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ARGH! Freaking carb issue!

USSkoval

USSkoval

Thornbirds look cool... Yeah, I said it
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I'm at the end of my rope with this Holley 670 truck avenger. No matter what I do there are drivability issues, and I haven't even left the driveway yet. Since I got it, there was an issue under hard throttle, where it would spit and sputter. It also ran rich while idling. I took it apart to inspect it, everything looked OK, so I blamed those issues on the single-plane intake and the cam that had just bit the dust.

Fast forward to today. New mild cam installed, along with a Performer intake installed. It was hard to get started, then it ran rich and sort of "popped" lightly out the exhaust for the first minute or so. Then it cleared up and the rest of the break-in period went well. Now it still runs rich at idle and starts to load up. Blipping the throttle clears it up temporarily. I CANNOT even get the idle adjusted properly. With it set a little fast, vacuum is around 17 or 18. If you turn the idle screw down a little, there is a dead spot and the idle does not decrease. Turn it a little more and it decreases too much and the vacuum drops down to around 10. The idle-mixture screws don't really change the vaccum, unless you screw them all the way in, then the motor starts to die. Float level seems to be OK. Why can I not even perform a simple task like set the idle? What is wrong here? I'm so sick of screwing with this thing:mad:
 
First make sure your timing is right. Second, make sure your choke is wide open. Now adjust your idle mix screws all the way in keeping an eye on how many times you had to go around. If one goes 2 and the other goes 3 full revolutions, back them both out to 2.5 turns and now they are even. Start it up and turn them both in 1/4 turns at a time until it starts to falter, back them out 1/4 to 1/2 turn. Clear out the throttle. Adjust the idle speed screw till you have approx. 700-800 rpm's in park.

All this is best to do with the engine warmed up to operating temperature. Don't burn yourself.

None of this will help you if you have a vaccum leak. Judging by your readings you don't, just throwin' it out there. Also assuming the float bowl levels are good and you have appropriate fuel pressure.

This is how i've always adjusted a Holley and most of the time they have run pretty good. If you have nuked the power valve it'll stumble when ya mash the pedal. Is the carb new or used? What jet size?
 
The carb was purchased new last year by the PO. When I originally took the carb apart to inspect it, the idle mixture screws were adjusted a little uneven. To start, I set the 1 1/2 turns out. They do not really change how the motor runs until they start to bottom out. The timing should at least be close, I won't be able to check with a light until tomorrow. The choke does open, and fuel pressure is not too high. I have the vacuum advance hooked to the timed port, should I try hooking it to manifold vac?
 
USSkoval said:
The carb was purchased new last year by the PO. When I originally took the carb apart to inspect it, the idle mixture screws were adjusted a little uneven. To start, I set the 1 1/2 turns out. They do not really change how the motor runs until they start to bottom out. The timing should at least be close, I won't be able to check with a light until tomorrow. The choke does open, and fuel pressure is not too high. I have the vacuum advance hooked to the timed port, should I try hooking it to manifold vac?
Click to expand...

No, not to full manifold vaccum. Keep it hooked to ported vaccum. If you hook to full manifold vaccum you will have vaccum advance at idle.

Sometimes when adjusting the idle mix screws it can take a couple minutes before there is any effect. Just went through all this on my Demon carb which is very similar in tuning to your holley.

When I set the idle screws I had to adjust 1/4 turn on both sides and wait a bit and clear it out, then repeat until they were getting in there pretty deep.

The whole process is somewhat fluid. Adjust the idle mix and reset the idle speed. Check the timing. Now everything is closer. You can fine tune it some more by repeating the whole process. Just keeping working it in till you can't make any adjustments without going the wrong direction. I need to go at mine again and get it all closer again. Keep working it you'll get it. Small changes, one at a time.
 
Checked the timing, it was way retarded:doah:. Got that taken care of, adjusted the idle speed, idle mixture, timing, etc. Vacuum is around 16-17 at idle now, and idle seems smooth. All that is good now, but there are still the same issues that were present when I got the truck. Well, maybe not all of them, as I haven't driven it yet. It still seems to run a tad rich. For instance, if I drop it in gear, foot on the brake and lean into the throttle, it starts to miss like it's starting to foul out. Put it back in park, and you can tell it is clearing out when you rev it, then it seems OK again. Am I missing something? This 670 carb shouldn't be too big for a mild 350. I have a Holley 750 vac secondary on the 327 in my Impala, and it worked fine right out of the box (literally). What gives?
 
