GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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?

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

What intake?

Re: GM Performance 350/290 motor question
 

Performer.

Re: GM Performance 350/290 motor question
 

Is it an air gap style or just a performer.

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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?

Re: GM Performance 350/290 motor question
 

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.

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

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

Re: GM Performance 350/290 motor question
 

I have more information regarding my saga.

When I had the new transmission put in last week, I noticed an exhaust leak when I got the truck back. I figured they crushed the doughnut gasket that connects the manifold to the Y-pipe. I got under the truck and pulled the old one off and I was right, the gasket was shot. I put a new gasket on and the "ticking" got noticeably better but, it didn't go away totally. So now I have an exhaust manifold gasket leaking (I hope it's that and not a cracked manifold). I just ordered a set of Earl's Pressure Master exhaust manifold gaskets (part# 29D03BERL).

I am amazed at how many issues I'm having to wade through after putting a new motor and transmission in this truck. I'm ready for this thing to start running like it should.

Once I have the exhaust leak fixed, I'll recheck the timing then the air/fuel settings, the carburetor vacuum, and then I'll go to the pump nozzle. Does this sound like the correct order of importance?

On top of all of that, I have to adjust the governor because my low RPM shifts aren't right.

Ok, I'm done whining for now.

As always, thanks for all of the help! It might not seem like it but I have learned a lot through this process and I am grateful for that.

I'll report back in a few days.

Re: GM Performance 350/290 motor question
 

I wondered what that whining noise was! Woke me and my cats up from our afternoon snooze!!
JK. LOL!!
You might think it never ends, but someday after you've adjusted the timing or tweeked the carb or changed that leaky oil pan gasket for the umpteenth time and it finally fixes it all, you'll wonder why it took so long. It's all just good experience.
It's all water under the bridge and in a few months when it warms up you'll get to tune your carb and your timing again. But you'll have a bunch more knowledge and it'll be easier. You'll do things in automatic mode and not even need to think what comes next.
THere is no secret for the combination of things to try. Just one at a time and keep notes on what does and doesn't work.
Keep pluggin' away! It's good for your brain!!
And now.
Back to your regular programming!!

Re: GM Performance 350/290 motor question
 

Yep, each project is different. Some go like warm butter, others are just buggers that never seem to quit dragging on with problems (for no good reason too!).

I found out the hard way that doing it yourself is the better way because when someone else does it, you end up redoing half their work yourself anyways. LOL. At least that's been my luck. And I've also had to redo my own work half the time too. LOL. But at least I learned from those mistakes. I wouldn't know as much as I do now without going through all of these trials. Most of the time, the next project is easier. But sometimes it isn't because new things STILL surprise me all the time. The rewarding part is finding correct solutions to new problems (rather than bailing wire and duct tape).

You are on the right track.

Re: GM Performance 350/290 motor question
 

This is a very true statement.

Re: GM Performance 350/290 motor question
 

A couple of things I noticed have not been mentioned here are float level and the fact that Crane made adjustable vacuum advance canisters. The float level will affect transition from the idle circuit to the part throttle circuit. Simply put a slight increase in float level up or down will effect the pull-over during transition. In your case I would raise the float level slightly to induce an earlier pull-over thus giving you a richer condition. So far as timing I would start at 12 degrees and fine tune,as others have mentioned, from there. I've been using adjustable vacuum cans for many years on all my SBC and BBC carb equipped engines . Crane was the only one that made them and I don't know if they stiil do. Good Luck. See Ya .Steve.

Re: GM Performance 350/290 motor question
 

I JUST found out that my distributor came with an adjustable vacuum advance. Yay, something actually worked in my favor! I'll be working on that soon too.

Re: GM Performance 350/290 motor question
 

Keep in mind there are two styles of adjustable canisters. On the MSD StreetFire (cheapo), for example, some came with adjustable vacuum advance, some didn't (and the documentation changed along the way too). Some came with stop plates instead. What brand is your distributor?

If you are lucky enough to have an adjustable canister, then you have to see what style it is. One style adjusts the "rate" of pull depending on vacuum level. The other style adjusts the "limit" of pull. If you have the former, then you may still need a stop plate.

So make sure you check yours carefully.

Re: GM Performance 350/290 motor question
 

Can you elaborate a little more on the part that I bolded?

I'm thinking I probably have the cheaper version. I'll go check to make sure.

Re: GM Performance 350/290 motor question
 

This truck is turning into the gift that keeps on giving. I just caught a glimpse of the number 7 plug boot arching to the manifold. Dang it! This stuff needs to settle down. I need to drive this truck every day.

What is a GOOD set of plug wires?

Re: GM Performance 350/290 motor question
 

I use Taylor wires on everything. The boots hold up extremely well near headers and the wires will outlast the rest of the ignition system. I use the cut to fit spiro pro 8mm wires because ignition wires hanging all over the place drives me nuts. I use looms that use the valve cover bolts and route the wires between the tubes. http://www.moroso.com/catalog/catego...?CatCode=40009 I use part number 72172 with small blocks. Taylor wires are part number 73251 for red cut to fit wires. They also come in a bunch of other colors.

Re: GM Performance 350/290 motor question
 

I have an update.

I got the size #35 and size #37 nozzles in today. My carb did come with a #31 nozzle, as I suspected. Tonight I installed the #35 and I also swapped the plug wires. As it turns out, I had 2 that were arcing. My exhaust manifold gaskets haven't come in yet so I haven't addressed that yet. I'll get to the results of tonight's work in a minute.

