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Old 11-01-2021, 01:17 AM   #1
Mercmarc
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Default Lean MisFire

Hello All,

Has anyone run across a lean mis fire under light throttle condition with new model 94's from Dennis Carpenter? Jet sizes from 49 to 62 and power valves from 5.5 to 4.5.

Background:

New 1950 239 built by Mike of H&H with the 94's on it. Ran well on the engine stand. Driving under light throttle just off idle to 1500 RPM it bucks, lean on the throttle and the bucking stops.
Parked in the garage you can raise the rpm and choke the carbs slightly and the engine speeds up slightly, even with the jets at 59.

Idles well, runs well under moderate to heavy throttle, plugs are still brownish.

Many thanks for any input, solid wires, HEI and point distributors, autolite plugs have been used with no change.

Respectfully

Marc
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Old 11-01-2021, 02:45 AM   #2
koates
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Default Re: Lean MisFire

Connect a test vacuum gauge to your intake manifold and check the readings against the test chart. That is an approved way of diagnosing problems. Regards, Kevin.
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Old 11-01-2021, 10:26 AM   #3
Bill OH
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Default Re: Lean MisFire

I had a pair of Hollys on a 239 back in the day and I experienced a flat spot in the transition from idle to mains. You might try opening up the idle mixture to see if that helps. I had vacuum leaks between the bowl and the base and with other problems I switched to 97s, which I also have on a 221 running stock mains and #71 pvs. Runs very well!!!
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Old 11-01-2021, 10:46 AM   #4
Mercmarc
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Default Re: Lean MisFire

Many thanks Gents,

I will do the vacuum test today and go from there. I was hoping to avoid going to the 97's because of cost and the fact that I just spent 1K on the two 94's. If that turns out to be the problem, then so be it. I will keep you posted.

Thanks again

Marc
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Old 11-01-2021, 11:37 AM   #5
flatjack9
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Default Re: Lean MisFire

94's are generally jetted with 49 - 51 main jets depending on your altitude. Under the conditions you describe, the idle system is still contributing. Might try opening the idle screws a bit.
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Old 11-01-2021, 12:23 PM   #6
51 MERC-CT
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Default Re: Lean MisFire

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Would be interesting to see what an AFR meter has to say about the condition.
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Old 11-01-2021, 08:41 PM   #7
Ol' Ron
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Default Re: Lean MisFire

What distributor do you have? I like the 94's better than the 97;s
Gramps
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Old 11-01-2021, 09:11 PM   #8
Mercmarc
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Default Re: Lean MisFire

I am running the original ford distributor with the vacuum advance. New Taylor solid wires, new coil with a new ballast resistor. New plugs that are Autolite 216.

I have run two electronic HEI distributors from different suppliers with the same results. ( still stumbles off idle up to 1500 RPM). Both versions failed within 50 miles of each other for different reasons. One had a cut wire by the advance plate moving, the other one failed because of a failed electronic board. That is why I am running the old original distributor with new points and condenser and the shaft is tight with no point variation while running a dwell meter. Condition is the same regardless of the vacuum advance connected or not. Ventury vacuum from one carb.
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Old 11-01-2021, 09:16 PM   #9
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Default Re: Lean MisFire

one distributor was made by TSP pro series ready to run HEI with an adjustable vacuum advance old style GM cap using
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Old 11-01-2021, 09:55 PM   #10
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Default Re: Lean MisFire

I shouldn't add, but I believe on a duel carb step up you want 3.5 pv's making it 7 all in vacuum dump. When you hit the throttle the vacuum drops.


Just an oldtimey thought. Idle good and revs in the garage, underload bucks. Sticky valves or condenser. New motor tight guides.

Last edited by Tinker; 11-01-2021 at 11:06 PM.
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Old 11-01-2021, 10:05 PM   #11
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Default Re: Lean MisFire

The stock distributor will not advance properly with 2 - 2's. You are probably running quite retarded. You need a Charlie N Y converted distributor with manifold vacuum advance.
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Old 11-01-2021, 10:40 PM   #12
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Default Re: Lean MisFire

Not familiar fully with HEI, but wouldn't that require spiral wound plug wires, even with points? Interesting setup.
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Old 11-01-2021, 10:59 PM   #13
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Default Re: Lean MisFire

Thank you's to everyone who has weighed in so far.

The results of todays efforts are this.
17 in of vacuum at idle. 4 degrees advance from the pointer on the crank pulley.
No vacuum at idle from the carb as noted above.
The engine idles well, throttles up well in the garage when revved.
The condition I am trying to solve is from just off idle to approximately 1500 RPM the engine stumbles.
While holding the pedal steady and the RPM around 1300 the engine runs lean. I can choke the carbs down and the vacuum increases to near 20 inches and the engine smooths out. Remove my hands from the carbs and it begins stumbling again all while holding the foot feed steady (second person) :-).