After a short drive, I'm happy to report that the previous heavy load/upper RPM issue is gone:D

Now I'm just left with this one last issue. It seems to run lightly rich at idle, and rich at very light throttle (like driving down the road). The exhaust currently ends under the truck, and the fumes that come in remind me of my lawn mower. For being a mild 350, it seems to pull very well, considering the 3.08 gear and the 33inch tires. What should I do next? Go down a jet size on the primaries?
 
USSkoval said:
After a short drive, I'm happy to report that the previous heavy load/upper RPM issue is gone:D

Now I'm just left with this one last issue. It seems to run lightly rich at idle, and rich at very light throttle (like driving down the road). The exhaust currently ends under the truck, and the fumes that come in remind me of my lawn mower. For being a mild 350, it seems to pull very well, considering the 3.08 gear and the 33inch tires. What should I do next? Go down a jet size on the primaries?
Click to expand...

You might try dropping a couple sizes in your primaries only first. Before doing that, I'd go back through the adjusting the idle mix one more time just to be sure.

Don't lean it out too much. Only way to really know for sure is to pick up a sniffer for the tail pipe. Can't remember the name of the device... Stoichiometric meter... Something like that.
 
i too had tons of problems getting my truck to run....

my holley had to have the jets changed... i will have to dig up my old threads to find out what i put in...
but i changed the jets and acc pump.... also my timing was super retarded...

i would set the timing and then the truck would idle at say 2800rpm.... no matter what..
then i turned the timing down to 800-900 and drove it... not knowing there was a high idle screw.... found that later.... adjusted but still had problems.... 4v to choke from a 12v source... so i got mad and ripped every wire out of my truck... currently for the last 9+ months I have putting off putting in my new ez wiring 21 kit.. that i am using the factory harness for too...
hopefully gonna work on that this weekend....


anyway lets start here..

http://www.holley.com/types/Accelerator Pump Kits and Service Parts.asp



everyone i know that has got a truck avenger to work said they had to replace the jets and acc pump cam...

medium%20pump%20cam%20intro.jpg



so i bought this
http://www.holley.com/20-12.asp

medium20-12.jpg


so i will leave that for now and try and find my old post on what I did...


oh my motor specs...

1991 caprice police L05 roller motor
1996 tahoe vortec heads
weiand power plus+ vortec intake
holley truck avenger 670
comp cams extreme 4x4 cam (small one .457" lift idle-1500rpm)
msd-6al and timing control kit
 
Try changing the power valve. Your symptoms sound exactly like a bad power valve.
If it is bad you will be rich at low rpm all the time no matter what you do.
Also
If the fuel level in the float bowls correct?
 
RootBreaker said:
here is mine...

.35 discharge nozzle and black acc pump cam recommended by holley

http://coloradok5.com/forums/showthread.php?t=195411
Click to expand...

The .35 nozzle has already been installed. I think the orange pump cam is what's installed, IIRC. Your motor specs seem to be close to mine, but you have better heads.

Thunder said:
Try changing the power valve. Your symptoms sound exactly like a bad power valve.
If it is bad you will be rich at low rpm all the time no matter what you do.
Also
If the fuel level in the float bowls correct?
Click to expand...

I thought a bad power valve would cause a hesitation when the secondaries open? When I had the carb apart, I put a hand-operated vacuum pump on the power valve. It seemed OK.

The thing runs pretty well overall. It only seems slightly rich at idle. It's just cruising at about 15-20 mph, it will start to load up a little. Just blipping the throttle a little here and there keeps it clear. If I romp on it, it pulls nice and smooth all through the power band.
 
I thought a bad power valve would cause a hesitation when the secondaries open? When I had the carb apart, I put a hand-operated vacuum pump on the power valve. It seemed OK.
Click to expand...

