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Old 09-01-2011, 03:34 AM   #1 (permalink)
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Default Experimentation with Rejetting my Weber Carb

For long I have been suffering from the engine overheating when I'm cruising on a highway in my Samurai. I've had this ever since I swapped the pulley fan with an electric one. The electric fan killed the overheating problem while idle, but brought in the heating at cruising. I replaced my engine with a good Japanese 1.6 G16A 8 valve, swapped in the carb and intake manifold from my old engine into the new one. The overheating was still there, but I also faced shortage in fuel supply at high load high RPM, especially when I climb a sand dune.

Driving on sand dunes really requires brutalizing the engine, especially when it is a small 1.6L engine.

I did the following to remedy the overheating at cruising, in the following order:
1. replaced the radiator with a new original Samurai radiator
2. replaced the thermostat with a Nissan thermostat that opens up at a lower temperature
3. adjusted the timing on the distributor to 8 degrees
4. put back the original Samurai transfer case (instead of the 6.5:1 rock crawler which I really don't need in sand driving) to keep the RPM lower

Nothing worked. The engine still overheated whenever it reaches 4000 RPM and stays there for a while.

So I decided it is likely that the problem is due to the engine running too lean on the Weber carb, which is calibrated for a 1.3L engine and I'm using it on a 1.6L. After taking advice from this forum, I ordered the rejetting kit from low range offroad, and started to play around with the carb jetting.

Here's a picture of the kit:


It was really interesting, and I learned a lot about carbs. I did the idle test according to the weber manual instructions, and found that I'm running perfectly ok on idle, which is expected since I had no idle problems. But I did benefit from one thing from this test. I got my mixture and idle screws adjusted properly, and now the car doesn't diesel anymore! That dieseling problem was driving me crazy for years, and turned out to be just the mixture and idle screws. I got my mixture screw at 1 3/4 of a turn out, which is within the weber specs of a maximum of 2 turns, so I did not need to do anything with the idle jets.

Then I started playing with the primary and secondary jets, and the air jets and pump jet.

This is where it got interesting. My stock setup was like this:
Air jets (air corrector): 160 (right), 170 (left)
Primary fuel jet: 140
Secondary fuel jet: 140
Pump jet: Single, 50

I thought since I am running lean, I should keep the air jets as small as possible, so I did not change the air jets. and I installed the largest fuel jets I have, to make it as rich as possible.

After some experimentation, the end setup I had was as follows:
Air jets: no change
Primary fuel jet: 155
Secondary fuel jet: 150 (the kit did not have identical jets, so I had to make them different)
Pump jet: Single, 55

I tried the double pump jet, but it caused the engine to cut off at 5000 RPM on third gear for some reason. I honestly have no idea what the pump jet is for or what it does, but I played with it nonetheless.

The result of the rejetting: The car now runs much better, has better power delivery and it seems to have stopped cutting off on load at high RPM, but I have yet to test it in the dunes. But IT STILL OVER HEATS AT 4000 RPM!

When I cruise at 4000 RPM, it overheats. It takes a bit longer to start overheating, but it does overheat.

I honestly couldn't bother taking pictures, so I found some pictures of a weber identical to mine in some other Toyota forum, which was really helpful in how to rejet my carb:


This is the picture of the float bowl:


The primary and secondary jets are at the bottom of the bowl, under the fuel, and the air jets are on the top. The pump jet is in front of the two air jets.

That is the richest I can get in this weber.

I wonder what will happen if I completely remove the jets, or probably drill them to make them even bigger?

Should I change the radiator to an Aluminum rad? Would that solve the problem? Or should I just fit a Chevy Lumina (Holden Commodore) 3.8L V6 and get an adapter from Marks adapters in Australia and completely change my Samurai to something different?
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Old 09-01-2011, 10:14 AM   #2 (permalink)
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It sounds like you really need to learn a bit more about carbs - I'm no guru but - let me share a little see if I can clear up a few grey areas.

Pump jet.

With a carburettor, it is the movement of air through the carburettor that causes a vacuum which draws the fuel from the float bowl, so when you open the throttle the air starts moving immediately and then the fuel gets pulled into the air stream, causing a momentary lean mixture condition on acceleration, so most carbs have an accelerator pump that sprays additional fuel into the air stream to compensate - this is what the pump jet controls - how much fuel is sprayed into the air stream every time you open the throttle.

