I have a new 406 engine that I am having problems with.   It starts easily and idles fine but won't take any throttle input while driving - it stumbles severely, to the point of stalling.  The engine is an almost exact copy of the Impersonator 406 from the article.  It has an Edelbrock RPM intake (not Air-Gap), a Holley 4010 750 cfm carb that I just rebuilt (twice), new fuel pump, new Mallory HEI distributor.   I can blip the throttle in neutral and it seems fine.  I can rev the engine to 2000+ rpm in neutral.  While driving, almost any throttle input causes a major stumble, sometimes with a backfire in the exhaust.  Closing the throttle allows the engine to keep running.  Given enough time, road and patience I can gradually accelerate.

The carb is jetted just as it came originally (67 primary, 75 secondary - which is also how the carb on the Impersonator 406 was jetted with the Vortec heads) with 6.5 power valves front and rear and a 30 cc pump on the primary side.  The accelerator pump is working - I've visually confirmed fuel discharging from the shooters.  I've got idle mixture screws 1½ turns out which provides best idle vacuum.  The engine is drawing right about 14" Hg at idle.

I tried swapping on a friends 850, mech. secondaries, Holley carb. to see if the problem would go away.  I changed the jetting to 67, 75 but it has a 3.5 powervalve up front.  Idle was relatively poor (smells rich) with the mixture screws 1 turn out.  The engine seems to take throttle a little better but not much - the problem is still there.

So now I'm thinking it's something else.  I've got initial timing set at 12 degrees.  The Mallory dist. is supposed to come preset with 24 degrees mech. advance that's all in by 3200 rpm.  I don't have a tachometer that reads above 2000 rpm but I have tried to check total timing and advance seems to peak at 35-36 degrees, which should be right.

Does anyone have any suggestions for me?  I really want to get the truck back on the road.  I've been riding a motorcycle to work everyday and the mornings are right at 32 degress and I'm freezing my butt off.

OM Gang

Backfire on accelleration is classicly associated with with a couple issues.

First is a lean mixture, however this causes backfires through the carb. Exposions in the exhaust are typical to rich mixtures and or late timing.

Second is ignition timing. Too much advance either in amount or rate causes severe hesitation.

Apparently where you live the weather is rather cold, the problem this leads into is that the Vortec heads have no exhaust heat to the intake manifold. At low atmospheric temps, the engine cannot vaporize enough fuel to run on. The problem gets worse when the throttle is opened as vacuum is reduced, vacuum helps vaporize fuel, so at idle the engine is just fine, but open the throttle and the vacuum drops, the manifold chills with lots of outside cold air and the fuel stops vaporizing. Now a strange thing happens. The mixture falls lean so the engine reacts like it's lean (hesitation, backfire, etc.) but the liquid fuel running through the engine unburnt makes the exhaust smell, and look rich. You get a lot of unused oxygen passing through with a lot of unburnt fuel which will sometimes ignite in the exhaust system. So you get a mix of indicators that tell you the engine is lean and rich all at the same time.

This is not to say you've got other porblems, but where the weather is chilly and you have no inlet heat you've got to go fix that problem. There's a couple ways from where you are: The Performer RPM should have a water bypass in the center where non Vortec designs have the exhaust cross over, you need to plumb this if you haven't so coolant will flow through the manifold. This will help when the engine is warm, you still have cold start and pre warm up to deal with. This leads to Two, a hot air inlet. This is simply solved with a factory air cleaner that has a thermostatic valve which when cold opens to draw intake air from a sleeve around the exhaust. The factory system also has a cold air inlet that should exit the radiator frame to pick up cold air from outside the engine compartment. When the engine reaches operating temperature the inlet valve changes position closing off the heated air and pulling cold in or at some in between temperature mixes both. You can always rig a cable or solenoid to give you on/off control of the valve. This also eliminates carb icing if you live where it's both cold and humid.

