CARB ADJUST?

[bmotojoe]

13 inches vacuum seems low to me, with a stock cam and no vacuum leaks it should be higher then 17 inches. Where did you attach the gauge to take your reading? Was your air cleaner assembly put on when doing vacuum test? Was the needle of the gauge steady?
Check this site out: After reading through the page there are animated scenarios explaining how to troubleshoot a vacuum related problem using a vacuum gauge.
Brian
http://www.secondchancegarage.com/public/186.cfm

 

[Vette79]

As Brian said, 13 is low. I am 4 turns out on the idle mixture screws and at about 16 on the vacuum guage. I have seen mine at 18 earlier in the year. Mike, does it backfire when you are accelerating? Brian, when I replace the cam and intake over the winter should I see a decrease in vacuum?

 

[mike82]

13 inches vacuum seems low to me, with a stock cam and no vacuum leaks it should be higher then 17 inches. Where did you attach the gauge to take your reading? Was your air cleaner assembly put on when doing vacuum test? Was the needle of the gauge steady?
Check this site out: After reading through the page there are animated scenarios explaining how to troubleshoot a vacuum related problem using a vacuum gauge.
Brian
http://www.secondchancegarage.com/public/186.cfm

i attached the gauge to the vacuum adv. hose.

needle on the gauge was steady as a rock.

no the air cleaner was not on. the air cleaner didn't really seem to make a difference.

i didn't notice the problems till i had the true duals installed.

 

[mike82]

As Brian said, 13 is low. I am 4 turns out on the idle mixture screws and at about 16 on the vacuum guage. I have seen mine at 18 earlier in the year. Mike, does it backfire when you are accelerating? Brian, when I replace the cam and intake over the winter should I see a decrease in vacuum?

it backfires if i hit the gas(wot) and let off the gas abruptly.

 

[bmotojoe]

Try hooking the gauge up either at the PCV line or at the intake manifold behind the carburetor. There should be a tee fitting there pull the top line going to the transmission and install the gauge line there. See if your vacuum readings change any. Sounds like you may be a little off on the timing as well. I run my L48 between 700 and 725RPM at idle and advance my timing light to 10 BTDC.
Brian

 

[bmotojoe]

As Brian said, 13 is low. I am 4 turns out on the idle mixture screws and at about 16 on the vacuum guage. I have seen mine at 18 earlier in the year. Mike, does it backfire when you are accelerating? Brian, when I replace the cam and intake over the winter should I see a decrease in vacuum?

I am no cam expert so I can’t tell you what vacuum readings you should expect with your new cam. I can tell you that the higher overlap you have between valves opening and closing your going to loose some vacuum. I would think that if you’re seeing 18 inches now to expect 15 or lower with the new cam. Also, you may notice your headlight doors opening on there own when you really tromp down on the throttle for an expanded period of time.
Brian

 

[minifridge1138]

A few things,

1) It really sounds like your timing is off. Carb settings should not cause backfire. Usually carb backfire means that the spark is getting there before the intake valve closes. This causes the backfire.

2) I know i originally said to use the vacuum going to the distributor's advance, but i think i was wrong. Over the weekend I adjusted my carb and I was only seeing 13 on the vacuum at the distrib (which IS too low). So I attached my gauge to the 'T' fitting behind the carb (it goes to the brakes and headlights) and I was reading 18.5 on the gauge. A little adjusting and I got 20 on the gauge.

3) I have had several people offer a slightly different way to adjust the carb. It is basically the same process i described, but instead of adjusting at idle, they say that you should get the car warmed up, and then adjust the idle speed screw (NOT IDLE MIX) until you get the car to the max RPM you want. Then use the process i described to adjust the mix screws until you get the highest vacuum. Then re-adjust the idle speed screw until you get the idle speed you want. They say that this gives you the best performance/fuel economy at cruising speed, rather than at idle.



So I suggest putting the gauge on a different vacuum line and seeing if it is still that low. Here is a VERY useful page describing what different vacuum readings mean: http://www.secondchancegarage.com/public/186.cfm.

 

[LanceB]

Mike,

Read my post in the carb work. I worked with a guy that help me simplify everything on my 81. We blocked off a ton of inlets and outlets. There are just so many parts added to the carb in my opinion. Mine is running better than it ever has with the same gas mileage and I feel like it is more dependable.

