GM Performance 350/290 motor question

[99-LS1-SS]

I have a 1984 K10 longbed with a slightly beefed up 700R4 and a 2400 RPM converter. My axles have the stock 3.08 gears running 33" tires. My truck originally came with a 305 and to say it was weak was putting it mildly (it wouldn't spin the tires on dirt).
I have recently installed the GM Performance 350 motor with 290 horsepower. The carb is a Holley Street Avenger 80670 (670 cfm) with vacuum secondaries. I'm running factory exhaust manifolds through the factory exhaust.

I have a couple of questions relating to the carb and the motor.

My engine question is; in the installation/setup instructions it states that the vacuum advance should NOT be used with this engine. I have copied and pasted directly from their sheet below.

"Set initial spark timing at 10º before top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide-open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve."

What are your opinions on this statement?


My carb question is related to a slight bog right off of idle that is more pronounced when the engine is cold. I've been watching a lot of Holley's videos and most of them state that you need to increase the accelerator pump nozzle size to cure this hesitation but, in the same Holley video it states that if you have a heavy vehicle with highway gearing then you need to lower the accelerator pump nozzle size. If I understand how these carbs work, when the choke is on the engine is in a richer state than when it is warmed up. If that is the case and the stumble/hesitation is worse when it is richer then it seems like I might need to decrease my accelerator pump nozzle size. I believe my carb came with #31 nozzle. If I follow Holley's 3 number jump at a time, I should go down to a #28 nozzle. Does any of this make sense or am I way off. I'm trying to learn more about carburetors and I want to learn the right way.

Do any of you guys run a 350/290 crate motor with a Holley 670? If so, did you have to alter it out of the box?

Thank you for any help.

[99-LS1-SS]

I called GM Performance and they specifically said that their performance motors were built and dyno'd without vacuum advance and were meant to run without vacuum advance. I unhooked it at lunch and it seemed to make my off idle stumble worse.
I honestly don't know if it is going too lean or too rich right off idle. I guess I'll buy a #34 nozzle and a #28 nozzle and see which one helps.

I would still like any input regarding the Holley carb.

[geezer#99]

THe no advance is a warranty problem. The bean counters are worried you'll put too much timing in it and hurt your motor and put in a claim on your warranty. This has been discussed here and several other forums.
As for your carb.
What's the outside temp where you live? Anything under 40* will cause a lean stumble.
Do you have the exhaust crossover in your intake working or have the stock air cleaner supplying heated air to your carb?

[99-LS1-SS]

It was 33 degrees this morning. I don't have the crossover or the stock breather. I have a Holley breather to because the stock breather wouldn't fit over the carb bowls.

[storm9c1]

I would CERTAINLY hook up vacuum advance, BUT it may need modified. You want to add a stop plate so that it only pulls in 10 degrees max. This is why they don't want you to use it because there is so much variation in vacuum cans and many will pull in TOO MUCH advance. Most folks don't know how or don't understand how to tweak this properly, so they just want you to ditch it. Without the vacuum advance, you will see a drop in MPG and some driveability problems, especially on the highway.

In your case, I would dial in the timing as instructed (vacuum advance disconnected). 10 degrees is good start. Make sure it is "all in" at 32 degrees at 3000 RPM. That will verify your weights are working correctly under the cap.

Then hook up the vacuum advance to a FULL manifold vacuum source with the engine idling and hit it with the timing light again and see how much it jumps up. If it jumps more than 10 degrees, you have to find a way to limit it to 10, either with a stop plate or an adjustable canister.

After you do all of this, readjust your carb idle mixture screws with a vacuum gauge. You want max vacuum reading, with the screws on the rich side of tipping the vacuum back down. Then take it for a test drive (yes, leave the vacuum advance connected to full manifold vacuum on the test drive, assuming you corrected the canister to bring in 10 degrees max).

If the stumble still happens, then I agree, next step is squirter on the carb. With temps that cold, you will have to also get the choke adjusted right, which can also be fun. Come summer, you will have to readjust it again unfortunately. Fuel injection can correct itself based on temps, but not a carb. There's always a compromise with big temperature swings with a carb, especially in a performance application that has the factory "workarounds" removed such as heat stoves, crossovers, heated intakes, mechanical chokes, etc.

