73johnson135 crank vacuum test

[SCO]

I've posted about this motor. I want to see if what the carbs see is adequate suction while cranking to get enough fuel into the cylinders to start adequately. I am thinking I can hook a vacuum gauge to a flat aluminum plate and duct tape that plate to the 2 barrel carb intake or even take carbs off and duct tape to intake ports one at a time to test. Problem is I have nothing to compare to, what woud be a good vacuum. Has anyone done this or other? What do you do to test this? Thanks SCO

 

[sparkroost]

Re: 73johnson135 crank vacuum test

the duct tape thing is not going to work. You will need to drill holes in your intake manifold or your carbs. It's not a number thing. It's like compression. You want to be the same vaccum on all 4 intakes, within 2 inches hg is ok.<br /><br />If it is off more that 2" top to bottom you will have to adjust your link rod that goes from the top carb to the bottom one. If it's not adjustable, get one that is or make one out of remote control parts from a hobby store.<br /><br />Try and remove the rod that connects the top carb to the bottom one for the throttle plates. Start engine(warm engine before doing this). Will it idle?(will be very low idle, like 400-500). If it sounds like it is trying to idle but is just a bit too low idle and stalls.... increase the idle, have someone start motor for you while you push up slightly on the lower throttle shaft. You will know when you get it in the right area.. If you are far off the engine will die and cough abruptly. Try and fine tune your lower throttle shaft until it idles........Did you get it to idle?..... it's now time to make an adjustable rod so you can synk your carbs together.

 

[clanton]

Re: 73johnson135 crank vacuum test

You can check the vacuum at the fuel pump pulse connection, which show the vacuum on the one cylinder, should be close to 6 lbs. Linkage is adjusted at the carb. If hard starting or rough idle, check timing sensor gap, and carb throttle shaft/body for wear.

 

[SCO]

Re: 73johnson135 crank vacuum test

So the timing sensor gap, if too great, would mean the starting rpms not adequate to put out a strong timing pulse I take it. I hope that gap is adjustable. Thanks. I will instrument this thing if all fails sparkroost. Thanks for the tips.

 

[Joe Reeves]

Re: 73johnson135 crank vacuum test

SCO.... I've been reading through the posts and am somewhat puzzled. These engines aren't really that technical or difficult to restore to the factory settings. Are you thinking about entering that engine in the Indy 500? (Grin)

 

[SCO]

Re: 73johnson135 crank vacuum test

I'm with you Joe. How can it be that tough? It must be something simple. I'll take it out this eve and load the barrels w premix and see if she starts better. That should isolate the problem to fuel delivery, pure and simple. Do you agree?

 

[Dhadley]

Re: 73johnson135 crank vacuum test

We built those plates a few years back for a looper. We just duplicated a carb base gasket in aluminum with a threaded hole over the venturi. Then with a vacuum gauge hooked to each we turned it over. I dont remember what the numbers were but they were all about the same.<br /><br />Your motor should have no problem starting if all is stock and as produced. Check the stuff Clanton mentioned. If you arent satisfied with the choke system you can install the newer injector style primer.<br /><br />Good luck!

 

[SCO]

Re: 73johnson135 crank vacuum test

I just finished testing on the flushette. I made up a plastic spray bottle of premix(spray and wash bottle I think), and gave it a shot in each of the 4 barrels prior to first start. I was surprised that it didn't catch immediately. It took the full 6 seconds to catch but started at first catch and stayed started. Yea. Thats the way I want the motor to run!! I still have to test in the water, but things look good. I had the carb cover off, and noticed that when I cranked the choke flappers opened slightly. I put it on manual full choke and again got the same result. I concluded that the suction was good and forced the choke flappers to open slightly. Let me know if that indicates a problem, and that maybe the choke closing force is not enough. Thanks getting near the end I think.

 

[sparkroost]

Re: 73johnson135 crank vacuum test

The choke flappers should stay closed, not patially closed, closed. This will draw in more fuel during startup. Adjust your linkage so when switched on they are fully closed.<br /><br />BTW, this has nothing to do with idle.

 

[SCO]

Re: 73johnson135 crank vacuum test

The idle sounds ok for now spark. The choke is designed to close via spring to a solonoid (or mechanical button). I think I can get a little more closing force from the solonoid spring as it is not set up exactly right, but think it may not be able to overcome that suction force by design. Ill check those brass choke air tubes also to make sure they are completely unobstructed. Once I get this thing starting I am going to recheck the timing and fine tune the idle.

