Holley tuning help

**tonyk72** · 04-05-04, 08:50 PM

Can anybody tell me & maybe put up a pic of how & where I adjust my air/fuel mixture on my 4150? It's running a little rich...wanna lean it up a bit. I have the little red Holley Handbook, but I need a more guidance.

Thanks,
Tony

**GerryLP** · 04-05-04, 09:47 PM

Originally Posted by tonyk72

Can anybody tell me & maybe put up a pic of how & where I adjust my air/fuel mixture on my 4150? It's running a little rich...wanna lean it up a bit. I have the little red Holley Handbook, but I need a more guidance.

Thanks,
Tony

Tony,

The jets are between the metering plate and the fuel bowl. There are standard jets P/N 122-XX, and these last two-position numbers designate the size of the orifice (i.e. 48 = 0.048").

There are also close limit jets. These are P/N 122-XXX. The first two of the last three numbers are the same as standard jets. The last number are: 1 = flows in the lean side of that jet size, 2 = middle flowing for that jet size, and 3 = flows in the rich side for that jet size.

If you are at sea level then your 4150 should function fine with the original jet size that came with the carb. One can change many sizes in the rich (larger orifice) direction. Going in the lean direction, one should not change more than one size smaller than stock at sea level. The carb could be functioning fine until you drive through a really cold air mass, and this will change your misture ratio to lean even more. A lean under no-load conditions is hard to detect, and thus most dangerous to your engine. Under load conditions, your carb will backfire, and that indicates a lean condition. So be careful

The best time to change jets is when you know that there is a fresh rebuild kit on the carb, for other situations (i.e. leaking main body gasket, clogging, sticking floats) can cause problems.

**GerryLP** · 04-05-04, 09:50 PM

p.s. There are other ways of changing your fuel/air mixture, but these are mostly seen around the track. A daily driver should not be modified in this way, for you'll turn the car more unreliable.

**JohnZ** · 04-06-04, 02:45 PM

There's a small idle mixture adjustment screw on each side of the front metering block, but they only affect mixture at idle; they're adjusted by connecting a vacuum gauge to a manifold vacuum source (like the fitting behind the carb) and adjusting for highest steady vacuum reading. If it's running too rich under normal operation (off-idle), it may be jetted too rich; changing the primary jets requires removing the front float bowl - they're on the float bowl side of the metering block. Or, you may have a blown power valve, which will cause a constant over-rich condition.

Check for a blown power valve by putting your finger over the end of the front bowl vent tube with the engine idling; if it takes several seconds for the engine to die, the power valve is OK. If it dies almost immediately, the power valve is blown/leaking and needs replacement. Putting your finger on the vent tube forces fuel into the engine immediately by way of the power valve vacuum passage if the valve is blown; if the PV is good, the fuel will be forced out of the boosters instead to stall the engine and this will take several seconds longer to occur.

**tonyk72** · 04-06-04, 05:34 PM

Gerry, thanks for the jet info...I thought I'd be able to turn a screw and adjust it that way...is this possible on some other carbs?
I had the carb rebuilt last year & had the shop put everything back to spec...so I hope my jets are OK. I assume I'd have to take the carb apart to make sure 100%.
John, I tried what you said about checking the power valve. It took 12 secs for my car to finally die. So that seems to check out OK.

but moving off topic a little...I still have problems with perculation...I think I FINALLY might have it figured out...see the pic.
I can't belive I never noticed this before until today! And this line was installed by my mechanic (same one who installed the fuel filters backwards).

**JohnZ** · 04-07-04, 05:58 PM

Originally Posted by tonyk72

Gerry, thanks for the jet info...I thought I'd be able to turn a screw and adjust it that way...is this possible on some other carbs?

There is no external adjustment on any carb for mixture, except for the idle mixture screws; main-circuit metering is controlled under all conditions other than idle by jet sizes (and metering rod sizes on some carbs), and those are all internal to the carb. What are the symptoms you see of it running rich?

P.S. - Unless you have a VERY tiny flat washer under the head of that forward-most intake manifold bolt, it's loose.

**joe1975** · 04-07-04, 06:09 PM

sorry to butt in but will a blown power valve cause a running rich gassy smell and the engine to run on after it's cut off

drags1998 · 04-07-04, 06:53 PM

Yes on#1 NO on #2 Ahigh idle will cause RUN-ON

**GerryLP** · 04-07-04, 08:09 PM

Originally Posted by joe1975

sorry to butt in but will a blown power valve cause a running rich gassy smell and the engine to run on after it's cut off

In tony's case, he seems to have enough intake vacume to be well above the power valve rating (Holley usually installs "6" or "6.5"'s power valves. If the cam is radical enough, then the power valve rating should be lowered to a value that when reached, will support the crossing over from the end of the main metering system to the point where the power valve is supplying most of the fuel for the WOT mixture. This value also prevents the power valve from opening at idle and turning your mixture rich.

Maybe John can explain it better. I believe it requires road testing and monitoring of the vacume drop at WOT.

**tonyk72** · 04-07-04, 08:14 PM

Symptoms for it running rich are sooty plugs. I am also still having some problems with perculation...a person I know suggested it the fuel/air mixture may be too rich and be causing this to happen.

As for my leaky carb & the perculation problem, I have sealed all the fuel fittings with some teflon pipe dope. I know this is not the way to fix the problem, but so far its working & I am at my wits end on this! Tonight I have also tightened up the fuel bowl screws just a bit & I also wired the heat riser to stay open. I took it out for a ride & parked it in the garage...I hope I don't smell gas in the morning.

John, it must be the angle of the shot, but I checked the bolt & it was just a tiny bit loose (1/4 turn to get it back to 30 ft lbs). Thanks

**GerryLP** · 04-07-04, 08:32 PM

Tony,

A couple of thoughts. I remember that I had a bad problem with perculation, and my solution the first time was to install a phenolic spacer (I used a 1", but there are 1/2" ones). The phenolic, as you may know, is a lamination of paper, plastic, or some other isolating materials with a bond agent. It acts as an insulator of heat on these types of situations.

The second time I had the problem (quite recent indeed), I found the non-stick gasket with a cut lenght-wise on some areas of the gasket (probably from overtightening). I had check floats and needles and evrything else. Anyway, I replaced the gasket and it cured the problem.

Give it a shot, if all else fails.

p.s. If you feel alot of heat rising from the engine (like while under a tree shadow), and you trust that your cooling system is OK, then check the timing. Settings close to TDC will yield more heat (from the exhaust manifold) than settings away from TDC (obviously before TDC).