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Username Post: What would cause an INCREASE in RPM while sitting at a light, in gear?        (Topic#352525)
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
08-16-18 10:35 AM - Post#2743265    

Never experienced this one before. I recently upgraded my fuel system (to great results apart from this intermittent issue) to a 255lph EFI fuel pump and Aeromotive bypass regulator (the one designed to work with a carb and EFI pump). Fuel pressure is rock solid at 6.5 psi. Holley Street HP 750VS carb. Feed and return lines are both 3/8" with no restriction based on the rock solid fuel pressure.
Every once in a while when I'm sitting at a light, the RPMs will increase with no input from me. I've seen as much as 1700rpm or so before throwing it in neutral and blipping the throttle, which cures it. It's only done it a couple times, and totally at random. There doesn't seem to be any surging going down the road. The car runs better than it ever has, as I was clearly having fuel delivery issues before with the mechanical setup. Where I used to get a decent second gear scratch on a manual shift, it will now kick the back end sideways. Definitely putting more power down.
Since you need more fuel to increase RPM, my guess is I'm getting a random leak from the carb, like fuel is getting pushed past the boosters. The only thing I have changed is from a mechanical pump to an electric EFI pump, and all related plumbing. I did have to pull and clean both needle and seat assemblies since the initial prime on the EFI pump was over 20 psi. This was after backing the adjustment screw off what I thought would be enough. WRONG. I found some rubber hose fragments lodged in both assemblies, but they looked and operated fine otherwise. Could a little piece of something be causing this issue?
I'd love to hear your thoughts. It's not a major issue, I'd just like to correct it if possible.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


Edited by Mr. Sinister on 08-16-18 10:38 AM. Reason for edit: No reason given.

 


Shepherd 
Frequent Contributor
Posts: 1789

Loc: Lake George, NY
Reg: 11-11-15
08-16-18 12:32 PM - Post#2743277    
    In response to Mr. Sinister

Generally for the idle speed to increase, more air must be introduced in some fashion. Worn throttle shafts in the carb, some kind of binding in the linkage, any vacuum leak..In one instance, and this very rare, a failing torque converter stator caused erratic idle speeds.



Edited by Shepherd on 08-16-18 01:08 PM. Reason for edit: No reason given.

 
acardon 
Ultra Senior Member
Posts: 11264
acardon
Loc: DFW TEXAS
Reg: 03-25-05
08-16-18 01:22 PM - Post#2743285    
    In response to Mr. Sinister

Since blipping the throttle fixes it, I would suspect a carb linkage problem, from bad motor/transmission mounts.
As mentioned, excess fuel won't increase RPM without AIR, it only makes it run rough.

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


 
aghaga 
"4th Year" Silver Supporting Member
Posts: 2205
aghaga
Age: 64
Loc: Altavista, Va.
Reg: 07-05-08
08-16-18 03:54 PM - Post#2743295    
    In response to acardon

I would check the centrifugal weights and springs in the distributor. If they are not free or the springs too loose the timing will fluctuate causing the rpm's to be unstable




 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
08-17-18 05:32 AM - Post#2743356    
    In response to aghaga

Thanks for the input!!

It's a steady increase of RPM from my in-gear idle speed of about 700rpm, up to as high as (I've seen) 1700 RPM. It doesn't jump around, just a smooth increase like I was giving it gas. It will typically start to increase in RPM after I've been sitting at the light for a bit.

The linkage thing is interesting, I did change to a new linkage rod with heim joints on either end. There's no binding that I can tell and I tested and adjusted it thoroughly when I installed it for smooth operation and full throttle. I think this is the best place to start looking.

There no slop or play in the throttle assembly, I had the carb off for a good cleaning when I had to pull the needle and seat assemblies.

I've had vacuum leaks before that will increase the RPM a little, but definitely not 1000 RPM in gear.

Engine mounts are good (solid steel) as is the trans mount. Minimal movement when I stab the throttle, and no banging under the car when I gas it going down the road.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


Edited by Mr. Sinister on 08-17-18 05:37 AM. Reason for edit: No reason given.

 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17356
grumpyvette
Age: 71
Loc: FLORIDA USA
Reg: 03-16-01
08-17-18 05:52 AM - Post#2743359    
    In response to Mr. Sinister

have you considered a rather simple test... adding an additional throttle return spring, as a temp test to see if adding additional resistance, it provides from the carb opening from the at idle set location prevents this from occurring?
carb and in some cases transmission shift linkage that goes to a carb must be free from binding.
if it does you've almost limited the cause to a binding or badly adjusted linkage ,
yes it might be a P.I.T.A. as the additional throttle resistance might be a problem but as a temp test it can provide you with info and facts



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 08-17-18 06:12 AM. Reason for edit: No reason given.

