Power loss around 3000 RPM

[71coop]

I'm trying this one again now that  the dust seems to have settled with the forum upgrade.

I'm  fishing for ideas and feedback on the problem described below. About 2 weeks  ago, I started noticing a drop in power cruising on the highway - it manifested as I slowly accelerated to pass traffic in a 4-lane highway. RPM dropped to around  2500, it felt almost like I was running out of gas, then recovered after a few seconds.

It seems to be load related. I can manually drop into 2nd, push the car up to around 4000 RPM or better, and if I hold that a bit, the same thing occurs. 

I can run the car up to 3700-3900 RPM in park and it revs just fine. Take the car on the road accelerate to 3000-3200 in drive, and it wants to fall on its face. 

I have gone over the fuel delivery from end to end. I swapped carbs with a spare, replaced all the old rubber (most of it was  not really old, but it was cheap), replaced the stock pump with a stock replacement (only $20 and was not sure hold the pump was anyway), checked the hard lines, replaced the filter, and pulled the sending unit (it was clean, the PO had replaced both tank and sender a few years ago).

I tried a new coil, temporarily swapped in a replacement Pertronix module, and both checked/re-gapped my plugs to  .035, then just for shits and giggles, replaced them (yes,  checked the  gap first). I made sure my timing did not change (I run around 14 degrees initial, all in @ 35/36 @ 2800). Vacuum adds about another 10-12 degrees,an have always run it off manifold vacuum.

I pulled the valve covers and did find some evidence that some valves are rotating as if insufficient seat pressure. I changed springs to a little stiffer set, seat pressures is close to 125 installed.

I road-tested after each change and found that nothing actually had any effect on the symptoms whatsoever. 

Last night I realized I had not looked at the radiator fluid, and found it was a little low -I am not sure it's related, but now wondering if I have a small head gasket leak.  The car has not  overheated. There is no evidence of any coolant leaks, and oil level is normal. No evidence of oil/coolant mixing  anywhere. If it is a leak, it has to be coolant leaking into a cylinder and  small so it  doesn't effect performance at low RPM (below 2500).

When putting a vac gauge on the motor, Idle vacuum holds steady at 16 Hg, which I think is to be expected with my cam profile, and I see no unusual behavior suggesting valve float when revving up to 3700 RPM, or evidence of worn rings or other head/valve wear.

What I  have not  done: When cranking w/out the coil hooked up,the car sounds like cylinder compression is even, but I have not checked actual numbers. I am not sure that a small head gasket leak would show up in that kind of crude check anyway. I have not changed plug wires.

Any additional forum thoughts on this? It's driving me nuts,and my wife really wants me to focus on finishing our deck vs messing with this an old car. I'm too OCD to  let it go, so before I either pull the heads myself, or take this to a mechanic and let him deal with it, I'm looking for  any ideas to help narrow my options down

If this looks familiar, I started the same thread just  before the forum upgrade,and my thread disappeared - right as I started troubleshooting, so that was frustrating in and of  itself.

 

[Tnfastbk]

Is your car an automatic car? If so could it be torque converter/transmission related possibly ?

 

[71coop]

Is your car an automatic car? If so could it be torque converter/transmission related possibly ?

Automatic. I thought of that, and I did put in a rebuilt trans a bout 2 months ago. However, after talking to the rebuilder, he is  sure that a trans issue would result in slipping (RPMs shoot up) ,not a loss in power/RPM drop. He's also a stand-up guy with a stellar local reputation, si I am confident he isn't trying to dodge a bullet.

I won't yet say it can't be the FMX, but right now, I am doubting it. I can't find any examples of a transmission dragging RPM down at  cruise, only slipping. Not sure how I can be 100% sure though.

 

[mezell29]

Sounds like you tried a lot.

I bought my latest very about 2 months ago and it hadn't been run in years. Every time I would take it out for a drive it would run fine for awhile then get sluggish and often die. Three things you haven't mentioned come to mind:

1. Bad Coil
My coil was an Accel and at least 10 years old. It was mounted to the engine and oriented on its side. It would get really hot. The connectors must be mounted horizontally to get proper cooling from the oil. I replaced it and relocated it away from the engine where it could get some cooling. Cheap insurance. 

2. Fuel starvation (Pump, fuel filter, etc)
Keep in mind sometimes mechanical pumps fail but don't show symptoms. I small hole in the diaphragm that pumps air in. A fuel pressure test might reveal a pump that isn't pumping adequately after the engine heats up too. The tank could have junk clogging up the screen as it moves around, there could be an old weak fuel line collapsing or some other restriction/blockage. 

