My vacuum at idle is around 10 with the needle flickering between 9 and 11. The motor is a 71 454 with a higher lift than stock cam installed by a previous owner. I found this while the checking air/fuel mixture on carb, the reading didn't change while adjusting the screws. As the rpm's go up the vacuum reading did also. I don't think this is normal. Any thoughts?
Thanks Fred

 

The usual way to adjust the idle mixture screws is to adjust them until you get the highest vacuum reading. Then, if the idle speed is too high, adjust it with the idle speed screw. If that's not clear enough for you, let us know, we'll go into a little more detail if you need it. Then we'll get started on the ignition timing!

Scott

 

Sorry, but I am unable to help except that I share the same issues you face. I have a 70 L46 350/350 and have rebuilt most things that move!
On my engine build I reborded the block, new pistons rings, valve seals, springs etc. I also thought it to be a good idea to change the cam, I chose a competition cam 12-246-3 which has a higher lift than stock, it was also recommended I change the valve springs and seals, so I did. My carb which is original was rebuilt.
My vacuum readings are similar to yours and I have great difficulty holding the recommended idle of 750 rpm. It doesn't seem to make any difference if the idle screws are all the way in or all the way out or somewhere in between. I returned the carb to the rebuilder twice who says the carb idles well on his crate motor plus I have taken the carb to a local "expert" who claims, all's well. I've tried running the engine with the inlet manifold vacuum services connector blanked off in case there is a leak in the head light/wiper system, but no difference.
I was told that the high lift cam could be the problem so I discussed this with the tech department of competition cams, they thought the cam should work fine and that I should look elsewhere into the carb or ignition.
On cold start with the choke engaged all's well. And the engine run's well in drive as long as when you stop and engaged the clutch the gas pedal needs constant pumping to prevent stalling.
I would be interested to know what you find ... I guess misery loves company!

Happy New Year

 

I suggest you guys read Lars' tuning articles. Worked wonders for me.

 

Determine the 'baseline' vacuum level (maximum) that your engine can provide with all vacuum-related connections disconnected and capped. Also, shoot puffs of carb cleaner at the carb and intake manifold to see if the engine revs (indicating that there are air leaks on the intake system).

If baseline vacuum is higher and/or you find intake system leaks, fix all the problems and your vacuum level should be adequate.

 

I did read one of Lars papers in fact I had just finished setting the total timing at 36*, dwell at 28 then went to the air fuel mixture. My real question was, is the vac reading at an average of 10 with a fast flicker of 9 to 11 is acceptable and any idea why the a/f screws have no effect. I have sprayed the carb and intake with starting fluid in case there was an air leak but nothing changed
Thanks
Fred

 

What kind of carb are we talking about here, anyways? Q-jet?

Scott

 

The idle mixture screws have no effect because you have a significant air leak INTO your intake system. Find and fix the air/vacuum leaks and you will solve your problem.

 

You haven't said what carb you're running.
If the opening point of the power valve(s) are not lower than your idle vacuum, they are starting to bleed fuel.
Easy to check on a Holley. The vacuum setting is stamped right on the valve.

 

I'm sorry to disagree with you, and please don't take this as a personal attack, but this is just an old wive's tale. The power valve has virtually no effect on the idle circuit.

Scott

 

OK, if you have followed Lar's instructions properly. your ignition timing should be in good order.

Be advised that a vacuum leak can occur as far forward as the headlights, because of the vacuum system used to activate the flip-up headlights. Also, the heater and A/C controls, and on older C3s, the windshield wiper flap are possible sources of a vacuum leak. Block off all of these connections and see if it makes any difference. If it does, re-attach vacuum connections one by one to track down the leak.

Scott

 

Sweet70 besure to read Lars articles on the distributor vaccum cannister and the different vacuum levels that the cannisters start to pull in. i had to replace my vaccum cannister to accomodate a bigger cam.

 

If you have a cam that only generates 9-11 in of vacuum then you need a vac can that is all in by 8 in of vacuum. Make sure you are connected to manifold vacuum. Then set your initial to 16*. That combined with a can that gives 16* of advance will then give you a total of 32* of advance at idle. If running an HEI with 20* of mechanical advance (standard) You'll get 36* advance all in at WOT.

A cam with low vacuum needs lots of advance at idle. Each engine will be different you'll have to find the happy spot for your cam. If it happens to be more than 16* initial and running an HEI then you may need to limit the mechanical advance so as to not ping at full throttle or cruise.
Your carb idle circuit will need to be modified to meet the new lower vacuum signal from the engine at idle. This usually involves larger jets or orfices to make it easier to pull fuel from the bowl.

 

I had similar problem with mine after build..383/525hp.. Big cam, .576 lift, 254* duration @.050.. only 6-7 inch Hg .. the wiggle on the needle is normal for a big cam, the inability to get any response to idle mix screws is exactly what I had. Screw all the way in or out , no effect.. No vacuum leak.. On my Quickfuel Carb ,the Air Bleeds are removable. I went two sizes larger on the bleeds , gained control of idle mix. idles great, lumpy but steady at 650 rpm with 1 1/4 turns out on all 4 screws . Yes 4 , it has 4 idle mix screws. ,, hope that helps

 

Thank you, when I said I had removed/replaced just about everything that moves, that was correct except for the distributor. I did change the points and cap, apart from that I believe it's original to the car with part # 1112021. I just looked at the vacuum advance and it has the markings of MS 360-12, not sure what that means. Looking at the manual for vacuum it states "Vacuum advance" (crank degrees @ in Hg). 0@7, 15@12. Centrifugal advance is given as 0@1000, 10@1700, 26@5000 and Ignition timing @ idle speed (degrees BTDC) = 8.
I just looked up a replacement vacuum control with Paragon and see where the 360# comes from ... it gives me a replacement but doesn't show the values. On my car at idle of 750 rpm I should have 8 degrees advance ... what advance do you think I should be looking for from vacuum?
I would like to read Lars notes on tuning etc, is there a tab or location I can find them?
Many thanks ... Sweet70.

 

ftf396 and sweet70,
You guys need the B28 vacuum advance can on your dizzy. Read this post below and good luck!!

http://forums.corvetteforum.com/c3-t...572-dizzy.html

 

Thanks for all the info guys. The carb is a Qjet. and it has a stock points distributer .I will check the vac lines and if can't find a leak will try the new can.

 

check the vacuum tank most of them are cracked it is becoming a very common repair for me about 1 a month poor idle is one of the symptoms.
Mark

 

Thx, my distributor is stock not MSD... Sweet70

 

Mark, thx for the interest, my problem persists even when the vacuum accessories are blanked off on the inlet manifold. I have sufficient experts opinions that the vacuum controller on the distributor needs replacing or changing to a different value. I'm going to work on that for a bit, then we'll see. Sweet70