Problems tuning my new motor.

[sixtyfour]

Having trouble getting my new motor tuned. I have timing set to 8 degrees, did this with the vacuum advance disconnected and the motor at idle, about 8 to 900 rpm. Have carb dialed in like the Edlebrock DVD explained. Just did my first 30 mile oil change. Symptoms are stumbles on take off and will sometimes backfire through carb, definitely missing at speed. Any thoughts on what to look at, advance it more, retard it more? I made my own plug wires which I've done on my other vehicles with no problems. What's the best way to test my wires? Thanks for any suggestions...

Mike

[ChiefRocka]

Have you tried setting the base timing to 10 or 12 degrees ??

Sounds like it may need some more advance.

Assuming everything else is cool.

[Chevy Fleetside]

Have to really hear it to know. The stumble might be the carb. Maybe there is a vaccum leak you missed and the backfire can be lots of things. If it sound like an explosion (Like fuel igniting) then it could be timing. You might be one tooth off on the dist, hard to tell without looking. If it sounds like a baseball bat hitting a wooden pole you might have a tight ex valve. I had a valve stud break on an exaust valve and I thought a rod let loose.

[sixtyfour]

The best way I can describe the back fire through the carb is it sounds like a sneeze. I advanced it a bit more just now, and it seems the sneeze and stumble have gone away a little but still missing. I'll know more tomorrow when I drive it to work. I think I may put my old plug wires on and see what happens.

[Chevysdominate]

Assuming your timing marks are accurate and you dont have any vacuum leaks, it sounds like you need to richen up your idle screws a little bit . That would probably get rid of your off idle issues. I havent used an edelbrock for a few years now, but i think you need to take the screws out to richen the mixture. Don't adjust the idle screws more then 1/8 to 1/4 turn without test driving it. The mis at speed may be a lean condition as well. If that's the case you might need to put different metering rods in. Just my 2 cents. Good luck.

[greg64]

Mike, does it idle okay? Sounds to me like weak ignition (which might be exacerbated by a lean mixture). If you're running points are they okay (gap and dwell?). If there's no vacuum leak then I doubt it's the carb. The factory tuning should be close enough to run, you just might stumble when you nail it.

Could pull a plug an have a look...

Good luck,
Greg

[protrash64]

The off idle stumble can be corrected by changing the position of the arm going to the accelorator pump to apply it sooner. I have used this on 2 edelbrock carbs and its always worked.

[sixtyfour]

Originally Posted by protrash64

The off idle stumble can be corrected by changing the position of the arm going to the accelorator pump to apply it sooner. I have used this on 2 edelbrock carbs and its always worked.

I'll look into this!

[sixtyfour]

Originally Posted by greg64

Mike, does it idle okay? Sounds to me like weak ignition (which might be exacerbated by a lean mixture). If you're running points are they okay (gap and dwell?). If there's no vacuum leak then I doubt it's the carb. The factory tuning should be close enough to run, you just might stumble when you nail it.

Could pull a plug an have a look...

Good luck,
Greg

It's a new GM 350ci/290hp crate motor with HEI ignition. This being a new motor to me I'm not so familiar with the characteristics, but the idle does sound strong from what I can tell although a bit lumpy like it has a nice little cam in it! I just put my old plug wires back on and it does run a little better but still misses at speed.

What's the best way to test for intake leaks? I've heard spray brake clean around the manifold and see if it stumbles. I hate the idea of spraying my new motor with chemicals that might remove the paint!

[TINKERDUKE]

Horrible job i know, but you might want to pop each plug out and check condition which may help diagnose problem and while your there clean and re-gap.
Steve.

[Chevy Fleetside]

Originally Posted by TINKERDUKE

Horrible job i know, but you might want to pop each plug out and check condition which may help diagnose problem and while your there clean and re-gap.
Steve.

Yeah like he said. It might even be too rich. If the plugs have black soot on them you are too rich. The suburban I bought was dumping fuel from one injector and is seemed like it had a lumpy cam and poped a few times untill the excess fuel was burned but ran fine at wide open throttle.

These are just suggestions. It might take a week or so to get it all right..

[badriand]

Did you check the carb make sure your floaters aren't sticking dumping to much fuel? I think you can adjust the needles all the way in then it is 2 or 2 1/2 full turns out. May be way off but mine was doing somewhat the same so i took my carb off and went back through it. Also had whole leaf stuck under one of the floats. Crazy huh



edit: But your carb is new right?

[Billla]

Timing, then carb. Be sure your static timing as well as mechanical and vacuum advance are all correct. You are running 12V to the HEI and not using the wires from the points distributor, correct?

