New motor has truck idling at 20 mph in gear.

[jtinpdx]

Hi All,
I've just finished with a full drivetrain overhaul on my truck including a TH350 rebuild with new torque converter and an engine swap to the 350 HO turnkey. The new motor has the Holley 600cfm carb on it. Picture attached.

I'm not sure it's set up properly yet though and would like to hear what you all think.

The motor runs great. Super strong. Starts right up. But I think it idles too high. And when it's cool, it stalls when put into gear. Also, in drive, it idles at about 20 to 25 miles an hour on a flat road. Seems excessive. Feel like I'm wearing out my brakes just driving around in the city.

It was put in by a GMC shop that seem like they know what they're doing. Service manager says that the engine is "cold-blooded". Runs cold. Takes a while to warm up. And doesn't act right until it's fully warmed up.

It's pretty frustrating though to have to let the truck run for 10 minutes before I can even get it out the driveway without stalling.

What do you think?

[69GMCLonghorn]

If adjusting the idle screw doesn't solve the issue, check for any linkage hang ups. Then I would tackle the timing. What I ended up doing on mine was capping off the vacuum advance and basically dialed in as much advance as possible without it pinging. I was able to turn the idle screw down a TON once I did that. Even with a cam, I could idle that thing at 500 RPM. I could fire it up when it was 35° outside with NO choke and it would idle within a minute. Give it a shot. If I was at a craps table in Vegas, I'd put $25 on a hard TIMING! Good luck!

[paul blair]

Electric choke adjustment?!

[Ironangel]

Originally Posted by jtinpdx

Hi All,
I've just finished with a full drivetrain overhaul on my truck including a TH350 rebuild with new torque converter and an engine swap to the 350 HO turnkey. The new motor has the Holley 600cfm carb on it. Picture attached.

I'm not sure it's set up properly yet though and would like to hear what you all think.

The motor runs great. Super strong. Starts right up. But I think it idles too high. And when it's cool, it stalls when put into gear. Also, in drive, it idles at about 20 to 25 miles an hour on a flat road. Seems excessive. Feel like I'm wearing out my brakes just driving around in the city.

It was put in by a GMC shop that seem like they know what they're doing. Service manager says that the engine is "cold-blooded". Runs cold. Takes a while to warm up. And doesn't act right until it's fully warmed up.

It's pretty frustrating though to have to let the truck run for 10 minutes before I can even get it out the driveway without stalling.

What do you think?

Double check the timing, 15 initial, 35 all in at 3000rpm. Dial your idle speed down to 700, 600 in gear. As for the warm up time, that's normal for an aluminum high rise dual plane intake. You have to wait for the crossover to warm the intake thus allowing the atomized mixture to reach the runners rather than condense on the bottom of the cold intake. If you want to just jump in and go get fuel injection, if you want to drive a classic, give the old technology time to warm up. I have practically the same motor in my daily driver and she needs time to warm up even in the summer, nature of the beast...

[71chevc10]

Originally Posted by jtinpdx

Hi All,
I've just finished with a full drivetrain overhaul on my truck including a TH350 rebuild with new torque converter and an engine swap to the 350 HO turnkey. The new motor has the Holley 600cfm carb on it. Picture attached.

I'm not sure it's set up properly yet though and would like to hear what you all think.

The motor runs great. Super strong. Starts right up. But I think it idles too high. And when it's cool, it stalls when put into gear. Also, in drive, it idles at about 20 to 25 miles an hour on a flat road. Seems excessive. Feel like I'm wearing out my brakes just driving around in the city.

It was put in by a GMC shop that seem like they know what they're doing. Service manager says that the engine is "cold-blooded". Runs cold. Takes a while to warm up. And doesn't act right until it's fully warmed up.

It's pretty frustrating though to have to let the truck run for 10 minutes before I can even get it out the driveway without stalling.

What do you think?

I had the same problem it was timing and set the idle in gear it runs great now

[jtinpdx]

Thanks all. Looks like I'll be dusting off my timing light this weekend. Appreciate the feedback!

-Jeff

[GASoline71]

Originally Posted by jtinpdx

It was put in by a GMC shop that seem like they know what they're doing. Service manager says that the engine is "cold-blooded". Runs cold. Takes a while to warm up. And doesn't act right until it's fully warmed up.

That is a total cop out by that shop... what a garbage answer.

Once the timing is nailed down, then you can adjust the idle down to where it needs to be.

Gary

[geezer#99]

And hopefully when he has an engine fire from the rubber fuel lines put in by the shop, they cover the cost of the melt down.

[my56chevytruck]

I think timing could be the answer, but Paul Blair said what i was thinking. sounds to me, if it starts right up, idles fast then after a period of time it idles down, the choke may be way off. And, I'd find another shop to go to if I were you. That's a ridiculous thing to say to a customer.

[brad_man_72]

Adjust timing, choke, and idle. Idle mph may also be a factor of gearing and torque converter stall speed.

[LockDoc]

-
I have a better idea. Take it back to the shop and tell them to fix it....

LockDoc

[jtinpdx]

Great advice here guys, except telling me I'll have an engine fire.

I've brought it back the shop once already and I think I'll keep hearing the same thing from them. So....

I'm savvy enough to do the timing myself. I'll start there aiming for 35 total timing and 10-15 initial. From there I'll tweak Idle screws while in gear and vacuum advance hose plugged down to something reasonable.

Then I'll see how she runs. If still funky, I'll play around with the choke.

Everything is brand new and seemingly functional so I'm sure it's a matter of fine tuning adjustments. I'll let you know how it goes.

