New engine problem.....just wont idle....dang!

[maydayneal]

Me and this chevy 350 are starting to get real close.....LOL!

Engine specs: 350 +.040, Brodix Trk-1x, Holley 4150 750CFM. Not sure of cam specs, but valve train is all roller. Timing is currenly at 34 advanced.

I've been doing some upgrades to the ignition system just because I dont know how old the stuff was when I got it home last week. I currently have an MSD 6AL with a Mallory Promaster coil. I've installed....new plugs (NGK), wires (msd 8.5mm), new dist cap & rotor (MSD).

After starting I hold the throttle at about 2000 rpm for a bit and bring up the temp. The throttle from there is very crisp and she ramps up very quickly. But if I let the idle drop below about 1800, the engine drops off quickly and dies. There's no smoke and the engine otherwise seems to run and accelerate very well.

So it seems that I might be having a vacum issue at idle....and as I understand it...vacum pressure is at its greatest at idle and necessary to draw the fuel/air down the intake.

I plan to get a vac tester tomorrow and pull the plug off of the vacum port at the rear of the carb and get a reading. I guess I will keep the idle at about 2000 to keep the engine running, hook up the vac gage and begin to slowly adjust the idle down until she begins to drop off to see what the vacum gage says.

Does this sound like a good place to start?....and what should I expect to see (normally)??

Anybody have this problem and can give insite to what the issue turned out to be??

Thanks!!

[TubbedTruck67]

Is the timing locked out? Or do you still have weights and springs in the distributer? It would be you have to light of spring or the wrong advance bushing in the distributer if you have a aftermarket one. A stock one wont have an advance bushing but will have weights and springs.

[maydayneal]

Is the timing locked out? Or do you still have weights and springs in the distributer? It would be you have to light of spring or the wrong advance bushing in the distributer if you have a aftermarket one. A stock one wont have an advance bushing but will have weights and springs.

I have the MSD Pro Billet Dist: http://www.msdignition.com/product.aspx ... erms=85551

I'm certainly no expert on engine ignition.....but the product discription indicates that you can lock it out or have the manual adjustments. I need to read more in the instructions to understand. But a great point to check because I know that the engine came out of another car and they were using a crank trigger system. So I need to learn how you lock out the timing and check that to see if it is locked or not.

At the track a last week....we used a timing light and moved the dist to advance the timing from 28 to 34 degrees.....not sure how that figures into the equation....but wanted to let you know that we adjusted the timing using the old fashion way.

One last observation....I removed the air breather to see the top of the carb....and when I turn off the motor (or dies) there is a lot of fuel vapor (like a fog) hanging over the bowl of the carb.

Thanks for the idea.

Update: Just pulled the dist cap and rotor...definitely locked out....no weights or springs....which is what I expected since it was a crank trigger system previous.

[coony278]

You might have a float sticking in the carb pull the brass screw on the side of the bowl and turn the fuel pump on and see if fuel comes out. If it does fix sticking needle and seat, and readjust the float level.

[maydayneal]

Update:

Had a good friend over (mechanic)....and we adjusted timing a bit...and he worked on the air/fuel mixture on the carb. Idleing much better although still seems to be running a bit rich....meaning when you "crack" the throttle she bogs just a split second.....like the engine is loaded up a bit on fuel.

I am beginning to suspect the power valve in the carb....it maybe leaking some and adding more fuel in during idle.

Next steps:

Going to purchase a temp gun to check the exaust temps on each cylinder just to see if we have anything funny going on. Those guns have become really resonable in price and a quick way to check individule cylinder issues.

My buddy is going to make some calls today and see if we can get our hands on another carb just to throw on to verify we have a carb issue......it may be time to do a rebuild kit.

Track is open tonight...so may take it up to test under load.

Still digging.....but making progress.

[TubbedTruck67]

Ya definently no springs with a crank trigger didnt know you were running a crank trigger. The vapor out of the carb usually means the throttle blades are open past the transfer slot in the baseplate.

[jmarkaudio]

T-slots are exactly what you need to check. Pull the carb and turn it over, there should be only about .020 of the transition slots showing, or should look square. If the idle speed screws are turned up too much it puts you in the transition circuit and will be too rich to idle right. And idle mixture screws will no longer seem to make a difference. The fix would be to put some bleed holes in the throttle plates to allow air in and allow you to turn the idle speed screws back out and cover the t-slots back square. Start with 3/32" in the primary and sneak up to what it needs, adding a pair in the secondary if needed. If they are not far off you can try adjusting the secondary blades open slightly before you start drilling, sometimes a little is all you need depending on how much cam you have. Also remember on a race engine you don't have a PCV valve, a controlled vacuum leak that needs to be compensated for. Newer Demon carbs have an Idle-Eze circuit that does essentially the same thing. The picture below shows the proper transition slot amount exposed above right of the X. X marks a good spot for the bleed hole as needed, just upstream of the idle port. As always, clean out the metal shavings.....

[maydayneal]

That was my problem....with the difficulty in adjusting the secondary idle screw I was opening the primary too far and getting into the transfer slots and thus bypassing the idle circuit all together. I ordered the secondary idle screw attachment for the 4150 and adjusted the secondary butterflies just a tick and was able to back off the primary and things got much, much better....I was able then to adjust the idle screws on all four corners and she's idleing fine now.

Whew.....what an education working thru all of this.

But thanks to all for the info and tips.