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

[clif1968]

Originally Posted by 67ChevyRedneck

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!

Just make sure that it don't ping under load. If you have to much advance it will ping usually in third gear at slower speeds with light acceleration. That's a sure way to blow a hole in a piston, or blow out a head gasket. I normally time engines by ear, advance a little at a time until I hear it ping then back it down a little. This is usually were I find the "sweet" spot in any motor.

[Jason915]

Originally Posted by clif1968

Just make sure that it don't ping under load. If you have to much advance it will ping usually in third gear at slower speeds with light acceleration. That's a sure way to blow a hole in a piston, or blow out a head gasket. I normally time engines by ear, advance a little at a time until I hear it ping then back it down a little. This is usually were I find the "sweet" spot in any motor.

I 2nd that I use the timming light just to give the engine about 10* to get it to run decent then I advance until I hear a slight ping then back it down til it's in the safe zone.

[MikeB]

Originally Posted by clif1968

A lot 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.

Next to vacuum advance in general, "ported" vacuum is probably the concept most misunderstood by hot rodders. Even some of the magazines get it wrong. (Gee, can we even imagine a car magazine getting something wrong?)

The only reason EVER to use ported vacuum is for an emissions controlled engine where the designers wanted very little, if any advance at idle. In fact, some engines built in the 70s were retarded at idle.

For a non-emissions, performance engine you want the FULL-TIME vacuum port connected to the distributor advance canister. (That's the driver side port on an Edelbrock Performer carb.) And yes, that means full vacuum advance at idle and "nearly full" vacuum advance under light-load conditions, such as cruising on a level highway. On the other hand, you should have NO mechanical advance at idle. It should start kicking in just above idle.

There's an article on the Internet by a former GM engineer who does a great job of explaining vacuum advance as well as ported vacuum vs. full-time vacuum. I will post it if I can find it.

I leave you with this: Why do you think the shop manuals always said something like, "Plug distributor vacuum advance line when setting timing." It was so you could set the initial timing w/o interference of vacuum advance.

[clif1968]

MikeB, I do agree with you, engines with radical cams can use manifold vacuum to gain additional timing advance at idle. Mild performance or stock cams do not need this.

http://home.comcast.net/~chadwick.robert/distributor.pdf#search='vacuum%20advance'

[67ChevyRedneck]

Dang it, now I'm confused. This is straight from Edelbrocks Q and A page:

Q:Which side of the carburetor do I put my dist vacuum line to?

A: Generally the distributor vacuum line goes to the timed (passenger side of carburetor) port. This is mandatory on emission controlled applications.

http://www.edelbrock.com/automotive_...f/carb_faq.pdf

Mines on the pass side with .45 NGK plugs, 8 degrees timing with no pings... I'm gonna leave her alone

[76bonanza]

I run full advance at idle and agree the reason you would want no advance at idle is for lower emissions at idle. mine runs much better and smoother with the vaccum advance hooked up to full vac.

This topic is discussed and debated often I tried it both ways and would not go back to ported or spark vac.

[MikeB]

Cliff,

Excellent article. Please note the author didn't say to use ported vacuum ONLY on race engines. He simply suggested using a modified vacuum advance canister as means to smooth out a radical idle w/o changing the initial or mechanical advance. (Normally, vacuum advance is not used at all on a race engine.)

You can get an engine to run OK using ported vacuum, but you will need more initial advance to compensate. Otherwise you will get a hesitation on light acceleration coming off idle. And compensating with more initial advance means you'll have to limit mechanical advance such that initial + mechanical = no more than 36.

If you have a Chevy V-8 with no vacuum advance at idle and it's set to 10-12 degrees inital advance, you have something unusual there if it: 1) idles smoothly, 2) doesn't heat up at idle, and 3) doesn't stumble off-idle.

Unlike some of my ramblings, this stuff isn't my opinion, it's simply fact.

[MikeB]

Originally Posted by 67ChevyRedneck

Dang it, now I'm confused. This is straight from Edelbrocks Q and A page:

Q:Which side of the carburetor do I put my dist vacuum line to?

[B]A: Generally the distributor vacuum line goes to the timed (passenger side of carburetor) port. This is mandatory on emission controlled applications.

