Pulsating at max RPM - help please.......

LeEarl · Post by **LeEarl** » February 13th, 2007, 1:52 am

OK let me set this up for you. I just rebuilt my daughters kitty cat engine and have it read to race again. One of my friends had all the parts new that I needed and I was able to put in NEW crank and rod, piston, rings, seals, gaskets and bearings and had the cylinder honed. The engine is pretty much totally new, now here is my problem. Compression is good (better then before).

When I bring the engine up to top end it will pulsate at max RPM. I was getting about 10,500 rpm before and now it runs at 9800 and climbs to about 10,100 and then drops back down to 9700 and then back up to 9900. I can not get it to run at a consistent rpm at the top end.

I did the break-in correctly and have checked the piston and rings to make sure everything looks correctly. Any ideas what I can do to get it up to speed? I have raced this sled before and it was good but I wanted better, but now it is a little worse then before.

Could it be ignition or maybe in the carb somewhere? I am at a loss at where to look.

thanks,

Engineer · Post by **Engineer** » February 13th, 2007, 1:53 pm

Sounds like it is running out of fuel or starving for fuel. Do you have the 2.0mm needle and seat in the carb or the stock 1.5mm?

LeEarl · Post by **LeEarl** » February 13th, 2007, 2:09 pm

It is not running out of fuel. I have the 2.0 and the float set up as high as I can before it starts to run out. Tank is lifted to the max and the fuel line is as short as possible. I have raced this sled before and it was pretty good but I wanted it better so I rebuilt the engine. No I am still at the same point if not a little slower.

Not sure what to try next........

Engineer · Post by **Engineer** » February 13th, 2007, 3:11 pm

The magnito might not be able to keep up with the RPMs. Do you have a scope or some way of looking at the coil voltage when its doing this?

LeEarl · Post by **LeEarl** » February 13th, 2007, 3:27 pm

I have never done that before.... You tell me what I need and what to look for and I will make it happen!

t-dub13 · Post by **t-dub13** » February 13th, 2007, 4:17 pm

I think Engineer's first assumption is right. That would be my first assumption anyway. But usually when an engine is rebuilt, it will run richer than before due to the better scavenging / compression...

I'd do a quick carb check anyway just to make sure there isn't a little piece of mung in the main jet. I've had to re-re-rebuild carbs before, especially ones with these tiny, easy to plug jets. Good luck!

Engineer · Post by **Engineer** » February 13th, 2007, 4:57 pm

An easy tool for checking is one of the Champion Spark plug brand neon tube pens. I looks like a ball point pen but it has a neon tube in the middle. You place it next to the the insulated part of the spark plug wire and the brightness tells you how high the voltage is. There is no scale, you just look and make sure it doesn't go out or too low. Then you know you have a problem. If it goes really bright, you have problem also. Another way is to use an inductive timing light and see if it stops sensing the signal. Those are the shade-tree mechanic ways, the best is an automotive scope like a SUN or BEAR.

LeEarl · Post by **LeEarl** » February 13th, 2007, 5:56 pm

t-dub13 wrote:I think Engineer's first assumption is right. That would be my first assumption anyway. But usually when an engine is rebuilt, it will run richer than before due to the better scavenging / compression... I'd do a quick carb check anyway just to make sure there isn't a little piece of mung in the main jet. I've had to re-re-rebuild carbs before, especially ones with these tiny, easy to plug jets. Good luck!

This is my 3rd race sled and the only one that acts like this. I have also replaced the main jet (70) and have cleaned it very well. I have even tried a good working carb on it with the same results...... Sled is getting plenty of fuel..

kidsperformance · February 13th, 2007, 10:38 pm

1. do you have point ign or CDI...A new crank..* the past when I have rebuilt other engines for other people and there new crank that they bought from the dealer was out of index....everyone I checked...They were all out .003+....take off the fan shroud and spin the flywheel and see if the flywheel bobs up and down....another ? what fuel are you using and how much oil and what type....Use Iridium plugs from NGK....like BP6 or 7HS...that plug takes 4000 less volts to fire over other plugs that I have tested....These motors only like to run on good clean pump gas...NO RACE GAS...but it's hard to find with all the alcohol base gases that are out there....VP makes a good clean pump gas called MS93....good stuff....

