350 Mag Still Bogging Down

[FreeBeeTony]

I don't know what else it can be.........<br /><br />Here is what I have done so far:<br />1) replaced plugs, cap and rotor<br />2) Set timing to 8* BTDC<br />3) Rebuilt Carb.<br />4) Checked for exhaust blockage, found nothing<br />5) Verified correct prop, 15" Pitch<br /><br />I can get up to ~3200RPM, engine sounds great.<br />Tonight I thought it was fixed, sounded as if the secondaries were opening (sounded kinda throaty), then it started to bog down when I tried to give it more throttle.<br /><br />Is it possible the timing isn't advancing properly up at these RPM's? It's the Thunderbolt IV Ignition. Can it be a thermal problem with the ignition module?<br /><br />I received the manual today and am going to go through it tonight.....<br /><br />I could really use some help here.........<br /><br />Any thoughts would be appreciated.

 

[tommays]

Re: 350 Mag Still Bogging Down

use a timeing light with and advance dial to be sure you are getting full advance<br /><br /><br />tommays

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

I was just looking at my new manual....<br /><br />On page 4B-12 it shows a diagram of the engine / firing order. It shows the number 1 cylinder on the dist cap at 12 o'clock. I think the #1 on my dist is more like 9 o'clock. If the timing is correct but the dist. is off by 90* and the wiring is off to compensate, would this matter? I don't know if I explained this well.<br /><br />ALso on the same page is a spark advance chart, it looks like this should reach full/max advance at ~2200RPM. My engine is running fine up to ~3200. Does this rule out not getting enough advance?

 

[Don S]

Re: 350 Mag Still Bogging Down

That doesn't hurt anything Tony, no problem where #1 is actually located as long as the proper firing order is used. You could change it if you wanted to, but it won't have any effect.<br /><br />I would start checking the fuel system from the tank to the fuel pump.<br />Check the fuel pickup tube for build up of gunk on the screen on the bottom of the tube. Could also be a partially plugged anti-siphon valve.<br />Another possibility is air leaks in the lines. Loose clamps or having the big 1/2" wide clamps on the fittings. They create flat spots on the hose and allow air to suck in. (Won't leak fuel)<br />Use the 1/4" wide mini clamps on fuel lines.<br />Also check the fuel filter and fittings. If they have teflon tape on them, get rid of the garbage and get some real pipe thread sealer, like this.<br /> Thread Sealant.<br />You can plumb in a short piece of clear hose in the fuel line just before it goes into the pump and run the engine at speed and see if you have any bubbles...... There shouldn't be any. <br />Also do a pressure test on the fuel pump.

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

So Don.....you think it's fuel related?<br />This is really driving my nuts......I was hoping to go fishing tomorrow. Would I do any damage if I ran the boat like it is at reduced RPM's?<br /><br />One other thing I noticed was that the fuel pump site tube was not connected.........

 

[Don S]

Re: 350 Mag Still Bogging Down

Would I do any damage if I ran the boat like it is at reduced RPM's?<br />

If you don't know what the problem is, how can you say if it will hurt it or not.<br /><br />I don't know what your problem is, but with all the work on the carb, timing, and tuneup and all, the fuel system seems like a good possibility anyway.<br /><br />That "Site Tube" is there to put fuel that leaks past the diapram in the fuel pump into the carb instead of the bilge. That is one of the big differences between an automotive and marine fuel pump.

 

[Limited-Time]

Re: 350 Mag Still Bogging Down

Did a repower with a 5.7 initial dist. installation was 1 tooth off. I rotated the dist far enough to compensate for it and set the timing at 6*BTDC as per spec. running TB V ing. The engine would not run correctly. I don’t know why, as Don stated it should make no difference...But when I corrected the dist position she ran like a champ. I really wish I knew why. Just a thought

 

[Don S]

Re: 350 Mag Still Bogging Down

L-T<br />If you move the distributor 1 tooth and change nothing else, of course it's going to be off.<br />But if you take all the spark plug wires off the engine and throw them in a box. Do the same with the cap. Now bring the engine up to #1 TDC on the compression stroke. <br />Now, put the distributor in the hole. Doesn't matter one bit which way it's pointing.<br />Now, put the rotor on the distributor then put the cap on to see which pole the rotor is pointing at. That is number 1. Now put on the rest of the wires in the proper rotation and firing order. Then crank the engine and turn the distributor a little bit back and forth till it starts. Use the timing light to set the timing and you are set to go.

