Ok, rekitted a 600cfm vac sec Holley on a 302W, and been struggling to tune it. Runs perfect with car sitting static, revs happily all through rev range but when driving it hesitates badly just off idle, and backfires thru carb, so you have to very gently accelerate up to road speed. I've tuned as per the book - steps so far:
Timed at 12 deg btc set idle mix around 1 1/2 turns out each side adjusted idle speed set float levels rechecked idle mix and running at around 18-20 In Hg manifold vacuum Set accelerator pump so comes in ok, and has 15 thou clearance at full throttle.
Why would it misbehave on the road, is it possible there's a vacuum leak occurring under load! Or could the secondaries be coming in real early or something! Have tried enriching mix, lowering float bowl levels etc etc but no real results. Looking at getting down to a dyno soon through frustration, but this would annoy me as the old Holley is usually pretty simple, and I am cheap. Any ideas!
extrayda,
Feb 22, 5:44pm
Are you sure it's the carb! Could be ignition, have you check plugs / leads / rotor / cap / points (if it has them :-).I had almost exactly this issue on one of my cars years ago, and it was a bad coil - issue only showed up under load. Do you have a spare coil you could try!
sifty,
Feb 22, 5:47pm
Hmm, brand new MSD dizzy and leads, so no points etc to worry about but I am using the old coil, could be a simple swap to try (had the carb on and off enough now), Will see if I have a spare one in the shed, cheers.
1fordluva,
Feb 22, 5:48pm
I thought it was 2 -2 1\2 turns at idle per side.could be wrong too. Power valve can cause a flat spot also.does it have the correct valve or even replaced on rekitting!
extrayda,
Feb 22, 5:51pm
Are you running a 6AL (or equivalent) with the dizzy, or is it one of those 'ready to run' ones!Did you change the carb & ignition all at the same time, or is the carb more recent! (basically can you pinpoint when the problem started).
sifty,
Feb 22, 5:51pm
yep new power valve, although it could possibly have been damaged by the backfires! Have tried backing the idle mix screws out further too but was obviously running rich at over 2 1/2 turns. Higher vacuum and cleaner plugs set between 1 1/2 and 2 turns
forgot to say all new plugs too.
extrayda,
Feb 22, 5:53pm
If the carb is relatively recent they have backfire protection now.So the new plugs are clean! What did you gap them at! I think from memory MSD's recommendations were a bit on the high side (for my car at least).
sifty,
Feb 22, 5:55pm
car had been sitting a few years and was set up to run flat out, wanted to use it as more sedate street car so replaced torker II intake with rpm air gap, ready to run dizzy and new leads, plugs etc. So lot of new stuff all at once added. Waiting on quick change kit for secondaries to arrive but shouldn't affect main jet/idle operation/transition!
xacoon,
Feb 22, 5:56pm
too much pump shot!
sifty,
Feb 22, 5:58pm
and carb dates from 89 I think, so prob pre-backfire protection job (did have a lot of bangs while getting going). But why would it run fine at rest from idle through full revs! That is what is bugging me most. It is drivable, but just has an annoyance and I know it's not right. Appreciate the quick replies though, I knew there must be a few that had spent time twiddling holleys out there.
extrayda,
Feb 22, 5:59pm
Did you replace all of the petrol, and have an inline filter! Could be a vacuum leak on intake gaskets - have you checked for that! Silly question, but did you also cap off any of the unused vacuum ports on the carb! Other possibility is that it has a hot cam that isn't happy with less timing etc - is the car auto or manual! High stall converter!
extrayda,
Feb 22, 6:00pm
Does the ready to run have vacuum advance, or mechanical!
sifty,
Feb 22, 6:02pm
Yep new fuel and filter, strong vacuum on manifold and no unplugged ports, might try setting up gauge on long hose and run into cab while road testing. The cam is a bit of a mystery, can't find the info but it is a 351 cam so has different firing order (checked and rechecked) think it was mildly hot spec so should run ok with manifold carb setup.
sifty,
Feb 22, 6:03pm
both, next trial will be to remove and plug vac advance to see what happens. As I understand it the vac advance should only really affect fuel economy
Oh I have played with timing too, from 10 deg to 15, settled on 12 as a reasonable static setpoint.
extrayda,
Feb 22, 6:09pm
In theory I think vac advance will improve both driveability and fuel economy by adding in timing when needed.I have always run mechanical though, so not 100% on that one.Did you set the timing with the vac advance blocked off, or connected! Interesting about the firing order, I didn't know that (I have only had clevelands & a 460 and Chevs). You said it runs and revs fine while static, so you must have that right.
sifty,
Feb 22, 6:12pm
Yep timing set with vac adv removed and plugged. Not sure why the firing order is different between 302, 351W. But just thought of something, might try rerouting cables so two firing consecutively are not run beside each other, again though, runs fine in the shed.
sifty,
Feb 22, 6:28pm
and cheers for the suggestions, gotta go but have a couple of things to try. will check in later
cuda.340,
Feb 22, 6:57pm
is your cam stock or a hot grind! my engine only runs well when the static timing is 22*, but then i have a hot street roller grind cam.
bigfatmat1,
Feb 22, 6:58pm
change the coil its not suited for electronic ignition
Tempted to try new coil but it's not a 40 yr old stock coil, is a few yrs old but was a high energy upgrade job and car runs fine flat out, and idling, why would it be different under load!
Might be a day or two before I get another try, wifey getting grumpy at time spent in the shed.
1fordluva,
Feb 23, 6:51am
At a guess,jetting would be the issue,you say it free revs fine,goes fine flat out,nothing or very shabby inbetween,jetting.
xacoon,
Feb 23, 8:52am
Is the coil for use with a resistor or non resistor type system! a high output resistor coil will not have enough juice to run a full hei setup to its potential, and it is most noticable under load when the most spark is needed.
sifty,
Feb 23, 9:36am
Might be worth putting a blaster 2 in, cheers.
sifty,
Feb 23, 10:01am
Oh crap, just reaised I may have wired up to ballast resistor inadvertently, as I think it is in the loom. Will check this out, might be as simple as bypassing resistor/running new 12V source.
Since the public registrations are closed, you must have an invite from a current member to be able to register and post in this thread.
Have an account? Login here.