4.3 Mercruiser backfires @ 4000rpm and above

[90stingray]

Hey all! Been a long time follower of this site... but just joined today
Got a 17.5' 1990 Stingray 4.3 w/ alpha one. Bought her early last year. It's always had a surge and backfire at any rpm above 4000. I have done new plugs, wires, cap, rotor, coil, fuel pump, new fuel lines, added water seperator, carb rebuild (mercarb 2bbl), and it runs exactly the same as the day i got it. Checked timing when i got her and it was timed 4 atdc! Retimed to 8 btdc and runs the same.

I found the service bulletin for the mercarb that described the exact symptoms... says to bump jets from 155 to 160. Boat came with 165's installed. Tried 170's still runs just the same.

Dont know what else to look at... just looking for some thoughts and suggestions. Thanks for the help! -JOE

 

[fossill]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

burn't valves, weak valve springs, worn cam lobes, loose timing chain, major intake air leak. make the mechanical end of the engine is ok.

 

[pwpns]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Is it backfiring through the carb or the exhaust? If through the carb I would first check for a vacuum leak you can do this by pushing the choke closed by hand. It should die quickly if the engine speeds up and then dies you have a leak. Another way to check is to spray carb cleaner around the edge of your manifold and base of the carb and see if the engine speeds up. Is your float level correct in your carb? What kind of fuel pressure are you getting? Like Fossill said you might want to do a compression check and a leak down test to help rule out any mechanical problems. Does the surge and backfire happen on acceleration above 4000rpm or is it steadly doing this at any rpm above 4000 rpm?

 

[christoner2002]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Deleted. Was going to mention to try the coil, but saw you already did.

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Forgot to mention did a compression test and got 180-185 across the board. I was thinking more fuel side since it does have a surge... which feels like lack of fuel. Once it is on the slow side of the surge, then it will backfire thru the carb a couple times. After that the rpm will surge up a little then start the cycle over again. Haven't looked at the antisiphon valve yet... any thoughts?

 

[twistedshadows]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

you never stated if it was backing firing through the carb (back fire) or backing firing through the exhaust (after firing) if through the carb you running lean which could be bad float level, carb needs to be retuned, fuel pump is bad, clogged carb/lines/filter/pickup tube, bad anti siphon valve.
if its through the exhaust its because ur running rich or you have burnt or bad valves (a leak down test will tell you if you have valve issues) or ur timing is WAY off.
joe

 

[twistedshadows]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Once it is on the slow side of the surge, then it will backfire thru the carb a couple times.

looks like i was wrong u did say it was through the carb. anyways what i listed will still cause those issues. id check the pickup/anti siphon valve first, then change the filter, then check the fuel pump, then pull the carb and tear it down and clean/adjust it. could also be a intake leak like previously stated and thats easy to find just spray around some starter fluid and see in the engine revs up.
joe

 

[pwpns]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Did you check for vacuum leaks? You might want to check your accelerator pump setting. You might also want to go to Must have technical info for DIY and look at number 3. You might not have enough fuel pressure at that rpm, or you might have an airlocked system leaning out your mixture. I had to replace a pump once because it wouldnt draw enough vacuum even though it still had 7lbs of pressure. Has the intake ever been off the boat?

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Did you check for vacuum leaks? You might want to check your accelerator pump setting. You might also want to go to Must have technical info for DIY and look at number 3. You might not have enough fuel pressure at that rpm, or you might have an airlocked system leaning out your mixture. I had to replace a pump once because it wouldnt draw enough vacuum even though it still had 7lbs of pressure. Has the intake ever been off the boat?

I would think at 4000 the accelerator pump would be out of the system. But there are no adjustments other than bending the rod. The intake has never been off. The vent is clear from the tank so its not trying to create a vacuum in the tank. It runs good at idle so i didn't try to hunt for a intake leak... i will give it a try. But keep the ideas coming. Thanks guys.

 

[pwpns]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

I was talking about the ability of the fuel pump to create a vacuum and draw fuel from the tank. check out the following link http://forums.iboats.com/showthread.php?t=283269. If it is backfiring through the carb it is either lean mixture or a timing issue problem. Do your plugs show a lean condition?

