454 crusader backfires thru carb at wot

[dvarnadore]

rebuilt the carb, checked the spark plug wires, replaced the distibuter cap, replaced the fuel pump, coil checks out ok,has fresh gas and new filters electronic ignition 1yr old starts with no problem-runs at low rpms great-idles down no problem-run the rpms up to about 2000 and the engine loses power and begins to backfire thru carb pull back on throttle a little bit and backfire stops and engine starts to smoothout ,I need to check compression and timing but what else could it be if those check out, other than a sticking intake valve? just bought the engine off the internet.

 

[bubbakat]

Re: 454 crusader backfires thru carb at wot

it is fireing on an open port ck valves to see if one might be adjusted to tight or a cam lobe worn down or lifter belled

 

[rodbolt]

Re: 454 crusader backfires thru carb at wot

hello<br /> yep what bubbakat says. timing is quick and easy to check and can cause that problem. its easy to eliminate it as a cause. a leakdown test will be more accurate than a compression test for this action. (you already know there is a problem). and if nothing else bring it up to the p[oint of missing and short out the cylinders one at a time until the problem quits, then you know what cylinder is affected.<br /> good luck and keep posting.<br /> also a broken valve spring will cause this same problem

 

[Bondo]

Re: 454 crusader backfires thru carb at wot

Another avenue to investagate is the Secondary Side of the Carb...... <br />It Sounds like it Could be getting Extremely Lean....<br />Plugged Secondary Jets, Maybe.....<br /><br />A Lean condition will Also cause a High Speed Skip....

 

[dvarnadore]

Re: 454 crusader backfires thru carb at wot

checked the timing and it was off- so off that trying to adjust it so that the mark would even get in to the gauge field would stall the engine . It appears as that the timing chain has jumped a tooth. It was backfiring thru the carb on all cylinders checked the port engine timing- and the timing mark will move 12Deg with only a slight throttle change from idle . So it appears as if that one is preparing to go.These engines supposedly have 750hrs on them is this normal wear?

 

[JB]

Re: 454 crusader backfires thru carb at wot

Ahoy, Capt. Dave.<br /><br />I have seen this symptom several times and each time the problem was that an exhaust valve was not opening sufficiently to scavenge its cylinder at higher rpm. Pressure remained and when the intake opened the pressure exited via the intake passages.<br /><br />Bent pushrod, rocker failure, bent valve stem could contribute.<br /><br />Good luck.

 

[Yepblaze]

Re: 454 crusader backfires thru carb at wot

First thing that comes to mind is water had gotten into an exhaust port causing a valvestem to rust in guide causing it to not retract properly and may have stuck a piston and bent slightly or just not sealing well at RPM.<br /><br />That was just a first thought!!!<br /><br />Then my next thought was the coil. How was it tested? And is it recieving full voltage at RPM to make spark adequitly?<br /><br />If it will not time to marks, it could simply be a spun ring ron the harmobnic balancer. A timing by ear by a experienced mechanic mich just be enough to make it "sing at speed"<br /><br />Anyway, these are just a few thoughts you might want to kick around.

 

[navigator336]

Re: 454 crusader backfires thru carb at wot

Do the engines have high energy ignition? If so, did you check the timing using a timing shunt? The shunt freezes the distributor advance module so that you can properly set advance at idle. If you checked advance without the shunt it will likely appear 10 to 15 degrees over advanced.

 

[dvarnadore]

Re: 454 crusader backfires thru carb at wot

thanks for the additional information I have timing gears and chain ordered for both engines they should be here in a couple days.still curious about normal wear on engines supposedly with 750hrs

 

[rodbolt]

Re: 454 crusader backfires thru carb at wot

hello<br /> before going to that expense why dont you check the chain? its an easy procedure that requires only turning the crank while watching the dist rotor. the timing chain only times the cam to the crank. it has nothing to do with timing advance. like I say. follow the manual closely and dont splatter gun the trouble shooting. it will save you much time and a lot of frustration. it is very very rare to see timing chain/gear failures on marine engines and 750 is barely broke in for that part.<br /> good luck keep posting and have a nice day.<br /> PS most people that power time a marine engine by ear by pistons.<br />that engine has a specific method of timing and checking the timing/advance curve. you must follow it.

