High speed bog

S

Shimjocky

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Jan 18, 2015
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Location
Michigan
Howdy folks. First post, but have been reading for a while. Did a bunch of searching here and elsewhere on my problem but never found what I was looking for.

I have an 83 Excel III 340 that I just did a motor swap on. I cleaned everything up, rebuilt the fuel pump, went through carb, etc.

It starts fine and idles nicely. Maybe a little bit of idle hang. My high end bog comes at wide open throttle. If I back off to about 3/4 throttle it runs pretty darn nice, but anything above that and it falls on its face.

It seemed to be lean, so I adjusted the float for more fuel. i actually had the carb on and off twice before I felt I was really close to the 25MM height. Still no joy.

Left plug is showing lean, so then I start to suspect PTO side crank seal. I warm it up and reach in behind the clutch with the wand on a can of MAF cleaner and sprayed the heck out of it. This had no effect on the idle speed.

I then checked the intake manifold, all good there too but I snugged up the bolts a bit anyway, no change.

Is the PTO side crank seal exposed on this machine? I started to wonder about it being exposed vs behind a plate or some such thing that might have rendered my test unreliable.

This one has me a bit stumped so far. What do you all think?

Thanks for any input. :)
 
Single,carb, single piped, twin cylinder motor lean on one side??? Air leak on the lean side. Could be the intake manifold gasket on that side or a head or base gasket. You should check for leaks there first. The crank seal test with starting fliud will not work on this sled since the oil pump housing is in front of the seal on the pto side completely incasing the seal.
 
Float level doesn't have anything to do with how much fuel the engine gets, that's decided by the mains, pilots, needles etc. The float level only keeps fuel in the bowls so the various carb circuits have fuel to pull from. Should be set to factory height.
 
YooperWoods said:
Float level doesn't have anything to do with how much fuel the engine gets, that's decided by the mains, pilots, needles etc. The float level only keeps fuel in the bowls so the various carb circuits have fuel to pull from. Should be set to factory height.
Click to expand...

My concern was that perhaps the float was set too low, which would potentially create a fuel starvation situation at high load. My issue was in fact trying to get that 25MM height set correctly. It was quite a bit low, but that did not solve the problem.

I was spending some time on the sled today. I could not find any air leaks. What I did find when I pulled the intake manifold was water in the bottom end! No clue how that got there. I used a fuel safe pump and a chunk of gas line to get it all out. I then dumped a bunch of gas in there several times followed by slowly turning the engine over to churn it all up, plugs out. Each time I would pump it all out and then do it again.

The gaskets were all good, but I used some good high tack gasket stuff anyway just to be sure.

I fired it up and decided to do a short run around the house before putting the air box back on. Thing ran like a scalded cat!

Back to the garage, and back on with the air box and belt guard.

Fired it back up and warmed it up to do my low speed adjustment. Everything seemed good so I went for a quick blast down the road. Same problem as before! It is a bit better since the water is out, but still has the high speed bog over about 3/4 throttle.

I was sure that the carb had the stock jetting, but perhaps I was wrong. The air box is clean with no obstructions. Is it safe to assume then that if this is down to jetting its running rich at WOT? The plugs sure never seemed to indicate rich.
 
YooperWoods said:
Float level doesn't have anything to do with how much fuel the engine gets, that's decided by the mains, pilots, needles etc. The float level only keeps fuel in the bowls so the various carb circuits have fuel to pull from. Should be set to factory height.
Click to expand...
For the most part yooper, you are correct, however the needle and seat CAN in fact influence how much fuel an engine gets. A friend of mine was fighting with a wot bog on an INDY storm a while back. He tried replacing every thing. It turned out to be that the needle and seat was too small. Most sleds run a 1.5, which is what this indy had in it, but it was supposed to have 1.8, there for it would run fine wide open for a short distance then more or less run out of gas. I would bet a float set too low would pretty do much the same thing.
 
Thanks for all the replies.
Just came in from a test ride and she is running great!

I could have sworn the jetting was stock, but it turns out to have been a bit rich. Main was 250 and pilot 80. I put in the stock 240/75 and off she went!

Whats really funny is that after stepping down (leaner) on the jetting, the plug color got better. Its a nice carboard brown now as opposed to the white ashy color it was before.

I am thinking that all the moisture in there may have borked its ability to move the fuel/oil mixture properly through the bottom end and up the ports, but that's pure speculation on my part.

Was a set of very specific circumstances that invoked unexpected symptoms. Learn something new every day I guess. :)
 
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