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| carb STILL backfiring...Mechanic gives up on me. https://slantsix.org/forum/viewtopic.php?t=48198 |
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| Author: | WagonsRcool [ Fri Mar 16, 2012 4:05 pm ] |
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There are inexpensive tools that screws onto a small propane tank (a valve with a hose) that lets you feed propane into the carb airhorn (it cost some more than just putting your hand over the choke plate, but it's a little more controllable) . If adding fuel makes the engine run better- then you look for vac leaks and/or carb problems. If it doesn't improve then you look at ignition OR engine mechanical. For ignition, you could have a (possibly) a weak coil- use a adjustable gap-type spark tester like Thexton# 458 to MEASURE coil output. Should be able to jump a 5/8" gap. Or it could be a timing fault- like slipped marks as Dan suggested. A silly question, did your "mechanic" get the plug wires installed onto the dist cap in the correct order? For the engine I would suggest a compression test on all cylinders, & a leakdown test as well. (what's a vacuum gauge read?) Possible culprits could be: intake valve just starting to leak/burn, int valve tight/ no lash (hyd lifters can fail "solid" & not collapse at all), int valve stem is sticking, broken int valve spring, exh valve isn't opening (wiped cam lobe). A methodical diagnostic procedure should narrow the possibilities down, quickly & without "throwing" parts at the problem. (why the heck would someone pull the timing cover just to CHECK the chain?) |
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| Author: | bigandbulky [ Fri Mar 16, 2012 4:08 pm ] |
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this is a stupid question but did he also change your distributor pickup? I have had this problem on bikes and one was the points were bad and one was the electric timing devise was bad ( same thing really) I would also look at the distributor being bad and when cheeking timing take the cap off and make sure you line up your mark with your #1 there may be multiple marks on your fly wheel ( this happened to me) and don;t just assume the one you are seeing is the one you need. |
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| Author: | ESP47 [ Fri Mar 16, 2012 4:42 pm ] |
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Lots of good advice here but you should start simple and move on to the complicated stuff later. Harbor Freight sells a decent cheap vacuum gauge. See if there are any ports on the carb that are capped off. With the engine running, pull the cap and put your finger over the port and see if it has vacuum. If it does then connect the gauge there and see what you're getting. You want at least 17hg at idle but 20hg is what you're looking for. If you don't have a capped port, you can use a T connector that comes with the kit and pull the vacuum pulloff hose on the carb and T off there to get your reading. If you want to do an almost mistake free vacuum check. Spray soapy water on the sides of the carb, the carb base, the intake runners where they meet the head, the EGR connection and any other ports on the intake. Then with the engine off, use a compressor to shoot air up the PCV hose toward the carb and you should see bubbles where you have a leak. Don't use a ton of air pressure, just enough to cause some bubbles. Either way, a vacuum gauge is essential to tuning your slant. |
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| Author: | bigandbulky [ Fri Mar 16, 2012 6:28 pm ] |
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that is some good advise. I will probably do that tomorrow myself. |
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| Author: | Bren67Cuda904 [ Fri Mar 16, 2012 6:43 pm ] |
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I vote for worn exhaust cam lobe. |
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| Author: | SlantSixDan [ Fri Mar 16, 2012 11:23 pm ] |
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Worn exhaust cam lobes do not cause spitback/backfire through the carburetor. |
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| Author: | Charrlie_S [ Sat Mar 17, 2012 4:51 am ] |
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Quote: Worn exhaust cam lobes do not cause spitback/backfire through the carburetor.
Beg to differ. If the exhaust lobe is worn enough, the cylinder pressure will escape out the intake when that valve opens. It will cause a "popping" through the carb. Just found this on a 360 recently. But I do think that this would be a last choice. Easy to check, pull the valve cover and watch the valves move. If one only moves a little, there it is.
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| Author: | SlantSixDan [ Sat Mar 17, 2012 11:21 am ] |
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Quote: Beg to differ. If the exhaust lobe is worn enough, the cylinder pressure will escape out the intake when that valve opens. It will cause a "popping" through the carb.
Okeh, yeah, I can see that happening.Quote: But I do think that this would be a last choice.
Me too. There's a big list of much more likely, much less ugly possibilities of what's causing the problem; those should be checked and ruled out or fixed first.
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| Author: | bigandbulky [ Sat Mar 17, 2012 1:29 pm ] |
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Do you have the car back yet. Could you record while someone holds the pedal down or see if you can make it happen? |
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| Author: | bigandbulky [ Sat Mar 17, 2012 5:50 pm ] |
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Quote: A silly question, did your "mechanic" get the plug wires installed onto the dist cap in the correct order?
After thinking about it. I Would try this first. I have no idea why I would say anything else until you looked at that.
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