Good stuff Viking, thanks for those Mikuni part #s...this will make sourcing very easy!Ive read through the Duraforce surging discussions and just some quick comment/logic on the butterfly vs jet issues;
Its assumed Toro/Lawnboy spec'd the butterfly plate with the smaller .055" dia. hole for the R-Tek (snowblower) carb, because LESS airflow is required during winter operation (cold air being more dense, containing more oxygen). Conversely, they spec'd the butterfly plate with the larger .100" dia. hole for the Duraforce mower carb, because MORE airflow is required during warmer summer operation (hot air being less dense, containing less oxygen).
Curiously, Toro/Lawnboy ALSO spec'd a larger 42.5 Pilot jet on the winter carb (R-Tek snowblower). Now this could be due to either the cold air containing so much more oxygen that the smaller air hole was still not enough to maintain proper air-fuel mixture ratio without surging at idle, or they ran a slightly richer mixture on the R-Tek due to the EPA's looser emission regulations on winter spec engines (relates to temperature -vs- atmospheric ozone depletion etc).
To point out the apples to oranges (R-Tek vs. Duraforce);
-The R-Tek is piston ported, Duraforce is Reed valve.
-The R-Tek is spec'd to run at 3800-4000 rpm, Duraforce 2900-3000 rpm.
-The R-Tek is spec'd at 50:1 gas;oil mix (richer mixture), Duraforce 32:1.
-The R-Tek pilot \ main jetting is 42.5 \ 73.8, Duraforce is 37.7 \ 77.5
The Duraforce mower engine should have the larger hole "summer" plate and be jetted accordingly. Using the smaller hole winter plate will starve the mower engine for oxygen in warmer weather causing a "rich-run" condition, unless it is "jetted down" to deliver less fuel as well. A check of the spark plug color will verify either way.
If you have "lean-run" surging (too much air, not enough fuel), then the order of business is 1) Make sure you have a correct and clean air filter in place 2) Check for vacuum leaks around gaskets, seals, air cleaner housing etc. 3) Make sure the inside of the carb is clean with no dust/dirt/debris clogging passages or jets 4) Try a larger pilot jet 5) Try a larger main jet.
My tinkerings with the Duraforce carb thus far have been limited, but apparently it shares the same pilot jet with the Suzuki 47P 2 cycle engine. Maybe someone can chime in if it also shares the same Main Jet design with the 47P as well. Here's some info on the 47P I've compiled; Carb info - so you can source jets anywhere, not just through mower $hops with Toro/LB/B&S pn's: Suzuki 47P 2 cycle, Mikuni B style butterfly carb, Mikuni part numbers;
Pilot jet (N101.043, 16mm long, 6.5mm head), available in increments of 2.5.
p/n: N101.043-37.5 (.375mm dia. holes), Pilot Jet #37.5 (LB "Duraforce")
p/n: N101.043-40 (.400mm dia. holes), Pilot Jet #40 (Toro "Suzuki 47P")
p/n: N101.043-42.5 (.425mm dia. holes), Pilot Jet #42.5 ("R-Tek", stock)
p/n: N101.043-45 (.450mm dia. holes), Pilot Jet #45 (Motorcycle shops)
p/n: N101.043-47.5 (.475mm dia. holes), Pilot Jet #47.5 (R-Tek high output)
Main jet (N102.221, 9mm long, 6mm head), available in increments of 2.5.
p/n: N102.221-75 (.750mm dia. hole), Main Jet #75 (Motorcycle shop)
p/n: N102.221-77.5 (.775mm dia. hole), Main Jet #77.5 (Toro 47P "High Alt")
p/n: N102.221-80 (.800mm dia. hole), Main Jet #80 (Toro 47P, stock)
p/n: N102.221-82.5 (.825mm dia. hole), Main Jet #82.5 (Motorcycle shop)
p/n: N102.221-85 (.850mm dia. hole), Main Jet #85 (Motorcycle shop)
One thing tinkerers discover quickly is the 2 cycle engine, for all its simplicity, is actually a finely tuned machine from top to bottom. This becomes apparent quickly when messing with air-fuel delivery systems engineered into designs by the factory, such as air cleaner housings, air filters, carbs, jets.
This is logical, considering the Duraforce is a 144cc engine and the Suzy 47P is 121cc, I'd expect the Duraforce to require more air & fuel to maintain the same engine speed, utilizing the same carb specs (cross sectional venturi area).Curiously most of the surging DFs were fixed with a #68 wire drill bit (which is just about .8xxmm) on the main jet and an R-Tek 42.5 pilot. I did swap in a couple of the "correct" throttle plates and they ran incredibly STRONG.
This one has me baffled and would it be interesting to know more. Presuming "don't have the updated plate" means having a Duraforce with the incorrect winter R-Tek plate (smaller holes), I would expect those small (holed) plate Duraforce engines to be running rich from the factory, and require smaller jets to match the smaller airflow, not larger jets. That is, unless of coarse LB happen to put waaay too small of jets in those small plate carbs (to meet EPA regs. etc.) as well, still leaving them running lean. If that is in fact the case, I'm surprised those small plate engines ran at all, or at least didn't come back to the dealers with scorched pistons or holes in them from running way too lean.I'd say in the 30+ DFs i've built the ANTI-SURGING path would be
1a-1b. If you don't have the updated plate i've had success replacing the pilot with a Suzuki 47P #40 pilot (these are twice the cost of the R-Tek jet)
Interesting you say that because it always consufed me as well as to why it "should " be rich yet richening it up seemd to alleviate the problem (not entirely correct it).This one has me baffled and would it be interesting to know more. Presuming "don't have the updated plate" means having a Duraforce with the incorrect winter R-Tek plate (smaller holes), I would expect those small (holed) plate Duraforce engines to be running rich from the factory, and require smaller jets to match the smaller airflow, not larger jets. That is, unless of coarse LB happen to put waaay too small of jets in those small plate carbs (to meet EPA regs. etc.) as well, still leaving them running lean. If that is in fact the case, I'm surprised those small plate engines ran at all, or at least didn't come back to the dealers with scorched pistons or holes in them from running way too lean.
yes we have. i somehow couldnt find those ones when i searchedDiddy. We have gone over this already do a search on dura force surging. Also click all the links on the treads. There is a great write up on what to do. If u think it uses too much gas running right then just change it back
Great thread below