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

I am going to "re-clean" the carb and then start checking the fuel delivery system.........<br /><br />Maybe it's just a loose clamp....... <br /><br />probably not with my luck!

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

The end of the hose connected to the water seperator seemed dried out........I am gong to try to cut it back........or I might have to replace it.<br /><br />Hope this is it!<br /><br />HOw do you get to the pick-up tube in the fuel tank? The hose is connected to a right angle fitting that doesn't appear to be able to be removed.......

 

[Don S]

Re: 350 Mag Still Bogging Down

That "Right angle fitting" is actually the antisiphon valve (what the hose is clamped on) that is screwed into a square block that is actually the pickup tube. The tube is threaded into the tank.

 

[Manipulator]

Re: 350 Mag Still Bogging Down

This may sound like a stupid question for someone with so much mechanical experience, but did you check your firing order? My 350 was bogging and I had trouble getting it to 3000 rpm. It idled fine so last thing I was thinking was firing order. Dad, came over and said it was the firing order. I had two plugs swapped (5 and 7) which I here is very common. They say the way the firing order is, and if you have these two cylinders crossed the engine will idle fine. Basically, I had a six cylinder until I switched them back and then it ran great. I too changed plugs, rotor, and points and rebuilt the Quadrjet. I know the disappointment you get after doing all that and it doesn't run right. Hopefully it is something simple.

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

OK.......I installed a clear piece of hose temporarily and saw NO bubbles.<br /><br />I rechecked the timing (~8.58 BTDC) and firing order.....I even moved the distibutor as talked about above to no aveil.........it didn't help.<br /><br />I checked the vacum....it was moving between 16-18......I adjusted the idle jets and it smoothed out even better and vacum came up to a steady 19.<br /><br />I am convinced it must be a blockage in the fuel system. The next thing I am going to do is run a seperate fuel tank and see what happens.<br /><br />The boat runs great at 3200RPM's, as soon as I try to go higher it seems to bog down. I ran at a steady 3200 for about 20 min with no problems....it's gotta be something between the tank and carb....if the seperate tank doesn't work I will have to check the fuel pump pressure (will need to buy a gauge).<br /><br />Wish me luck........hope to do this test later today........wife is getting "annoyed"!<br /><br />Don, the pick-up tube is threaded into the tank? It seemed "flush" the the top of the tank....I will have to look at it more closely. I can "unscrew" the anti-siphon valve from the tank?

 

[Reel Poor]

Re: 350 Mag Still Bogging Down

I can "unscrew" the anti-siphon valve from the tank?

Yes,... and you should. Also remove the pick-up tube. Thats what the antisiphon valve is screwed into.<br /><br />Antisiphon valves are a common fuel restriction problem. Its a fitting with a check ball and spring built in.

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

OK....I replaced the anti-siphon valve, can't really tell if it was bad........need to go out on the boat tomorrow.<br /><br />When removed the valve there was a 90* fitting. I removed that and then there were threads sticking out of the top of the tank with a screen on it, doesn't appear that I can remove this unless it has a plate like where the fuel gauge is installed. Is it possible that this tank doesn't have a removable pick-up tube?

 

[haliescomet]

Re: 350 Mag Still Bogging Down

what happened is it fixed? was it feul tank

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

don't know yet.........will know tomorrow after test ride. After doing a search on anti-siphon........it sure sounds like the problem.<br /><br />I will post results tomorrow!

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

well....the anti-siphon vale didn't work.....I might try eliminating it next.

 

[Don S]

Re: 350 Mag Still Bogging Down

Before you start replacing everything in the fuel system, get yourself a 6 gal outboard tank and hook it up to the fuel pump. Then run it and see if it works. If it does, then you have a problem in the fuel tank to fuel pump system. Probably the screen on the fuel tank pickup. Why don't you unscrew it and look?

 

[FreeBeeTony]

Re: 350 Mag Still Bogging Down

I can't seem to figure that out Don. The anti-siphon valve is screwed into a 90* fitting, the other side of the 90* is screwed onto threads sticking out of the tank. If this is the pick-up tube, I don't see how I could remove it.<br /><br />And I do plan on doing just what you said with a seperate tank.<br /><br />Do you have any diagrams of a tank/pick-up tube? That might help.