PanzerAce Posted April 8, 2008 Share Posted April 8, 2008 I posted this in originally in the L6 forum, forgetting that there was a fuel specific forum. http://forums.hybridz.org/showthread.php?p=878024 The short version of my current question is this: Once my engine has fully warmed up (~half hour of driving), if I let it idle, after about a minute it starts missing a beat every so often. This will get worse the longer it sits, to the point that it has died on me twice. There is no noticeable problem when I'm on the throttle. Also, if it helps: if I take the engine up to speed, it seems to 'reset' the problem clock, if that makes any sense. Quote Link to comment Share on other sites More sharing options...
Tony D Posted April 15, 2008 Share Posted April 15, 2008 Sounds like you may be percolating, or vapor flashing in the bowls from heat generated in the engine bay soaking the bodies. Mikuini made cooling bodies to put under the carbs to allow the fuel return line to pass through the chamber, acting as a coolant keeping the body's temperature lower during periods of low airflow through the engine bay. Gas boils, bubbles, and can cause rich or lean conditions resulting in a misfire. Cooling bodies solve a lot of that kind of stuff. It was 102 here (Riverside) yesterday, and I'm assuming up in Merced you guys beat us to that mark by about a week... Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted April 15, 2008 Author Share Posted April 15, 2008 Wow, I don't think ambient has hit 100 yet, but with an L30 in the engine bay, and a 20 year old at the controls, I'm sure it gets hot in the engine bay. I have the cooling bodies under the carbs, but I'm not quite sure yet how to integrate it into my current fuel line setup. First step is to tear out the insulation that the previous owner put on the hood... Thanks for the help Quote Link to comment Share on other sites More sharing options...
zcarnut Posted April 15, 2008 Share Posted April 15, 2008 Make sure you have some type of thermal insulator between the carb and the intake manifold. You can use the plastic anti-vibration kits for this. Also, a sheet metal heat shield mounted under the carb bodies is very effective to reflect the radiation from the exhaust manifold/header. Quote Link to comment Share on other sites More sharing options...
brokebolt Posted April 18, 2008 Share Posted April 18, 2008 My bell top SU's did the same thing, mainly on the rear carb. The fuel bowl would actually go dry (long story). I agree with what has been said, try one more thing to confirm your suspicions. Use a inferred tempature gun and get a reading off each carb top and then the bottom just after you stop then after the problem starts and just after it dies. Record these numbers so you know what the optimal temp is for your car. Now you can base line any mods and confirm that the mods are working. A good heat sheild is always a good idea. Buy a starter thermal blanket and bolt it to the bottom of the heat sheild for added protection. You'll have to cut off the velcro flap. Looks like hell but if done right nobody will see it without a mirror. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 18, 2008 Share Posted April 18, 2008 What are you using for the fuel pump and rail? The only time that I had vapor lock was when I ran the mechanical pump and the stock rail with SUs and those problems completely disappeared when I switched to an electric pump and got rid of the fuel rail. I ran my Mikunis with the stock asbestos insulators and no heat shield in 105 degree weather and never had any vapor lock issues. I did use a longer Cannon manifold which moves the carb bodies a little farther away from the header tubes. Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted April 19, 2008 Author Share Posted April 19, 2008 What are you using for the fuel pump and rail? The only time that I had vapor lock was when I ran the mechanical pump and the stock rail with SUs and those problems completely disappeared when I switched to an electric pump and got rid of the fuel rail. I ran my Mikunis with the stock asbestos insulators and no heat shield in 105 degree weather and never had any vapor lock issues. I did use a longer Cannon manifold which moves the carb bodies a little farther away from the header tubes. I'm slightly confused. Are you referring to vapor lock in the fuel lines, or the fuel in the carburetors vaporizing? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 19, 2008 Share Posted April 19, 2008 I never tracked it down that closely to be honest. I just know that I was racing that day, I'd bring the car in from a session, pop the hood, and then as it idled with the fan running the exhaust would start to sound flat and farty, and then after a couple minutes it would die. Let it sit for an hour or so, it would fire right back up and run great. In the afternoon when it got hotter I also noticed it was bogging at the end of the longest straightaway too. I don't really think it matters too much where the fuel gets hot. The problem is that it gets hot. I think the mechanical pump and stock rail put a LOT of heat into the fuel. Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted April 19, 2008 Author Share Posted April 19, 2008 I never tracked it down that closely to be honest. I just know that I was racing that day, I'd bring the car in from a session, pop the hood, and then as it idled with the fan running the exhaust would start to sound flat and farty, and then after a couple minutes it would die. Let it sit for an hour or so, it would fire right back up and run great. In the afternoon when it got hotter I also noticed it was bogging at the end of the longest straightaway too. I don't really think it matters too much where the fuel gets hot. The problem is that it gets hot. I think the mechanical pump and stock rail put a LOT of heat into the fuel. Sounds like actual vapor lock in the fuel lines to me... Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted April 21, 2008 Author Share Posted April 21, 2008 And where can I pickup a heat shield? I'm not having much luck finding one... Quote Link to comment Share on other sites More sharing options...
brokebolt Posted April 22, 2008 Share Posted April 22, 2008 My best guess is you will have to make one. I do not know of any aftermarket product that fits the bill. I would suggest using a very thin sheet metal so you can easily bend it with your hand and also use a prick punch to put holes where you need them; better than trying to drill it, and quicker too. Once you got a basic shape and bends than you can make a permenant heat shield with a thicker material and aluminium too. My Webers have a bolt stud on the front of the carb so it makes mouting them quicker and easier on the air inlet side of the carb. I would mount it in the rear as a main mounting point and use some kind of support on the strut tower side...somewhere. Best of luck. Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted April 27, 2008 Author Share Posted April 27, 2008 thx. Looks like just riping out the insulator on the hood and running some carb cleaner through was enough for now. Quote Link to comment Share on other sites More sharing options...
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