89 Ebbtide 7.4 454 backfire out carb

[brad.wolf]

Hello all,
I have recently put a gently used automotive truck motor into the boat due to a cracked block. I have used the marine parts from the old motor, starter, carb, ignition components, etc.. The motor in the truck ran great before we took it out. Now that I installed it in the boat I get a consistant popping out the carburetor around 2100 rpms. It runs and sounds great at idle, it only pops under load. I have replaced with new ignition coil, new plugs, verified and ohmed out plug wires, checked for crack in distributor, verified timing is set to 8* btdc. New fuel and water seperator, cleaned screen on carb. I still get the consistant pop or backfiring out of the carburetor. While running we removed plug wires one by one to try to locate if it was coming out a specific cylinder, didnt have any luck still maintained the same pop. "would this eliminate valve train issues" It has the same heads that were on the motor in the truck and again it sounded and ran good in the truck. I had thought it was maybe stale fuel, so i put approximately 15 gallons of premium fuel along with octane boost and it didnt help. Think it could be the carb? Possibly ready for a rebuild? Is there any suggestions out there, maybe something I am overlooking? Any help would be greatly appreciated.

Thanks

 

[Bondo]

Re: 89 Ebbtide 7.4 454 backfire out carb

Ayuh,... My guess is, there's Crud in the carb, causing a Lean condition,...

Rebuild it, or replace it...

 

[JustJason]

Re: 89 Ebbtide 7.4 454 backfire out carb

Might want to throw in a compression test to for S&G's

 

[zbnutcase]

Re: 89 Ebbtide 7.4 454 backfire out carb

A shop I know ran into that on a reman 454. Turned out it had a heavy duty truck cam in it instead of the correct marine one. Fought that SOB for a week! That said, you may have carb issues as well.

 

[6meter]

Re: 89 Ebbtide 7.4 454 backfire out carb

Double check your firing order. If it ran good in the truck and not now, its probably one of the items you took off the old motor and put on the new one. See if you can borrow a coil off a known good running boat. New doesn't always mean good.

 

[brad.wolf]

Re: 89 Ebbtide 7.4 454 backfire out carb

Thank you all.
I verified firing order, and wires all correct. Planning on getting the carb rebuilt. Sometimes I have to shoot ether in her to get her to fire. Could be the carb. I will post once I have the carb rebuilt and let you know the status. I will also look into the cam to verify. Thanks for all the responses!

 

[cr2k]

Re: 89 Ebbtide 7.4 454 backfire out carb

What was the timing setting for the truck? You may need to use that since you are using the truck cam. I would triple check the firing order 1-8-4-3-6-5-7-2

 

[fossill]

Re: 89 Ebbtide 7.4 454 backfire out carb

Using ether to get it started is never a good thing, especially for a gas motor. One major issue is that it is very dry and as such is hard on valves, pistons and cylinder walls. You're better to squirt a bit of gasoline down the carb.
Sounds like you got severals problems.
Before you get too involved with the carb, I'd be checking out the mechanicals of that engine, ie: compression test, leakdown test and pull the valve cover and check the valve lift to make sure the cam hasn't a worn down lobe.
Backfiring through the carb usually almost always points to a valve or cam problem. Since you installed an old automotive engine I assume you marinized completely, ie: proper head gaskets. Also, if you kept the original truck intake manifold make sure that egr valve is removed and blocked off. They also can cause backfiring through the carb.

 

[brad.wolf]

Re: 89 Ebbtide 7.4 454 backfire out carb

I will verify timing on the truck motor to ensure that it too is supposed to be.
I also did a compression check. Not great results, but if low compression was the issue, wouldnt it cause it to run bad at idle too??
Compression results
1 150 2 135
3 150 4 125
5 130 6 130
7 130 8 130

I haven't had a chance to do a leakdown test.

Would this have eliminated the valve train, when I pulled plug wires off one at a time and the pop didnt go away?

Fossil, I also used the old intake off the boat motor to put onto the automotive engine. Also, thanks for the heads up on the ether.

 

[brad.wolf]

Re: 89 Ebbtide 7.4 454 backfire out carb

Compression results
1 150 2 135
3 150 4 125
5 130 6 130
7 130 8 130


I am not much of a mechanic, but could anyone tell me if these compression results would indicate a major problem and cause my 7.4 merc to backfire out of the carb? I tried one other carburetor and it was still backfiring. If these results are reasonable, i plan to replace the cam, but if these are not within tolerance then i dont want to waste anymore time and money on the motor?
Thanks

 

[dockwrecker]

Re: 89 Ebbtide 7.4 454 backfire out carb

The general rule is 5% difference in compression ratios and you've got a couple of holes well outside of that. Could be leaky valves/ bad rings but something is definitely amiss here. 125 is really low. I'd try a wet test of compression by squirting a couple of shots of 30W down the low holes and see if it improves. If it does, it's bad rings, if not, it's probably a valve burnt. Either way, it doesn't look good. A carb rebuild may help for a bit, but you've got a combustion balance problem going on here. And it sounds like it's inherently lean if you're having to use ether to start it, and probably weak valves because of it.

 

[JustJason]

Re: 89 Ebbtide 7.4 454 backfire out carb

Your compression is fine.

 

[PitchFork]

Re: 89 Ebbtide 7.4 454 backfire out carb

Your compression is fine.

That is what I thought. If I remember right it is suppose to be 15-20% of spec which I believe is 150psi.

 

[JustJason]

Re: 89 Ebbtide 7.4 454 backfire out carb

Technically it's the lowest can be no lower than 70% of the highest, and all cylinders must be over 100psi.
But that's just being all technical about it.

 

[dockwrecker]

Re: 89 Ebbtide 7.4 454 backfire out carb

Glad I'm not buying a motor from either of you guys.

 

[PitchFork]

Re: 89 Ebbtide 7.4 454 backfire out carb

Number of 70% is right from Merc Spec.
http://www.boatfix.com/merc/Bullet/97/97_25.pdf

 

[brad.wolf]

Re: 89 Ebbtide 7.4 454 backfire out carb

Great, thank you guys. I appreciate the feedback. I plan to have a neighbor and myself look more in depth of this issue this weekend. I will post updates if I get this corrected. Plan on doing a pressure test first, then moving on from there.

Thanks again