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Thread: bigger engine wanting less fuel?

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    bigger engine wanting less fuel?

    My 250rs top end bit the dust a few weeks ago out in the dunes (age/low compression, no catastrophic failure) and I opted to go for the 310 set up from ESR. The previous motor was a simple .20 over bore with a 38mm air striker carb, fmf pipe, and air box removed. It ran perfect (maybe a little lean on the pilot) with the carb set up from esr at about 1100ft ( I think the esr setting is ddg needle on 4th clip, +-182 main, and like a 50 pilot?). When I put the 310 kit on it with their reed set up and trx7 house port I bumped up the pilot 1 size (52 maybe?), the main to a 185 to start. I also went from running straight pump 91 to 50/50 91 and 110 vp mixed at 32:1 w/ klotz on esr's 93oct dome.

    With that said I took it on its maiden voyage today and the bottom end to 2/3 throttle was near perfect but around 3/4 throttle it blubbered out. I ended up having to jet the main down to a 172 to get it to even take over 3/4 throttle, but it is still fat and the plugs are coming out very dark and fat from the top end. I am concerned that a bigger motor wants so much of a smaller main jet. Is this just a side effect from the greater air mass, reeds that arnt 30 yrs old and running race fuel or am I missing something else in the tuning that is causing the main to be fat?
    Last edited by 87r; 12-18-2016 at 10:31 PM.

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    Racer atvmxr's Avatar
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    What I have been told is the bigger motor makes more vacuum pulse through the carb and thus can pull more fuel through the jets.

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    Kind of along the lines of what I was thinking, I just wanted to verify that I wasn't nuts/ going to melt this shiny new aluminum down.

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    Racer Usmoneylover's Avatar
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    Make sure your plug is gapped to around .018 a weak spark can have similar symptoms.

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    I ran 3 different b9es plugs at +- .016-.020 and a b8es for laughs and giggles but didn't notice any difference. My only other concern is that I had to replace my flywheel in the mix of the rebuild as my original stripped when trying to do the dc Conversion and I had to weld a puller to it to remove it. The replacement is off of a 88 trx according to the seller and mine is a 87 long rod. The part numbers are different but my understanding is any of the Trx flywheels interchange anyways. I hadn't gave the ignition much thought however as it appeared to clean up by leaning it out. It feels pretty cool ( I can hold my hand to the jug for a few seconds) even after a couple of hard pulls and doesn't have any symptoms of being lean, I'm just terrified of burning up a 1300$ top end.

    Thanks for the thoughts

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    Quote Originally Posted by atvmxr View Post
    What I have been told is the bigger motor makes more vacuum pulse through the carb and thus can pull more fuel through the jets.
    A stronger vacuum pulse on the jet can be also be caused by the intake (air filter/intake tube) pipe stinger/muffler inside diameter, shape of the intake port, RPM, and how quickly the reeds petals open as well as many other design factors.

    It takes a certain amount of fuel to produce one horsepower. If it is not getting this minimum amount of fuel to support the power, power will not be produced!!! If the 310 engine is making more power than the 250, it is pulling more fuel through a given size main jet than the 250.

    If leaning the main jet makes the engine run with less misfiring near your shift point at high RPM, continue to lean the main jet until it runs clean
    Last edited by Jerry Hall; 12-19-2016 at 06:16 PM.

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    Quote Originally Posted by 87r View Post
    I ran 3 different b9es plugs at +- .016-.020 and a b8es for laughs and giggles but didn't notice any difference. My only other concern is that I had to replace my flywheel in the mix of the rebuild as my original stripped when trying to do the dc Conversion and I had to weld a puller to it to remove it. The replacement is off of a 88 trx according to the seller and mine is a 87 long rod. The part numbers are different but my understanding is any of the Trx flywheels interchange anyways. I hadn't gave the ignition much thought however as it appeared to clean up by leaning it out. It feels pretty cool ( I can hold my hand to the jug for a few seconds) even after a couple of hard pulls and doesn't have any symptoms of being lean, I'm just terrified of burning up a 1300$ top end.

    Thanks for the thoughts
    Jetting does not have much of an effect on coolant temperature and or cylinder temperature. Outside air temperature and the amount of time the engine is run at full throttle are the conditions that have the largest affect on engine temperature.

    I think that the main differences in the years of flywheels are in the diameter of the flange where the rivets are located.

    If an engine is rich enough to misfire at or near the peak RPM, I would not worry about a piston seizure if the engine clearances were set where they should have been and the pipe stinger/muffler restriction is where it should be. A lot of the 310s will seize when new or because the recommended piston to cylinder wall clearance was too tight after a new piston and bore job. The piston to cylinder wall clearance should always be checked before assembly on any new top end. The clearance should be at least .0030" if you are going to put a lot of break-in time on it and jetting it not to produce max power. If you are going to jet it for max power it will need .0035" to .0040" or more depending upon how long it is held wide open at any one time.

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