Engine Issue that appears only when hot . Or so it seems

[IOW BOB]

So we've had our 215 for 8 weeks now .
She's a 1998 5.0 v8 mercruiser
I'm systematically going through it to make it a good reliable boat .

I've changed manifold and risers , thermostat , fully serviced it when I first got her

We went out the other day and was crusing along at 3200rpm for about 45 mins

When all of a sudden it started back firing / popping through the carb . So slowed down instantly

Anything above 1100rpm and it would pop and bang through carb . We were a few miles from shore so limped to safe water , turned it off and all appeared okay . Restarted it and idled lovely .

As we had nowhere to moor up we started heading home at 1000rpm after about 20 mins I thought let's see if it is still doing the problem .
Throttled on and sat at 3000RPM for 5 mins or so until problem came back .
Carried on at 1000rpm for 20 mins tried throttle Again and off she went no problem for about 5 mins ..
eventually we got near home after going through this on off throttle thing.
We anchored up so the kids could have a swim . Switched off the engine for about 45 mins ..

When we got moving again all was good all the way home 15 mins ish at 3200rpm ..


So my question is where do I start looking to resolve this issue .
In my eyes going slowly aloud the engine to cool allowing somthing to work until it got hot again .

Temp guage sat at 175 and didn't really move much
It has a 71c thermostat in it now and the one I took out was 60c
So I'm going to find a new 60c one and fit that as I prefer an engine that runs cooler

I was thinking distributor cap, rotor arm , leads , Coil , fuel pump

Any help greatly appreciated

 

[Jimmy Buoy]

Cracked coils will seem fine until heated, the crack opens up more and causes your symptoms. Sounds spark related. (possibly a crack in the distributor cap if the coil is ok)

 

[romanservices]

do you have a fuel cooler? could it be vapor lock?

 

[CliffA]

I assume this engine is carbureted so I suggest identifying the issue as being related to fuel supply or ignition as a first step.....this is relatively easy to do by pouring a little fuel down the carb when the engine starts to stumble.....if the additional fuel causes the engine to smooth out then your issue is fuel related....if it does not you need to look at the ignition system components....

good luck....

cliff

 

[skunkman]

Sounds like you are running lean. Possibly the accelerator pump in your carb. Might be as simple as a new accelerator pump diaphram. If you look into the carb (with engine off) pump the throttle and see if you get a smooth delivery of fuel.

 

[IOW BOB]

I also had the carb stripped and cleaned when we first got her .

I'm gonna order the parts I mentioned and see how I go

 

[scoflaw]

Complete tune up to start with, don't forget the plug wires. Do a compression test when the plugs are out and record those numbers for future reference.

 

[Espos4]

Any follow up? Did you find the problem?

I see in another post of yours that you replaced the coil. Did that cure the backfiring issue?