3. Vapor Lock
I had replaced everything from the tank to the carb. In the end my biggest enemy was vapor lock. After a lot of testing I solved that problem. I had limited under-hood space due to the RPM Air-Gap and the plain hood, but bought a little thicker gasket and put on the Holley Heat Shield. I put an electric fuel pump at the tank to push the fuel, removed the mechanical pump to stop heat soak. I re-routed the lines to the rear of the carb rather than the front connecting to the first metal line in the engine compartment. The biggie was there was a large inline filter. With the electric pump on and the fuel draining into a can, I could watch vapor lock happen in the fuel filter with no pressure. Fuel travelling from higher to lower pressure would cause air bubbles to form. I replaced the larger fuel filter with a really small fuel filter.

Mine seemed to be a combination of issues and after spending a couple weeks pinpointing and ordering parts, all seems to be good even in 90+ degree Florida heat with my new AC running. I plan on adding a NACA hood which will give more under hood airflow as well.

Could even just be bad gas. Bad ethanol gas in the warming heat of summer. You are in Ohio so if you had some winter gas with the lower boiling point still, even that could be an issue. Our cars weren't meant for this crappy ethanol blend. 

Hopefully something in there helps. 

 

[71coop]

Mezell29 - I did swap coils, and always mount them with terminals horizontal. Coils match the ignition requirements for Pertronix.

Also swapped fuel pumps and checked both tank and sender. all good there, did not change anything.

I don't think it's bad gas, I drive this enough that it's always fresh. I fill up at least every other week, usually at the same place. I won't rule out the need for a heat shield, though, I did experience that on my vette, so will check that option.

 

[mezell29]

I ordered this one:

https://www.amazon.com/Holley-108-70-Carburetor-Heat-Shield/dp/B00062YTRC/ref=sr_1_2?dchild=1&keywords=holley+heat+shield&qid=1592494471&sr=8-2

Put as thick a gasket as I could on both sides. Percolating gas in the line or bowl will do it. I have AC and the brackets block a lot of airflow on the top. 

Let us know!

 

[71coop]

I actually have that heat shield, used it on the corvette.

I may give it a shot, but to be honest I'm just not hopeful. The power loss is almost entirely predictable by RPM, and has only been an issue the last couple weeks. Plus I noticed this in the morning (maybe 68-70 degrees on a warmer day).

I could be wrong, maybe my local gas station just pulled the rug out from under me in May and I am boiling my fuel. I may try a return line too (I had to go that route with the other car along with the heat shield).

 

[c9zx]

If you have access to a leak down tester do a long duration leak down test while watching for bubbles in the radiator coolant. I found a very small head gasket failure this way. It took several minutes for it to show bubbles. I assume you have checked the advance curve and float levels in the carb. Let us know what you find, it may help others in the future. Chuck

 

[Stanglover]

Okay, I just read through all this again. I was following it when the switch over happened, but I lost track of the conversation.

Can you refresh us on exactly what "mods" your engine has. I see you have Trick-Flow heads, cam upgrade, but I don't remember what intake you have or the carb you're using. Does the intake have the factory stile exhaust heat cross over that was supposed to heat the old Autolite 4300 (D) carb. This definitely will be a problem on other carbs as I found out on my own car. I was having a load of trouble with gas percolating off causing hard starting when hot. My fix was to block off the heat cross over and add a 1" fiber spacer. No ram air, so lots of room. Mine is a 4 speed, but since I learned how to build my own factory stile distributor and curve it, I can easily pull it trough 5500 rpm without any loss of power or pinging or....  I also don't believe in running the vac advance on manifold vacuum. I find ported is best for mine. I only add 4-5 degrees of vac advance to the 34 deg. mechanical it runs at. 

One dumb-ass question; is your fuel filter on  correctly with the flow  toward the carb? Stupid mistake that I've made myself.

 

[71coop]

Holley 4160 vac secondary with electric choke (the one with  the single front inlet). Stock (for holley) jetting and Power Valve. Intake is the Edelbrock 2665,  it does have an exhaust crossover, and i did not block it. Ports match up reasonably well with the TF heads.

I did swap carbs with a newer 4160 and did a test run, no change.  I've run into percolation issues  on the vette, but never had that issue on the Mach - but haven't completely ruled the possibility out yet.

On the vacuum advance, I tried both ways and my combination seems to like manifold. I think every setup responds differently. I was thinking I might dial the amount of advance back a bit - might as well add that to the list now.

 

[Stanglover]

With the mods. you have, all I can say is it has to be something odd as it ran ok before, correct?  You've look at practically everything and I know first hand how frustrating it can get. A process of elimination which you seem to have done. 

