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Changing jets in Holley Carburetor on 265 Chevy Engine


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pikalpha
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Aug 4, 2012, 11:44 PM

Post #1 of 14 (1342 views)
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Changing jets in Holley Carburetor on 265 Chevy Engine Sign In

I have an old four barrel Holley carburetor maybe a 730 or 750 on a 265 Chevy engine. The Holley is a double pumper and it is putting too much gas into the engine. We have adjusted the carb but it still is getting too much gas and plugs are fouling quickly.

I have been told changing the jets in the carb to a smaller size will help with the amount of gas the engine is getting. My question is how difficult is it to change the jets out and what size jets do I need to install? Or do I even need to do this? The Holley has been on the car for about 25+ years and has not fouled out plugs like this before. I think the rebuild I had done on the carb awhile back and tinkering on the carb to adjust it awhile back may be the problem. I don't really want to buy another carb because this one ran right once and I am hoping it can again with new jets or maybe an adjustment somewhere.

If anyone has any insight or suggestions or advice about this, you have my full attention.

Thank you.


re-tired
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Aug 4, 2012, 11:56 PM

Post #2 of 14 (1329 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

If it's been on the car that long and has not fouled plugs and it suddenly does, i dont think it is the jets. I would recheck the following ,,,,,,, check both float levels while eng is at idle. ,check accellerator pump diaphrams . check choke for proper operation. If all else checks ok I would replace power valve.


LIFE'S SHORT GO FISH


Discretesignals
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Aug 4, 2012, 11:59 PM

Post #3 of 14 (1328 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

If that is a 750 or 730, I'd say the carb is too big for the engine size.





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Tom Greenleaf
Ultimate Carjunky / Moderator


Aug 5, 2012, 12:20 AM

Post #4 of 14 (1312 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Hmmm? If it worked for 25+ years and now rich I think float level is the issue as RT just mentioned or other. Might be set properly dry with finger/device but some will "sink" being too heavy saturated with fuel.

Not a chance from me to find the spec for weight but should be able to observe that fuel level in bowl(s) are too high hence rich fuel delivery. Weak on old paper diagrams but some memory of a side plug on bowl that you could open, set floats till fuel just spilled out like a gearcase fill plug idea.

Pic may show carb you are dealing with....



T
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Discretesignals
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Aug 5, 2012, 12:27 AM

Post #5 of 14 (1306 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

265 with a double pumper 750 on it? Even if it did run, it must of run like a bag of poop, especially if it is in a big heavy car with an automatic. A 450 cfm rated carb on mildly modified 283 is more than adequate.

Here is a calculator for figuring out what size carb to have on your engine. Obviously too large of a carburetor for your engine is actually going to cause the too big carburetor symptoms such as: sluggish response, possible stalling, poor fuel economy, etc. Unless your supercharged/turbo charged and running the engine up to 9000 rpm all the time, a 750 or 730 double pumper is too big for anything less modified.

http://www.csgnetwork.com/cfmcalc.html





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(This post was edited by Discretesignals on Aug 5, 2012, 1:43 AM)


Tom Greenleaf
Ultimate Carjunky / Moderator


Aug 5, 2012, 12:50 AM

Post #6 of 14 (1289 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

I hear you - too many CFMs for a low CID V8. Seems just '55,'56+'57 were the years when OE. Plain not sure but displacement is effectively much more if/when the red-line RPMs of the things is designed thru the roof.

Really not sure but pics archived of the dang things looked like this......

or this.....



Signed.....

Bewildered,

Crazy T
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Hammer Time
Ultimate Carjunky / Moderator


Aug 5, 2012, 8:00 AM

Post #7 of 14 (1257 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Although I agree that the carb is oversized for that engine, the extra CFM isn't going to foul the plugs. It may make it fall only it's face and bog out because it can't process that much air that fast, but it's raw fuel that is fouling those plugs and the jet size ran before without fouling so that's not the problem now. Either the floats are adjusted wrong as pointed out already or the metering plates aren't sealing properly.




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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator


Aug 5, 2012, 9:43 AM

Post #8 of 14 (1247 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Agree. My take on carbs is they are responsive to to the airflow thru them. If the engine can't consume (make vacuum for) what is available it should just deliver to the circumstance at the time. Largest CFM carb on a huge displacement motor still idles and adjust to that. Too small would be limited to that. It's matching it all up that has the potential for maximum performance.