There's other possibilities but best not to get too much going at once. One additional comment regarding ignition timing. The term "supposed to......." really isn't good enough, you should get the distributor on a Sun machine to really trim it for the proper curve to RPM rate and full amount.

Bogie

'Bogie, you have been a great help throughout this build-up.  The temps have been in the mid 30's in the evenings, I am running an open element air cleaner and I DON'T HAVE THE HOOD ON.  Did I mention I live in western Oregon where it is damp 366 days a year?

I imagine that simply installing the hood will improve, somewhat, the problem I've been having.

What is the best method to plumb the coolant bypass in the intake manifold?  Run the heater hose (that is now connected to the intake manifold) to one side and connect the other side back to the intake?

Thanks,

OM Gang

Your Vortec Performer RPM intake should have 4 tapped bypass ports. Two are in the coolant return at the front and two are at the base of the plenum. The latter pair are intended to supply hot coolant under the plenum to vaporize fuel.

A little theory; vaporized fuel burns, liquid fuel does not. Lots of writings extol the virtues of reducing intake charge temp as a way of increasing mixture density and thus power. All of this is correct except for fact that within about 35-45 milliseconds the fuel has to burn, the stuff that enters as blobs doesn’t. Putting heat in the plenum base is a long accepted practice to force vaporization of fuel that tends to collect there. This improves combustion, power and efficiency but you end up walking a compromise of where’s the optimum power between a cold charge and a well mixed and vaporized charge. Obviously too much heat or too much cold reduces power for different reasons. This is less of a problem for a competition engine because the high RPMs will force a homogeneous mixture by the extreme turbulence. That doesn’t happen at commuter RPMs, so you need some heat to do the job.

To plumb this, install a fitting into both left side coolant ports and connect them with a length of ½ inch ID hose (some use a 5/8s so check out what your heater uses). At port below the plenum on the right side install another ½ inch fitting and run a hose to the heater inlet side. From the heater outlet side run a return hose 5/8s or ¾ inch return (again depends on model which size you’ll find) from the heater to either the heater return nipple on the water pump or to the pump inlet side heater fitting on the radiator. Different years and different models have unique configurations. Some may also return to a fitting in the pump inlet hose. A plug is placed in the unused left side manifold coolant port.

This can be designed to right side if you prefer, a lot will depend on space and clearance issues. The important thing is to make sure coolant has a flow path under the plenum.

Models with air-conditioning may include a heater shutoff or proportioning valve. In this case it may be necessary to install Tee fittings into the heater hoses to provide the plenum plumbing with a shunt around the control valve, other wise there will be no flow unless the heater is on.

You can also plumb this so there is a shut off valve to the manifold such that in the summer or at the track, if manifold heat is unneeded or unwanted it can be turned off.

I suppose I should provide an opinion about open element air cleaners. At the risk of upsetting a lot of people, they’re a waste of good money and reduce performance. In spite of the need to put some heat into a street driven manifold to aid fuel vaporization, it’s still a good idea to source inlet air from somewhere beside the hot under hood environment. To do this doesn’t take any rocket science nor huge expense. Any factory closed air cleaner housing can be used. A simple duct needs to connect the inlet snorkel to a location behind the grill where outside cool air can be brought in. You gain roughly 1% power for every 10 degree reduction in air temp at the carb inlet. Typical under hood on top the engine temps run from 160 to a couple hundred degrees F. If the outside air temp is 80 degrees, you pick up an 8% power increase for free. You can spring for the rather expensive cold air kits or just fab up something using bits and pieces of 4 inch duct work and 4 inch flexible plastic or aluminum dryer vent tube from the hardware store. Then put your cash into a K&N replacement, cleanable air filter. You can blow their doors off and laugh all the way to the bank.

Bogie

Is there benefit to using either ribbed tubing or smooth tubing?

