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Old 11-22-2005, 01:20 PM   #1
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Default Engines leans down a lot once tank gets to 1/4 full

Hey guys,

I originally posted this on the Ofna forums but thought I would have better luck finding a solution here.

I'm having some problems with my Jammin X1CR buggyr / engine (RB WS7 II). Everything runs excellent until the car starts to run out of fuel. The engine likes to lean down a lot once the tank gets to about 1/4 full. The engine goes from 212F to 255F and loses low-end (overheating). But once I get the tank above around 1/4 full everything goes back to normal.

I have tried replacing the fuel tank and fuel tubing and rerouted the fuel lines but the problem is still there.

I know that the engine is tuned correctly since everything runs fine and then suddently it starts overheating once the tank gets to the end.

P.S. I'm using a JP1 pipe
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Old 11-22-2005, 02:19 PM   #2
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It might be and this is a maybe, your pressure tap on your pipe is clogged.gravity will flow the fuel when tank is full but when level goes below carb height. your fuel flow is cut down
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Old 11-22-2005, 02:29 PM   #3
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Good idea, but I've already checked the pipes outlet and it's fine also replaced the tubing and tank already, so that's not it.
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Old 11-23-2005, 12:44 PM   #4
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Start with a full tank. Run it down to 1/4 when it starts to heat up, fill the tank back to full. Don't stop the motor durring this process. If the temp goes back down, you know the problem is in the fuel delivery. If it stays hot, you have either a tuning issue, or an air leak in your carb or back plate. I had a similar problem a while back and found that I had gradually "tuned" the carb for a rich bottom and a lean top. My track is small, so the problem didn't really show itself until the end of the race. I reset the needles and the problem dissapeared. Now I have a spare tank and new fuel line.

I am not a newbe and thought I was pretty good at tuning, but I had made a mess of the tuning.
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Old 11-23-2005, 03:06 PM   #5
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Thanks for the comments... I think I had the same problem as you but I noticed it became better once I removed the fuel filter. I think I was being trown off since when I let the car idle and refilled the tank the temps dropped a lot (cause of the rich bottom), making me believe that the tank was the culprit.
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Old 11-23-2005, 07:42 PM   #6
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do you have a long pressure line? try shortening it. the problem probably got better when you removed the filter because you removed some volume from the pressure line.

if the engine does not create enough vacuum on it's own to draw fuel into the the carb with a long pressure line, you can help it by shortening the line- adding pressure in the tank. with a new engine the line could be very long, but as the engine starts to lose seal a long pressure line will run lean.
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Old 11-23-2005, 07:49 PM   #7
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The pressure line isn't too long, about 10 inches or so... The engine is fairly new as well, only 1 gallon through it (good compression too)
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Old 11-23-2005, 08:26 PM   #8
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10" is probably not too long. I have at least that much + a pressure chamber. Try a 6-8" pressure line and just see what happens. Do you have a pressure chamber?

How long is the inlet to the carb? Try shortening that some if it is more than 3- 4". I know you off road guy's like to have long carb inlet lines for when you end up on your lid, but mine is only about 2" (no filter)- direct from tank to carb in my MRX-4.

Start with all the lines run fuel/pressure lines run direct and see what it does. If it's OK, then try lengthening the lines one at a time to where you want them and see where the problem shows up again.
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Old 11-23-2005, 08:47 PM   #9
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I'm not using a pressure chamber and the fuel outlet tubing is about 12" or so.
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Old 11-23-2005, 09:13 PM   #10
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it has been my experience that much more than 6" from tank to carb causes problems you are having when fuel is consumed. there is really no need for that much line. unless you have to marshall your own car from the drivers stand, 4" is enough to keep it running for 20- 30 seconds on it's lid.

you have quite a long fuel delivery system. in my opinion you are asking too much of the fuel pump (which is your engine). when tank level gets low the exhaust pressure from the engine cannot keep the 10" pressure line charged enough to push sufficient pressure into the tank's empty space. along with that fact, your engine must draw the fuel through 12" of line basically on it's own if you can't keep the tank pressurized enough.

The probelm lies in the length of your fuel system
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Old 11-23-2005, 09:18 PM   #11
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Remember that we're talking about a .21, so I doubt you can get 20 seconds out of 4" of fuel tubing. Also, everyone I see at the track is running huge lines with 1 or 2 filters and not having any problems.

But of course, I will try running a shorter line just for the hell of it.
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Old 11-24-2005, 07:11 AM   #12
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yep, I know you're talking about a .21. that's what would be in my MRX-4
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Old 11-24-2005, 07:39 AM   #13
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Sorry, I noticed that after I posted the reply... I'll try shortening the tubing to about 4 inches and give her a try in the 10cm of snow we got last night.
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Old 12-20-2005, 10:28 PM   #14
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Cdelong gave you good advice also try a pressure chamber!
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Old 12-21-2005, 09:41 AM   #15
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I will tell a very simple solution.

Change the fuel tank, it probably have a small crack, that will not leak fuel but will lost pressure.
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