6 degrees BTDC is a little low. try a little more timing. should stop your backfire/cough and improve the vacuum.
351c Tuning Tips, Suggestions
Hey guys looking for any suggestions. I get backfire thru the carb on acceleration (just revving up the motor in park) and under load.
Engine is a 1973 351c 2V.
Timing is set at 6 BTDC
Brand new plugs (Autolite AP25's)
Brand new wires (Ford Racing 9mm)
Brand new distributor (GM HEI type setup)
After running and testing and idling, I pulled the plugs, they were totally black and fouled out. Cleaned them and reinstalled them, got out the vacuum gauge and adjusted the fuel mixture. I wound up turning both mixture screws in 2 full turns before it had any effect on vacuum.
On test drive after this the engine still backfires from time to time when the gas pedal is pressed.
Plugged all vacuum ports (including large PCV port on back of carb spacer). Checked carb mounting nuts, both of the back nuts were a little loose, I was able to tighten them both down a bit. Vacuum reading was 14 +/- 1hg at idle. On rechecking the idle mixture using the vacuum gauge I was able to turn both mixture screws in another half turn and got my vacuum to 16 +/- 1hg, but as I adjusted this my engine speed kept increasing, so that when I was done, I needed to reduce the idle speed. Turning it back down to about 550rpm, it idles smoothly, but as I reduced the idle speed my vacuum went back down as well, now I am at 13 +/1 1hg.
If I hold the brakes and put it in Drive & give it gas (simulating load) it does not backfire, however with no load and stomping the gas pedal, I still get the pop. I am getting frustrated with this, would love some suggestions. I know sometimes when this happens it is fuel related (lean backfire) but everything so far has indicated it was way too rich, or electrical related (however everything is new runs very smoothly, starts at the first touch of the key, etc, except when the gas pedal is stomped).
Engine is a 1973 351c 2V.
Timing is set at 6 BTDC
Brand new plugs (Autolite AP25's)
Brand new wires (Ford Racing 9mm)
Brand new distributor (GM HEI type setup)
After running and testing and idling, I pulled the plugs, they were totally black and fouled out. Cleaned them and reinstalled them, got out the vacuum gauge and adjusted the fuel mixture. I wound up turning both mixture screws in 2 full turns before it had any effect on vacuum.
On test drive after this the engine still backfires from time to time when the gas pedal is pressed.
Plugged all vacuum ports (including large PCV port on back of carb spacer). Checked carb mounting nuts, both of the back nuts were a little loose, I was able to tighten them both down a bit. Vacuum reading was 14 +/- 1hg at idle. On rechecking the idle mixture using the vacuum gauge I was able to turn both mixture screws in another half turn and got my vacuum to 16 +/- 1hg, but as I adjusted this my engine speed kept increasing, so that when I was done, I needed to reduce the idle speed. Turning it back down to about 550rpm, it idles smoothly, but as I reduced the idle speed my vacuum went back down as well, now I am at 13 +/1 1hg.
If I hold the brakes and put it in Drive & give it gas (simulating load) it does not backfire, however with no load and stomping the gas pedal, I still get the pop. I am getting frustrated with this, would love some suggestions. I know sometimes when this happens it is fuel related (lean backfire) but everything so far has indicated it was way too rich, or electrical related (however everything is new runs very smoothly, starts at the first touch of the key, etc, except when the gas pedal is stomped).
1 - 13 of 13 Posts
6 degrees is what it calls for in the Haynes manual and the Chiltons, so that's what I was going with. If you think bumping it up to 8 or 10 might improve things I will definitely give it a shot. But first thing tomorrow I am pulling the plugs to read them and see if they look ok now, check & verify the gap, and then do a compression test while I have them all out.6 degrees BTDC is a little low. try a little more timing. should stop your backfire/cough and improve the vacuum.
Hello Cracker289,
A compression test is always helpful - BUt I don't think
you will find an issue there.
I know you have done a LOT of quality posts here and I have
read many and learned a lot. BUT
As cindys_sn95 has said - move the timing to 10BTDC.
The specs all call for 6 but she will run much better at 10.
Next - - even the PRO's miss the little stuff sometimes.
I would make sure the wires are all run correct. You would be
surprised at how well that motor will run even with 2 wires in the wrong
spot on the cap (firing order).
TRUST me Cracker - -I know you know a LOT but sometimes a fresh set
of eyes can pick up something little.
After the compression test - -I would turn the mixture screws all the way in
to a gentle seat - -then I would back both off one and a half turns. This is a
good starting point. Then watch the vacuum and set for the highest number
as you know.
For kicks - -I would try to eliminate a SLIGHT vacuum leak. Take a can of
starter fluid and spray around the base of the carb when she is running.
If after checking things and checking for vacuum leaks - -it may be time to
rebuild or replace the carb. I WOULD NOT do any major changes until
you are certain the issue is in the carb.
Figured I offer a few little tips to get you going - - It may be just something little.
Trust me on this - -I don't think you have a major issue just a frustrating one.
Print DAd
A compression test is always helpful - BUt I don't think
you will find an issue there.
I know you have done a LOT of quality posts here and I have
read many and learned a lot. BUT
As cindys_sn95 has said - move the timing to 10BTDC.
The specs all call for 6 but she will run much better at 10.
Next - - even the PRO's miss the little stuff sometimes.
I would make sure the wires are all run correct. You would be
surprised at how well that motor will run even with 2 wires in the wrong
spot on the cap (firing order).
TRUST me Cracker - -I know you know a LOT but sometimes a fresh set
of eyes can pick up something little.
