Holley Help
Holley Help Engine is a stock LY6 with custom cam, GMPP dual plane intake, and headers. Carb is a box stock Holley 770 Street Avenger. Combo has about 5k miles on it and has run fine until recently. I started to notice a slight hesitation at tip in, most noticeable when the engine was cold, so I put a wideband on the truck and noticed the AFR would hit mid 15s on tip in. Checked the accelerator pump mechanism and it was fine, so I installed a .031 discharge nozzle on the primary side, stock was .025.
Now the AFRs jump as high at 17 at tip in and the problem is much worse. This is the opposite of what I expected to happen, so Im stumped. Any help would be great.
AW
Now the AFRs jump as high at 17 at tip in and the problem is much worse. This is the opposite of what I expected to happen, so Im stumped. Any help would be great.
AW
Accel pump is only for quick throttle movements. Get the AFR's steady with slow throttle movements first then tune the squirters last.
I fixed that problem with my double pumper with two changes. First on my carb I needed to open up the idle air bleeds two sizes and using one size smaller idle feed restrictor. You may or may not need to do that. My idle screws where at under half turn before I modified the IFR.
The other thing is to make sure you have 20 thousands of the transfer slot showing in both primary and secondary when the carb is at idle. You need to pull the carb off to check this. In my case with a stock LS2 cam a lots of idle timing forced me to back the idle speed screw out to the point where the butterflys were sitting on the stops. This caused the carb all sorts of issues when coming off idle. To fix this I set the idle speed screws with the carb off so exactly 20 thou of the transition slot was uncovered front and back. Put the carb back on and pulled timing below 500 RPM to get it to the idle speed I wanted. Cleared up my issue.
I fixed that problem with my double pumper with two changes. First on my carb I needed to open up the idle air bleeds two sizes and using one size smaller idle feed restrictor. You may or may not need to do that. My idle screws where at under half turn before I modified the IFR.
The other thing is to make sure you have 20 thousands of the transfer slot showing in both primary and secondary when the carb is at idle. You need to pull the carb off to check this. In my case with a stock LS2 cam a lots of idle timing forced me to back the idle speed screw out to the point where the butterflys were sitting on the stops. This caused the carb all sorts of issues when coming off idle. To fix this I set the idle speed screws with the carb off so exactly 20 thou of the transition slot was uncovered front and back. Put the carb back on and pulled timing below 500 RPM to get it to the idle speed I wanted. Cleared up my issue.
I took the primary bowl and block apart. Found a small metal shaving in the bowl. Dont know if that was the culprit or not. Cleaned out the pump passages anyway. Waiting for the weather to clear before I can test it out
Yea its weird. you put too big a squiter on there and you guage will real lean. I believe its bc the fuel is burning in the exhaust tubes. Something like that. I have learned that as well. lol. You will really run a much small squirter than you anticipate.
ok I have to back track here. This was very strange for me as well. with a 31 there was a slight off idle stumble but would bull through the low end of a gear, went to 37 and there we much less off idle stumble and it had a hard time pulling through the low end of the gear and would go lean, was really wierd. So I figured out it was because I was using too small a pump cam with the bigger squirter. Believe it or not, I'm runing 52/52 (its just what my car wants i guess) with no lean or rich spots on idle rev and only a slight lean spot pulling through a the low end of the gear, but it completely got rid of the off idle stumble and runs soooo much better. If I were you I would try stepping up to a 40-42 with a bigger pump cam like blue or green. and then slowly back down on the nozzles until you get your desired size.
I havent had much time to work on it lately. I decided I would get the jetting where I wanted it first, then fine tune the pump shot. So far Ive gone up two sizes primary and secondary and it needs more on the secondaries, still too lean at WOT. Ive got the new jets, just havent had time to install yet.
