Fixing the infamous off idle Holley flat spot?

I finally got my flathead running again. It runs better than ever without those stuck valves, but the isky cam didn't hurt! I'm running dual holley 94's. I got it tuned in pretty well but it seems to have a flat spot off idle. It idles nice and once it picks up the rpms it pulls nice too, but it takes a second for it to get going when it almost wants to die. It was better after I put both accelerator pumps down on #1 from #2. I'm running #46 jets and whatever power valves came in the rebuild kits. It was only about 35 degrees and I was raising hell with no air cleaners or hood and the chokes open. Motor was at about 140. I only went for a quick spin so i'll have to check it out more. But what kind of things should I look at first? BTW the plugs were black when I pulled them.

 

I'm thinking the power valves are too big. Where can I buy ones around 4.5 - 5.5 ?

 

I don't know about 94's, but on a regular holley the power valves don't really do anything immediately off idle, that's the job of the accelerator pump.

Is it a "flat spot" or is it a stumble?

If your accellerator pump is spraying too much fuel, it will mostly flood (stumble)for a second, until the engine speed comes up enough to accomodate the fuel.

 

If the engine temp was 140, it wasn't sufficiently heated up. Hold off on the radical diagnosis until you check it out at 180 or "normal" operating temperature. A cold engine is always stumbly.

Let us know how that goes, and we'll continue the virtual tune up then.

FYI- Every Holley carb I've ever "cured" of the flat-spot was way out of proper adjustment or was mis-timed.

IMO, there is no "Holley flat spot", but there are plenty of people who don't know how to properly tune them.


Scotch~!

 

I had a similar problem, I got my 5.5's from vintage speed. I've heard some bad things about him here, but havent had any problems yet. You can get them at any auto parts store though if you know the part #'s since its the same ones holley still uses. The problem I was having was the stock sized 7.5 for one carb power valves were dumping too much from two carbs and loading it up. Hope this helps. This may not be your problem, but it fixed mine.
Drew

 

apparently a lot of people are confused about power valves. While some brands/styles flow a little more fuel than others, there really are no "larger/smaller" power valves.

The number on the power valve is actually how many inches of vacuum the valve opens at. Ex: a 5.5 is closed until you open the throttle enough to drop he vacuum to 5.5 in/Hg, then the power valve dumps in the additional fuel need to deal with the load on the motor without leaning out. a 7.5 open at 7.5 in/Hg, etc....

 

I'm glad you said something because that's what I was thinking, but I haven't "tuned" a holley in such a long time I wasn't 100% sure on that. So, in theory, he'd want a 3.5 right? So that the powervalve doesn't open as soon(if it's running rich).

I didn't even know that they shared the same powervalves as 4150's etc. Learn something new everyday on the hamb!

 

The way that I select a power valve is to go for a drive with a vacuum gauge hooked up to th emotor. Just do regular cruising, light acceleration etc, don't hot rod it. Watch the vac. gauge. Pay special attention to the average vacuum level while cruising, then nail it and see how low the vacuum goes.

I usually select a power valve that opens about 1 in/Hg LOWER than the lowest "old lady" vacuum reading I got... just make sure that it's HIGHER than your WOT vacuum reading.

 

Remember to buy a good quality powervalve. A lot of companies are sourcing things overseas and tolerences aren't what they used to be. One of my good accounts went through 3 5.5 powervalves that were marked correctly but measured as 10.5!

Spend a couple of bucks more to avoid chasing your tai!!!

 

I run 42 jets and plugged power valves on my Flatty. Runs fine...but does'nt have a radical cam.

 

Power valves dont do anything for the idle circut or off idle, they only enrichen the main circut. Check the arm for the accl pump if you have too much slop you will get a flat spot or if you are getting too much pump it will be fat and stumble.

 

Thanks guys. My flathead dosn't have thermostats and it's winter so it's never going to warm up this time of year. I have already got the accelerator pumps on the lowest setting. I know how power valves work, but between the cam and dual carbs I'm not pulling a lot of vacuum, so I think I need ones that are rated at less vacuum so they arn't opening prematurely.

 

Isn't WOT vacuum 0 in/Hg? Or, is that dependant on how big your cam is?

 

I guess I could have worded that better, too soon would be more accurate a description. As the power valve would prematurly "dump" with dual carbs. to be honest my method of tuning the power valves was much less scientific than others. I simply bought a few different sizes and tried them all to get rid of that "load up" on initial throttle. Flatheadpete, did you loose any top end from plugging the powervalves? I seemed to lean out a bit at the top end with plugs.
Drew

 

Actually at WOT there is a small amount of vacuum. Otherwise any vacuum secondary carb would never open the secondaries. The amount of vacuum does depend on a lot of factors, but just to clarify there is vacuum even at WOT. A mechanical secondary carb may be able to cause a nearly zero vacuum when you first crack the throttle open at lower engine speed, but it too will develop vacuum once engine speed is up.

 

Thanks 38Chev... The only car that I had with a vacuum gauge also had a supercharger on it, and it'd zero in/Hg w/ a flash of the pedal - but not hit boost. So I wasn't sure.

 

WoW...lotta stuff I've never heard before in here...

the PV is marked 1/2 of what it opens at...so a 65 is a 6.5 and opens at like 13...or atleast thats what I've always been told...

w/ my holleys I fi**ed the off idle stumble by goin w/ a bigger squirter...went from a 28 to a 31 and made all the difference in the world...I tired jets and adjusting...but you need something from the transition of the idle circut to the main...