Usually a bad power valve wont cause a hesitation unless it is stuck closed.
More common for them to leak vacuum and be open at idle.
Power valve is held closed by vacuum. When the throttle is opened manifold vacuum drops allowing the power valve to open and richen up the mixture. If the diaphragm has a hole in it or the PV is the wrong size it can open up and cause a rich mixture at idle/low rpm.
It is possible your carb has the wrong power valve. They are sized according to vacuum at idle.
A backfire thru the carb will usually take out the power valve. The newer carbs have a check ball that is supposed to protect the PV but it does not always work.
 
The saga continues:mad:

I put some gas in the Blazer today so I could take it for a longer drive and make sure everything was OK (besides the minor issue it had). I made the short drive into town to the gas station without any troubles at all. I put gas in it, when I pulled out of the lot, there was a real fast stumble and then it popped once out the carb and immediately died. Fired it back up, revved it a couple times and started to drive off. Then it started loosing power, running a little rough and had what felt like a dead miss whenever the accelerator was pushed. At this point, I was seriously PO'd at that truck avenger so I drove back home and pulled it off.
On went the 670 Street Avenger that I had gotten from a buddy. It had been screwed with and sitting for a while, but it was worth a try. It behaved a little better, but not by much. However, the rear float level did not want to adjust and I think the needle was not seating completely. So I pulled that carb back off too.
So then I installed my trusty old Holley 750 vac secondary. It was on the 327 in my Impala, which was running fine a year ago (sitting since then). Of the three carbs, this one works best, by far. It ran the smoothest, but still ran rough under heavy acceleration. The more I drove it, the worse it became (but still was fine at light throttle). Oddly, there was now smoke coming from the left exhaust pipe, that seemed to somewhat go away.

I have come to the conclusion that there is more than carb issues going on here. But what the heck could it be? It has crappy old rotted mufflers on it, could one of them possibly be plugged up? It idles fine, but misses under heavy accel.
 
I just started tinkering with carbs....I just went through a bunch or problems with my truck avenger...mine had a 28 nozzle and a orange accelorator pump cam also the black secondary spring. i bumped it up to the 35 nozzle, brown accelorator cam and the larger yellow spring in the kit.( pump cam kit from aed 15.00, secondary spring kit 12.00).......mine had many of the same symptoms....after i set it up 5 or 6 times messing with it, calling holley, i adjusted the accel pump linkage to .015 with a feeler guage and at that the gas squirts emidiately........
check your power valve if the boosters drip it will load up and hesitate, i did alot of reading and asking around mine runs that wild that in 3rd i can stand on it and spin the 39.5s...dont give up...offroad its awesome....
 
I'll give it some time, maybe I'll try and get the TA to work out, but probably not. I'm just too sick of that thing.

My crusty but trusty 750 is working OK, but there is still what feels like a constant dead miss under heavy accel. After driving a minute or two, it gets a little worse. The whole ignition system is relatively new, but maybe it needs new plugs/wires again? I sawed off the mufflers since they need replaced anyway, and the problem still is there. There is a little more power now though.
 
If you have the same or similar symptoms with different carbs, maybe there is a different issue. Fuel pump or sock filter in the tanks or something along those lines.
 
cybrfire said:
If you have the same or similar symptoms with different carbs, maybe there is a different issue. Fuel pump or sock filter in the tanks or something along those lines.
Click to expand...

Yeah, that's what I'm thinking now, but what?

About everything is new or recent on this truck. I wouldn't think it is a fuel issue, the pump is new, I installed a known good working carb, and the previous carbs have proven that there is plenty of fuel flow.

The distributor, cap, rotor, plugs, wires are not very old. I suppose it could be a bad wire or two, as a couple are close to the headers.

In general, what would cause a miss under acceleration, but runs fine under light load?
 
Just a long shot here, but maybe your vacuum advance is sticking. I've seen a couple vacuum advance canisters go bad. I had an 86 K-10 4x4 with a frozen distributor and it would buck and snort under heavy acceleration but run smooth at idle and cruise.

Get a timing light on it and rev it up with the advance hooked up to see if the dizzy is advancing.
 
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