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Username Post: Engine falls flat at 4k, but only in first gear, and only from a dig.        (Topic#346752)
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-22-17 08:58 AM - Post#2705199    

Pretty specific conditions, right?
I was messing around the other night, and went full throttle from a dead stop. This car has never been a terror off the line, and will really only bark the tires if I plant it from a dead stop. Engine really doesn't come alive until 3000-3500 rpm. So it did its normal lazy takeoff, but when I hit about 4k in first, it really fell flat on its face. I shifted to second, and it picked right back up, and pulled strong through 6200 where my shift light comes on. Third felt strong as well.
Tried another pull, but walked it up to about 3k in first before matting it, and it pulled well all the way up in first and second.
Combo is 10.5:1 350 Chevy, Comp XR294HR cam, performer rpm air gap intake, performer rpm heads, holley 750 street hp vacuum secondary. Yes, the carb is a tad big, but it's what holley recommended with my setup due to 6500 max rpm. It's stock apart from a 4.5 power valve, since I'm only making 9" of vacuum at idle. Th350 trans, 2400 stall, 3.70 gear.
I suspect a vacuum secondary issue, but I'm all ears. It's odd that it only does this in first gear. I can walk mid-12 second cars from a roll, but can't beat a low-13 second car from a dig because of how lazy it is down low. My impression is it's lazy down low due to the combo and there's not much I can do about that with current parts, but the falling flat thing has me stumped.
I'm really thinking about putting the Sniper EFI on it this winter.......

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


Edited by Mr. Sinister on 08-22-17 09:12 AM. Reason for edit: No reason given.

 




grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17101
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
08-23-17 04:11 AM - Post#2705285    
    In response to Mr. Sinister

when your trying to isolate and test any issue it helps to mentally step back and ask yourself ,
"what condition has changed or been induced that might explain this"
then test your theory...Id start by installing a fuel pressure gauge sensor for the remote dash mount fuel pressure gauge at the carb inlet port and having someone watch it carefully as you made a few hard acceleration runs,
have you adjusted both the fuel bowl fuel levels so the idling engine shows fuel just level with the sight plugs lower edge


now it may be something else but Id suspect that either the fuel pump has a hard time over coming the initial inertial loads that are reduced once the cars launched,Id suggest you install a fuel pressure gauge where you can see it hooked up to the carbs inlet port and verify you maintain a consistent 6-7 psi , if the pressure drops noticeably at any time, during a run, that fuel system needs upgrading.
http://garage.grumpysperformance.com/index.php?thr...

http://garage.grumpysperformance.com/index.php?thr...

or the oil pan contents sloshing to the rear uncover the pick-up and your oil pressure sensor is cutting electric to the fuel pump or ignition, until the oil pressure returns
Id sure throw an extra quart of oil in the engine and install a fuel pressure gauge to isolate those two potential sources before looking further

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-23-17 04:18 AM - Post#2705286    
    In response to grumpyvette

Well, it's definitely not an oil pressure sensor cutting fuel, it doesn't have one. Fuel pressure is stable (I've always had a gauge at the carb inlet). Were fuel pressure the culprit, I'd suspect it would always do this, not just in first from a dig.


It's going to be a tough one to crack (as has been the custom with this car).

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17101
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
08-23-17 04:23 AM - Post#2705287    
    In response to Mr. Sinister

have you adjusted both the fuel bowl fuel levels so the idling engine shows fuel just level with the sight plugs lower edge , its simply a process of isolate and test until you locate the problems source, just never assume, test
btw a carb mount fuel gauge alone without a remote sensor and dash gauge does little good simply because you can,t see whats going on while the cars accelerating under load
I'm sure we can locate the source of your problem , but it will take some time and testing to do so as usual

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 08-23-17 05:16 AM. Reason for edit: No reason given.

 
acardon 
Ultra Senior Member
Posts: 10862
acardon
Loc: DFW TEXAS
Reg: 03-25-05
08-23-17 07:01 AM - Post#2705297    
    In response to Mr. Sinister

  • Quote:
It's stock apart from a 4.5 power valve, since I'm only making 9" of vacuum at idle.



I think your problem may be related to vacuum and gearing. There is less vacuum with a load on the engine. Without a load, the vacuum is highest. When accelerating in first gear the load decreases when the vehicle and engine are moving at the same speed, so the vacuum peaks, which may be causing a fuel/air imbalance. In other gears, there may still be a load at 4,000 RPM.

Don
66 Corvair (driving)
57 2dr HT (driving)
56 2dr HT (waiting to be restored)


 
Shepherd 
Contributor
Posts: 933

Loc: Lake George, NY
Reg: 11-11-15
08-23-17 07:04 AM - Post#2705298    
    In response to grumpyvette

Agreed, if it at launch only, sounds like fuel starvation as the bowls momentary empty. Grumpy Jenkins had this issue back then in his early Pro Stock car, using a 1/2 fuel line, the weight of the fuel column in the line on hard launch stalled the fuel pump.



 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-23-17 09:18 AM - Post#2705306    
    In response to grumpyvette

Float levels are good.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-23-17 09:23 AM - Post#2705307    
    In response to acardon

  • acardon Said:
  • Quote:
It's stock apart from a 4.5 power valve, since I'm only making 9" of vacuum at idle.