After the compression test - -I would turn the mixture screws all the way in
to a gentle seat - -then I would back both off one and a half turns. This is a
good starting point. Then watch the vacuum and set for the highest number
as you know.
For kicks - -I would try to eliminate a SLIGHT vacuum leak. Take a can of
starter fluid and spray around the base of the carb when she is running.
If after checking things and checking for vacuum leaks - -it may be time to
rebuild or replace the carb. I WOULD NOT do any major changes until
you are certain the issue is in the carb.
Figured I offer a few little tips to get you going - - It may be just something little.
Trust me on this - -I don't think you have a major issue just a frustrating one.
Print DAd
Well I don't think the compression test will show me anything (at least I hope not, because if it does then it means something serious!) but it will be good to do one and know what I have (engine has not been rebuilt so I am not assuming much, but will be nice if they are all relatively close to each other), and since I'll have all 8 plugs out to read & check gaps its as good as time as any.Hello Cracker289,
A compression test is always helpful - BUt I don't think
you will find an issue there.
I know you have done a LOT of quality posts here and I have
read many and learned a lot. BUT
As cindys_sn95 has said - move the timing to 10BTDC.
The specs all call for 6 but she will run much better at 10.
Next - - even the PRO's miss the little stuff sometimes.
I would make sure the wires are all run correct. You would be
surprised at how well that motor will run even with 2 wires in the wrong
spot on the cap (firing order).
TRUST me Cracker - -I know you know a LOT but sometimes a fresh set
of eyes can pick up something little.
After the compression test - -I would turn the mixture screws all the way in
to a gentle seat - -then I would back both off one and a half turns. This is a
good starting point. Then watch the vacuum and set for the highest number
as you know.
For kicks - -I would try to eliminate a SLIGHT vacuum leak. Take a can of
starter fluid and spray around the base of the carb when she is running.
If after checking things and checking for vacuum leaks - -it may be time to
rebuild or replace the carb. I WOULD NOT do any major changes until
you are certain the issue is in the carb.
Figured I offer a few little tips to get you going - - It may be just something little.
Trust me on this - -I don't think you have a major issue just a frustrating one.
Print DAd
I did check the plug wires to make sure the firing order was correct. Also made sure my HEI is getting 12 volts while the engine is running (I connected the original positive wire for the coil to a relay, and then let the relay keep a steady flow of power to the ignition).
I am definitely going to take your (& previous poster) advice on moving the timing up a few degrees.
Regarding the vacuum leak possibility, I am thinking if it was loose & leaking under the carb, there is every possibility that simply tightening it won't fix it now, so the next thing I'm thinking of is pulling the carb off and putting a new gasket down, that should only be a 10 minute job and even if it doesn't solve the problem, I'll rest better knowing that is one less possible issue.
The carb itself is relatively new, as in, it might be a few years old but has very little run-time on it. But it's still possible that it may have so gummed up passages from sitting so maybe it's worth it to take it apart for a good cleaning, but oh what a hassle, especially if it turns out that its not the carb!
the original positive wire for the coil is a resistor wire! NOT full 12 volts. with it running, check the voltage.
From earlier: "Also made sure my HEI is getting 12 volts while the engine is running (I connected the original positive wire for the coil to a relay, and then let the relay keep a steady flow of power to the ignition). "the original positive wire for the coil is a resistor wire! NOT full 12 volts. with it running, check the voltage.
i saw what you posted john, that is why i stated the original positive wire is NOT 12volts! you know what....nevermind.
Oh I thought you were under the impression that I was feeding the HEI with the original 8 volt points power feed. I'm only using it to switch the relay on and off.i saw what you posted john, that is why i stated the original positive wire is NOT 12volts! you know what....nevermind.
I'll try the timing advance this afternoon as you suggested (if weather cooperates) and post my results here. Thanks.
I pulled and read the spark plugs, 6 of them look perfect, and 2 of them look a little rich, but not too bad.
Cranking compression test is pretty good for most cylinders, but a little low on #6.
1 = 140
2 = 145
3 = 145
4 = 140
5 = 150
6 = 120
7 = 145
8 = 155
Next up is dinner time but after that, will clean and re-gap spark plugs, and then advance the timing as suggested earlier.
Cranking compression test is pretty good for most cylinders, but a little low on #6.
1 = 140
2 = 145
3 = 145
4 = 140
5 = 150
6 = 120
7 = 145
8 = 155
Next up is dinner time but after that, will clean and re-gap spark plugs, and then advance the timing as suggested earlier.
My spark plug gaps were all a little too wide. Per the tune up specs in my Chiltons it should be 0.034 and per a HEI troubleshooting website I found, gap should be kept to 0.035, so thats essentially the same. However mine were 0.040 - 0.045. I've corrected the gaps and reinstalled the plugs, engine runs pretty much the same. Thanks to the rain I am not going to be able to try the timing change tonight, but I will tomorrow.
I think you will find that bumping up the timing will help a lot. My 351C 2v has never had a rebuild and has lost a lot of compression over the years, so it actually runs best at about 14* BTDC!
So, I'd like to thank you all for the tips and suggestions. Those who said timing was the issue were right on the money, the engine is running 100% better. The real problem was that some dern fool marked the harmonic balancer at 10 degrees AFTER TDC instead of 10 before. And yes I was that dern fool. :shiny:
Timing now set at 12 BTC and she's running like a champ!
Timing now set at 12 BTC and she's running like a champ!
1 - 13 of 13 Posts
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