Fuel, oil, carburetor jetting info

[Bill Becker]

I'm new to this site, I joined this one since it seems to have more users and information. I'd like to share my experience with performance problems and solutions for fuel and carburetors. I welcome others input and advice. I own a tree service in northern Virginia, over the last two years I have had more trouble than the previous 18. I have rebuilt nine carburetors this year alone (some less than two years old). We have several Stihl saws that see daily use. The fuel is always fresh yet with pump gas we experience water droplets in the fuel tanks and have to clean contamination from the carburetors frequently even with fuel additives. The diaphragms, fuel and impulse hoses deteriorate. We started using VP Racing Fuels SEF94 50/1 Premix (synthetic oil, no ethanol) two months ago, costs about $70.00 per five gallons. I add .64 ounce of Amsoil Sabre or Stihl HP Ultra per gallon to make it 40/1. I also changed the jets and exhaust muffler covers on some of the saws. They start easier, rev quicker and have slightly more power although they consume a little more fuel (no increase in smoke). I plan to update this post later in the year after we gain more experience and take apart the carburetors to inspect their condition. We are also using SEF94 without premix oil in our small 4 - stroke equipment.

044 (15 years old, original piston, rings, cylinder, solid exhaust muffler cover, quieter) installed MS460 carburetor with .70 jet (original carburetor did not have a replaceable jet)

066 (17 years old, original piston, rings, cylinder, dual port muffler cover) installed .74 jet (was .64)

046 (5 years old, dual port muffler cover) installed .70 jet (was .64)

MS460 (2 years old, dual port muffler cover) installed .70 jet (was .64)

MS460 (new) installed dual port muffler cover, .70 jet (was .68)

MS650 (new) installed dual port muffler cover, .74 jet (was .62)

MS361 (new) this will be a climbing saw for larger limbs. I do not plan to modify this saw. According to the parts manuals the MS361 has a replaceable jet (comes with .52), the MS362 does not have a replaceable jet.

I use the Walbro and Zama rebuild kits, they include the inlet needle.

Dual port muffler covers:
044/046/MS460 1128 140 0801
064/066/MS650/MS660 1122 140 0800

Jets:
.66 1128 121 5605
.68 1128 121 5603
.70 1128 121 5602
.74 1122 121 5607

Carburetors with replaceable jets and fully adjustable fuel mixture screws:
044/0460MS460 1128 120 0624 with .70 jet
064/066/MS650/MS660 122 120 0621 with .64 jet

Adjustment of carburetor fuel mixture screws will depend on altitude, temperature, humidity, engine size, jet size, condition of air filter, type of exhaust and type of fuel and oil.

Now about ultra low sulfur diesel.......wait, that's for another forum.

 

[blsnelling]

Welcome to the forum. I'm curious as to why the larger jets. I've not seen an issue with getting enough fuel even on a ported saw.

 

[Bill Becker]

I believe in the pursuit to meet EPA requirements the manufacturers have made the newer equipment run too lean. My experience with motocross bikes and chainsaws as well as other small equipment is that they start easier and run better with larger jets. This is not always the case however many of my older engines have larger jets. You also have more adjustment range. The part numbers I listed come from the Stihl parts manuals. I also believe that running richer on two strokes will prolong their life, larger jet = more fuel = more oil = more lubrication and cooling. I've had to change spark plugs a little more frequently, not a big expense. Local small engine mechanics I talk to say they see a lot of seized engines on the newer products (2 and 4 stroke). I recognize some of this might be due to misuse as well as lower quality manufacturing although some is probably due to running too lean. Case in point: my oldest 066 and 044 still run as strong as they did when new and they have the original pistons, rings and cylinders. They also out perform some of the newer models.

Bill

 

[mtngun]

Welcome to the forum. I'm curious as to why the larger jets. I've not seen an issue with getting enough fuel even on a ported saw.

+1 :agree2: What Brad said. Dunno what main jet my saws have, but I have no problem tuning them plenty rich. The bigger mains sound like a solution in search of a problem.

If you were having problems with pump gas, I suspect it was because of ethanol and/or water in the gas. Carbs may have to be rebuilt more frequently with ethanol, fact of life. But, carb kits are cheap.

Ethanol will soak up water. There is almost always a little water contaminated gas in the bottom of the jug/barrel. If it looks cloudy, milky, or watery, it's probably contaminated. Toss it.

When I am getting gas from a 55 gallon barrel, I usually end up dumping the last 5 gallons due to water contamination. Or the last few ounces from a 5 gallon or 2 1/2 gallon jug.

Your racing fuel is probably ethanol free, and that's all well and good if you can afford it. Most of us learn to live with ethanol's quirks, even though we grumble about it.

Biggest problem I've had with ethanol is vapor lock in hot weather. It can boil like crazy and make the saw hard to start.

 

[stinkbait]

I read where JJ was talking about the larger jet in the old 066 being better. There may be something to it.