Hesitation issues - need help

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mflynn

mflynn

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Tried to fix this myself but I can't seem to get it worked out. 318 with a Holley 670 cfm Street Avenger, CompCams 268 XE, dual exhaust, running original PCV but do not currently have the breather side hosed into the air filter. Original 904 trans (rebuilt) with original torque converter, B&M SuperShifter. Lokar throttle and kick down. Getting a little belt screech on startup, so I plan to tighten the alternator up. Idles nice, especially in park, and I have had it adjusted so it can idle at around 700 rpm in gear at lights, but now it's idling lower than that and I can't seem to get it back to 700 range when in gear. I've had a pretty consistent hesitation issue when I first step on the gas, which seems to hit right in the 1200-1400 rpm range. Sometimes this also happens when coasting at cruising speed and I hit the gas to accelerate. Most times I can get through this by "feel" - backing off a little then giving it some gas, or sometimes there is no hesitation if I really gas it through the 1200-1400 range. Sometimes though it stalls. Starts right back up but a real concern and PITA. I have fiddled with the idle mixture - when this first started I thought just getting it a little richer would do the trick - but I messed with it so much that I have lost track of exactly where I am at right now and am thinking of resetting and starting from scratch. I also thought maybe the choke was coming off too soon, so I have also backed the electric choke setting off so that it is at its limit. I should also note that it runs great on the highway at rpm approaching and over 2000. I'm now thinking this is either a vacuum issue or maybe there is something wrong with the secondary? Any ideas?

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Secondaries don't come into play that early in the RPM's so I doubt that's it.
I would be looking at accelerator pump shot adjustment from your description.

If it's worse when the engine is colder this may very well be what it is.
There is also something about the power valve that may not be right also, but I'm not well versed on that being an Eddie owner.
 
What is your initial timing at? I have the same carb on my 318 currently with stock cam. My air bleed screws are 3/4 turn out for the most vacuum, anymore and vacuum drops at idle. Also had to adjust my accelerator pump, it was giving too much fuel too soon off idle
 
I would try changing the power valve but only after you check the vacuum at your normal idle with the vacuum advance blocked off. When you changed your idle speed (down to 700) you probably changed you idle vacuum. The power valve lets extra gas into the primaries when your vacuum drops (when you accelerate). If the power valve is the wrong one it won't give the engine the boost of gas when it needs it causing a hesitation. Advance also has a great effect on idle vacuum. You can change your advance and put the vacuum out of range for the power valve. PVs aren't very expensive so it's a cheap fix.

treblig
 
All the above post offer a lot of useful info. You should be able to solve your issue with the above listed:
Ignition timing (initial)
accelerator pump adjustment (including the squirter size and accelerator cam)
Power valve
 
Id throw all your tinkering out the window, get a rebuild sheet and put everything back to spec including the idle screws (seat then give them 1.5 screws out). pretty much everything under the fuel line depends on the proper float level so thats what needs to get checked first. Time it then set idle, then start turning screws equally to set highest vacuum, then reset idle, back and forth until there is no change. Most Holley carbs come jetted right, out of the box even for slightly lumpy cams. PV should be about 2-3 inches below criusie reading. 6.5 is about stock. The old "PV should equal 1/2 the idle vacuum reading" is somewhat of a ballpark now. If you have a vacuum gauge you can watch in the readings at cruise. You want the PV to be 2 below cruise, anything less and your running rich off the PV unnecessarily.
 
I used to have a 670 street avenger. I also had a hard time getting it to idle at 700 rpm. The thing is, if you keep turning the idle screw to get it to idle at higher rpm you mess up the transition from the idle circuit. You can check this by removing the carb and look underneath it to see how far in you are in the transition slots. Holley recommends not being more into them than to make them look like a "square". So check that, set them to square and reset the idle mixture screws. Hook up a vacuum gauge and tune for best vacuum. If it still idles real low you can compensate by bumping up you initial timing (just keep an eye where it ends up in full). If it's still low you can compensate by opening up the secondaries just a tiny little bit. If I remember that screw is located underneath the carb flange and is tricky to reach. I got it to idle good without messing up the idle circuit. Also check for vacuum leaks before doing anything.
 
You also need at least an 1800 rpm stall converter and 3.25 gears or numerically higher and headers with the cam to get much benefit out of it.

An XE262h would have been far better for your app.
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The AF screws dont affect idle transfer slot, its the idle set screw that does that. Normally, when you are tuning the AF, you are decreasing the idle screws, not turning them up, but the 'square transfer slots' base line is good advice that I forgot to add. Like I said, most new Holleys are pretty good out of the box. I think the HP carbs have the secondary adjustment up top now instead of under the flange like the old ones and have 4 corner idle so there are 4 transfer slots to square up. If it is rich at idle, the idle air bleeds can be altered in extreme cases if they are screw in. Just give them a shot of carb cleaner to make sure they are open. If they are plugged, youll get a very rich idle that cannot be adjusted with the AF screws as there is no vent to dilute the idle feed circuit vacuum signal. Set it back to stock and start all over and keep a log of what you do so you can refer back to it if something doesnt work.
 
