Jump to content
Server maintenance Read more... ×
4vxb

302c fueling issue??

Recommended Posts

So im running a holley 600 vac secondary on a slightly worked 302. Noticed a big difference in pick up from the 465 holley it was running. But have noticed that once the secondarys open up, it boggs down, pops and farts and just turns to crap really. Only once has it ever picked up its skirt and got off it ass when they have opened. Gave it a go shortly after, and it turned to crap again. Was thinking along the lines of a inadequate fuel supply from the standard spec fuel pump? Anyone got any advice? 

Share this post


Link to post
Share on other sites

i'd be checking the secondary actually is opening... (on My gemini with original carby the diaphram was leaking and not opening.. = starving for fuel and backfiring etc)
@gerg would be the holley guru at the moment, 

 

could also be ign breaking down. it's like deja vu.. i'm sure this was just covered recently or very similar at least

Share this post


Link to post
Share on other sites



So im running a holley 600 vac secondary on a slightly worked 302. Noticed a big difference in pick up from the 465 holley it was running. But have noticed that once the secondarys open up, it boggs down, pops and farts and just turns to crap really. Only once has it ever picked up its skirt and got off it ass when they have opened. Gave it a go shortly after, and it turned to crap again. Was thinking along the lines of a inadequate fuel supply from the standard spec fuel pump? Anyone got any advice? 


Firstly, check that the secondary bowl is sitting at the right float level. If you have externally adjustable floats, do the usual sight plug procedure, if no sight plugs, adjust the level up until you see fuel dribbling out of the boosters and wind it back down a full turn. This is a good ballpark method to start from, then you fine-tune from there. You may have to drop it more if you get flooding under brakes.

Next, check what secondary spring you are running. The standard one is purple from memory. On a warm 302, it shouldn't be opening before around 3800 rpm, and if it does, the spring is too weak, which causes bogging due to insufficient airspeed through the venturi. Install the next stiffer one and run it again.

If running a secondary plate (not a block with jets and power valve) there's not much you can adjust for jetting except drill out the metering orifices in the bottom. You really shouldn't have to do this if everything else is working right on the carby.

Make sure your secondary float has free movement up and down and that the needle moves with it and the seat housing isn't blocked. You'll need to pull the secondary bowl off to check this.

I very recently did up a mate's 600 vac and when I pulled it down, found a whole bunch of things wrong. Secondary throttles were closed too much, secondary linkage not pushing them open slightly at WOT, but most importantly, this carby had sat with bad fuel for a long time and the secondary booster feeds were blocked. Had to scrape them out with a drill bit. Also, a secondary air bleed was blocked, which would cause flooding once that circuit came on. Another thing, one side of the squirter was blocked.

This carby looked quite shiny and new from the outside, but things were pretty ugly inside.

I'd start from there rather than automatically blame the fuel pump, which rarely give trouble.

Sent from my CPH1903 using Tapatalk

Share this post


Link to post
Share on other sites

Cheers guys, float levels are fine and its had a holley rebuild kit throwen at it. There is a possible chance of a definite maybe that the secondary diaphram hasnt been touched though. Will have a look at it at some stage. I thought it may of been the stock fuel pump wasnt enough to feed it, or possibly the fuel lines too small. Will check the diaphram first though. 

Share this post


Link to post
Share on other sites

Strong possibility it could be an ignition issue too. Its only got a crappy old 40amp alternator, which was ony thrown on to get it running after i rebuilt the engine 10+ years ago. Have got a 100amp unit to replace it but just havent got round to swapping it over haha.

Share this post


Link to post
Share on other sites

There's a saying in old hot rod circles: "carb issues are often ignition issues"

A hole in the secondary diaphragm will simply stop them from opening. Shouldn't cause popping and farting.

Sent from my CPH1903 using Tapatalk

Share this post


Link to post
Share on other sites

Cheers bud. Will get off my ass and swap that alternator out for the 100 amp one and see what happens

 

Share this post


Link to post
Share on other sites

Its not running anything out of the ordinary. I do know that the headlights are dim and when the headlights are on, the alt light on the dash flashes on and off when i turn the indicators on. It was probably a 40 amp alt when it was new, think 39 of them escaped over the last 40 odd years. Noticed tonight when i had a look at it and found that one of the wires out of the back of the alt is pretty lunched so will be replacing that. 

Share this post


Link to post
Share on other sites

Im running a 750 holley on a warm 351 using a RP mechanical fuel pump and dont have any issues. I also run an ICE ignition sytem.

 

Only issue when  i put my foot down is the fuel needle follows suit and drops very quickly.

Share this post


Link to post
Share on other sites

Check the ignition system. Leads, cap, ignition module or points.... Sounds like the timing is going out at revs. 600cfm should be sweet on a 302. 

 

Check plug gaps, don't run leads 5 & 6 parallel to each other, cheap leads can crossfire. Tried running her on 98 PULP.?

Share this post


Link to post
Share on other sites

I had a similar story where I bought an off the shelf 600 vac sec to replace an old one and and things went backwards bigtime. I transferred the jets  over amongst other things but with my limited knowledge i couldnt get it anywhere near as good as the old carby. I eventually gave up and took it to a dyno tuner who got the timing 100% right and diagnosed an incorrect power valve. I cant remember the exact size that went in but it wasnt a small adjustment. I couldnt believe how far out the carby's stock power valve settings were at the time for a mild 302C I just assumed it would perform like the old one? The engine purrs like a kitten now it was the best thing I ever did.

Share this post


Link to post
Share on other sites
I had a similar story where I bought an off the shelf 600 vac sec to replace an old one and and things went backwards bigtime. I transferred the jets  over amongst other things but with my limited knowledge i couldnt get it anywhere near as good as the old carby. I eventually gave up and took it to a dyno tuner who got the timing 100% right and diagnosed an incorrect power valve. I cant remember the exact size that went in but it wasnt a small adjustment. I couldnt believe how far out the carby's stock power valve settings were at the time for a mild 302C I just assumed it would perform like the old one? The engine purrs like a kitten now it was the best thing I ever did.

Stock power valve is 6.5, on all Holleys except maybe Dominators, etc. so I believe. If you have good vacuum at idle and part throttle, say up around 17-18", then a 6.5 will come in too late, and will be even more noticeable if you've jetted down a bit from factory for a bit more economy. You'd need to go to an 8.5 or so to have the correct fuel enrichment come in under load. The rule of thumb on a street car is take your idle vacuum and halve it to get your PV rating. On a more performance-oriented tune, say with a lumpy cam with poor idle, go 2' less than your idle vacuum. This is because your cruise vacuum will likely be more than your idle.

 

As a point of interest, the 5.0 Mustangs in the 80s used a specially calibrated Holley that had a dual-stage power valve. The first stage cut in at around 11, and second stage was at around 8.5. this allowed nice lean jetting for cruise (emissions, etc) but was able to supply some enrichment at part throttle without drowning it.

 

Sent from my CPH1903 using Tapatalk

 

 

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×