Q Jet vaccum line and 87 Mercruiser

[Liberator 6]

All,
I've been having problems with my carb bogging down, running extremely rich and refusing to pull a 21 ft boat out of the hole. I was talking to someone I work with who said most likely a timing advance tube from the distributor to the carb wasn't holding a vaccum. Sure enough - I see a bronze vaccum line coming off the distributor with nothing attached. There are also two vaccum lines coming off the front of the QJet which are just plugged off. Which one should the distributor vaccum line go to on the carb? The one above the fuel or air adjustment screw?

I just replaced all my plugs, and will be re-building the carbs this afternoon. If anyone can help with that vaccum line question I'd appreciate it!

Rob

 

[Haut Medoc]

Re: Q Jet vaccum line and 87 Mercruiser

If it is a marine distributer it has no vacuum advance.....
Possibly a vent though......
Marine distributers use mechanical advance.....
Those holes on the Q jet should remain plugged....
Is this a marine carb?......
If either of these items are not, you need to replace them or risk explosion &/or fire.....
Have you checked the timing?.....
I'm assuming you have a TB IV.....
8* degrees before @ idle.....
One other thing to check for the rich condition you describe is the fuel vent tube from the fuel pump to the carb......
If there is fuel present there, your pump has a ruptured diaphragm & needs to be replaced......
That would certainly cause your "rich " problem & save you from a possibly unnecessary carb rebuild......​

 

[Liberator 6]

Re: Q Jet vaccum line and 87 Mercruiser

I'll check the timing, it does have a cold start "issue" where it takes a little more starter effort than natural. Here is a picture of the distributor "thing" I was talking about, I've never seen anything like it. There is a sticker which says Marine, but.... I took this picture as I was replacing the plugs/wires (there is obviously one wire not on the distributor at the moment).

I bought the boat used, and the engine needed to be replaced. It's been in and out of a few boat shops in several states (MO, VA, TX) and nobody has said "hey idiot, this isn't Marine stuff on here" which I'd hope they'd do...

Fuel vent tube?? I don't know what that is, but I'll look around that area. If you can give me an idiot 101 on this I'd appreciate it!

Rob

 

[Don S]

Re: Q Jet vaccum line and 87 Mercruiser

That isn't a vacuum fitting, all it is is a breather vent. Nothing goes onto it.
That is typical Mercruiser thunderbolt cap. The newer caps have a black plastic fitting instead of the brass one.
When was you last full tuneup? Might want to run a compression test and check the eng. vacuum for any signs of trouble.

 

[Haut Medoc]

Re: Q Jet vaccum line and 87 Mercruiser

That's a vent, nothing more.....
The fuel tube I describe should be clear or yellow & goes (should go) from the fuel pump to the carb.....
It's purpose is to dump excess fuel from a ruptured diaphragm into the carb instead of the bilge....
You better have one....​

 

[Liberator 6]

Re: Q Jet vaccum line and 87 Mercruiser

That fuel line is there, it's yellow and seems to be flexible and in good order. Last full tune up is... RIGHT NOW. It had one but sat in storage for three years while I was out of country. I replaced the plugs and wires yesterday, ironically the older wires were only pushing about 9 ohms of resistance, about the same as the new ones. Figured it couldn't hurt anyway.

It's been awhile since I did timing last (older Mustang). Never done it on a boat before, any tips or sites/posts/tips to suggest?

THANK YOU!
Rob

 

[SuperNova]

Re: Q Jet vaccum line and 87 Mercruiser

If that cap is as old as it looks, you may want to put a new cap and rotor on there also.
--
Stan

 

[Liberator 6]

Re: Q Jet vaccum line and 87 Mercruiser

Cap and rotor are ordered thru a local marine shop. I understand that at idle, timing should be at 8 degrees - but since it has a mechanical advance, what should it be once all the advance is out - say at 3000 RPM? Someone told me 38 at 3000RPM but this person deals with hotrod setups and I'd just like to know - THANKS!

 

[Don S]

Re: Q Jet vaccum line and 87 Mercruiser

The Thunderbolt ignition does not have a mechanical advance, it's electronic. Total advance should be 28? to 32?

 

[SuperNova]

Re: Q Jet vaccum line and 87 Mercruiser

I don't believe there is any mechanical advance in that distributor. It is all done electronically by the TB IV ignition box. You should set it a 8 degrees at idle and you're done. The box takes care of the rest of it.
--
Stan

 

[SuperNova]

Re: Q Jet vaccum line and 87 Mercruiser

Oops, Don was a little quicker.