600 Holley part throttle lean situation

[junkyardjeff]

This carb has acted this way since day one and I am finally getting around to dealing with it,its on a 1970 ford 302 in my 65 custom 500 with shorty headers and 2 1/4 inch exhaust so the motor was real lean with all plugs burning white.  According to the Holley website this carb was supposed to come out of the factory with 66 jets but had 64s (I bought some 68s today but had a bunch extra and found the 66s) so I installed a set of 66s and seems to run a little better but just off idle it seems to be lean and surging a little untill I give it a little more gas,do I need a different pump cam or squirter to fix it.   As soon as I dry out a little (raining today) I am going on another test ride and see how it runs.

[chimp koose]

I know its basic but are the float levels set a bit low?

[junkyardjeff]

This carb has the non adjustable floats so I cant change anything there,I think there might have been some dirt in the carb and has been in there for awhile.  When I first put the 66 jets in I took it for a drive and it was better but still had the part throttle surging so I took it back apart and blew out the metering plate,I did not remove it when changing the jets and did put new gaskets on it that time.  I just got back from a 30 mile highway drive and some in towm miles and the new plugs dont have enough color to see how they are burning so I will drive it to work next week and then next weekend pull a couple plugs and check them again,now its running alot better with hardly any part throttle surge so I think maybe a little longer pump stroke and possibly one step larger on the jets and I am done.

[enjenjo]

you are aware that the pump cam has two possible mounting positions?

[junkyardjeff]

I did not know there were two positions for the pump cam and untill today the carb has not been touched since I took it out of the box,I know why there was dirt in the carb as the bowl gasket had sunk down on the top middle a little.  There had been a little staining on the top of the fuel bowl which I did not pay much attention to and now I know why I always smelled gas,I was able to set the timing back to specs after the jet change as I retarded it a couple degrees since it pinged a little.  In a way I wish I never changed it to a 4 bbl as it ran excellent with the 500 2 bbl I had on it before but its getting better.

[wayne petty]

[wayne petty]

if you look at these two pages....   15 and 16


http://www.mustangandfords.com/techarticles/mufp_0603_ford_carburetors/photo_15.html

http://www.mustangandfords.com/techarticles/mufp_0603_ford_carburetors/photo_16.html


you might want to ...   just for a test....   if you have some thin welding wire... MIG...   double loop it around the bowl vent.. and stick the ends into the outer 2 air bleeds....  the outer 2 are for the idle circuit... the inner two are for the mains.. and the mains are not causing your problem...


this photo shows the pointer directing you to the main air bleeds..

http://www.mustangandfords.com/techarticles/mufp_0603_ford_carburetors/photo_25.html


its the other bleed... you want to part way block... not all the way.. just part way...    that will richen up the idle transition mixture..

if the hesitation goes away...   you might have a blocked idle feed restrictor.. thats embedded in the metering body.. under the not easy to remove plugs...


metering circuits.. are like straws with a hole in the side...

if the hole is small... or blocked.. you get more fuel than air...

blocked is not good...

if the hole is larger.. you get less fuel...

if you restrict the IDLE air bleed .. you get more fuel during the transition from idle to the mains.. which is where i think you are having your problem..... this is just for a test...

you might also want to verify the pump adjustment after you have the float bowls off.. they can get installed a little high or a little low.. this throws off the proper adjustment..    engine off.. WOT.. and 0.015" feeler gauge between the spring loaded screw and the end of the lever..


just understand... we are trying to adjust the fuel mixture on the idle circuit and the idle transition circuit...    

the main jets control the fuel that gets pulled over the boosters...   not so much on the idle ...  the size of the main jets adjusts power and economy while on the mains...

if you only operated the carb at idle and wide open throttle...   you would have it made...  but there are a bunch of different things that can effect the drivability..

how about some intake manifold vacuum readings...

at idle....  

in drive at idle...

when the problem is happening...

at cruise... where it runs good...

you might end up swapping a different vacuum number for your power valve...   the power valve system bypasses the main jets.. if it opens to early in the throttle...  you can get weird problems...

[junkyardjeff]

I will try the wire trick and see if it improves,I think other then needing it richened up due to the good flowing exhaust I have on it I think most of the part throttle issues was dirt related because of the faulty fuel bowl gasket which I think came from the factory like that since yesterday is the first time I had this carb apart.  On my last drive I think I have 95 percent of the issue solved nd its acceptable at this point,how long do I need to drive it untill the plugs get some color on them so I can tell if I need more or less in the jets.

[junkyardjeff]

It ran great this morning with the richer jets,this car was so cold natured and needed the choke on most of the way to work (5 miles) and now I can push it in when the cold light goes out.  The real test is when its about 20 degress outside instead of the mid 40s today,I think I am done tuning untill I remove the one head and get the guide repaired as I think I put the restrictors in the exhaust passage under the carb in the manifold and want to remove them and install a plastic carb spacer. I think doing that will allow it to warm up quicker and I get a better feel on how its running and then continue the tuning if it will be needed,I installed a 84 mustang dual snorkel aircleaner and hooked up the heat tubes to the headers thinking it would improved the running when cold but the motor was just too lean but now might be working.  I got the timing back to specs as it used to slightly ping but thats gone now so I think I am getting somewhere now.