I'm sure they are still out there. Adaptors to put a 4V carb on a 2V manifold that could only rise to the ability of the two hole manifold! Not sure why people thought that would work but many were sold. Conversely putting a smaller 2V on a 4V manifold was thought to save fuel and really couldn't by the concept of how they work?!

T
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Discretesignals
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Aug 5, 2012, 3:01 PM

Post #9 of 14 (1234 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Just wondering why you would even bother repairing the probably worn out carb that is too big for the engine anyway? Instead of wasting money on attempting to rebuild it get you a new carb that is sized and tuned for the engine. You'll be a lot happier in the long run because you won't be driving around in a gutless soggy pig. Misconception is that people think more is better, when in fact, as carbs go more isn't better. There is a lot more than just slapping on a fat Holley and expecting to gain tons of power.

Hopefully the engine isn't worn out or causing a signaling problems to the carb otherwise you'll be wasting your time no matter what carb you have on it.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 5, 2012, 3:24 PM)


Tom Greenleaf
Ultimate Carjunky / Moderator


Aug 5, 2012, 3:32 PM

Post #10 of 14 (1227 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Hence Quadrajets got a nick-name "Quadrabog" for a while. It's only taking in atmospheric* pressure at best as shown by the 83% such that you would need mirror glass passages (polished) to gain on that or force feed them. Matched is the key for correct behavior.

All the games in the market out there don't work better than the weak link. Some GM prototype small blocks were said to handle insane high RPMs and can't prove that,

T

* Ever hear of "Summer vs Winter" horsepower? Air is denser when cold - see hot air balloonCool
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Discretesignals
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Aug 5, 2012, 3:45 PM

Post #11 of 14 (1223 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

It sorely depends on what you want from the engine? Sure you could have a smaller engine that is capable of twisting the crank to insane rpms that would require a larger carb, but something like that would be useless for everyday driving in traffic to and from work. I guess the OP just wants to figure out why the plugs are fouling out and I am getting carried away(like usual). Maybe he/she is happy with a larger carb that has been operating on the vehicle for decades not realizing it could be better and more efficient at producing torque with a better match of components. OP never stated which vehicle this thing is in. Could be a full blown drag car.

The Quadra's I read where mostly 750 cfm rated carbs, but had a smaller primaries for low speed operations. Being vacuum operated secondary, the carb only displaced what was actually needed by the engine?? Or was it that flipper flapper valve that looked like a choke plate that was used as a displacement controller? I guess GM thought they would make a universal carb that could work on both big and small blocks.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 5, 2012, 3:54 PM)


Tom Greenleaf
Ultimate Carjunky / Moderator


Aug 5, 2012, 8:59 PM

Post #12 of 14 (1203 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Right DS - we went astray as if that never happens. This whatever creation has a carb and is flooding so let's get on that. Carbs are in basically a bottom, middle and top - sounds kinky.

So........ If choke isn't somehow on all the time (the top) then it pretty much leaves the bottom (poor middle gets left out so far) so it's getting idle mixture and perhaps too strong which would take assertive misadjustment or fuel level too high from leaking needle valve, floats too heavy to shut off incoming fuel at proper level.

Pretty much everything involved with the middle is blending idle settings to just mid range, blend again till WOT and would run leaner with faults except for choke or possible bent or broken parts in the scene somehow??

T


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pikalpha
New User

Aug 5, 2012, 11:08 PM

Post #13 of 14 (1189 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In

Thanks, guys for all your help. Everyone really came through on this like gangbusters.

I have a friend who is an old school mechanic and will pass along your ideas on the problem to him. Maybe he can figure out what is happening with the floats and valves.

Your suggestions about getting another carb for the car is an idea I have been thinking about but have been putting off this expense until I know this big Holley won't work out. I have several smaller 4 barrel Quadrajets that can be rebuilt but I am just not a great fan of these carbs.

When we get the problems ironed out, I'll let you know what we did and if it worked.

Thanks again for all the time and help.


Hammer Time
Ultimate Carjunky / Moderator


Aug 6, 2012, 6:09 AM

Post #14 of 14 (1181 views)
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Re: Changing jets in Holley Carburetor on 265 Chevy Engine [In reply to] Sign In


Quote
I have several smaller 4 barrel Quadrajets that can be rebuilt but I am just not a great fan of these carbs.


How are you going to do that? What manifold are you running. Unless your Holley is a spreadbore with a matching manifold, those carbs are not interchangeable.




~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.





Changing jets in Holley Carburetor on 265 Chevy Engine


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