Car is fine off the line. Just when you reach the point where she getting near the top end she hesitates and drops off. Its as if there is a governor on the carb. What gives? Is my float getting stuck? Carb had a total rebuild not long ago and is getting these annoying little quirks which are driving me crazy! :af:
I agree with the fuel delivery assessment. In working with these carbs over the years, the typoical complaint when I dealt with clogged filters, dying fuel pumps or debris in the fuel tank clogging that filter, it was typically about top-end complaints. One easy way to determine the issue might be to duplicate the issue, then simply switch the ignition off, (do not lock the steering wheel!) shift to neutral, and let the vehicle coast to a stop. Then try restarting the vehicle, or measure the float level. If the carb is almost empty, I would address delivery issues. It might be as simple as a defective fuel cap, not allowing the fuel to exit the tank. It might be safer to simply replace the in carb fuel filter, (they quickly clog if water or moisture is in the fuel), and try the same thing with the gas cap off, or ajar. Filters are cheap. If the filter is clogged, I would definitely suspect water, and be sure that you use the correct filter for the application. I have seen parts stores only carry the later filters with the roll-over valve. This filter required a different pump, delivering an additional 1/2 to one pound difference in fuel pressure. Ray
Changed the carb fuel filter when problem first started. As far as the gas cap on the tank she's from 71. I should mention that the car was originally a 2bbl. I replaced the fuel pump when she leaked awhile back before the carb and manifold swap. Bought the 4bbl and had her cleaned and rebuilt fully. Initial problem I had was that the float would get stuck and she would pee gas all over the place. Had her taken apart and redone a second time at which time all seemed OK. Then this problem. Could it just be that I have a bad rebuild??
One of the first things to do is to install a high flow needle/seat assembly. If the carb was "redone" or "rebuilt", is probably got the parts from an over the counter kit, which will NOT contain the high flow seat. Most kits will have .110" or .125" seats in them, not the .135". Install the larger seat assembly, re-check the float level, and test it again. If it still runs out of fuel, the fuel delivery system has a problem or is not adequate for the power level of the engine.....Cliff
Since "all seemed normal" for a while, I suspect something changing recently. Is there any possibility that the colder, denser air is leaning the mixture, meaning that the metering was marginal initially? Something else to consider is the possibility of bad gas. Be sure of the supplier, as even gas from the same manufacturer can vary station-to station. The condition of the in-tank fuel filter could be an issue, as could be the diaphragm in the pump being affected by the crap liberally called gasoline. There are those stations out there whose fuel quality could , at best, be called marginal. To address Cliff's contention, it is very possible that, when the temps drop, the denser air demands additional fuel, (it will), and the needle and seat assy is marginal. Note: air density will directly affect the fuel the carb delivers. It is not fixed, just limited. K&N air filters, for example, can create a surge, due to the additional air available. This can be especially true if the engine mods lead to a true increase in performance. In short, things change in the winter, especially in colder climates, and contrary to Al Bore, this has been one of the coldest, and earlier than normal, causing all kinds of problems, for both us, and the fuel industry. "Going fast costs money. How fast do you want to go?" Ray
Now that you mentioned it I did install a K&N open element filter set up in place of the stock one recently. Would it really make that much difference? On the open road with gradual acceleration the car gets up there with no problem. It was only under hard acceleration while checking the shifts in my tranny that I hit the upper end problem. Maybe I'll reinstall the old air cleaner before messing with the carb. Still haven't been able to locate the carb adjusting tool that works perfect with these carbs.
Just to give you an idea of the difference a K=N filter makes, I recently worked on a new Explorer with a check engine light. The code was for the fuel trim being insufficient. That means that the computer could not add sufficient fuel to satisfy the O2 sensor. A complete check out indicated that everything was fine. Questioning the customer indicated that he had recently installed a K=N filter, which was when the light went on. When I put a new paper element back in, everything returned to normal, and fuel trims returned to factory settings. When I reinstalled the K+N, it took all of four minutes to re-illuminate the light. I could watch the fuel trims rise on my scan tool, until it topped out. Two minutes later, the code returned. Questions? Ray
Does that mean I should eliminate the K&N set up or just lean out the carb? Obviously I was looking to somewhat beef up performance. In this crazy weather I now have a slight backfire just hitting the pedal a little. Noticed it today when I got the car back from the tranny shop after getting a leak fixed. Frustrating to say the least. :af:
Steve, there are a lot of potential problems with your 40 year old fuel system, and they start from the tank, where the fuel sender's filter sock is located. It may be restricted, as well as the rubber hoses connecting it to the fuel feed line. They are known to degrade as well. So tell us about your fuel system. Is it all original? Devon
System is all original as far as what I know except for the rebuilt 4bbl and fuel pump. The carb has had numerous issues since the rebuild and I'm wondering if it may actually be the cause of my misery. First the float was sticking now the linkage is sticking. Maybe it's time to have the rebuild checked. When its good its good but when its bad its bad!
