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Low speed (1500-1800 RPM) driveability issue

General engine tech -- Drag Racing to Circle Track

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GRTfast
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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Wed Sep 11, 2019 9:12 pm

David_viny wrote:
Wed Sep 11, 2019 9:09 pm
Light flywheel/clutch? That will definitely contribute to the pulsing feeling at low rpm.
I used to, but it’s got a 28 lb steel flywheel now. That smoothed things out a lot.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Wed Sep 11, 2019 9:12 pm

BILL-C wrote:
Wed Sep 11, 2019 8:01 pm
With a light car and 3.25 rear gear, 1500-1800 rpm, and such a large carb, you are still cruising on the idle circuit . You can make 30+ hp before pulling a drop of fuel through boosters if the secondary throttle plate stop screw is opening plates even slightly at idle. It is very common for 1 booster to start pulling fuel before the others, causing light throttle fuel distribution issues. Full throttle fuel distribution has absolutely nothing to do with light throttle fuel distribution. Very common to see the fuel distribution change with rpm and throttle position. Set up a GO-PRO or similar camera over carb and go for a test drive. I'll bet you will be surprised at what you see. Oversized carb and intake ports contribute to problem also.
Interesting, I may do that.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by travis » Thu Sep 12, 2019 1:05 am

That cam is listed as 2200-5800 rpm range, just FYI. You’re trying to operate outside of its listed rpm range. 235/249@.050 on a 107 lsa...lots of overlap and low speed reversion there for sure.

Maybe cruise in the next lower gear? :wink:

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Thu Sep 12, 2019 8:08 am

Update. Last night I made a temporary divider that works with my open spacer. It definitely made a difference in the right direction, but with the open spacer, there is a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations. It looks like the 4 hole spacer (for throttle response) with a divider filler (like the one Brian M showed) will be the sweet combo. I'll report back once I have all that in place and have the carb tuned. Thanks for the help.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by BobbyB » Thu Sep 12, 2019 8:12 am

Can you post a picture of the divider you made?

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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 » Thu Sep 12, 2019 8:39 am

Make or get a combo spacer. (2 holes on the deep side
A oval on the shallow side) that supports your divider.
(Divider attached to combo spacer)

Good effort and progress.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by F-BIRD'88 » Thu Sep 12, 2019 8:46 am

Likely a better camshaft for your purpose and driving would be the factory hyd roller cam from the GMPP 502-502 hp crate motor. 226-230 110 LSA 8 deg overlap. @.050
Verses current 28° overlap.

Or another cam like that.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Thu Sep 12, 2019 8:49 am

BobbyB wrote:
Thu Sep 12, 2019 8:12 am
Can you post a picture of the divider you made?
I will when I take it out, I have a new 4 hole spacer on the way from summit. I made this open spacer out of the 4 hole I had (I have a mill). I milled the spacer open, then milled a couple slots in it that the divider fits into. The divider divides the spacer opening and has a tab that sticks down into the notch in the manifold divider. It's all pretty basic, it was intended as a proof of concept, not as something I would use permanently.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Thu Sep 12, 2019 8:51 am

F-BIRD'88 wrote:
Thu Sep 12, 2019 8:46 am
Likely a better camshaft for your purpose and driving would be the factory hyd roller cam from the GMPP 502-502 hp crate motor. 226-230 110 LSA 8 deg overlap. @.050
Verses current 28° overlap.

Or another cam like that.
Yeah, sometime soon I need to have Cam King make me a custom bump stick.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by murre » Thu Sep 12, 2019 3:47 pm

if I did not misunderstand your symptoms. then I had the same problem. the solution was. I taped half the air filter (180 degrees). I try to turn it forward. then backwards. then for fun I turned to one side. to my great surprise, the problem disappeared. it works just as well on the left as the right side. a very easy test to do.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by Tuner » Thu Sep 12, 2019 4:20 pm

GRTfast wrote:
Thu Sep 12, 2019 8:08 am
Update. Last night I made a temporary divider that works with my open spacer. It definitely made a difference in the right direction, but with the open spacer, there is a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations. It looks like the 4 hole spacer (for throttle response) with a divider filler (like the one Brian M showed) will be the sweet combo. I'll report back once I have all that in place and have the carb tuned. Thanks for the help.
I had this problem, "a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations." a while back and the cure was to completely close the secondary throttles and readjust the primary idle accordingly.

