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Hunterb
07-04-2006, 05:46 PM
I have a question for the tuning guru's.

My boat runs great almost all the time. It idles fine, pulls up most skiers without complaining and overall runs flawlessly. This weekend I was pulling a large friend (220 lbs.) up on a slalom ski and the engine would backfire immediately after I punched the throttle to pull him up.It would pop once through the carb and then throttle up normally. It only seems to do it with the heavy friend behind the boat. Should I be looking for something electrical or fuel related? I thought it might be the timing or possibly a slightly lean condition. The ignition has been converted to electronic and the carb (4160) was renewed last summer.

Anyone have any ideas? Thanks in advance if you do.

Bruce

Hoff1
07-05-2006, 07:52 AM
I've had my 1990 with the same engine/carb backfire on me a few times. The power valve was blown. Not sure if this happened before or during the backfire, buy after I replaced it and adjusted the fuel/air on the carb, no more problems. Backfire during the hole shot makes me think it's running a little rich at idle. I imagine your renew kit had a new power valve in it, but either way around it, a backfire will usually cause you to need a new one. Can be picked up at most Autozone's or similar places.

rektek
07-05-2006, 09:56 AM
sounds lean, second the power valve idea. any hard starting warm after sitting ?

Jeff :twocents:

Hoff1
07-05-2006, 09:58 AM
sounds lean, second the power valve idea. any hard starting warm after sitting ?

Jeff :twocents:


my mistake, backfire through exhaust is too rich.

Hunterb
07-05-2006, 02:44 PM
Thanks guys,

I will turn the mixture screws out a bit and see if it fixes it. It has only happened when I try to pull by big friend up so it's only under heavy load. I'm going to check the dist. cap and plugs as well as someone suggested that they may be the culprit.

I'm heading to the lake today and won't be around a computer for a week or so. I'll fiddle with the boat (one change at a time) and report back which one fixes it.

Thanks again

Bruce

rektek
07-05-2006, 04:50 PM
the mixture screws are for the idle circuit, off idle they don't have any effect, bring a power valve and some carb gaskets with you.

tommcat
07-05-2006, 06:22 PM
a lean condition will cause an intake backfire(among other things) and on that carb all it takes is one backfire and your powervalve is blown. they make anti backfire type powervalves that solve that problem.

as far as the lean condition goes, is it only right when you hit the throttle or does it do it when at a steady throttle position?
if it is only as soon as you hit the gas i would look at accelerator pump flow, if it is just generally lean it needs to be jetted up. read your plugs to see if it's lean or not.

Hunterb
07-07-2006, 08:34 PM
Thanks for the replies.
I adjusted the mixture screws out a bit and then adjusted the idle screw slightly. Seems to havefixed it. Took the big man out today and had no problem. Power valve seems to have survived.

I'm on a blackberry so typing is a pain.

Thanks again to those who responded.

Bruce

Hoosier Bob
07-07-2006, 08:49 PM
You can always roll into the throttle. Any four stroke can suffer from lag or a lean with a big gulp of air and no gas. Referring to updraft motorcycle carbs. Down draft carbs I believe do the opposite. Big gulp of gas and no draw created for air=rich. You may just be overzealous! Run some quality carb cleaner (I use Gumout once a year) and ROLL into the throttle. I know the guy is 220 and probably asked for it all but my guess is he could use a little less. :twocents:

Brent
07-08-2006, 07:00 AM
I've had a few back fires. What are the symptoms of a blown power valve ? Can I pick up the correct gaskets & valve from a auto parts store, or would it be wiser to do the skidim route?

Hoff1
07-10-2006, 10:20 AM
Blown power valve will cause bogging on acceleration from idle. Black smoke at idle with a lot of incomplete combustion. Drives fine at high speeds. More likely to backfire as well. You can pick up everything you need at an auto parts store. Last time I replaced my powervalve, I bought some of the blue gaskets that are reuseable.

Brent
07-10-2006, 03:35 PM
Thanks great info. I'm sure that is my problem!