Brian

'Bogie, thanks again for the response.  I purchased ½" pipe fittings and attempted to route the coolant as you described.  I have the passenger side connected to the heater core inlet with a ½" pipe to 5/8" hose fitting and 5/8" hose.  I tried to install a ½" 90° pipe adapter to the tapped hole adjacent to the plenum on the drivers side but there is not enough clearance to turn it (the hose connector hit's the plenum).  I tried a ½" pipe to 5/8" hose straight fitting (like on the passenger side) but the 5/8" hose then interferes with the throttle linkage.  I am considering trying AN fittings - installing ½" pipe to -10 (-12?) AN adapters in the threaded holes and then using 90° hose ends to connect.  The local speed shops don't carry anything in stock larger than -8 AN, which I believe is ~3/8".  I think -10 AN is ~9/16" and -12 is ~11/16".  How important is it that I maintain a 5/8" connection to the plenum?  Will dropping down to a -10 AN increase pressure in the cooling system?  Or will it just reduce flow to the heater core somewhat?  Is there another/better method to make the connection that I am missing?

My fustration level is high - I have put less than 10 miles on my brand-new Impersonator 406 engine in the two weeks since I've installed it.  My wife think's I'm an idiot (but doesn't say it out loud).  I rode my motorcycle to work today at 30°.  It was 34° when I rode home.  I'm still cold.

OM Gang

I haven't come up with a satisfactory way to route water in these manifolds either. By the time you get something that works, it's ugly and unprofessional looking at best.

On the drivers side, clearance to the throttle linkage drove me to putting a spacer under the carb to jack it up high enough to clear the linkage. But this was on a pick up so vertical clearance wasn't the problem that this would be on a pasenger car or 'vette.

The 5/8s hoses are just to be compatible with the heater hose sizes. There's no reason whay you couldn't back these down to 1/2 or maybe even 3/8s.

AN would offer some adjustability as they come in things other than straight or 90s. Same goes for SAE fittings, most similar to AN but use a 45 degree flare rather than 37. The difference between 37 for airplanes and 45 for everything else was to give a mechanic an immediate feel for what is tested and qualified to fly and what isn't. This had nothing to do with one angle seals better than another. Therefore, if you live reasonably close to heavy equipment shop or something similar that sells SAE fittings, like some NAPA stores, "use 'em if they got 'em".

I forgot to mention that the fittings should really be either aluminum or stainless steel otherwise you get a voltage between the dissimilar metals in the presence of water which corrodes the aluminum manifold casting. 

I think that a hot air stove as previously described would be a quick fix. You can use almost all junk yard parts. Perhaps not as cool looking as your open element filter, but an effective way of getting some warm air into the carb during the winter.

I hear ya about the cold on a bike. I rode from Seattle to Portland and back in January a couple years ago to visit my youngest daughter. Riding during a sunny but cold day wasn't too bad, but coming home with temps in the twentys or teens was killer. I got as far as Centralia when I ran into freezing fog. I pressed through that to the south end of Olympia and got a motel room where I stood in the shower and didn't get warm enough to even shiver for 5 to 10 minutes, then I shook so bad I fell down in their tub.

My wife thinks I'm crazy, and doesn't keep it too herself.

Bogie

'Bogie, thanks again for the reply.  I've purchased some fittings that I think will work.  Ideally I'd like to keep from using a carb spacer although I have plenty of hood clearance since it's a '76 Blazer.  I'll post back with some pictures when I get it done - should be early next week as I will be out of town this weekend.

Are you in the Portland area?

OM Gang

I live in Seattle. I have a daughter and her family that lives on the south west side of Portland and an ex-wife in Dayton.

Bogie

I've finally got the truck running.  The lack of heat under the plenum was not the main problem I was suffering.

