<![CDATA[JR Race Car: Latest News]]> https://www.jrracecar.com Mon, 15 Jun 2026 07:40:56 +0000 <![CDATA[Carburator Tune-up]]> https://www.jrracecar.com/blog/carburator-tuneup/ Wed, 23 May 2018 09:59:59 +0000 https://www.jrracecar.com/blog/carburator-tuneup/ This time of year we get a lot of questions about tuning up carburetors.  Most engine building and fellow racers will ask about tuning indicators to help change the tune-up.  EGT (exhaust gas temperature) and CHT (cylinder head temperature) are two of the most common tune up indicates. There are other indicates that racers and crew chiefs describe such as a bog or stumble, misfire or pop, surging or unstable RPM.

The Basics

The carburetor is where air and fuel are mixed and sent down the intake manifold to the engine.  Air velocity through the throat or venturi of the carburetor draws fuel from the bowl and up into the air stream.  The fuel pump only flows fuel into the bowl, it is the air velocity through the throat that draws fuel into the air stream.

The tune up of the carburetor will change with changes to atmospheric pressure.  Traveling to other tracks and going up or down in elevation my require a change to your carburetor.    Going up in elevation with less atmospheric pressure will need less fuel (smaller jet), while going down in elevation will need more fuel (larger jet).

Jet Selection (Mikuni Carburetors)

There are a variety of jets available, generally speaking the larger the jet number the bigger the passage through the jet.  There are two fuel jets that control how much fuel passes from the bowl into the venturi, a pilot jet and main jet.  The pilot jet is used at when the slide is low is closed, or nearly closed.  The pilot jet controls many of the staging indicators such as temperatures and RPM stability.

The main jet controls the fuel at wide open throttle, in drag racing this is everything past about 3 ft.  The main jet controls surging or miss firing on the big end of the track.  The balance tween the pilot and main jets controls the off idle response of the carburetor.  If the two jets are out of balance then there may be a bog or stumble when the throttle goes from idle to wide open.

The air jet, or air adjustment on the carburetors is is a passage that lets air into the fuel stream about half way between the bowl and venturi.  This air helps brake the fuel up into smaller droplets.  The more air (bigger jet, or adjustment screw out) that passes through the less fuel is drawn from the bowl.


The Tune-Up

Tuning a carburetor is a little of an art form.  Each person may do it a little differently or have different options on different indicators.  Two of the same carburetors may have a different jetting and they both run well. The real test of the tune-up is did it go down track smooth and consistent?  If your car is running with no pops, bogs, misfires, surging, or hunting at staging, and is consistent in time; Don't fix it!  CHT and EGT go out the window, they don't matter if the car runs good and consistent.  

If you need tuning advice please call or e-mail us with your question, there are far too may specifics to cover in this article.  We also have a chart in our Tech Department page for tuning of basic indicators. 

]]> This time of year we get a lot of questions about tuning up carburetors.  Most engine building and fellow racers will ask about tuning indicators to help change the tune-up.  EGT (exhaust gas temperature) and CHT (cylinder head temperature) are two of the most common tune up indicates. There are other indicates that racers and crew chiefs describe such as a bog or stumble, misfire or pop, surging or unstable RPM.

The Basics

The carburetor is where air and fuel are mixed and sent down the intake manifold to the engine.  Air velocity through the throat or venturi of the carburetor draws fuel from the bowl and up into the air stream.  The fuel pump only flows fuel into the bowl, it is the air velocity through the throat that draws fuel into the air stream.

The tune up of the carburetor will change with changes to atmospheric pressure.  Traveling to other tracks and going up or down in elevation my require a change to your carburetor.    Going up in elevation with less atmospheric pressure will need less fuel (smaller jet), while going down in elevation will need more fuel (larger jet).

Jet Selection (Mikuni Carburetors)

There are a variety of jets available, generally speaking the larger the jet number the bigger the passage through the jet.  There are two fuel jets that control how much fuel passes from the bowl into the venturi, a pilot jet and main jet.  The pilot jet is used at when the slide is low is closed, or nearly closed.  The pilot jet controls many of the staging indicators such as temperatures and RPM stability.