I think your problem may be related to vacuum and gearing. There is less vacuum with a load on the engine. Without a load, the vacuum is highest. When accelerating in first gear the load decreases when the vehicle and engine are moving at the same speed, so the vacuum peaks, which may be causing a fuel/air imbalance. In other gears, there may still be a load at 4,000 RPM.




This is kind of what I was thinking. Walking it up and then flooring it is allowing time for the the carb to compensate for the vacuum drop, where as just matting it is causing an imbalance. I bought a holley VS spring kit, and I'm going to try tinkering there first. It certainly does not feel like a rich bog, and I can't see that being the case at 4000rpm at WOT anyway. It literally feels like I lose half my power all of the sudden, like it's not getting fuel.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


Edited by Mr. Sinister on 08-23-17 12:18 PM. Reason for edit: No reason given.

 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17101
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
08-23-17 11:16 AM - Post#2705320    
    In response to Mr. Sinister

http://garage.grumpysperformance.com/index.php?thr...
swapping to a BLUE Holley Accelerator Pump Cam might help
[


IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-23-17 12:20 PM - Post#2705324    
    In response to grumpyvette

  • grumpyvette Said:
http://garage.grumpysperformance.com/index.php?thr...
swapping to a BLUE Holley Accelerator Pump Cam might help
[





I imagine it could use a better shot off idle, which would help with the sluggishness down low. It won't have any effect on the 4k loss of power though, right?

I appreciate the input so far, guys.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-28-17 11:38 AM - Post#2705884    
    In response to Mr. Sinister

So after some digging around, it sounds like the 4k issue may be a case of the secondaries opening too early, and causing it to lean out further up the rpm band. I'm going to try some stiffer springs and see how that goes. Sound logical?
I'm also going to try different pump cams and replace the 31 squirter with a 35 to address the heavy throttle stumble off idle.
Hopefully this saturday. Does this all make sense?

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
08-31-17 05:37 AM - Post#2706266    
    In response to Mr. Sinister

OK, since I'm kinda just going at this with what makes the most sense to me, I started tuning. Changed the squirter from a 31 to a 35. Definitely didn't like that. Popping when I'd give it light throttle.
Put the brown spring (second heaviest) in the secondaries. Seemed to like that, as the dead spot was lessened on all 3 of the dig pulls I did. Still there, however. One pull was clean all the way through, though. So it doesn't fall flat every time.
Thought I had a set of pump cams, but did not. Using the stock orange cam, I changed the screw location on the arm from 2 to 1. Seemed to like that, as it will now spin the tires when I punch it from a slow roll or dead stop.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Johnny468 
"9th Year" Silver Supporting Member
Posts: 770
Johnny468
Loc: Richmond, Va.
Reg: 10-24-08
08-31-17 08:23 AM - Post#2706291    
    In response to Mr. Sinister

I've had the best results on my set-up with the black pump cam, which I believe is the most aggressive.

Johnny

1965 Impala SS
505/4-speed/3.90


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
09-08-17 12:07 PM - Post#2707051    
    In response to Johnny468

OK, purely hypothetical, I'm not looking for anything other than a yes or no.

Is it POSSIBLE, that at higher RPM in first gear, when load decreases and engine vacuum rises again, that vacuum could rise enough for the power valve to close?

(Keep in mind I went from a stock 6.5 to a 4.5 pv due to low idle vacuum. Since then, I went to a slightly looser convertor and added gear and a smaller rear tire. I also went from a fairly restrictive Pypes street pro mufflers run out the back, to a set of Dynomax race bullets dumped before the axle. Load is definitely less than it was before, and I don't recall this happening before the convertor and gear change. I understand how to measure for a power valve, but it occurred to me that maybe I'm right on the line of where the 4.5 is too soft, and closing when the load decreases just enough. Wouldn't that also explain why it pulls strong in 2nd and 3rd, where load would be higher? I'm betting I could also jet down a little as well, as plugs are reading a fair bit more rich than before. What do you think?)

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


Edited by Mr. Sinister on 09-08-17 12:15 PM. Reason for edit: No reason given.

 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
09-08-17 03:18 PM - Post#2707073    
    In response to Mr. Sinister

Well, nuts to that theory. I must have put the 6.5 back in at some point. Seems like I had it jetted low on the secondary side. Was 78, now 82. Also dropped the primary from 76 to 74 as the plugs are reading rich.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
09-09-17 06:03 AM - Post#2707142    
    In response to Mr. Sinister

If you're still following, the changes I made made a nice improvement. Put the 4.5 pv in, I must have never gotten around to installing it initially, that's all i can think could have happened. Primaries are cleaner and more crisp, and it pulls to 6k real fast now in first and second. The bog around 4K was still there on one of the dig pulls. Still some more tuning to do, but I'm definitely moving in the right direction.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Shepherd 
Contributor
Posts: 933

Loc: Lake George, NY
Reg: 11-11-15
09-09-17 11:11 AM - Post#2707176    
    In response to Mr. Sinister

Keep notes, like I do. Lol



 
Mr. Sinister 
Contributor
Posts: 555
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
11-10-17 01:36 PM - Post#2713934    
    In response to Mr. Sinister

Haven't messed with the car in a while, but I'm going to try jet extensions on the secondary side to see if it clears up that bog. If that doesn't do it, I'm totally stumped.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 




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