Ideally you use the smallest pump jet that gets rid of the flat spot on acceleration - any larger increases your fuel consumption.

Primary & secondary main jets.

Unless I'm mistaken you have some sort of progressive carburettor - the two chokes are different sizes and the primary (the smaller choke) opens first, and then as you open the throttle more, the secondary comes in to play - the norm is to jet the primary slightly lean for economy, and the secondary slightly rich (for power) - you have a 155 primary main and a 150 secondary main - reverse them and see what happens - in fact you can probably go back to the original 140 primary main, since there was no problem with low end power.

What happens if you completely remove the jets will depend on which jet you remove - main jets or pump jet - the engine will probably flood and not run, air jets - it probably will start but run lean. Drilling the jets out can be done IF you have the right drills.

I also have a question about your electric fan.

If I understand correctly, with the stock fan you had an overheat problem at idle, but not at cruise, and with the electric fan, the situation reversed, overheat at cruise, but not at idle - is that correct?

Where does the electric fan mount - in front of the radiator - or - between the engine & the radiator like the stock fan did?

Here's the thing - at cruise speeds, the forward movement of the vehicle should push enough air through the radiator that a fan is not necessary - I seem to recall previous discussions on the fan moving air in the correct direction (front to back), but I don't know if the possibility of the fan itself blocking airflow - which is possible if it uses a flat pancake motor and is mounted in front of the radiator.
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Old 09-01-2011, 01:28 PM   #3 (permalink)
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Ok, let me get things clear. Although this is not a technical forum, you people sure seem to have a whole lot technical knowledge Yes, I do crave for more information, thank you

I have a 32/36 Weber carb. I'm not sure what type of carb is that, whether it is a CV carb or a progressive carb.

Pump jet: Does it make sense to you that putting a larger pump jet resulted in better response after 5000 RPM on third gear? That is what happened.

Pri and Sec main jets: How would I know which is the primary jet and which is the secondary jet? Both were of size 140. There's a left one and a right one.

What about air jets (air correctors)? I assume air moves through the big throat on top of the carb, so what does the air corrector do? And was I right to keep it as small as possible to enrich the mixture?

About the fan, I'm 1000000% positive it is blowing in the correct direction. It is mounted between the engine and the radiator, and I can feel the air blowing when I put my hand behind the fan or around the engine. The mechanical (pulley) fan had no viscous clutch on it, it is directly bolted on the pulley itself and does not move when the engine is off at all. This is our local Saudi spec of the Samurai. My theory is that when the mechanical fan was attached, it was turning at the exact same speed as the engine, and increased in speed as the engine RPM went up, providing air flow even higher than that of the wind that blows into the radiator when the car is cruising at high speed. The electric fan (pulled from a Subaru) has a maximum RPM of about 2800, if my memory serves me well, which is not nearly as fast as the maximum speed of the mechanical fan at 5000 RPM. So I think that is why it kept it cool when it was on.

Quote:
Originally Posted by fordem View Post
It sounds like you really need to learn a bit more about carbs - I'm no guru but - let me share a little see if I can clear up a few grey areas.

Pump jet.

With a carburettor, it is the movement of air through the carburettor that causes a vacuum which draws the fuel from the float bowl, so when you open the throttle the air starts moving immediately and then the fuel gets pulled into the air stream, causing a momentary lean mixture condition on acceleration, so most carbs have an accelerator pump that sprays additional fuel into the air stream to compensate - this is what the pump jet controls - how much fuel is sprayed into the air stream every time you open the throttle.

Ideally you use the smallest pump jet that gets rid of the flat spot on acceleration - any larger increases your fuel consumption.

Primary & secondary main jets.

Unless I'm mistaken you have some sort of progressive carburettor - the two chokes are different sizes and the primary (the smaller choke) opens first, and then as you open the throttle more, the secondary comes in to play - the norm is to jet the primary slightly lean for economy, and the secondary slightly rich (for power) - you have a 155 primary main and a 150 secondary main - reverse them and see what happens - in fact you can probably go back to the original 140 primary main, since there was no problem with low end power.

What happens if you completely remove the jets will depend on which jet you remove - main jets or pump jet - the engine will probably flood and not run, air jets - it probably will start but run lean. Drilling the jets out can be done IF you have the right drills.

I also have a question about your electric fan.

If I understand correctly, with the stock fan you had an overheat problem at idle, but not at cruise, and with the electric fan, the situation reversed, overheat at cruise, but not at idle - is that correct?