I installed an Aeroquip ½" male pipe to -10 male AN 45° fitting into the manifold on the drivers side.  This fitting will rotate fully without contacting the plenum so it can be tightened down completely (the 90° fittings I tried previously wouldn't).  To that I connected an Aeroquip -10 female AN to -10 hose barb 45° adapter (for use with AQP Socketless hose - no clamps needed).  This setup give's the 90° bend we're looking for under the throttle linkage.  This setup cleared the throttle linkage on my Holley 4010 750 with a 3/8" thick carb. gasket - no spacer required.  The setup clears the throttle linkage on an Edelbrock 750 with a standard gasket and no spacer (just barely).  I tried to install an identical setup to the coolant port on the drivers side front.  The two adapters end up being less than 2 inches apart and approx. ¾" different in height.  I couldn't get a satisfactory hose connection without a crimp so I ended up using the passenger side coolant port with a ½" pipe male to 5/8" hose male 90° fitting.  This setup works but is not as clean looking as it would be connected to the drivers side.  I haven't tried it yet, but I think a ½" male pipe to -10 male AN 90° on the drivers side with a -10 female AN to -10 hose barb straight adapter will work.  I think the height of the two setups will be much closer and the distance between adapters closer to 3 " making a hose connection much easier.

Two of the 45° fittings and two of the 45° adapters plus 2 ft. of AQP hose cost me $90 so this isn't a low cost solution but the result is clean and leak free.

Anyway, after I had all the above installed the engine still refused to take throttle.  I began trouble-shooting the distributor (a new Mallory HEI unit) by recording the advance in 100 rpm increments all the way to 3600 rpm.  I found the distributor only had 18° of mech. advance requiring 18° initial to get the total I wanted.  Still, the curve was relatively consistent and I determined wasn't the cause of my problems (I am exchanging this one for a good one though).

I had rebuilt my Holley 4010 with a genuine Holley rebuild kit and had already gone through it once again to confirm I had re-assembled everything correctly but everything was sure pointing at the carb. as the problem.  The engine wouldn't take small throttle openings, particularly under load.  I had discovered if I tapped the throttel a few times and put my foot into it (either getting into the power valve or the secondaries) the engine would accelerate pretty smoothly.  The engine wouldn't take cruise or any kind of small throttle - It either was idling or going like a bat out of hell.  At wits end, I finally broke down and purchased a new Edelbrock 750 vac secondaries carb - keeping my fingers crossed that I didn't just blow $300 on something I didn't need.  The engine now runs like a top.  I have no idea what is wrong with my 4010; it's as if there is not transition or cruise circuit, only idle and power.

Regardless, my Blazer is now driving again.  I have some tuning to do once I get a functioning distributor installed but it runs relatively well right now.  The extra cubes mellowed out the 280° cam and the truck sounds only slightly lopey at idle (nice though).  The power, what I have sampled so far, feels great - much better than the tired 350 from before.

Thaks to everyone who helped my along the way, particularly to old Bogie.  Let me know if you'd like picks or part numbers for the Aeroquip pieces I used for the coolant cross-over.

OM Gang

Ribbed usually proves to be easier to form around things but probably doesn't have as much corrosion life as solid tubing. But in the end the first problem is to make the connection.

Bogie

A bazillion years ago I had  Holley 3150 3 barrel, it developed a problem where it would idle or it would scream, but would give nothing in between. I never could fix it, finally stripped it for parts.

Bogie

I'm pretty sure that's where I'm at with the 4010.  I've gone through it twice already.  Too bad it has a brand new, $50.00, strip kit in it though.

OM Gang

Yeah, go figure, I'd soak the castings in a bucket of carb cleaner, rinse everything, blow out the passages with compressed air, inspect every part under a lighted magnifyer, put it back together with everything adjusted just so. Then I'd inspect the assembled carb with the magnifyer, hunting for cracks that might open up with assembly, looking for missmatched parts and all that could go wrong. Bolt it on the engine and get zip in the middle with a great idle and a screaming full on.

I also had a Q-jet that went that route. Search and hunt for the problem in either carb over a long period of time never turned up anything that looked wrong, yet you're frustrated because you know the problem is there in front of you, somewhere. Drives me nuts when it happens.

Bogie