351c miss/backfire under load?

[39deluxecp]

I finally got to drive my 1955 Thunderbird with a rebuilt 351 cleveland and a c4 transmission driving. It starts and runs fine, until I try to give it more than half throttle. then it cuts out and almost backfires, but not a real backfire. runs fine up to max rpm as long as it is a gradual acceleration. I put it together with new rings, bearings, heads rebuilt with proper springs, new hydraulic cam and lifters. timing chain and sprockets. It has a new 3310 750cfm Holley, new high energy distributor and coil, plugs and wires are new also. new fuel pump, gas tank and fuel lines. I put bigger jets in primary and secondary on the carb with no change. the timing is at 38* at about 3 grand(no tachyet). It runs fine at part throttle, just when I push it down it misfires. any ideas??

[40 Deluxe]

Weak spark causing misfire-causes: defective coil, bad wiring (on the primary side), high resistance in the ignition switch or other connections; Lean fuel mixture-causes: partially plugged lines (anywhere from tank to carb), debris in tank or carb, low float level, weak fuel pump. Tee in a fitting in the fuel line to check fuel pressure while at full throttle. Duct tape the gauge to the outside of the windshield I see you have new parts,but--My boss had a plaque on his office wall with 2 Cummins rod bearings on it with the words "Trust but verify". Seems they were mis-boxed or mis-labeled, were for a different engine-I.D. OK but O.D. too small. The guy had dropped the pan like three times before he measured and found the mistake. The lesson? Double check even new parts when there's a problem!

[39deluxecp]

I just got back from the shop. I used a spark tester ( like a sparkplug from snap-on) and it has a red sperk. i checked voltage to coil, was 13.5 volts engine running. It could be the new NAPA coil I installed. it is from a 1988 taurus i think, but ut is new. i will substitute another coil tomorrow.

[KULTULZ]

new high energy distributor and coil

H.E.I. (GM) was available in two styles, coil incl in the DIST or a remote coil. Your DIST must be the remote style as you are using a FORD TFI E-COIL?


Where did you find the DIST?

[Ford blue blood]

I think that is way too much carburetor I have been running a 600 CFM Holley and a Dura-Spark ignition on a 351C/FMX for years, good performance, easy starting hot or cold and absolutely no stumble. Give a smaller CFM a try, initial timing at 12 - 14 degrees and do a thorough check for vacuum leaks.

[scicala]

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The 750 CFM 3310 Holley shouldn't be too big for a 351 C, especially since it has vacuum secondaries. Not saying it can't be a carb issue, but not because it's a 3310 Holley. I would lean towards the weak spark theory and put the stock 72 jets back in the primary side of the carb at least. Also make sure the accel pump on the carb is adjusted correctly and squirts with the slightest opening of the throttle.


Sal

[KULTULZ]

... and correct POWER VALVE matched to the engine manifold vacuum.

[39deluxecp]

I am using a self-contained electronic distributor from Speedmaster, new NAPA coil from a early taurus. I did adjust the accelerator pump to the 2 position for max fuel. It did help a little. Timing does not make any difference as far as this problem as i tried different settings. The 351c has a mild cam(204*&214*), headers, 360* aluminum intake manifold. The manifold is not what I really want but will have to do until I get my stimulus check ,LOL. I want to check the coil with my ohm meter. What should the readings be for the resistance?? thakyou all for your help.

[39deluxecp]

the power valve is a 6.5 from the factory. the engine has 14" of vacuum at idle. so the power valve should be okay, right??

[KULTULZ]

Originally Posted by 39deluxecp

the power valve is a 6.5 from the factory. the engine has 14" of vacuum at idle. so the power valve should be okay, right??

Should be - https://www.holley.com/blog/post/hol..._valve_tuning/

It only does it while under load on the highway? Are you sure of the DIST ADV CURVE? Have you plotted manifold vacuum (on the road)? Are you using manifold vacuum or ported vacuum as a DIST signal source?

