Holley experts help please...

[jlkendri]

I'm trying to adjust my 770 street avenger. Engine is 351w/435hp from engine factory. Victor jr. heads and victor jr. single plane intake. Cam is Comp cams XR288rf-hr10. I have done LOTS of reading on adjusting this carb. But at idle the vacuum gauge is jumping all over the place between about 12 and 8 and I can't tell much from it. I think I'm running a little rich at idle and cruise speed. I have the idle mixtures 1.5 turns out with a 6.5 power valve. Does someone have a similar setup they tuned on a dyno or with O2 sensor? Or, does someone have enough knowledge to set me in the right direction? I can't set the idle mixture with the wild vacuum reading and I'm wondering if I would be better off with a 4.5 power valve or so. With a low vacuum, I could be leaking gas past the power valve at idle. Valve is not torn. Thanks alot!

[Bob Taylor]

My best answer is call the engine factory they usually dyno test everything . Maybe they can give you a baseline?

[FFR5452]

You shouldn't have had to adjust anything unless you are at a high altitude like Denver. Then you're going to have to lean it out quite a bit.

Is the engine running poorly or are you just fiddling with it? The street avenger is a very simple carb to adjust. Set your floats first. Then Set the idle air screws to the highest idle speed. Don't worry about a vacuum gauge. Now, adjust the idle speed. Now test drive it. If running rich, check the choke operation. Make sure it is opening completely.

But you probably will end up swapping it out for a double pumper when you're ready for more power and better throttle response.

[jlkendri]

The engine is running poorly AND i'm fiddling with it, haha. Acts like it's running rich at idle and cruise. Runs great at wot and over about 2500 rpm. I took it apart last week and cleaned it and all looked well. I just wonder if a 4.5 power valve would be better than a 6.5 with my low idle vacuum. I will adjust for idle speed instead of vacuum. I checked the choke and it's opening all the way. You can tell it's a little sluggish at idle and cruise and the plugs are trying to foul again, just changed them. It was very responsive when I changed the plugs last week, now it's sluggish again. I looked at one today and it's looks fairly clean but a little wet (gasoline). I think you are right on the double pumper. Thanks Jim.

[Ray]

I had a 670 on my 302 and was over-carb'ed (same symptoms as you're having.) That, along with some vacuum leaks (header gaskets) was causing a lot of problems. Once I got the proper carb (dropped down to a 570) and fixed the vacuum leaks, the carb was mush easier to dial in.

Not saying that this is your trouble, but you might want to check them just to eliminate them as a trouble spot.

Ray

[Avalanche325]

Is the idle surging? A power valve will do that. It is easy and cheap to change it. Just make sure you don't have any vacuum leaks first.

[jlkendri]

not surging but doesn't like to exactly stay put. I guessed this was from it loading up.

[Frank Messina]

I went through this same drill. You need to figure out where the excess fuel is coming from. After plugging both power valves (I have an HP750) I was able to isolate the excess fuel as coming from the primary needle and seat which was sticking. I tried cleaning it but finally gave up and just replaced it. Once that was done I was able to set the idle mixture and got a nice steady 14.5 In vacuum.
Frank

[johnnybgoode]

Have you checked the T-Slot (Transition Slot) opening? You will have to take the carb off to check it. If you do a search on T-Slot you should find a bunch of information on the subject. If the T-Slot is uncovered too much the carb will run very rich and the Idle Mixture Screws will be pretty ineffective. Scott

[CS14]

Originally Posted by Ray

I had a 670 on my 302 and was over-carb'ed (same symptoms as you're having.) That, along with some vacuum leaks (header gaskets) was causing a lot of problems. Once I got the proper carb (dropped down to a 570) and fixed the vacuum leaks, the carb was mush easier to dial in.

Not saying that this is your trouble, but you might want to check them just to eliminate them as a trouble spot.

Ray

As Ray has posted, I also think that carb is little to big for your engine, take a look on holly.com and put in your set up in their calculator. I have a Holly 600 on my 302", it runs perfect.

[FFR5452]

Where are your floats set?

[Cone Basher]

IMHO the best thing you can do is get a mechanical secondary carb. My Roush 402-R motor came with a 770 street avenger carb and had the vr. jr single plane intake. I fiddled with it for a year, changed plugs, jetted it down etc. It was constantly loading up but ran great once the revs came up, i think that was caused by the combo of the cam and the single plane intake. I swaped it out for a Holley HP pro series carb and it is now fine once it warms up since the HP carbs have a milled choke horn. I would recommend a DP carb for a light car. I'm running a 650 CFM on mine and it works fine for Auto-X since I added road race floats. Be consevative on the cab CFM if you want throttle response.
HTH
CB

[jlkendri]

floats are set right (where fuel is level with holes). Wish I had a wideband, that would tell me a lot.

[jlkendri]

ya, if i had it to do again, i would get a mech. secondary. Just for kicks, i did holley's carb selector and it said a mech. secondary would be the best match for my car.

[FFR5452]

It sounds like the carb is adjusted properly. Now, Focus on why that vacuum reading is jumping around so much. Ask Gordon, he might have a clue. Too many reasons for a vacuum fluctuation to list.

[jlkendri]

that's funny you mentioned that. I was wondering the same thing, if vacuum wildly wagging back and forth from 7-12 was normal. I couldn't even think about trying to read that vacuum gauge. Maybe I have a bad vacuum leak. That might explain the carb running rich at idle.

[FFR5452]

A Vacuum leak would lean out the mixture as it would add more air to the mixture.