BBK

Tire pressure and misfire on highspeed

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Hey, 

 

I read in the chassis manual that the Riv needs a tire pressure from 24 psi which is about 1,65 bar. Actually I had 33 psi (2,3 bar). On 24 psi the tire was so flat that I don't trust to drive and fill in 33 psi again. I have a 225 75 r15 tire. 

 

What tire pressure you drive? 

 

Also I drive some 1/4 mile times and recognized some heavy misfiring when I reach about 80 mph. Before 80 miles I don't have problems. What can happened? 

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I go with the tire manufacturer psi (on side of the tire) Not what's in the manual. I highly doubt you are running the factory tires that came on the car when new.

 

As far as misfire kinda tough without driving it. Possiably a low float level or weak fuel pump. Points, ignition etc.. Could be multi things.

Edited by demon452 (see edit history)

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It helps to tell us the year of your Riviera.

 

First-generation Rivieras used biased ply tires when new, at 24 psi to give a very smooth ride.  In fact, there is a Dealer Service Bulletin that specifically addresses new car owner complaints of "harsh ride": Step 1 is check tire pressure.  If over 24 psi cold, reduce pressure and instruct owner to not go above that.  So soft tires were designed into the overall ride comfort of the day.  Buicks were known for their comfortable ride.

 

I now have radial tires on my '63.  I have increased the tire pressure to 31 psi front (engine weight up there) and 29 psi rear.  The higher tire pressure improves fuel economy.  You do not want to go above the pressure on the side of the tire when cold, which is 32 psi on my tires.

 

I remember as a child when radial tires first came out and everyone put the original factory tire pressure in them, they "just looked low".  It's because of the tire design, compared to the old bias ply tires.  We got used to the new look.  Later the factory tire pressures were increased to improve fuel economy and we got used to that look.

 

Tires with lower pressure run hotter.

 

YMMV.

 

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21 hours ago, Jim Cannon said:

Tires with lower pressure run hotter.

My Grandfather taught me how to use a vulcanizing machine. Then he told me "We put them together with heat and pressure, they come apart the same way. I have always ran 35 PSIG in mine.

 

Misfire at 80 can be fuel starvation. The LEAST likely place mechanics look is the rubber hose at the fuel tank sending unit and the rubber hose at the fuel pump suction. Sometimes fuel won't leak out, but under demand air will leak in.

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If your pump is pulling fuel from the tank via a vacuum, would it stand to reason that at 80 mph the pump is pulling enough vacuum to collapse 50+ year old rubber?

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On 2/17/2019 at 5:50 AM, BBK said:

I read in the chassis manual that the Riv needs a tire pressure from 24 psi which is about 1,65 bar. Actually I had 33 psi (2,3 bar). On 24 psi the tire was so flat that I don't trust to drive and fill in 33 psi again. I have a 225 75 r15 tire. 

 

What tire pressure you drive?

24 psi is for the tires that were originally equipped on the car.  For modern radial tires, I would not exceed the maximum tire pressure recommendation on the sidewall as this is for the maximum load rating of the tire.  Each application is different so I would use a pressure that allows the tire to wear evenly across its face and this will require you to keep checking.  What pressure recommendation did the tire shop give you?

 

On 2/17/2019 at 5:50 AM, BBK said:

Also I drive some 1/4 mile times and recognized some heavy misfiring when I reach about 80 mph. Before 80 miles I don't have problems. What can happened? 

Fuel starvation will appear as gradual loss in power and, if the fuel pickup in the tank has any flow, there is no way for it any significant vacuum to build and will vapor lock first.  I would check your condition of your ignition system first.  If you're not stuck on originality, I would seriously consider an HEI upgrade if it still has a points ignition system.  See HEI Ignition Upgrade.

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35-42psi front, about 2-4lbs less in the rear. Experiment and see what works best. If you have really good shocks(Viking or QA1) you can run a lot of pressure and the car will still ride great.  

 

The "misfire" sounds like detonation. Does it sound like marbles in the motor?   Verify timing, put a pertronix in distributor(I like the 3 version) and put a new fuel pump and soft lines in the car if more than 3 years old. 

Edited by DualQuadDave (see edit history)

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The misfire on full throttle sounds like a machine gun. When I move the gas pedal smoothly it seems to be OK. 

 

What is to prefer the TA Performance HEI for 100, pertronix about 200 or msd 6 AL for 250? Will I get some more power? 

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You might have spark jumping inside the cap. Take a close look for cracks in the cap, and make sure the contacts are clean. Could also have voltage leak on the plug wires. One or more might have voltage jumping from a wire to something metal near by. 

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3 hours ago, BBK said:

The misfire on full throttle sounds like a machine gun. When I move the gas pedal smoothly it seems to be OK. 