LeEarl · Post by **LeEarl** » February 13th, 2007, 10:56 pm

It is a newer engine, so I think it is s CDI (1998). When did they switch? The crank was built buy crank pro that trues most all the kitty cat cranks in our area. He told me it was within .001... There is no vibration in the engine when it it running. Same gas and mix as my other daughters race kitty cat and that one runs fine.... I just installed a new plug after I rebuilt the thing - NGK.

I will check the voltage tomorrow night....

kidsperformance · Post by **kidsperformance** » February 14th, 2007, 8:02 am

.001 won't kill that much RPM..When I do a new crank they are very easy to make them dead on...CDI I think started in 1985..there's something missing....Measuring the voltage is done through a scope and fed into my laptop in with conjunction computer software that I have...also was the rod clearanced?? They are clearanced to tight when they are new and have to be opened up(honed) .0005 to .0007 is they clearance you need..esp if you want to spin a stock motor 10K + RPM

t-dub13 · Post by **t-dub13** » February 14th, 2007, 11:00 am

Is the governor completely eliminated? Stock carb or round slide Mikuni? If you used the common method of governor elimination where you just loosen the pinch bolt on the shaft, is there any way that the governor is actually trying to govern still by the shaft hanging up in the clamp or something???

Interesting about the ignition theories. I think I need to get a Champion Neon Tube Pen!!! Neat tool.

dantheman · Post by **dantheman** » February 14th, 2007, 11:24 am

does it surge the whole time its running or just after its warmed up????

LeEarl · Post by **LeEarl** » February 14th, 2007, 1:34 pm

The rod should have been set correctly. This guys does many of these a year for the local racers. The governor is completely tied off the same way my other 2 race sleds are.

Guys, this is not my first race sled and I can usually get them running pretty good. This one had me stumped..... it is like the engine is trying to get up to top rpm but just can not make it.

dantheman, I have never held the throttle wide open when it is cold, so I have no idea if it is surging at top RPM when it is cold.........

it only pulsates at top end, idle is good and holds pretty good even rpm until I get up around 9800 then starts to pulsate....

kidsperformance · Post by **kidsperformance** » February 14th, 2007, 1:40 pm

LeEarl....doe you live in Wis. ? and if so will you be at the state championship race in Minocqua....????

LeEarl · Post by **LeEarl** » February 14th, 2007, 3:13 pm

No I live in MN... Way up north were we have had 13 days strait of -20˚ or lower temps in the morning.

I need to work on this sled tonight all night long. I will let you guys kow what i find out tomorrow.....

t-dub13 · Post by **t-dub13** » February 15th, 2007, 10:39 am

Don't be offended by our simple or silly questions / suggestions. I've been wrenching on sleds for 30+ years and whenever I'm stumped it's usually because I'm thinking too hard about the problem. It's something so simple that I've failed to see it. Case-in-point: '74 T'nt 340, fuel problem, rebuilt the carb 3 times, checked lines, and pulse numerous times, checked everything else too. Turned out to be a clog in the steel elbow that goes throught the grommet coming out of the tank....couldn't see it and with new fuel lines and filter, I was positive there wasn't any problem....boy was I embarrassed!!!!

Sometimes it's hard to see the forest through the trees....

LeEarl · Post by **LeEarl** » February 15th, 2007, 1:10 pm

I hear ya... I have been through most EVERYTHING on the sled and nothing stand out to me.

I worked on the 120's yesterday instead of the Kitty Cat, so no answers yet. I will get there and will let everyone know when I do!

Kitty Cat Snowmobiles

Pulsating at max RPM - help please.......

Pulsating at max RPM - help please.......

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