Lance

 

[JohnZ]

2) I know i originally said to use the vacuum going to the distributor's advance, but i think i was wrong. Over the weekend I adjusted my carb and I was only seeing 13 on the vacuum at the distrib (which IS too low). So I attached my gauge to the 'T' fitting behind the carb (it goes to the brakes and headlights) and I was reading 18.5 on the gauge. A little adjusting and I got 20 on the gauge.

That's correct - all Corvettes from '68-up use a "ported" vacuum source for the distributor vacuum advance, not a full manifold vacuum source, for emissions reasons. You must have a full manifold vacuum source for the vacuum gauge.

 

[minifridge1138]

JohnZ,

Didn't i read in an article (that you wrote) that it is better to use full manifold vacuum for the vacuum advance?

I swear that the last time i tuned my carb, I plugged my gauge into the line going to the distributor and was reading full vacuum, but maybe I was wrong.


Thanks!

 

[petes74ttop]

That has to be a typo.

I'm sure he ment manifold vacuum. PG

 

[GerryLP]

A back-fire when it flashes out through the carb throttle is caused by a lean condition. The back-fire (I believe normally called "after-fire") is primarily caused by a timing mal-adjustment.

 

[mike82]

it backfires if i hit the gas(wot) and let off the gas abruptly.

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i re-read what i posted and maybe i didn't say it right. ishould of said;

the carb does not backfire, if i go WOT the car will backfire at the exhaust.
the car will backfire slightly(thru the exhaust) if i rev the motor(while in park).

 

[mike82]

Try hooking the gauge up either at the PCV line or at the intake manifold behind the carburetor. There should be a tee fitting there pull the top line going to the transmission and install the gauge line there. See if your vacuum readings change any. Sounds like you may be a little off on the timing as well. I run my L48 between 700 and 725RPM at idle and advance my timing light to 10 BTDC.
Brian

yes sir i know exactly where your talking about behind the carb. i'm assuming i should still unplug the vacuum advance and then plug the hose going to it.

i have followed the directions of the decal under the hood which is, 700rpm in drive and 12 BTDC.

 

[bmotojoe]

If your adjusting the carburetor then keep the vacuum advance hose connected to the distributor. If your adjusting the timing then disconnect that hose and plug it. Is your vacuum advance hose connected to a ported connection on the carburetor or is the hose connected to full vacuum?
Brian

 

[mike82]

If your adjusting the carburetor then keep the vacuum advance hose connected to the distributor. If your adjusting the timing then disconnect that hose and plug it. Is your vacuum advance hose connected to a ported connection on the carburetor or is the hose connected to full vacuum?
Brian

vac. adv. hose is connected to ported connection.

 

[Vette79]

By accident, I have left my vacuum advance hose disconnected from my carb after making adjustments to my carb. The result was the backfire at the exhaust when at WOT. Brian, are you saying that when adjusting the idle mixture screws to connect vacuum guage to manifold vacuum and keep vacuum advance hose connected to carb? I'll try this later today and see what reading I get.

 

[bmotojoe]

Yes, connect the vacuum gauge directly to manifold. Keep ported vacuum advance line from carburetor to distributor connected. I’ve tried adjusting my Q-jet so many ways in the past but the lean-best idle procedure I explained on the first page has worked the best so far for me. I don’t keep track of the turns made to each screw anymore; I used to and was always concerned if one was more then another, not anymore.
Brian

 

[mike82]

Yes, connect the vacuum gauge directly to manifold. Keep ported vacuum advance line from carburetor to distributor connected. I’ve tried adjusting my Q-jet so many ways in the past but the lean-best idle procedure I explained on the first page has worked the best so far for me. I don’t keep track of the turns made to each screw anymore; I used to and was always concerned if one was more then another, not anymore.
Brian

hey Brian,when you are adjust your carb, are you in park or drive?
also, do you know about how many turns out you are with the IMSs'.
thanks,,,,mike

 

[JohnZ]

JohnZ,

Didn't i read in an article (that you wrote) that it is better to use full manifold vacuum for the vacuum advance?

I swear that the last time i tuned my carb, I plugged my gauge into the line going to the distributor and was reading full vacuum, but maybe I was wrong.


Thanks!

Yes, connecting the advance line to full manifold vacuum works MUCH better, but they came from the factory with it connected to "ported" vacuum for emissions reasons. In the early 70's the TCS system switched the source between full and ported depending on what gear the transmission was in and the engine temperature. Stick with a full manifold vacuum source.