Technical Running rich: Rejet or hotter plug?

Like the title says, is it better to re-jet or hotter plug if it's on the rich side? I would think re-jet for better fuel economy. My question is how to pick between the two?

 

Opening the plug gap and advancing 2 degrees are 2 other options. I used all four at different times when racing. Good luck....

 

Changing the plug is a small incremental change. If you close, it may be worth it. If a larger change is needed and you aren't shoving more air into the cylinders, better to pull fuel and re-jet.

 

A lot depends on your cars combo I guess. If you are running too much carb for the motor changing the plugs will not help and you have to think about your air fuel ratio when you decide to rejet. I.E. a 750 CFM carb on a stock 283 would be an extreme example.

 

Beaner,
Trying to learn. It seems to me a carb/throttle body that is rated at 750 cfm would serve a good size engine if jetted correctly.

Why wouldn't jetting it correctly also work on a 283, as long as air/fuel ratio was 13, or whatever is considered optimum?

Phil

 

From the interwebs :

Cold plugs are ideal for high rpm engines, forced induction applications, and other instances where the engine produces high operating temperatures. Conversely, hot plugs are good for applications that operate mainly at low rpms. Because they have a longer insulator nose length, heat is transferred from the firing tip to the cooling system at slower pace. This keeps the spark plug temperature high, which allows the plug to self clean and prevent fouling.


The right jets will not hurt you one bit

 

Because if it is running fat on a 283 and it is too much carb to start with changing the jets will not make it less carb, it still flows the same amount of air. If the carb is close to the correct size for the motor than re-jetting it will make good sense. If it is too much carb the only reason that jetting it down makes good sense is that it is all you got, you have smaller jets and you need to get to work in the morning.

I just used an extreme to make my point.

Most engines are over carbed in my experience. I made over 400 HP on a 355 with a *600 CFM carb. Everyone will tell you that I needed more carb but they would be wrong.

*the carb probably flowed closer to 650 I altered the venturis.

 

Beaner it sounds like you knew you could use a larger carb and didn't have one so you made one.

 

I made it work with the intake that I altered. The plenum was just a little too big for 600 CFM and too small for anything bigger than 650-660. 600 Holleys were a cheap as dirt at the time and I own a dremmel. LOL

Note: this falls under do not do this at home kids. I have screwed up a bunch of carbs and intakes learning to do it and I still screw up sometimes.

 

This can be kind of a "Catch 22" situation.

If you've decided that it's too rich because the plugs are a bit sooty, you could go to a hotter plug. That may help keep the plugs cleaner, but it's still too rich. So then you fix the choke or adjust the floats or lean out the jetting and it's no longer too rich. But now you've got a set of plugs in it that may be a step hotter than they need to be.

Is this street driven engine or something that sees some track time?

 

It seems a 283 will only flow as much air as it needs.
However, if the venturi are too large for the engine the velocity of the air may be insufficient to run properly/efficiently.
Am I on the right track?

Phil

 

What carb bad power valve high float level will make it run rich.

 

Yep but it depends on how deep a breath the 283 takes too. For example a 283 horse 283 takes a deeper breath than a 220 horse motor, both are stock motors. End of the day if the carb is too big no amount of re-jetting will actually cure it. You may get a good (fair?) fuel ratio but it still wont run right.

There is more than just engine size to carb sizing, way more. The carb needs to match what the intake will handle, that is determined by plenum and runner size ( length and cross section), there is cam profile and compression and heads to think about. Add to that volumetric efficiency.

Its easy to ball park one for example if I know you got a 283 that is mildly built I may tell you that a 500 CFM carb is good. But even that depends on which 500 CFM carb, for example certain carb manufacturers flow their carbs dry, others wet. A wet flowed carb @ 600 CFM flows 600 cfm ( or will) but a dry flowed carb @ 600 cfm flows less when there is fuel in it.

TMI? not for you, maybe for some.

Oh in answer to your question yes. Too big a hole on the throttle body end slows the air velocity.

 

#1 - post pics of motor, with air cleaner off # - post pics of all spark plugs / what plugs they are - #3 give info of how engine is built, age/miles, etc #4 - why running current carb & has it been worked on - #5 post pics of ride & info on drivetrain - #6 if tuning a motor to have it run right only took 1 of 2 changes we would all be cruising more

 

Basically plug heat range has nothing to do with the AFR, it's just a lot of people think they need a hotter plug if the plugs are fouling. In most cases the stock plug is optimum for all around use though if you're running long hours on the freeway in summer a step cooler plug range is a good idea.

 

283's ran pretty good with a Rochester 4GC or Carter WCFB. Neither had 500cfm and were in the mid 400's. A 245 HP 283 had 2 of these Carters with the same cam and compression as a single 4 barrel engine. The jetting was the same for either. It's fuel to air ratio and efficiency. A correctly jetted 750 and a correctly jetted 600 my be close with the 750 cfm making more hp at 6000 rpm but who drives there?

 

I haven't had to read plugs for awhile, but from what I've read, you can't do it like the old days with the newer reformulated stuff called gas.

An inexpensive AFR meter with a wired O2 sensor, will get you as close as most of the OBD1 cars up through the early 90's. Save the money you'd spend on a wideband sensor and buy some Jets

 

Here's the motor 1953 239ci, EAB heads, good even compression, carbs are 97's, size 50 mains, 65 pv,
Plugs Auto lite 216 gaped at .035. timing is 12 degrees advanced

It's definitly running rich. I threw in a set of NGK B-4L, set the gap at .030 and it's running better I'm going to start dropping the main size down and keep reading the plugs.

 

It's a stock cam and stroke also. I wanted to make everything fit first and enjoy it before I started modifying the motor.

 

I installed a O2 sensor and wideband gauge on my truck, 250 inline 6 with dual Carter W-1's. I had been driving the truck for a couple years without the gauge fine tuning best I could. I thought I was close till the first trip with the wideband installed. Turned out I was way rich all across the rpm range even though the plugs looked good and the exhaust seemed clean. I changed float levels, metering rods, and timing to get the A/F where it needed to be at each rpm point. It took three different metering rod sizes, and a couple float level adjustments, and lots and lots of test runs before I considered it tuned up. Now it idles at 14.5, mid throttle 15.0 and cruise 16.2 @ 70 mph. Full throttle drops to 13.6, fuel smells are gone, plugs are clean, starting is easier, and fuel mileage has gone up.

What I found is a 1/16" float drop will change the whole ratio range quite a bit, like 2 points on the gauge. Same with metering rod hight, just .050" will make a huge difference once you are close.

Spend the money for a wideband, it will be your new best friend!

 

Back to the over sized carb discussion a bit.

When I dynoed the 352 ci FE fro my '62 Merc, I brought along a few carbs ranging from a 600 cfm up to a 750 Holley to see what would work best. Even with the A\F Ratios dialed in and jetted properly, the 750 was always 40 to 50 HP behind the 600. There just wasn't enough cam in that motor to support the bigger carbs and it always gave up a ton of power to the smaller carb.

On the other side of the coin. The OP is running a blower motor, and assuming the rest of the motor can handle it, he could change up his pulley sizes and shove a bit more air in there as well to help correct the rich condition.

I also agree with the suggestions to add an O2 meter and gauge to really help dial things in. It is a flathead, and they have poor burn characteristics to begin with due to the chamber design, what with that extremely long flame travel distance and all. Reading the spark plugs likely isn't telling the whole story.

See this video of a acrylic cylinder head on a Briggs to under stand some of the bad burn characteristics of a flathead.

 

what is fuel pressure at? dual 97's too much carb for motor? before changing things might want to look at: www.myrideisme.com/Blog/6-things-you-may-not-know-about-stromberg-97-carbs/

 

A lot of people over carburate their engines. Here is a Holley carb selector program that will give you a better idea of the carb size you should be running for your situation.
The selector chart is for 100% efficiency, so you need to add approximately 10% for a better idea regarding the actual size of the carb you would need. This is a rough guide, but it will give you a better idea as to whether you are running a carb that is too large.

https://www.holley.com/retailer/carbselector/

 

Right on.

The only way to get it right is to monitor the exhaust either this way or on a chassis dyno.

 

I pulled this old girl out of the mothballs and been learning how to use the scope. The timing light, dwell meter and condenser tester are of great benefit. It it also has an combustion efficiency tester. I'm going to give her a try.

 

Fuel pressure is at 2.5 PSI, Floats are set correctly also, carbs are synchronized. I don't think it's to much carb, S.Co.T. superchargers were designed to run on fathead motors with no notifications and two carbs. I'm going to jet down to .48's and see if that improves the color of the plugs.

 

What jets are in there now? Dropping a size or two isn't going to do much, that thing is black as coal.

 

Interesting reading the comments here

I think you putting an AFR meter (wideband O2 sensor) on it will help you figure out what's going on.

Keep in mind that the main jets don't directly affect idle and low speed mixtures, and it seems to me that often the soot build up is from operating at these conditions.

 

I fabricated a crude throttle position gauge to go along with my A/F gauge, and vacuum gauge to help in tuning. I simply tied string to the throttle pedal, and tied the other end to a spring loaded movable slide in a cardboard piece taped to the dash board. With a few marks for idle and full throttle I was able to divide the rest into equal parts. This let me see just how much throttle pedal movement was needed to keep up with traffic and normal driving. I found that 90% of my driving was under 30% throttle and of that, 1/2 was just off idle. By adjusting the idle mixture, I was able to change the A/F ratio of the first 10 to 15% of the throttle opening ( Carter W-1 carburetors ). As Jim pointed out, main jets did not change the idle A/F ratio.

 

Thought I'd throw this out there. I remember in a lot of repair manuals had a page with pictures of spark plugs. Showed different conditions and possible causes.
I'm not a know it all, so I always refer to manuals to figure things out. If I listened to everyone's opinion (everyone has a different one) - just too confusing.