That is correct. I was refering to the idle set screw and not the idle mixture screws, but I might have been a little unclear.
 
Has the accelerator pump actuator cam been adjusted and fine tuned or replaced if needed?
 
Thanks for all the great advice, some of which may require further explanation/education. For now, here's what I've done: checked the float bowl (it spilled a small amount so it was a little high), pulled the float needle and made sure it was clear, dropped the float a little, replaced the fuel filter, replaced the coil and checked the timing. Factory spec for the 318 is 10 degrees BTDC - I was advanced beyond that, so I brought it back and actually on last check last night it was retarded a little (probably around 8 degrees BTDC). Wondering where the timing should be with the cam I'm running - any suggestions? (Also, barnett468 - I was going to go with the 262XE but as a newcomer to this game, maybe got "greedy?"). Took it for a spin and everything is running much better, although I got a couple of backfires when re-accelerating after slowing down a bit. This morning it was tough to start, then after it warmed up, wicked high curb idle in park (but not in gear). But hesitation is virtually gone and it cruises fine now between 1200-1400. I got new ignition wires and plugs that I'll probably put in this weekend too, and maybe replace the points while I'm at it. Then if things are still even the slightest bit funky, I'll start looking at all of your suggestions about the accelerator pump, power valve, transition slots, etc.
 
Looks from the picture the vac adv is connected to ported.Try hooking it up to manifold vac.(under front bowl).The idle will be higher ,back it down to normal try setting initial timing(vac. hose disconnected) to 12-14 deg.Also you might have to turn idle mix screws out further to richen it up to make it go in gear without dropping alot of rpm,the richer the less it drops with a perf cam.You cant always set idle mix screws by highest vacuum.
 
I have had it adjusted so it can idle at around 700 rpm in gear at lights, but now it's idling lower than that and I can't seem to get it back to 700 range when in gear.
Click to expand...

Did this change all by itself?
A gradual change is usually an A/F thing; like a vacuum leak, or a plugged air-bleed or crappy gas, or the secondaries not staying closed,or a faulty PCV, or sometimes; tight valve lash.
A sudden change is often slipped timing, or a blown PV.
You do know that the acc.pump linkage needs to be reset after almost every idle speed screw adjustment, right?

Assuming the ignition system is capable, then, to recap; Is/was
The hesitation from a gentle tip-in? If yes,then this is all in the low-speed circuit,which starts with the fuel level.
If the hesitation occurs when stomping it, that would be the Acc pump.
If it occurs under moderate primary only feeding, the then the PV may be too slow to open.
If it happens every time the secondaries open, well, that would be the secondary actuating spring.
10* initial is not enough for the 268 in a teener, but that will get worked out in the T-Port sync.Don't forget to limit the power-timing to 32 to 36 degrees
Yes there is a slim chance that the PV is leaking fuel.

AJ's guide to Transfer Port Synchronization
 
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I have my Holly vacuum advance port connected the same as "mflynn". That's where it's supposed to be according to this diagram:

Carburator Adjustment for Holley 4v 4160

Unless you're saying to connect it to the "full vacuum" port to change the way the vacuum advance is supposed to work???

treblig
 
10 degrees is a bit low on timing, clue was you said it was hard to start afterward. I run 15 initial and 34 total mechanical with my stock cam 318. Getting your initial timing and fuel level sorted first as suggested previously would be a good starting point. Writing everything down in a log as suggested is also a great idea so you're not chasing your tail and forgetting where you're at in tuning. Do you have a vacuum gauge?
 
So I put in new ignition wires. Reset the idle mixture screws all the way in and then backed them out 1 1/2 turns each. Got initial timing set to 10* BTDC, but need to redo it because I don't think I did it correctly. Anyway, ran it to the beach and back over the last couple of days, a total of about 70 miles. Starts but then rough and low idle, difficult to keep running. After about 10 minutes of driving - stalling, surging and a backfire or two - it runs great. No hesitation, no stalling, smooth acceleration from stop and at highway speeds, 700-800 idle in gear. Then when I park it idle is high in "Park", after an hour or so rest it repeats above scenario. So I'm going to make sure on the initial timing and advance it to 15*, then take a shot at the total timing and get it advanced to 34*. Hopefully that does it, but if not, I'll go through the other suggestions, although I'm also thinking about upgrading the PCV from stock.

