Poor performance when warm, vapor lock? Fuel Pressure?
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
Likes: 15
From: Las Vegas Nevada
Poor performance when warm, vapor lock? Fuel Pressure?
One of my latest struggles in my attempt to get decent performance out of my engine. I realized my performance gets worse as the engine heats up within a few minutes as well, that problem gets exacerbated as the ambient temperature gets hotter through the day. When I run my car in the morning before it heats up, it feels most powerful by quite a large margin. Makes me think I'm getting fuel-flow issues related to temperature. Now, I have a Holley mechanical fuel pump which has an output of about 7psi at idle, but no fuel return. I assume those two are connected and should be dealt with before I continue fiddling with tuning.
As well with this, I'm struggling to choose a fuel pump. I have a mild/aggressive 350 with a Holley 670 Street Avenger that should eventually make between 350hp and 450hp at the crank.
I was looking at this one: https://www.summitracing.com/parts/c...make/chevrolet
And I notice three things. A) The price is $16 which seems too good to be true. B) Summit lists a range of 5.5-6.5 PSI but the Q&A reports a max of 8psi and I'm concerned that's too much and could flood my Holley? C) Is 40GPH sufficient? I'm reading conflicting reports on Google about this, but it seems 40GPH is good for 400hp?
As well with this, I'm struggling to choose a fuel pump. I have a mild/aggressive 350 with a Holley 670 Street Avenger that should eventually make between 350hp and 450hp at the crank.
I was looking at this one: https://www.summitracing.com/parts/c...make/chevrolet
And I notice three things. A) The price is $16 which seems too good to be true. B) Summit lists a range of 5.5-6.5 PSI but the Q&A reports a max of 8psi and I'm concerned that's too much and could flood my Holley? C) Is 40GPH sufficient? I'm reading conflicting reports on Google about this, but it seems 40GPH is good for 400hp?
Drifting
Joined: Jun 2012
Posts: 1,990
Likes: 183
From: Port St. Lucie Fl
I believe 7psi is higher than most carbs like, it could need a regulator but with lack of return you could also have a vapor locking issue you are fighting. Doyou have the old pump laying around you could test with?
Questions about that fuel pump indicate it may need a regulator also.
Questions about that fuel pump indicate it may need a regulator also.
Last edited by Kacyc3; Jun 14, 2019 at 10:35 AM.
Le Mans Master
Joined: Aug 2005
Posts: 6,544
Likes: 1,977
From: Temple Terrace Florida
Is your choke fully open when the engine is warmed up? Does your car have an exhaust pipe gate that may be stuck closed? Is your vacuum advance in the distributor working? Check that with a hand held vacuum pump. Check your timing to see if the dist. is advancing throughout the rpm range. Jerry
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
Likes: 15
From: Las Vegas Nevada
Do not have the old pump, might have to just order this one anyway to test that out. I guess I'll see if it needs a regulator as well!
Don't really have a way to test with the gauge since it's attached to my carb hard-line. What makes me suspect is that as it gets hot, the engine struggles more and more to reach higher RPM. It doesn't seem to stumble or bog, just feels like it's not getting gas. It only has that issue when the engine is hot, if it's freshly started she revs up and runs great.
I do have a return line, but the thing is not connected to the fuel pump as-is. I'm not terribly sure about jetting, but Holley tech told me I should be pretty spot on. I'm at 2500ft and for the time being I'm running the car in ambient temps between 90 and 107 degrees which as I understand has me sitting at 60-65 for primary and 65-70 for secondaries. Right now it's stock at 65/68 I believe.
Don't really have a way to test with the gauge since it's attached to my carb hard-line. What makes me suspect is that as it gets hot, the engine struggles more and more to reach higher RPM. It doesn't seem to stumble or bog, just feels like it's not getting gas. It only has that issue when the engine is hot, if it's freshly started she revs up and runs great.
I do have a return line, but the thing is not connected to the fuel pump as-is. I'm not terribly sure about jetting, but Holley tech told me I should be pretty spot on. I'm at 2500ft and for the time being I'm running the car in ambient temps between 90 and 107 degrees which as I understand has me sitting at 60-65 for primary and 65-70 for secondaries. Right now it's stock at 65/68 I believe.
