4.3 Mercruiser Bogging issues

[mbrasch]

Been a long time reader and decided to post my first issue. Purchased a '93 crownline this spring and have been chasing a bog issue since. Original motor came with the 4 barrel weber carb, so instead of rebuilding, just purchased the Edelbrock 1409 and installed the 1485 tuning kit. Results were still a heavy bog in the secondaries. Had my local marina adjust the timing to 14 degrees at idle as Edelbrock recommended. My mechanic said it was getting way to much gas when the secondaries opened. Lowered the secondary jets from .089 to .086.

The boat ran great after that. WOT fealt great, was hitting 46MPH at around 44-4500 RPM until the afternoon. Then the bog came back. It was an extremely hot day, so I'm wondering if temperature has anything to do with it?

The next day, I lowered the secondaries one more step to .083. This produced an obvious loss of power over the previous morning and at WOT was still producing a bog. I would have to throttle back to 80 percent to get a burst of power.

Does anyone have a recommendation for next steps? I am looking at putting the .086 secondaries back in and adding the Edelbrock heat insulator gasket at Edelbrocks recommendation, but I would like to get second opinions, or other things to try this coming weekend so we can start enjoying the boat instead of working on it every weekend.

 

[cr2k]

Re: 4.3 Mercruiser Bogging issues

Start with the heat insulator for sure.

 

[fossill]

Re: 4.3 Mercruiser Bogging issues

Old carb had a bog and the new carb also. Did that engine come oem with a 4bbl or did someone else convert it.

 

[mbrasch]

Re: 4.3 Mercruiser Bogging issues

I believe it was OEM with a 4bbl. The boat is spec'd at 190 hp with the original documentation which I believe in that year was the 4 bbl.

 

[fossill]

Re: 4.3 Mercruiser Bogging issues

And you've gone through a compression test and timing check? Ignition system up to par?

 

[mbrasch]

Re: 4.3 Mercruiser Bogging issues

Timing has been checked and adjusted to 14 degrees at Edelbrocks recommendation. My marine mechanic wasn't worried about compression, so that was not checked. I found out this past weekend the spark plugs are one heat range hotter than they should be from stock, so I will be replacing those this weekend as well.

 

[fossill]

Re: 4.3 Mercruiser Bogging issues

Since you are going to have the plugs out anyway, might as well check the compression. If anything you will have numbers for future reference.
Skipping steps or assuming something when troubleshooting an issue more than often leads parts being replaced needlessly and or the issue never getting resolved 100%. I find it odd that it was bogging with the old carb and that replacing it did not fix the problem. That tells me that you have a problem that probably isn't in the carb. Also is fuel pump flow and pressure ok?
Only thing I cansuggest is to grab the oem manual and follow the troubleshooting steps in the flow chart to a tee one by one.

 

[Dsilvers112]

Re: 4.3 Mercruiser Bogging issues

i agree with fossill. if it boged with the old carb and still does it with the new carb chances are problem is not in the carb. I would also consider it being the ignition module which controls the spark advance. Ive got a 4.3 that had a similar problem and the module was not advancing the time when you throwed the fuel to it so it would bog not burning the fuel. I would deffinately do a compression check on all cylinders too. if you keep running it like this it could cause serious engine damage. My module caused mine to melt plug ends and bent a valve. Do you know if it is Thunderbolt ignition?

 

[Dsilvers112]

Re: 4.3 Mercruiser Bogging issues

Also the timing is supposed to be set at 8 degrees. When you replace the plugs i would go back with what it calls for should be AC-MR43T plugs.

 

[mbrasch]

Re: 4.3 Mercruiser Bogging issues

Thanks. I appreciate your help. I will be replacing the secondary jets this weekend back to the .086, installing the heat gasket, and replacing the plugs. If that doesn't cure the problem, then I will start concentrating on the ignition system. It sure seems like it is a carb tuning issue though. It did run great until later in the day.

To answer your question, the fuel pump is good. The whole fuel delivery system has been checked out prior to replacing the carb.

 

[Dsilvers112]

Re: 4.3 Mercruiser Bogging issues

yeah sometimes an ignition problem will fool you into thinking its a carb problem. Good luck. I know how it feels to work on your boat all the time and seems like you cant ever enjoy it

 

[mbrasch]

Re: 4.3 Mercruiser Bogging issues

Dsilvers, its a Thunderbolt IV ignition. Thats correct, I am going to be putting in either the Delco MR43T's or NGK BR6FS plugs.

 

[Dsilvers112]

Re: 4.3 Mercruiser Bogging issues

K i would reccomend the AC's. The module should be mounted on the distributor or on the exhaust riser BTW

 

[mbrasch]

Re: 4.3 Mercruiser Bogging issues

My mechanic did say it was advancing ok. A little high for his taste also, but we were just configuring to what Edelbrock recommended to find the issue. Once we get it worked out, we made plans to lower the timing when we winterize it.