351w backfire during accel
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=33936
Printed Date: October-08-2024 at 10:56am
Topic: 351w backfire during accel
Posted By: lx.crowell
Subject: 351w backfire during accel
Date Posted: June-23-2014 at 12:36pm
| I have a 1974 cc southwind with a 351w, just put electronic ignition with a brand new coil, also i rebuilt the carb this winter. when i start to accelerate it will back fire once and then run great all the way through the power band. The backfire comes through the carb and the carb is a holley 4160. Could this be timing preignition, carberation, or something more serious like a sticky valve? It has also been running very rich after i rebuilt the carb any ideas could that be what is causing the backfire? |
Replies:
Posted By: backfoot100
Date Posted: June-23-2014 at 1:37pm
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Did you time it after installing the EI? ------------- When people run down to the lake to see what's making that noise, you've succeeded. Eddie |
Posted By: lx.crowell
Date Posted: June-23-2014 at 1:51pm
| Yea i retimed the motor after i put the EI in i set it at 12 degrees btdc but ended up at closer to 25 btdc because thats were the motor would run at anything less and it wouldnt run. |
Posted By: Bri892001
Date Posted: June-23-2014 at 2:56pm
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25 sounds like a lot of initial to me. Double check your spark plug wiring order. I suspect you might not be hooked up to your actual #1. Another possibility is that you have a timing light with a dial back knob improperly adjusted? |
Posted By: backfoot100
Date Posted: June-23-2014 at 3:12pm
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Wow, something else is definitely off then. Brian is right. Firing order with correct rotor rotation, #1 plug configuration, etc. Until you can set the timing correctly, you'll have problems. You need to confirm that everything is correct. ------------- When people run down to the lake to see what's making that noise, you've succeeded. Eddie |
Posted By: lx.crowell
Date Posted: June-23-2014 at 5:00pm
| Was i correct before with 12 degrees btdc |
Posted By: Bri892001
Date Posted: June-23-2014 at 6:13pm
| Yup, 10-12 is the norm. |
Posted By: lx.crowell
Date Posted: June-23-2014 at 6:16pm
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It seems to running really rich at that timing any suggestions on that |
Posted By: SNobsessed
Date Posted: June-23-2014 at 8:33pm
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Running rich has nothing to do with timing. You should pull some plugs & look at color. Black would definitely be rich. Light tan is best. White is too lean. Sounds like your float is set too high. Other possibilities are improper idle mixture adjustment (easy to fix) or ruptured power valve (unlikely). ------------- “Beer is proof that God loves us and wants us to be happy.” Ben Franklin |
Posted By: lx.crowell
Date Posted: June-26-2014 at 1:13pm
| Ok retimed it ran better then leaned it out and it ran like a top but it still has a small backfire when you go to plane out, then after it backfires it runs great gps said i was doing 38-40mph with three people in it best that boats ever run. I noticed when I slowed down off of plane it had alot of black smoke checked my oil which was coal black after about 2 hours of use on a fresh oil change. is it possible that the motor has alot of carbon build up from running so rich which is causing the backfire? |
Posted By: mamigacz
Date Posted: June-26-2014 at 1:56pm
|
When you say you leaned it out, I'm assuming you turned the idle mixture screws, right? The mixture screws affect idle and a little bit of off idle. This adjustment does nothing for mid range or wide open throttle. Typically different power valves and jets are used to change fuel mixture at mid range and wide open throttle. When you bought the rebuild kit did it have the same power valve as the one you took out? Is it a 2.5? Carburetors have different fueling circuits. The transition from one circuit to the next can be difficult without the help of the accelerator pump. Also, a back fire on hard acceleration is usually the result of lean spot between carberator circuit transitions. Make sure your accelerator pump is working and adjusted properly. As Snobsessed pointed out, make sure the float are adjusted properly. Too low and you will get lean issues, too high and gas will drip out the venturi's. |
Posted By: lx.crowell
Date Posted: June-26-2014 at 2:39pm
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Yes when i said I leaned it out I only adjusted the low speed idle screws and on the low end it was definetly rich beacuase it was dumping unburnt gas out the exhaust and the plugs where black. How do I check and or adjust my acclerator pump? Not sure on the power valve I'll check the vavle and float tonight when i get home. Anything else that I could be missing? |
Posted By: Bri892001
Date Posted: June-26-2014 at 5:03pm
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The Accelerator pump check/adjustment, is about the easiest one you'll make. Don't even have to take anything apart. The adjustment is made on the lever, which is kind of like a seesaw. One side rides along the accelerator pump cam, and the other side actuates the pump. One side, I believe it's the pump side had an adjustment screw and nut. Loosening it, takes up the slack. The only slight gotcha is (on mine at least) you need two of the same size box wrench. I believe it was 3/8. There wasn't room to get a socket on unless you removed the carb. Here's a pretty good pic. It's not a marine carb, but the idea is the same: 
|
Posted By: Mountain Man
Date Posted: June-26-2014 at 6:20pm
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Eliminate the easy things first, especially those that don't require parts. Confirm timing, proper firing order, check the condition of spark plug wires (no burned through plug wires that short between cylinders). Holley carbs built before 1992 lack a check valve that protects the power valves during a back fire. Simple test for the power valve. Crank the engine and allow it to warm, then close (turn in) all idle mixture valves, if the engine dies the power valve is good. If engine continues to run replace the power valve. The power valve opens under acceleration and is opening is initiated by a drop in vacuum. The number on the power valve indicates at what vacuum, (inches Hg) it begins to open). The lower the number - the lower the vacuum required. A 75 opens at 7.5", a 65 at 6.5", etc... If you can get a vacuum reading at idle speed simply divide that by 2 and that's your size needed. 13" of vacuum at idle you would / 2 = 6.5 or "65". Good Luck! |
Posted By: mamigacz
Date Posted: June-26-2014 at 7:10pm
Start out by tightening the adjustment screw until there is slack or a gap between the lever and the adjustment screw. Then, loosen the adjustment screw until its barely touching the lever. No play or slack is the goal. |
Bri892001 wrote:Posted By: SNobsessed
Date Posted: June-26-2014 at 7:55pm
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Another thought - if the carb has foam floats, replace them! I just helped a friend with a merc outboard, one float was cracked & saturated with gas. Runs much better with good float. ------------- “Beer is proof that God loves us and wants us to be happy.” Ben Franklin |