Last edited by Shdggsdv; Jun 14, 2019 at 11:51 AM.
Race Director
Joined: Mar 2016
Posts: 14,423
Likes: 8,231
From: Napa Valley California
Vapor lock occurs when the fuel sitting in the fuel line and carb boils away when the engine is not running.
As long as the fuel is passing through the line and carb, it isn't an issue.
My 73 bb has the factory set up and did not come with a return line and I've never had an issue.
My guess would be that your coil is failing.
When a coil starts to fail they will function normally when cold and will degrade as they warm up causing poor performance , or as time goes on the engine will stop running altogether until the coil cools and you can start the engine after several hours.
Coils are cheap and are easy to replace.
As long as the fuel is passing through the line and carb, it isn't an issue.
My 73 bb has the factory set up and did not come with a return line and I've never had an issue.
My guess would be that your coil is failing.
When a coil starts to fail they will function normally when cold and will degrade as they warm up causing poor performance , or as time goes on the engine will stop running altogether until the coil cools and you can start the engine after several hours.
Coils are cheap and are easy to replace.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Loss of power with temperature sounds more like your taking air for the carb from inside the engine compartment. Or possibly a vacuum leak that increases with temperature because of expanding metal.
But you should understand all cars/engines loose power as air get hot and it's density goes down.
But you should understand all cars/engines loose power as air get hot and it's density goes down.
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
Likes: 15
From: Las Vegas Nevada
Choke is definitely fully open pretty quick, both the choke plate and the fast idle come down as they should. I have headers and true duals so no pipe gate. Vacuum advance, I haven't checked and I'll try that out and report back. Dizzy is advancing no problem and I'm connected to manifold vac.
I have a year old MSD Streetfire HEI with the light springs so I don't think I'd have any issues with ignition on that front. Though, is it possible my dizzy's power wire isn't a thick enough gauge? And with that I wonder how I'd even test that. Hm
I totally understand that engines should loose some power, but it's to the extent that I don't even feel WOT unless the engine is cold, unfortunately. I'll definitely check out the possibility of a vacuum leak, but how do I check for it pulling inside the engine? That almost sounds like you're talking about the intake gasket I assume?
I have a year old MSD Streetfire HEI with the light springs so I don't think I'd have any issues with ignition on that front. Though, is it possible my dizzy's power wire isn't a thick enough gauge? And with that I wonder how I'd even test that. Hm
I totally understand that engines should loose some power, but it's to the extent that I don't even feel WOT unless the engine is cold, unfortunately. I'll definitely check out the possibility of a vacuum leak, but how do I check for it pulling inside the engine? That almost sounds like you're talking about the intake gasket I assume?
Last edited by Shdggsdv; Jun 14, 2019 at 12:44 PM.
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Melting Slicks
Joined: Feb 2017
Posts: 3,232
Likes: 1,232
From: PHX AZ
Do you have a thick, insulated Carb spacer???? That's what its there for to prevent heat soak and float bowl evaporation. Esp aluminum manifolds.
Throw some insulation on the fuel lines inside the eng compt.
unkahal
Throw some insulation on the fuel lines inside the eng compt.
unkahal
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
Likes: 15
From: Las Vegas Nevada
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
I totally understand that engines should loose some power, but it's to the extent that I don't even feel WOT unless the engine is cold, unfortunately. I'll definitely check out the possibility of a vacuum leak, but how do I check for it pulling inside the engine? That almost sounds like you're talking about the intake gasket I assume?
Too bad you don't have an O2 sensor with an air/fuel meter. So a rich motor will stumble when it's drunk with fuel and a lean motor will bog. I think yours reacts more like a bog. Have you checked your carbs fuel bowel levels? Could even be the inlet needle and seats are to small (can't recall if holleys use them so I'm guessing).
O2 monitors are almost pocket change now days and if you don't have cat converters then you can install O2 sensor temporarily in the end of the pipe to measure. Really a modern tool for tuning and troubleshooting.
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
Likes: 15
From: Las Vegas Nevada
Usually a intake vacuum leak will increase RPM when a flammable like either or propane is sprayed on the area. I've even had WD40 show RPM increase but no guarantees with that.
