First I'd like to thank those of you who have assisted in getting this far. Unfortunately I'm not home yet....

I have a '73 L82 4sp with a Holley DP. When cruising around town, she is running fine and if I accelerate at a constant rate, she'll climb right to 5,000 RPM without any issues. However when I punch it, she'll climb to about 3,500-4,000 RPM then I get hesitation, sputtering and some minor popping.

I replaced the fuel pump with a Holley 110 gph as my fuel pressure was running in the 3-4 psi range so now she is running right around 6-6.5 psi and Holley says that this carb likes 5-7 psi range so I think I'm good there. I thought that maybe the secondaries were opening too soon so I tried a stiffer spring but found no appreciable difference. I've also moved my engine timing up and down and although it obviously affects the performance this issue has remained. (btw, also changed the fuel filter and confirmed my lines aren't kinked)

I had the carb off this winter when I replaced the valve springs but did nothing to it so nothing should have really changed. Although it's possible that this issue was present previously since I couldn't really push it above 4,000 RPM due to a bad clutch and bad valve spring.

I'm perplexed.....any thoughts/suggestions?

Jim

 

Is this a double pumper (DP) or is a vac secondary Holley? Double pumpers don't have a spring on the secondaries but are actuated by a linkage.

 

I would start by checking the advance curve - timing first, then carb.

What size is the carb? I have to wonder if potentially it's just a low fuel level in the carb - worth checking the float level before looking upstream for a fuel flow/pressure issue...

I'd agree that it's likley not a secondary opening issue - especially given where it's happening in the RPM range.

 

Spark plugs? My 68 427 would do this. Changed the plugs=problem eliminated.

 

The carb is the 670 CFM Avenger with dual feeds....my bad, probably bastardized their branding

http://www.holley.com/0-80670.asp

The plugs (AC Platinum) are new as are the wires (Taylor 10.4 MM Thunderbolt). I checked the level in the primaries and they're fine (not surprisingly). Have to check the secondaries today and am going to test the coil and check the distributor cap next.

 

OK, so it's not a DP (double-pumper) so secondary opening *could* be an issue. It might be worth trying a *lighter* secondary spring as it's possible that the secondaries aren't opening all the way when needed at the top end of the RPM range.

As noted, I would check the timing first...

 

You didn't mention it, so I am assuming you haven't done this yet. It's the first thing you should always do with just about any carb; Make sure the float level is set correctly. Take care of that and get back to us. In the meantime, check out these links. Good luck!

http://www.junkyardgenius.com/holley/tune01.html

http://www.nastyz28.com/~ericf/tech/htune.pdf

http://www.bob2000.com/carb.htm

Scott

Edit: Hey!! You added to your post while I was writing mine!! No fair!! LOL!!

 

Lars Grimsrud has written a paper specifically addressing stumble and hesitation issues with Street Avenger carbs. E-mail him at v8fastcars@msn.com and ask him to send you a copy. Don't be shy, he's a great guy to deal with. Ask him for the paper on tuning Holley carbs in general, too. One of the links in my prior post is an old version of that paper, but he updates them periodically, so ask him for the fresh version.


Scott

 

I had a similar issue that turned out to be a problem with the wire that feeds voltage to the coil in my 71 (points ignition).

My car has auto tranny. I could make it repeat the problem by power braking it. The wire on my 71 is a resistor wire that was breaking down, don't know if its the same on your 73.

Roger

 

How did you determine the resistor wire itself was the culprit? I suspect mine as well, even though it is a fairly new Lectric Ltd harness. Their engineering/tech dept told me it had to be my coil and would not tell me the specs on the wire or provide me with a replacement wire...only a new harness. I've changed coils twice. I even tried an entire known good stock vette points ignition system, including wires and new plugs. Haven't gone to the fuel system yet and really don't want to spend the money on a non-stock distributor.

What did you do to replace your wire?

 

Using a volt meter I measured coil voltage while power braking the car in my driveway. With car in neutral revving engine I measured 9 volts. With car in drive I would hold the brakes then step on the gas and around 1500rpm my voltage would drop to about 6 volts and car would start missing and back firing. I ran a new wire from the alternator with a Mallory inline resistor that gave me about 9 volts at the coil and it ran fine. Once I knew it was my wiring I started wiggling the wiring near the fusible link and it fixed my problem. The fusible link was partially burnt. If you need more help let me know.

Roger

 

Great catch; I have been bit by that one a few times...

 

Thanks, I had a lot of help from forum members to get it figured out.

 

OK, here's an update...I checked the timing. The instructions which came with my Edelbrock Performer cam says to start at 14* BTDC and although she runs best around 12, I tested it from 8* (where the original timing should be) to 18* and while there was a variance in how the car ran, the issue when pressing her remained so I'm thinking it's not the timing unless someone has other thoughts.

I also tested the coil (MD Blast 2) with an ohm meter against an identical one that was brand new and the readings were almost identical so I can rule that one out. (I have not checked the wires leading to/from the coil, although mine is a non-points car it's worth the check).

Finally, when I rock the car and look in the sight window, the gas level in both bowls appears to be right at the bottom of the window although admittedly that may not be the most accurate method.

I will send Lars an email as I'm thinking it's either something in carb or something in the ignition. I'm also going to re-gap my plugs on the off chance one is messed up. Any other thoughts are welcome!

 

Checking the timing isn't just static, but seeing what you're getting for mechanical advance as well...

 

OK....stupid question:

How do you check just the mechanical advance? Total advance at 3,000 RPM (vacuume advance not connected) is running around 30* with the initial advance at 12.

 

Not a stupid question at all. The vacuum advance isn't in play here since we're looking for an issue at WOT. Generally for mechanical advance (which you were checking with the vacuum disconnected) we're looking for 36 degrees before 3,000. Might be worth checking for sticky advance weights and looking to see if someone had installed a limit bushing on the advance pin. But I don't think that 6 degrees is the source of your bog as it's not that far out of the ballpark.

How did you check the total advance - timing tape or advance timing light?

I just always take a moment to check the timing before looking at anything else - it's saved me a bunch of time over the years

Sounds like the fuel levels are reasonable too...

 

Thanks...I checked the timing with a timing tape and a standard timing light.

I'm currently traveling for business and airplane time is always good for thinking. I think you're right, that this is timing/distributor related. I recall back in college I had a '76 Dodge Aspen with that ironclad 225 Slat 6 & a 3 spd OD. At one point, it ran fine except when you hopped on it at which point it bogged down and lost all power. Drove us nuts trying to figure out what was wrong and ultimately discovered that the distributor was off a tooth.....sounds eerily familiar.


I will be back in town on Wednesday and can look to see if there are sticky weights but I doubt there is a limit bushing as this wasn't a problem before I took her apart.

 

OK, now, don't be insulted, just making sure. Did you measure the advance at 3000 rpm, or did you see the timing stop advancing at 3000 rpm? The stock springs can have the curve go as high as 5000 rpm. This might explain why you only have 18* of mechanical advance. Like I said, don't be insulted, just trying to eliminate variables here.


Scott

 

Scott, I am one person you rarely have to worry about insulting!

That is a great question and although I can verify this on Wednesday, if memory serves me correctly, the timing stopped advancing somewhere between 2,000 and 3,000 RPM. Initially I had my wife rev the engine to 2,000 RPM and the total advance was around 28*, then I had her take it to 3,000 and it went maybe another 2*. After that every time I checked the total I was rev'ing it at the carb and regardless of how hard I pushed it she pegged around 30*......you're piquing my curiosity.