engine luls and backfires over 4500 rpm only

[Ed R]

Re: engine luls and backfires over 4500 rpm only

sounds like float level too low; motor will start missing over 4900 ; do to cam, timing, air/fuel mix etc.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

sounds like float level too low.

How do you check the level? Where is the float level adjustment on these carbs? Any online diagrams?

 

[Bondo]

Re: engine luls and backfires over 4500 rpm only

Any online diagrams?

Ayuh,...

Right Here.....

 

[ken_23434]

Re: engine luls and backfires over 4500 rpm only

You mentioned replacing the inline water separating fuel filter, but did you replace the one in the fuel pump? The fuel pump on the 3.0l mercruisers have a fuel filter in them under that cone shaped part of the fuel pump. There is also a "fuel strainer" right at the fuel inlet to the carb. If either one of them was getting clogged, that could lead to your problem.

A fuel filter that is getting clogged up cannot pass as much fuel as a new one. It might be able to pass enough fuel for the engine to perform up to a certain load. Once the load gets above some particular threshold, the engine will start to starv for fuel and run lean.

I am just making up #'s, but lets say the filter is partially clogged and will only pass 5 gph. Well, your engine doesn't use 5 gph until it reaches 4,000 rpm. So, as long as the engine stays below 4,000 rpm the engine runs fine. Well, when you open it up to 4,600 rpm the engine now wants more than 5 gph, but the filter will only give it the 5 and the engine starts to ping and run bad.

I hope that explains why people have mentioned the fuel filter. If the mechanics told you the engine would run bad at ANY rpm with a clogged fuel filter, I would look for advice from another mechanic.

 

[drrm123]

Re: engine luls and backfires over 4500 rpm only

I'm trying to look at a manual for this engine, but i dont think it has a fuel return line on a engine with a carb, are you sure that was not vent line that was full of crud? if that was a vent line and it was plugged up that may be your problem? just finished down loading manual, clear tube is a vent tube that goes from carb to fuel pump. crud had to come from someplace could be in carb also.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

I'm trying to look at a manual for this engine, but i dont think it has a fuel return line on a engine with a carb, are you sure that was not vent line that was full of crud? if that was a vent line and it was plugged up that may be your problem?

Its a clear plastic line that runs from the carb back to the fuel filter.

I *IF* you really want to be able to work on your boat you gotta invest in some equipment and tools. I can't tell you how many people I see waste silly sums of money on parts and time because they didn't want to spend $20 or $30 for the tools needed to troubleshoot a problem.

Money is not an issue for me but time is as well as space to store stuff. Thats why I want to collect all the info I can before I spend my only day off running around buying stuff. When I am sure what I need I WILL buy it. I am no mechanic but am mechanically inclined and figure if I can fix I will and if not take it to someone who can.

If your mechanic buddies didn't tell you that, then aren't marine mechanics

They told me not to run it over 4600. These engine don't handle over reving very well, even if it is only 600RPM.

If the mechanics told you the engine would run bad at ANY rpm with a clogged fuel filter, I would look for advice from another mechanic.

They said that if it was water or bad gas the problem would be at any RPM

but did you replace the one in the fuel pump? The fuel pump on the 3.0l mercruisers have a fuel filter in them under that cone shaped part of the fuel pump. There is also a "fuel strainer" right at the fuel inlet to the carb.

No I didn't know that, I'll check that out, thanks.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Ayuh,...

Right Here.....

Awesome!

Lots of great info that will come in very handy! Thank you.

 

[chiefalen]

Re: engine luls and backfires over 4500 rpm only

There is one other place you could look for a clog.

The fuel pickup tube should have a screen on it, and the anti-siphin valve could be sticking.

This is all assuming that the fuel pump is giving 4-6 pounds.

But the very first post i asked you to run it on a external tank so we can ether discount the tank or verify that the tank was the problem.

So the fitting bye the carb has a filter, clean it. The filter in the fuel pump, clean it, the fuel pickup tube screen, clean it, and verify the anti-siphin valve is not clogged and working, blow thru it.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

There is one other place you could look for a clog.

The fuel pickup tube should have a screen on it, and the anti-siphin valve could be sticking.

This is all assuming that the fuel pump is giving 4-6 pounds.

But the very first post i asked you to run it on a external tank so we can ether discount the tank or verify that the tank was the problem.

So the fitting bye the carb has a filter, clean it. The filter in the fuel pump, clean it, the fuel pickup tube screen, clean it, and verify the anti-siphin valve is not clogged and working, blow thru it.

Thanks again for your help. I'll check out the screen on the pickup tube. Where is the anti-siphin valve?

 

[Bondo]

Re: engine luls and backfires over 4500 rpm only

Where is the anti-siphin valve?

It's the Hose Barb on the outflow of the fuel tank....

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Well I have spent a lot of time and some money trying most that has been suggested on here with no luck. Finally took it in to my LBS and they put it on the dyno. My tach is acurate and the motor is capable of cranking up to 5300 rpm at WOT. It started cutting and lulling and he said that it has a rev limiter and that is what is cutting the engine and is the cause which is good. The motor should not go over 4800 rpm.

Thanks again for all the suggestions and help everyone.

 

[Bondo]

Re: engine luls and backfires over 4500 rpm only

October 2nd, 2008, 08:09 PM
May 8th, 2009, 07:24 PM
he said that it has a rev limiter and that is what is cutting the engine and is the cause which is good.

Ayuh,... Wow,... Whata Trip eh Bob..... Thanks for stopping by,+ educating Us,....
I guess We should have questioned the Igntion system......