Wait a minute,,,, if a k&n filter is less restrictive that means that the carb is running leaner due to the increase in oxygen,,,, so you are getting a lean out condition and lean backfire????? and it shows up at higher rpms.... when carbs are known to lean out even when they are performance carbs.... richen up the mixture some and see what happens....
Thought about that but she sure smells like she's running rich and dumping. Once we get back to normal temps here in Florida I'll play around a little bit. Guess its best I start from the beginning just to avoid making unecessary changes. Thanks
So far I have had 2 diff 350 engines. Both had that backfire prob on cold days. On the first engine I couldn't figure out the problem.Tthe second is my Buick 350. Larry suggested that the cold air was leaning out the mixture and stated that I needed a heat stove and hose from the factory air cleaner to the heat stove... Problem solved:TU: No more backfire or loss of power on cold days. I keep the flap on the factory air cleaner open on cold days to alow heat at all times. It works for me
Had the car out today. It was 50 degrees so I didn't let her warm up much. Just went for a run to see what was going on. First run under some load, she missed then was OK. Second, third and fourth runs I was hard on her and couldn't get her to backfire whether she was hit hard at midway or off the line. Nothing was changed or touched!! Linkage still grabbed on the first push of the pedal when I went to warm her up but after that all was moving freely. Think I might take a second look at the Petronix ignition module I put in to eliminate the points. Heard some guys on the board had some issues with the system. I swear the car is possessed!
Question regarding when this backfire occurs. Does it occur cold, or during warm-up? Are we talking about a full fledged rifle report, or a fart through the carb? If the issue is one of a "fart," you are likely looking at a choke coming off too fast. If, however, this is an issue of the air filter causing the lean-uot, address the warm metering before continuing. One caveat here, guys, is the possibility of ignition issues causing these things. On too many occasions, an individual gave up on metering problems, brought it to me, and I fixed an ignition issue. Then, the carb had to be returned to normal. Issues with plug wires are easily mis-read, and a new plug wire set is easy to install, and relatively cheap, especially when chasing issues. When I scope the engine, I can see the plug wire issue, and can easliy show the customer. You would be surprised at how many other driveability problems disappear as well. If you are sure that it is one of fuel delivery, fix the problems hot before chasing cold issues. For the record, it is definitely an issue with free-flowing air filters to lean mixtures, especially when combined with the already lean mixtures the gov't required. These mixtures were marginal at best the the crap the manufacturer used for an air filter. Ray
Problem occurs hot and cold. Is more of a fart than a repeat. Was thinking that maybe the carb linkage sticking on initial startup had something to do with the choke not working properly. However, when somenone sits in the car and depresses the pedal for the first time it sticks but I don't see anything binding. Keep in mind that the problem is not occuring all the time which is why its so annoying. If the wires were bad wouldn't the problem be all the time? Thats why I was concerned about the Petronix unit I installed being a possible issue as well.
I have seen my share of throttles sticking in the bores. I would try opening the throttles slightly with the idle speed screw, regardless of the idle to determine whether the throttles are sticking. With respect to the plug wires, they usually show up when spark requirements are maximum, like when short shifting a standard trans... It appears that you may have plug fouling issues, as, from what I read, when you legged the car a little, the problem cleared up. I suspect that the car has insufficient time to clean the plugs after a cold start. This might be due to a choke staying on a little too long, a thermostat opening too soon, lowering the operating temps, or anything lowering the operating temps or enriching the mixtures. Have you tried using plugs with a wider gap? Not simply opening the spark gap, as that would create its own issues, but going to a plug with an X or 6 suffix after its designation. Ex R43tsx is the same as a R43ts, but with a wider gap. With the increased spark energy promised with the Pertronix unit, this might help. Some food for thought... Ray
Usually, a "fart" is the result of a lean-out. This can happen when the throttle is opened too quickly or too far during warm-up. The additional air supplied by the carb during warm-up pulls the choke open wider, due to the offset of the choke blade on the shaft. It is possible that the wider plug gaps I suggested in the earlier post might cause a better burn of the leaner mixtures, and keep the plugs cleaner. I have seen my share of cold flooded engines require plug replacement just to get the engine running due to the fouling. I strongly recommend the replacement of plugs at 6,000 mile intervals with factory ignitions, and at 20,000 mile intervals with the better ignitions. Ray