 

Power valve is indeed just an on/off valve--metering is done at little restrictions in p***ages. Off-idle is way before they matter--you'rw running on idle and off-idle hole, plus a bit of accel pump, which has three settings..you have a 59A--make sure the advance is working on this, where the cam pivots on the shaft..advanmce starts at about 400 on these, and is an important part of the equation.

 

Bruce, I have the accelerator pumps backed off to the lowest setting, that's where it seems best. About the distributor, I set it up in a KR Wilson tool, so I know the timing and points are right. I havn't messed with the vacuum advance setting, I only cleaned and lightly greased the piston when it was apart. I have it hooked to manifold vacuum like it was. Should it go to one of the carbs?

 

Thanks for clearing that up. I'm sure the distributor is working properly, I had it all apart, cleaned and lubed everything. I didn't mess with the vacuum brake, though. Maybe I'll back it off and go from there. I still think this is a carb issue.

I keep hearing my #46 jets might be too lean, but then why would the plugs be black, and why would it run better after I set the accelerator pumps on the leanest positions?

The weather isn't helping me get out and troubleshooting any.

 

Two things--if you are showing signs of richness with jets that are indeed way leaner than stock, are you running air cleaners?? Test with them off--both early Ford and Hotrod aircleaners often actually flow about like corks. Then, get some stats and get the thing up to 180 or so.
It is possible your power enrichment is coming in way too early if wrong valves are in for your particular case, but that fuel goes in via the main system, which isn't active til long past idle--the circuits you're interested in in the carb are idle and off idle, both fed via the idle jet that runs down at the front of each nozzle bar. Also, you need exhaust heat in manifold in this weather.

 

Bruce is right on, as usual.

Again- It was running COLD, and it's breathing winter air, which is atypical and less than optimal to tune with....at 140 degrees coolant temp....!

I think you may be better off just waiting for the warmer weather, when it's a more typical driving condition, for your fine-tune.

I'd bet a little accelerator pump adjustment is all it'll take once it's breathing summer air with the coolant at 180 degrees...


Things change A LOT.

My more radical cars have changed tune dramatically from day to night, as I'm sure many others have.



S~

 

I had the same problem when I adjusted the carbs (94's, 49 jets, 4.5 power valves) and found that having no air cleaner on the carbs caused the rough transition. Once I had the aircleaners back on it was a lot smoother.
Also, my supposedly rebuilt carburators leaked like crazy (vacuum) because the gasket surfaces were warped because of overtightening. Never trust a rebuilt carb. If you have a vacuum leak the power valve might stay open all the time and dumps raw fuel into the intake. You can tell that the idle mixture screws will have hardly any effect (engine still idles when mixture scews are fully closed).

 

I appreciate everyones advice, and I know Bruce is the guru so I'm paying special attention!

When I had it out I did have the aircleaners off, plus the chokes were wide open and there was no hood on it. It might have been almost 40 out, with the motor around 140-150. Maybe if I even closed the chokes a little it would have helped.
After it stops snowing again I'll have to fool with it more before I go pulling things apart and changing stuff.

 

Using chokes (and for the opposite taking off the vac lines for a leak) gives a quick way to alter mix for diagnosis. 140 degree temp bad for oil, bearings, and probably mixture...
Gasket leak at seat of power valve would allow gas to go down vac p***age directly into carb--any other PV failure shouldn't matter until well above idle, as the normal path for this gas is through the venturi activated main delivery system. Idle and off idle circuits run everything--off idle slots are main fuel source til about twice idle speed, and supply some fuel up to maybe 1500 as the venturis begin to function--this last bit might be quite important with duals, as far less air is going through each venturi and venturi function is delayed by this. Stock 59A jets would have been .051.

 

I meant to add: Check the gasket and fit of the PV carefully, and make sure you have chosen a gasket that actually seats on the flat places--there's LOTS of room for confusion, as the current rebuild kits contain a hatfull of extra gaskets intended to fit all three million variants of the old Fod carb. There may well be several possible variants of PV in there, also--I believe the current supply is coming from the Holley 4150 types, and if I remember correctly there are a couple variants of configuration with different gaskets for those--so just eyeball your hardware thoroughly and don't ***ume anything is going to match up like Ford did it in 1948. A basic lesson from years of fooling with old cars is to be VERY su****ious of recently manufactured parts--no one out there seems to know or care much about what they are selling. ***ume that each part in the kit was actually manufactured to fit a Subaru until you have proven otherwise to your own satisfaction--and keep all the old gaskets for comparison (***uming the carb probably worked in its previous life) until you are sure the new stuff is right.

 

Bruce and others, today I checked out the distributor and noticed the screw was turned pretty far in. I figured what the heck, I backed it out by hand until it stopped, and tightened the jamb nut there. I started it up in the garage but there wasn't much of a difference from before.

I'm still tweaking the idle speed. It seems when hot idle is set right, when you pull the choke out fast idle is much to fast. I do have that just about right now, where fast idle isn't too high, but hot idle is still a little slow.

Regardless, even after you get it idleing nice with the choke and idle speed, if you just give it a little gas it wants to die unless you give it more gas and rev it a little to keep it going until the idle levels out again. You can't smoothly give it throttle. After you get past the initial stumble it will rev up. If you just start to pull the choke out it will rev up without dying too. Today this was in the 70* garage with the door open a little and the motor got to about 140, although I didn't have too long to fool with it.

Thanks guys. Help me get this ford back on the road after 30+ years!

 

btt, If I don't get this running right I'll have to push it into the woods out back or put a chevy in it. I don't know which would be worse

 

The more I hear you talk about it, the more I think you don't have enough accelerator pump squirt. When you bump the linkage, do the accelerator pumps squirt?

It sounds like the carbs are opening, but not getting sufficient pump shot to bring the motor up to speed.

Scotch~!