BTW: my original hesitation issues were at "gentle tip-in." Tromping on it has not been the problem.
 
You're on the right track, but the stalling surging and back firing got me curious. I have to wonder if your timing is really where you think it is. If the balancer is starting to slip, your marks are off and can only be verified by a piston stop and knowing where top dead center is. The stalling and surging has me wondering if there isn't crap getting stuck in your float needle and floating around in the bowls. I have heard of the chrome on the inside of carb bowls flaking off and plugging jets. Surging can also happen if the gas tank is not vented. Maybe some of the gurus will chime in with some other suggestions. For what it's worth, I run the same xe268h cam in my 350 and it's initial and total is 18/34.
 
racerdan80 said:
You're on the right track, but the stalling surging and back firing got me curious. I have to wonder if your timing is really where you think it is. If the balancer is starting to slip, your marks are off and can only be verified by a piston stop and knowing where top dead center is. The stalling and surging has me wondering if there isn't crap getting stuck in your float needle and floating around in the bowls. I have heard of the chrome on the inside of carb bowls flaking off and plugging jets. Surging can also happen if the gas tank is not vented. Maybe some of the gurus will chime in with some other suggestions. For what it's worth, I run the same xe268h cam in my 350 and it's initial and total is 18/34.
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On the timing issue, that's exactly what I'll be shooting for this weekend, and why I don't think I got it correct the other day. As for the stalling/surging, I did check the float needle the other day before getting into the electrical and it was clean. That said, I had this carb apart a few months ago after my secondary was blowing gas like a geyser and it had the exact issues you described, but pretty much concentrated to the secondary bowl. So I replaced that with a new one. I'm assuming that the primary bowl is going to eventually end up the same way, and would otherwise think it could be causing the problem, but I don't know if that makes sense because once it warms up everything seems to flow well, and the primary bowl had not yet started to flake when I had it apart. I do plan on returning the carb to Holley over the winter when I take the car off the road (there was a manufacturing defect concerning the bowl liners, and they will be sending a new one back).
Thanks for the timing numbers on yours!
 
So, long story short, I sent the carb back to Holley just before Labor Day weekend, got the new one back from them today. Put it on tonight and everything is solved! Started right up, idled, took it out and no problems under any conditions - cruising, city traffic or highway. What a feeling! Thanks for all of your suggestions - I learned a lot through this process.

The only issue now is it's idling high and I've got to work out some issues with the throttle cable/pedal (not enough throw in pedal so not getting WOT). Piece of cake.

I did some searching around and found a fair number of threads about Holley issues similar to mine, and I'm thinking most are probably related to the same defect. Don't know if everyone who has experienced these problems knows about the defect, but certainly the word should be spread. And big kudos to Holley - they stood behind their product (that's what Made in the USA is all about brother!) and made it wicked easy to get my new carb.

Finally, a correction on my set up. I don't know what I was thinking but my cam is an XE262, not an XE268. Can't explain the brain cramp on that - I started a thread on this when I was trying to figure out which cam to go with a while back, got a lot of great replies and great discussion and the 262 was pretty much the consensus. I had initially been leaning toward the 268 but was ultimately convinced that it was too aggressive so maybe that's why it came back into my mind.
 
10* initial isn't enough timing. Turn it up to 16-18 and see how it reacts. Then tailor the mechanical advance to hit your total number.

I bet it has pretty significant rpm drop when put in gear from your p/n idle RPM.

Solved to some is a horrible tune up to others. If it smells like a fuel truck out the tailpipes at idle, it's not right.
 
Yep the Holley Street Avengers are a great 'out of the box' carb for the most part, I have one on a 245ci 'Hemi' straight-six in another Valiant and it loves it. Only thing I had to weak was the idle (a tiny bit) and the power valve - just one size earlier (higher vac) than standard ended up being perfect.

I tend to time at idle, then turn the idle down, then advance the timing again followed by an idle down adjustment. This usually gets you where you need to be. If it makes it hard to start when hot (say you took timing to 16BTDC at idle) then wind it back 2 degrees and see how you go from there.

Cheers - boingk
 
crackedback said:
10* initial isn't enough timing. Turn it up to 16-18 and see how it reacts. Then tailor the mechanical advance to hit your total number.

I bet it has pretty significant rpm drop when put in gear from your p/n idle RPM.

Solved to some is a horrible tune up to others. If it smells like a fuel truck out the tailpipes at idle, it's not right.
Click to expand...
You are right - it did have significant rpm drop during first drives after install. But I did advance the initial timing a bit from 10* to 14*-16* range, and richened it up a little (about a 1/2 turn out)
(I also adjusted the choke and fast idle too) and it's running about 800 in gear when warm, then about 1200 in park.

Can you describe how to "tailor the mechanical advance?" Thanks!
 
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