New crate 350, backfires...stumbles...

[Daubs]

The crate 350 is in and running...but not very well. Here's link to complete details:

http://67-72chevytrucks.com/vboard/s...d.php?t=198802

We set timing to 4 BTDC and she runs, but idles rough. When you drive her she stumbles low in the RPM range, and even backfires at times. But once you get her reved up a little it screams!

It's got Holley 1850-2 600 CFM carb that came off old engine. Didn't do much to carb besides de-grease. HEI from old truck. New cap / rotor / plugs and wires.

Suggestions? Could the harmonic balancer have slipped and not be giving me true timing mark? Should I be timing differently with the HEI dist.?

[jamesi20]

If you put the key in the balancer it probably is ok

[76bonanza]

I would advance it more untill the idle smoothes and see how it runs. 4 seems really low to me but not sure. I think mine is around 12 but mine is not stock.

[Hart_Rod]

Sounds like timing to me. Did you replace the distributor? Some time the weights and springs get worn. Maybe check all of your vacuum lines.

[Daubs]

HEI dist is off the old engine. I'll check vac lines. Other thoughts?

[smbrouss70]

Did you run 4 degrees of advance on your old engine with the HEI? I have the same engine in my truck with an HEI in it and the timing is set at 14 IIRC.

[Daubs]

Originally Posted by smbrouss70

Did you run 4 degrees of advance on your old engine with the HEI? I have the same engine in my truck with an HEI in it and the timing is set at 14 IIRC.

Not sure where old truck was set timing wise. Just purchased timing light for the new crate install.

14 degrees behind top dead center? (IIRC = if i recall correctly?)

[smbrouss70]

14 BEFORE top dead center and yes you got IIRC correct. When I put my engine in is when I switched to HEI, I did alot of internet research and found that HEI's like more advance than the old points type distributors, most guys were running betwwen 12 and 16. I finally found my sweet spot at 14 (after 2 weeks of messing with it.)

[byrd]

I agree with smbrouss7, the PO of my truck had already installed a 290 hp crate 350 with HEI and it was set around 12-14 btdc, well after replacenig the harmonic balancer I found that out. I have been running mine around 12-14 depending on the weather for 2 yrs and it runs great, has plenty of power and with an edlebrock 1406 it's pretty good on fuel 14mpg for my 69 and has 3:73 non posi rear.

[Daubs]

12-14 degrees before top dead center sounds like a lot. Wouldn't retarding the timing hurt low end, but help high end RPM?

This is manual tranny truck (4spd). It backfires under load at low RPM's, just off idle. Seems to love the higher end of the RPM scale at this point in time.

What plugs you running? I think the autozone guy sold me R44TX plugs.

(I'm setting the timing with the vac advance hose disconnected from dist)

[clay68c10]

I've always started with 10 deg and adjusted from there, 14 isn't too much unless you're pinging. Do you have vacuum advance? If so, did you pull and block the line to the dist when you timed it? I usually set advance w/o vacuum. Then I connect the vac line to the ported vac fitting (no vac at idle, vac when the throttle opens up).

[smbrouss70]

Going from 4 to 14 degrees before top dead center is advancing the timing. I don't remember exactly which plugs I'm running, I know they are splitfires. Don't be afraid to play with your timing. Just make small adjustments of about 2 degrees each time, then go for a drive and see how it responds. Your engine will let you know when you are getting too far out of whack (backfiring, pinging, etc.) Also just because we both have the same engine part number doesn't mean EVERYTHING is exactly the same in our engines, not to mention we live in different climates, so our timing settings could be different. But, I do think that you're going to end up with at least 12 degrees of advance.

[smbrouss70]

I just thought of something... did you re-use your harmonic balancer by any chance? If so, the outer ring could have slipped causing your readings to be off. Based on the time and effort you have put in to your engine I'm doubting that that's the case but I just have to check.

[Daubs]

Yep, using the old harmonic balancer.