I've been obsessed with making this truck run better for the last few weeks. I'll go to sleep thinking about it and I'll wake up thinking about it. I suppose I probably have some form of OCD that won't let me have something that doesn't work correctly if I am able to fix it. Anyway, I was reading and thinking and I believe that some of my issues are coming from the fuel percolating in the bowls. That might explain why my truck died in the middle of stop and go traffic and it would explain why warm starts are so tough. I wrapped the fuel line coming from the pump to the carb in a foil based adhesive wrap. I also purchased a phenolic spacer that I'm going to install in the morning. Since I have a dual plane intake and I'm not going to be drag racing this truck I got the 4 hole version instead of the open version. I'm hoping that between wrapping the fuel lines and installing the spacer, I'll fix my warm start issues and dramatically reduce the chances of vapor locking this summer. I'll report my results tomorrow regarding the warm starts.

Since the transmission was installed I haven't been happy with the low RPM shifting. I have been on the phone with Hughes a few times and I've done all of the adjustments that they've recommended and it didn't help at all. The 1-2 shift wasn't happening until about 21 MPH and the 2-3 shift was coming in almost immediately after the 1-2 shift (somewhere around 25 MPH). Hughes ended up sending me a new governor and I installed that tonight. It shifts MUCH better. I think I can fine tune the shifting with the throttle valve cable now, where before it didn't affect the low RPM shifts at all. That's one headache gone!

Back to the nozzles and plug wires. With the #35 nozzle installed, the truck didn't stumble nearly as bad. It still has a slight stumble but, it is noticeably better. I think I'm going to bump it up to the #37 and see how that does. If that doesn't help it, I'll put the #55 power valve in that I got today. If the power valve doesn't help, I guess I'll learn how to re-jet a Holley 4150...lol.

I have an economy related question for the Holley experts. Given the fact that my truck is a heavy pig with terrible gears and tall(ish) tires, should I put a stiffer vacuum secondary spring in it to keep the secondaries from coming in so soon? I know percolation can can hurt gas mileage and I suspect that it is a factor in my excessively terrible MPG. I don't expect this thing to ever get good gas mileage but, it is horrible at the moment. So, do you guys think that a stiffer secondary spring will help, on top of fixing the fuel percolation will help my gas mileage?

Thanks.

Re: GM Performance 350/290 motor question
 

Progress was made today!

Instead of going up to a size #37 nozzle, I decided to leave the #35 nozzle in and switch pump cams. It had the orange cam in the number 2 position. I switched it to a blue cam in the number 2 position. The ramp rate of the blue cam is steeper than the orange cam, so the fuel squirt gets delivered faster. I also added a 1" phenolic spacer to help minimize the fuel bowl percolation. Between the spacer and the blue cam, my stumble is MUCH better. It isn't 100% gone yet but, it's 95+% gone. I think I'll need to increase the jet size for the primaries and the secondaries before it is totally gone. But, I can tolerate with the way it drives...for now.

I think my truck likes to make sure I'm never done showing it attention. After having significantly reduced the stumble, my truck decides to spring a small leak in the heater core. /sigh

So far, here is what has been done to my truck to help the 350/290 engine with Holley Street Avenger that has a off idle stumble in the order that were done in.

1. I set the initial timing to 14 degrees BTDC. Currently, I'm not running vacuum advance. I will hook it back up once I know I can control it.

2. I ran the air/fuel mixture screws in all the way and backed them out 1.5 turns to get a baseline. I ended up backing them out a total of 2.5 turns.

3. I set the curb idle screw down where it should be with the truck in gear.

4. I installed a 1" phenolic carburetor spacer to help reduce the chance of fuel percolation. I also wrapped the fuel line coming from the mechanical fuel pump with a heat reflecting foil wrap to help reduce the chance of vapor locking.

5. I changed the accelerator pump nozzle from a #31 to a #35.

6. My carburetor came with a orange pump cam that was in the number 2 position and I switched it out for a blue cam that is in the number 2 position.

These steps helped my off idle stumble greatly. While it isn't totally gone, it is much, much better.

Re: GM Performance 350/290 motor question
 

There is more tuning to be done with the idle mixture. Just randomly picking a point to turn the screws out to wont help you. You need to slowly turn the screws until you get the highest idle. Once you adjust one screw and the idle rises bring the idle back down with the idle screw. Now adjust the mixture screw on the other side for highest idle. Turn the idle back down with the idle screw. This should be done in park. Not in gear. Truck should idle at 750-800 in park. Now with idle speed set recheck and make sure timing is still at 14. Don't be scared to put another degree or two of timing in it. It might clean up your stumble completely. A lot of tuning comes with experience and knowing what to listen for etc. The only way to learn is to just try different things. There probably isn't anything you can screw up bad enough that we cant help you get it straightened back out. There is a lot of years of experience that read this board and contribute.

Re: GM Performance 350/290 motor question
 

I ran the screws out until I got the highest vacuum. I didn't do it randomly, I just forgot to put that in the list. I will try the additional timing too.

Re: GM Performance 350/290 motor question
 

OK good. It read as though you just turned them out 2.5 just trying to save you grief. Good deal then. Do you know where your total timing is set? Have you put a spring and weight kit in the distributor? You will want 36-38 degrees of total timing. Most distributors have very stiff advance springs and don't allow the mechanical advance to come in near as soon as it needs to.