My carb vehicles are set for a summer tune. I never drive them on the roads in the winter due to salt, but I do run them around my property to gain access to garages. If one sits outside below 40 degrees for more than a few hours, I have to let them warm up for 10 minutes on fast idle with choke. Otherwise they run poorly when first started. This is expected with the factory parts removed. FWIW, I run the same carb as you, and yep, it needs to warm up just the same.

[99-LS1-SS]

My truck is stumbling after I've let it warm up and it has been driven for about 10 minutes or so.

[geezer#99]

What intake?

[99-LS1-SS]

Originally Posted by geezer#99

What intake?

Performer.

[geezer#99]

Is it an air gap style or just a performer.

[99-LS1-SS]

Originally Posted by geezer#99

Is it an air gap style or just a performer.

It's the standard Performer. The model number is 2701. Here's a link to it.
http://www.edelbrock.com/automotive_...ormer-sb.shtml

[99-LS1-SS]

Originally Posted by storm9c1

I would CERTAINLY hook up vacuum advance, BUT it may need modified. You want to add a stop plate so that it only pulls in 10 degrees max. This is why they don't want you to use it because there is so much variation in vacuum cans and many will pull in TOO MUCH advance. Most folks don't know how or don't understand how to tweak this properly, so they just want you to ditch it. Without the vacuum advance, you will see a drop in MPG and some driveability problems, especially on the highway.

In your case, I would dial in the timing as instructed (vacuum advance disconnected). 10 degrees is good start. Make sure it is "all in" at 32 degrees at 3000 RPM. That will verify your weights are working correctly under the cap.

Then hook up the vacuum advance to a FULL manifold vacuum source with the engine idling and hit it with the timing light again and see how much it jumps up. If it jumps more than 10 degrees, you have to find a way to limit it to 10, either with a stop plate or an adjustable canister.

After you do all of this, readjust your carb idle mixture screws with a vacuum gauge. You want max vacuum reading, with the screws on the rich side of tipping the vacuum back down. Then take it for a test drive (yes, leave the vacuum advance connected to full manifold vacuum on the test drive, assuming you corrected the canister to bring in 10 degrees max).

If the stumble still happens, then I agree, next step is squirter on the carb. With temps that cold, you will have to also get the choke adjusted right, which can also be fun. Come summer, you will have to readjust it again unfortunately. Fuel injection can correct itself based on temps, but not a carb. There's always a compromise with big temperature swings with a carb, especially in a performance application that has the factory "workarounds" removed such as heat stoves, crossovers, heated intakes, mechanical chokes, etc.

My carb vehicles are set for a summer tune. I never drive them on the roads in the winter due to salt, but I do run them around my property to gain access to garages. If one sits outside below 40 degrees for more than a few hours, I have to let them warm up for 10 minutes on fast idle with choke. Otherwise they run poorly when first started. This is expected with the factory parts removed. FWIW, I run the same carb as you, and yep, it needs to warm up just the same.

The vacuum advance added between 12-15 degrees of timing.

[99-LS1-SS]

I ran my idle screws in all the way and then backed them out 1.5 turns and started messing with them to get as much vacuum as possible. I got about 3/4 of another turn and that was as much vacuum as I could get. The most vacuum I can get is 12". That seems low for no more than that motor has done to it. HERE IS A LINK to the spec sheet. The vacuum didn't go up any more regardless of how much I backed the idle mixture screws out. I tried to back them out another 1 full turn to see if that helped the off idle stumble and it didn't. It almost felt like it was worse.

[cdowns]

a 3.08 and 33" tires are gonna make that dog sluggish all around

[99-LS1-SS]

Originally Posted by cdowns

a 3.08 and 33" tires are gonna make that dog sluggish all around

I agree about the sluggishness. I'm more concerned with the shuttering/bogging right off of idle.

[geezer#99]

since you added vac advance and it got better, maybe you need more initial.
PLug the vac advance and bump your timing up to 16 and see if that changes the bogging.

[99-LS1-SS]

Originally Posted by geezer#99

since you added vac advance and it got better, maybe you need more initial.
PLug the vac advance and bump your timing up to 16 and see if that changes the bogging.

