76 sno twister problems

250 twister · Post by **250 twister** » Wed Nov 16, 2005 7:58 pm

ok,i have a 76 s/t 250 that baisically has a new engine.i bought mostly N.O.S parts from image enterprise.the major problem is the plugs foul out after a few seconds and raw fuel is dumping out of the exhaust.the cylinders are not losing compression,but i seem to think it needs a leak down test,this engine has 5 mins on it and not even riddin on. so has anybody had this problem?and what do i do about it?

Post by **Mercury** » Wed Nov 16, 2005 8:24 pm

What is the compression on each side?

250 twister · Post by **250 twister** » Wed Nov 16, 2005 8:53 pm

they are both the same,it was a cold compression test but i cant test now because its been sitting for a couple years and it needs to be cleaned and i should have it running by tomarrow but i seem to think its in the crank case,i have a extra case which is the original but i have to do some work to it.could it be the crank seals?its gotta be in the case because they are both fine and the carbs are set right and everything,i have size 25 jets in it.

SHewelt · Post by **SHewelt** » Mon Nov 28, 2005 10:35 pm

First scenerio: The crank seals. Here's what happens; The upstroke, the piston draws a vacuum / fuel into the crankcase. The piston starts down, the crank seals allow the pressure to leak past the crankshaft & the piston starts up again, drawing very little fuel into the cyls, while eventually filling the crankcase with fuel. The fuel will eventually pour out the exhaust port. I also had this happen with a bad center seal on the crank-shaft (the crankcase pressure leaks to the other cyl past the bad center seal...same results). So when you pull it apart, check the center seal (alum) too. It shouldn't have excessive rocking & should be almost snug, but free to rotate. Good luck.

SHewelt · Post by **SHewelt** » Mon Nov 28, 2005 10:37 pm

Consider this too. I had bad seating float needles, which didn't stop fuel flow when the floats raised. Changed both the needles & seats ($15) in both carbs and that solved my problems...

Post by **Mercury** » Tue Nov 29, 2005 10:10 am

Diagnostic help can be obtained by slathering a generous portion of Vaseline Petroleum Jelly where the crank shaft meets the seal on the clutch side.

It will offer a temporary additional air seal to see if that is your problem. It may also help in visually finding "bubbles" from leaking air.

Use on Clutch side only...you don't want to get this mess on the inside of the flywheel.

NOTE: IT IS A DIAGNOSTIC TOOL ONLY...NOT A FIX.

250 twister · Post by **250 twister** » Mon Dec 05, 2005 6:26 pm

ok we got it running no problem,heres the issue..plugs are wet with raw unburned fuel,its also coming out the exhaust ports,we have not drove just started it in our basment with drive belt off and a custom washer and dryer exhaust system!we ran it for 5 mins and raw fuel was already coming out.we also leaned out the oil mixture to 32:1 and we are running turbo blue gas at 110 octane with spectro 2t oil,this engine has a half hour of idling and never driven since rebuilt with new cylinders,rings pistons, carbs,gaskets,new crank case and a rebuilt crank,and new coils and plugs,also the carbs are leaned out with leaned jets 25 on the pilot and 250 on fixed wich was recommended by mercury for racing.any clues now?its snowing and driving me insane not being to drive this and owning it for 15 years..........please help thanks

250 twister · Post by **250 twister** » Wed Dec 07, 2005 9:08 am

wel i seem to think it could be the middle crank seal,is there any silicon i can use on it to seal it to see if it will run better?and if we do a leak down test will that prove its leaking out of the middle seal?also to could it be the fuel pump because of a old or bad diaphram?i cant remember if i rebulit it or not because its been a long time.

phil brehmer · Post by **phil brehmer** » Sat Jan 07, 2006 5:59 am

simmer little 250. Now lets take a look at what we are doing here. First off what is the temp. in the basement where you are attempting to run this high perf. machine? The jetting you mentioned is for outside air temps of around what? 15-20 deg f. or so? If you are starting the engine in 60-70 or even 50 deg f temps it will almost imediatly foul the plugs and exibit exactly the symptoms you discribed. My suggestion is to drag her out in the cold where she belongs and watch her do her stuff.

SHewelt · Post by **SHewelt** » Wed Jan 25, 2006 1:32 pm

Pull the carbs, bring the piston down to BDC on the same cyl that has the crankcase pulse line for the fuel-pump. Pull the pulse line & blow air through a piece of line into the crankcase with your mouth...not a compressor! If you can clearly hear the air passing into the other cyl (which has the piston up at TDC, and you can see into the crankcase under the piston skirt)...you have a center seal leaking....

250 twister · Post by **250 twister** » Sat Sep 16, 2006 9:43 pm

oh hey i havent checked this in awile due to warm weather,i was wondering if bad ignition system would cause this because i havent replaced any of it excpt the coils and plugs. and i already ran it in 30 degree weather and it made no difference

SHewelt · Post by **SHewelt** » Wed Oct 11, 2006 1:28 pm

The thunderbolt stator & cdi wire insulation will crack & fall off with age, (even if it's an NOS stator in a box), causing wire shorting between them. What is required, is cut back the wires (carefully) as close to the stator & re-solder new wire in place. The old wire appears to be either alum or a really crappy grade of tin/copper...it's difficult to solder to. If you lok at the wiring, I'm sure you will see what I mean. This could be your problem, but usually lot's of backfiring occurs when this happens & engine won't rev.

250 twister · Post by **250 twister** » Sun Oct 15, 2006 12:50 pm

no,it runs smooth no problems except the fact it fouls out the plugs after 2 mins of running, its completly wet.......its confusing

fulltime101 · Post by **fulltime101** » Sun Oct 22, 2006 7:41 am

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by 250 twister
 no,it runs smooth no problems except the fact it fouls out the plugs after 2 mins of running, its completly wet.......its confusing

<hr height="1" noshade id="quote"></blockquote id="quote">If the fuel is good, float needles not leaking and the outer crank seals are good with good compression and ignition is good then it is probably safe to say that the center seal on the crankshaft is faulty. I had one of these on a cat with a kawi engine when I was a kid and it was the same thing.

250 twister · Post by **250 twister** » Sun Nov 05, 2006 4:48 pm

so how would i find out if it really is bad?