 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
08-17-18 05:55 AM - Post#2743360    
    In response to grumpyvette

I haven't, but that's too simple of a test to not try. I have a fairly heavy spring on it now, and I'm pretty sure it was part of a 2 pack I bought however long ago. I'll see if I can find the other one and stick it on there too.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


 
IgnitionMan 
Valued Contributor
Posts: 3625

Reg: 04-15-05
08-17-18 10:35 AM - Post#2743382    
    In response to Mr. Sinister

Fix the air leak under the carb.



 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
08-17-18 11:49 AM - Post#2743397    
    In response to IgnitionMan

  • IgnitionMan Said:
Fix the air leak under the carb.




Wouldn't that be a constant source of high RPM and other vacuum leak related issues? Apart from the very occasional idle speed increase only in gear, it's running damn near perfect, and when it does its new trick, it does it very smoothly. Idle speed otherwise is exactly where I had it set before removing the carb for maintenance. It doesn't jump up, it steadily builds RPM until I blip the throttle, which settles it back down to normal. It's only done this 3 times in a couple hundred miles of driving.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


 
IgnitionMan 
Valued Contributor
Posts: 3625

Reg: 04-15-05
08-17-18 04:04 PM - Post#2743417    
    In response to Mr. Sinister

Free idle vs pulling against a torque converter load.



 
Rick_L 
Honored Member
Posts: 27634
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
08-17-18 07:27 PM - Post#2743440    
    In response to IgnitionMan

Huh?



 
IgnitionMan 
Valued Contributor
Posts: 3625

Reg: 04-15-05
08-18-18 08:23 AM - Post#2743498    
    In response to IgnitionMan

Yes, have seen it happen, even does so on EFI engines when the mixture is right on out of gear and engine is at ops temps, and an idling load is applied. Doesn't take much. A very small vacuum leak can have little to no effect at idle out of gear, but cause idle to rise to overcome loading of the engine.

Stop questioning me as if I have no brains, I am not stupid, and have worked in these fields for over 45 years, some of them at GM, and Holley.



 
acardon 
Ultra Senior Member
Posts: 11264
acardon
Loc: DFW TEXAS
Reg: 03-25-05
08-18-18 09:50 AM - Post#2743508    
    In response to Mr. Sinister

  • Quote:
I've seen as much as 1700rpm or so before throwing it in neutral and blipping the throttle, which cures it.



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


 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
08-20-18 06:56 AM - Post#2743717    
    In response to acardon

I'll pull the carb and spacer just to be sure, since that's an easy thing to check.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


 
IgnitionMan 
Valued Contributor
Posts: 3625

Reg: 04-15-05
08-20-18 10:01 AM - Post#2743753    
    In response to Mr. Sinister

IF you have a propane torch, might flow a low volume of UNLIT propane around the carb and adapter bases when it is idling high, and see if it drops down by itself.



 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
08-27-18 10:12 AM - Post#2744552    
    In response to IgnitionMan

Update: Pulled the carb and spacer, found no apparent leaks. Both gaskets looked good.
Added my second spring to the linkage, and it seems to have changed what's happening when it happens. Instead of increasing RPM on its own now, if I tap the gas a little at a light in gear, it will rise a few hundred rpm. Once it hung at about 1300rpm, and once it rose to about 1300rpm, then dropped back down. It idles really strong at about 700-750 in gear with the EFI pump setup, which I'm pretty pleased with, but it does cause the idle to hang at about 1100rpm when free revving sometimes. I'm thinking I need to adjust that throttle linkage rod a little, I think it may be a hair too short.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


Edited by Mr. Sinister on 08-28-18 07:33 AM. Reason for edit: No reason given.

 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17356
grumpyvette
Age: 71
Loc: FLORIDA USA
Reg: 03-16-01
08-27-18 10:34 AM - Post#2744555    
    In response to Mr. Sinister

sure sounds like binding linkage

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!!


 
jdk 
"10th Year" Gold Supporting Member
Posts: 443
jdk
Age: 69
Loc: columbus, ohio
Reg: 02-15-10
08-27-18 03:41 PM - Post#2744590    
    In response to grumpyvette

could be the air cleaner housing or something along
the accel rod. jmho jim



 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
10-01-18 11:07 AM - Post#2747617    
    In response to jdk

Well, I seem to have found the issue. Was out putzing around on Saturday and the car was running like an animal all day, with no issues at all. Made a hard 3 gear pull up a hill close to home. Let off the gas at the top of the hill and coasted about an 1/8 mile to the back entrance of my neighborhood. I could tell right away something was off by the way it was coming down the RPMs. Coughing, sputtering, then died at a stop sign. Had to hold it wide open to get it to fire. So, flooding badly. Switched off the fuel pump and it all cleared up. Switched it back on, same problems. Got it home and pulled the air cleaner. Keeping it running with the fuel pump on resulted in fuel pouring out of the secondary throttle rod holes in both sides of the carb, and a steady drip from the secondaries at idle. Primary rod holes are bone dry, and no leaking from the boosters. Turn the pump off, no issues.