I will try what you suggested when I get home tonight. I left my timing light in the garage or I would do it at lunch.

After I posted last night, I went back to the air/fuel screws and re-adjusted them. I turned them in and then backed them out 2 full turns. That's where it pulled the most vacuum (12") and no matter how far I backed it out it didn't pull more vacuum and the stumble didn't get any better so, I feel like that is probably a good baseline the those screws.

IF the timing adjustment doesn't help, I'm going to start looking at a larger accelerator pump nozzles. If I read the description of my carb correctly, I have a size 31 nozzle. They recommend jumping up of 3 sizes but, they don't make a size 34. They make a size 32 and then the next size up is a 35. I think the 35 will be a descent jump. If it helps any I will know if that is the issue and if it gets worse I'll know that I need to go down to a size 28.

I'm also wondering if my accelerator pump cam is the correct cam for my application (heavy truck with terrible highway gears). My carb came with the orange cam and it's screwed into the number 2 slot. I could screw it into the number 1 slot and that should make the fuel shot come in sooner but, I don't think that is the issue. I can look down into my carb and if I move the throttle at all, gas squirts out of the nozzle. Do you have any opinions on my current cam or a better cam choice?

Thank you very much for the help that you've provided so far. I sincerely appreciate it. I want to learn how to manage this carb and get it running correctly.

[geezer#99]

From the spec sheet and your adjustments you shouldn't expect more than 12" of vacuum. While your cam specs aren't that aggressive, with the low vac reading it tells me you need way more timing.
Whenever I see someone trying to eliminate a stumble I always look to the distributor.
Actually the rule of thumb has always been 'Timing first, carb second'.
Your motor might like up to 20 degrees initial. Don't be afraid to try that. Don't worry about your total timing yet. If more initial cures your stumble you can modify the mechanical timing later.
Leave the carb alone for now.

[99-LS1-SS]

I thought I had the timing taken care of when I set it to 10 degree BTDC. I will go up to 15 degrees and drive it. If that helps I'll start stepping up 2 degrees.

[99-LS1-SS]

I upped the timing some and it didn't touch the off idle stumble. It did seem to help the overall power slightly.

I'm thinking I need to start looking into larger a pump nozzle. I think I will swap the cam to the number 1 hole to try to get the squirt of gas in just a bit sooner.

[cableguy0]

Off idle stumble is almost always a lean condition. I would bump up the accel pump nozzle.

[storm9c1]

Yep, sounds like you need to mess with the carb next.

When you fix the carb and then are ready to go back to timing, if you need to run more than 12 degrees initial, then it will become critical to be careful with vacuum advance. If the "can" is adding 15 degrees more at idle (which is odd that you gave us a range and not a fixed value) with the vac advance hooked up, that's going to be too much -- you need to find a way to hold that back to 10 degrees, either with a stop plate or an adjustable canister. We can help you with the details later when needed. For now, get it running right first.

[99-LS1-SS]

When I adjusted the timing earlier I went up to 16 degrees BTDC. I didn't mind the HP boost but the idle seemed like it would hang when I would push the gas and let off. It wasn't screaming but, it wouldn't settle back down unless I blipped the throttle pretty quick. I dropped the timing down to 14 degrees and that seemed to address the idle hanging.

The reason I gave the vacuum advance range is because the lines on my balancer aren't real easy to read. My timing pointer covers the numbers on the balancer. I know where 0 is and my pointer goes to -14 degrees. Anything beyond that is hard for me to see. I can't get a good angle on it. I need to climb under the truck and mark the various increments with different colored Sharpies. That would tell me exactly where it is with the advance plugged up. But, for now, I know it is 12 degrees or more of timing added when the vacuum advance is hooked up. Until I can get a stop plate for my advance I'm going to leave it unplugged.

I'm going to go ahead and order the larger nozzle and I'll probably order a cam kit too. Do you think it's worth ordering jets?

[geezer#99]

I just had another thought! No, it didn't hurt much! LOL!!
Power valve.
Do you know what the power valve is rated at?
With low vacuum at your tip in point you could be lean due to lack of fuel added by the power valve.

[99-LS1-SS]

It's should have a 65 power valve in it.

[99-LS1-SS]

According to Holley's video, I probably need a 55 power valve.
Here's the video.