351pcm bogging at idle and when take off
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=2386
Printed Date: November-23-2024 at 10:21am
Topic: 351pcm bogging at idle and when take off
Posted By: 75nautique
Subject: 351pcm bogging at idle and when take off
Date Posted: September-03-2005 at 7:34pm
I have no problem trying to start up the engine but after its fired up it starts to sound like its bogging (coughing). When you sit for a while at idel it will conk out on you or if you sit for a little bit and then put the throttle down it will conk out. Also when you take off it hesitates and doesnt give full performance. sometimes it will take off fine and then other times it will bog. What could be the problem.
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Jake Henning
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Replies:
Posted By: stang72
Date Posted: September-04-2005 at 1:03am
Have you done anything recently...remove carb,change fuel line,filter Etc ?
I would check the easy stuff first...
Fuel filter...fuel lines (for air leaks),vacuum leaks...line obstructions and fuel tank vent.
Ignition...plugs,wires,points,cap and rotor.
Might need to advance the timing.
When did you last replace the coil and the resistor?
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stang
Face plants are not that funny when it's you face!
http://www.correctcraftfan.com/diaries/images/3720/photo1.jpg" rel="nofollow - The Super Air
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Posted By: S.T.
Date Posted: September-04-2005 at 3:01am
Similar thing happened to me all of a sudden. It turned out the small spring on the centrifugal advance broke and I was getting 26-degrees of advance at idle. It ran fine at speed (where it should have that much advance) but idled horribly and wanted to die when I put it in gear.
...just another thing to check. You can look at the springs but the ridiculous idle advance will also give it away when you throw a timing light on there.
If that happens to be your problem, you can't get parts for the Prestolite distributors. I found that the weakest spring in a Moroso advance curve kit for a Chevy HEI distributor matches the old spring very closely.
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Posted By: topspd
Date Posted: April-25-2006 at 9:08pm
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I have a similar prob. Boat will start up fine, and idle good. If I stand on the throttle it will take off fine, but after about 3-5 seconds will start to hesitate on and off for the next 10 seconds, but will eventually clear up and continue to full speed. Timing was initally 6 degrees, increased to 10 and still no changes. Boat runs great after about 10 seconds, and like I said WOT is fine. There's plenty of gas visable through the site hole when the problem occurs. I can give enough gas to pick up a large skier, but only hesitates if I stand on it. Any ideas? New holley carb 4160, lines, filter, mach. pump, clean tank. I've raised the fuel level on the float just above the bottom of the plug, but didn't change the performance. Also, no black/blue smoke during start, or during the hesitation so I assume it's not running rich. Still smells like a classic so I don't think it's lean either. Thanks to all.
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Posted By: tjackle
Date Posted: April-26-2006 at 3:18am
I had that problem. I rebuilt the carb (Holley 4160), replaced the fuel filter(which was corroded badly) and checked the timing(it was a little off). Ran great after that.
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http://www.correctcraftfan.com/diaries/details.asp?ID=1520&sort=&pagenum=1&yrstart=1976&yrend=1980 - 78 Nautique
If you're going to do it...
Do it in a boat
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Posted By: topspd
Date Posted: April-26-2006 at 3:43am
The carb's been on the boat for maybe 3 months, if that. Can't have more than 10 hours on it, and the carb was brand new, I also replaced all the lines, mach pump, and filter. I also had someone clean out the tank, there was all kinds of crap in there...maybe there's more. I just can't imagine there being much, seeming that I can still see fuel through the site hole when the problem occurs...but maybe something made it's way to the carb. Although, it runs fine WOT, speedo creeps up slow, but I'm at about 4000-4200K RPMS, and it sounds great. I don't like holding her there for long, but I love the sound of older boats. And what do you mean by timing was a bit off? What did you have it at, and what did you change it to? Like I said, I was at about 6, and moved to 10 advanced, didn't do anything besides slow down the starter.
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82 Ski Nautique
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Posted By: Tim D
Date Posted: April-26-2006 at 8:13am
Check the intake for leaks. Spray carb cleaner down each side where the intake meets the heads. If it revs up or levels off you have an intake leak. Does it have black smoke coming out the exhaust when it is bogging? Also, I would replace the power valve and the accel pump in the carb.
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Tim D
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Posted By: 79nautique
Date Posted: April-26-2006 at 9:19am
Jake when was the last time the carb was rebuilt?
Look for a vaccum leak around the intake and the carb, tighten all of the bolts on the carb, bowls and baseplate. Look at the accel pump and make sure it's giving you a full squirt of gas for the full stroke and the float maybe a shade high. But it sounds like your ready for a re-build.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: 79nautique
Date Posted: April-26-2006 at 9:21am
Topspd lower your float a little your timing is where it should be, but you could have an issue with the mechanical advance and not the base timing.
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http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
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Posted By: skyhawkflyer
Date Posted: April-26-2006 at 12:33pm
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As the engine warms is your choke opening all the way?
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Posted By: 91nautique
Date Posted: April-26-2006 at 5:10pm
I had a similar problem with my gt40 351,it was a vaccum leak under the carb.The thick gasket fitted from the factory has a stainless spacer impregnated into it to cover the exhaust crossover port allowing the exhaust to preheat the carb in cold conditions and prevent exhaust gasses being drawn into the carb.
When I rebuilt the motor I replaced the original gasket with a standard holley type gasket and the exhaust gasses burnt through it.
The factory gasket seems unavailable,but skidim have a fix for the problem.
http://www.skidim.com/prodinfo.asp?number=EDL-2732
And two of these(one either side of the plate)
http://www.skidim.com/prodinfo.asp?number=RM0054B
Ive had another similar promlem with eratic engine performance,turned out being the anti syphon valve in the top of the gas tank,so maybe have a look there as well.
Hope Ive helped
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