Seeking Advice Concerning How To Correct An Engine Misfire

To follow along with Marlin and JT....

 Remember the the story of George Holley and Henry Ford working with the first “iron pot” carburetor in Henry’s lab in Dearborn, MI......

Henry Ford:

“ Goddamnit George, this stupid carburetor is not adjusted correctly!”

George Holley

“Henry – if I’ve told you once, I’ve told you 1000 times - 90% of “carburetor problems” are caused by the ignition! 

I have have seen this myself - not the discussion, but chasing probs in the carb caused by ignition issues  

Rocky

rocky posted:

To follow along with Marlin and JT....

 Remember the the story of George Holley and Henry Ford working with the first “iron pot” carburetor in Henry’s lab in Dearborn, MI......

Henry Ford:

“ Goddamnit George, this stupid carburetor is not adjusted correctly!”

George Holley

“Henry – if I’ve told you once, I’ve told you 1000 times - 90% of “carburetor problems” are caused by the ignition! 

I have have seen this myself - not the discussion, but chasing probs in the carb caused by ignition issues   

Rocky

 ...Sorry, I can't resist. Most people don't know, that way before George Holleys' First Single Barrel Carburetor; at the Dawn of a Internal Combustion Engine, ever Powering a Four-Wheeled Vehicle....The VERY First 'Carburetor' was a SPONGE! We've come a Long Way, Gentlemen!

Marlin

Many thanks for sharing your experience which is very much appreciated. I checked all of them on my engine but found everything okay (as far I can judge). On the week-end I started playing with the jets of the secondary and found that the misfiring rpm is depending on the jet size of the secondary. I got following: Jet size 74 provides heavy misfiring @ 4700 rpm, 78 misfiring @ 5200 rpm and 81 light misfiring @5500 rpm (I probably need to go up to 85 to shift misfiring to 6000 rpm). Seems that the mixture is leaning out with increasing rpm. The jet size for the prim is 70 and there is no misfiring at the full range of rpm. Was reading that leaning out at high rpm can be corrected by the air bleeds (smaller size provides richer mixture at increasing rpm) but on my carburetor those air bleeds are pressed in and cannot be changed. It is still difficult to understand why the required jet size between prim and secondary of my carburetor are so significant in size (Holley advises that the maximum jet size difference should be in the order of 4)

Brgds

SIG

Sig have you re-adjusted the floats since you received the carb from Bob?

 

Don't remember which Holley you have, but know there are some Holley vac-secondary 4bbls that are made more for emissions than for performance, and they are not necessarily ID-ed as 'emission carbs'. Such Holleys are extremely difficult (or impossible) to tune on a performance 351-C. Double-pumpers are similarly difficult as they ALWAYS run rich. I had one brand new Holley vac-sec on a Pantera in for work, and after weeks, I gave up on getting it to run right and bought a Tuner-Holley with some 18 separate modifications built in. That instantly fixed the problem and yielded better power and added 2 mpg on long trips. May be worth the extra money to you as well.

Hi Sig -

 Based on your description above, I think you have mechanical secondaries. You said you had a “double pumper“.

 

Anyway I had a problem with my Holley vacuum secondary carburetor, where the diaphragm the pools in the secondaries was stiff and not opening.

 

That might be something else you want to check, that your secondaries are opening smoothly as you ramp in the power.  It might explain why you are having to go so much richer than the primaries.  

 

We we made a little flag, and affixed it to the secondary linkage.  This let us see that the secondaries we’re sticking.

Easy to do.

 

Rocky

 

 

 

 

Good morning,

I did not re-adjust the floats only checked the correct fuel level in the bowls. 

The carburetor is new, secondaries are working smoothly. It is a double pumper Holley and a Tuner-Holley which is from competition carburetion.

Brgds

SIG

Found finally the reason for the misfiring. I had a PowerValve 35 installed (in Prim) and replaced it by a 65. 

Brgds

SIG