Holley 750 VS tuning

[Ziegelsteinfaust]

I got my truck running last week, and have slowly started tuning.

My old engine was a 383 LT1 with unknown smog legal cam, but with about .500 lift with 206 duration. Along with AFR 195 heads.

The newer engine is a 350 LT1 with a Comp 306 cam. Same heads, and roller rockers as above.

My old engine ran great with 72 jets in the primaries with a roughly 13.0-13.7 afr at cruise. Rather street or highway.

The new engine I am down to 66 jets, and finally settled into a 13.3-14 afr cruising the streets.

Is a 6 jet change normal for a smaller engine? I never moved that many jets before much less down on a engine before. The 383 had way more vacuum, but needed bigger jets? I find that odd. As I would of bet I needed bigger jets for less manifold vacuum before hand.

The reason I switched engines was the 383 had 10.7-1 compression with alot of dynamic compression, and didn't like 91 octane. The 350 runs great on 91, and is easier to start. Even though it is 10.5-1, butnthe 306 cam bleeds alot off at idle. But is much more cold blooded. Along with the trans wiping out the thrust bearing. I had to just go all the way.

[garyd1961]

I'm not a carb expert in in way but 66 seems mighty small to me. I have a l31 350 with stock cam and 770 holley. Im running 70 primary jets down from 72. The only gauge I used though was to put my foot in it and see which ones ran best.

[crazy longhorn]

If you are tuning a 3310 Holley, I do agree with Gary.....on most apps, i went down 2 on the numbers on the front jets. I never (on the street) messed with the metering plate in the rear, but did opt for the purple spring under the diaghargm on the secondaries. that seemed to work well 355/383, & a 455 Olds. All of my engines , had a fairly mild camshaft.....(.224/.230 @ .050) on duration, & pulled decent vacumn. Depending on the duration & overlap on your cam, you might look @ ign timing 1st....maybe she need a little more(quicker advance)? The other thing (ign 1st), would be maybe a power valve change Can you spec us the cam numbers, & also what the engine pulls vacumn @ idle? Longhorn

[Ziegelsteinfaust]

The carb and my engine is a s*** show for some reason.

Same carb on the 383 with a smog legal cam I could control afr quite well through the rpm band. The cam timing, and high static was its issue.

The fresher 350 ran great at 13.5 afr till 2750-3000 rpm then it goes lean bad. I added 4 jet sizes, and it still does it bad, but now I am fat in the 2000 rpm range. While still being 15-1 at 3000 rpm for some reason. If I stab the throttle it is fine, but part throttle crusing is no good.

I think it could be air bleeds or something. As my 750 was the cheap model I upgraded over the years. So I may of hit a wall with what it could do. 20+ years ago I had a 421 sbc with a circle track flat tappet mechanical cam "similar specs to my now 306 cam" that had similar issues, but I switched to a pro-built 700cfm double pumper. Issues went away as did mpg, but let's be honest. It was not a DD motor anyways.

Either way I will work on it more Friday when I get it back for a bad axle seal.

[68post]

https://www.smicarburetor.com/news/14/

Click on the download, "HotRod 3310", to bring up the article on tuning.

[crazy longhorn]

That was a really nice read from 68post......still I will say, that most of the carb issues, come from IGN timing.....you can go there, no argue, but IGN timing 1 st! You may be in a spot, that you need more initial timing to lite the fuel @ low rpms. As asked, shoot the timing specs.....initial/total/ vac advance. You can (chase ) that carb as. long AS you want.....but you will not tune that carb,untill you have the IGN timing right on Longhorn

[68post]

Originally Posted by crazy longhorn

That was a really nice read from 68post......still I will say, that most of the carb issues, come from IGN timing.....you can go there, no argue, but IGN timing 1 st! You may be in a spot, that you need more initial timing to lite the fuel @ low rpms. As asked, shoot the timing specs.....initial/total/ vac advance. You can (chase ) that carb as. long AS you want.....but you will not tune that carb,untill you have the IGN timing right on Longhorn

From all I've learned ..I agree with the above, ignition has to be ruled out first !

[Ziegelsteinfaust]

Originally Posted by crazy longhorn

That was a really nice read from 68post......still I will say, that most of the carb issues, come from IGN timing.....you can go there, no argue, but IGN timing 1 st! You may be in a spot, that you need more initial timing to lite the fuel @ low rpms. As asked, shoot the timing specs.....initial/total/ vac advance. You can (chase ) that carb as. long AS you want.....but you will not tune that carb,untill you have the IGN timing right on Longhorn

The ignition timing is 12* advanced, and 32* all in.

When the engine was apart I marked the timing pointer when I installed the dampener.

I can handle a little more initial timing, but the head design typically should not want more then 32*. When I get over the money I just paid. I have a dyno tuning shop a friend of mine likes alot in the OC

[garyd1961]

Originally Posted by Ziegelsteinfaust

The ignition timing is 12* advanced, and 32* all in.

When the engine was apart I marked the timing pointer when I installed the dampener.

I can handle a little more initial timing, but the head design typically should not want more then 32*. When I get over the money I just paid. I have a dyno tuning shop a friend of mine likes alot in the OC

I would still up the timing a couple degrees and see if it helps.

[Ziegelsteinfaust]

It starts to diesel when I shut it off about 1/3 the time. It rarely does it at 12*.

Plus I have run it at 15*, and could not get a meaningful change.

Right now the truck is down as I am swapping out the steering column, but I have made lots of head way when I opened up the secondary transfer slots.

I have added jet size to the primaries, and run rich at 2000 rpm. Or about 12.5 afr. Yet still a little lean at 3000rpm cruise, or 14.0 +- .3 afr.

If I floor it. The afr drops to 11.5-12 range, and I have full rpm again.

[crazy longhorn]

Ok, let us talk about vac advance? Do you have that on your dizzy? Most of my toys liked to idle ,with 22-24 degrees timing. If you say it runs best @ 32 degrees total/ 12 initial.....with the vac advance, you can add another 10 -14 degrees & get the idle timing right. I would hook it to a manifold vac source on the carb.....easy start/ nice idle, with less burn on the eyes! When you open the throttle, the vac advance drops out....back on your total that you wantIdle & cruise will love you for it! At this computer, I am just an arm chair Quarterback, & will listen to all ideas.....I just dont know it all Best of fun,Longhorn And a big HELL YES, for 68 post, for the info.....you might need to fine tune that carb, & he showed you how to do it

[Ziegelsteinfaust]

The truck is sidelined right now due to a column swap that went sideways. Plus I had to work 12 hour days at the plant so I won't be able to work on it again till Tuesday. My next day off.

I was beginning to think my vacuum advance is not right since it only advances my distributor 5-7 degrees.

Hopefully I resolve this quick I miss driving the truck alot.