Where does the electric fan mount - in front of the radiator - or - between the engine & the radiator like the stock fan did?

Here's the thing - at cruise speeds, the forward movement of the vehicle should push enough air through the radiator that a fan is not necessary - I seem to recall previous discussions on the fan moving air in the correct direction (front to back), but I don't know if the possibility of the fan itself blocking airflow - which is possible if it uses a flat pancake motor and is mounted in front of the radiator.
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Old 09-01-2011, 05:28 PM   #4 (permalink)
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A 32/36 Weber is a progressive carb - the numbers refer to the choke sizes, so the primary is a 32mm choke (right side or top in the picture below) and the secondary is a 36mm choke (left side or bottom in the picture below) - the corresponding main jets will be right & left also.



A larger pump jet should have no effect on "steady state" power, because the pump only works when the accelerator is rapidly depressed - so - if what you're describing as a better response over 5000 rpm is throttle response, then yes, that is to be expected, you're spraying more fuel in - the fuel is sprayed through the little nozzles alongside the pump jet.

Air correctors.

Air correctors are a little harder to explain - not all carburettors have them - I deliberately did not mention them in my last post because I'm not 100% certain how they work - on other webers, the air corrector screws in to the top of an emulsion tube, my assumption is that on this carb, the emulsion tube is not changeable - fuel goes in at the bottom of the tube, air goes in at the top and they form this emulsion or mixture, which then comes through a hole in the side of the emulsion tube and into what I call the "spray bar" - that little brass tube visible in the center of each choke - it has holes in the underside through which the fuel enters the air stream.

What I think the air corrector does is lean out the mixture as the air flow increases, so yes, keeping them small will make for a richer mixture at higher rpm, which is where your problem is.

Regarding the fan - it's not so much the rpms as the air flow - some electric fans will move more air even though they turn at a lower rpm than the stock fans, but, since there is no spec published for the stock fan, fitting an electric fan is almost always a "best guess" process.
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Old 09-02-2011, 12:31 PM   #5 (permalink)
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In that case, I do have the larger jet in the secondary main fuel jet. So I'll try and put back the small jet for the primary main jet to save fuel. But that probably will not solve my problem.

When exactly does the secondary take over the fuel supply from the primary jet? At what RPM?
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Old 09-02-2011, 04:03 PM   #6 (permalink)
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Quote:
Originally Posted by alternator View Post
When exactly does the secondary take over the fuel supply from the primary jet? At what RPM?
Aaahh - The primary main jet supplies the primary choke, the secondary main jet supplies the secondary choke - the engine runs on the primary choke only at low rpm, and both primary & secondary chokes at higher rpm - but exactly when the secondary will open is a question I cannot answer - the opening mechanism for the secondary choke is different from one carb to another and I do not know what how your weber works.

On some carburettors, the secondaries are vacuum operated, and will open based on two conditions - the first being that the primary must be fully open (the primary is controlled by the accelerator cable) and the second being the airflow through the primary - the opening of the secondary will therefore not be rpm related, but load related.

On other carbs, the secondaries are controlled by the accelerator and open after the primary - regardless of load or rpm.

Another thought that crossed my mind...

If I'm not mistaken, the intake manifold you are using has a flange to which the carburettor bolts, and the flange has two holes, one smaller than the other - these holes need to be as large as the carburettor chokes otherwise they disrupt the airflow - depending on which way you positioned the carb, this may not be so - and you may need to open up those holes for proper air flow.
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Old 09-02-2011, 11:28 PM   #7 (permalink)
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That's a very good point. Actually, I recall when I fitted the carb the first time that the intake manifold flang sizes don't correspond to those of the carb. I need to fit the carb backwards to have the same size holes match, but I couldn't because the electric choke mechanism interfered with the head valve cover.
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Old 09-03-2011, 09:33 AM   #8 (permalink)
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Maybe you should consider removing the carb and the manifold from the engine and "port matching" the manifold to the carb base - if the hole in the manifold is larger than the carb, leave it alone, but if it's smaller, open it up to the same size.

You could also consider some sort of tapered adapter - so that there is no abrupt step to a smaller diameter opening
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Last edited by fordem; 09-03-2011 at 10:10 AM.
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Old 09-03-2011, 12:06 PM   #9 (permalink)
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Or I can just flip the carb 180 degrees, and remove the electric choke. Then the holes will match. That way I can also remedy the stalling on steep hills problem.
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