Okay, I'm confused...Time for "stupid engine question #1":

Timing set at "0" top dead center mean's it's getting spark as the piston is at highest in the cylinder. "4 degrees Before top dead center" means sparking before the piston is at top dead center, correct? The spark is actually firing before the piston has reached the top-dead-center, or still moving up in cylinder(?).

[smbrouss70]

correct, it's only a stupid question if you don't ask

[clif1968]

Originally Posted by Daubs

The spark is actually firing before the piston has reached the top-dead-center, or still moving up in cylinder(?).

The air/fuel has to start burning before the piston get to TDC so that the flame travel reaches the piston top when it gets to TDC. If you advance the timing to much the flame reaches the piston before it gets to TDC causing the piston to rattle in the cylinder, aka "ping". If the flame does not reach the piston until after TDC you get a very big power loss, rough idle, and it will sometimes spit and cough until you get the RPMs up enough for the vaccum/mechanical advance to get the timing up.

[68Stepbed]

My engine runs best with 10 deg. BTDC, ACDelco R45TS plugs with .045 gap, with HEI.

[meathead95]

Originally Posted by 68Stepbed

My engine runs best with 10 deg. BTDC, ACDelco R45TS plugs with .045 gap, with HEI.

mine too!

[Daubs]

Originally Posted by clif1968

The air/fuel has to start burning before the piston get to TDC so that the flame travel reaches the piston top when it gets to TDC. If you advance the timing to much the flame reaches the piston before it gets to TDC causing the piston to rattle in the cylinder, aka "ping". If the flame does not reach the piston until after TDC you get a very big power loss, rough idle, and it will sometimes spit and cough until you get the RPMs up enough for the vaccum/mechanical advance to get the timing up.

Thank you. That completely makes sense when you explain it that way.

And thanks 68Stepbed and meathead95 for plug recommendataion.

[67ChevyRedneck]

A question that I have that may help Daubs as well is this: When you set the timing, you should unhook the vacuum advance to the dizzy, right? Once it is unhooked, does the line back to the carb need to be plugged off or left open? So for those who set their timing at 10 or 12 or whatever, that's with the vac. advance unplugged?

[clif1968]

Alot of people unhook it and plug it off to set the timing, but if it is connected to the correct vaccum port there should be no vaccum at idle anyway and you could leave it hooked up. If it is hooked straight to manifold vaccum you would have full vaccum advance at idle and it would be useless.

[Chevrolet4x4s]

Stupid question#2 how many times does ther crank turn for one turn of the cam ......4?????
Shane

[MikeB]

Originally Posted by Daubs

...she runs, but idles rough. When you drive her she stumbles low in the RPM range, and even backfires at times. But once you get her reved up a little it screams!

Classic symptoms of a vacuum leak. As you open the throttle more and more, the leak becomes less significant. I had the same symptoms with a new Edelbrock carb mounted on one of their 5/16" heat-insulating fiber gaskets. A little more torque on the carb bolts solved the problem.

Another symptom of a vacuum leak is the idle speed keeps increasing when you turn the idle screws out way past 2 turns. Lets just hope it isn't a leak under the intake manifold. I saw that happen once, but only because someone RE-USED the intake gaskets!

[clif1968]

Originally Posted by Chevrolet4x4s

Stupid question#2 how many times does ther crank turn for one turn of the cam ......4?????
Shane

Crank turns 2 revolutions to the cams 1 turn.

[67ChevyRedneck]

Originally Posted by clif1968

Alot of people unhook it and plug it off to set the timing, but if it is connected to the correct vaccum port there should be no vaccum at idle anyway and you could leave it hooked up. If it is hooked straight to manifold vaccum you would have full vaccum advance at idle and it would be useless.

Thanks! I put a new edelbrock on a crate motor two years ago and made sure I hooked the vac adv. to the port that has no vac at idle, and left it hooked when I timed it to 8 degrees BTDC. I thought 8 was the max your supposed to go on a stock motor, but I might bump it up to 10 to see what it does! It runs great now, maybe I'll be able to free up some horsies!