Pulled the carb and found there's a ton of slop in the secondary rod bores. This carb is 5 or 6 years old, so not old enough to have this amount of slop in the secondary side. Primary was nice and tight.

What the hell could have caused this?
In any event, looks like I'm rebuilding the carb.......

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


 
Jims56chevy 
Frequent Contributor
Posts: 2431
Jims56chevy
Age: 65
Loc: chino valley az
Reg: 11-23-04
10-01-18 11:44 AM - Post#2747619    
    In response to Mr. Sinister

You either have to much pressure from the pump or you have a carb full of crap that is holding your seats open.

56 210 2dr


 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
10-01-18 12:19 PM - Post#2747621    
    In response to Jims56chevy

Pressure is regulated to 6 psi, and I know I blew a bunch of junk into the carb and floated both needles when I put this new EFI pump on it because the Aeromotive regulator designed to run this setup with a carb was preset at over 15psi (where my gauge maxes out). I cleaned it out once, looks like I'll be doing it again.
What concerns me is all the slop on the secondary throttle rod, and the gas pouring out of the bores. There's at least a millimeter of movement on the passenger side where the rod enters the carb base.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


 
japete92 
"5th Year" Silver Supporting Member
Posts: 1223
japete92
Loc: No. Virginia
Reg: 01-18-13
10-01-18 01:21 PM - Post#2747628    
    In response to Mr. Sinister

  • Mr. Sinister Said:
Pressure is regulated to 6 psi, and I know I blew a bunch of junk into the carb and floated both needles when I put this new EFI pump on it because the Aeromotive regulator designed to run this setup with a carb was preset at over 15psi (where my gauge maxes out). I cleaned it out once, looks like I'll be doing it again.
What concerns me is all the slop on the secondary throttle rod, and the gas pouring out of the bores. There's at least a millimeter of movement on the passenger side where the rod enters the carb base.




How old is your gas tank? Any tank that has been left for a long time is subject to corrosion. Any fuel left in it to evaporate can leave residue. Fuel lines too.

That's one possibility. It's one I dealt with on my '63. The new carb, on the new crate engine kept getting 'gunked up'. A very fine residue was getting past the fuel filter and clogging the carb. On my car it was coming from the original gas tank. Blew out and flushed the fuel lines, and replaced the tank and the filter. The carb problem ceased. Can't say that is your problem, but it may be.

Pete



 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
10-02-18 05:41 AM - Post#2747680    
    In response to japete92

Thanks Pete.

Tank was in the car when I bought it 13 years ago. It's not the original tank, and it's spotless on the outside. It looked pretty clean inside when we dropped it to add the return-style pickup.
Fuel lines are 100% new last Spring, but they're soft lines currently until I can get some custom hard lines made up. They are ethanol safe material, supposedly. I have noticed if I get fuel on the outside sheathing of the fuel lines, it leaves black gunk on my hands. I'm hoping that's not the case with the inside of the lines as well. I haven't noticed any residue buildup inside the carb.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


Edited by Mr. Sinister on 10-02-18 05:42 AM. Reason for edit: No reason given.

 
Mr. Sinister 
Contributor
Posts: 580
Mr. Sinister
Age: 41
Loc: Fair Hill, MD
Reg: 05-18-09
06-14-19 12:41 PM - Post#2768456    
    In response to Mr. Sinister

Figured it out finally!!
Wasn't a big enough annoyance to really go nuts chasing it down, but I finally decided to get to the bottom of it the other day.
I noticed it would not do it in reverse, only in forward gears. So I had my wife sit in the car and run it through the gears while holding the brake. When she slid it into drive, there was a noticeable jump in the engine (motor mounts are fine, I checked), enough to jerk the throttle linkage and pull the throttle blades open a hair. The linkage was adjusted too short!! Added a little length to the adjustable linkage and all is well again.

Bill -
1955 Chevy 210 2 Door Sedan
http://imgur.com/a/CfOc6
1978 Dodge Tradesman 200 Boogie Van
https://imgur.com/a/E2nmwhW


 


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