The main jet controls the fuel at wide open throttle, in drag racing this is everything past about 3 ft.  The main jet controls surging or miss firing on the big end of the track.  The balance tween the pilot and main jets controls the off idle response of the carburetor.  If the two jets are out of balance then there may be a bog or stumble when the throttle goes from idle to wide open.

The air jet, or air adjustment on the carburetors is is a passage that lets air into the fuel stream about half way between the bowl and venturi.  This air helps brake the fuel up into smaller droplets.  The more air (bigger jet, or adjustment screw out) that passes through the less fuel is drawn from the bowl.


The Tune-Up

Tuning a carburetor is a little of an art form.  Each person may do it a little differently or have different options on different indicators.  Two of the same carburetors may have a different jetting and they both run well. The real test of the tune-up is did it go down track smooth and consistent?  If your car is running with no pops, bogs, misfires, surging, or hunting at staging, and is consistent in time; Don't fix it!  CHT and EGT go out the window, they don't matter if the car runs good and consistent.  

If you need tuning advice please call or e-mail us with your question, there are far too may specifics to cover in this article.  We also have a chart in our Tech Department page for tuning of basic indicators. 

]]> <![CDATA[High Powered Parts]]> https://www.jrracecar.com/blog/high-powered-parts/ Fri, 11 May 2018 13:21:19 +0000 https://www.jrracecar.com/blog/high-powered-parts/ JR Race Car is currently the only manufacturer for the Hercules Chassis and the original manufacturer for the Shockwave Clutch. As an engine builder, JR Race Car has made the ZR, and ZR-4 and Titan Engines (with roller lifter technology). We have the setup for every jr drag racer whether you want to race 12.90, 8.90 or 7.90. We supply everything from turn key chassis, to engine parts and clutch parts. JR Race Car is also the only manufacturer to make a full billet block for the "open class" Briggs and Stratton animal for Karting.

]]> JR Race Car is currently the only manufacturer for the Hercules Chassis and the original manufacturer for the Shockwave Clutch. As an engine builder, JR Race Car has made the ZR, and ZR-4 and Titan Engines (with roller lifter technology). We have the setup for every jr drag racer whether you want to race 12.90, 8.90 or 7.90. We supply everything from turn key chassis, to engine parts and clutch parts. JR Race Car is also the only manufacturer to make a full billet block for the "open class" Briggs and Stratton animal for Karting.

]]> <![CDATA[Our Proud History]]> https://www.jrracecar.com/blog/our-proud-history/ Fri, 11 May 2018 10:01:03 +0000 https://www.jrracecar.com/blog/our-proud-history/ In 1992, JR Race Car began manufacturing parts for the newly accepted NHRA Junior Drag Racing. JR Race Car initially offered components such as turn key Jr dragster chassis, engines, clutches and chassis components (manufactured in house from the beginning!). Over twenty years in Jr Drag Racing, JR Race Car has led the way in Jr Dragster manufacturing from initially machining a large portion of the CNC parts for other vendors in the market, to designing and machining its own ZR, Titan, and ZR-4 Engines and revolutionizing the market with the Shockwave and Shockwave OD CVT (centrifugally variable transmission) clutches and the innovative Hercules Chassis. JR Race Car is the only place to go for NHRA, IHRA, and ADRL Jr drag racing components.

]]> In 1992, JR Race Car began manufacturing parts for the newly accepted NHRA Junior Drag Racing. JR Race Car initially offered components such as turn key Jr dragster chassis, engines, clutches and chassis components (manufactured in house from the beginning!). Over twenty years in Jr Drag Racing, JR Race Car has led the way in Jr Dragster manufacturing from initially machining a large portion of the CNC parts for other vendors in the market, to designing and machining its own ZR, Titan, and ZR-4 Engines and revolutionizing the market with the Shockwave and Shockwave OD CVT (centrifugally variable transmission) clutches and the innovative Hercules Chassis. JR Race Car is the only place to go for NHRA, IHRA, and ADRL Jr drag racing components.

]]> <![CDATA[The CVT Clutch System]]> https://www.jrracecar.com/blog/the-cvt-clutch-system/ Wed, 25 Apr 2018 10:05:00 +0000 https://www.jrracecar.com/blog/the-cvt-clutch-system/ The CVT clutch system is a simple system. The clutch balances the engine RPM with the jack shaft RPM, that is it. All the clutch does is keep the balance between engine and jack shaft.