Well, saying "generally" is just plain wrong. They should have said to use ported vacuum only if the carb you're replacing uses it. Or, more specifically, to use it only if the engine and ignition system were designed to use ported vacuum to meet emissions requirements. Guess they're trying to cover their rears with the EPA, state of California, etc.

I wonder how many "my engine stumbles off-idle" tech support calls they get because of this?

[67ChevyRedneck]

Hmmmmm, interesting info MikeB. My truck doesn't idle "prefectly smooth" at idle, but it's not what I would call rough either, this may be due to my adjustment of my idle mixture screws. I've never noticed a "stumble" on take-off, but if I try to hammer it from idle it will "hesitate," AND it ALWAYS went up to about 205 degrees after startup and stayed there for a few mins before dropping to 190 degrees. It would also climb to 200 on a hot summer day in traffic, but never overheated (I'm talking 103 degree Columbia SC summer days.) I think I'll try it on the other port (what could it hurt?) What would you recommend as a start for degrees BTDC with the full ported vacuum so I have something to shoot for? (I have a timing light and I fully understand what pinging and a bad idle sound like.) With my current set-up I was getting about 14mpg city and 16-17mpg hwy with a 700R4/3.73 rear. Thanks all, I hope daubs gets his problem fixed too

[clif1968]

Originally Posted by MikeB

Well, saying "generally" is just plain wrong. They should have said to use ported vacuum only if the carb you're replacing uses it. Or, more specifically, to use it only if the engine and ignition system were designed to use ported vacuum to meet emissions requirements. Guess they're trying to cover their rears with the EPA, state of California, etc.

Good point. On a stock engine with a stock cam using ported vacuum will not cause an engine to stumble if the timing is set correctly. I've always used ported vacuum.
Well, obviously there are different veiws on this issue. I've been turnin' wrenches on SBC engines for 20+ years, there are many ways to tighten a bolt, but the end result is always the same.

[Jason915]

I have always gone by the rule of less timming during load for less chance of detonation and that is why I use the ported vacuum thinking that when vacuum is low then timming is retarded. Interesting, now my Grandma's 79 C20 with a 350 has the vacuum advance pulled at idle and I always thought that was just a wierd emissions deal.

[Chevrolet4x4s]

Originally Posted by clif1968

Crank turns 2 revolutions to the cams 1 turn.

Thank You,Was wondering about that.
Shane

[Greubin]

I have a timing issue as well. Here's my post in the tech forum

Today, I reset the timing to 12deg BTDC and retarded the adjustable vac advance completely counter clockwise. With this setting the engine starts fine, idles smootly, but drives like a complete dog - very little acceleration. I checked total advance at 3000rpm which was 22deg. Isn't it supposed to be more like 36deg?

I tried connecting to the unported vac port on the carb. But the engine runs like it's being rev limited at idle, if I advance the vac. adv at all, the engine won't idle or start.

I ordered the HEI from Summit Racing, it comes with medium weights and adjustable vacuum advance. The best way I have the engine running is to adjust timing by vacuum gauge to highest vac ( 15"HG @ idle ) and back it off 2", this gives me approx. 16deg BTDC.

[cowboy33713]

Howdy;
when I first got to working on my truck after limping it home after buying it, I got one of those Mr Gasket advance curve kits for around $9, with the new weights and three sets of springs, for my HEI. I ended up using the lightest set of springs, initial advance of 12 degrees, and a plug gap of .060"
For whatever reason, my engine liked .060 better, and it was inexpensive to try. Went from stumbling hesitation to ,well, really decent!
Just my 0.02
cowboy33713

[Greubin]

Thanks Cowboy - I'll look into the recurve kit

[Daubs]

Yea, guy at Autozone recommended .060 gap on my plugs. I thought he was a little crazy cause he sold me some "it will do" parts in the past. I might have to open up those gaps and give her a try. They are @ .045 right now.

[Greubin]

I regapped at .060" and that made an improvement. I gave up on running the vac. advance, truck runs like crap with it connected. I guess I'll call Summit and have them send me a replacement HEI dist.

[69-350]

might as well bump this thread to the top...

