Motor stumbling, input please!

['70 C/10 SS500]

I got the exhaust done on my '76 500/Th-400 swap into my '70 Chevy truck this weekend. I finally got to run the motor for more than a few seconds now. The motor was pulled from a running '76 CDV, cleaned up on the OUTSIDE, and only a new non-roller steel timing set for the inside. A new water pump, oil pump, plugs, wires, cap, rotor, 425 pulleys, and re-built stock Quadrajet carb top off the other mods to the motor to get her to where she's at currently. The motor has just under 110,000 miles on her. I set the timing in neutral (no vacuum adv.) to 8 deg. at a yet unknown idle rpm. No tach in the truck and no tools/gauges to measure rpm. If the shop that re-built the carb didn't mess with the idle speed then it's stock from the running car before. In neutral, the motor revs fast and sounds strong. No hesitation or stumbling at all. With the new exhaust she sounds wicked! (Stock manifolds into 2 1/2" head pipes to dual Flowmaster Delta Flow 40 series mufflers). In gear though, I can only throttle to about 1/3 to 1/2 throttle with good manners, but anything past that and the motor stumbles and bogs and doesn't recover until I let up. The truck was originally equipped with an inline 6, 250 cid/Th-350 and HAS (currently also) 5/16" fuel line from the fuel tank. I transitioned to 3/8" just before the fuel pump inlet and use the stock pump outlet to carb hard line to supply the carb. When the shop re-built the carb, they left the jetting all stock. The vacuum advance is pulled off the carb nipple directly above the fuel inlet (the 45 deg. downward angled nipple,) and goes directly into the vac. adv. pot without any tees to anything else. The carb only has two nipples used, that -45 deg. nipple to the vac. adv., and the tranny modulator valve in the back. The power brakes is plugged, and all other nipples capped. The carb shop says that the ported vacuum is the 1/4" nipple front and center of the carb at the base. But the stock Caddy vac. adv. line came off the -45 deg. nipple, so that's what I used. The egr valve is blocked off, and the vacuum manifold under the dist. on the front of the motor has all 5? nipples capped. The passenger side exhaust manifold heat riser buttlerfly has been removed and the hole for the buttlerfly shaft is welded to (effectively be) plugged. The weather outside when I was trying to tune her was California 60+ deg. and sunny. What could the stumbling problem be from? Anybody have any ideas? I am still in the begining phases of tuning since I just started this last Sunday, but it is stumping me at the moment. Please help me get this big Caddy past part throttle!!! I hope this makes sense and will help you help me. Thanks in advance!

[rage'nrat638]

so does this new engine have HEI?????
if it does did you change the resistor wire to a regular wire feeding the 12 volts to it.....

try holding your hand over the carb with it running and see if it speed's up if so u got a vacuum leak.....

['70 C/10 SS500]

Yes, it has HEI but no, I didn't change the wire. The truck had HEI when I got it, so I figured it was changed by the P.O. Maybe not though. How do I determine what type of wire I have currently? If the wire wasn't changed, how would that effect the motor? Not supply enough voltage to the HEI and have poor high speed spark? Is that possibly why when in neutral it revs high and fast but under load it stumbles? Would changing to a capacitive discharge system like a MSD 6-Al or so help? Are those ignitions easily installed on our trucks? Need more input!

[rage'nrat638]

i think the stock wire is green.....
just run a new wire from the battery fuse box any where and see if that fixes the problem.....first....

yes if it's the stock wire it won't supply full 12 volts to the system

and yes MSD kick's butt.......

so guy's just install the MDS parts inside the stock dizzy.
i would change the complete dizzy and go with a 6AL BOX...

but fix that stumble first

[stelth2002]

Yes haveing a resistor wire would could cause that as there is a higher demand for amps when your putting your engine under load. So to test your wire and see, pull the main hot wire off your HEI, now turn your ignition switch to running and get a volt meter or multimeter of some kind. Measure the voltage. If its 12 or higher then your good. If its lower than 11 i would run new wire and if thats not it maybe your battery. it sounds like a carb problem though.

['70 C/10 SS500]

Tonight I'll check the color of the wire and put a voltmeter on it and see what I get. The battery is a brand new Optima red top that was installed Saturday afternoon. Battery was checked at the shop before I bought it and was great. I'll keep working on this, and my brakes problem and hopefully have this truck tearing up the streets soon. BTY, with even only light throttle, this 500 Caddy motor has some pretty good get up and go! Can't wait to get her running right. :

[John Fabris]

Did you put on new spark plug wires too??

I have had several cars that ran great in neutral, only to stumble in gear. When under load, the spark jumps from the wire. Changed plug wires, and all was good.

['70 C/10 SS500]

Well, it's not the wire to the distributor. I checked it with a voltmeter and it's putting out darn near what the battery is across the poles. Just for good measure though, I also tried a jumper wire directly from the positive terminal on the battery straight to the dist. and the problem persisted. So now I'm again looking at either the fuel delivery system, or the carb itself. The problem seems to come when the vehicle is in gear and I open the throttle enough that the secondaries start to open. As long as I don't open the secondaries, to motor runs fine. However, as soon as I get into it hard enough to open the secondaries, then it immediately starts to stumble. If the sec. float level is not right would that be the cause of this problem? Remember, in neutral, it will rev to the moon without any hiccups. In gear, even from a total standstill, secondary bowl (should be) full, and flooring it, it will immediately stumble, but I can drive all day without any problems at light throttle. I think I'm going to check the secondary float level and see if that may be the problem. Any ideas would still be appreciated!

P.S. Yes, it does have new wires as well as plugs, cap, & rotor.

[FRENCHBLUE72]

Sound's like the problem I am having.. I can run the motor up to when the secondarie's start to open and it then bogg's down almost to an idel... The only thing I have thought of was when the secondaries are opening there is no fuel getting squirted into the secondaries there for making for a lean condition form too much air anybody else have any suggestion's???? That's just my 2 cent's worth ...

[jhow66]

Q-Jet does not have but one fuel bowl. Your problem seems be that secondary air valves are opening up to soon. Check out the photo and adjust about 1/8 a turn tighter until the stumble goes away.. When you have it right there will be a smooth tranistion as the back to barrels open. (The rear butterflys open with the primarys-it is the air valves that controls when the rear barrels start to operate).

[jhow66]

Sorry, red text should read "turn clockwise to increase tension on air valves".