want to say thanks again for all the comments from everyone. I'd have given up long long ago and went back to playing with the k5 if it wasn't for you all and this board.

 

[Solace in Solitude]

This is in the FAQ, at the top of this forum. Vacuum 101 - Land Cruiser Tech from IH8MUD.com

 

[sggoat]

if you have the stumble normal but works ok with choke, you have fuel delivery issues your carb is gooked up

 

[65swb45]

It is low vacuum, for sure.

 

[Pin_Head]

Vacuum leaks and lack of sufficient idle fuel behave the same and are hard to distinguish. Both typically result in low manifold vacuum and fuel dripping from the primary main nozzle because you have to bump up the idle speed screw to get it to idle. You can tell if it is getting some idle fuel by disconnecting the solenoid and seeing if it stalls. The brake booster and cracks in the bottom of the manifold are hard to find vacuum leaks. The EGR can also cause a vacuum leak on engines after '78.

 

[chuckost]

Disconnect distributor advance line at carb. Put vacuum to it(suck)! Does air just come through? Blow in it does it seem open?
Didn't see year but if vacuum retard you might not easily notice a leaking diaphragm. I pulled the hose at the dizzy and plugged with a bolt. Wow! Power! Lol

 

[Oliver]

I'm with Strait Cruisin here - every vacuum check known to modern man seems to have been suggested and done over - all to no avail. With a stutter on take-off (high spark load) I'd put my money on an ignition hardware problem, like plug gap, leads, cap, (coil?), etc. IMHO pulling the choke just makes for a richer mixture and therefore easier for the spark to jump the plug gap

Hope you get the Jump on it!

Cheers

 

[elliott10]

Hey guys...

I didn't want to muddy the waters any more till i got a vacuum gauge and it came in today. So i hooked it to the booster hose and fired it up. Cold it was hanging right on 19 with about a 1 psi bounce. It wasn't "glued in place" looking like someone said it should be. After about 10 min idling it rose to a steady 20psi with the same flutter in the needle. So I'm assuming that all means I'm good on vacuum and dont actually have a leak? Called my dad over and i tried to determine if the vac advance is working by putting the motor under a load and watching the bb with a timing light. It never moved. Reving the motor didn't change anything either. I'm not sure thats how you test the advance or not but i dont have one of the hand held vacuum pumps so thats all I could do. I called Marshall yesterday and talked to him again and he suggested i check the power valve. I pulled the top off the carb and pulled it out and it blows through the hole fine so maybe the "tube?" To the underside of the butterfly is stopped up. I have to give a big thanks to @Trollhole he immediately offered to send me a new carb no question at all and took time out of his day to talk to me for 20-25 minutes and was happy to do it. No matter what from here forward if he sells it i will only buy from him.

Thanks again for all the comments too. It's very rewarding to have small accomplishments working on this old heap myself.

 

[kruisinkid]

if you can afford the $170 or whatever it is, get a new dizzy from marshall. i promise its worth every penny.

 

[emac]

if you can afford the $170 or whatever it is, get a new dizzy from marshall. i promise its worth every penny.

agree. Mine works great.

 

[Pin_Head]

With a stutter on take-off (high spark load) I'd put my money on an ignition hardware problem, like plug gap, leads, cap, (coil?), etc. IMHO pulling the choke just makes for a richer mixture and therefore easier for the spark to jump the plug gap.

Except this isn't correct. There is no particularly high "spark load" off idle and making the mixture richer doesn't make it significantly easier to spark.

The spark voltage goes up with the air fuel charge and engine load, like wide open throttle. Ignition problems typically show up at very high RPMs where the coil magnetization doesn't have time to saturate.

Off idle stumble is a classic symptom of a vacuum leak or lack of idle fuel. If you can't find the leak (and you really have checked everything), then it is probably a lack of sufficient idle fuel.