Engine hesitations @ cruising speeds
Thread Starter
Instructor
Joined: Jan 2007
Posts: 157
Likes: 0
From: Austin TX
Engine hesitations @ cruising speeds
So this has been a problem that I have neglected to really look into for over a year. I have a 350 L48 w a holly carb and some other small mods.
At idle my vette sounds smooth and when accelerating there are no hesitations at all, feels and sounds very solid.
Whenever I am cruising at or around 40 mph with an rpm around 2000. The needles starts to bounce up and down around a 100 rpms. You can feel these small hesitations until you start "giving" it more fuel.
I just had side pipes put on so it is more pronounced with the sound right next to my ear.
Am I looking at a carb issues or a transmission issue? Or something else?
At idle my vette sounds smooth and when accelerating there are no hesitations at all, feels and sounds very solid.
Whenever I am cruising at or around 40 mph with an rpm around 2000. The needles starts to bounce up and down around a 100 rpms. You can feel these small hesitations until you start "giving" it more fuel.
I just had side pipes put on so it is more pronounced with the sound right next to my ear.
Am I looking at a carb issues or a transmission issue? Or something else?
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
I agree, just had this issue. Rough off idle performance, low rpm cruise issues. I was running two soft springs on the advance weights and swopped out one soft to a medium and problem solved. Combination of too much centrifugal and too much vacuumn advance.
Thread Starter
Instructor
Joined: Jan 2007
Posts: 157
Likes: 0
From: Austin TX
Thanks guys. I'm not very automotive savvy so I will have to do some research and see if I can fix this. If you guys have any pics or videos of this please send, if not I will see what I can find in my corvette repair manuel and Youtube.
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
I originaly was dealing with what I thought was a miss fire. I had a smooth 600rpm idle and very good acceleration performance, but if I raised the rpm to 1,000 - 1,200 range it had a terrible roughness and miss firing. depending whether you take it to a garage or fix it yourself, your timing is certainly something you should look at. Look for the 'Lars paper on timing' this is the best information you can get for working on timing for the SBC.
Thread Starter
Instructor
Joined: Jan 2007
Posts: 157
Likes: 0
From: Austin TX
I originaly was dealing with what I thought was a miss fire. I had a smooth 600rpm idle and very good acceleration performance, but if I raised the rpm to 1,000 - 1,200 range it had a terrible roughness and miss firing. depending whether you take it to a garage or fix it yourself, your timing is certainly something you should look at. Look for the 'Lars paper on timing' this is the best information you can get for working on timing for the SBC.
Thread Starter
Instructor
Joined: Jan 2007
Posts: 157
Likes: 0
From: Austin TX
Burning Brakes
Joined: May 2014
Posts: 1,094
Likes: 16
From: Los Angeles California
What kind of fuel are you using. I was having hesitation problem around the 20 to 30 mph. I usually use 91 but gas was going up switched to 87 and car started stalling went back to 91 and it stopped. Just a suggestion.
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Melting Slicks
Joined: May 2012
Posts: 2,192
Likes: 343
From: Allenton Wisconsin
hesitation
Holley carb you say? Accelerator pump maybe faulty or you may have the wrong cam that actuates the pump. Holley sells a kit with numerous cams for experimentitation purposes. After the fourth time I think I got it right. Easy to change. Also in agreement on that octane and timing other CF members mentioned. I also had that problem when I had to small of a jet size in the primaries. Gotta love these Holleys for their tunability to every engine. Running a 650 spreadbore 2101 Eldebrock on a 69 L48. T
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
I would say that here lies your problem, If you have an adjustable vacuum canister you can tune it to give less advance, or you can control the rate of centrifugal advance by changing the springs in the distributor. A good timing light will help you work out what needs to be done. follow Lars instructions and you cannot go wrong. Let us know how you get on.
Thread Starter
Instructor
Joined: Jan 2007
Posts: 157
Likes: 0
From: Austin TX
I have never used anything but premium gas and I always make sure it is a reputable gasoline and not some mom and pop high octane gas. The problem was happening whenever I had my rear exhaust too, the sounds are just more pronounced now that I have the side pipes.
I think it is the vaccuum advance, but then again, I am not engine expert so. I'm just tired of taking it in to get adjusted when I am getting charged $100 hr. I need someone to show me the ropes with this stuff so I can do it myself. I'm joining a local C3 club, so hopefully I can make some friends that won't mind showing me the ropes.
These ideas are helpful though and I appreciate them!
I think it is the vaccuum advance, but then again, I am not engine expert so. I'm just tired of taking it in to get adjusted when I am getting charged $100 hr. I need someone to show me the ropes with this stuff so I can do it myself. I'm joining a local C3 club, so hopefully I can make some friends that won't mind showing me the ropes.
These ideas are helpful though and I appreciate them!
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
If I lived in Austin I would give you a hand, but I'm a long way away!
I would suggest that you install stiffer springs in the distributor as an interim measure, it will cure the issue, your timing may not be at an ideal setting, but at least you will not be running too much advance which can do damage.
I would suggest that you install stiffer springs in the distributor as an interim measure, it will cure the issue, your timing may not be at an ideal setting, but at least you will not be running too much advance which can do damage.
Le Mans Master
Joined: Oct 2010
Posts: 6,932
Likes: 845
From: melbourne florida
2023 C3 of the Year Finalist - Modified
the B.T.U rating of alcohol is less than gasoline,if it affects your mileage ( and it does) it will alter your A/F ratio. some cars aren't impacted much and some drivers are just clueless , but a lot of cars get really badly out of wack. we used to have a phrase for stuff like this " the sh-- is Junk.
Remember this stuff come November.
Remember this stuff come November.
Race Director
Joined: Dec 2004
Posts: 10,332
Likes: 63
From: PARADISE
St. Jude Donor '09-'11-'12-'13-'14
the B.T.U rating of alcohol is less than gasoline,if it affects your mileage ( and it does) it will alter your A/F ratio. some cars aren't impacted much and some drivers are just clueless , but a lot of cars get really badly out of wack. we used to have a phrase for stuff like this " the sh-- is Junk.
Remember this stuff come November.
Remember this stuff come November.
You are so right! Here in N. Nv, we have winter and summer blends. I don't drive the Vette much in the winter, so I usually buy about 20 gallons of summer gas to get through winter. The difference is amazing, but you're right, the gas station gas is S--T, winter or summer.