I am running 52 main jets with 4.5 PV. I took the carbs completely apart today which are new from Dennis Carpenter, looking for an obstruction in the venturies of any sort. None found, only a throttle plate issue that I resolved. No change in performance or the issue that I am addressing.

This engine was built by Mike at H&H with these carbs on it. On the stand it ran well, once it was in the car this condition became evident. I have 175 miles on car and engine at this point. I have tried multiple distributors, both HEI and points with no change in the condition. I have tried jets from 49 to 62 with no significant change except for a better rev at the richer condition. The plugs remain brownish and at idle I get 1 inch of vacuum variation while at 17 inches plus or minus 1/2 inch. I don't know what cam Mike installed other than to say it is the same as he runs and he was certain I would be very happy with it. (no help I know) Editorial humor:-). H&H are great people.

Tomorrow I will drive the car again with the vacuum gauge attached to the manifold port and see what driving reading I get. More to follow

Again thank you to all

Sincerely

Marc
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Old 11-01-2021, 11:02 PM   #14
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Default Re: Lean MisFire

I have had no issues with the solid wires that I purchased through Summit and I was told that the Taylor wires would work well with the HEI setup. I wish the HEI distributors held up.

I will check in with Charlie NY to see what he needs from me.
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Old 11-01-2021, 11:15 PM   #15
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Default Re: Lean MisFire

It's really a cross over electronic and point dizzy. So I have no idea on wires. Solid for points, spiral for electronic ignitions. I like the 97's too so.


Think Flatjack makes a great point on duel 94 carbs and where you pull vacuum from. CharlieNy is a good cat.


I like your idea of running a vacuum gauge in the cab also. I run them full time on my basic 36.
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Old 11-02-2021, 12:15 AM   #16
Mercmarc
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Default Re: Lean MisFire

Thank you Tinker, I just received a replacement (warranty) for the HEI system that failed on the electronic portion. I may give it one more try while working with Charlie Ny. I will try running the manifold vacuum to the advance on the electronic version just to see what affect it has overall. The vacuum advance begins pulling at 8 in vacuum but can be tightened up to begin moving closer to 13 in. Either way, at 17 in vacuum at idle it will be full on until leaning hard on the pedal. May not be a bad thing. Still have the centrifugal also. Light springs max out at around 3000 RPM if I remember but I will check the data sheet to be certain. If I recall correctly late 60's SBC used to run full advance at idle.

More to work through.

More updates to follow.

Respectfully

Marc
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Old 11-02-2021, 12:20 AM   #17
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Default Re: Lean MisFire

Flathead runs idle 18-21 vacuum. 19-20 normal idle.

But with blowers and multicarbs it changes, lowers.

Beyond me, best of luck!

Not sure I know much about sbcs, but I'm not sure they run full advance at idle rpm's.

Just because a motor preforms on a stand or in the garage, Doesn't mean that it will do the same thing when you put load on it.

If you are smoking the electronics, it might not be vacuum.

Interesting to see how it goes for you.


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Old 11-02-2021, 09:08 AM   #18
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Default Re: Lean MisFire

Is this a 6 or 12v system? Charley can make a points style dist for 6v. You just can't use a stock dist for 2 carbs.
Gramps.
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Old 11-02-2021, 09:50 AM   #19
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Default Re: Lean MisFire

Engines used to run with full vacuum at idle. As emissions laws came in to effect, vacuum was changed to ported to help lower emissions.
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Old 11-02-2021, 10:02 AM   #20
Charlie ny
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Default Re: Lean MisFire

Merc,
Per chance are you running progressive linkage ? Best to run direct linkage,
progressive 2x2 can be a tuning headache with 94's. I will suggest running one carb

at a time... use a carb block off plate if you have one, set the mixture screws for the
smoothest idle and set idle speed at 500. Square that carb and go for a ride....ditto
all for the other carb.
4.5 PV's might be to high in/hg. If your PV's have the rectangular outlets
and big soft gaskets take heed, they are not correct for 94's even if they came in the kit's you purchased.

The vac operated power circuit on 94's can be a pain with multi carbs especially when a non stock cam is introduced. Thru the years on 2x2 setups I plug the PV's and run 52 jets.

The Chevy conversion distributor is a simple upgrade. Once installed the rate of adv and total can be changed without removal from the motor, pretty cool.
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All this is my opinion only...I absolutely respect alternative ideas and opinions
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