Choking Out

[WarHammer]

Hello Fellow Mopar Enthusiast,

I’m having some trouble with my new Blueprint 408 and hoping to get some advice on diagnosing and fixing an issue with the choke and high idle system.

Here’s the situation: the engine is brand new with about 30 miles and just a few hours of run time. When performing a cold start, I fully depress the gas pedal to engage the high idle cam, then release to start. However, the engine starts extremely rough (to the point I'm concerned it may cause damage) — vacuum is at about 5, it’s blowing smoke, and sounds like it’s firing on only four cylinders with what feels like completely retarded timing (seems like not actually).

Once the engine warms up and the throttle plate opens, it runs perfectly with about 13 on the vacuum no smoke, no hesitation stumbling or weak performance. Here are some additional details:

• Timing is set at 16° initial, 36° total advance.
• Firing order is correct.
• Carb is a classic Holley 750 with a 12v keyed electric choke (confirmed).
• Cam is a hydraulic roller: .544/.541 lift, 236/242 duration at .050", 110 LSA.
• Curb idle is set at approximately 900 RPM.
• I’ve tried adjusting the high idle cam screw on the throttle arm, but this doesn’t affect the initial high idle RPM (about 650–700 at startup).
• The idle speed increases in rough increments as the engine warms. (kinda jumps a couple hundred rpm at a time)
• The high idle won’t step down automatically; it only drops when I blip the throttle.
• If I do not feather the throttle after the initial startup, it will stall out up until the point in which the RPM's have reached around 1,000

Any input you or troubleshooting steps to assist in fixing the problem would be greatly appreciated.

71 Duster, Blueprint 408 (491hp) 727 with balanced converter

 

[perfacar]

what rpm is your timing set at?

 

[WarHammer]

At idle 900 rpm and all in around 3-3500

 

[dano]

That cam probably wants more initial timing.

 

[TT5.9mag]

This is all carb tuning.
How does it run if you cold start it with the choke disconnected and out of the equation?

 

[toolmanmike]

Pull off the air cleaner before you start it cold and watch the choke. Do your normal start procedure by opening the throttle to set the choke and fast idle. The choke butterfly should close fully with slight tension. Now start the engine and watch the choke butterfly. It should open just a slight amount when the engine fires and slowly open to wide open as the engine warms up. If it doesn't go all the way closed, it is set too lean and needs to be richened up. If it doesn't open fast enough when the engine starts, it is set to rich and needs to be leaned out.

Now, you may also have a carb or fuel pressure issue that you are unaware of but check the choke operation and adjustment first.

 

[413]

What is the air temp there this time of year?

Is the choke too rich? Sounds like it if It runs bad when cold and great when hot.

Open the choke after it starts and see what happens.

The fast idle should stay on until the throttle pedal is touched.

Once it is hot and running good go out and DRIVE THE CAR! Put some miles on it, put a load on it, make it work. Seat those rings. Do not rev it too high just yet, go uo a hill accelerating the entire time.

 

[WarHammer]

Dano: you may be right but blueprint dictates no more than 16-36 no less than 10-32 or if something happens its not covered under their warranty.

TT5: Havent tried that yet but I'll give it a go

Toolman: adjusting lean and rich in relation to the choke plate, im assuming that is done through the choke housing and not referring to air idle mixture screws?

 

[WarHammer]

413: may be to late on the rev part. Ambient temp in this area is 60's in the morning high 70's in the afternoon. The high idle cam I thought transitions down automatically from high to mid to curb without touching the throttle is that not a thing?

 

[TT5.9mag]

What carb is on it? Got a Holley list number? You may only need to adjust the spring tension on the choke flap and you’ll be poopin in tall cotton. Beyond that idle afr tuning might be necessary.

 

[toolmanmike]

Dano: you may be right but blueprint dictates no more than 16-36 no less than 10-32 or if something happens its not covered under their warranty.

TT5: Havent tried that yet but I'll give it a go

Toolman: adjusting lean and rich in relation to the choke plate, im assuming that is done through the choke housing and not referring to air idle mixture screws?

Yes at the choke housing. The round black cover with a power lead coming out of it. Watching the operation from throttle opening to after engine start is very important to know which way you need to adjust. (if any)

 

[toolmanmike]

The high idle cam I thought transitions down automatically from high to mid to curb without touching the throttle is that not a thing?

