I think my 1977 GTB has a split personality This morning in the Phoenix desert I drove about 15 miles. I listened to the awesome exhaust note with an occasional pop from the exhaust. Lustful After lunch and about 20 more degrees, the engine could not hold an idle and it was backfiring having trouble accelerating. Once at speed on the highway it was 90%, but not like the morning. Almost night and day. Same gas, same spark, same everything except the heat. Is this possible? I have had many injected cars in the heat but not carbed. What do you think? Is it the heat causing poor performance?
I once had very similar heat stroke symptoms in my old Jag XKE. It was a long time ago but if I remember correctly it was fuel starvation due to something like vapor lock in the fuel line. I think it was fixed by changing a fuel tank valve that vented to the air?
+1. My next guess would be a vacuum leak caused by temp change but they usually get sealed up by higher temps. Try running some wire harness insulation around your fuel lines. See birdmans carb sync tutorial for a pic. Finally let me know when you want to go for a run. I'm in.
At first try to open the fuel cap and try to do a small run. If the engine improves, check the venting system in the fuel tank. But in my opinion it could be a too rich air mixture too. When air temperature increases your mixtures simply becomes richer. Check a spark plug: If it's black you have to tune the carbs, if it's brown you just have to fix the fuel tank venting system. In any case, carbs are very sensible to big temperatures and pressure change Ciao
vac leak. When the car gets warm, it runs lean. You don't hear it when the car is running because you are putting so much fuel through the carbs. Whole reason the carbs are off my car and being rebuilt. Sealed bearings on the throttle shafts and this goes away.
No expert, but isn't popping, overrun, such as you saw in the morning a condition associated with too rich a mixture? If you're too rich when cool, not sure how that will get better as the ambient temp goes up. I think it gets worse because warm air density is less. People think overrun is "cool", but it can be from less efficient tuning.
Not to belabor the point but if it is vapor lock then the cause is a pressure issue in the fuel system. I think there could be a potential fire risk if any fuel lines are softening up and expanding allowing the gas to vaporize. How old and what type fuel hose do you have in the car. Something to just double check.
Take the car for a drive, and when it gets to the temperature it starts running poorly, pop the lid and listen (put your ear as close to the airbox as you can without burning yourself), do you hear gurgling as if the gas is boiling in carbs? Must be very quiet to hear, like in your garage...
The temperature conditions you describe will not make a difference i.e. the effect will be the same carbed or fuel injected. if the engine is tuned right. So something changed. Did it get better after sitting overnight or does it still have an issue? I usually like to start with the ignition system, I my experience a large number of carb issues are caused by the ignition system. First I take an ignition wire off and make sure I have a spark that will jump a 1/2 inch gap. It is best to do this while the car is having a problem. Coils can be sensitive to temperature, work when they are cold and failing when hot. Since your issue is idle and cruise related the next thing I would check is the idle jets, if one or two of these is clogged it will cause the issues you describe. The idle jet is one of the smallest passages in the carb and is the most likely jet to get plugged. After that it gets more difficult to figure out. Cheers Jim
Are you running points or electronic ign. What plug heat range are you running and what do they look like. I ran BP5's in my 77 and it ran pretty good.
" I my experience a large number of carb issues are caused by the ignition system." The standard old saying was, "90% of carburetor problems are ignition!"
Yep I pretty much agree as I learned this on my 66 XKE. With the 77 308 I did the carbs once in 14 years and after redoing the ignition system didn't have any problems.
Except in the case of old Italian Weber carbs. Do you know if your carbs have ever been rebuilt? If the answer is "NO" then it's the throttle shaft bearings. The originals were grease packed, and when the grease finally dries out after 40 years, you get a vac leak. The car runs better cold, because it's running rich and hides the vac leak. As it warms up, the fuel gets warmer/more volatile, and the vac leak becomes more relevant. Get some carb cleaner out and spray it around your carbs. I'm betting that's the issue.
I only clicked on this thread to post the same thing. Check your points or better yet get a Pertronix ignition. Aaron
Thanks for all the responses Distributors and coils are new,newpointsand bearings, carbs were recently rebuilt, and new idle jets were installed. With all the comments, and condition of the carbs, I lean toward the venting