You haven't answered the question, "How much does the intake vacuum increase from the curb idle to the level road cruise at 1500-1800?"

You should try closing the secondary idle mixture screws completely and only using the primary idle mixture screws to set the idle mixture.

I can't imagine any reason to change the camshaft when tuning the carburetor is what to do. Funny how if somebody thinks they are a hammer everything they see is a nail.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by bobmc » Fri Sep 13, 2019 8:56 am

have you used the carb spacer heatshield like the factory used? I would think the shorter spacer the better. MSD type boxes have been taming race cam idle since the 1970's, I don't think you could foul plugs with the idle settings

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Re: Low speed (1500-1800 RPM) driveability issue

Post by ClassAct » Fri Sep 13, 2019 10:11 am

Tuner wrote:
Thu Sep 12, 2019 4:20 pm
GRTfast wrote:
Thu Sep 12, 2019 8:08 am
Update. Last night I made a temporary divider that works with my open spacer. It definitely made a difference in the right direction, but with the open spacer, there is a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations. It looks like the 4 hole spacer (for throttle response) with a divider filler (like the one Brian M showed) will be the sweet combo. I'll report back once I have all that in place and have the carb tuned. Thanks for the help.
I had this problem, "a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations." a while back and the cure was to completely close the secondary throttles and readjust the primary idle accordingly.

You haven't answered the question, "How much does the intake vacuum increase from the curb idle to the level road cruise at 1500-1800?"

You should try closing the secondary idle mixture screws completely and only using the primary idle mixture screws to set the idle mixture.

I can't imagine any reason to change the camshaft when tuning the carburetor is what to do. Funny how if somebody thinks they are a hammer everything they see is a nail.


I'm glad you pointed that out. It's a tuning issue. The OP is making progress and needs to keep moving forward.

Addressing the question you asked is the first step. Once that information is provided, it's will most likely be a simple fix.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 2:24 pm

ClassAct wrote:
Fri Sep 13, 2019 10:11 am
Tuner wrote:
Thu Sep 12, 2019 4:20 pm
GRTfast wrote:
Thu Sep 12, 2019 8:08 am
Update. Last night I made a temporary divider that works with my open spacer. It definitely made a difference in the right direction, but with the open spacer, there is a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations. It looks like the 4 hole spacer (for throttle response) with a divider filler (like the one Brian M showed) will be the sweet combo. I'll report back once I have all that in place and have the carb tuned. Thanks for the help.
I had this problem, "a tip in stumble that I could not cure with a bunch of different accel pump cam/squirter combinations." a while back and the cure was to completely close the secondary throttles and readjust the primary idle accordingly.

You haven't answered the question, "How much does the intake vacuum increase from the curb idle to the level road cruise at 1500-1800?"

You should try closing the secondary idle mixture screws completely and only using the primary idle mixture screws to set the idle mixture.

I can't imagine any reason to change the camshaft when tuning the carburetor is what to do. Funny how if somebody thinks they are a hammer everything they see is a nail.


I'm glad you pointed that out. It's a tuning issue. The OP is making progress and needs to keep moving forward.

Addressing the question you asked is the first step. Once that information is provided, it's will most likely be a simple fix.
I will get the vacuum measurement during a test drive this weekend and report back.

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Re: Low speed (1500-1800 RPM) driveability issue

Post by GRTfast » Fri Sep 13, 2019 6:49 pm

BobbyB wrote:
Thu Sep 12, 2019 8:12 am
Can you post a picture of the divider you made?
You do not have the required permissions to view the files attached to this post.

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