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Help chasing an exhaust backfire when engine gets hot #1319651 Mon Jul 29 2019 03:51 AM
Joined: Dec 2016
Posts: 14
K
k4cooper Offline OP
New Guy
Details: 1950 3100, 216 engine, running a Carter YF carb, aside from the Carter carb, all else is stock.

I recently starting chasing down what seems to be a fairly quick onset of a backfire.

I noticed it at first on deceleration. I tracked that down to being a little too rich on the carb and leaky exhaust manifold donut gasket.

In the process, replaced worn points, reset timing, points gap, cleaned and regapped all plugs, readjusted the carb and it seemed to go away.

I have it running smooth as can be, until the engine gets really fully warmed up and the backfiring seems to return.

It's consistently not backfiring when it's first started and just getting warmed up. Runs fine for five minutes or so with choke wide open, then starts popping under a little load at maybe 2500 RPM (guessing, didn't check it yet).

I realize diagnosing such in post can be difficult, but I've read (and learned) much from the experts here so I'm hoping someone can help get me focused on the next steps more quickly.

Thanks in advance.

Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319655 Mon Jul 29 2019 06:14 AM
Joined: Feb 2002
Posts: 2,922
Woogeroo Offline
Socket Breaker
well, you are in the right place... someone should be along with more questions and suggestions.

-Woog

Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319658 Mon Jul 29 2019 09:02 AM
Joined: Feb 2001
Posts: 6,669
W
WE b OLD Offline
Master Gabster
Maybe too rich of a mixture??????? Or a weak return spring not shutting the throttle soon enough,


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Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319664 Mon Jul 29 2019 11:55 AM
Joined: Feb 2000
Posts: 4,053
J
Joe H Offline
Shop Shark
Could be exhaust manifold to block leak. When it gets hot, the manifold expands exposing a leak. The leak allows fresh air in that ignites from heat. Could also be a weak exhaust valve spring or leaking exhaust valve seat allowing fresh fuel / air mixture into the exhaust.

Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319667 Mon Jul 29 2019 12:21 PM
Joined: Apr 2005
Posts: 8,112
Justhorsenround Offline
Grease Monkey, Moderator General Truck Talk & Greasy Spoon
I might suggest adjusting the valves. One may be just a touch tight and when the engine gets warm it doesn’t seal. Just throwing out an idea.


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Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319672 Mon Jul 29 2019 01:25 PM
Joined: Mar 2010
Posts: 8,850
5
52Carl Offline
Master Gabster
Check the operation of the flapper valve (bi-metallic spring) which deflects heat away from the bottom of the intake once the engine heats up.
What did the spark plugs look like before you cleaned them? Where they covered in soot as though it was running very rich?
YF carbs have a diaphragm for an accelerator pump which are compromised by the blend of fuel available today. They start to develop a pinhole which causes a rich condition. As they fail further, it becomes extremely rich. I have had this happen 2 times with mine. I was able to drive the truck home by pulling the throttle knob out to keep the throttle plate open far enough to burn the excess fuel so that it would continue to run.
Carl

Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319674 Mon Jul 29 2019 01:35 PM
Joined: Jan 2013
Posts: 1,523
N
ndkid275 Offline
Shop Shark
Carl,

I too was going to suggest the heat riser. I had a similar issue that cleared up once I got it working correctly. It was an easy fix, but caused me s lot of issues.

Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319732 Mon Jul 29 2019 10:48 PM
Joined: Dec 2016
Posts: 14
K
k4cooper Offline OP
New Guy
Many thanks for the quick suggestions.

@WE b OLD: I guess it could still be running rich, but when I first started chasing it, I leaned out the mixture as much as I could. Prior to that the plugs were all a little sooty. I haven't thought about a stronger return spring. Will investigate that.

@Joe H and justhorsenround: Checking valve adjustment was next on my list of things to try, just haven't had time to work it. Never done it, so any advice here beyond what I've read already would be appreciated too.

@52Carl and ndkid275: I'm not familiar with the function of the heat riser valve two of you reference. I'll pull out the shop manual and do some reading, but if you know a good way to test functionality and fix if broken, please advise. Carb was rebuilt approximately a year ago and it's never had ethanol gas in it, but it's possible that the pump has a pinhole, also something to check.

Again, many thanks. This site is invaluable for those of us who have worked on vehicles for many years, but new to these specific trucks.

Re: Help chasing an exhaust backfire when engine gets hot [Re: k4cooper] #1319832 Tue Jul 30 2019 05:04 PM
Joined: Mar 2002
Posts: 8,926
truckernix Offline
Master Gabster
I assume that you changed the condenser with the points? I had a similar problem with mine and it was the condenser that was six months old. I hooked up a timing light and pointed it into the windshield. Running the engine in the garage I could see the spark fail as it bucked and died.


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Re: Help chasing an exhaust backfire when engine gets hot [Re: truckernix] #1319914 Wed Jul 31 2019 02:51 AM
Joined: Dec 2016
Posts: 14
K
k4cooper Offline OP
New Guy
Interesting. I did change the condenser when I changed and reset the points.

Based on the above, I think I'm going to (in this order):

1). Check function of heat riser, free up if stuck
2). See if I can lean out the carb a little more
3). Check/replace throttle return spring
4). Replace accelerator pump, in case it has a leak
5). Check/adjust valves

Again, many thanks for the advice.

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