I'm not at all familiar with any of the upgrades you've made. but  I did have a similar issue, maybe not quite as bad. Mine just had this persistent  'bog'  when hitting the gas plus it was running really rich at idle. I knew my timing was as good as it was ever going to get with a factory dizzy and Pertronix Ignitor II (and coil) I'm sure you've read about me taking the car to a tune up specialist who drilled a 3/32" hole in each primary plate, the set the transfer slots, idle mixture screws and curb idle. After stripping the Holley SA670 (4150) several times playing with power valves, jets, float level etc.etc. I finally installed a larger squirter, 3 -4 numbers higher if I remember and all my issues went away. That of course was all related to acceleration, not cruising, but it would seem that there is a lack of fuel  at some point.

Got to go now, out for Thursday night coffee with the boys and our cars. I'll catch up later.

 

[Kilgon]

Seems like you covered all the obvious things and then some.   What type of air intake system do you have.  Any chance something is stuck or loose in it.   Noted that you mentioned you can rev the engine up in park and it runs fine.  If there is something loose in the air intake it could be that the extra pressure of the air being forced in while driving is causing the air flow to be blocked thus causing a very rich mixture and loss of power.    

 

[71coop]

With the mods. you have, all I can say is it has to be something odd as it ran ok before, correct?  You've look at practically everything and I know first hand how frustrating it can get. A process of elimination which you seem to have done. 

I'm not at all familiar with any of the upgrades you've made. but  I did have a similar issue, maybe not quite as bad. Mine just had this persistent  'bog'  when hitting the gas plus it was running really rich at idle. I knew my timing was as good as it was ever going to get with a factory dizzy and Pertronix Ignitor II (and coil) I'm sure you've read about me taking the car to a tune up specialist who drilled a 3/32" hole in each primary plate, the set the transfer slots, idle mixture screws and curb idle. After stripping the Holley SA670 (4150) several times playing with power valves, jets, float level etc.etc. I finally installed a larger squirter, 3 -4 numbers higher if I remember and all my issues went away. That of course was all related to acceleration, not cruising, but it would seem that there is a lack of fuel  at some point.

Got to go now, out for Thursday night coffee with the boys and our cars. I'll catch up later.

Geoff, it did run good before - It has to be something that I'm overlooking or missing. I was leaning heavily towards lack of fuel before I went all over the fuel delivery system. I may revisit the carb one more time just to be sure. 

 

[71coop]

Seems like you covered all the obvious things and then some.   What type of air intake system do you have.  Any chance something is stuck or loose in it.   Noted that you mentioned you can rev the engine up in park and it runs fine.  If there is something loose in the air intake it could be that the extra pressure of the air being forced in while driving is causing the air flow to be blocked thus causing a very rich mixture and loss of power.    

I'm running the Ram Air setup these cars came with (although mine is a reproduction). I checked for binding or loose parts, nothing was evident. Will keep looking. 

 

[Stanglover]

Geoff, it did run good before - It has to be something that I'm overlooking or missing. I was leaning heavily towards lack of fuel before I went all over the fuel delivery system. I may revisit the carb one more time just to be sure. 

Ok back from Coffee. On the way there, I had a thought and later I talked to a buddy who was a mechanic. His thought was what I had thought,  possibly a partially blocked exhaust. He also suggested to run a vacuum gauge into the car so you can see what it is doing whilst driving. If not your exhaust then somehow the fuel is being restricted, check all the fuel filters including the sintered bronze one(s) in the carb inlets. Basically is ain't breathin right is what he said.

Hope that helps,

Geoff.

 

[71coop]

If I get time today I will dig deeper into this suggestion. It might even be a chance to run through town with  open headers......

 

[Stanglover]

Aww, that'll hurt some ears!!

What are you using for mufflers? I switched from Flowmaster 40's to Flowmaster FlowFX which are glass packed straight through. Far less drone (but not gone completely unfortunately) quieter at idle speeds, but boy do they sound good when I hit the loud peddle! Car takes off like a rocket and probably sounds about the same from the outside. I have factory manifolds, not headers btw.

 

[71coop]

They look like old Flowmasters, but I am really not sure - they were on the car when I got it in October of 2018. If I end up finding a restriction, I will have to think about the FlowFX.

 

[Stanglover]

They look like old Flowmasters, but I am really not sure - they were on the car when I got it in October of 2018. If I end up finding a restriction, I will have to think about the FlowFX.

Look at my walk around video. It's not the best sound, but it might give an idea of what the FX sound like at idle (fast in this case, not yet fully warmed up)