 

[NHGuy]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Surging will often happen if you are getting air in the fuel feed, or it could happen from inadequate fuel flow at your higher RPMs.
Anti siphon valve is easy to check. Get 2 wrenches and hold the tank fitting with one while you turn the anti siphon valve with the other. Remove it, move the check ball with something that won't bung it up, preferably something hard plastic. Rinse it well with carb cleaner or fuel. If there is any question just replace it.
I had some carb popping when I needed a fuel water separator this summer. That could be involved.
You were smart to do the fuel lines etc, did you change the little cylindrical filter on the carburetor entry? On edit, that would go with the carb rebuild.
You might be smart to pull the fuel sender & look in the tank to check for crud too.

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

I like the idea of plumbing in a vacuum gauge in the fuel system. Also, i have not looked in the tank. I think i will replace antisiphon and have a look in the tank. Maybe adjust the carb float level? I just got her winterized for storage. But i am ready to get this thing running like it should. If anyone has any more thoughts... they are all very appreciated.

 

[Aloysius]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

It's probably NOT fuel related. There is a constant reservoir of fuel in the float bowl..momentary loss of fuel pressure, or air in the fuel, won't affect it. Very seldom is backfiring fuel related.

don't rule out valve float. Stock springs aren't designed for prolonged high speed operation, and are good for around 5000 rpms at best. Lose some spring tension, rpm max. will drop.

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

It's probably NOT fuel related. There is a constant reservoir of fuel in the float bowl..momentary loss of fuel pressure, or air in the fuel, won't affect it. Very seldom is backfiring fuel related.

don't rule out valve float. Stock springs aren't designed for prolonged high speed operation, and are good for around 5000 rpms at best. Lose some spring tension, rpm max. will drop.

I am not running at high rpm for extended times. I would think the springs would be good for 4000-4500... even if some were weak. Do you think they would cause a backfire? So what do you suggest i do with the valvetrain?

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Bump

 

[6meter]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Just rereading your post. What brand plugs did you use? Recomend staying away from Champions. Not judging you or others, but was the carb done completely and correctly? Does it happen at all engine temps? Or only warm or cold? My last thought is you replaced a bad part with another bad part.

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Took out automotive champions and used the marine ac delcos. Used the full rebuild sierra brand rebuild kit. Never had a problem rebuilding carbs before... i thought about going back into it to. That mercarb bulletin wants the spring loaded needle and seat assy, i used the solid. But i think that is to help a flooding problem. But like i said thought about opening her back up to do eveeything mercarb has changed. Choke is always open...but starts no problem. So i never messed with it. But surges and backfires warm or cold, makes no difference.

 

[Aloysius]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Please clarify..the surging and backfiring only occurs above 4000 rpms? Surging can be caused from a "rich" condition, but backfiring essentially nevers happens because of a rich mixture.

Valve springs..it will have the same problem with or without a load on the engine.

Vibration can cause a bad electrical connection also.

Is the polarity on the coil correct??

 

[90stingray]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Please clarify..the surging and backfiring only occurs above 4000 rpms? Surging can be caused from a "rich" condition, but backfiring essentially nevers happens because of a rich mixture.

Valve springs..it will have the same problem with or without a load on the engine.

Vibration can cause a bad electrical connection also.

Is the polarity on the coil correct??

Yes it will run all day long at 3900. As soon as you cram the gas all the way she will begin the surge and on the low (slower rpm) it will backfire thru carb then pick up a few hundred rpm then start all over. The motor feels like it wants to go but just wont. I don't feel any excessive vibration.
Yes, coil is hooked up per the manual. It's even mounted correct too

 

[fossill]

Re: 4.3 Mercruiser backfires @ 4000rpm and above

Might be unorthodox but assuming its not the 4.3TKS leave the flame arrestor off and reach in and apply some choke (with a long screwdriver or something) when it starts doing that and see what happens (keep fire extinguisher handy). If it quits backfiring you know its a fuel mixture issue. If you got the TKS, it may be possible you have an issue with that.