View attachment 1716464322

No, you need to tap the throttle to get it to come off of fast idle. It will not slow down to curb idle by itself.

 

[TT5.9mag]

 

[WarHammer]

Thanks fellas. I'll dig into this and report back.

 

[toolmanmike]

Thanks. You beat me to it. I have been watching videos looking for a good one. This is a good one.

 

[413]

Lay some rubber! 4000 RPM max for a little bit.

Yep pedal needs a tap to go off fast idle.

Once it starts open the choke by hand some or all the way. Maybe wire it open just to see what it does.

 

[TT5.9mag]

Lay some rubber! 4000 RPM max for a little bit.

Never understood this. What’s the difference between 4000 and 4500? The engine has no idea what rpm it’s at nor does it care. Why limit the rpm to a specific number for break in?

 

[413]

Never understood this. What’s the difference between 4000 and 4500? The engine has no idea what rpm it’s at nor does it care. Why limit the rpm to a specific number for break in?

The engine does care, alot! It's called piston speed. Yes it knows the difference, and it's tight because it's new. 3000 RPM would be better. Make it work with a load, don't rev the **** out of it. Unless it's built for drag race only then let it rip!

Most guys don't know their piston the wall clearance, and most shops make them tight so they won't make noise or burn oil and smoke.

Go ahead and redline the ******, I don't care. Just trying to help a guy that may not know any different, like you. Many folks read and don't post and if every fool wants to redline a street motor and then wonder why it does this or that later on, that's why!

 

[PRH]

Blueprint 408 complete engine?

Should have been dyno tested before delivery…….I’m sure it’s already seen the high side of 5500rpm.

As for the problems…….sounds like the choke is set too rich, and/or the pull off isn’t working.

With air cleaner removed so one can see what’s going on, the choke blade should crack open a bit as soon as the engine is running.
If not, the pull off isn’t doing it’s job.

 

[CFD244]

the pull off isn’t working.

This would be my guess as well. In my experience with choke pull offs, if it runs rough chugging along then picks up after it starts to open a bit not enough pull off. If it starts and revs up initially then stalls, too much pull off. Good luck.

 

[TT5.9mag]

The engine does care, alot! It's called piston speed. Yes it knows the difference, and it's tight because it's new. 3000 RPM would be better. Make it work with a load, don't rev the **** out of it. Unless it's built for drag race only then let it rip!

Most guys don't know their piston the wall clearance, and most shops make them tight so they won't make noise or burn oil and smoke.

Go ahead and redline the ******, I don't care. Just trying to help a guy that may not know any different, like you. Many folks read and don't post and if every fool wants to redline a street motor and then wonder why it does this or that later on, that's why!

Meh, fire it up and put on the chip in the burnout box.

 

[mbaird]

Possibly the floats are set too high allowing fuel to drain into intake when you shut it off?

 

[WarHammer]

Thank you everyone for chiming in. A lot of great feedback and I really appreciate it. Had a little bit of time after work to piddle with it and the consensus was right. I activated high idle, started and the choke plate stayed closed. Adjusted the choke one notch plate opened and that seemed to clean everything up. I'll continue to play around with it to dial it in more when I have some more time but overall I think we are good or at least heading in a better direction.

 

[mopar head]

You also don`t need to floor it to set the choke, possibly dumping more fuel than it needs to start. I use two 1/4 throttle pumps on both of my engines.
They fire right off. have fun.

 

[toolmanmike]

Thank you everyone for chiming in. A lot of great feedback and I really appreciate it. Had a little bit of time after work to piddle with it and the consensus was right. I activated high idle, started and the choke plate stayed closed. Adjusted the choke one notch plate opened and that seemed to clean everything up. I'll continue to play around with it to dial it in more when I have some more time but overall I think we are good or at least heading in a better direction.

Good to hear. You just need to watch the operation and understand how it works and what the engine likes. Except for fuel evaporation, My 2 Mopars start easily, fast idle (not too fast) and don't die from being too lean, or stumble from being set too rich. It's a fine tuning thing but it's pretty nice when it works well. I got some experience at a GM dealer in the late 70's with carbs that had primary and secondary choke pull off's controlled with Thermo-vacuum switched screwed into the intake. A bit more complicated but the operation was basically the same. Just more parts that needed to work together.