88 454 backfires at higher rpms

[jeffssgtusmc]

I bought a crate motor with domed pistons advertised as 415hp rather than 330. With orginal carb i am getting popping and backfireing anything above idle once it warmz up. I am thi king lean but im wondering if i neex a bigger carb. Problem wasnt as bad 20 hrs ago bit getting worse. I mostly troll so when im done fishing and try and go under power it starts popping. It will do it at 2000rpm and higher. Or am i way off and need to adjust timing again.

 

[HT32BSX115]

Re: 88 454 backfires at higher rpms

I bought a crate motor with domed pistons advertised as 415hp rather than 330. With orginal carb i am getting popping and backfireing anything above idle once it warmz up. I am thi king lean but im wondering if i neex a bigger carb. Problem wasnt as bad 20 hrs ago bit getting worse. I mostly troll so when im done fishing and try and go under power it starts popping. It will do it at 2000rpm and higher. Or am i way off and need to adjust timing again.

Howdy,


Well, you're going to have to reveal a little more info. What kind of an engine is this, what drive, ratio, and prop. Also what size boat are we talking about?

Where did you get the engine? Is it a marine engine? does it have a marine cam? compression ratio? , what sort of proof did the seller give you that proves it's a 415hp engine? RPM, torque, dyno results etc.

Just from your description, it sounds like either the valve timing or the ignition timing (or both) is all wrong. If you have a "wet" exhaust and you're using the wrong cam, it won't be long before you damage the engine.


Regards,


Rick

 

[Bondo]

Re: 88 454 backfires at higher rpms

I bought a crate motor with domed pistons advertised as 415hp rather than 330. With orginal carb i am getting popping and backfireing anything above idle once it warmz up. I am thi king lean but im wondering if i neex a bigger carb. Problem wasnt as bad 20 hrs ago bit getting worse. I mostly troll so when im done fishing and try and go under power it starts popping. It will do it at 2000rpm and higher. Or am i way off and need to adjust timing again.

Ayuh,.... What killed yer old motor,..?? Burnt pistons,..??

My guess is there's crud in the fuel system, causin' it to go Lean, 'n yer backfirin'...

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

Ok it is a 26ft slickcraft that i bought with a cracked block so the last guy never winterized it. It is a mercruiser bravo one unsure of gears. Bought long block from us engine they sold it as a marine engine so i dont have any reason to believe otberwise. Unsure of exact specs of engine. Just wondering if a bigger carb would be a good idea. If so which one.

 

[Bondo]

Re: 88 454 backfires at higher rpms

Ok it is a 26ft slickcraft that i bought with a cracked block so the last guy never winterized it. It is a mercruiser bravo one unsure of gears. Bought long block from us engine they sold it as a marine engine so i dont have any reason to believe otberwise. Unsure of exact specs of engine. Just wondering if a bigger carb would be a good idea. If so which one.

No,... ya gotta make Sure the carb there now is workin' properly...

If that carb fed a BBC before, it can Still feed a BBC...

If anything, it's probably full of crud, 'n needs rebuildin'...

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

Ok with that said i plan to buy a rebuilt one. (Tried to rebuild one and jacked it up once before) should i get a bigger or no.

 

[Bondo]

Re: 88 454 backfires at higher rpms

Ok with that said i plan to buy a rebuilt one. (Tried to rebuild one and jacked it up once before) should i get a bigger or no.

Ayuh,... Pretty tough question to answer, considerin', We have No Clue as to what carb is on it now...

I'll venture a Guess, that it's a Rochester spread-bore carb,...
those will flow 850cfms, so I doubt yer gonna find a Bigger carb...

If I were replacin' it,...
I'd swap the manifold to, 'n bolt on a 750cfm Edlebrock Performer...
It's a carbon copy of the carb Merc now uses...

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

The stock quadrajet is whats on there now. Any recommendations would be helpful

 

[HT32BSX115]

Re: 88 454 backfires at higher rpms

I live sort of "Down the road" from USENGINES and they definitely know how to build a marine engine............. What did they say? They claim a 7 yr warranty.

Also, let us know when you do get it running right. I am interested in one of their 454's.

 

[Bondo]

Re: 88 454 backfires at higher rpms

The stock quadrajet is whats on there now. Any recommendations would be helpful

Ayuh,... guess ya didn't like My answer,..??

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

I do like ur idea bondo but would my stock intake work im gonna look intothe performer carb also i saw the carb that is stock advertised as 600 cfm im not understanding how youd get 850 oit of that

 

[Bondo]

Re: 88 454 backfires at higher rpms

I do like ur idea bondo but would my stock intake work im gonna look intothe performer carb also i saw the carb that is stock advertised as 600 cfm im not understanding how youd get 850 oit of that

Ayuh,... The Stock carb on the Merc 4.3ls, 'n the 5.7ls are the 650 cfm models...

Merc uses the 750 cfm model of the same carb on the newer, Post-Quadra-jet, 7.4l motors...

The ole Quadra-jet is a variable carb that fed everything from little 6 cylinders to Monster BBCs....
It'll flow "Up to" 850 cfms...

The 750 cfm model performer would be a Great choice, but I'd go for the matchin' Intake....

The only carb that fits yer intake, is the Quadra-jet...

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

Im on board with what ur saying what intake do u suggest is a marine one different

 

[Bondo]

Re: 88 454 backfires at higher rpms

Im on board with what ur saying what intake do u suggest is a marine one different

Ayuh,... Freshwater, or Salt,..??

In saltwater, ya need an iron manifold, or bronze lined aluminum...

In sweetwater, you can get by for a long time with a nice aluminum manifold...

For what you say ya do,... Just replacin' yer carb, 'n cleanin' out the entire fuel system will keep her runnin' as good as it has ever run...

You don't need a Bigger carb,... ya need yer's doin' it's Job...

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

Talked to a tech from quadrajet.com and said to try and partly close choke when engine starts to backfire and that will tell you if its to lean and then rebuild carb with bigger jets. I thought that was a great idea what do u guys think

 

[speedygee]

Re: 88 454 backfires at higher rpms

Talked to a tech from quadrajet.com and said to try and partly close choke when engine starts to backfire and that will tell you if its to lean and then rebuild carb with bigger jets. I thought that was a great idea what do u guys think

Jeff did you get your backfire issues fixed? I have the exact same problem on a 94 gen V 7.4 with a Weber carb. I repowered over the winter and been struggling with this ever since.

 

[jeffssgtusmc]

Re: 88 454 backfires at higher rpms

Speedy email me at ****************** this website is a pain to use on my mobile.

 

[Bondo]

Re: 88 454 backfires at higher rpms

Speedy email me at ****************** this website is a pain to use on my mobile.

Ayuh,.... Please read the forum rules... postin' email addresses in the open forum is illegal...

Please use a pm to convey that info....

Thank you...

 

[Bondo]

Re: 88 454 backfires at higher rpms

Jeff did you get your backfire issues fixed? I have the exact same problem on a 94 gen V 7.4 with a Weber carb. I repowered over the winter and been struggling with this ever since.

Ayuh,... Welcome Aboard Speedy,... Start a thread of yer own, 'n We'll see if We can help ya sort it out...

 

[Bondo]

Re: 88 454 backfires at higher rpms

Talked to a tech from quadrajet.com and said to try and partly close choke when engine starts to backfire and that will tell you if its to lean and then rebuild carb with bigger jets. I thought that was a great idea what do u guys think

Ayuh,... If everything else Checks out,... It's worth a try...

Gettin' it Remaned at a qualified Rochester Shop might be Better yet...