1980 Excel V: Big flat spot upon opening the throttle?

ws6transam

ws6transam

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Joined
Sep 4, 2007
Messages
69
Age
59
Location
Haslett MI
When warmed up, as soon as I crack the throttle open, the 8H8 540cc engine dies out. If I coax the throttle open, it'll work itself through the flat spot and wail like a motorcross bike, spraying snow out the back like a jet ski. A day's worth of riding shows spark plugs that are not wet, sooty, but show an even tan across the insulator. A little bit of oily residue resides around the threads and plug gasket of one cylinder, but the other cylinder shows no oily residue: Just a clean, tan plug. Same color as the other.

The engine is freshly rebuilt with original bore, just a light hone and some new pistons and rings. The carb is original, unmodified, and sports the stock #145 main jet. The engine has a stock airbox, and the exhaust is also stock. The primary and secondary clutches are, as far as I know, factory original. The float level is set to stock specifications. I pulled out the low-speed jet and cleaned it. All orifices that I could find were cleaned out. The pilot jet is turned out 1 1/4 turns, and I've experimented with it, and it doesn't seem to help. (It does change the idle though).

So why the flat spot? It's like the carb is in serious need of an accelerator pump or something. My OEM Yamaha Excel V book has only one clue, and it's troubleshooting guide points to the slow speed jet. Yet I didn't see any troubles with it. Is there anything else that I could check before tearing the carb apart again?
 
Check the secondary clutch and make sure its not sticking open a bit and the belt is riding high. My excel V has a brass sleeve on the shaft in the secondary that wedged it open about an 1/2 an inch. It caused the machine to feel like it had dead spot until it would wind up enough power to turn the track.
 
I don't exactly know the size of the carb, but I think it's a Kehin 38mm.

As for the clutch, it's worth a look, but I've gone over them pretty well and they slide freely. When I open the throttle, it's not a bog where the engine sounds like it's lugging or anything; Rather it is a >whuff!< and the engine stops firing until I let off the throttle and ease into it slowly. If I keep the throttle blades open, the engine just stops spinning. It's almost like it leaned out, which is what I think is happening. Closing the throttle results in the engine instantly re-lighting and going into a nice idle. Once the engine is past 3000 RPM or so, wide-open throttle results in tremendous power, enough to spin the track silly at anything less than 20 MPH.

I'm going to have to pull that carb again, I think, and go over it one more time. I just dont know what I'll be looking for.
 
when you have the carb opened again make sure you blow everything out
with shop air,all jets and passages,take your time do a good job..

any jets you can remove remove them and blow 'em out,held up to a
light you want to see through them

make sure your fuel pump is working properly too
 
Ya sounds like some kind of blocage somewhere, or the floats are set too low, make sure the tab is even with the metal around it or maybe the pick up is clogged. I have an extra kiehin off an enticer, I know the enticer 340 and exciter 440 use the same size carb, just different size jets -- i should leave my manual here, hmm. I thinks its a 34 but not certain either. If they all use the same size you could swap jets and be all set. I you want to try it let me know i am in owosso, I will let you have it for twenty bucks.
 
I made some progress tonight!

Yeah, there is a blockage: Right in the front of the carbuerator throat is a passage called the air main jet circuit, I think. It leads into the center of the carb to the main jet pickup tube, and it was totally blocked. I couldn't get it unclogged so I disassembled the carb, turned on the oxy-acetylene torch, set the tip for fine pinpoint, and heated up the brass pickup tube until the carb cleaner flashed into flame. After putting out the fire a few times, it stopped flaming and got nice and hot. Then I sprayed carb cleaner into the port a little at a time until I was able to get the red plastic tube into the port without melting it. I met with some success, as now the carb cleaner blows through and bubbles out the main jet. It might still be partially clogged, but I dont think it can be cleaned without pressing the pickup tube out of the carbuerator for a thorough soak.

After reassembly, I took it for a ride and sure enough, the dead spot is now much improved! I can't kill the engine with the throttle unless it's idled for half a minute and I completely whack the throttle to WOT. That'll stall the engine, but as long as the snowmobile was moving, I didn't experience the dead spot. I'll live with it the way it is for now I think, and just try running a few tanks of seafoam through the system during next week's vintage trail ride in Cadillac Michigan. (See sticky notes). Now that the port is slightly open, perhaps the seafoam'll finish the job.
 
Well, the Cadillac trip was less than thrilling in terms of performance. I got left behind on the trails a few times because the engine was coughing backfiring and dying: It idled just fine but as soon as I attempted to put any power into it, it just plain died. I thought it was loading up, but when I pulled the plugs they looked fine! Then I thought it was electrical, until someone showed me that if I pulled open the starter circuit, the engine would regain its power. I could then flip the starter circuit off and it would run with power like crazy, until whenever it was that I shut down the throttle again. I was only able to finish the ride by flipping that darned lever up & down: It had to be off when I idled to a stop or the engine would die. Yet it had to be on in order to open the throttle. It was bad, like having to use two throttles at the same time....

So, it's definitely carb related, it seems. I don't want to burn up the engine: It seems most likely that it's starving for fuel at times, as if the fuel isn't getting up the jet. Funny thing, it ran better with the #135 jet than it does with the #145 jet! The plugs still looked tan and dry, not much carbon, but that was on the trail when looking for fouled plugs... (I haven't looked at them since getting home: With all that lever flipping I've no clue what color they are now).

All I can think of is that main air jet... It's the only port I can find that's still blocked but it looks as if I need to press the main jet pickup tube out of the body to get at it.

Before doing that, I probably ought to find a new carbuerator body.

This is a Kehin 8H800 RF07, 44mm throat, with #145 main jet, #95 slow jet. The float height is set to spec of .600" height which is within .010" of nominal. The air box is stock, as are the pipe, reed cages, and the ports.

Someone once told me that this is the same engine as the SRV540. Can anyone confirm this? I need to know what crosses so that I can start my search. This snowmobile handles nice, if only I can get the throttle response to follow, this'll be a heck of a fun sled!
 
FOUND IT!


It was indeed the main jet pickup tube that was plugged up with black goo. Probably was a mixture of mouse droppings and gasoline that did it in. I was previously unable to get the emulsion tube out of the carb body, but some vice grips and some careful back & forth wiggling managed to break it free. I cleaned out the little holes, pipe cleanered out the black sludge that was packed in there, and then lots of compressed air through the air main jet circuit. After pressing the tube back in place, I was able to spray lots of carb cleaner through the passage and it was spraying into the pickup tube whereas before it would only trickle.

I haven't had a chance to try it out yet, but I can say for sure that the flat spot is completely gone, and it tries to climb up and out of the trailer when I nail the throttle. The problem was, as far as I can tell, that without the air flow, the pickup tube isn't able to suck the fuel up through the main jet. Thus, major leanout, stumble, and dieout. I'm surprised I didnt nuke a piston when this thing plugged up.
 
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