Starting from a stop:

At the start, aka 'the hit', the primary clutch engages on the belt to provide driving force to the belt. The belt will slip through the primary as the car starts to move forward. The belt slips because the driven clutch is not moving or starts to move very slowly and the engine is turning several thousand RPM. If the belt did not slip, the engine would come to a stop and the car would not move.

Full Engagement:

As the jack shaft gains in RPM the belt will stop slipping through the primary and become fully engaged. The engine RPM will start to increase as the jack shaft gains more RPM. At this point the clutch is still in 'low gear', that is the belt is still in the bottom of the primary and the top of the secondary. The engine will gain in RPM until the clutch reaches the shift point, just like driving a car in 1st gear.

The Shift Point:

The shift point is where the driven clutch starts to open and the belt moves up off the bottom primary, the belt is now 'floating' in the clutch, meaning the belt is not at the top or bottom of either sheave. The shift point is where the CVT clutch starts to work. The weight of the arm in the primary apply clamping force to the belt and this clamping force keeps the engine RPM constant. The engine RPM will fall quickly to a stable (flat line) RPM, this quick decrease in RPM is due to the clutch changing the ratio between the engine RPM and jack shaft RPM. The jack shaft will continue to gain in RPM as the vehicle speed increases while the engine is held to a stable RPM due to the flyweights clamping force.

One to One point (1:1 point):

Until this point the engine has been turning faster than the jack shaft. The belt has been going through the primary sheaves at a smaller diameter than the driven sheaves. The 1:1 point is where the engine and jack shaft are turning the same RPM, the belt is going through both sheaves at the same diameter.

Overdrive

As the jack shaft increases in RPM with vehicle speed and the engine is held stable due to flyweight clamping force, the jack shaft will turn faster then the engine. This is known as Overdrive because the clutch is driving the jack shaft RPM faster than the engine RPM. The clutch will continue to shift until full shift or high gear is reached. Height gear is where the belt is at the top of the primary sheave and the bottom of the driven sheaves.

]]> The CVT clutch system is a simple system. The clutch balances the engine RPM with the jack shaft RPM, that is it. All the clutch does is keep the balance between engine and jack shaft.

Starting from a stop:

At the start, aka 'the hit', the primary clutch engages on the belt to provide driving force to the belt. The belt will slip through the primary as the car starts to move forward. The belt slips because the driven clutch is not moving or starts to move very slowly and the engine is turning several thousand RPM. If the belt did not slip, the engine would come to a stop and the car would not move.

Full Engagement:

As the jack shaft gains in RPM the belt will stop slipping through the primary and become fully engaged. The engine RPM will start to increase as the jack shaft gains more RPM. At this point the clutch is still in 'low gear', that is the belt is still in the bottom of the primary and the top of the secondary. The engine will gain in RPM until the clutch reaches the shift point, just like driving a car in 1st gear.

The Shift Point:

The shift point is where the driven clutch starts to open and the belt moves up off the bottom primary, the belt is now 'floating' in the clutch, meaning the belt is not at the top or bottom of either sheave. The shift point is where the CVT clutch starts to work. The weight of the arm in the primary apply clamping force to the belt and this clamping force keeps the engine RPM constant. The engine RPM will fall quickly to a stable (flat line) RPM, this quick decrease in RPM is due to the clutch changing the ratio between the engine RPM and jack shaft RPM. The jack shaft will continue to gain in RPM as the vehicle speed increases while the engine is held to a stable RPM due to the flyweights clamping force.

One to One point (1:1 point):

Until this point the engine has been turning faster than the jack shaft. The belt has been going through the primary sheaves at a smaller diameter than the driven sheaves. The 1:1 point is where the engine and jack shaft are turning the same RPM, the belt is going through both sheaves at the same diameter.

Overdrive

As the jack shaft increases in RPM with vehicle speed and the engine is held stable due to flyweight clamping force, the jack shaft will turn faster then the engine. This is known as Overdrive because the clutch is driving the jack shaft RPM faster than the engine RPM. The clutch will continue to shift until full shift or high gear is reached. Height gear is where the belt is at the top of the primary sheave and the bottom of the driven sheaves.

]]>