strange engine issue from 900-1400 rpm

[67 chevelle]

After watching the video , I would not rule out a valvetrain issue

[Bocephus69]

well, i am convinced that my issue is not timing related. even though i did make an adjustment that was recommended and my issue seems to be less "obvious", the problem still exists. it seems as if i need alot of initial timing to get it to idle nice (just by sound). at 600-700 rpm, if i unhook the vacuum advance and time it, it idles best at upwards of 20 degrees what it "sounds good". i know thats not the correct way.....so here are some numbers for you guys. it idles really nice by sound at 20-26 degrees, decent at 16 degrees, and 12 is about as low as i can go before the engine starts bogging down, losing RPM, and sounding like it wants to die. i loosened up the distributor and tested in multiple positions with no luck.


i asked for the help of one of my dads buddies over the weekend, and he agreed with you guys in saying "i had the idle too high, and it was perhaps not running on the idle circuit of the carb." so what we did was lower the idle until the engine wouldnt even hardly run on its own and tested out both a/f screws. this thing was idling well below 500 rpm. the drivers side screw made immediate changes for the good/better of the way the engine idled. the passenger side a/f screw still did nothing. all the way out, all the way in.....it made no difference in the way the engine idled or even sounded.


we took it for a drive, and he seen how nice it ran at cruising speed (or anything over 1/4 throttle for that matter). he is convinced i have issues with the idle to 1/4 throttle portion of the carb.


i did check for vacuum leaks around the carb. i did find one on the back side where the brake booster vac line hooks up to. i got that fixed, but it didnt change anything in regards to the issues i am asking about.


and getting back to the a/f screw on the passenger side. at this point, not matter what the idle speed/timing/after small vac leak fixed............that a/f screw does nothing. all the way out, all the way in....no changes. should my next step be to remove the carb and attempt to clean out that side of the carb?

[Clavo106]

ok my friend.my 2 cents.. if its engine in searching at idel i would looking for a Frist vac leak. then timing... now iv never rebuild a q jet but I have rebuilt plenty of motorcrafts... I few years a go I had similar experiences with a friends of Cadillac he bought a eldbroc carb and slap it on left it seting for a few years and ask me to get it running.. I got it fired up thru it in drive ran for a bit then died. when I thru it in reverse admittedly died.. I did everything . look and adjust timing about more time I could count sprark. vac leaks spark. coil dis.... and how many time I adjust the A/F screws a bout a hundred time it did not change a thing.... I finely gav in and told him if he had the original carb he said he did.. so I slap that bad boy on and worked solid. drove great.. not saying it would work for u but im thing looking at that carb another time would not hurt.. good luck..

[Bocephus69]

i removed both a/f screws.


drivers side (the side i know works) - if i blow air into the a/f screw hole, it will geyser fuel out of a small, vertical hole on the front of the carb. whe blowing carb cleaner into the screw hole, i get a light mist of vapor inside the carb below the choke flapper.

passenger side - when i blow air into the a/f screw hole, i dont get a geyser, but i get a mist inside the carb. spraying carb cleaner into the a/f screw hole yields about the same results as the drivers side....a light mist of vapor inside below the choke flap




i took some small wire and cleaned out what i could. resprayed air, resprayed carb cleaner, put the a/f screws back in and i have no change at all.



i talked to another guy that asked if i replaced these when i rebuilt the carb. i said no. what are you thoughts? http://quadrajetparts.com/idle-picku...ice-p-291.html

[Bocephus69]

Well I got the carb completely apart and cleaned before I had to go to work this evening. I found nothing obvious, but still used a lot of carb cleaner and compressed air. I did a little research about float height, and plan to set/check it tomorrow before reassembling the carb

[Bocephus69]

well guys, i got this 350 purring like a kitten. as i mentioned above, went through all the fuel/air passages again with carb cleaner and compressed air. i didnt find/see anything that threw a red flag at me, but who knows....i could have had a blockage someplace.


one thing i did do some research on was float level. i found some interesting info. i found that pre 1975 carb float levels are set at a different height than 75+ models. well, when i rebuilt the carb, the kit's instructions said to set the float at 5/16ths (.312).


now, the 350 in this pickup was a donor from a 76 caprice. the info i found online suggested setting 75+ q-jets float in the .400-.410 range.....so thats what i did.


i fired this baby up, and knew right away something changed. constant rpm idle, less engine vibration. i put it in gear with the foot on the brake, and it runs PERFECT with a load on it. no more vibration, no more resonating sounds in the cab that make your ear drums hurt (haha).


i set it for 35-36* total timing, which put me at about 11-12* initial. the idle a/f screws are about 2.5 turns each (and yes both work now). i love it when things work out as planned

[Sklptrljay]

Always nice when an annoying problem like that goes away!