Hello all, so the gm 350 is running great and I'm happy with it.. With the 2 bbl holley it has a smooth stead idle in park..here's the issue.. I swapped it out to try the Holley street avenger I have and it runs great and seat of the pants gain is noticeable for sure however at idle in park the rpms are jumping up and down by about 50 and it tends to be worse as soon as I start it up. The oil pressure moves about 3 psi with the rpm moving around I haven't made any adjustment just bolted it on. Its a 570 cfm vacuum secondary idle set to 700 and about 550-600 in gear the problem is either less or non existent once the car is in gear I've looked around for vacuum leaks and can't find any....any ideas what's going on here and is it okay to run like this everything else seems to be a okay it starts right up....also has a manual choke....thanks guys!
Thanks for the tip ill get the 6.5 and try it out...how would the power valve influence the way it idles with what I'm assuming is a lighter valve? Just trying to figure how it all works.. Thanks again!
Did you try adjusting the mixture screws first? You would only need to change the power valve if the motor idles at less than 8 inches of vacuum... Stock motor should be up close to 20 so the 8.5 is fine. Sent from my Moto G Play using The H.A.M.B. mobile app
I didn't know the mixture screws being out of whack would influence the carb like that but if it can ill Definitely tweek with the screws a bit...so would you think they're too rich or too lean causing the surge? And I do have about 18 inches of mercury at idle
Check out the Holley you tube videos on adjusting the secondarys. Watch them all. https://www.google.com/search?q=adj...ome..69i57.10247j1j4&sourceid=chrome&ie=UTF-8 You can also buy a little adapter to make turning the secondary idle screw real easy without having to remove the carb. Or, replace the OEM screw with one that you can adjust yourself with a small wrench or allen wrench.that you you
What manifold and or adapter did you have, take off, change, use????? Try richer on the idle screws, being lean makes a surge. Vacuume leak makes a surge too (because it's lean with a leak)
I take it your talking about this screw? They seem to always make for issue's. Most common is throttle shaft rocking on the screw (because screw is pointing up rather than down like primary and to sloppy of a bore for throttle shaft) making it hard to hold a steady rpm.
Hey guys thanks for all the responses ill be trying to adjust the mixture screws before I mess with the power valve and I'm going to triple check for a vacuum leak....and I'm using an edelbrock eps intake on a gm350 and I had a 4 bbl to 2 bbl composite 3/4 inch adapter under the old holley and it ran great which is why I suspect the new avenger carb. And how can I check to know if i have a sloppy bore that the throttle shaft rides in?(shaft on screw) does anybody have a diagram on this? I'm going to YouTube school on holleys now
Technically they're known as "idle mixture screws". They're adjusted to provide a smooth and steady idle speed and have little or no effect on mixture once the throttle is opened a bit. Make your adjustments with the engine at operating temperature with the choke fully open. Connect your vacuum gauge to a full manifold vacuum source and adjust for a high, steady manifold vacuum reading.
Is it a new carb? Do the basics before you start changing parts and messing with screws that seldom need screwed. Put your vacuum gauge on it and turn the mixture screws in until the idle drops and vacuum drops. Then back them out one at a time until it starts smoothing out and highest vacuum is achieved. Do both sides a couple of times to make sure it is idling correctly. Check your timing before adjusting the carb!! Set it to factory specs with vacuum advance hose pulled of the distributor and plugged. Make sure the vacuum advance hose is connected to the correct part on the carb too. Do the basics first !!! Sent from my Moto G Play using The H.A.M.B. mobile app
Different carburetors have different tweaks or adjustments. Anything that will bolt up will probably run, but that doesn't mean it's running right. Check and adjust each carburetor baseline. Start with the "mechanical zeroing", make sure everything physically moves freely, no binding when pedal is floored, butterflies vertical, and is set to the bench adjustment i.e. Mixture screws & float height etc. Fuel height in the bowl. (For example). It may be fine with one carburetor, for whatever reason a different carburetor it may end up being too high or too low. Fuel height is set by adjusting the float level. It is an important adjustment because it affects the operation of everything else downstream in the carburetor to some degree. If engine is pulling 18" to 20" of steady vacuum at idle RPM it's unlikely there are any external vacuum leaks.
Sorry for the late response its been freezing outside here for the last 2 days but today I turned the idle screws in and then with the vacuum gauge hooked up I tuned them until I got the most vacuum at idle and I made sure my vacuum advance was set to about 10 degrees before it was about 13 with the advance plugged so I set to 10 and adjusted the carbs idle mixture and its cured the problem! Thanks again to everyone for their help!
And you didn't have to buy a bunch of stuff you didn't need or take it apart. Always start at the basics. Sent from my Moto G Play using The H.A.M.B. mobile app
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