 

What is to prefer the TA Performance HEI for 100, pertronix about 200 or msd 6 AL for 250? Will I get some more power? 

Have you done a basic tuneup INCLUDING wires?

Tom Mooney

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21 hours ago, BBK said:

The misfire on full throttle sounds like a machine gun. When I move the gas pedal smoothly it seems to be OK. 

 

What is to prefer the TA Performance HEI for 100, pertronix about 200 or msd 6 AL for 250? Will I get some more power? 

The strength of the spark depends upon many factors including thee the ignition system's ability to saturate (ie, build a full magnetic field) the coil.  High engine speeds and incorrect dwell (related to points gap) adversely affects coil saturation.  High combustion chamber pressures (ie at full throttle) increase the voltage required for the spark to jump the sparkplug gap in the engine.  Points & condenser ignition system need good components and proper tune to work properly.  A high performance aftermarket ignition system will not cause the engine to make more power than a old points ignition system in good working order under normal driving.

 

Variable dwell electronic ignition systems (like the GM HEI system) were designed to provide a hot spark with a minimum of maintenance.  I would say that the cheapest and most effective ignition upgrade would be to switch to the GM HEI system and most of the parts required can be sourced from the junkyard.  I upgraded to an HEI system last year and it was quite straightforward.  See HEI Ignition Upgrade.

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Regarding tires...when I was in high school I worked at a full service Exxon station so we checked the oil and tires of every car that came in. Most of the same cars came in every week or so. If customers told us they thought their tires looked flat, we'd fill them to 28PSI.  If customers told us the car was riding hard, we'd lower the tires to 28PSI. Basically, whatever they said, we'd fill them to 28PSI and they never had another complaint. Radial tires had been around for a while, but people still looked at them and worried they were low. Over the course of two summers, I bet I filled thousands of tires to...28PSI. Today, no matter what the side of the tire says...guess what PSI I fill them to? PRL
 

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In the summer, I run some 245/45ZR20 BF Goodrich tires on my wagon.  Talk about not being able to tell if a tire is low on air?  

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The tire pressures stated on the sidewall specify the pressures at maximum rated load (for the TIRE).  The correct tire pressure as installed on the car is found in the owner's manual (or the door-jamb decal on late model cars).

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14 hours ago, EmTee said:

The tire pressures stated on the sidewall specify the pressures at maximum rated load (for the TIRE).  The correct tire pressure as installed on the car is found in the owner's manual (or the door-jamb decal on late model cars).

 

The pressure stated in the owners manual is good to use IF the tire that you are installing is the same construction type and size as the original factory tire.  When you deviate from original tires to something else, like radials instead of bias ply tires, you need to go more with the manufacturer's guidance.

 

You are correct about the pressure stated on the sidewall being the pressure at which the tire can sustain maximum rated load (also stated on the tire).  So you can look at that rated load compared to the tire's actual load, on the car, with luggage and passengers, and decide if you want to reduce the tire pressure by a few psi below that max pressure value. 

 

In general, tires with more inflation pressure run cooler for the same load.  Low pressure in tires for a given load (especially loads close to rated load) run hot.  Hot tires come apart inside and can fail.  So I prefer to err on the higher pressure side.

 

I never see excess wear in the center of my tires due to overinflation.

 

HTH.

 

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The 3827S is the carb the factory put on the 300 cubic inch V8 in 1965 with a standard transmission.

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On 2/17/2019 at 2:50 AM, BBK said:

Hey, 

 

I read in the chassis manual that the Riv needs a tire pressure from 24 psi which is about 1,65 bar. Actually I had 33 psi (2,3 bar). On 24 psi the tire was so flat that I don't trust to drive and fill in 33 psi again. I have a 225 75 r15 tire. 

 

What tire pressure you drive? 

 

Also I drive some 1/4 mile times and recognized some heavy misfiring when I reach about 80 mph. Before 80 miles I don't have problems. What can happened? 

Check your coil. Many high speed misfires are do to the coil breaking down. If you  have the old style coil. At lower rpms the coil can handle the load. But, as speed increase spark needs to coincide. And if it doesn't then you'll have a misfire.. Try this simple test. After freeway driving. When you park. Feel your coil. If it's unusually hot, that's an indication the coil is failing. If the coil isn't the problem then it may be fuel starvation. Hope that helps. 

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Your comparing apples and oranges.  Original bias ply tires vs. newer radial tires.  

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I have heard that apples to oranges comparison a lot of times. I just have trouble figuring out which apples to compare to which oranges.

1890950243_TypesofApples.jpg.ebea9e5ff9df3b76f5a5aac1eefa4b82.jpgTypesOrange.thumb.jpg.9e6c72c37576185071454c51d8a1607f.jpg

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Granny Smith to Navel.

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