I've got a question. I've been toying around with my truck lately, and since a few days ago, on my 600cfm E-Choke Edelbrock Carb, the High-Idle Cam has seemed like it's been loose and fallen back in a way that the adjusting-screw on the throttle plate there isn't contacting any of the notches on the Cam. Anyways, this isn't such a big deal now that she idles at that, but now around ~2000+/- RPM she'll pop like a Civil War battlefield. I've played with the timing a bit, but she's still doing it. I still need to get a timing light on her, but til then, anybody got suggestions? Is this timing, or am I off on that too? Could it be something in the carb, too?

[Greubin]

Originally Posted by Greubin

I regapped at .060" and that made an improvement. I gave up on running the vac. advance, truck runs like crap with it connected. I guess I'll call Summit and have them send me a replacement HEI dist.

My problem turned out to be a defective HEI distributor. Summit Racing replaced it under warranty. I installed the replacement dist. today, runs like a champ.

[Daubs]

Problem solved! Turned out to be carb problems.

When I pulled the Holley from the old engine I tried to clean her up as best I could. Engine cleaner to get all the gunk off. I probably got some grime down in the carb, etc. After initial start up and break in, we drove the truck around the block. She sputtered and backfired off idle, but seemed to run good once you built RPM's. Just never did well off idle.

I posted another question here on the board and asked if I should ditch the Holley. Some suggested going with an Edelbrock...which I did. And boy am I glad. Went with the Edelbrock 1405, 600 CFM manual choke carb.

After re-routing the fuel line, getting a spacer for the air filter and getting her all hooked up, she fired righ away. Re-set the timing and took her around the block. Wow! Amazing difference. Super strong off idle and power all the way up. Carb really woke up that motor!

I still got some tweaking to do, but I can spin the tires turning a corner in second gear (right off idle). I like.

Many thanks to all who helped me trouble shoot this new engine. Running great now. This is what I expected when I opted for the new crate over a rebuild. Thanks, thanks, thanks!!!

[71chevytruckguy]

i had the same problem with my q-jet when i put it on the my new 305 in my '69. just drove the haeck out of it from a few days and problem solved. chris.

Originally Posted by Daubs

Problem solved! Turned out to be carb problems.

When I pulled the Holley from the old engine I tried to clean her up as best I could. Engine cleaner to get all the gunk off. I probably got some grime down in the carb, etc. After initial start up and break in, we drove the truck around the block. She sputtered and backfired off idle, but seemed to run good once you built RPM's. Just never did well off idle.

I posted another question here on the board and asked if I should ditch the Holley. Some suggested going with an Edelbrock...which I did. And boy am I glad. Went with the Edelbrock 1405, 600 CFM manual choke carb.

After re-routing the fuel line, getting a spacer for the air filter and getting her all hooked up, she fired righ away. Re-set the timing and took her around the block. Wow! Amazing difference. Super strong off idle and power all the way up. Carb really woke up that motor!

I still got some tweaking to do, but I can spin the tires turning a corner in second gear (right off idle). I like.

Many thanks to all who helped me trouble shoot this new engine. Running great now. This is what I expected when I opted for the new crate over a rebuild. Thanks, thanks, thanks!!!

[Greubin]

I'm still tuning the engine, looking to get best possible MPG with reasonable performance. Getting back to the vacuum advance discussion...

My engine specs
GM GoodWrench 350 / 290HP, HEI, eddy 1406 ( primary jets 1426 / rod 1456: secondary jet 1425 ) 700R4 w4.10

I recently rejetted my 1406 carb to the aforementioned specs and achieved 15mpg, up from 12mpg thru tuning of the carb alone, vac. adv connected to the non-ported vacuum connection ( drivers side ). My initial timing was set to 12*BTDC and I experienced that off idle hesitation that MikeB mentioned ( full time vacuum ).

After reading several articles about vac. advance, most of the articles argued that an increase in MPG would be seen from running "ported vacuum" on the vac. advance. I switched the hose back to ported vacuum, MPG dropped to 11.5MPG, but the off-idle hesitation went away.
Today, I moved the initial timing up to 16*BTDC and left the vac. advance connected to ported vacuum. Truck ran great on the way into work, I'm hoping to see an MPG increase. I have yet to check full advance, perhaps tomorrow.

I'll post my MPG once I fill 'er up in a few days.