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Roadtrip thoughts: Loss of performance after modifications

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Jesse73
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Roadtrip thoughts: Loss of performance after modifications

Post #1 by Jesse73 » Sat May 18, 2019 7:59 pm

Hello again everyone,

Yesterday I posted about having tuning issues during a roadtrip in my '73 240 powered F100. I made a few modifications recently and have quite frankly been disappointed with the results. I had a couple of thoughts while driving home today, kind of spitballing here but maybe this will make sense to someone.

A few weeks ago I replaced the head gasket in the truck, and while I had everything apart I went ahead and put on EFI exhaust manifolds and planned on putting in stamped Chevy rockers as well. Ultimately did not get the rockers in the mail before work started, so we skipped putting them in and simply replaced the gasket and put on the new exhaust. Also put on a 2 1/2 inch walker 2-1 downpipe at this time. Truck ran a lot better and I was pretty pleased with the work.

Last week I installed the Chevy rockers and drove around town. Everything seemed about the same so I thought I was good to go. This weekend we took off for a roadtrip and quickly found out that we were running out of steam around 50 mph (2100 rpm by doing the math on the 30.2 inch tires and 3.73 rear end ratio)
Tried messing around with the fuel mixture by adjusting the metering rod (Carter YF) and adjusting the timing. Nothing really changed that much, adding fuel probably helped a bit. Still not much power running on the highway. Still has pretty good acceleration up to 45-50 miles an hour.

This leads me to my thoughts. My wife said "after you put on the new exhaust the truck was quick, What's new since then?"
She is right. After the exhaust swap the truck WAS quick, and it accelerated great from 50 mph to 60 easily. The new thing is the rockers.

Am I holding the valves open longer with the Chevy rockers and losing too much vacuum for everything to work properly? Vacuum advance, carburation etc?

That's my guess anyway, the only change I made was the rockers.

Not sure what everyone thinks!

-Jesse

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THE FRENCHTOWN FLYER
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Re: Roadtrip thoughts: Loss of performance after modifications

Post #2 by THE FRENCHTOWN FLYER » Sat May 18, 2019 8:26 pm

The additional degrees that the greater rocker arm lift holds the valve open is minuscule. Put the original rocker arms back on and see if the problem goes away. That will tell you if the rocker arm change was the problem. Do not change anything else. I suspect it is not the rocker change but something else is going on.
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

jason832
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Re: Roadtrip thoughts: Loss of performance after modifications

Post #3 by jason832 » Sat May 18, 2019 10:06 pm

I remember my fiascos when I had the stock carter carb, never ending....

Have you checked other simple things?

Blocked fuel filter? Maybe while cruising the bowl empties then it struggles?
Spark plugs gapped and still look good?
Good electrical grounds tight and clean?
Timing?
Try a spare ignition module (you should always have one with you, they will die and leave you stranded at some point)
Vacuum leaks?

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B RON CO
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Re: Roadtrip thoughts: Loss of performance after modifications

Post #4 by B RON CO » Sun May 19, 2019 8:59 am

Hi, one guess is it also sounds like a clogged exhaust.
Maybe something in the muffler broke.
I might unhook behind the down pipe and try it.
What the heck, it is a lot easier then changing the rockers.
Good luck
B RON CO. Still workin' on it!

1933 Ford Pickup - 59A Flathead V8
1966 Ford Bronco - U14 - 170/200 Straight 6
1966 Ford Mustang - 289 V8

pmuller9
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Re: Roadtrip thoughts: Loss of performance after modifications

Post #5 by pmuller9 » Sun May 19, 2019 9:20 am

If the engine is idling smoothly at 600 rpm then it is not the rockers or rocker adjustment.
The Chevy rockers will almost always results in more power if everything else is OK.

Make a line on the harmonic balancer 1 1/8" to the right of the timing notch.
That will be about 38 degrees BTDC.
Check the timing by reving the engine without the vacuum advance connected till the timing stops advancing.
The timing light should show a smooth advance up to that line.

If the timing is ok then the carb needs some work.

Jesse73
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Re: Roadtrip thoughts: Loss of performance after modifications

Post #6 by Jesse73 » Sun May 19, 2019 12:35 pm

Hmm. Well, I am fairly confident I have good fuel flow. Brand new tank, line, and filter was replaced this spring when I took the truck out of storage. I will get a fitting and hook up a gauge to the pump and see if the pressure is correct.

