How much Vac at Idle?? (1 Viewer)

[MudHunter]

**** UPDATED ******



72 F Engine

I have around 16 steady. I think I have the flooding issues resolved, timing is set good now also. I am now experiencing popping and backfire under acceleration, none in 1st, some in second and alot in 3rd under load.

I have re-tourqed the manifolds...etc....so whats the deal??

 

[laguna80]

Normal Vac is supposed to be 16.5 mmHg. Low vac can be due to a lot of things. Here's a good link to the tech forums. http://www.ih8mud.com/tech/vacuum.php

 

[MudHunter]

So after reading this...and hearing what my mechanic friend who helped me set the timing and such...he suspected that my dizzy was part of the problem.

Also from this chart...I have stable vacum. It does not jump around, etc.

Now how would the dizzy make it back fire? Letme take a stab at it. If the dizzy misses one cylinder, that cylinder still gets fuel but it does not ignite, then gets forced out the exhaust side and backfires when it hits the headers etc??

thanks for any help,
wannabe shadetree guru

 

[trekker]

You are narrowing it down...

Do some other obvious thinds:

If your cap and rotor have not been replaced in a while... replace them.
Also check your exhaust system and ensure that there is good airflow. An impeaded airflow causes the symptoms you describe. Did the 72 have a cat converter?

 

[ibewebbman]

maybe check your vac under load - it might be a leak say at the carb insulator that gets worse under hard acceleration. I am no expert but experienced something similar to what you describe and a re-torque of the carb to manifold bolts resolved the issue. I duck tapped my vac guage to the windshield to figure this one out - thanks to Jim C for that idea

good luck
Alex

 

[flippin sweet cruiser]

Have you checked your points? Mine did the exact same thing, and when I pulled the points the really wasn't anything left of them. Check the gap or replace them and that should hopefully solve your problem.

 

[MudHunter]

I have a newer cap and rotor...but honestly have not looked at the points yet.... I'll check that tonight.

I do have a GM HEI dizzy I could swap in...but would rather the toyota part work if at all possible.

Trekker...if it had a cat...it does not now. This truck is pretty much engine, battery, alternator, distributor and coil, radiator. Thats it. Bare Bones.

 

[trekker]

No cat. Did not appear till '79.

Do however check and make sure you have airflow through the exhaust system. Is your muffler pretty old? It could be restricting airflow.

Additionally, a soft popping noise, together with a rough engine idle, could indicate a suspect valve or ignition timing. If the motor misses under load, e.g. when accelerating, it may be defective ignition leads or spark plugs.

A louder popping during speeding up or slowing down is often a sign of wear in the muffler or a leak at the exhaust joints. It might also indicate a leak at the carburettor to the manifold.

I hooked up a wet/dry vac to my tail pipe (engine off of course!). I then took my cancer stick (Marlboro works just fine) and gingerly waved it around all joints including the manifold. Found a few leaks this way. Use caution when near gas sources!!!!

In the day, I checked for blockage in my exhaust system the same way. Although you will get air flow to the vac (remove tube to valve cover to allow air to enter) you really can't tell if your muffler is bad or there is blackage somewhere. To do this, unbolt your exhaust pipe from the manifold and then crank up the vac. If its blocked, you will know! There should be a subtle change in pitch in the vac motor if any.

I betcha you have a blockage somewhere.

 

[brider]

vac reading

I have a 71 F engine and at idle it pulls 21 in Hg. Your 15 seems on the low end, and the fact that you KNOW there are problems tells me your vac reading will go up once you sort them out. My vote is for timing/points/coil.

 

[Cruiser_GT]

Just for giggles try setting your timing using the vac gauge... go for max vac. Back off from that a half or maybe an inch from that point. My desmogged '78 runs best right at 20 inches.

If you have pinging under load, then back the timing off a bit more. Repeat until you are amazed at how well tuned your truck is... but, if you have any of the problems others have mentioned above, this probably won't fix your missing. It did correct mine. hth

 

[MudHunter]

All good suggestions...will be testing out a few tonight!!

 

[laguna80]

Just for giggles try setting your timing using the vac gauge... go for max vac. Back off from that a half or maybe an inch from that point. My desmogged '78 runs best right at 20 inches.

If you have pinging under load, then back the timing off a bit more. Repeat until you are amazed at how well tuned your truck is... but, if you have any of the problems others have mentioned above, this probably won't fix your missing. It did correct mine. hth

This is something that I've never been able to figure out if I'm doing it right. Whenever I adjust for max vacuum the engine revs and the RPMs go up. It would be nice to keep the vacuum at around 20, but I can't figure out how to make the engine idle at 650 RPMs.

 

[MudHunter]

Originally Posted by Cruiser_GT
Just for giggles try setting your timing using the vac gauge... go for max vac. Back off from that a half or maybe an inch from that point. My desmogged '78 runs best right at 20 inches.

If you have pinging under load, then back the timing off a bit more. Repeat until you are amazed at how well tuned your truck is... but, if you have any of the problems others have mentioned above, this probably won't fix your missing. It did correct mine. hth


This is what I think the PO did. When we checked the timing to begin with...it was around 20 degrees...and we had to back the carb settings WAY DOWN. Now...this took care of the rrunning rich problem...but now I have popping and backfire under acceleration.

I checkd my points and they look great. I did order a new set since they were only $6 though.

My exhaust is reall simple...header to what looks like a glasspack to tail pipe. Have not played with the idea of a blockage here yet.

I may just try and advance the timing and see where that gets me???

 

[trekker]

I'm still betting on the restricted exhaust.

Try to undo the junction btween the headers and the glasspack. Move the pipe over a little so there is free flow. Run it. See if that glasspack is your problem.

And you have headers... Nevermind what I mentioned about undoing your exhaust from the manifold...

 

[MudHunter]

I'll be trying this tonight also Trekker!!!

 

[Cruiser_GT]

MH - 20 degrees advance? That surprises me a bit.

Maybe this is a dizzy problem... do you have any cruiserheads you could call on to loan you a good distributor so that you can eliminate that as the problem?

 

[nat]

Replace the cap, rotor, points and condenser.............then recheck the timing. Are you timing it with the vacuum advance plugged? Did you redo the air fuel mixture?

If you still have problems, take apart the dizzy, maybe the centrifigal advance is having problems.

 

[LT]

No real tech on your engine problems, but my 2f runs at 21 inches of vacuum at idle.

 

[MudHunter]

OK....I unhooked the exhaust at the collector. No change in condition. Reconnected exhaust. Replaced Coil, now I have a no power to the dizzy. For kicks...reinstalled the old...nothing. Re-installed new Coil. My points will be in tommorow. Will be getting a condensor...and external resistor for coil also.

Cant for the life of me think of why the thing wont start now unless the external resistor for the coil is somehow shot now. It was running right before I replaced the coil. Engine turns...NO FIRE.

Tired and stressed from work...maybe not the right day to work on her.

 

[MudHunter]

MH - 20 degrees advance? That surprises me a bit.

Maybe this is a dizzy problem... do you have any cruiserheads you could call on to loan you a good distributor so that you can eliminate that as the problem?

Dont know of anyone close enough to swap for the evening. I do however have a GM HEI sitting in the garage. IT's VERY TEMPTING to go for the swap.