 

[sparkroost]

Re: 73johnson135 crank vacuum test

Ditch the spring. Be creative.

 

[jim dozier]

Re: 73johnson135 crank vacuum test

I'm with sparkroost on the choke plates. I don't know exactly how your engine works but when I choke an engine I want to choke the mother. I will be the one to undo the choke not some namby pamby spring!! Anyway, on my Suzuki the choke linkage is direct and when I choke it it stays choked. Completely.

 

[SCO]

Re: 73johnson135 crank vacuum test

I agree with you both in one sense, that it is the best, but, if the engine allows some flap movement when at factory spec then shutting that choke flapper manually could be considered a workaround, and an avoidance of the real problem. <br /><br />Joe, Dhadley, Clanton,<br />Do ya'll notice some choke flapper opening because of the suction airflow forcing them open silghtly when cranking as a normal condition on these v4's?

 

[sparkroost]

Re: 73johnson135 crank vacuum test

I believe you have a thermister on your engine connected to the choke. That thermister will create a votlage drop across it when it's cold. This is for warm up purposes so that the choke is partially closed to create more vaccum. The spring is used so that if the thermister is working and you floor the engine, the vaccum will overcome the spring and open the plates.<br /><br />Remove thermistor and spring(or shorten the spring) and you will have much quicker starts.. That is what you wanted, right?<br /><br />Heck, by now that spring does not have the tinsel strength it used to...

 

[SCO]

Re: 73johnson135 crank vacuum test

Sparkroost, these are good suggestions, and I will employ them or some other way to get that choke closed all the way when cranking. I agree that the automatic choke function is worthless! But for now, I want to keep problem solving till I get the motor to factory spec, then I will improve from there. I think the spring might be to avoid damaging the choke mechanism if misadjusted. The throw of the solonoid gos past the close limit of the choke and spring load s it shut.

 

[sparkroost]

Re: 73johnson135 crank vacuum test

I don't belive the choke solenoid is strong enough to break the choke plates or linkage. If this is a concern, adjust the linkage with the choke engauged, then you won't have a problem.<br /><br />Factory spec... you will need a rebuild for that to make things "a perfect world"..<br /><br />I find it amusing that you want factory spec, but don't want to improve your condition. You are asking for quicker starts, but want it the factory way. The motor is 30 years old! Make it work for you.

 

[sparkroost]

Re: 73johnson135 crank vacuum test

Just remember.. the choke solenoid plunger is a piece of metal. The solenoid housing is a coil of wire. When 12V is applied it acts as a magnet and sucks the plunger down. There are no mechanical parts connecting the plunger to the choke housing.

 

[SCO]

Re: 73johnson135 crank vacuum test

Sparkroost,<br />I am not saying what will happen if I change the spring design, I am offering a speculation about the unintended consequences that go with modifying an engineered design. We don't know what went into the design of the mechanism. Also I find it amusing that you do not see the logic of getting the motor to factory spec prior to making these improvements. I want to exhaust that possibility first. <br /><br />quote:"There are no mechanical parts connecting the plunger to the choke housing. " true, but the plunger has a significant mass and gets to high speed when coil is activated. A rigid linkage could transfer a big force to a slamming choke flapper. You may be correct about the thermistor, but maybe also there is more to the design. <br /><br />Don't think I'm angry here, I actually enjoy jawing about it. <br /><br />Also, for now, I am not just asking for quicker starts, just starts. It just barely starts , and I want to make sure there is not another problem that I am overlooking. I took the boat on a campout, and was almost stranded in the boonies because of this start problem.

 

[sparkroost]

Re: 73johnson135 crank vacuum test

Sometimes things look good on paper, but in the real world there function does not equate to the paper it was written on.<br /><br />You proved that you have a fuel delivery problem with the pre-mix. Go a step further and you will have that problem solved. Then you can move on to other things.<br /><br />I am just trying to help ya SCO. If you want to keep the spring... buy a new one. Adjust the linkage so that they close all the way, and be done with it

 

[SCO]

Re: 73johnson135 crank vacuum test

NP Sparkroost, youre an iboats brother, and I appreciate your help. Maybe I am too cautions, but, I pay the price if things go wrong. I don't mind taking time.