Super Jet Tuning abnormalities

[Waternut]

I'm kind of curious if my problem is carb related or something else. I bought an engine a few months back and it ran well. It had instant clean hit on the bottom end even after idling for a while and the plugs looked perfect on the top end. However, if I'm going slow, there is a wierd transition somewhere around 1/4 throttle (rough estimate) where the motor just yanks me if I pull the throttle a hair. I thought maybe it was the porting on the last motor and ignored it. Then I bought a more powerful motor, tuned it, and it does the exact same thing. Only the new motor does it with both stock 760 carbs and aftermarket GRS carbs. The GRS carbs provide better power all around but that funky transition is still there.

Any ideas what the problem is and is it fixable? Could it caused by carbs, porting, pipe, ignition, or what?

 

[madscientist]

are you running flow control valves? they can create that "HIT" as we seem to call it. the change in the back wave is dramatically changed when it goes from dry to wet.

 

[Dustin Mustangs]

I say pipe tuning as well. I can get mine to be light switchy or linear depending on how I have my screws tweaked.

 

[Waternut]

I do have the red FCV. I could never get it to open up at any decent speed so I had to cut a coil off of it. Maybe it's still opening at too high of an rpm creating that funky spot. The waterbox is usually only slight cooler than the chamber and head pipe. Any ideas which way I should go to try and fix it? Wetter or drier?

 

[munki63]

Sounds like a problem I had last summer, for me it was a bad needle/seat

 

[chriselmore_1]

maybe try to run your stinger dry? A buddy had me set up mine no water in the stinger top and middle screws closed and bottom opened half a turn. It's bean the best setup I have tried thus far instant hit off idle and smooth all the way up.

 

[madscientist]

you could try running just a restricter in the stinger instead of the fcv. it will keep the power more liner by not having a dry/wet effect that the fcv produces.

i run a 75 main in mine.

 

[Matt_E]

Can somebody explain to me why there should be little power without water in the waterbox (FCV off), and then there is a big power increase when water is present?

This doesn't make a whole lot of sense to me personally.

The purpose of the FCV is to dry out the wb at low RPM to prevent loading up.

It sounds to me like the OP found the exact spot where his motor gets on the pipe, abruptly.

 

[Dustin Mustangs]

^ That's what I was talking about. ^

You can change when and how abruptly your pipe comes on with how much water you are putting into the chamber. Or at least that is what I have found when playing with mine.

 

[Matt_E]

Yes, but the FCV to the water box has nothing to do with that.

 

[orangefinger]

We had a similar problem with a ski last year, unknown configuration...there was nothing and then came everything all the power like arm janking...it was very annoying, anyway this is what we tried.
Engine was 701 with dual 38's

- FCV different setting, and eliminated >> no/minor results
- B pipe , screws diff settings >> no/minor results
- Exchanged 2 different set of carbs & numerous jetting configurations >> no/minor results
- Exchanged waterbox >> no/minor results
- Raised/Lowered compression >> change but did not adress problem.

Which left us to

Airleak ?
Porting ?
Electrical ?
Pitch impeller ?

Then summer was over...I am curious to what it could have been

 

[Waternut]

If this happened during the summer I wouldn't have any issues tinkering with this and finding the sweet spot. However, since it's so cold here right now, I have limited time before I get too cold to ride.

So with that said... Doesn't the FCV just regulate water going into the waterbox and therefore doesn't affect how the pipe operates? Next, would adding more water to the pipe lower the rpms when I get on the pipe or raise them? I would think if I could get on the pipe a little sooner, it would be better for both power and for cruising around since right now my cruise zone is very close to this transition.

 

[Matt_E]

Say, what compression are you running?
Sounds like someone took a motor built for high compression/race gas /advanced timing and detuned it.

 

[chriselmore_1]

exactly my thoughts

 

[Waternut]

Say, what compression are you running?
Sounds like someone took a motor built for high compression/race gas /advanced timing and detuned it.

Someone has a little birdie on their shoulder. This engine is actually supposed to produce around 220psi. Currently the engine is producing about 205 on one cylinder and 180 on the other so it's down on compression but still runs incredibly strong. I have the next size pistons and rings that I plan to put in there after Daytona. I'm currently running straight avgas. I left the static timing at 30° but I started backing off the timing earlier than I normally would (4500rpms I think).

 

[suva]

The last ported cylinder I had ran like that. 'Race' porting without high compression.

 

[Matt_E]

I'd say that's your problem right there.

Higher porting makes for much better mid and top range power, and you compensate for the bottom end loss with compression and timing.

 

[Waternut]

Well that explains the one engine but the other engine didn't have any exhaust port work as far as I could tell and only the intake ports were cleaned up.

 

[Polish jet pilot]

widening exh. port doesnt change duration BTW. I'd say needle and seat or/and lac of proper compression for the porting you have.

 

[Matt_E]

Who says the exhaust port was only widened?