Are the plugs wet or dry? And what kind of run 1/8th, 1/4 mile or something else? Is this electric or push start? Where do you shut off and do you or not drive back to the pits?Joshua Lee said:No matter what I do I cant get a clean plug on my current setup...
830hp carb with a 71/72? jet and 3.5" PV's
Timming locked out at 36*
After I make a pass these plugs stay dark... Any ideas?
Joshua Lee said:wet
Wet says there's too much fuel somewhere in the process or the plugs arn't hot enough. This could happen on the drive back or the back off after the traps. 1/8th drags don't get the engine very warm either, it could just be a situation where the engine temp needs to come up more. Being too cold for drag engines is a common problem. Everybody wants a cold engine for power, but cold engines really aren't doing as well as they could. You want cold induction into a warmed engine, where the clearances are optimized and the oil heated.
Race engines are hard to id where these problems originate but your operating cycle allows you to make some changes to localize the problem. I'd suggest shutting down and disconnecting the engine from the drive line after the traps rather than coasting down then towing back, this should tell you if the wet plugs are a result of coasting after the traps and idling back to the pits or whether it's occurring during start up, staging and the run.
This is an edgy issue, it's not uncommon to see the main metering dripping on idle. Race engines tend to idle at a high RPM which can excite the mains into feeding. This gets into all sorts of issues in terms of jet sizes (not only mains and power, but air correction jets as well) and throttle position at idle (whether to drill the butterflies for bypass air or not)BBCMudbogger said:I have been told that you will see fuel dripping in a race carb......
sometimes it smells like fuel?? whats this about?baddbob said:Bogie, Nice writeup!!!! 4-6" of vacume at idle makes me wonder what the total engine package is??? timing? cam specs? compression ratio?
If your running the hottest plugs NGK makes then the carb must be way rich especially considering the way the plugs look. How does the oil smell?
oldbogie said:This is an edgy issue, it's not uncommon to see the main metering dripping on idle. Race engines tend to idle at a high RPM which can excite the mains into feeding. This gets into all sorts of issues in terms of jet sizes (not only mains and power, but air correction jets as well) and throttle position at idle (whether to drill the butterflies for bypass air or not)
When we think of jet sizes we tend to think of main metering jets. But there are jets on the power valve as well, I’m assuming a Holley or clone there-of, not a Carter or Q-Jet. With a big cammed engine not pulling much idle vacuum there is a tendency for it to trip in the power valve way too early, you need to look at the number stamped on the valve to determine at what vacuum it tips in. Generally you want to start tuning this circuit with a valve that tips in at about 1/2 the vacuum the engine pulls at idle. With Carters and Q-jets this is a function of the main metering rods and is controlled by a combination of rod diameter and spring tension against the power piston and lever arm length or shape. The other often over looked jets are the air correction jets, some carbs allow replacement others require drilling. The latter being more permanent so you just don't want to get the Craftsman out and knock holes in those jets. Generally the need is where more fuel is delivered to the motor; the larger the air correction jets need to be. This can go further into the emulsion tubes but messing with this is really for the carb pros as you can easily reduce these expensive hard to get parts to junk.
Setting up idle for a clean transition to main metering on a race engine can be more of a pain than the street because of the need to avoid bogging the engine out of the hole. Typically a carb has three basic circuits involved in the transition from idle to main metering those are the idle, transition, and main circuits; hopefully at this point the power circuit is not functioning. The engine with a race cam in it wants to idle high which requires more air than is typical for a less cammed street engine. A 4 corner idle Holley or clone is nice for this as you have enough area in the 4 butterflies to keep them positioned under the transfer slot. This keeps the carb from feeding the idle with both the idle and transition circuits and helps keep the mains on the primary side from dribbling. The Carter has an idle air bypass screw which gets around this problem entirely. Going back to Holley types, even the average primary idle only Holley has an adjustment screw to position the secondary throttle butterflies; this can be stolen and modified to allow an adjustable air leak into the secondaries to add idle air which is compensated for with the main idle fuel screws. This again allows the primary throttle plates to position themselves below the transfer slot and keeps the primary air flow down to where the mains aren’t dribbling fuel. Increasing the size of the main’s air correction jet will also help stop dribbling. The old Hot Rodders trick of drilling a small hole in the primary throttle plates can also be used to add idle air thus allowing the primary throttle plates to be positioned below the transfer slot. This, however, does nothing to stop so much air flow from starting to draw fuel from the main circuit. Also lowering the fuel level in the bowl will reduce the dribbles but has other problems like high end lean out which will toast the pistons.
The Holley needs help when drag racing with jet extensions and stuff like that, I highly recommend purchasing one or more of the tuning books out there, the book recommendation would extend to Carters and Q-Jets as well if that’s appropriate to what your running.
When you visualize how a carb works and what it’s sensitive to and compare that to the dynamics of racing, it becomes apparent that if these things work at all it’s a minor miracle. Coming out the hole the fuel on the primary side is jammed against the metering plate which has the same action on the jets as increasing the bowl’s fuel level causing the primaries to go rich. On the secondary side all the fuel rushes to the rear of the float bowl reducing the height above the jets on the secondary plate causing that end to go lean. This is an event you have to balance the jet sizes for, however, will cause the metering when the car is not accelerating hard to be a little off. Circle track racers have the problem of forces causing the fuel to migrate to the right side of the fuel bowl thus leaning the left side cylinders and enriching those on the right, assuming a single plane manifold. This is compensated by staggering left and right jet sizes. A dual pane manifold produces the same problem but now the lean and rich cylinders are determined by which side of the plane they’re feeding from.
One nice thing about fuel injection is that for a lot of money you get completely around these vehicle dynamic induced fueling problems.