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One of these days...soon
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Discussion Starter #1
I have read all the posts on the site and tried everything I could come up with and my motor is still running a little rich, hesitates a bit when revving, is lacking power, and just all of sudden dies for no reason.

It is a 406 sbc, appox. 9.7:1 CR, comp cam XE274, 2400 stall, performer RPM intake, speed demon 750 cfm carb, msd hei distributor, AFR195 heads, th350 tranny, 308 gears in the rear end.

I have the timing at 28 deg advanced no vac. advance, idle is about 1000rpm’s, and all 4 mixture screws are turned out half a turn, the demon instructions indicate that it is best to have the mixture screws turned out 1 to 2 ½ turns, but it would be way to rich if I did that.

Any help would be appreciated.

406chevelle
 

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What do you mean by 28o advanced. Is that what you are reading at the balancer with the timing light? Or is that the total timing when it is all in?

If you have 28o initial, your idle will be pretty high. Then you would have to close the throttle blades down a bunch at idle to make up for the highe idle. That would richen everything up and you would only need a 1/2 turn, like you said on the mixture screws.
 

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Hotrodders.com Moderator
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timing

What is your intial timing and you total timing??? Let us know that first and we can get started helping you.

Keith
 

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One of these days...soon
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327 Posts
Discussion Starter #4
Sorry for the slow reply….work, kids, wife….you know the story.

28 degrees is initial timing measured at the balancer with a timing light while idling at 1000rpm's, I don’t know what the total timing is, what RPM should I be checking it at? Doing this with no help so should I just keep turning the idle screw until the timing stops advancing?

406chevelle
 

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One of these days...soon
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327 Posts
Discussion Starter #6
thanks for the quick respone Ilucian077,

The vac. advance is not connected at this time, and the ports are plugged.

If I reduce the advance the motor won't start, it just turns over.

sorry to be such a smuck but I am kind of lost on this problem.

406chevelle
 

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Did you check that the balancer is correct? They can shift.

When I assembling my engine, I checked that the pointer pointed to 0o when the engine was at TDC. I used a straight edge and feeler gages to make sure TDC was TDC.

There is another problem. I can't put my finger on it, but maybe someone else on this board can read between the lines. Your engine shouldn't run better with 28o initial. The starter should kick back when you try to start it or it should ping as you accelerate.

Check that everything is right with the carb. Make sure your distributor's mechanical advance is working.
 

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One of these days...soon
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327 Posts
Discussion Starter #8
Ok I have got it running good at 12 deg. advanced at 1000rpm idle and the total timing is about 24 deg. advanced (mechanical, vac. advance is plugged), and I tried adjusting the mixture screws, it is still too rich and when I rev it up I can hear some popping in the exhaust, like a mello backfire.

I do need to change the thermostat it gets up to about 210 deg. I have a 180 thermostat in it, and hope a 160 will bring the temp down about 20 deg. Could running it this hot be part of my problem?

406chevelle
 

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406chevelle said:
Ok I have got it running good at 12 deg. advanced at 1000rpm idle and the total timing is about 24 deg. advanced (mechanical, vac. advance is plugged), and I tried adjusting the mixture screws, it is still too rich and when I rev it up I can hear some popping in the exhaust, like a mello backfire.

I do need to change the thermostat it gets up to about 210 deg. I have a 180 thermostat in it, and hope a 160 will bring the temp down about 20 deg. Could running it this hot be part of my problem?

406chevelle
210 is normal operating temp. on newer engines. I would run the engine you have at 180o.

Changing thermostats isn't going to bring the temp down necessarily. At 210o, the 180o and the 160o thermostats will both be open all of the way.

Your total timing should be at 34o to 36o.
 

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Hotrodders.com Moderator
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adjustments

It looks to me like your distributor has 12 degrees of advance in it. So you would set your total timing at 36 to 38 degrees to get a starting point. That would make you initial 24 to 26. It's ok to run it like that as long as you have the proper heat plugs and the correct octane fuel. I run mine locked at 36 degrees and never had any starting problems. You will need fresh spark plugs in it after every change. I know it cost alot of money throwing away plugs but if you have one glazed over you might fix the problem but not know it. Does this motor have a pvc valve??? If you get the timing set and install a fresh set of plugs and it is still rich. I would remove the carb and look at the transfer slots on the primary side. They should look like a square. If there is more then .025 exposed that is the reason it is running rich. Did you open the secondary side at all when you first installed the carb???? Is you speed demon a newer model with the idle ease option???? What plugs are you running in it now???

Oh yea as 77 stated take a few minutes and conferm that your balancer is marked correct for the timing points at TDC

Keith
 

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One of these days...soon
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327 Posts
Discussion Starter #12
Thanks for the help so far, it is badly needed.
I bought a speed demon because my research indicated that it would be the best performing carb for my motor, and I was fully a wear that it would be difficult to tune, but was willing to learn.

The balancer is good.
It is still at 12 deg. advanced.
I have just been adjusting the mixture screws.
I am now getting a backfire through the carb when I rev it, and still rich.

I guess I should have said something sooner…. I am use a summit A/F gauge and sensor to check lean or rich; I have one setup on each header.

I think everything is OK; it is just my lack of experience especially with carb adjustment. It is getting very frustrating.

I am thinking that maybe the secondary’s should be opened up a bit so more air will flow into the carb, but the only way I see to make that happen is to bend the linkage, I have read that it is sometimes necessary, but hate to bend things up.

406chevelle
 

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Hotrodders.com Moderator
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carb

On the demon carbs they have a screw to open the secondary just like the primary. It is on the passinger side of the carb at the secondary shaft.

Does your carb have the idle ease feature????? Look down the hole where the air cleaner stud goes. If there is a slotted head screw in there you have the idle ease... Let me know and i can help you adjust it.

I personally don't thing 12 degrees is enough initial timing for your set up. It should be a minimum a 18 degrees.

What are the numbers on the plugs you are running???

Keith
 

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Technical Support Barry Grant
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Rich Idle Condition

If you're only running 12 degrees initial timing on your combination, this is most likely causing a lot of your situation. Increasing your initial timing to 16 or 18 degrees should take care of most of your situation. Increasing the timing will allow the engine to burn the fuel better, so you can reset your mixture screws, and butterflies. This should eliminate the rich idle condition, and your hesitation problems.
 

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WrenchingAce
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116 Posts
Hey Bud,
I have an advance light that I would be happy to bring by and help you out with the timing issue. Give me a call and let me know when would be a good time.
 

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Registered
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There is a carb expert that's a regular on the corvette forum. He writes papers on tuning Qjets on Holleys (and distributors).
His name is Lars Grimsrud. I rebuilt & tuned my q-jet per his instructions and it came out great.
Note: before you start adjusting the carb, make sure the distributor is set up correctly as the others have mentioned.

With his permission I am posting his paper.

Cheers

EDIT:
Note: I can't post his paper - too big. Please email me and I'll send it to you.

[email protected]
 
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