Somebody mentioned torque converter. That's where my head went to, but it's a stock replacement as far as I know. I don't think I'd want to go with a higher stall converter on a fun daily driver would I? But I need to correct the statement. It idles at 25+. It accelerates uphill from 20pmh!

Thanks again! I love the site and the incredible knowledge and guidance you get here.

[Devil Pup]

Originally Posted by LockDoc

-
I have a better idea. Take it back to the shop and tell them to fix it....

LockDoc

I agree with LockDoc and the others that have said to take it back. I don't pay for people to work on my stuff normally BUT when I do I expect them to do it correctly. Even if you're capable of working on it, you paid them to do it. Sorry I cant be any help on the issue though.

[everettelane]

I cant really tell from the picture where did they hook the vacuum to the distributor if its hooked to full manifold vacuum it could be causing some of the problems your describing and poor performance.

[RUN GMC]

Just one more thing to through out there.....I had this same problem with my old S-10 it idle high and I would adjust the idle then it would be to low.....finally I noticed that gas pedal wasn't returning to zero so I put stronger return springs on it but it still stuck at times so I took pedal links and rods apart cleaned, sanded, greased, and re-assembled it and that did the trick for me........so when you crank it next time reach down and pull up on your gas pedal to see is totally return to rest or zero.......

[jtinpdx]

Hey All,
Thought I'd circle back with the results.

I took the truck back to the shop and got one of the more experienced (older) guys to take a look at it with me. He was great.

He painted the 0 and 32 degree marks on the balancer. They were nearly impossible to see before. We timed it to 32 degrees at 2000-4000 rpm. It's all in already at 2000. Left the vacuum lines plugged.

Then he adjusted the warm idle down to something reasonable on the Holley. I'm guessing it's in the 800 range now when warm. Then he adjusted the choke.

Runs like a dream now. Haven't had a chance to put my foot in it much, but it feels great. Much more well-mannered. Next job is to change out the belt-driven fan to electric. Parts on the way.

Thanks all of your for help. Much appreciated.

[67 chevelle]

32 total is where you want to be with that engine , not 35

[i82much]

If you get some light throttle hesitation, pull the carb off, square the transition slots on the primary side, and use the secondary idle speed adjustment to get your idle speed back where you want it.

[jtinpdx]

Originally Posted by i82much

If you get some light throttle hesitation, pull the carb off, square the transition slots on the primary side, and use the secondary idle speed adjustment to get your idle speed back where you want it.

I do have some throttle hesitation. Would love to learn more. What do you mean by "square the transition slots on the primary side"?

[i82much]

Originally Posted by jtinpdx

I do have some throttle hesitation. Would love to learn more. What do you mean by "square the transition slots on the primary side"?

So if you look down the throttle bore on a Holley, you see a slot. That slot is called the "transfer slot" or "transition slot." Fuel and air comes into the transfer slot through the idle circuit.

There are two ways to set the idle speed on a Holley style carb. You've got the primary idle speed screw on the driver's side that controls how far open the primary throttle blades are at idle. Then you've got a secondary idle speed screw that controls how far open the secondary screws are at idle. The secondary idle speed screw is accessible via the bottom of the carb using an allen wrench. On some carbs, you can reach it without removing the carb, on others, you can't.

Anyway, by "square the transition slots" I mean take the carb off, look at the transition slots from the bottom of the carb, and adjust the throttle blades until the only part of the transition slot you see looks like a square. Then, use your secondary idle speed screw to regain any idle speed you might lose by adjusting the primary idle speed screw.

A lot of times, people adjust their idle speed with the primary idle speed screw. This can impact part-throttle performance because it reduces the amount of transfer slot that is available for part-throttle enrichment.

Anyway - if you aren't too worried about getting it perfect and the truck runs pretty good otherwise, definitely check the accelerator pump first. If it doesn't trickle fuel at the slightest movement of the primary throttle blade, you will have part-throttle hesitation no matter what you do with the transfer slot.

[i82much]

http://www.onallcylinders.com/2012/1...ning-problems/

[jtinpdx]

Originally Posted by i82much

So if you look down the throttle bore on a Holley, you see a slot. That slot is called the "transfer slot" or "transition slot." Fuel and air comes into the transfer slot through the idle circuit.

There are two ways to set the idle speed on a Holley style carb. You've got the primary idle speed screw on the driver's side that controls how far open the primary throttle blades are at idle. Then you've got a secondary idle speed screw that controls how far open the secondary screws are at idle. The secondary idle speed screw is accessible via the bottom of the carb using an allen wrench. On some carbs, you can reach it without removing the carb, on others, you can't.

Anyway, by "square the transition slots" I mean take the carb off, look at the transition slots from the bottom of the carb, and adjust the throttle blades until the only part of the transition slot you see looks like a square. Then, use your secondary idle speed screw to regain any idle speed you might lose by adjusting the primary idle speed screw.

A lot of times, people adjust their idle speed with the primary idle speed screw. This can impact part-throttle performance because it reduces the amount of transfer slot that is available for part-throttle enrichment.

Anyway - if you aren't too worried about getting it perfect and the truck runs pretty good otherwise, definitely check the accelerator pump first. If it doesn't trickle fuel at the slightest movement of the primary throttle blade, you will have part-throttle hesitation no matter what you do with the transfer slot.

This is awesome. Thank you for the great write up. Another thing on the list of tinkering projects. I'll take a look into this and let you know how it goes. Much appreciated!

-Jeff

[i82much]

Originally Posted by jtinpdx

This is awesome. Thank you for the great write up. Another thing on the list of tinkering projects. I'll take a look into this and let you know how it goes. Much appreciated!

-Jeff

No worries, this stuff is fun!

[paul blair]

What was the main problem--carb adjustment?!