Quick help needed: Holley stumbles after leaving line

[v8astregt]

Sorry for being last minute here, but I just got back from a day at the track, and am going for another day in the morning. Gotta get all my $160 worth!!!!!!!!


Anyways, my chassis is doing very good this time. The only difference otherwise from the last time I ran probably March, is that today it was well over 100* and in the middle of the day, rather than the last time being 25-30* cooler and at night. Anyways, it'd doing well except after I leave the line, it'll get to around 3-4000 RPM's and it just bogs, and falls on its face for half a second. It'll catch after that, and run fine for the rest of the run. It'll also bog during the burnout after I shift it into second.


Late notice I know, but any ideas? My thoughts are that it might be related to the secondaries kicking in? Maybe the spring is too soft? Weird though, I've NEVER had a bogging issue before. Could it be somehow related to the change in temp/air density?

[TopNX75]

If you are saying it "stumbles" just after leaving the line, and you don't already have jet extensions in the secondaries, I would start there.

When you leave hard, the inertia pulls the fuel in the secondary bowl "away" from the jets, causing fuel starvation just until the fuel sloshes back. They sell little extension pieces that simply tap onto the jets inside the secondary bowl. It's an easy job to do but sometimes it requires replacing the float also. If your float already has two notches in the bottom, it is designed to allow room for the extensions and will not need to be replaced.



Hope this info helps!

Happy vega-ing
Sean

[v8astregt]

Makes sense. I'll play around with it today.

But I hope I already found the problem. I was swapping my secondary vacuum actuator with a quick change one from another 750 I had laying around, and I found the vacuum plug on the metering block was barely held together. I know that port is used under hard load, so hopefully that is the problem. We'll see, I'm gonna hop in the shower, then head off to the track again.

[HI WINDING MONZA]

Old gas in the tank in AZ ( only a month old or more ) does the exact same thing. Try some fresh fuel. Its happened to my Monza at the track , just a little new fuel and it does the trick. Doug in Az

[v8astregt]

We'll see how it goes later. It was better today, but then again I did replace the vacuum plug that was completely destroyed.

I did run 4-5 passes yesterday, and I started with 3 gallons of pump gas, a few months old, then I added about 3 gallons 110 Sunoco. It seemed to bog all the same during the runs I got in yesterday. I'm not sure today since I couldn't hear anything with the cars I was running against. I'll freshen up the carb (been 4 years), and wait till it's cooler, and try it again.

[BillPappy]

The vacuum secondary spring can weaken over time. Did you do something with your timeing? advancing to fast?
Did you change your Positive Crankcase Ventilation System? If carb was dealed in before. More vucuum will cause
a Leak and mid lvl stumble.... With that car and engine Im surprised your running vacuum secondary.

[v8astregt]

The car was untouched since I last raced it in May. I did change the timing, I advanced it a little after 3 runs Friday... it had the bog before and after.

I'll mess around with the assortment of vacuum springs sometime when it gets a little cooler out.

I do have a vacuum secondary carb on there since we have about 4 of them laying around. Don't have any extra double pumpers though.

[Sirshredalot]

Check your filters too...sediment that gets sucked into a screen type filter can really restrict flow...and cause a bog.
Do you have a fuel pressure gauge mounted where you can see it when you launch?

Jet extensions are a good idea, and worth looking into, even if thats not the problem right now anyway.

Vacuum leaks can be all kinds of fun to chase....How do your plugs look?
What kinda ignition module do you have? A failing ignition module will also cause a mid range stumble...and they dont have to have a reason to go out either.

4 years is about time for an overhaul anyway...at least a gasket kit and new diaphragms...if you do it, do yourself a favor and get the reusable nylon bowl washers, viton end transfer tube seals, and nitrophyl floats.

When you get it apart look for corrosion in the main jets, once they sit a while gasoline and alcohol with react with the brass and can cause a chalky white build up...Ive mostly seen it on marine carbs( a little far fetch for you climate eh?). It could still happen though.

God bless
-Shred

[1fourosix]

Have you tried increasing shooter size? It worked for me - 75 monza,406,th400,2.93 10bolt with drag radials-so far 12.20-115mph and its driven to the track

[chevyart]

heyv8astregt last guy is probably right. first and most sensible place to start with a bog off the line is the shooters. you should have minimum of size 31s in there for race applications. check your floats and make sure your fuel pressure is ok. that should be a good place to start to check for your problem.. good luck chevy art