 

[dvarnadore]

Re: 454 crusader backfires thru carb at wot

rechecked the timing and it is timed correctly now, but problem still exists. Checked for any vacum leaks and found none. tried a minor adjustment on the valves still no help? (I guess I'll have a couple spare timing chains if the need actually doe arise.) I had swithed the failed mechanical fuel pump with a generic electric fuel pump that maintains aprox 6 lbs of fuel pressure on the line could it be that this is not adequeate and the engine is being fuel starved at high rpm's

 

[navigator336]

Re: 454 crusader backfires thru carb at wot

You might try swapping the coils on the two engines to see if the problem switches sides. <br /><br />Did you re-jet the rebuilt carb the same as it was? Why were the engines for sale on the net?<br /><br />What's the fuel pressure on the other engine? <br /><br />If the EST distributors weren't new (1 year old), I might suspect the module pick-up.

 

[rodbolt]

Re: 454 crusader backfires thru carb at wot

hello<br /> well if you really want to check the timing chain a quick method is to rotate the engine in its normal direction until the timing mark is at about 10 degrees BTDC. remove the distributor cap carefully watch the rotor while you rotate the engine backwards. stop at the first sign of rotor motion. now look at the balancer. if it moved much more than 8 degrees or so the timing chain is starting to wear.remember the dist rotaes at 1/2 crank speed<br /> most problems like you describe are valvetrain related. but if someone has installed the wrong ign module that could do it as well. its time for a compression as well as a leakdown test. also carefully inspect all the intake valve springs. they can break and still try to work.<br /> good luck and keep posting<br /> while the valve cover is off check all the valve spring installed heights. if one is to low that is the one that is stuck.

 

[dvarnadore]

Re: 454 crusader backfires thru carb at wot

thank you for the feed back it is greatly appreciated. I switched coils as well as switched carbs and the problem stays the same <br />I believe you are correct about the valve train I will do a compression test tomorrow.<br />How can you tell if you have a wiped lobe on the cam?

 

[Walt T]

Re: 454 crusader backfires thru carb at wot

Capt Dave you probably put the wires on one tower off. Re check #1 firing position, wires and timing mark.

 

[dvarnadore]

Re: 454 crusader backfires thru carb at wot

double checked the wires and they check out OK<br /><br />If the electronic ignition is not advancing during acceleration could this cause the problem?

 

[navigator336]

Re: 454 crusader backfires thru carb at wot

I doubt it. I think your engine would just be a slug. You could check with your timing light at idle,1000,1250,1500,1750, 2000,2250, 2500 rpm's. Don't use the shunt for this check. If the engine timing scale doesn't go past 30 degrees of advance, you'll have to use a dial type timing light to figure your total advance.

 

[SS MAYFLOAT]

Re: 454 crusader backfires thru carb at wot

Not an expert by any means, but to elimante the idea of a "lean condition", swap the carbs from one to the other. Problem moves, problem detected. Prolonged lean condition can cause some serious damage.<br /><br />Good Luck....SS

 

[Walt T]

Re: 454 crusader backfires thru carb at wot

I dont care if you triple and quadruple checked the wires. Verify #1 TDC and re wire your cap accordingly. Verify it by removing the spark plug in #1 cylinder, with your finger over the hole bump the engine over until you feel pressuire on your finger. Then as the timing mark comes up to TDC align it at 0. Pop the cap and verify the tower the rotor is pointing at is the #1 plug wire tower.<br />You may be right, but I have seen this hundreds of times. The owners did a tune up and it wont run right. They swear on their mothers graves they did the wiring right and they checked it and re checked it. First thing I do is check the wires and find the problem. Then they say "Hey you're not going to charge me for that are you it only took you 5 minutes"<br />All I am saying is be POSITIVE the basics are covered before you start spending major dollars. I myself have made bonehead mistakes because I assumed something was okay.

 

[DJ]

Re: 454 crusader backfires thru carb at wot

Capt Dave,<br /><br />Check out this thread, you'll be surprised at the outcome.<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=004987