Changed my Jets and now it Bogs at Cruise/Off-Idle

Langadorf · 04-13-2003, 10:03 PM

I recently put some race fuel in my tank and the plugs were reading leaner. I changed the jets (up 4 sizes) in my Holley 750 DP to try to optimize WOT performance. I didn't touch anything else. Now, WOT performance is much better, but there is a strange bog at both cruise and off-idle. I can be cruising along at a steady state, and intermittantly there will be a bog. Also, everytime I gently come off idle easing away from a light, it bogs. It didn't do either of these things before the jet change. Any ideas on what is going on?

L79vette · 04-14-2003, 09:54 PM

Sounds like your power valve is opening and closing intermittently.

Bearcat · 04-19-2003, 10:17 PM

4 JET SIZES??? :eek: :eek:

That is WAY too much. When you get close on your carb jetting, you change it by only ONE jet size, and then vary other things. Consider the jet size as the "coarse" adjustment. Then the squirter size and cam lobe selection as the "medium" adjustment, and the fuel mixture screws and air bleeds as the "fine" adjustment. And do not play with air bleeds unless you're a professional.

What was your carb doing before? If you were only *slightly* lean on the plugs, and it was stumbling at full throttle, then you probably did need a jet size enrichment, but only one step at a time. Try dropping back down 3 jet sizes. You'll probably still run good at full throttle. Punch the throttle and see if it hesitates. If so, jump up 3 steps higher in the squirter size. Then adjust your fuel mixture screws all the way in, and back each one out about a turn and a half. Hook up a vacuum gauge to manifold source (NOT ported vacuum). Adjust mixture screws evenly for an increase in vacuum reading.

Now once you get part throttle operation running nice, with no bogs or hesitations, then go out and do a full throttle run and immediately shut down. Pull your easiest plug and read it. It should be brownish tan, or a little gray. If it's black, it's too rich. If it's dark but not black, it's a little rich, but that's safer than a lean condition.

Try these steps and get back to me.

sinistervette · 04-20-2003, 04:55 AM

4 sizes is not all that major, just depends on what ones you did. If you changed all 4 of them by 4 sizes then yes that is major even just the front 2 by 4 sizes would be major as well as unneccesary for your condition, but 1 front 3 rear or 4 in just rear would be an ok adjustment. If you had good driveability before, just upper rpm/wot issues you should concentrate more on the secondary side instead of primary, and also consider the possability of your powervalve closing as your vacuum builds back up at higher rpms. Go back to your origional settings, and install a vacuum gauge on it so you can monitor engine vacuum during your wot run. Make sure that you are not building enough vacuum to close the powervalve. The reading on the gauge should be at least 1" vacuum less than the rating of your power valve. If you are not building the vacuum, then change just the rear jets 4 sizes to equal the change that you have already made before and found out wot was much better. Now your driveability should still be there as well as the wot performance. If you are doing this at the track, take some extra time trials and play with the secondary jetting to get your optimum mph. Does your 750 just have primary idle mixture screws or is it 4 corner idle? If it is 4 corner, readjust your idle mixture screws after the jet changes, and make sure if it has a secondary power valve or not. :cheers: