91 or 96 octane in a '47 flattie?

6

66miles

Senior Member
Been messing lately with plug fouling issues in my '47. Until today she wouldn't idle more than 5 mins before the plugs were covered in black soot and starting to foul out. Have finally gotten her running at idle for extended periods without fouling by leaning the idle mixture right back so the screws are only a quarter turn off the seats. She is running NGK B6L at the mo but has also had the same prob with Champion H10C plugs. Now, with the carb leaned right out, the plugs aren't sooty or wet, but are looking a healthier chocolate-brown and dry. The car also idles much smoother now too.
I was about to get hold of a hotter set of plugs like NGK B4L or Champion H12 and try them with a slightly richer idle setting because I think my carb is probably a bit lean where I've got it set right now. Then I read a post from a guy who says the gas we are using in NZ these days is crap and it's not so much the fault of the plugs but the crappy gas.
I've been shoving 91 octane in the thing because that's what everyone seems to use these days. However, I was wondering if 96 octane might burn a little cleaner (and hotter). Also, every station has it for a few cents more. PS: Idle screw tapered ends have slight wear grooves showing. Might be time to get the carb recond.
 
The problem is we're running better gas than was available back in the flattie day, but 97 is right in that it's not suitable for our low compression, inefficient, carburated engines. I have the same problem you do, and I've tried 96 octane, made little difference. I've also tried H10s and B6Ls, made no difference, so I've got a set of Autolite 216s coming from the US. I dont really hold much hope for an improvement, time will tell. I'm picking it's something we're going to have to live with, unless Brian or Ian or one of the other knowledgeable Kiwis chimes in. Dang, I just came through Wellsford a couple of days ago, it would have been cool to meet up.
 
66miles said:
Been messing lately with plug fouling issues in my '47. Until today she wouldn't idle more than 5 mins before the plugs were covered in black soot and starting to foul out. Have finally gotten her running at idle for extended periods without fouling by leaning the idle mixture right back so the screws are only a quarter turn off the seats. She is running NGK B6L at the mo but has also had the same prob with Champion H10C plugs. Now, with the carb leaned right out, the plugs aren't sooty or wet, but are looking a healthier chocolate-brown and dry. The car also idles much smoother now too.
I was about to get hold of a hotter set of plugs like NGK B4L or Champion H12 and try them with a slightly richer idle setting because I think my carb is probably a bit lean where I've got it set right now. Then I read a post from a guy who says the gas we are using in NZ these days is crap and it's not so much the fault of the plugs but the crappy gas.
I've been shoving 91 octane in the thing because that's what everyone seems to use these days. However, I was wondering if 96 octane might burn a little cleaner (and hotter). Also, every station has it for a few cents more. PS: Idle screw tapered ends have slight wear grooves showing. Might be time to get the carb recond.
Click to expand...
We have the same crap gas you guys have. Most and myself run 87 octaine . Sounds like you need a carb rebuild. 1/4 turn out doesn't sound right usually 1 turn is good. Somethings wrong in there.You need carb work in OMO. ken ct usa.
 
Might I suggest looking at the ignition timing. Most of these engines are set by ear. If you continue running very lean you run the risk of burning valves. Considering the octane available when the engine was made 91 is more than sufficient unless the heads have been cut.
 
A few notes.

Higher the octane, the slower the burn. High octane helps avoid pre-ignition from high compression. Therefore, low octane gas ignites quicker.

The plugs indicate if the motor is running lean or rich, not the setting of the idle screw. The setting of the idle screw indicates a problem with the carb.

Neal
 
High octane is for high compression engines. Flatheads are low compression. I'd say look elsewhere for the problem.

I don't know the price difference in NZ (you say a few cents, here it's like 40-50 cents/gal more), but in any event you're wasting money.
 
Octane rating isn't your issue as others have said. However there is also a language problem here (no, not a joke about either of us talking funny english :) )

In testing octane rating there are two typical tests and one U.S. imposed "standard". The standardized tests are for RON (Research Octane Number) and MON (Motor Octane Number). The US gov't imposed rating is AKI (Anti Knock Index). The same gasoline sample will yield two different values for RON and MON because of test parameters being different. For example, the RON value might test at 91, the more severe test method on the same sample could yield a value of 83 MON. Our gov't created AKI to simplify consumer comparison :rolleyes:. The formula for AKI is RON + MON / 2 = AKI which is the number mandated be posted on the pumps here in the US. If you look closely at that label the above formula is printed in small text in the corner. Using the above values of RON/MON you'd get 87 for example. I suspect you folks down under don't utilize the AKI rating. So it's likely your 91 is the RON value which would be identical, or nearly so, to our 87. I don't know if you guys are mandated to use alcohol as an oxygenate as we are.
 
I agree with the carb problem. I run 87 octane in my 276 inch flathead and B6L plugs. No problems. What carb are you using?
 
Hmmm... not trying to hijack 66miles thread, but sounds like we have the same situation. I'm running a 94, and I've tried two fully rebuilt carbs, and one was by a carb builder who has been doing flattie and Model A stuff all his life. Both carbs made no difference to the sooty plugs situation. :mad: I have brand-new leads and a Bubbas dissy. The C59A is a stocker with the exception of iron heads cut to .045 clearance over the pistons.
All cars, new or old, in New Zealand have black tailpipes, the days of nice light-coloured tailpipes is long gone, but I feel that the plugs should not be as sooty as they are.
 
I might suggest your problem could be a "blown" power valve. A common problem with holley 94 carbs. If the power valve is blown it will run extremly rich. No matter how you adjust the air mixtures.
The power valves are made for various altitudes. They are inexpensive. Take the time to inquire about one for the altuide which you live.
Buy an extra one !!
Have fun.
 
Thanks a heap guys. Sounds like this fouling prob has got a lot to do with the dubious state of the old carb and less to do with anything else. As you will all be able to tell, I'm not much of a carb guy, and would probably leave any tricky carb work to a pro. So far as I can tell, the carb looks to be a Ford/Holley of a later type. It has that done-a-few-miles look to it, and has probably never been touched, so I guess I've got nothin to loose by stripping it down for a look. Next stop from there would be the carb shop!...
 
I am sort of going along with the consensus, here and I would be looking at what might be causing a general over rich condition, that has to be compensated by adjusting the screws leaner than normal. There are a few things that can cause the problem, and I've personally encountered most of 'em.

Blown power valve.
Punctured float.
Float level too high.
Fuel pressure to high due to fuel pump.
Needle valve not closing fuel supply off fully due to a foreign body in the seat.
Needle valve not sealing due to a poor quality rubber tip.

There may be others, but here is what I have had to do to overcome similar problems:

Exchange punctured float with a good one. (This caused such an overrich condition that it wouldn't run at all when hot).

Replace the rubber tipped needle valve with a steel tipped one from an old carb.

Replaced the spring in a new fuel pump with one from an old high mileage pump.

Generally the old flatheads will run fine on the lowest octane fuel available at the pumps. As stated your octane ratings are probably set differently than the USA ones, here in the UK we use 95 as a standard and 98 as a "super". I don't know what the equivalent is in the USA or NZ but I know the UK and USA numbers are different.

Mart.
 
Uncle Bob said:
Octane rating isn't your issue as others have said. However there is also a language problem here (no, not a joke about either of us talking funny english :) )

In testing octane rating there are two typical tests and one U.S. imposed "standard". The standardized tests are for RON (Research Octane Number) and MON (Motor Octane Number). The US gov't imposed rating is AKI (Anti Knock Index). The same gasoline sample will yield two different values for RON and MON because of test parameters being different. For example, the RON value might test at 91, the more severe test method on the same sample could yield a value of 83 MON. Our gov't created AKI to simplify consumer comparison :rolleyes:. The formula for AKI is RON + MON / 2 = AKI which is the number mandated be posted on the pumps here in the US. If you look closely at that label the above formula is printed in small text in the corner. Using the above values of RON/MON you'd get 87 for example. I suspect you folks down under don't utilize the AKI rating. So it's likely your 91 is the RON value which would be identical, or nearly so, to our 87. I don't know if you guys are mandated to use alcohol as an oxygenate as we are.
Click to expand...


I think you're right Uncle Bob. Our 91 Octane is closer to your 87.

Gb
 
About to start pulling the thing apart for a bit of a look. Will be paying particular attention to the state of the power valve, float and needle valve. Unfortunately I dont have ready access to things like vacuum or pressure guages so everything is gonna be done by eyeball. Not exactly scientific but it's a start. PS: How the hell do the carb guys verify the level of fuel in the float chamber with the engine running and the top bolted down on the carb. This car's fitted with an electric fuel pump at the tank outlet which feeds through the stock mechanical pump (which also operates). I seem to recall the pump was rated at 2-3 pounds output. Maybe the fuel feed pressure is a pound or so too high also. I guess I could try running it with the electric pump disconnected for a while, but I would really love to be able to verify that the needle valve isn't being overcome by too much pressure... I can already feel a trip to the carb shop coming on!
 
The original is a tad weak and often would run the battery flat when trying to pull the fuel through from the tank after she had sat for a while and fuel has drained back through the lines. Likely the old pump isn't all that bad but maybe just a little tired. The electric pump sure helps with the infrequent starts the old gal gets, but maybe a better idea would be to just start her more often. Even better idea would be to get her road legal so I can drive her couple of times a week!
 
Just use the electric pump to prime the carb, then turn it off for normal usage. As long as the mech pump can pull the fuel through the electric one when it is not running, then it should work ok. If the mech pump ever failed you could get home with the electric one.

I don't know quite what happens, maybe someone could give an answer, but if the electric pump puts out lets say 3 lbs, does the mech pump then put an extra (lets say) 3 lbs on top, equalling 6 lbs (roughly)? or does the fuel pump add nothing as it would only go to 3 lbs and the pressure is already at that.

I dunno.

Mart.
 
Thanks Mart. Don't know why I didn't think of that. You could be quite right about the 2 pumps working in tandem and thereby delivering too much pressure. I've never really given it a lot of thought till now. Maybe someone out there knows.
 
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