Jonsered 670 Chainsaw (Porting & Builds)
A Jonsered 670 can be a strong 67 cc-class platform, but a reliable build starts with the exact saw, bar, chain, and service limits—not a guessed port map or carb setting. Inspect and measure it first, repair faults, then have porting and tuning done by a qualified saw technician. Test the finished saw under load before regular cutting.
If your 670 is dull, low on bar oil, or not running right, porting is not the first fix. A sharp chain and sound engine matter more than modified ports. Porting changes how the engine breathes, so a mistake can mean poor performance, unsafe chain speed, or a seized piston.
I treat a build as a sequence: identify the saw, inspect it, decide whether the parts are healthy, and only then consider modifications. The details below help you plan that work without guessing at fuel mix, chain fit, or engine limits.
Equipment Setup & Safety Preparation
A build begins with the saw as it sits, not with a grinder or a new muffler. Identify the exact 670 version and inspect its safety parts, fuel system, cutting gear, and service information. The 670 family is about 66.8 cc, but parts and specifications can vary with model and configuration.
Identify the saw and cutting setup
Read the model and serial markings, then compare them with the parts information or manual for that saw. Do not use a newer Jonsered manual as a stand-in unless it confirms compatibility. Before ordering parts, record the bar mount, chain pitch, gauge, and drive-link count.
A 3/8-inch pitch is common on these saws, but confirm the pitch stamp on both bar and drive sprocket. Chain gauge is set by the bar groove; some setups use .058-inch chain, but .050 and .063-inch options exist. The chain must match the bar and sprocket. Engine displacement alone does not determine what fits.
| Item | What to confirm | Why it matters |
|---|---|---|
| Bar and chain | Pitch, gauge, drive-link count | Prevents a mismatch that can bind or derail |
| Drive sprocket | Pitch and wear | Must match the chain; heavy wear can damage a new chain |
| Bar length | Bar stamp and condition | An 18–24-inch bar is a practical starting range, not a universal limit |
| Fuel mix | Exact saw manual and oil specification | Ratios can differ; do not infer one from another model |
| Carburetor | Exact carb type and service limits | Porting can change tuning needs |
A longer bar puts more load on the engine and chain. Use one only when the wood, chain, saw condition, and your skill allow it. Keep the bar oil reservoir filled with quality chainsaw bar-and-chain oil, then check that oil reaches the chain. Flow varies with setup and conditions, so do not rely on a supposed universal flow rate.
Inspect before modifying
With the engine off and cool, inspect the chain brake, throttle lockout, stop switch, anti-vibration mounts, chain catcher, fasteners, sprocket, bar rails, and oil delivery. Repair defects before testing or porting. Check compression and fuel-system condition using the correct service procedure; a basic inspection cannot prove that every internal part is sound.
Wear a helmet with eye and face protection, hearing protection, chainsaw-rated leg protection, gloves, and boots. Set an exclusion zone and clear escape routes before cutting. Follow applicable requirements, including ANSI Z133 and OSHA 29 CFR 1910.266 for logging work. Do not fell trees unless you have the training, equipment, and escape route for the job.
Field Cutting & Landscaping Operation
A modified saw still needs the same controlled work habits as a stock saw. Before starting, assess the tree, lean, wind, dead limbs, footing, and nearby targets. Confirm chain tension, fuel, bar oil, brake function, and a clear work area; do not use extra power to compensate for poor setup or technique.
Start and test with control
Start the saw on stable ground with the chain brake engaged and the bar clear, following the model’s starting procedure. Once it runs, confirm that the chain stays still at idle, test the brake as directed by the manual, and warm the engine without prolonged idling or high-speed revving with no load.
Keep two hands on the saw and maintain a firm stance. Keep the bar nose clear of the work, and do not cut above shoulder height. Use suitable felling and limbing methods, and watch for wood closing on the bar. If the chain binds, stop and assess the cut; do not force the saw through it.
Check the build under load
A ported engine must be checked for stable operation, adequate lubrication, and safe tuning under cutting load. A tachometer helps a qualified technician compare engine speed with the limit for the exact saw and setup. Do not apply a generic maximum-RPM or idle-RPM figure to a modified engine.
Stop work if the engine surges, runs unusually hot, loses power, or will not return to idle. A lean-running two-stroke can overheat and seize. Stale fuel, a restricted fuel supply, an air leak, or a poor post-porting carburetor setting can contribute. Shut it down and diagnose the cause before restarting.
Representative build scenarios
These examples show how I would approach common workshop decisions. They are not records of particular customers or a promise that a given change will produce a certain result.
| Scenario | Sensible next step | Avoid |
|---|---|---|
| A 670 burns wood and feels weak | Sharpen the chain, inspect the bar and sprocket, check air filter and fuel supply | Porting before ruling out basic faults |
| A sound saw is being considered for a build | Confirm model, compression condition, carburetor, ignition, and service limits; consult a qualified builder | Copying another saw’s port dimensions |
| A modified saw surges or gets hot | Stop cutting and check for air leaks, fuel restriction, and carburetor calibration | Leaning the mixture to chase higher speed |
| A longer bar is planned | Verify bar mount and chain match, then assess load and oil delivery | Choosing bar length by displacement alone |
After each cutting session, engage the brake, shut the engine off, and let it cool before refueling or maintenance. Check chain tension, oil level, loose parts, and damage. Clear debris from the cooling fins, air filter, bar groove, and sprocket cover as needed.
