Technical At wits end... early Chrysler Hemi tuning

Hello all,

Looking for opinions on a relentless cut-out when applying throttle to a stock 354 Hemi. It has been doing this for years.. I have done and redone everything I can think of without luck. Now I just feather the throttle every time I start off from a stop. This makes it not so fun to drive. It never stalls, just cuts out very bad... feels like accel pumps... but I am 99% sure they are good.

Starts great cold. Sometime a little sluggish to start hot but I think that's just heat soak.
Idles nice (good fast idle, and curb idle once warm). Good vacuum at idle.

Carburation: Dual WCFBs rebuilt. Recently pulled again for new idle mix screws and reset of the metering rods and accel pump stroke range setting. Accel pumps seem to be doing great.

Ignition: distributor rebuilt (mechanical/vac advance confirmed), timing checked and rechecked a dozen times. (I had convinced myself it was the distributor... timing not advancing properly, but the rebuild didn't really change the behavior.)

Fuel: rebuilt mechanical pump, fuel bowls are full with new ceramic filters. running 90 octane ethanol free fuel.

Any ideas?

Thanks for reading!
JJ

 

timing advance ? But it sounds like squirters aren't big enough.

 

I'd try a few degrees of initial timing then maybe a few more.

 

Carbs out of sync

 

Did you replace the harmonic balancer? On Dads 392 we installed a new balancer, only to find the timing marks were off by 9 degrees. This problem could happen to an original balancer as well.
Your problem does sound more like a fuel/air issue.

 

Carb linkage ?
Who built the carbs?
Fuel pressure ?
Volume?
Air filter (a)?
Have you confirmed static timing and all is correct ?
My guess is air or fuel

 

Could the torque of the engine be tilting the block or exhaust over against the +12v wire that feeds the coil and shorting to ground? I had that happen to me once.

 

Do you ever get a lean “pop” from the carbs?
If not, my guess would be secondary ignition.
“Tip in” puts the most stress on secondary ignition parts. Look for evidence of arcing on the rotor, plug wires, coil, plugs.
Tighten up your spark plug gap a bit and see if the issue changes at all.
Run the car in the dark. A spray bottle with water will show you real quick if you need wires.

 

Wow... lots of good thoughts.

I don't think it's air... and the mech pump is keeping the carb bowls full... so it's not fuel pressure. I built the carbs with Echlin kits.

1) Accel pump not pushing enough? I tried different plunger arm settings which should change the pump stroke, but still no difference. Could pull them again and give the leather an oil soak...

2) Carb adjustment? I admit that this is a challenge.. I haven't been able to find a suitable way to measure air volume flow on the large WCFB airhorns. The throttle plates on each carb are adjusted with the wire gauge method described in the shop manual. Then, once installed on the manifold, the carb/carb linkage was installed so there was no slack through the linkage and both throttles opened at the same time. The vacuum reference I am using is off the manifold, so again, no definitive evidence that they are synchronized. But, they should be.

3) Timing? The harmonic balancer might be a good places to start; I am using the original timing marks. I have thought about replacing it with a modified 340ci version from HotHeads.

4) Torque reflex pinching wire? That must have been a real pain to discover!

5) Secondary ignition? This is one thing I haven't considered........ I'll be researching this... unless you can elaborate Trendz. I do not hear any pops when the cut out occurs. I am using custom made Taylor Spiro Pro wires with plastic plug side boots from Doug Herbert.

 

Timing makes a huge difference on a quick or slow response from throttle opening. I'd bump it up a few degrees and see if it is improved. You can advance it right up until it pings, the back it back down.

Are the accel pump plunger arms forced down, or are they gravity pulled down? If the leather cup is spread too far out, it can roll up and takes forever to gravity feed down. The forced down plungers generally do a better job when the throttle is opened quickly. I'm not sure what the WCFB has, but a lot of the early Carter carbs were gravity down pump plungers with a slot the linkage fit into.

Does it start to go, then flatten out, or is it flattened out from the beginning? Just because the pump can supply enough fuel at an idle, that doesn't mean it can supply enough fuel with two carbs wide open. In addition to a fuel pressure test, there is also a fuel volume test. Do the volume test. Gene

 

If I am reading correctly, your issue is major hesitation from idle.

One of the most common causes of this, especially with an automatic transmission, is setting the idle mixture control screws for too RICH an idle.

This can cause "puddling" of fuel in the intake plenum(s) and runners, as their is virtually no air velocity in the intake system. When the throttle is opened rapidly, the increased air velocity in the intake sweeps all of the puddles into the cylinders, creating a momentary over-rich condition (so rich it will not fire). Many will assume this is a lean condition, and adjust the accelerator pump(s) for maximum squirt, which simply makes the condition worse.

Try resetting both the idle mixture screws (a good starting point would be 1 turn from lightly bottomed) AND the throttle positioner screw(s) (giving just a wee bit more air) and see if this helps.

Generally when this condition is present, the car will idle well (sometimes with a trace of tailpipe smoke) and run well, just will not accelerate from a dead stop. Also the engine will start exceptionally well cold, less so when hot.

Jon.

 

Gene-Koning: Forced down accel plunger. And it seems to run great at WOT, it's just falls flat on its face for a second or two right at tip-in.