[39deluxecp]

i tried it with the vacuum advance connected and disconnected, no difference. I have checked total advance with the timing light, 38*. When i checked the spark with the spark tester, it only had a red spark. That is why i am testing the coil.

[KULTULZ]

Originally Posted by 39deluxecp

i tried it with the vacuum advance connected and disconnected, no difference.

Absolutely no difference in advance curve @ idle with vacuum source?

I have checked total advance with the timing light, 38*. When i checked the spark with the spark tester, it only had a red spark. That is why i am testing the coil.

38 degrees at what RPM?

I hate to ask, but what is the source of the H.E.I. DIST?

The 14HG manifold vacuum. Was that at idle or with the engine in gear (AT)?

[39deluxecp]

no difference in miss of engine with vacuum advance connected or not. The distributor is a China quality product. I got a reading of .04 ohms and 7.38 ohms on the coil. limits are .04 to 2.0 . and 10,000-16,000 ohms. when the digital ohm meter says 7.38, is that 7,380 ohms??

[39deluxecp]

14" vacuum at idle in nuetral. 38* advance at 3,000 rpm, vacuume advance disconnected.

[KULTULZ]

Originally Posted by 39deluxecp

no difference in miss of engine with vacuum advance connected or not.

The distributor is a China quality product. I got a reading of .04 ohms and 7.38 ohms on the coil. limits are .04 to 2.0 . and 10,000-16,000 ohms. when the digital ohm meter says 7.38, is that 7,380 ohms??

There is a miss now?

I hate to be negative, but there is no quality made CHI-COM product. It sounds like there is no vacuum advance and/or the DIST has manifold vacuum signal instead of ported vacuum signal. What did the instruction sheet say to source the vacuum signal? If manifold, that is for GM. The DIST has to be setup differently for a FORD.

Originally Posted by 39deluxecp

14" vacuum at idle in nuetral. 38* advance at 3,000 rpm, vacuume advance disconnected.

Is the car an automatic? The manifold vacuum has to be measured while idling in gear. You then fine tune the power valve from there with road testing.

Please read this and prior referring URL - https://www.aa1car.com/library/ignition_coils.htm

[KULTULZ]

I just saw the photo of your engine. It does not appear as a GM style HEI DIST to me. Do you have a receipt giving PN and vendor?

What you are experiencing is LEAN TIP-IN hesitation and can be caused by carb, dist advance, inoperative (sticking) EGR and/or vacuum leak.

H.E.I. is terminology for GM design electronic ignition.

[cousinjd]

Just a couple of comments/observations of the picture of engine: Following the fuel line from the front of engine, it appears to be crimped or smashed at vacuum inlet to distributor. Was the photo of engine taken of a cold or warm engine as the choke looks to be partially closed. Changing of jets was mentioned, how about float levels. Vacuum to dist was also mentioned but not directly addressed. Just my two cents worth.

[KULTULZ]

Originally Posted by cousinjd

Just a couple of comments/observations of the picture of engine:

Following the fuel line from the front of engine, it appears to be crimped or smashed at vacuum inlet to distributor.

Was the photo of engine taken of a cold or warm engine as the choke looks to be partially closed.

Changing of jets was mentioned, how about float levels.

Vacuum to dist was also mentioned but not directly addressed. Just my two cents worth.

You know what I hate? Young guys with good eyesight ...

All Good Points!

The vacuum advance signal is ported. Can't figure out that DIST.

[scicala]

I was actually quite surprised when I saw the words China and Quality in the same sentence. Oxymoron of all time.


Sal

[KULTULZ]

MSD was bought by HOLLEY, and HOLLEY has pulled a few tricks in the past.

MSD is MADE IN CHINA, period. The once quality control is not there as the production is given to independent manufacturers and they have no concern other than making big bucks. Where do you think all of the counterfeit parts come from?

And we buy foods, food additives and pharmaceuticals from them?