Too bad you don't have an O2 sensor with an air/fuel meter. So a rich motor will stumble when it's drunk with fuel and a lean motor will bog. I think yours reacts more like a bog. Have you checked your carbs fuel bowel levels? Could even be the inlet needle and seats are to small (can't recall if holleys use them so I'm guessing).
O2 monitors are almost pocket change now days and if you don't have cat converters then you can install O2 sensor temporarily in the end of the pipe to measure. Really a modern tool for tuning and troubleshooting.
Too bad you don't have an O2 sensor with an air/fuel meter. So a rich motor will stumble when it's drunk with fuel and a lean motor will bog. I think yours reacts more like a bog. Have you checked your carbs fuel bowel levels? Could even be the inlet needle and seats are to small (can't recall if holleys use them so I'm guessing).
O2 monitors are almost pocket change now days and if you don't have cat converters then you can install O2 sensor temporarily in the end of the pipe to measure. Really a modern tool for tuning and troubleshooting.
I'll test the intake with a flammable, but how would I check for a leak facing into the engine?
Last edited by Shdggsdv; Jun 17, 2019 at 04:54 PM.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
I don't know if you have a cat converter and a cross over pipe or not. Most owners put the sensor and bung in the collector but getting the sensor close to the collector still works. Don't know why you can't take that first piece of pipe off and have the bung welded in - any muffler shop can do that.
I don't know what you mean by "facing into the engine "?
I don't know what you mean by "facing into the engine "?
5th Gear
Joined: Apr 2019
Posts: 5
Likes: 0
i'm having this exact same issue on my 75 350...been pulling my hair out trying to figure it out...i know i have low compression on no.6 cylinder..currently working on that to see if its valves or rings...i do know i'm having vapor lock issues even after carb spacer and fuel line insulation and running non-ethanol gas...think my vapor lock is coming from mechanical pump...gonna try electric pump out of engine bay to see if that cures vapor lock..i'm following this thread cause i'm at the point of yanking the motor and i really don't have the funds right now to do that!!!
Drifting
Joined: Jun 2012
Posts: 1,990
Likes: 183
From: Port St. Lucie Fl
i'm having this exact same issue on my 75 350...been pulling my hair out trying to figure it out...i know i have low compression on no.6 cylinder..currently working on that to see if its valves or rings...i do know i'm having vapor lock issues even after carb spacer and fuel line insulation and running non-ethanol gas...think my vapor lock is coming from mechanical pump...gonna try electric pump out of engine bay to see if that cures vapor lock..i'm following this thread cause i'm at the point of yanking the motor and i really don't have the funds right now to do that!!!
5th Gear
Joined: Apr 2019
Posts: 5
Likes: 0
didn't have any left over..may order some more..yes i have a return line factory installed..don't know if it matters but the engine is the original l48 350 with 72,000 miles on it. have brand new proform high output coil dist. brand new holley 600 vac. sec.brand new stock gm mech. fiel pump.timing set perfect..initial and total.have checked for vaccum leaks everywhere...like original poster said ..it runs strong when first started and a little while after..once i take it out and get on it hard a few times and it gets hot..it starts to lose power and engine shakes really bad...let it cool down for an hour or so and runs great again..i can see bubbles in clear fuel filter coming from pump when i get it back in my garage hot
Last edited by Keith Stokes; Jun 18, 2019 at 10:02 AM.
Drifting
Joined: Jun 2012
Posts: 1,990
Likes: 183
From: Port St. Lucie Fl
didn't have any left over..may order some more..yes i have a return line factory installed..don't know if it matters but the engine is the original l48 350 with 72,000 miles on it. have brand new proform high output coil dist. brand new holley 600 vac. sec.brand new stock gm mech. fiel pump.timing set perfect..initial and total.have checked for vacuum leaks everywhere...like original poster said ..it runs strong when first started and a little while after..once i take it out and get on it hard a few times and it gets hot..it starts to lose power and engine shakes really bad...let it cool down for an hour or so and runs great again..i can see bubbles in clear fuel filter coming from pump when i get it back in my garage hot