I am going to see if I can rent a timing light today that has the "set back" dial or the whatsit that allows you to set total timing. If not I will try marking the pulley as suggested. We did mark the flywheel with a new TDC mark by measuring the piston when the head was off.

Muffler, exhaust etc is all brand new. I can take off the muffler and give it a shake fairly easily.

Maybe I will take off a rocker and measure it somehow and make sure I put in the correct rockers? I agree with what has been said, the Chevy rockers seem tried and true.

Just seems like I had issues after adding the rockers and all fine beforehand.

Thanks for the thoughts so far!

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #7 by pmuller9 » Sun May 19, 2019 12:42 pm

You don't need to remove a rocker.
Just measure the valve lift.

The timing needs to be checked anyway since the distributor was moved around.

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #8 by bubba22349 » Sun May 19, 2019 4:35 pm

:hmmm: did you happen to do a compression test on your engine before the mods, or after? I have to agree with FTF, pmuller9, and others that the Rocker Arm and exhaust Mod's you did shouldn't have anything to do your lost performance. There has to be something else that's the real cause for this loss in performance. Good luck :nod: :thumbup:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #9 by Jesse73 » Tue May 21, 2019 7:57 pm

Compression test shows ~135 psi across the board.

Jesse73
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Re: Roadtrip thoughts: Loss of performance after modifications

Post #10 by Jesse73 » Thu May 23, 2019 3:31 pm

I have found out the exact problem, anyone care to take a guess? FTF was kind of right in a way it was not the rocker arms!

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #11 by pmuller9 » Fri May 24, 2019 3:17 pm

So what was the problem?

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #12 by bubba22349 » Fri May 24, 2019 3:35 pm

I going to guess ignistion like a weak coil. :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #13 by guhfluh » Sat May 25, 2019 10:34 am

Nobody will know without more info.

I'll guess carb linkage.
1967 F-250 Crew Cab 2wd, 300 6cyl, T-170/RTS/TOD 4-speed overdrive
240 head, Offy C, EFI exhaust manifolds, Comp 268H, mandrel 2.5-3" exhaust, Edelbrock 500, Pertronix ignitor and coil, recurved dizzy. 200whp/300wtq

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #14 by sandboxer » Sat May 25, 2019 11:02 am

Emergency brake was stuck. A bit of a drag...?

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #15 by Jesse73 » Sat May 25, 2019 11:11 am

sdiesel wrote:in such a scenario, ur best friend is a vacuum gauge.
time to peak vacuum. factory spec. is no longer a valid number.


check carb is opening butterflie(s) fully.
this is a very common problem and often overlooked wit the above customer complaint.
even check the pedal .
more than once I have found debris blocking pedal actuation. (construction equipment).


of course vacuum leaks, and loose parts, that have rattled loose on ur driving trip..

lastly , I ALWAYS, check my plug wire schematic.
there has never been a soul on this board who has chased gremlins for extended periods, only to find misrouted plug wires (that's a joke son).



Sdiesel nailed it. Pedal was on the floor and the throttle wasn't wide open. I started poking around with it and asked my friend Dave ( he helped me change the head gasket) if he did anything with the throttle linkages. Yes he said I started taking it off and gave up and just unpinned the whole thing. So the linkage was unthreaded quite a ways.

Still have a mid throttle lean spot, been slightly raising the metering rod to try and clear that up.

-Jesse

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #16 by guitarsnracecars » Fri Jun 14, 2019 12:36 am

Did the same thing circle track racing and didn't learn it was the throttle linkage til after I sold it... REALLY? REALLY? SEROIUSLY? I Really did something that dumb?
So, problem found and solved... what did you gain with the rocker arm swap?

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Re: Roadtrip thoughts: Loss of performance after modifications

Post #17 by Jesse73 » Sat Jun 29, 2019 8:27 pm

The truck is pretty snappy now. I would say the EFI manifolds, and Chevy rockets were worth it. Still working on fine tuning but I'm doing about 13-14 city driving and around 18-18.5 on a couple of highway trips. I will post a new topic with my results also.

Thanks!

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