Mystery loss of power

[Richard]

I'm hoping someone out there will have some information on this!

I have a 56 100-4 with converted TR3 carbs (1 3/4"), recently rebuilt engine, and carbs. The car runs great from a standing start, but when I am on the highway, at a steady speed in 4th or O/D at 3000 RPM or above, it feels like 2 of the cylinders stop firing. Originally, my mechanic and I both thought it was fuel related, but the fuel pump puts out consistent pressure, even when it loses power, and the carbs seem to be operating correctly (we ran it into an exhaust analyzer), so the fuel system does not seem to be the problem. When it loses power, raw fuel can be smelled from the exhaust.

It also displays this symptom when accelerating hard on a grade.

Points, condenser, cap, rotor and coil are all new, plugs show no fouling. We're considering maybe going to an electronic system, but this shouldn't be necessary.

If anyone has any ideas on possible causes, I would really appreciate it- we've been working on this for a few months now, trying to figure it out!

 

[Dave Russell]

Try narrowing the plug gaps a bit. If it is engine load/ignition related this should help. Gaps should be .025" but you could go to .015 or .020 for a try. Might give you a clue.

A distributor point gap which is too narrow can cause the same problem.
D

[ 04-11-2004: Message edited by: Dave Russell ]</p>

 

[Stretch]

The miss on a heavy accelerator load implies an ignition problem to me. The higher cylinder pressure increases makes the spark more difficult. Other possible problems to add to Dave's list include a crack in the distributor cap or oil/dirt inside. Weak or defective ignition wire insulation can cause problems under load. Sometimes spark plug insulator problems can exist even when new, or from being stessed after gapping. Make sure the plug insulator is clean on the outside

 

[Richard]

Hey guys, thanks for the info, will try these and see what happens.

 

[aeronca65t]

Good suggestions above.

Also, check valve adjustment, igniton timing (too retarded might cause this), fuel tank vent (in the cap, I'd guess) and any extra "hidden" fuel filters that have been added over the years.

 

[Steve P.]

I had a similar problem with my BJ8 many years
ago. Spent several weeks trying to figure it
out. The symptoms were the same but I did not
have gas running out of the exhaust. The problem
revealed itself one day when I was crusin down
the freeway just a *tad* over the speed limit.
All of the sudden...POW...from the back of the
car. Suddenly I had lots of power. One of
the mufflers was partially blocked with the
guts that had rusted and accumulated near
the outlet! Check your exhauset system backpressure.

 

[Michael Oritt]

Richard--

I had very similar symptoms (power loss at high speed and up hills) a few years back that I finally traced to a partially clogged pickup in the tank. BTW, I seldom hear of folks cleaning the fuel filter built into the bottom of the SU pump--it does not have a great capacity for debris and is easily cleaned.

In any case, please fill us in when you straighten things away.