Backfire through carb under load

Mahoney

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Aug 2, 2004
Messages
537
Well I got my engine running and idling well. On the hose in the driveway everything is perfect, can rev up to 3500 and timing advances, running at 2000 rpm sounds perfect. Idles at 550-600 rpm<br /><br />Dropped it in the water for the first time and up to about 2000 rpm everything sounds great while underway, right at 2000 rpm I get backfire through the carb. If I back it back down to 1200-1500 it goes away.<br /><br />Any suggestions?
 

tommays

Admiral
Joined
Jul 4, 2004
Messages
6,768
Re: Backfire through carb under load

when my filter has its fill of water i get this type of problem<br /><br /><br />tommays
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I just cleaned out the entire tank, removed and everything, brand new gas, new fuel water seperator, new carb fuel filter. Not sure why in driveway totally fine, under load intermittent backfire.
 

Bondo

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Apr 17, 2002
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Re: Backfire through carb under load

You've got Sh!t In the Carb.....<br />Probably in the Secondary Jets........<br />It runs Great in the Driveway,... Because there's No Load on it........<br /><br />Open the Carb up,+ Blow it out,.. Reset the Needle Valves, See what you've got.......
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I plan on doing that again tomorrow, I had taken it apart and reassembled with carb rebuild kit, but the carb probably needs replacing. Its going to be rainy here anyhow, couldn't hurt taking it all apart again, cleaning it, and blowing it all out.<br /><br />So I will not notice this problem even gunning it under no load to 3500 rpm for a second and backing off, but will while it is underway?<br /><br />I am hoping it is something like this, if it is a stuck valve or something I would assume I would notice that problem even with no load.
 
D

DJ

Guest
Re: Backfire through carb under load

So I will not notice this problem even gunning it under no load to 3500 rpm for a second and backing off, but will while it is underway?<br />
That's correct, no load, in the driveway. BIG DIFFERENCE, in the water.
 

Mahoney

Chief Petty Officer
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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Thats encouraging, I was hoping it was something like that and not a valve problem or something. The tough part is, I can't really test out my solutions till I get back on the water. argg
 

Mahoney

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Aug 2, 2004
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Re: Backfire through carb under load

One more thing, you guys are great, I have learned so much using these forums getting this boat back under way.<br /><br />While I have the carb apart(rochester 4 brl, mechanical choke), is there anything specific I should look for? I will prolly post pics of things I think are suspect if I can get around to dissasembling it tomorrow.<br /><br />Thanks again.
 

Bondo

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Re: Backfire through carb under load

Well,.......<br />With My Experince with Rottenchesters,.......<br /><br />I Prefer the "Australian Float Test"............. :D <br /><br />When achris sees this, He can Explain it Much More Clearly than I............
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Haha, yah yah, I am not there quite yet. I did find something suspect just now when I took the carb apart, there was some old gasket material partially clogging some holes by the primary jets, which can't be good.<br />Soaking that carb plate in cleaner overnight then making sure I got all pieces clean then putting it back together.
 

trog100

Senior Chief Petty Officer
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Dec 1, 2004
Messages
751
Re: Backfire through carb under load

snapping the carb open very quickly should show up the lean mixture problem if its still there without a load.. snapping it open very quick provides its own load and enough air all in one go to be noticable if the mixture is too lean..<br /><br />u might have to disconnect the linkage to do this quick enough thow.. the engine will pick up quick and clean or will kinda gasp and possibly backfire before it picks up..<br /><br />if it still as problems try putting your hand over the carb air intake as u rev the engine a few times as well.. this can help clear minor jet blockage..<br /><br />trog100
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Thanks Trog, I will try that when I get it back together, I have never heard it backfire since I got the timing right in the driveway even "revving it up". I did find some blocked passageways while taking the carb off, I provided some pics of some areas.<br /><br /> http://client.webshots.com/album/311524204nYrPcp
 

mercrewser

Petty Officer 1st Class
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Dec 4, 2003
Messages
367
Re: Backfire through carb under load

I Like the choking method that trog100 mentioned
 

Bondo

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Re: Backfire through carb under load

Ayuh,........ I Don't Think Hand Choking That Carb. would have done Enough.........<br />Man,..... That Carb. is Filthy.........<br /><br />Check for any Play where the Throttle Shafts pass thru the carb Body,........<br />I understand they develop Vacuum leaks at these points........<br /><br />Good Luck.............<br /><br /><br />BTW, trog, You're describing an Accelerator Pump Test............<br />Plugged Secondary Jets will Only show being Plugged,... Under Load..........
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Its soaking right now, she is gonna be clean as a whistle when I am done. I think running her before I cleaned out the tank fouled it up pretty quick.
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I got a 17 gallon 5 horse compressor ready to get at er!
 

achris

More fish than mountain goat
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May 19, 2004
Messages
27,468
Re: Backfire through carb under load

Mahoney,<br /><br />Check the fuel pump pressure AND volume. If it's low then you get a lean condition at higher engine speeds (with load on only).<br /><br />Bondo,<br /><br />At the moment a Rochie could well be going on my engine! I'll be giving the Weber the 'Aussie float test'!<br /><br />Chris............
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Fuel Pressure test is scheduled for tomorrow as well. Thanks guys, I hope this takes care of it!
 
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