Carbking: What you describe here sounds dead on... major hesitation from idle. But more than just a stumble; my wife thinks it stalls every time. But after a moment or two it picks back up and cruises fine. I have been chasing a timing related issue so fervently because it feels like the spark totally cuts out (erratic timing advance or something). But it never backfires, which is probably a nail in the timing coffin. Intake tract puddling at idle... that is a really interesting thought. In my opinion, the idle mix setting on a pair of WCFBs is the hardest thing to get right; 2 carbs, 2 mix screws per carb. It's probably a fair guess that I am not doing it right!

 

A little more info does the car just bog or do you feel it missing out. If it is a bog look at the throttle plates at idle are they open enough to be using the transition slots? If they are try adjusting the idle mixture to get the highest rpm then adjust the idle screw to the right rpm and repeat. The idea is to get to to idle with the least amount of throttle plate opening.

 

Twin carbs split vacuum signal in half making it hard to pull fuel out at tip in. You need to open up what ever circuit that is. Not familiar with how those carbs work but on some you have to drill out passages to make it easier to pull fuel. Easy test if the intake an open plenum, if it is block one carb and see if it still does it. Then block other and re check. If it runs great on one at a time you need to open up the correct circuit with a correct size drill.

Sent from my SM-A102U using The H.A.M.B. mobile app

 

I may be doing it "wrong" but on my 392, the rear WCFB is the "primary" one, and it is the only one that the engine idles off of. On the front carburetor, the idle mix screws are turned all the way in. The rear carburetor is also the only one with a choke, and the setup uses progressive linkage. Think of it as running off a single WCFB until I have my foot down, when the second one opens up.

Now keep in mind that I pieced this setup together from a bunch of parts and have no idea what I'm doing. But it does seem to be working. You might give that a shot, assuming you're running a manifold that is completely open underneath, like my stock 300C intake is.

 

Bog or miss? I'd say bog. It doesn't get rough... just a total cut out.

'57 392 twin carb setups DID switch to progressive linkage (rear carb was primary with choke only on the rear). '56 354 was not progressive. Each carb has a choke and idle circuit. Rear carb has the port vacuum reference for advance.

 

Is the mechanical advance stuck?

 

This was one of my leading suspicions... until I had the dist reconditioned and the mech/vac advance both checked vs RPM on a distributor machine.

 

jj94tt, you never said straight or progressive linkage. I run dual WCFB’s as progressive (they are not on a Mopar or GM) and idle on both. I do not and never had a hesitation. I set them at idle with a UniSyn both in neutral and in gear. Since the aluminum carb cover I use is a larger diameter I siliconed an adapter to it and then the UniSyn to it. (See photo) At idle both red balls are exactly the same and near center. My idle in gear is just under 600 and my engine is bone stock. My accelerator pumps are on the stock location and the slightest movement allows a dribble of fuel in.
To me it sounds like a fuel problem or an added air problem. I have never heard of the over rich puddles carb king described unless it was on an old Smokey Ram manifold but he knows way more than me. I recently changed my intake gaskets and was able to lean out both my carbs by the UniSyn.
I have my vacuum advance usesVenturi vacuum at idle. Good luck.

 

Straight linkage - not progressive. I need to rig up a UniSyn adapter like that.

 

Ordered a UniSyn today. Regarding the vacuum advance, I said "port vacuum", but I think it is technically venturi vacuum when taken from the nipple between the WCFB idle screws, right?

 

I have dual 97's on my flathead. Went through the same deal changing everything. It started and ran good, but when I got on the throttle hard it would hesitate and I would have to feather the throttle. One day after making changes I left the air cleaners off and it worked great. Got less restrictive ones, instead of the hat style ones and that was it.

 

jj94tt ...you said these on a 354 Chrysler; are they original Mopar carbs? Do they match or have the same tag #’s that may make a difference. Mine are the same tag number from a 1953 Olds and since they were jetted for a 8-1 303” and I have a 297” 8.4-1 they work great. I had what I interpreted as heat boil with today’s fuel, I installed 1/4” phenolic spacers I bought off e-bay and that went away. Since I’m progressive I choke only the rear, I only partially close the flap enough to raise the idle step. 1 pump at 1week cold and start up. 2 pumps if it sits 2 weeks. Never touch hot or even warm.
Have you tried progressive. Once running just disconnect the front one and drive and see if it stumbles. Anything is worth a try. Photo of my linkage. Home made.

 

JJ .... how about some good engine pictures and even a video to help us out?

Based on what you have wrote, my money is on what @carbking Jon says, he knows.

 

Try running it with the throttle linkage disconnected on one carb. If the hesitation goes away, I'd be looking at a progressive setup.

 

Turning the mixture screws all the way in does not shut off the idle system off You are shutting off air making the system go rich. Start with the screws about one turn out

 

I think I have the mix screws set to 1 1/2 turns out. So, it should be leaner... I like the puddling idea, but this doesn't seem to support that theory.

Regarding straight vs. progressive - there is probably a reason Chrysler switched from the straight twin quad setup on '55 & '56 to the progressive setup on '57...

I will have to get some pics and a video.

 

Had the car out today... not sure if this video will help. Next time I'll get one from under the hood, just working the linkage.

 

It would help with a more detailed explanation of what we're supposed to be looking for in the video.

 

Have you tried blocking off one carburetor?