truck idles fine dies when put in gear

[iforsman]

I'm looking for some pointers. i recently rebuilt the engine and broke it in. Engine runs and idles great, however when I put it in gear it dies. I've tried several different things but no changes. Timing is currently at 8 BTDC (have tried from 4-12 but 8 seems to be the best), checked for vacuum leaks and found one hose that was cracked (transmission vacuum line)replaced it and no change. current intake vacuum at idle is 14/15 which seems a little low to me. Any suggestions on what else to check as i'm at a lost.

specs: SBC 400, stock heads, running a Comp cam 4X4 extreme 218/226 cam, Edelbrock 1406 carb

 

[MrHorsepowerLSx]

What is the motor RPM at hot idle?

 

[iforsman]

It’s sitting at 850.

 

[MrHorsepowerLSx]

What happens exactly when you shift? Does the motor attempt to stay running and then dies, does it sputter and die when the gas is applied? How low does the RPM drop when you initially shift from park to drive?

 

[iforsman]

When you shift the rpm drops to 650 then starts to struggle and dies. When I watch the rpm it goes from idle( 850) to 650 then bounces down to 300 back to 600 and then dies. It doesn’t backfire when this occurs just sputters and dies. I’ve tried to get on the gas as soon as the gear is engaged but still does the same thing

 

[MrHorsepowerLSx]

Do you have a mechanical fuel pump or inline electric pump? Motor could be starving from low fuel pressure when the engine is under load. You could have a restriction in the fuel line or filter. The choke could be stuck on ( it shouldn’t die tho ) or there is the possibility of your timing actually being wrong. Did the motor run well before the rebuild? Did it run after the rebuild? Is this an issue that just occurred after the rebuild. What balancer did you use on the motor? Did you verify that the timing tab lines up with the mark on the baa car when the motor is at TDC?

 

[Ricko1966]

Did you get it to idle on the mixture screws or do you have the throttle stop screw cranked in. Sounds like you need to adjust mixture and back the throttle stop screw down.

 

[Matt69olds]

Do you have the vacuum advance connected? If so, disconnect it while the engine is running, is there a huge drop in rpm? If so, you need to rework the vacuum advance. Don’t leave it disconnected, if you do you’re missing out on a huge improvement in part throttle drivability and fuel economy.

If there is a big difference in idle speed with/without vacuum advance, you either #1: need a canister that doesn’t add so much advance, or 2: you need a canister that takes move vacuum before it begins to advance timing.

What might be happening is too much vacuum advance at idle, as soon as you put a load on the engine the vacuum drops, the vaccum advance pulls a bunch of timing, the engine stumbles and dies.

Read this:

Timing & Vacuum Advance 101 -good article by GM engineer

www.460ford.com

 

[iforsman]

It is a mechanical fuel pump. prior to engine rebuild didn't have this issue. I did rebuild the carburetor while it was off. Fuel seems to flow fine through the clear filter but haven't tested the fuel pressure. Balancer is a pro sport #34266 i did set the engine at TDC and the timing marks all lined up.

with the vacuum advance hooked up timing is at 17 BTDC at idle.

 

[iforsman]

A little update. played around today with the vacuum advance and tried a new one. engine definitely idles better but still a little rough when going in gear, but at list it didn't die. The only vacuum advance I could get today was right at the peak of my intake manifold vacuum at idle. got another vacuum advance on order that is slightly below that so we will see if that fixes the problem.

Thanks for the article @Matt69olds.

 

[Matt69olds]

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If you’re patient, and don’t mind fabricating tools and parts, you can make and adjustable vacuum advance stop.

If the canister adds 20 degrees of timing, and you mechanically prevent the canister from relaxing all the way, you puke drop the amount so advance to say 15 degrees. And since the canister rod is now prevented from relaxing to its original starting position, it now takes more vacuum to overcome the spring tension.

Look up some articles on limiting vacuum advance on GM HEI.

 

[Ricko1966]

The reason I asked about your throttle stop screw and mixture screws is your vacuum readings seems a little low to me.

 

[iforsman]

I got the mixture screws set at 1 and 1/4 turn. The throttle stop screw doesn’t seem to be very far in. Maybe 4 threads out the back(going towards the throttle linkage). It is definitely less with the new vacuum advance than with the old one. I am sitting right at 15 on the vacuum gage now when in idle. When I put in gear I drop from 800rpm to 600rpm and have about 12 of on the vacuum gauge.

 

[MrHorsepowerLSx]

Have you tried adjusting your idle mixture screws with a vacuum gauge to see if you can improve your vacuum numbers

 

[iforsman]

I have. The best I could get was 15 at idle.