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Need input on Carburetor

HawK5

HawK5

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I have a new Jasper 350 with mild cam in my 1980 Jimmy, it’s got a 6” lift and 35s, sm465, and will soon have 4.56s, tires will eventually be bigger.
Previous owner had motor installed by a local chain that is notorious for poor work. They told him the carb needed to be rebuilt but that’s all I know. The carb is a Holley Truck Avenger 670 and seems to be having
a couple issues:
1. the electric choke works when it is cold but randomly kicks on when it is warm, I have disassembled and reassembled a couple of times and I think the tab that goes into the electric wound coil keeps coming out.
2. It occasionally diesels or has run on when I turn the truck off.
3. It smells rich and seems to have a flat spot in the middle of the power band.
Should I A: rebuild it and replace the electric choke with a manual. ($100)
Or B replace the whole carb with Edelbrock AVS 2 off road (part# 1915) ($500)
From what I’ve read reviews are all over the map. I intend to do all kinds of wheeling and running around town, sometimes I drive 100 miles on the hwy to get to my wheeling spots, so mpg is also important, not concerned with max HP, just enough for merging on to Hwy or the occasional heavy footed wheeling.
Fuel injection is out of my price range for now, so I’m curious what you guys think I should do. Rebuild the truck avenger or swap to an off road avs2 eddy.
 
Either way, new carb or rebuild you'll need to tune the carb to engine.
I haven't fooled around with a holly that wasn't 40 years old, the avengers are probably different.
First thing tighten the fuel bowls, they come loose after a few heat soaks from new. Second to cure the run on or dieseling lower the idle speed, counter clockwise on screw. If idle is to low attempt to adjust with air fuel ratio screws, and distributor timing.
Now the flat spot is an air fuel ratio inbalance at that particular throttle position. Accelerator pump adjust should be checked. Secondary opening timing. Main jet size, Holley have always been the rich side of the scale out of the box, if it still burns your eyes after tightening the bowls, 1 or sizes smaller on the main jets.
As for the choke if you can't solve the coil issue sure go manual.
 
eodcoduto said:
Verify the timing first before messing with the carb, that way it is isolated and you aren’t chasing two potential issues.
Click to expand...
I’m kind of inexperienced when it comes to tuning.. I have a timing light, what should I be looking for on base timing, and all in timing? And I disconnect the vac advance right?
 
Start at 8-12 degrees before tdc and go from there. Disconnect the advance and that will set your base line. With the cam, that adds another variable to both timing and the carb. You might want to find a local guy to help too.
 
eodcoduto said:
Start at 8-12 degrees before tdc and go from there. Disconnect the advance and that will set your base line. With the cam, that adds another variable to both timing and the carb. You might want to find a local guy to help too.
Click to expand...
Good info, but we should probably be slightly more specific as he said that he hasn't done much tuning.
(Yes, I know that Google can explain some, but I have the time to type right now. )

Disconnect the vacuum line to the can on the distributor and plug the line.
This may require turning the idle speed on the carb up to keep the engine running.

I would recommend starting at 10* before TDC on an unknown engine, which may have some carbon in it, even though I normally end up at 12*.

Once the timing is set, reconnect the vacuum line and reset the idle to a speed that you like. Usually 600 to 750 rpm.

Check for timing advance when you reconnect the line and when raising the RPM. All in should get you to 35 to 40 total if the centrifugal advance is working.

Then you should be able to go to the carb adjustments.
 
The engine has documentation, it’s a jasper level one 350, basically one step up from their base 350. However Jasper doesn’t give any cam specs and had no advise for tuning when I called them to try and find out details.
Check for vac leaks.. I have tried using propane in the past but never really experienced the dip in rpm or know how subtle it might be. Also running explosive gas around a running engine always makes me nervous. Any other ideas? I know they have smoke machines but I’d have to buy one or figure a way to back yard engineer one
 
If your engine is dieseling after you shut it off that sounds like the primaries are hanging slightly open, and continuing to feed a small amount of air and fuel after you shut the engine off. This problem on a Q-jet came from the throttle shaft bore being worn out, but with your carb I am not sure. You may want to see if your carb has a linkage step adjustment for the primaries, in order to close them all the way when engine is shut off. Also as mentioned tune the carb. There are some good YouTube videos on how to get all the right jet settings, and accelerater pump adjustments. There is a YouTube channel called JunkerUp by a guy named Chris Birdsong that has some plane English tutorials on jetting and tuning a carb correctly.
 
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Thank you I will check those out! My plan is to replace the electric choke with a manual, as I know there are problems with it. Watched a YouTube vid on how check if the power valve is blown, something like adjust idle air screw to see if engine rpm changes as it should, check for vac leaks with some brake cleaner, confirm base and mechanical timing. From there I’ll dive into the carb.
 