Maintenance, Sharpening & Tuning
Maintenance protects the engine and makes a ported saw easier to assess. Start with a correctly matched, sharp chain and an intact bar. Then check fuel delivery, ignition, cooling, compression, and lubrication. A modification cannot compensate for a dull cutter, worn sprocket, blocked oil path, or engine air leak.
Sharpen the chain to its own specification
Use the chain manufacturer’s file size and filing angle, not a value guessed from the saw model. Those specifications depend on the chain type. File cutters evenly, keep the correct angle, and use a depth-gauge tool matched to the chain. A .025-inch depth-gauge setting is common on many full-size chains, but it is not universal.
If you do not know the chain model, check its markings or packaging before filing. Replace a chain with cracked, damaged, badly worn, or incorrectly filed cutters. Do not force a dull chain through wood; it can bind, increase kickback risk, and make the engine work harder.
Inspect the bar rails for spreading, burrs, and uneven wear. Clean the groove and oil hole, and turn the bar if its design permits. Replace a worn sprocket rather than fitting a new chain to one with severe wear. These checks are a better first step than adding engine power.
Plan porting and carburetor work carefully
Porting changes the cylinder’s gas flow. It may involve reshaping ports, but the safe dimensions depend on the exact cylinder, intended use, and measured condition. I would not copy internet port timing or remove material based on a generic recipe. A small measurement error can change power delivery and reliability.
Before porting, a qualified technician should identify the cylinder and carburetor, inspect the engine, and agree on the intended work. Afterward, the technician should verify fuel delivery, ignition, compression, cooling, carburetor calibration, and chain lubrication. Use a tachometer and the applicable service limits for the exact saw. A ported saw may need a different carb setting, but that is not permission to exceed its safe specification.
Match the fuel and bar oil
Use the fuel ratio and two-stroke oil specification stated in the manual for your exact saw. If the manual is missing, find reliable model-specific service information before mixing fuel. Do not guess from a newer Jonsered model, and never run straight gasoline or an unverified oil ratio. Inadequate lubrication can quickly damage the piston and cylinder.
Use fresh fuel handled and stored as directed by the manual and fuel-oil maker. Keep the bar-oil tank filled with proper bar-and-chain oil. After work, check that the oil system is delivering to the chain; a full tank alone does not prove that the bar is being lubricated.
Storage & Seasonal Care
Storage affects the next start and can prevent avoidable fuel-system trouble. Let the saw cool, then clean it and follow the exact manual’s storage instructions. Depending on that procedure, drain fuel or use the specified method; do not leave stale fuel in the carburetor without checking the manufacturer’s guidance.
Clean the air filter, cooling passages, bar groove, and sprocket area. Inspect the chain and bar before storage, cover the chain, and keep the saw dry. Store fuel away from ignition sources and in an approved container. When bringing the saw back into service, inspect the safety parts and cutting gear before starting.
If the engine surges, runs hot, loses power, or will not settle to idle, do not put it away as a problem to solve next season. Diagnose it first. An air leak or restricted fuel flow can make a two-stroke run lean and seize, sometimes with little warning.
Bottom line: Get the exact model and parts configuration, restore the saw to sound condition, and use chain and fuel specifications from the correct sources. Porting should be measured and professionally tuned, then checked under load. If a basic fault remains, fix it before modifying the engine.
FAQs
How much displacement does a Jonsered 670 have?
The 670 family is approximately 66.8 cc. Confirm the exact model and parts configuration before ordering parts or selecting a service specification.
What bar length should I use on a 670?
An 18–24-inch bar is a practical starting range, but fit depends on the exact mount, chain, sprocket, wood, and operator skill. A longer bar increases load.
What chain pitch does the 670 use?
Three-eighths-inch pitch is common. Check the bar and drive-sprocket markings to confirm they match before fitting a chain.
What chain gauge fits a Jonsered 670?
Gauge depends on the bar. Some setups use .058-inch chain, but check the bar stamp and chain packaging rather than assuming a gauge.
What fuel ratio should I use?
Use the ratio and oil specification in the manual for the exact saw. Do not guess or borrow a ratio from a newer model.
What file size and angle should I use?
Use the chain maker’s specifications for the exact chain. The saw model does not set the file size or filing angle.
Is a .025-inch depth-gauge setting correct?
It is common for many full-size chains, but it is not universal. Use the chain maker’s specification and a matching depth-gauge tool.
Can I use a generic RPM limit after porting?
No. Identify the exact carburetor and service limits, then have a qualified technician check the modified saw with a tachometer.
Should I port a saw that cuts slowly?
Not before checking chain sharpness, bar condition, sprocket wear, air filter, fuel supply, and engine health. Fix basic faults first.
What should I do if a ported saw surges or runs hot?
Stop the saw and diagnose it before restarting. Check for issues such as air leaks, fuel restriction, or incorrect carburetor calibration with a qualified technician.
(This article was written by one of our staff writers, David Hensley. Visit our Meet the Team page.)
