New motor in, hoping for some tuning advice

[HawK5]

Well so far, the new to me “Jegs 350hp/350ci” I bought off CL for $1000 actually seems to be panning out, she’s in and fires up, cams got a little lope, and it seems to pull pretty good in the upper rpm range. There’s been a few kinks to work out, no big deal, typical stuff with a motor swap. I have gotten a few neighborhood spins in and I’m thinking I want to make sure the tuning is close. As far as specs on the motor, it’s kind of mystery, other than I verified it’s got world product torquer heads, not sure if they’re the 67 or 76 cc, .040 thick head gaskets, pistions have 4 valve reliefs, I’m told the cam is a “comp 270” specs listed below, and I have a Holley truck avenger 670. Checked the timing throughly here are the results:
Rpm-degrees of advance
Timing without vac advance
1000-10
1500-20
2000-27
2500-27
3000-32

Timing with ported vac advance
1000-10
1500-30
2000-45
2500-52
3000-55

Comp 270 cam specs:
Basic rpm range: 1800- 5800
In/ex duration at .050: 224
Advertised duration 270
In/ex valve lift .470
Lobe separation 110

The engine seller seemed to know his stuff recommended jet sizes: 73 front and 76 rear, with a 45 power valve, but admitted he wasn’t super familiar with the truck avenger 670.. I didn’t bother taking the carb apart so I have no idea what’s in it, I was too anxious to get it in and started but I assume whatever it left the factory with.. she sounds healthy and starts pulling decent in the upper rpms, but does have a little stumble off idle with a quick pedal flash, and it feels a little lack luster in the lower rpm range, but maybe that’s just the cams power range..

Any of this stand out to you guys as way off?

 

[HawK5]

Bump

 

[Bent77]

the vacuum advance should be adding timing at steady state, to lean it out at cruise.
It should not add any during acceleration or you will probably see it ping

55 total seems a bit high. TBI at lean cruise runs high 40's

 

[Bent77]

As for the cam, that is pretty mild. Your off idle stumble may be fuel delivery related

Here's a quick test. Start it and make sure it won't roll

Under the hood, no air cleaner, pull the throttle open so that you get the same stumble.
Now put your other hand over the primary side and repeat the throttle cycle

If that makes it worse, the accelerator circuit is too rich (you're cutting air with your hand)
If it makes it better, it needs more fuel sooner. Usually either a bigger accelerator pump or change the acc pump cam timing
Also is a sign of a power valve issue or mismatch. I typically run a 8.5 or 10.5 power valve




What is your idle vacuum and rpm?

 

[76zimmer]

You could try bumping the timing up about 4*, watch your total (w/o vac) and limit to 36*, listen for any pinging. Back off a bit on the vaccum advance too. Limit the amount of movement or get an adjustable vac unit.

 

[HawK5]

thanks for the direction, I’ll check some on everything you guys suggested tomorrow or Wednesday and report back..
It also occurs to me that the motor came with a billet distributor but I opted to use the one from the old engine.. I should probably swap to that one and recheck timing. The last engine died from a cracked head, it had gm light weight cast heads and if the timing was advancing too much, over time that would increase the heat right? Maybe a contributing factor..

 

[Blue85]

Is 1000 the idle speed? (because the port is still shrouded). That's pretty high for a cam like that and maybe the high idle is covering up a tuning issue. That ported advance is giving you a 20* jump in timing, which could contribute to the off-idle stumble. You could try full vacuum, but start the base timing lower to see if that helps.

I would say that if you move the throttle very slowly and there is no bog, it might be worth adjusting the accelerator pump.

 

[HawK5]

No idle speed is set around 750-800 Which also has 10* now that I think about it I’m not 100% sure if it’s ported or full, I attached a picture showing the vac port, I assumed it was ported as it appears to be above the throttle blades

 

[imiceman44]

That was my thought

the vacuum advance should be adding timing at steady state, to lean it out at cruise.
It should not add any during acceleration or you will probably see it ping

55 total seems a bit high. TBI at lean cruise runs high 40's