High Speed Fuel Jet Question -- MarineDoctor?

[turtle1173]

Hi all,<br /><br /> I took the carbs off of my 1972 85 HP for probably the 12th time today (literally in the past 3 months) to see if I can figure out what is wrong with them. The symptoms are: it will run full speed for about 1 1/2 minutes, then it will start missing. If I back down to around 1/3 - 1/2 throttle it runs fine. Any more than that and it starts missing. The motor is getting good spark and the compression is good. Timing is also dead on. It has New points, new fuel lines, & a new fuel pump. So that's why I feel it must be the carbs.<br /><br /> OK, I never knew that much about the main jet in a carb until I read TheMarineDoctor's post about what he encountered with the motor getting too much fuel and therefore performing at a lower RMP (I believe it was at 4400 instead of 5500, or something close to that).<br /><br /> I checked my manual and my motor is supposed to have .072 jets on the carbs at sea level (which is what I'm at). My jets are .064 for a difference of .008 on the small side.<br /><br /> If, in MarineDoctor's scenario, a difference of .004 too big made a motor perform over 1,000 RPM's too low, would .008 too small have the opposite effect and starve my motor for gas when I'm above 1/2 throttle???<br /><br />I would appreciate your input,<br /><br />Shane

 

[The Marine Doctor]

Re: High Speed Fuel Jet Question -- MarineDoctor?

If your carbs are indeed running lean...the spark plugs will definately show the problem.<br /><br />They will be a white/tan and dry. This is due to heat and possible detonation.<br /><br />A way to check for this while the engine is running...or to check any cyl that may be lean.<br /><br />Remove the cover while operating the boat.<br /><br />Take the choke rod out from the carbs so you can activate the choke individually.<br /><br />2 people<---<br /><br />Start the engine...warm it up and accel to full speed. Have the second person slowly move the choke by hand on each carb. If the RMP picks up on any carb you will know its lean, You should not have to close the choke very much. <br /><br />If this does not find anything for you.<br /><br />You may have broken rings...Yes I know...no scoring n such. But old Chrysler's would break rings and still run and not score.<br /><br />The only sign of a problem was being down on power.<br /><br />Also timing should be set at 30 deg...NO HIGHER. I know the book says 32...but that was when the fuel had lead in it.<br /><br />Also..check the fuel tank for the vent...might be obstructed.<br /><br />1 min to 1.5 min...you might be using up what is in the hose until such time as it gets empty enough to start restricting the RPM of the motor.<br /><br />TMD

 

[walleyehed]

Re: High Speed Fuel Jet Question -- MarineDoctor?

How old are the spark plugs you are using???<br /> Are they relitively new?<br /> 2 things you cannot simulate very accuratly checking spark is resistance caused by heat, and, by compression. It may sound a little crazy, but I have fixed this exact problem a few times on customer rigs with new plugs(Champion, only). Just a thought. Also, can you determine the validity of the jet size?? I have run accross Main jets that have been drilled by someone thinking more fuel is more power....008 leaner than stock-sealevel Jets should run real good at Grandlake, Colorado....8500ft, but I don't think your engine would last very long if they are truely that lean. If this is a problem that has just started showing up, I would seriously consider new plugs, and a try-out first, if you haven't already...... <br /> Also, pull a plug and look at the piston top with a good flashlight, and see if the top of piston shows any signs of pitting (detonation)<br />

 

[walleyehed]

Re: High Speed Fuel Jet Question -- MarineDoctor?

Also, forgot to add that TMD has a very possible condition with the fuel not venting properly....

 

[turtle1173]

Re: High Speed Fuel Jet Question -- MarineDoctor?

Hey guys, thanks for the suggestions.<br /><br /> MarineDoctor, I have trouble getting a good feel for my spark plugs when using the non-gap type. The other day I took the bottom plug out and put a "normal" plug in (L82C) and ran it for a bit. At the ramp the insulator of that plug looked kind've an off white to a light pink color.<br /><br /> I haven't choked the carbs individually but when it gets to the level where it starts missing, if I hit the choke momentarily, it revs up for a half-second. That seemed to suggest to me a lean condition and certainly a lack of fuel. What do you think?<br /><br />Walleyehed, the plugs are new. You have a good idea with verifying the size of the jets. .072 should be between 1/16" and 5/64" but is much closer to 5/64". Also, I never have had this engine running good since I've had it. I've been working on it since April (blown head gasket, warped head, etc.)<br /><br /> I will also check the fuel vent (good ideas).<br /><br />Thanks again,<br /><br />Shane

 

[The Marine Doctor]

Re: High Speed Fuel Jet Question -- MarineDoctor?

I also did some checking on the spark plugs at work.<br /><br />Champion recommends L82c<br /><br />While NGK recommends BUHW or the equivalent of L76V.<br /><br />White/tan is lean or hot.<br /><br />TMD

 

[turtle1173]

Re: High Speed Fuel Jet Question -- MarineDoctor?

marinedoctor,<br /><br /> I've been using L20V in the motor but I will change to L76V once I get these "issues" worked out.<br /><br /> So it sounds like that jet could very well be my problem.<br /><br />Thanks again,<br /><br />Shane

 

[turtle1173]

Re: High Speed Fuel Jet Question -- MarineDoctor?

My local mechanic used to mainly work on Chrysler outboards in the '70s at the local boat shop (which hasn't been there in years). Anyway, I thought since I needed 3 jets for my carbs and I'm missing that main nozzle on the one carb, I would go to him and see if he had any spare parts he would sell me.<br /><br /> Anyway, He opened up a little metal tackle box and he had all kinds of Chrysler Carb parts. I pulled out 3 .072 jets and found a nozzle that looked like mine. He told me he didn't work on many Chryslers anymore and for me to just take them. I was so excited I could hardly contain myself, so I thought I would let everyone here know about my supreme joy <br /><br />Shane

 

[The Marine Doctor]

Re: High Speed Fuel Jet Question -- MarineDoctor?

<br /><br />Crossing fingers.<br /><br />TMD

 

[turtle1173]

Re: High Speed Fuel Jet Question -- MarineDoctor?

I thought some might be interested in the latest update on my motor.<br /><br /> To make a long story short, it appears that the nozzle and right sized jets did the trick. It ran wide open all the way to my fishing spot and all the way back (15 minutes each way). I am very pleased.<br /><br /> I do have another question. I didn't have new cork fuel bowl gaskets, and the ones I had were really falling apart. So I purchased this gasket maker from the auto part store that said it was resistant to gas, oil, diesel, etc. Anyway, I assembled the carbs the day before I took it out. I figured that would be plenty of time for it to cure (didn't say on the package how long to allow). No such luck. The stuff looked like it dissolved about 5 minutes into my first run and my carbs were all leaking like crazy.<br /><br /> Question: What kind of gasket maker can I use that will work on this? Or can I buy some gasket material (cork?) from an auto parts store? I've bought gasket material from stores before but this probably wouldn't hold up to gas would it??<br /><br />Thanks,<br /><br />Shane