[PrerunnerRob]

Originally Posted by Billla

Timing, then carb. Be sure your static timing as well as mechanical and vacuum advance are all correct. You are running 12V to the HEI and not using the wires from the points distributor, correct?

ditto, and did you change the hei wire to a large 12g wire?

[jchav62]

Make sure the spark plug gap is .045 with that HEI also... One thing I've overlooked on the Edelbrock carbs in the vacuum port in the back of the carb. Is the port plugged/used?

[sixtyfour]

So much to look at, kinda overwhelming for a rookie like me! Tonight I took a can of brake Kleen and sprayed the manifold at 1000rpm idle and I the motor did not stumble so I think the intake is sealed good. I ordered a piston stop tool to determine true top dead center. It should be here by the weekend along with the proper plug wires.

To answer some questions, the plug on the back of the carb is installed in the vacuum port. Also the plugs are gapped to .45. Tomorrow I'll pull the plugs to see what they look like. The old motor had a HEI ignition and I'm using the same power supply, but I'll double check the voltage to the coil tomorrow as well.

It runs like I have some plug wires crossed but I've double and triple checked that and they look right according to this site. http://boxwrench.net/specs/chevy_sb.htm
I'll keep ya'll posted.

[ZBS 64]

recheck your plug wire order,I had the same problem and the #5 and #7 where crossed

[PrerunnerRob]

Oh, and I almost forgot, pull your valve covers and make sure that your rockers are moving, I just had to deal with a loose one recently, my truck was doing the same thing your was.

[greg64]

Like Rob said, make sure the wire to the HEI isn't the old resistance wire. Heck, just temporarily hook the HEI to the battery + with a jumper wire and see if it helps. The HEI will put out a real wallop compared to points, but only if fed enough juice.

For the vacuum leak check, something that works is a propane cylinder (the small ones for soldering copper water pipe). Crack the valve and wave it around likely spots. No mess either.

I really think the plugs will tell more of the story.

Float level in the carb?

[sixtyfour]

Made some progress tonight. Pulled the plugs, they all looked good except #3 which looked pretty lean. Turns out that plug was gapped to about .47, I went ahead and cleaned all the plugs and re-gapped. Also bought a plug wire tester, they were all working. The rough idle smoothed out a little and it runs so much better, no more miss.

Still more tuning to do. Still hesitates on take off and pops through the carb, tomorrow I'm gonna to adjust the acceleration pump and play with the timing some more. Will also look at some of the other things you guys are suggesting this weekend. I'll post up what I find...

[Billla]

Originally Posted by sixtyfour

Still hesitates on take off and pops through the carb, tomorrow I'm gonna to adjust the acceleration pump and play with the timing some more. Will also look at some of the other things you guys are suggesting this weekend. I'll post up what I find...

I'd strongly suggest starting with timing and then working to the carb. A bigger "shot" from the pump may cover up a bog...but in almost every case I've seen it was just covering up for either incorrect timing/advance OR vacuum secondary springs. You didn't note the specific carb number, but my 1406 required 1 step heavier vacuum secondary (pink) spring.

[nonos64]

check the dwell of the points if you have a points distributer I went through the same problem I thought it was the carb but my screw that holds the points were striped and it would do the same thing good luck

[joe231]

Originally Posted by nonos64

check the dwell of the points if you have a points distributer I went through the same problem I thought it was the carb but my screw that holds the points were striped and it would do the same thing good luck

He has HEI, no points

[sixtyfour]

On the way to work this morning the motor started missing again, so when I got home I sprayed the manifold with brake clean and got it to stumble. Seems I have a manifold leak. After looking at the Edelbrock site I've found I didn't install the manifold to a tee per their instructions. I used end gaskets, Edelbrock says not too. They say only use silicone sealant. Edelbrock also says I need to use some tacky glue between the head and the intake gaskets, I didn't. I put them on dry. Looks like I have my work cutout for me this weekend.

Question, what is the proper torque sequence for the manifold bolts? I started at the center and worked my way out in a criss cross pattern? Was this right?

[Billla]

Originally Posted by sixtyfour

I used end gaskets, Edelbrock says not too. They say only used silicone sealant. Edelbrock also says I need to use some tacky glue between the head and the intake gaskets, I didn't. I put them on dry. Looks like I have my work cutout for me this weekend.

Question, what is the proper torque sequence for the manifold bolts? I started at the center and worked my way out in a criss cross pattern? Was this right?

Yep, never - ever - use the rubber gaskets. Lay down a nice 3/8" or so bead of RTV, overlapping the intake gaskets on the heads. You don't really need to use a dressing on the intake gaskets, but I always use Gaskacinch. You know that you also need to run a thin bead of RTV around the water passages, right?

Proper sequence is in your service manual Remeber to always work up to the final torque value in at least two steps.