A blown power valve will cause rich condition, and was pretty common in older Holley's. I thought the issue was addressed in the Avenger
 
Yeah I’ve heard that as well, this carb is fairly new, 2018 I think? But I’ve also read some guys that say it can still happen. So I’ll see if that’s a possibility.
I think there’s a combination off issues at play, it runs rough and low rpm on a cold start with choke on, but after a minute or so RPMs increase to about 1500. Once it’s warmed up and the choke is off, it seems to run pretty good, but smells rich and there’s a lack of mid rpm power, then occasionally the choke starts kicking on again, and occasionally it diesels when I turn it off, but this seems to be somewhat rare.
 
If you do end up replacing the power valve, be gentle tightening the new valve, over tighten and the gasket can split. Split gasket is the same as blown power valve or worse. After a few heat cycles re-tighten the bowls. I haven't ever used them but I hear there is new metering block and bowl gaskets that are reusable. I have strong memories of tedious scrapping the gasket materiel off metering blocks. Might be why I haven't messed with a Holley in years.
 
Here’s my .02, I have ran 3 truck avengers over the years. They are meant to run at extreme angles off road and thus are set very rich out of the box. I personally think a 670 cfm is too much for a 350, my current 350 is running a 470 avenger and I also ran a 470 on an AMC 360. The only 670 I ran was on a 400 BB in a Dodge and I had to jet that down 4 sizes from factory. I agree to set timing first but I wouldn’t be surprised if that carb is just dumping fuel. Truck avengers do well at what they are meant for which unfortunately makes them a non street friendly carb.

Side note, if you wheel the truck you will hate the AVS carb, even with the off road kit it sucks.
 
Chickntrk said:
Here’s my .02, I have ran 3 truck avengers over the years. They are meant to run at extreme angles off road and thus are set very rich out of the box. I personally think a 670 cfm is too much for a 350, my current 350 is running a 470 avenger and I also ran a 470 on an AMC 360. The only 670 I ran was on a 400 BB in a Dodge and I had to jet that down 4 sizes from factory. I agree to set timing first but I wouldn’t be surprised if that carb is just dumping fuel. Truck avengers do well at what they are meant for which unfortunately makes them a non street friendly carb.

Side note, if you wheel the truck you will hate the AVS carb, even with the off road kit it sucks.
Click to expand...
To be clear there is an Avs, and an “AVS 2 off road” part # 1915 or 1916. I’ve heard lots of poor reviews on the Avs for off roading, and how to mod them for off road. But I’m not sure if the reviews are relevant to Edelbrock’s version of the off road carb.

I’m leaning towards running the truck avenger. I’ve run a tank of gas with Carb cleaner additive through it and it seems to be improving. I’ll ck timing and vac leaks to rule anything else out.
I’m still learning a lot about carbs. It smells rich AF. Is that a necessary evil for off camber wheeling? Or should I try leaning it out? As a side note i installed headers and it seems to have responded well to those. I assume it’s clearing the unburned fuel out of the combustion chamber sooner if it’s running rich. I also had a bung installed for AFR guage thinking that may be helpful for tuning?
 
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AFR gauge is definitely helpful.....the AVS 2 off road, is an AVS with the off road kit installed by edlebrock instead of you installing it. The Holleys are jetted very rich out of the box as part of helping them run better off camber, however they can be jetted down some and still perform well.
 
Ok an update: did a compression test, had ~120psi on all cylinders and verified spark, the number 4 spark plug was wet and wasn’t sparking, see the pic of the single plug. I dried it off and it started sparking. The fluid seemed to thick to be fuel, seemed like maybe coolant or oil, but it was pretty clear and coolant was 1.5 gallons low.. Also found the middle pair of intake manifold bolts on both sides were about 1/4 turn loose. Looking at all of the plugs, any thoughts?

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I’d throw some known good plugs (no autocraft junk) in it and run it some and pull them and check again. How are your plug wires? May as well do a full time up if one hasn’t been done. Do I see green on the electrode on #4?
 
The plugs and wires look to be low mile, probably installed new with the motor in 2020. I didn’t check gap ‍oops! and I smelled it but there were lots of competing smells due to an oil leak on the valve cover above and my dirty gloves. Which leads me to think coolant or oil, if it was fuel it would be a lot stronger. I did noticed an orange/rust color on a lot of the plugs, I assume this is from long periods of sitting and moisture accumulating in the cylinder? one of my buddies recommended a coolant pressure test and cylinder leak down test..
 
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