where to hook up vacuum gauge

[Rellik546]

Where exactly do you hook up a vacuum gauge to check cylinders, and also to adjust idle mixture? I know its suppose to be hooked up to the manifold, but where? is there a barb it can hook up to? i cant seem to find any info on where specifically to attach the gauge in the manual, it just says attach to manifold. the manifold is directly under the carb, but i cant seem to find anything to attach it to. i have a 85' mercruiser 140 181 with a rochester 2 barrel carb. also if testing my fuel pump, it test the line between the pump and carb which is a steel tube. so i just get 2 thread to barb connectors and a tee fitting. thread appropriate sized threaded barb adapters to threaded connections, then attach tee fitting to vacuum/fuel pressure gauge. and use spare gas line pieces and attach to the barbs on the tee fitting and then to each of the threaded barb fittings i just installed. is this the proper way?

 

[fat fanny]

Re: where to hook up vacuum gauge

No clue on the vacum to the manifold but if it were me I would have a new steel fuel line line made up to go from the pump to the carb with the gauge fitting in between and leave it there! Better safe than sorry in this case. Glad to hear your back on the water!! Are you trying to track down that hesitation issue after the carb re-build? Just remember most carb re-builds are a 50/50 chance of success to solving all acceleration issues. Just a side note I soaked my carb 3 x and washed it with soapy water between each soak before re-assembly. Also if your looking for some sort of vacum leak try a bottle of water while running the engine and spray it on various conection locations around the engine and if it stumbbles ther it is. Hope this helps.

 

[scoflaw]

Re: where to hook up vacuum gauge

Base plate on your carb might have the fitting your looking for. PCV hose, could be reduced to fit.

 

[achris]

Re: where to hook up vacuum gauge

Attached is a picture of your manifold. I have put an arrow to the plug you should remove. The thread is a 1/4"-18. You should be able to get a fitting with a barb from any reasonable hardware shop.

For the fuel pressure test I would get an old fuel tube, cut it and insert a tee for the gauge.

Good luck,

Chris..........

 

[Rellik546]

Re: where to hook up vacuum gauge

Achris, thank you!!! I spent an hour today looking at the manifold and was just clueless. Fat fanny, long time! I'll send you a pm. I'm kinda hesitant about spraying electrical connections with water, unless u mean fuel connections. Either way kinda feel uncomfortable doing that. All the carb related issues are fixed. I just need to set dwell gap and angle and set timing. The reason for vacuum tests is to just tune, or look for other existing issues if there are any snd fix them now. Basically just want to get to know my engine and get it running as best as possible. Since we started tuning and addressing issues I've gained about 7mph. Oh fat fanny, I like the fuel gauge idea, and I need a new steel gas tube as someone had kinked an area of the hose right before thd carb inlet. It's not bad, it almost look like someone tried to use wire cutters on it? And I believe the outlet on fuel pump is slightly stripped, so I'll remove it, drill and tap the hole.


If I want to adjust carb with a vacuum hose. Do I use the barb that connects the clear sight tube on the carb to the see tube barb on fuel pump. It's on the opposite side of the mixture screws, and above the throttle plate. Other than that barb, I can't find anything to connect to. Are u suppose to use the choke heater tube?

 

[achris]

Re: where to hook up vacuum gauge

The only vacuum point you can use to hook up a vac gauge for carb adjustments is the one I pointed at in the picture. The barb with the sight hose from the fuel pump is not a vacuum point. That's for fuel to flow into the carb if the fuel pump diagram fails (as opposed to filling the sump with 'bang juice'). The choke heater tube is just that, a choke heater. The other end of it is open to the world, under the manifold, so no vacuum there.

If all the carb problems are sorted, and you only need is to set points dwell angle, ignition timing and idle speed and mixture, why not just use a dwell meter, timing light and tacho? All this faffing around with a vacuum gauge is for naught...

Cheers,

Chris......

 

[Rellik546]

Re: where to hook up vacuum gauge

thanks chris, i just bought a multimeter that has a tach and dwell meter included, so i will use that. the main reason for vacuum gauge is to test internal engine parts...i dont have an issue, just want to get to know my motor. also the motor was rebuilt, and the compression numbers after rebuild were written on the block for each cylinder. 135 130 120 135...the 120 one seams a bit low so i wanted to see what the possible reason was (piston rings, valve, ect.)

i was told that the most ideal fuel air mixture is when you achieve maximum vacuum? is this true? if true this is why i wanted to use the vacuum gauge to set the mixture.